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Functional Organization of Motoneuron Pool and its Inputs

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Abstract

The sections in this article are:

1 Morphological Considerations
1.1 Columnar Arrangement of Motoneuron Pool
1.2 Dimensions of α‐Motoneurons and Distribution of Cell Size
1.3 Scaling of Motoneurons
1.4 Initial Segment of α‐Motor Axons
1.5 Axon Collaterals of α‐Motoneurons
1.6 Recurrent Inhibitory Feedback from Motoneurons
1.7 Direct Synaptic Interconnections Between Spinal Motoneurons
1.8 Species of Motoneurons
1.9 Terminals of Motoneurons in Muscle
1.10 Morphology of Neuromuscular Junctions
1.11 Matching the Properties of Motoneurons and the Muscle Fibers They Supply
1.12 Concluding Comments
2 Firing Patterns of Individual Motoneurons and Motor Units
2.1 Functional Significance of Size of Motoneurons
2.2 Measurement of Total Output of Motoneuron Pools
2.3 Critical Firing Levels of Motoneurons
2.4 Relation of Critical Firing Level to Axon Diameter and Motoneuron Size
2.5 Effects of Inhibitory Inputs on Critical Firing Level and Rank Order During Repetitive Firing
2.6 Recruitment of Motor Units in Humans
2.7 Evidence Regarding Alternative Patterns of Recruitment
2.8 Evidence Regarding Voluntary Selective Control of Motor Units
2.9 Size Principle in Other Species
2.10 Modulation of Firing Rate
3 Organization of Input to Motoneuron Pools
3.1 Anatomical Studies
3.2 Techniques Used to Study EPSPs Elicited by Impulses in Single Afferent Fibers
3.3 Amplitudes of EPSPs Elicited by Impulses in Single Fibers
3.4 Boutons of Ia‐Fibers on Motoneurons
3.5 Physiology of Ia‐Terminals
3.6 Distribution of Ia Excitation to Motoneuron Pools
3.7 Comparison of Projections to Homonymous and Heteronymous Motoneurons
3.8 Correlations Between Morphology and Function
3.9 Latency of EPSPs
3.10 Other Examples of Divergence in Inputs to Motoneurons
3.11 Ia‐Projections to Motoneurons Controlling Other Parts of the Body
3.12 Group II Input From Secondary Endings in Muscle Spindles
3.13 Inhibitory Inputs to Motoneurons
3.14 Group Ib Input From Golgi Tendon Organs
3.15 Monosynaptic Input From Descending Pathways
3.16 Topographic Factors Governing Development of Connections of Ia‐Fibers to Motoneurons
3.17 Concluding Comments
4 Nonuniformity of Motoneurons
4.1 Early Classification of Tonic and Phasic Types of Motoneurons
4.2 Significance of Nonuniformity of Muscle Fibers
4.3 Motoneuron Properties Independent of Size
4.4 Differential Responses of Motoneurons to Injected Currents
4.5 Influence of Muscle on Developing and Mature Motoneurons
4.6 Evidence From Human Disease
4.7 Concluding Comments
5 How Size of Motoneurons Determines their Susceptibility to Discharge
5.1 Properties of Motoneurons That Influence Susceptibility to Discharge
5.2 Role of Input in Determining Susceptibility to Discharge
6 Some Principles Underlying Organization of Motoneuron Pools
6.1 How Sensitivity in Gradation of Tension is Achieved
6.2 Basis for Relation Between Motoneuron Size and the Force Its Motor Unit Develops
6.3 Actual Sensitivity in Grading Muscular Tension
6.4 Mathematical Derivation of a “Principle of Maximum Grading Sensitivity”
6.5 Recruitment Order and Minimum Energy Principle
6.6 Collective Action of Motoneuron Pool: Role of Input
6.7 The Size Principle in Ia and Group II Sensory Fibers
6.8 How Does the Central Nervous System Use the Motoneuron Pool?
Figure 1. Figure 1.

Reconstructions of medial gastrocnemius (MG) and soleus (SOL) nuclei from serial sagittal sections in cat. Right, dorsal view of spinal cord outline (white matter–pia boundary in heavy lines) on which are superimposed positions (dots) of MG (left hemicord) and SOL motoneuron cell bodies (right hemicord). Boundaries between L6, L7, and S1 segments (identified by dorsal root entry zones), are indicated by horizontal heavy lines; midline denoted by vertical dashed line. Dashed lines across the cord denote levels (labeled A–E) at which reconstructions of cross sections were made. Left, reconstructions of cross sections at levels A–E showing white matter–pia boundary in light lines and gray‐white matter boundary in heavy lines. Dashed lines at most lateral parts of cross sections indicate estimated outline; the most lateral parts were lost in sectioning. All diagrams drawn on same scale. Neurons indicated on each cross‐section diagram are cells located within 300 μm rostral and caudal to that level.

From Burke et al. 47
Figure 2. Figure 2.

Distribution of motoneuron sizes in the 7th cervical segment of 6 normal rhesus monkeys.

Adapted from Hodes et al. 161
Figure 3. Figure 3.

A: diameters of sciatic α‐motor axons of cat at initial segment (IS) are plotted against mean diameters of their parent cell bodies (n = 68). Linear correlation coefficient (r) = +0.522 (P < 0.001). B: mean diameters of sciatic α‐motor axons in white matter are plotted against mean diameters of their parent cell bodies (n = 42). Linear correlation coefficient (r) = +0.516 (P < 0.001).

From Cullheim 80
Figure 4. Figure 4.

Axon conduction velocities in cat are plotted in A against size of parent cell bodies (n = 24), in B against axon diameters at initial segment IS (n = 24), and in C against axon diameters in white matter (n = 33). In A the linear correlation coefficient (r) = +0.673 (P < 0.001); in B, r = +0.869 (P < 0.001); and in C, r = +0.804 (P < 0.001).

From Cullheim 80
Figure 5. Figure 5.

Relation between axonal diameters in spinal cord white matter and diameters of initial segments (IS) for 39 sciatic α‐motor axons in cat. Linear correlation coefficient (r) = +0.698 (P < 0.0001).

From Cullheim and Kellerth 82
Figure 6. Figure 6.

Relation between maximum tetanic tension of individual motor units in cat soleus muscle and conduction velocity of their axons in 3 different experiments.

From McPhedran, Wuerker, and Henneman 253
Figure 7. Figure 7.

Relation between logarithm of tetanic tensions developed by motor units and the conduction velocities of their motor axons in 4 leg muscles of the cat. Heavy dots on vertical bars indicate mean value of tension ± SD. For peroneus longus and tibialis anterior, intervals corresponding to conduction velocities under 85 m/s contained too few motor units to allow valid calculation of mean tension. Range of tensions developed by these motor units are represented by a vertical bar. Open circle represents tension of a single unit. For soleus, 4 motor units innervated by axons, either faster than 90 m/s or slower than 60 m/s. are not represented.

From Jami and Petit 185
Figure 8. Figure 8.

Distribution of maximal tetanic tensions of motor units of cat's soleus (A) and medial gastrocnemius (B).

A, from McPhedran, Wuerker, and Henneman 253; B from Wuerker, McPhedran, and Henneman 353
Figure 9. Figure 9.

Smoothed version of histogram in Figure 8B, illustrating distribution of some of the properties of motor units in typical limb muscles. Height of columns indicates relative numbers of motor units with the properties listed below.

From Henneman 154. Skeletal muscle: the servant of the nervous system. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980
Figure 10. Figure 10.

Reflex discharges recorded from a filament of 7th lumbar ventral root in response to stimulation of ipsilateral sciatic nerve of cat. Numbers at right of each tracing indicate relative intensity of stimulation. Initial deflection on left of each tracing is the early discharge referred to in text.

From Henneman 153
Figure 11. Figure 11.

Stretch‐evoked responses of 2 α‐motoneurons recorded from a filament of 7th lumbar ventral root of cat. Amount of tension applied and developed reflexly is indicated by separation of 2 top beams in each frame. Several seconds elapsed between successive frames while muscle was stretched 1,2,3,4,5 and released 6,7,8,9.

From Henneman et al. 159
Figure 12. Figure 12.

Stretch‐evoked responses of 5 α‐motoneurons recorded from a filament of the 1st sacral ventral root of cat. Numbers above action potentials indicate rank of units according to size.

From Henneman et al. 159
Figure 13. Figure 13.

Frequency distribution of thresholds of tonic responses to stretch in 3 experiments on cats. Abscissa, threshold tension of deefferented triceps muscle. Ordinate, number of units whose thresholds fell between values indicated on abscissa.

From Henneman et al. 159
Figure 14. Figure 14.

A: orderly recruitment in cat of 3 triceps motoneurons of different sizes in response to increasing degrees of stretch. Tension produced by stretch of triceps muscle indicated by separation of top traces in each frame. B: orderly inhibition of same 3 units during a constant stretch of 4 kg. Records 1–4 on left show control responses of stretch before, between and after 3 inhibitory stimulations. Records on right were obtained during 100/s stimulation of ipsilateral deep peroneal nerve at 3 intensities. Largest unit was silenced first (line 1), intermediate unit next (line 2), and smallest unit last (line 3). Note lasting effects produced by brief inhibitions, i.e., failure to recover to previous levels in lines 2 and 4.

From Henneman et al. 160
Figure 15. Figure 15.

Graphic representation of discharge frequency of 3 motoneurons of different sizes (AC) in triceps surae of cat, and of contractile tension developed by the muscle itself (D) in response to varying degrees of excitation and inhibition: x‐axis, intensity of inhibitory stimulation applied to ipsilateral deep peroneal nerve; y‐axis, frequency of discharge in AC and contractile tension in D; z‐axis, stretch of triceps surae, in millimeters. Data for all 4 graphs obtained simultaneously. Cells were subjected simultaneously to various mixtures of excitatory (z‐axis) and inhibitory (x‐axis) stimuli. Each plotted point represents mean of 2 successive determinations. Inhibitory stimuli were 100/s shocks applied for 0.9‐s periods at 4‐s intervals. Note that unit A (smallest) fired spontaneously without stretch, unit B began to discharge between 0 and 5 mm of stretch, and unit C (the largest) between 5 and 10 mm.

From Henneman et al. 160
Figure 16. Figure 16.

Scheme of experiment used to measure maximal monosynaptic discharge of a motoneuron pool. DR, dorsal root; VR, ventral root.

From Clamann, Henneman, et al. 59
Figure 17. Figure 17.

Pairs of responses in cat recorded as in Figure 16 from proximal and distal halves of 1st sacral ventral root in response to single‐shock stimulation of combined nerves to medial and lateral gastrocnemius (MG and LG). Left and right deflections of each pair are antidromic and reflex responses, respectively, displayed on superimposed traces of a 2‐beam oscilloscope. First 10 pairs in top row recorded before 12‐s tetanus (500 shocks/s) to MG and LG nerves. Subsequent pairs recorded at 2‐s intervals after tetanus.

From Clamann, Henneman, et al. 59
Figure 18. Figure 18.

Simultaneous recordings in cat of a series of monosynaptic reflexes of triceps surae pool (top) and a single triceps motoneuron (bottom) showing critical firing level of the latter. Top trace, time integrals of monosynaptic reflexes recorded at 1/s from proximal half of 1st sacral ventral root. Height of each vertical line measures size of population response. Bottom trace, monosynaptic reflexes of a single triceps motoneuron recorded from a small filament of 7th lumbar ventral root. Records were made during decline in posttetanic potentiation following a brief train of conditioning shocks that were applied to the combined medial gastrocnemius and lateral gastrocnemius and soleus nerves at 500/s.

From Henneman et al. 155
Figure 19. Figure 19.

Critical firing levels of 3 triceps motoneurons in cat. Single unit in bottom left had a sharp threshold at 39%. The 2 units in bottom right tracing had thresholds of 60% and 33%. Plateau in top left record indicates 100% discharge level of triceps pool.

From Henneman et al. 155
Figure 20. Figure 20.

Susceptibilities of 2 pairs of cat plantaris motoneurons with known critical firing levels (CFLs) to repetitive discharge in cat. Electrical stimulation of plantaris nerve at 300 stimuli/s in both cases. Stimulus intensity (i.e., voltage) indicated by level of trace a with reference to base line d. Responses of plantaris units recorded on traces b and c. CFLs indicated at right end of these traces. Time marks at 1‐s intervals for both top and bottom records.

From Henneman et al. 155
Figure 21. Figure 21.

Differences in susceptibility to repetitive firing of 2 cat plantaris motoneurons whose critical firing levels were 12.0% (top) and 15.5% (bottom). A: shocks applied to plantaris nerve were supramaximal for all group Ia‐fibers. Frequency was switched from 20 shocks/s initially to 30 shocks/s at 1st arrow, and to 40 shocks/s at 2nd arrow. B: frequency of stimulation was 500 shocks/s throughout and intensity was increased gradually as trace moved from left to right.

From Henneman et al. 155
Figure 22. Figure 22.

Relation between axon size and critical firing level (rank order) of 21 cat plantaris motoneurons isolated in a single experiment.

From Clamann and Henneman 60
Figure 23. Figure 23.

Critical firing level (CFL) of a cat plantaris motoneuron with and without inhibitory input from lateral popliteal nerve. Top, time integrals of monosynaptic reflexes of plantaris population elicited at 2‐s intervals during declining phase of posttetanic potentiation. Bottom, simultaneous responses of a plantaris unit with a critical firing level of 61%. Alternate reflexes preceded by a 200‐ms train of pulses to lateral popliteal nerve. Horizontal line indicating critical firing level drawn as described in text.

From Clamann, Gillies, and Henneman 58
Figure 24. Figure 24.

Effects of recurrent inhibition (A) and lateral popliteal inhibition (B) in cat on monosynaptic reflexes (traces 1, 2) and repetitive firing (traces 3, 4) of a pair of plantaris motoneurons with critical firing levels of 18.6% (top) and 18.4% (bottom). A: in traces 1, 2 the underlined reflexes were preceded and accompanied by a 100‐ms train of shocks (100/s) to proximal portion of 7th lumbar ventral root. Antidromic volleys set up by stimulation in A1, A2 resulted in recurrent inhibition mediated by Renshaw cells. Of the 18 reflexes in trace 1, 9 were inhibited by this input; in trace 2 only 1 reflex was silenced. In A3, A4 the same pair of units is seen responding rhythmically to repetitive stimulation of the plantaris nerve. When continuous recurrent inhibition was begun, indicated by thick base line in A4, top unit ceased firing and bottom unit slowed to about one‐half original rate of discharge. B: same pair of units was subjected to inhibitory input elicited by stimulating lateral popliteal nerve at 100/s. In B1, B2 the underlined monosynaptic reflexes were somewhat more susceptible to this inhibition than to the recurrent inhibition. Of the 16 reflexes receiving this inhibition, 15 in trace 1 and 4 in trace 2 were silenced. In B3, B4 the same pair of units firing rhythmically in response to 100/s stimulation of the plantaris nerve was subjected to an inhibitory input whose intensity was gradually increased as the traces moved across the oscilloscope. The top unit was finally suppressed by this input whereas the bottom unit was only slowed in rate.

Clamann, Gillies, and Henneman 58
Figure 25. Figure 25.

Records illustrating quantitative relationships between intensity of excitatory stimulus causing 2 cat motoneurons with critical firing levels of 18.8% and 12.7% to discharge repetitively and intensity of inhibitory stimulus required to silence them. Stimulus strength of excitatory input (100/s stimulation of a plantaris nerve), indicated by level of line E with reference to base line B, was constant during each test but varied in different tests. Stimulus strength of inhibitory input (100/s stimulation of lateral popliteal nerve), indicated by line I, was increased as trace moved across screen. Intensity of inhibition required to silence each unit was measured at time of their last discharges, indicated by dashed lines. Time signals at top of record represent 1‐s intervals.

From Clamann, Gillies, and Henneman 58
Figure 26. Figure 26.

Graph showing linear relation between strength of excitatory stimuli causing each of 2 cat motoneurons to discharge repetitively and strength of inhibitory stimuli required to silence them. Data obtained as illustrated in Figure 25. Data for lower unit, with critical firing level of 12.7%, are plotted with filled circles and solid line; data for upper unit, with a critical firing level of 18.8%, are represented by open circles and dashed line.

From Clamann, Gillies, and Henneman 58
Figure 27. Figure 27.

Number of human motor units having the twitch tensions indicated. A: linear scale. B: logarithmic scale. Distributions are similar for all 3 subjects. The computed best‐fitting line on the semilog plot (B) indicates an approximately exponential relation between number of motor units and twitch tension.

From Milner‐Brown et al. 258
Figure 28. Figure 28.

A: firing pattern of a high‐threshold unit recorded from human extensor indicis during isometric contractions with different rates of rise of tension (A–D). Each registration shows (top to bottom) the spike record, isometric force, and tracking signal which was visually displayed to the subject. Time scale in bottom 2 records is one‐half that of top 2 records. During increasingly faster contractions, unit starts firing at successively lower force levels. B: dependence of time between beginning of muscle contraction and firing onset of 3 motor units on rate of rise of tension. Double logarithmic scale. Faster contraction causes earlier recruitment. C: change of threshold force of recruitment (ordinate) associated with variation of rate of rise of isometric tension. Recordings from 7 motor units of extensor indicis of 3 subjects (▪, ▴ •). D: regression line of change of threshold force of recruitment on rate of rise of isometric tension (y1 = −0.067x + 198.66) calculated from 12 motor units of extensor indicis. The increase of muscle force that occurs within the mean contraction time of the muscle is shown for comparison (y2 = 0.060x).

From Büdingen and Freund 30
Figure 29. Figure 29.

Records of EMG activity and force outputs of medial gastrocnemius (top 2 traces) and soleus (bottom 2 traces) muscles of cat, together with simultaneous record of activity of a small number of medial gastrocnemius (MG) motor axons in a natural filament of MG muscle nerve (3rd trace). During MG vibration (160 Hz, 90 μm) activity in whole MG increased and that in soleus (SOL) showed a smaller increase. Pinching the skin over the lateral ankle with toothed forceps produced 2 bursts of MG activity, with recruitment of large amplitude motor axons in the MG filament, but caused quite complete suppression of SOL activity. Isolated activity of 5 identifiable MG motoneuron axons is shown in A–E. Note slowing of unit A discharge during each burst of activity in the higher threshold units B–E.

From Kanda et al. 192
Figure 30. Figure 30.

Simultaneous recording of one continuously firing long‐interval motor unit and one intermittently firing short‐interval motor unit in human. A: increasing contraction. B: prolonged maximum effort. C, D: rapid twitches. Time bar, 100 ms.

From Grimby and Hannerz 139
Figure 31. Figure 31.

Properties of a small (A), medium (B), and large (C) motor neuron of lobster innervating the main power stroke muscle. 1, Soma diameters; 2, axonal conduction velocities; 3, amplitudes of action potentials recorded with 2 extracellular electrodes at different positions on the motor nerve; 4, amplitudes of excitatory junctional potentials (EJPs) recorded from a single muscle fiber (top trace in each record); 5, adaptation to a maintained intracellular depolarizing current; and 6, facilitation properties of extracellular EJPs (top trace in each record) during 50‐Hz stimulation. Note antifacilitation produced by the largest motoneuron. Time marks in row 5 (lowest trace), 0.1 ms.

From Davis 85
Figure 32. Figure 32.

Morphology of Ia‐collaterals to motoneurons and arrangements of boutons on dendrites. A: sagittal section through lumbosacral cord of 40‐day‐old kitten (section shown at right). Primary fibers in dorsal columns (a) send ventrally directed branches to ventral horn (b) to terminate in region of motoneurons (c and d) that have sagitally running dendrites. Primary afferent branches also give off branches to dorsal horn and intermediate zone; these are described in more detail in the original illustration of Scheibel and Scheibel 310 from which this figure is modified. B: scale drawing illustrating morphology of collaterals of a single triceps surae Ia‐fiber as revealed by horseradish peroxidase (HRP) filling. This 3‐dimensional representation provides both a cross‐sectional (transverse) and sagittal perspective. The Ia‐fiber in the dorsal columns is drawn as a thick black line along with the dorsal root ganglion cell and dorsal root portion of the cell. Four collaterals emanating from the dorsal column region are illustrated, each one coursing through the dorsal horn (in a ventrallateral and rostral direction) to terminate in the cylindrical motoneuron region (lamina IX). Other terminals are given off by the same fiber in laminae VI and VII. C: morphology of a single soleus Ia‐fiber in the dorsal columns with 9 branches given off to ventral horn; HRP technique. Two motoneurons—soleus (SOL), which innervates type S motor unit, and lateral gastrocnemius (LG), which innervates a type F, fatigue‐resistant motor unit (R)—were also filled with HRP. Circles and stippling denote somata and maximum dendritic extents of these neurons. Note that at most only 2 of 9 Ia‐fiber branches have access to each of these motoneurons. Also ventrally directed collaterals of this fiber do not exhibit rostral component shown by Brown and Fyffe (Fig. 32B). Top and bottom solid lines denote dorsal and ventral borders of cord. Dotted lines represent dorsal and ventral limits of ventral horn. D: origins of primary collaterals of MG, LG‐S, and posterior tibial (PT) Ia‐fibers. E: schematic drawing of disposition of Ia‐fiber boutons on α‐motoneurons. In each case, both Ia‐fiber and motoneuron filled with HRP. Note in SOL motoneuron that bouton A is on proximal dendrites in contrast to B‐F, which are on distal dendrites. Terminals on LG‐type FR motoneuron are of en passant variety (triplet) and of terminal variety.

A, adapted from Scheibel and Scheibel 310; B and D, from Brown and Fyffe 26; C and E, from Burke et al. 51
Figure 33. Figure 33.

A: arrangement for spike‐triggered averaging. Muscle‐stretch activated group Ia impulse recorded from dorsal rootlet (a) is used to trigger an averaging computer, using a pulse‐height analyzer to select impulses of constant height produced by a single fiber. Each trigger pulse causes averager to process the next several milliseconds of signal recorded from the microelectrode impaling the motoneuron. Motoneuron is identified by antidromic invasion following muscle nerve stimulation (b). If Ia‐fiber acting as source of trigger signal sends terminals to motoneuron, then an EPSP is averaged. B: examples of single 20‐ms records (sweeps) from a medial gastrocnemius (MG) motoneuron triggered by an MG group Ia impulse. The EPSP is seen at start of sweep superimposed on highly variable synaptic activity. Note variability in amplitude of Ia‐evoked EPSP. CF: 4, 16, 64, and 256 sweeps are averaged, respectively. Note improvement in resolution of EPSP as number of sweeps averaged is increased. Only potentials synchronized with the MG afferent are preserved. Others are asynchronous with respect to the trigger signal and so are averaged out. G, H: dorsal root recording (G) and simultaneous intracellular recording from motoneuron (H) at slower sweep speed. Note 2 Ia impulses in G and synaptic noise in H. Calibration, 500 μV for BF, and H; 0.7 ms for BF; 5 ms for H.

A, adapted from Mendell and Henneman 255
Figure 34. Figure 34.

Amplitude histograms of single‐fiber EPSPs in homonymous and heteronymous motoneurons. A: histogram obtained using electrical stimulation method. B: histogram with spike‐triggered averaging method. Arrows represent values of less than 1% of N. Afferents and motoneurons are mainly from triceps surae and semitendinosus muscle groups in A and exclusively so in B.

A, from Jack et al. 82; B, from data of Mendell and Henneman 255, Nelson and Mendell 269,270, Scott and Mendell 315
Figure 35. Figure 35.

Plot of mean amplitudes of postsynaptic population potentials (PSPPs) elicited by afferent impulses with mean condition velocities shown on abscissa. The PSPPs were evoked by impulses in single group Ia and II fibers of cat medial gastrocnemius muscle, using spike‐triggered averaging. Responses recorded from 1st sacral ventral root and lower part of 7th lumbar ventral root. The 153 data points were grouped in pentads of conduction velocity and mean and standard deviations of PSPPs for each group are shown. Mean data points fell into 2 groups, each of which was fitted with a straight line by method of least squares, using point at 72.5 m/s for both calculations.

From Lüscher, Henneman, et al. 244
Figure 36. Figure 36.

Dependence of amplitude of single‐fiber EPSPs on size of motoneuron in cat. Top, relationship between EPSP amplitude (obtained by spike‐triggered averaging) and motor axon conduction velocity determined for 4 separate group Ia afferent fibers. Axonal conduction velocity determined by antidromic stimulation. Medial gastrocnemius (MG) Ia‐fibers and motoneurons in each case. Note inverse relationship. Bottom, each point represents median value of amplitude distribution of a presumed single‐fiber EPSP evoked in a motoneuron during stretch of triceps surae. The EPSPs are not restricted to those generated by Ia‐fibers. Many of these EPSPs are probably produced by interneurons. It is assumed that variation of these EPSPs with motoneuron input resistance as shown would also occur in the subpopulation of Ia‐evoked EPSPs. Small cells with large input resistance 203 generate larger EPSPs. ○, Gastrocnemius motoneurons supplying type F (rapidly contracting) motor units; ⊕, MG motoneurons supplying type S (slowly contracting) motor units; and •, soleus motoneurons.

Top from Mendell and Henneman 255; bottom from Burke 36
Figure 37. Figure 37.

A: EPSP shapes computed for input restricted to specific regions of the dendritic tree of cat. All EPSPs assumed to be recorded from compartment 1 (the soma) of a 10‐compartment motoneuron. Compartments 2–10 represent dendritic regions of equal electrotonic length but increasing distance from the soma. Top, time course of the conductance transient generating EPSPs is given by dotted line; a is EPSP expected for a transient delivered to all compartments simultaneously. Bottom, b–d are EPSPs expected for transients delivered to compartments 1, 4, and 8, respectively. Ordinate is in arbitrary units; to produce EPSPs with equal peak amplitudes as shown, intensity of conductance changes for d > c > b > a. Note differences in time course of EPSPs, and for b–d, note differences in time of their onset. B: end plate potentials at different locations in same muscle fiber (frog sartorius in vitro). Microelectrode was inserted sequentially into curarized fiber and end plate potential in response to motor nerve stimulation was measured. Numbers at left of each trace represent distance in millimeters from end plate focus. S is the stimulus artifact. Time is in ms.

A, from Rall 287; B, from Fatt and Katz 112
Figure 38. Figure 38.

A: single‐fiber excitatory postsynaptic potentials (EPSPs) obtained using spike‐triggered averaging produced by the same medial gastrocnemius (MG) Ia‐afferent fiber in 6 different MG motoneurons in cat. Note different shapes of the 6 EPSPs. Calibration pulse at the end of each sweep is 200 μV. B: single‐fiber EPSPs (by spike‐triggered averaging) produced in the same LG motoneuron by an LG Ia‐fiber (top) and a soleus Ia‐fiber (bottom). Calibration pulse is 100 μV, 1 ms. Note differences in shape of these EPSPs. C: example of a single fiber EPSP (MG Ia‐fiber and MG motoneuron) with compound shape (spike‐triggered averaging). Calibration is 1 ms, 50 μV.

A, from Mendell and Henneman 255; B, from Scott and Mendell 315; C, from Mendell and Henneman 255
Figure 39. Figure 39.

A: plot of ratio of excitatory postsynaptic potential (EPSP) amplitudes against ratio of their rise times (RT2/RT1). Each point was obtained by consideration of 2 averaged single‐fiber EPSPs (spike‐triggered averaging) evoked by different Ia‐fibers in the same motoneuron. Medial gastrocnemius (MG) Ia‐fibers and MG motoneurons throughout. By definition RT1 < RT2 so that RT2/RT1> 1. Furthermore , the ratio of the amplitudes, is defined so that amplitude of faster rising EPSP is in numerator. Majority of points have suggesting a tendency for faster rising EPSPs to be larger than slower rising ones. Correlation between and RT2/RT1 is very weak. B: plot of single‐fiber EPSP amplitude (obtained by electrical stimulation) against distance (in units of space constant δ) between the terminals and the electrode (assumed to be in soma). The EPSPs are chiefly from hamstring and triceps surae motoneurons with both homonymous and heteronymous combinations.

A, from Mendell and Weiner 256; B, from Iansek and Redman 176
Figure 40. Figure 40.

Model of projection of single Ia‐fiber to homonymous and heteronymous motoneurons (circles). Systematic differences in mean number of boutons provided by single Ia‐fibers to homonymous and heteronymous motoneurons would lead to differences in projection frequency and excitatory postsynaptic potential (EPSP) amplitude in these 2 types of projections. Mean bouton number was chosen using estimates of Iles 177 as a guide. Numbers below each circle represent number of boutons provided by single Ia‐fiber to each motoneuron.

Figure 41. Figure 41.

Scatter diagram of relationship between rise time and latency for single‐fiber excitatory postsynaptic potentials (EPSPs) in 78 medial gastrocnemius (MG) and 23 lateral gastrocnemius and soleus (LG‐SOL) motoneurons. MG Ia‐afferent fibers throughout. Dashed vertical line at 0.27 ms denotes minimum latency recorded in heteronymous LG‐SOL motoneurons. Inset shows intracellular (A), extracellular (C) records from a motoneuron with spike‐triggered averaging. Note reversal of EPSP, but not diphasic prespike following withdrawal of electrode. Calibration pulses 50 μ V, 1 ms. B is same as A with both scales expanded by 4. Latency was measured from negative (downward going) peak of prespike to onset of EPSP.

From Munson and Sypert 263
Figure 42. Figure 42.

Comparison of synaptic delay associated with single‐fiber excitatory postsynaptic potentials (EPSPs) produced by group Ia and group II fibers in cat. Left, trace A represents potential recorded inside a medial gastrocnemius (MG) motoneuron with spike‐triggered averaging from a group II afferent fiber. The large upward potential is EPSP, which is preceded by a brief diphasic potential considered to arise from activity in presynaptic terminals of group II fiber. Trace B is record obtained by spike‐triggered averaging from a position just outside motoneuron. The early potential is not inverted, indicating that it is produced outside motoneuron and unlike EPSP, which reverses, showing that it is generated across the membrane. Trace C is averaged record recorded from group II fiber in dorsal rootlet. Interval from dorsal root spike to presynaptic spike is conduction time (CT) and remaining time to EPSP onset is synaptic delay (SD). Right, histograms of conduction time and synaptic delay for group II fibers (top) and group Ia‐fibers (bottom). Note similarities in synaptic delay for both types of fiber and trend for shorter conduction time in (the larger) group‐Ia fibers.

From Stauffer et al. 328
Figure 43. Figure 43.

Inhibitory postsynaptic potential (IPSP) in a cat motoneuron analyzed by spike‐triggered averaging. A: top trace is intracellular record from a posterior biceps or semitendinosus motoneuron. Bottom trace is extracellular spike activity from an interneuron satisfying the criteria required for a Ia‐interneuron. Interneuron is excited by iontophoretically applied glutamate and discharges rhythmically. Each spike is associated with an IPSP (overlaid by row of dots). B: averages of 512 sweeps triġgered by interneuronal spike from motoneuron (above) and interneuron (below). Trigger level was set so that averaging began at beginning of interneuronal spike.

From Jankowska and Roberts 189
Figure 44. Figure 44.

Influence of entry level of Ia‐fiber of cat on amplitude of postsynaptic population potentials (PSPPs) its impulses elicit.

Data are from a single experiment. Schematic drawings in top of figure illustrate recording arrangements on ventral roots (VR); CV represents conduction velocity. Arrow points at spinal segment that the afferent fiber enters. The PSPPs recorded from caudal part of 7th lumbar ventral root (L7VR) and 1st sacral ventral root 1 and 1st sacral (S1 VR) 2 are reproduced below. A: Ia‐fiber arising in medial gastrocnemius (MG) enters through S1 dorsal root. B: Ia‐fiber from MG enters spinal segment L7. C: Ia‐fiber from lateral gastrocnemius (LG) enters spinal segment L7. Each PSPP was averaged 4,096 sweeps. From Lüscher, Ruenzel, and Henneman 246
Figure 45. Figure 45.

Three‐dimensional graph illustrating relationship between amplitude of postsynaptic population potentials (PSPPs) and conduction velocity and spinal entry point of afferent impulses evoking them. Amplitudes were averaged over 4,096 sweeps and were recorded from 1st sacral ventral root. Data were derived from a single experiment. All afferent fibers had origin in medial gastrocnemius muscle.

From Lüscher, Ruenzel, and Henneman 246
Figure 46. Figure 46.

Maximal firing rates of 4 plantaris motoneurons of cat with approximately equal critical firing levels recorded simultaneously during first 940 ms of their response to electrical stimulation of plantaris nerve. Stimulus parameters adjusted to produce maximal firing rates. Bottom unit fires significantly slower than top 3 units, giving impression of having been sampled from a different population than the others.

From Harris and Henneman 145
Figure 47. Figure 47.

Firing rates (averaged over first second) of 2 pairs of units of cat with similar critical firing levels, compared simultaneously. Data obtained by applying a wide range of stimulus intensities and frequencies to plantaris nerve. A: for each pair of units firing rate (FR) of 1 unit is plotted versus FR of other unit. Dashed lines indicate linear regression. B: ratio of FRs of each pair plotted as a function of the rate of 1 of the units expressed as a percentage of its maximum rate. Bottom pair (open circles) had very similar FRs (ratio ≃ 1.0); top pair (closed circles) had widely differing FRs (ratio > 2.0). Linear regression lines with slopes near zero indicate ratio of FRs remained constant for all these inputs.

From Harris and Henneman 145
Figure 48. Figure 48.

Examples of aggregate excitatory postsynaptic potentials (EPSPs) recorded from 3 medial gastrocnemius motoneurons of cat to illustrate effects of posttetanic potentiation on cells of different input resistance (IR) and size. Bottom traces illustrate that average size of aggregate EPSPs before potentiation varied directly with IR of motoneuron. Top traces are averages of 16 EPSPs following tetanus. Percent potentiations in AC were 123, 92, and 11, respectively.

From Lüscher, Ruenzel, and Henneman 245
Figure 49. Figure 49.

Relation between input resistance and amplitude of aggregate excitatory postsynaptic potentials (EPSPs) before (A) and during (B) posttetanic potentiation in cat. Circles, medial gastrocnemius motoneurons; triangles, semitendinosus motoneurons.

From Lüscher, Ruenzel, and Henneman 245
Figure 50. Figure 50.

Plot of changes in rise times of excitatory postsynaptic potentials (EPSPs) versus changes in their half‐widths following a tetanus to cat muscle nerve at 500/s for 10 s. Arrows indicate directions of change.

From Lüscher, Ruenzel, and Henneman 245
Figure 51. Figure 51.

Some features of a motoneuron pool. Heights of vertical lines represent sizes of motoneurons. Their spacing indicates relative numbers of cells of different sizes. Horizontal line below base line shows combination of motoneurons firing in response to a given input. Largest cell discharged denoted by X.

From Henneman 361. Organization of the motoneuron pool: the size principle. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980
Figure 52. Figure 52.

Concomitant changes in mean firing rates of units i and j (fi and fj) during slow volitional changes in an isometric contraction in human. The fi‐fj relationship of each pair is approximated by 1 or 2 straight (dashed) lines. The same i unit was used in pairs 5–8.

From Monster and Chan 261


Figure 1.

Reconstructions of medial gastrocnemius (MG) and soleus (SOL) nuclei from serial sagittal sections in cat. Right, dorsal view of spinal cord outline (white matter–pia boundary in heavy lines) on which are superimposed positions (dots) of MG (left hemicord) and SOL motoneuron cell bodies (right hemicord). Boundaries between L6, L7, and S1 segments (identified by dorsal root entry zones), are indicated by horizontal heavy lines; midline denoted by vertical dashed line. Dashed lines across the cord denote levels (labeled A–E) at which reconstructions of cross sections were made. Left, reconstructions of cross sections at levels A–E showing white matter–pia boundary in light lines and gray‐white matter boundary in heavy lines. Dashed lines at most lateral parts of cross sections indicate estimated outline; the most lateral parts were lost in sectioning. All diagrams drawn on same scale. Neurons indicated on each cross‐section diagram are cells located within 300 μm rostral and caudal to that level.

From Burke et al. 47


Figure 2.

Distribution of motoneuron sizes in the 7th cervical segment of 6 normal rhesus monkeys.

Adapted from Hodes et al. 161


Figure 3.

A: diameters of sciatic α‐motor axons of cat at initial segment (IS) are plotted against mean diameters of their parent cell bodies (n = 68). Linear correlation coefficient (r) = +0.522 (P < 0.001). B: mean diameters of sciatic α‐motor axons in white matter are plotted against mean diameters of their parent cell bodies (n = 42). Linear correlation coefficient (r) = +0.516 (P < 0.001).

From Cullheim 80


Figure 4.

Axon conduction velocities in cat are plotted in A against size of parent cell bodies (n = 24), in B against axon diameters at initial segment IS (n = 24), and in C against axon diameters in white matter (n = 33). In A the linear correlation coefficient (r) = +0.673 (P < 0.001); in B, r = +0.869 (P < 0.001); and in C, r = +0.804 (P < 0.001).

From Cullheim 80


Figure 5.

Relation between axonal diameters in spinal cord white matter and diameters of initial segments (IS) for 39 sciatic α‐motor axons in cat. Linear correlation coefficient (r) = +0.698 (P < 0.0001).

From Cullheim and Kellerth 82


Figure 6.

Relation between maximum tetanic tension of individual motor units in cat soleus muscle and conduction velocity of their axons in 3 different experiments.

From McPhedran, Wuerker, and Henneman 253


Figure 7.

Relation between logarithm of tetanic tensions developed by motor units and the conduction velocities of their motor axons in 4 leg muscles of the cat. Heavy dots on vertical bars indicate mean value of tension ± SD. For peroneus longus and tibialis anterior, intervals corresponding to conduction velocities under 85 m/s contained too few motor units to allow valid calculation of mean tension. Range of tensions developed by these motor units are represented by a vertical bar. Open circle represents tension of a single unit. For soleus, 4 motor units innervated by axons, either faster than 90 m/s or slower than 60 m/s. are not represented.

From Jami and Petit 185


Figure 8.

Distribution of maximal tetanic tensions of motor units of cat's soleus (A) and medial gastrocnemius (B).

A, from McPhedran, Wuerker, and Henneman 253; B from Wuerker, McPhedran, and Henneman 353


Figure 9.

Smoothed version of histogram in Figure 8B, illustrating distribution of some of the properties of motor units in typical limb muscles. Height of columns indicates relative numbers of motor units with the properties listed below.

From Henneman 154. Skeletal muscle: the servant of the nervous system. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980


Figure 10.

Reflex discharges recorded from a filament of 7th lumbar ventral root in response to stimulation of ipsilateral sciatic nerve of cat. Numbers at right of each tracing indicate relative intensity of stimulation. Initial deflection on left of each tracing is the early discharge referred to in text.

From Henneman 153


Figure 11.

Stretch‐evoked responses of 2 α‐motoneurons recorded from a filament of 7th lumbar ventral root of cat. Amount of tension applied and developed reflexly is indicated by separation of 2 top beams in each frame. Several seconds elapsed between successive frames while muscle was stretched 1,2,3,4,5 and released 6,7,8,9.

From Henneman et al. 159


Figure 12.

Stretch‐evoked responses of 5 α‐motoneurons recorded from a filament of the 1st sacral ventral root of cat. Numbers above action potentials indicate rank of units according to size.

From Henneman et al. 159


Figure 13.

Frequency distribution of thresholds of tonic responses to stretch in 3 experiments on cats. Abscissa, threshold tension of deefferented triceps muscle. Ordinate, number of units whose thresholds fell between values indicated on abscissa.

From Henneman et al. 159


Figure 14.

A: orderly recruitment in cat of 3 triceps motoneurons of different sizes in response to increasing degrees of stretch. Tension produced by stretch of triceps muscle indicated by separation of top traces in each frame. B: orderly inhibition of same 3 units during a constant stretch of 4 kg. Records 1–4 on left show control responses of stretch before, between and after 3 inhibitory stimulations. Records on right were obtained during 100/s stimulation of ipsilateral deep peroneal nerve at 3 intensities. Largest unit was silenced first (line 1), intermediate unit next (line 2), and smallest unit last (line 3). Note lasting effects produced by brief inhibitions, i.e., failure to recover to previous levels in lines 2 and 4.

From Henneman et al. 160


Figure 15.

Graphic representation of discharge frequency of 3 motoneurons of different sizes (AC) in triceps surae of cat, and of contractile tension developed by the muscle itself (D) in response to varying degrees of excitation and inhibition: x‐axis, intensity of inhibitory stimulation applied to ipsilateral deep peroneal nerve; y‐axis, frequency of discharge in AC and contractile tension in D; z‐axis, stretch of triceps surae, in millimeters. Data for all 4 graphs obtained simultaneously. Cells were subjected simultaneously to various mixtures of excitatory (z‐axis) and inhibitory (x‐axis) stimuli. Each plotted point represents mean of 2 successive determinations. Inhibitory stimuli were 100/s shocks applied for 0.9‐s periods at 4‐s intervals. Note that unit A (smallest) fired spontaneously without stretch, unit B began to discharge between 0 and 5 mm of stretch, and unit C (the largest) between 5 and 10 mm.

From Henneman et al. 160


Figure 16.

Scheme of experiment used to measure maximal monosynaptic discharge of a motoneuron pool. DR, dorsal root; VR, ventral root.

From Clamann, Henneman, et al. 59


Figure 17.

Pairs of responses in cat recorded as in Figure 16 from proximal and distal halves of 1st sacral ventral root in response to single‐shock stimulation of combined nerves to medial and lateral gastrocnemius (MG and LG). Left and right deflections of each pair are antidromic and reflex responses, respectively, displayed on superimposed traces of a 2‐beam oscilloscope. First 10 pairs in top row recorded before 12‐s tetanus (500 shocks/s) to MG and LG nerves. Subsequent pairs recorded at 2‐s intervals after tetanus.

From Clamann, Henneman, et al. 59


Figure 18.

Simultaneous recordings in cat of a series of monosynaptic reflexes of triceps surae pool (top) and a single triceps motoneuron (bottom) showing critical firing level of the latter. Top trace, time integrals of monosynaptic reflexes recorded at 1/s from proximal half of 1st sacral ventral root. Height of each vertical line measures size of population response. Bottom trace, monosynaptic reflexes of a single triceps motoneuron recorded from a small filament of 7th lumbar ventral root. Records were made during decline in posttetanic potentiation following a brief train of conditioning shocks that were applied to the combined medial gastrocnemius and lateral gastrocnemius and soleus nerves at 500/s.

From Henneman et al. 155


Figure 19.

Critical firing levels of 3 triceps motoneurons in cat. Single unit in bottom left had a sharp threshold at 39%. The 2 units in bottom right tracing had thresholds of 60% and 33%. Plateau in top left record indicates 100% discharge level of triceps pool.

From Henneman et al. 155


Figure 20.

Susceptibilities of 2 pairs of cat plantaris motoneurons with known critical firing levels (CFLs) to repetitive discharge in cat. Electrical stimulation of plantaris nerve at 300 stimuli/s in both cases. Stimulus intensity (i.e., voltage) indicated by level of trace a with reference to base line d. Responses of plantaris units recorded on traces b and c. CFLs indicated at right end of these traces. Time marks at 1‐s intervals for both top and bottom records.

From Henneman et al. 155


Figure 21.

Differences in susceptibility to repetitive firing of 2 cat plantaris motoneurons whose critical firing levels were 12.0% (top) and 15.5% (bottom). A: shocks applied to plantaris nerve were supramaximal for all group Ia‐fibers. Frequency was switched from 20 shocks/s initially to 30 shocks/s at 1st arrow, and to 40 shocks/s at 2nd arrow. B: frequency of stimulation was 500 shocks/s throughout and intensity was increased gradually as trace moved from left to right.

From Henneman et al. 155


Figure 22.

Relation between axon size and critical firing level (rank order) of 21 cat plantaris motoneurons isolated in a single experiment.

From Clamann and Henneman 60


Figure 23.

Critical firing level (CFL) of a cat plantaris motoneuron with and without inhibitory input from lateral popliteal nerve. Top, time integrals of monosynaptic reflexes of plantaris population elicited at 2‐s intervals during declining phase of posttetanic potentiation. Bottom, simultaneous responses of a plantaris unit with a critical firing level of 61%. Alternate reflexes preceded by a 200‐ms train of pulses to lateral popliteal nerve. Horizontal line indicating critical firing level drawn as described in text.

From Clamann, Gillies, and Henneman 58


Figure 24.

Effects of recurrent inhibition (A) and lateral popliteal inhibition (B) in cat on monosynaptic reflexes (traces 1, 2) and repetitive firing (traces 3, 4) of a pair of plantaris motoneurons with critical firing levels of 18.6% (top) and 18.4% (bottom). A: in traces 1, 2 the underlined reflexes were preceded and accompanied by a 100‐ms train of shocks (100/s) to proximal portion of 7th lumbar ventral root. Antidromic volleys set up by stimulation in A1, A2 resulted in recurrent inhibition mediated by Renshaw cells. Of the 18 reflexes in trace 1, 9 were inhibited by this input; in trace 2 only 1 reflex was silenced. In A3, A4 the same pair of units is seen responding rhythmically to repetitive stimulation of the plantaris nerve. When continuous recurrent inhibition was begun, indicated by thick base line in A4, top unit ceased firing and bottom unit slowed to about one‐half original rate of discharge. B: same pair of units was subjected to inhibitory input elicited by stimulating lateral popliteal nerve at 100/s. In B1, B2 the underlined monosynaptic reflexes were somewhat more susceptible to this inhibition than to the recurrent inhibition. Of the 16 reflexes receiving this inhibition, 15 in trace 1 and 4 in trace 2 were silenced. In B3, B4 the same pair of units firing rhythmically in response to 100/s stimulation of the plantaris nerve was subjected to an inhibitory input whose intensity was gradually increased as the traces moved across the oscilloscope. The top unit was finally suppressed by this input whereas the bottom unit was only slowed in rate.

Clamann, Gillies, and Henneman 58


Figure 25.

Records illustrating quantitative relationships between intensity of excitatory stimulus causing 2 cat motoneurons with critical firing levels of 18.8% and 12.7% to discharge repetitively and intensity of inhibitory stimulus required to silence them. Stimulus strength of excitatory input (100/s stimulation of a plantaris nerve), indicated by level of line E with reference to base line B, was constant during each test but varied in different tests. Stimulus strength of inhibitory input (100/s stimulation of lateral popliteal nerve), indicated by line I, was increased as trace moved across screen. Intensity of inhibition required to silence each unit was measured at time of their last discharges, indicated by dashed lines. Time signals at top of record represent 1‐s intervals.

From Clamann, Gillies, and Henneman 58


Figure 26.

Graph showing linear relation between strength of excitatory stimuli causing each of 2 cat motoneurons to discharge repetitively and strength of inhibitory stimuli required to silence them. Data obtained as illustrated in Figure 25. Data for lower unit, with critical firing level of 12.7%, are plotted with filled circles and solid line; data for upper unit, with a critical firing level of 18.8%, are represented by open circles and dashed line.

From Clamann, Gillies, and Henneman 58


Figure 27.

Number of human motor units having the twitch tensions indicated. A: linear scale. B: logarithmic scale. Distributions are similar for all 3 subjects. The computed best‐fitting line on the semilog plot (B) indicates an approximately exponential relation between number of motor units and twitch tension.

From Milner‐Brown et al. 258


Figure 28.

A: firing pattern of a high‐threshold unit recorded from human extensor indicis during isometric contractions with different rates of rise of tension (A–D). Each registration shows (top to bottom) the spike record, isometric force, and tracking signal which was visually displayed to the subject. Time scale in bottom 2 records is one‐half that of top 2 records. During increasingly faster contractions, unit starts firing at successively lower force levels. B: dependence of time between beginning of muscle contraction and firing onset of 3 motor units on rate of rise of tension. Double logarithmic scale. Faster contraction causes earlier recruitment. C: change of threshold force of recruitment (ordinate) associated with variation of rate of rise of isometric tension. Recordings from 7 motor units of extensor indicis of 3 subjects (▪, ▴ •). D: regression line of change of threshold force of recruitment on rate of rise of isometric tension (y1 = −0.067x + 198.66) calculated from 12 motor units of extensor indicis. The increase of muscle force that occurs within the mean contraction time of the muscle is shown for comparison (y2 = 0.060x).

From Büdingen and Freund 30


Figure 29.

Records of EMG activity and force outputs of medial gastrocnemius (top 2 traces) and soleus (bottom 2 traces) muscles of cat, together with simultaneous record of activity of a small number of medial gastrocnemius (MG) motor axons in a natural filament of MG muscle nerve (3rd trace). During MG vibration (160 Hz, 90 μm) activity in whole MG increased and that in soleus (SOL) showed a smaller increase. Pinching the skin over the lateral ankle with toothed forceps produced 2 bursts of MG activity, with recruitment of large amplitude motor axons in the MG filament, but caused quite complete suppression of SOL activity. Isolated activity of 5 identifiable MG motoneuron axons is shown in A–E. Note slowing of unit A discharge during each burst of activity in the higher threshold units B–E.

From Kanda et al. 192


Figure 30.

Simultaneous recording of one continuously firing long‐interval motor unit and one intermittently firing short‐interval motor unit in human. A: increasing contraction. B: prolonged maximum effort. C, D: rapid twitches. Time bar, 100 ms.

From Grimby and Hannerz 139


Figure 31.

Properties of a small (A), medium (B), and large (C) motor neuron of lobster innervating the main power stroke muscle. 1, Soma diameters; 2, axonal conduction velocities; 3, amplitudes of action potentials recorded with 2 extracellular electrodes at different positions on the motor nerve; 4, amplitudes of excitatory junctional potentials (EJPs) recorded from a single muscle fiber (top trace in each record); 5, adaptation to a maintained intracellular depolarizing current; and 6, facilitation properties of extracellular EJPs (top trace in each record) during 50‐Hz stimulation. Note antifacilitation produced by the largest motoneuron. Time marks in row 5 (lowest trace), 0.1 ms.

From Davis 85


Figure 32.

Morphology of Ia‐collaterals to motoneurons and arrangements of boutons on dendrites. A: sagittal section through lumbosacral cord of 40‐day‐old kitten (section shown at right). Primary fibers in dorsal columns (a) send ventrally directed branches to ventral horn (b) to terminate in region of motoneurons (c and d) that have sagitally running dendrites. Primary afferent branches also give off branches to dorsal horn and intermediate zone; these are described in more detail in the original illustration of Scheibel and Scheibel 310 from which this figure is modified. B: scale drawing illustrating morphology of collaterals of a single triceps surae Ia‐fiber as revealed by horseradish peroxidase (HRP) filling. This 3‐dimensional representation provides both a cross‐sectional (transverse) and sagittal perspective. The Ia‐fiber in the dorsal columns is drawn as a thick black line along with the dorsal root ganglion cell and dorsal root portion of the cell. Four collaterals emanating from the dorsal column region are illustrated, each one coursing through the dorsal horn (in a ventrallateral and rostral direction) to terminate in the cylindrical motoneuron region (lamina IX). Other terminals are given off by the same fiber in laminae VI and VII. C: morphology of a single soleus Ia‐fiber in the dorsal columns with 9 branches given off to ventral horn; HRP technique. Two motoneurons—soleus (SOL), which innervates type S motor unit, and lateral gastrocnemius (LG), which innervates a type F, fatigue‐resistant motor unit (R)—were also filled with HRP. Circles and stippling denote somata and maximum dendritic extents of these neurons. Note that at most only 2 of 9 Ia‐fiber branches have access to each of these motoneurons. Also ventrally directed collaterals of this fiber do not exhibit rostral component shown by Brown and Fyffe (Fig. 32B). Top and bottom solid lines denote dorsal and ventral borders of cord. Dotted lines represent dorsal and ventral limits of ventral horn. D: origins of primary collaterals of MG, LG‐S, and posterior tibial (PT) Ia‐fibers. E: schematic drawing of disposition of Ia‐fiber boutons on α‐motoneurons. In each case, both Ia‐fiber and motoneuron filled with HRP. Note in SOL motoneuron that bouton A is on proximal dendrites in contrast to B‐F, which are on distal dendrites. Terminals on LG‐type FR motoneuron are of en passant variety (triplet) and of terminal variety.

A, adapted from Scheibel and Scheibel 310; B and D, from Brown and Fyffe 26; C and E, from Burke et al. 51


Figure 33.

A: arrangement for spike‐triggered averaging. Muscle‐stretch activated group Ia impulse recorded from dorsal rootlet (a) is used to trigger an averaging computer, using a pulse‐height analyzer to select impulses of constant height produced by a single fiber. Each trigger pulse causes averager to process the next several milliseconds of signal recorded from the microelectrode impaling the motoneuron. Motoneuron is identified by antidromic invasion following muscle nerve stimulation (b). If Ia‐fiber acting as source of trigger signal sends terminals to motoneuron, then an EPSP is averaged. B: examples of single 20‐ms records (sweeps) from a medial gastrocnemius (MG) motoneuron triggered by an MG group Ia impulse. The EPSP is seen at start of sweep superimposed on highly variable synaptic activity. Note variability in amplitude of Ia‐evoked EPSP. CF: 4, 16, 64, and 256 sweeps are averaged, respectively. Note improvement in resolution of EPSP as number of sweeps averaged is increased. Only potentials synchronized with the MG afferent are preserved. Others are asynchronous with respect to the trigger signal and so are averaged out. G, H: dorsal root recording (G) and simultaneous intracellular recording from motoneuron (H) at slower sweep speed. Note 2 Ia impulses in G and synaptic noise in H. Calibration, 500 μV for BF, and H; 0.7 ms for BF; 5 ms for H.

A, adapted from Mendell and Henneman 255


Figure 34.

Amplitude histograms of single‐fiber EPSPs in homonymous and heteronymous motoneurons. A: histogram obtained using electrical stimulation method. B: histogram with spike‐triggered averaging method. Arrows represent values of less than 1% of N. Afferents and motoneurons are mainly from triceps surae and semitendinosus muscle groups in A and exclusively so in B.

A, from Jack et al. 82; B, from data of Mendell and Henneman 255, Nelson and Mendell 269,270, Scott and Mendell 315


Figure 35.

Plot of mean amplitudes of postsynaptic population potentials (PSPPs) elicited by afferent impulses with mean condition velocities shown on abscissa. The PSPPs were evoked by impulses in single group Ia and II fibers of cat medial gastrocnemius muscle, using spike‐triggered averaging. Responses recorded from 1st sacral ventral root and lower part of 7th lumbar ventral root. The 153 data points were grouped in pentads of conduction velocity and mean and standard deviations of PSPPs for each group are shown. Mean data points fell into 2 groups, each of which was fitted with a straight line by method of least squares, using point at 72.5 m/s for both calculations.

From Lüscher, Henneman, et al. 244


Figure 36.

Dependence of amplitude of single‐fiber EPSPs on size of motoneuron in cat. Top, relationship between EPSP amplitude (obtained by spike‐triggered averaging) and motor axon conduction velocity determined for 4 separate group Ia afferent fibers. Axonal conduction velocity determined by antidromic stimulation. Medial gastrocnemius (MG) Ia‐fibers and motoneurons in each case. Note inverse relationship. Bottom, each point represents median value of amplitude distribution of a presumed single‐fiber EPSP evoked in a motoneuron during stretch of triceps surae. The EPSPs are not restricted to those generated by Ia‐fibers. Many of these EPSPs are probably produced by interneurons. It is assumed that variation of these EPSPs with motoneuron input resistance as shown would also occur in the subpopulation of Ia‐evoked EPSPs. Small cells with large input resistance 203 generate larger EPSPs. ○, Gastrocnemius motoneurons supplying type F (rapidly contracting) motor units; ⊕, MG motoneurons supplying type S (slowly contracting) motor units; and •, soleus motoneurons.

Top from Mendell and Henneman 255; bottom from Burke 36


Figure 37.

A: EPSP shapes computed for input restricted to specific regions of the dendritic tree of cat. All EPSPs assumed to be recorded from compartment 1 (the soma) of a 10‐compartment motoneuron. Compartments 2–10 represent dendritic regions of equal electrotonic length but increasing distance from the soma. Top, time course of the conductance transient generating EPSPs is given by dotted line; a is EPSP expected for a transient delivered to all compartments simultaneously. Bottom, b–d are EPSPs expected for transients delivered to compartments 1, 4, and 8, respectively. Ordinate is in arbitrary units; to produce EPSPs with equal peak amplitudes as shown, intensity of conductance changes for d > c > b > a. Note differences in time course of EPSPs, and for b–d, note differences in time of their onset. B: end plate potentials at different locations in same muscle fiber (frog sartorius in vitro). Microelectrode was inserted sequentially into curarized fiber and end plate potential in response to motor nerve stimulation was measured. Numbers at left of each trace represent distance in millimeters from end plate focus. S is the stimulus artifact. Time is in ms.

A, from Rall 287; B, from Fatt and Katz 112


Figure 38.

A: single‐fiber excitatory postsynaptic potentials (EPSPs) obtained using spike‐triggered averaging produced by the same medial gastrocnemius (MG) Ia‐afferent fiber in 6 different MG motoneurons in cat. Note different shapes of the 6 EPSPs. Calibration pulse at the end of each sweep is 200 μV. B: single‐fiber EPSPs (by spike‐triggered averaging) produced in the same LG motoneuron by an LG Ia‐fiber (top) and a soleus Ia‐fiber (bottom). Calibration pulse is 100 μV, 1 ms. Note differences in shape of these EPSPs. C: example of a single fiber EPSP (MG Ia‐fiber and MG motoneuron) with compound shape (spike‐triggered averaging). Calibration is 1 ms, 50 μV.

A, from Mendell and Henneman 255; B, from Scott and Mendell 315; C, from Mendell and Henneman 255


Figure 39.

A: plot of ratio of excitatory postsynaptic potential (EPSP) amplitudes against ratio of their rise times (RT2/RT1). Each point was obtained by consideration of 2 averaged single‐fiber EPSPs (spike‐triggered averaging) evoked by different Ia‐fibers in the same motoneuron. Medial gastrocnemius (MG) Ia‐fibers and MG motoneurons throughout. By definition RT1 < RT2 so that RT2/RT1> 1. Furthermore , the ratio of the amplitudes, is defined so that amplitude of faster rising EPSP is in numerator. Majority of points have suggesting a tendency for faster rising EPSPs to be larger than slower rising ones. Correlation between and RT2/RT1 is very weak. B: plot of single‐fiber EPSP amplitude (obtained by electrical stimulation) against distance (in units of space constant δ) between the terminals and the electrode (assumed to be in soma). The EPSPs are chiefly from hamstring and triceps surae motoneurons with both homonymous and heteronymous combinations.

A, from Mendell and Weiner 256; B, from Iansek and Redman 176


Figure 40.

Model of projection of single Ia‐fiber to homonymous and heteronymous motoneurons (circles). Systematic differences in mean number of boutons provided by single Ia‐fibers to homonymous and heteronymous motoneurons would lead to differences in projection frequency and excitatory postsynaptic potential (EPSP) amplitude in these 2 types of projections. Mean bouton number was chosen using estimates of Iles 177 as a guide. Numbers below each circle represent number of boutons provided by single Ia‐fiber to each motoneuron.



Figure 41.

Scatter diagram of relationship between rise time and latency for single‐fiber excitatory postsynaptic potentials (EPSPs) in 78 medial gastrocnemius (MG) and 23 lateral gastrocnemius and soleus (LG‐SOL) motoneurons. MG Ia‐afferent fibers throughout. Dashed vertical line at 0.27 ms denotes minimum latency recorded in heteronymous LG‐SOL motoneurons. Inset shows intracellular (A), extracellular (C) records from a motoneuron with spike‐triggered averaging. Note reversal of EPSP, but not diphasic prespike following withdrawal of electrode. Calibration pulses 50 μ V, 1 ms. B is same as A with both scales expanded by 4. Latency was measured from negative (downward going) peak of prespike to onset of EPSP.

From Munson and Sypert 263


Figure 42.

Comparison of synaptic delay associated with single‐fiber excitatory postsynaptic potentials (EPSPs) produced by group Ia and group II fibers in cat. Left, trace A represents potential recorded inside a medial gastrocnemius (MG) motoneuron with spike‐triggered averaging from a group II afferent fiber. The large upward potential is EPSP, which is preceded by a brief diphasic potential considered to arise from activity in presynaptic terminals of group II fiber. Trace B is record obtained by spike‐triggered averaging from a position just outside motoneuron. The early potential is not inverted, indicating that it is produced outside motoneuron and unlike EPSP, which reverses, showing that it is generated across the membrane. Trace C is averaged record recorded from group II fiber in dorsal rootlet. Interval from dorsal root spike to presynaptic spike is conduction time (CT) and remaining time to EPSP onset is synaptic delay (SD). Right, histograms of conduction time and synaptic delay for group II fibers (top) and group Ia‐fibers (bottom). Note similarities in synaptic delay for both types of fiber and trend for shorter conduction time in (the larger) group‐Ia fibers.

From Stauffer et al. 328


Figure 43.

Inhibitory postsynaptic potential (IPSP) in a cat motoneuron analyzed by spike‐triggered averaging. A: top trace is intracellular record from a posterior biceps or semitendinosus motoneuron. Bottom trace is extracellular spike activity from an interneuron satisfying the criteria required for a Ia‐interneuron. Interneuron is excited by iontophoretically applied glutamate and discharges rhythmically. Each spike is associated with an IPSP (overlaid by row of dots). B: averages of 512 sweeps triġgered by interneuronal spike from motoneuron (above) and interneuron (below). Trigger level was set so that averaging began at beginning of interneuronal spike.

From Jankowska and Roberts 189


Figure 44.

Influence of entry level of Ia‐fiber of cat on amplitude of postsynaptic population potentials (PSPPs) its impulses elicit.

Data are from a single experiment. Schematic drawings in top of figure illustrate recording arrangements on ventral roots (VR); CV represents conduction velocity. Arrow points at spinal segment that the afferent fiber enters. The PSPPs recorded from caudal part of 7th lumbar ventral root (L7VR) and 1st sacral ventral root 1 and 1st sacral (S1 VR) 2 are reproduced below. A: Ia‐fiber arising in medial gastrocnemius (MG) enters through S1 dorsal root. B: Ia‐fiber from MG enters spinal segment L7. C: Ia‐fiber from lateral gastrocnemius (LG) enters spinal segment L7. Each PSPP was averaged 4,096 sweeps. From Lüscher, Ruenzel, and Henneman 246


Figure 45.

Three‐dimensional graph illustrating relationship between amplitude of postsynaptic population potentials (PSPPs) and conduction velocity and spinal entry point of afferent impulses evoking them. Amplitudes were averaged over 4,096 sweeps and were recorded from 1st sacral ventral root. Data were derived from a single experiment. All afferent fibers had origin in medial gastrocnemius muscle.

From Lüscher, Ruenzel, and Henneman 246


Figure 46.

Maximal firing rates of 4 plantaris motoneurons of cat with approximately equal critical firing levels recorded simultaneously during first 940 ms of their response to electrical stimulation of plantaris nerve. Stimulus parameters adjusted to produce maximal firing rates. Bottom unit fires significantly slower than top 3 units, giving impression of having been sampled from a different population than the others.

From Harris and Henneman 145


Figure 47.

Firing rates (averaged over first second) of 2 pairs of units of cat with similar critical firing levels, compared simultaneously. Data obtained by applying a wide range of stimulus intensities and frequencies to plantaris nerve. A: for each pair of units firing rate (FR) of 1 unit is plotted versus FR of other unit. Dashed lines indicate linear regression. B: ratio of FRs of each pair plotted as a function of the rate of 1 of the units expressed as a percentage of its maximum rate. Bottom pair (open circles) had very similar FRs (ratio ≃ 1.0); top pair (closed circles) had widely differing FRs (ratio > 2.0). Linear regression lines with slopes near zero indicate ratio of FRs remained constant for all these inputs.

From Harris and Henneman 145


Figure 48.

Examples of aggregate excitatory postsynaptic potentials (EPSPs) recorded from 3 medial gastrocnemius motoneurons of cat to illustrate effects of posttetanic potentiation on cells of different input resistance (IR) and size. Bottom traces illustrate that average size of aggregate EPSPs before potentiation varied directly with IR of motoneuron. Top traces are averages of 16 EPSPs following tetanus. Percent potentiations in AC were 123, 92, and 11, respectively.

From Lüscher, Ruenzel, and Henneman 245


Figure 49.

Relation between input resistance and amplitude of aggregate excitatory postsynaptic potentials (EPSPs) before (A) and during (B) posttetanic potentiation in cat. Circles, medial gastrocnemius motoneurons; triangles, semitendinosus motoneurons.

From Lüscher, Ruenzel, and Henneman 245


Figure 50.

Plot of changes in rise times of excitatory postsynaptic potentials (EPSPs) versus changes in their half‐widths following a tetanus to cat muscle nerve at 500/s for 10 s. Arrows indicate directions of change.

From Lüscher, Ruenzel, and Henneman 245


Figure 51.

Some features of a motoneuron pool. Heights of vertical lines represent sizes of motoneurons. Their spacing indicates relative numbers of cells of different sizes. Horizontal line below base line shows combination of motoneurons firing in response to a given input. Largest cell discharged denoted by X.

From Henneman 361. Organization of the motoneuron pool: the size principle. In: Medical Physiology (14th ed.), edited by V. B. Mountcastle. St. Louis, MO: The C. V. Mosby Co., 1980


Figure 52.

Concomitant changes in mean firing rates of units i and j (fi and fj) during slow volitional changes in an isometric contraction in human. The fi‐fj relationship of each pair is approximated by 1 or 2 straight (dashed) lines. The same i unit was used in pairs 5–8.

From Monster and Chan 261
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Elwood Henneman, Lorne M. Mendell. Functional Organization of Motoneuron Pool and its Inputs. Compr Physiol 2011, Supplement 2: Handbook of Physiology, The Nervous System, Motor Control: 423-507. First published in print 1981. doi: 10.1002/cphy.cp010211