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Control of Locomotion in Bipeds, Tetrapods, and Fish

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Abstract

The sections in this article are:

1 Biomechanical and Electromyographical Information
1.1 Single Limb During Locomotion
1.2 Interlimb Coordination
1.3 Treadmill Versus Overground Locomotion
1.4 Trunk Movements During Locomotion
1.5 Pathological Gaits
1.6 Summary
2 Neural Generation of “Basic Locomotor Synergy”
2.1 Central Versus Peripheral
2.2 Parts of CNS of Primary Importance for Neural Control of Basic Locomotor Synergy
2.3 Spinal Centers for Locomotion—Behavioral Results
2.4 Reflex Control of Basic Locomotor Synergy
2.5 Activity in Certain Spinal, Cerebellar, and Brain Stem Neurons and in Certain Reflex Pathways During Locomotion
2.6 Cerebellum and Locomotion
2.7 Initiation of Locomotion—Brain Stem Circuitry
2.8 Central Organization of Spinal Pattern Generation
2.9 Possible Rhythm‐Generating Mechanisms and Models—Facts and Fiction
2.10 Developmental Aspects
2.11 Summary
3 Adapting Basic Locomotor Synergy to Animal's Needs
3.1 Changing Speed
3.2 Goal‐Directed Locomotion—Turning and Walking Along Curvatures
3.3 Modifications of “Locomotor Posture”
3.4 Positioning of Limb in Each Step
3.5 Reflex Adaptation of Step
3.6 Summary
4 Concluding Remarks
Figure 1. Figure 1.

Step cycle. Joint angles in ankle and hip are plotted throughout 1 step in a slow walk (left) and a fast gallop (right). Below graphs period of foot contact is indicated by hatched region. Note striking difference in duration of contact period (stance phase) between walk and gallop, whereas swing phase remains constant. Different terms used for different phases of step cycle are listed below left graph. Under right graph approximate position of limb is drawn for the different phases as indicated by interrupted lines. Drawings do not take into account movement of lower spine.

Adapted from Goslow et al. ; see also ref.
Figure 2. Figure 2.

Duration of step cycle (left) and support phase (middle) at different velocities of walking and running in human. Each line indicates observations on one individual. The right graph shows the duration of flexion and extension vs. cycle duration.

From Grillner et al.
Figure 3. Figure 3.

The support length (ssup, given in meters in A) and the support phase (Tsup, given in seconds in B) vs. velocity of running (m/s). Solid line (alt in A and B) indicates one extreme hypothetical case in which the velocity was only changed by varying the support length; dotted line (alt 2) shows the other extreme case where only Tsup is modified. The dashed line indicates the normal linear relation between ssup and velocity of running in humans and the related changes in Tsup. See text.

Part A from Grillner et al.
Figure 4. Figure 4.

Vertical and longitudinal shear forces during walking in humans. Note the initial breaking (decelerative) force and the late accelerative force (left). The peak values of both the decelerative and accelerative forces increase with walking speed (right).

From Herman et al.
Figure 5. Figure 5.

Position, force vectors, and torques during support phase in walking cat. Cat was walking over a force plate showing amplitude and direction of different force vectors and origin of mechanical axis; position of leg was recorded simultaneously. The mass of the limb segments were obtained postmortem. Each drawing depicts the hindlimb and represents the calculation from 1 “frame.” The frames, represented from left to right, were recorded at 160/s, the 1st 2 after contact is represented and then every 10th frame following, i.e., no. 10, 20, 30, 40, and so forth. Filled columns at ankle, knee, and hip joints represent the torque, and the dotted envelope the maximal positive and negative values obtained (calibration in lower right corner). Note difference between ankle and knee joint in relative torque, and net braking torque in the hip between frames 1 and 40. The contact forces are shown below with their resultant and computed forces acting at the hip above the hip joint.

From Zomlefer, unpublished data
Figure 6. Figure 6.

Simplified scheme of electromyographical activity in cat hindlimb. Most extensors (gen. ext, hip‐toe) show a similar pattern, but definite small differences exist. The toe flexor (toe fl) represents extensor digitorum brevis, whereas extensor digitorum longus behaves like the ankle flexor tibialis anterior. Some differences also exist between the different flexors (gen. fl). The knee flexors (knee fl), e.g., semitendinosus, have a short early flexor burst and a more variable burst during Te1 and Te2. Some muscles like sartorius or rectus femoris may also display double bursts.

Figure 7. Figure 7.

Interlimb coordination during walk and trot in cat. Periods of foot contact are plotted for 1 step cycle starting with foot strike of left hindlimb (LH). Bars indicate period of contact. LF, RH, and RF indicate left forelimb, right hindlimb, and right forelimb. Below contact of each limb is plotted phase in which limb is put down in relation to step cycle of LH, i.e., time from onset of step cycle to foot contact divided by duration of entire step cycle.

Adapted from Stuart et al.
Figure 8. Figure 8.

Pattern of foot contacts during different types of gallop. LH, left hindlimb; LF, left forelimb; RH, right hindlimb; RF, right forelimb.

Adapted from Stuart et al.
Figure 9. Figure 9.

Intersegmental coordination in fishes—phase coupling. Graph shows lag between activation of 2 consecutive segments vs. duration of 1 cycle (i.e., from onset of 1 electromyogram burst on 1 side to onset of next burst). Lag between activation of 2 segments thus varies with cycle length; i.e., at fast speeds of swimming, the intersegmental lag becomes shorter. Lag is always a constant fraction of cycle length; i.e., it is a phase coupling. Graph is plotted from results from spinal dogfish. On right is shown an eel‐like fish, with phase lag of 0.5. Phase lag remains constant at different velocities of swimming.

From Grillner
Figure 10. Figure 10.

Forward velocity, frequency, and amplitude of lateral displacement of the body in fishes. A: frequency (ordinate, Hz, triangles) of alternation (tailbeat) of an eel swimming at different forward velocities (abscissa) and also the corresponding cycle duration (ms, circles). Fishes were swimming in a closed chamber, 0.15 m × 0.15 m × 0.70 m, in which the water flow could be varied up to velocities of 3.3 m/s by a motor‐driven propeller in a closed system. The wires of electromyogram (EMG) electrodes were passed through an aperture in the swimming chamber and the fishes, on which spots of light‐reflecting material were sewn, were filmed (80 frames/s) through transparent chamber with synchronization between EMG and film. B: lateral displacement of tip of tail fin of a 280‐mm long trout swimming at different speeds of forward velocity. C: amplitude of lateral displacement at different points of an eel (distance from tip of head is shown to right). Amplitude of lateral displacement is increasing from head to tail. Scale (ordinate in mm) given for lowest curve applies to all. Bars below record for points corresponding to 25 mm, 129 mm, 183 mm, and 239 mm indicate the period of EMG activity on 1 side at same level as point where lateral displacement was measured. Crossing points at the different levels are connected by thin lines to emphasize the propagated nature of the wave.

From Grillner et al.
Figure 11. Figure 11.

Control of lower trunk during walking (cat). Movement of different points on back and hindlimb has been recorded together with the electromyographical (EMG) activity represented in its rectified and filtered version. A: movement in ankle joint (degrees) and activity

ipsi‐ (i) and contralateral (co)] in knee extensor, vastus lateralis (VL), and ipsilateral erector spinae muscles, multifidus (MfL) and longissimus (LoL) at 4th lumbar segment. Note the 2 bursts in the back muscles. B: the 2 bursts are well correlated to onset of EMG activity in the ipsi‐ and contralateral extensors at different period lengths (k). C: simultaneous lateral movements at 9th thoracic (Th) and 3rd and 5th lumbar (L) segments and, below, the changes in angle between these points. E: corresponding sagittal (up and down) displacement. Horizontal (side) displacement is largest at lower velocities (long cycle duration, part D), whereas sagittal displacement changes less (F). A, C, and E are the same step cycles; vertical lines indicate touchdown. [From Carlson et al.
Figure 12. Figure 12.

Longitudinal body movement of horse and cheetah during gallop and lateral movements of newt. Note flexion and extension of lower spine for horse and cheetah. Bottom, period of foot contact for galloping cheetah. Right, lateral body movements of walking newt. Note how these movements prolong step.

Data for cheetah from Hildebrand ; data for newt from Gray
Figure 13. Figure 13.

Generation of movement according to a chain reflex scheme and a central pattern generation scheme; mu., muscle.

Figure 14. Figure 14.

Effect of dorsal root transection on the electromyographical (EMG) pattern during walking for mesencephalic cat. Periods of EMG activity in 12–15 consecutive step cycles are compared before and after ipsilateral and then contralateral transection of all dorsal roots (3rd lumbar‐4th sacral). Bars indicate main period of EMG activity during successive cycles. Each cycle starts with the onset of activity in the ankle extensor (lateral gastrocnemius); dot to right shows termination of cycle. Hip flexor is iliopsoas; knee flexor is semitendinosus. In addition to its main activity in flexion, this muscle has a smaller and more variable burst during the extension phase, which is indicated by two dots. Lowermost muscle is lateral sartorius (hip/knee), which is a flexor of the hip but also has an extensor function at the knee. Data were obtained in a high mesencephalic cat stimulated in locomotor region. Treadmill speed was 1.6 m/s in left panel and 2.0 m/s in center and right panels. Note that characteristic short period of activity of semitendinosus remains after transections and extensor bursts, as in the intact cat.

Adapted from Grillner and Zangger
Figure 15. Figure 15.

Mesencephalic cat on treadmill. Drawings show arrangements for fixation and recordings including lateral tilt conditions.

From Orlovsky and Pavlova
Figure 16. Figure 16.

Location of subthalamic (SLR), mesencephalic (MLR), and pontine (PLR) locomotor regions in cat brain stem. Brain stem is drawn against Horsley‐Clarke coordinates (in millimeters) in rostrocaudal plane (ANT, anterior, POST, posterior). CS and CI, colliculus superior and inferior; NR, nucleus ruber. Lines I, II, and III refer to brain stem transections referred to in text (see ref. ).

Figure 17. Figure 17.

Averaged EMGs from spinal and intact cats. Rectified and filtered EMGs from muscles indicated are averaged after normalization of step‐cycle duration. Mean is indicated as a continuous line and standard deviations as a dotted line. Recordings are from different cats (chronic spinal, n = 2; intact, n = 2) and experimental sessions but may still be comparable, since the cycle durations are chosen to be about 800 ms (± 20 ms) and the onset of the step cycle is triggered by movement in the ankle where flexion (F) and first extension phases (E1, E2) have been used as trigger points. Different phases are marked as lines. Number of averaged locomotor bursts varied between 8 and 12. IP, iliopsoas; SART, sartorius; GM, gluteus medius; ST, semitendinosus; Q, quadriceps femoris; TA, tibialis anterior; FDL, flexor digitorum longus; LG, lateral gastrocnemius; SOL, soleus; EDB, extensor digitorum brevis.

From Forssberg, Grillner, and Halbertsma
Figure 18. Figure 18.

Comparison of electromyographical (EMG) activity of flexors and extensors under intact and spinal conditions in the turtle. Duration of EMG activity is plotted as function of cycle period during swimming movements elicited by electrical stimulation of dorsolateral funicle. Circles represent data obtained from hip retractor and knee flexor. Squares represent data obtained from knee extensor. Closed symbols indicate data obtained in intact condition. Open symbols indicate data obtained in spinal condition. Cycle periods were grouped into bins of 200 ms each, and each point indicates the average of EMG durations for each bin. Vertical bars indicate ± 1 SD of the EMG duration in each bin.

From Lennard and Stein
Figure 19. Figure 19.

Phase lag and the effect of dorsal root (DR) transection in the swimming spinal dogfish. A: time lag between onset of electromyographical (EMG) activity between 2 electrodes 25 segments apart in red lateral muscle (normalized to delay per segment) vs. time duration of burst (equivalent to period length) in a swimming spinal dogfish. r, correlation coefficient. B and C show same type of representation of EMG activity and cycle length as in Figure at 6 different levels (segment number indicated to left). In C, dorsal roots of segment numbers 30–50 have been transected bilaterally (spinal dogfish).

Data in A modified from Grillner ; data in B and C modified from Grillner et al.
Figure 20. Figure 20.

Abolition of burst activity in ankle flexor motoneurons by stretch of ipsilateral ankle extensor muscles (iE) in a mesencephalic cat walking on a treadmill. Top traces, force in ankle extensors (calibration = 2.6 kg); middle traces, electromyogram (EMG) recorded from ankle extensor muscles (triceps surae); bottom traces, EMG recorded from ipsilateral ankle flexor muscle (tibialis anterior; iF). B is continuous with A. Ankle extensor muscles were initially stretched so that inhibitory pauses in extensor activity occasionally did not occur. Note that abolition of inhibitory pause in extensor activity was always associated with abolition of a flexor burst. Near end of B, extensor muscle length was reduced and there was a corresponding marked decrease in extensor EMG.

From Pearson and Duysens
Figure 21. Figure 21.

Locomotion in spinal cat. Reflex effects in support phase. Electromyogram (EMG) and hip movements during lift‐off responses. A: EMG and hip angle. Ipsilateral (i) and contralateral (co) gastrocnemius (G) and semitendinosus (St) recordings. During period where ipsilateral limb is held, iG is tonically active; it is silent while contralateral leg walks. Continuous measurement of hip angle was retraced with help of a computer to transform the cosine output to an angle output. B: average hip angle and EMG during 40 normal walking cycles normalized to 1 from the onset of iG activity. Vertical lines indicate ± SD of the mean; dotted horizontal lines indicate SD of mean EMG duration. C: hip angle at initiation of swing and lift‐off responses. Hip angles corresponding to onset of flexor EMG St were measured in 100 normal walking cycles and 48 lift‐off responses. These angles are plotted as histograms (open areas, swing of normal locomotion; hatched areas, lift‐off responses) as percentage of relative frequency. Mean angle at onset of St during walking was set at 0 (±SD of 4.8°). Angle for lift‐off responses is − 5.6° (±SD of 10.9°).

From Grillner and Rossignol
Figure 22. Figure 22.

Peripheral effects on centrally generated activity in a curarized low spinal cat with fictive locomotion (dopa and nialamide). Artificially applied movements of femur (hip), as during locomotion, drive efferent burst activity. Neurograms (Pb, posterior biceps; TA, tibialis anterior; Sol, soleus; co, contralateral) are rectified and filtered. All branches to sciatic, obturator, and femoral nerves are cut. Hip joint and muscles around hip are innervated; iliopsoas may be transected.

Unpublished data cited in Andersson, Grillner, et al.
Figure 23. Figure 23.

Summary of reflex effects acting on central pattern generators for locomotion. Left panel shows effects that may be important during different parts of hip flexion; right panel shows corresponding information during hip extension.

From Grillner
Figure 24. Figure 24.

Single motor unit activity during locomotion in mesencephalic cat. Units are recorded in ventral root filaments. A and B both show pattern of activity in 3 different cycles correlated to lateral gastrocnemius (LG) electromyographical activity. A and B are at different treadmill velocities and consequently at different cycle durations. Note initial doublets in A and B, clearly distinguished at higher time resolution in a and b.

From Zajac and Young
Figure 25. Figure 25.

Ia‐interneurons recorded during fictive locomotion in mesencephalic cat. A: interneuron response to activation of homonymous muscle nerve during resting conditions at low (continuous recording) and high (vertical record) time resolutions. Corresponding data are shown in B during an episode of locomotion. Note that there are bursts of activity and that the neuron responds to nerve stimulation only during burst periods.

From Fel'dman and Orlovsky
Figure 26. Figure 26.

Interneuron with asymmetric activity during fictive locomotion in spinal dopa preparation. Note that activity is related to that of ipsilateral flexor, but large increase in activity occurs only in last part of burst. At bottom is schematic representation (as in Fig. ) of periods of activity in several successive cycles of filaments and of interneuron. In left graph, cycle starts with onset of activity in interneuron. In right graph, zero point is moved to end of interneuron burst. It can be seen that termination of activity in the interneuron is related tightly to termination of flexor burst. IN, interneuron; fl., flexor; ext., extensor; i., ipsilateral; c, contralateral; int., rectified and filtered. For comparison both total duration of interneuronal activity in each cycle and duration of main burst are given.

From Edgerton, Grillner, et al.
Figure 27. Figure 27.

Activity of ventral spinocerebellar tract (VSCT) neurons. Antidromic invasion (from the ipsilateral vermis) of the VSCT neurons is shown in A (single stimulus) and B (400/s). C shows activity of a VSCT neuron together with movements in both ipsilateral (Hipsi, flexion up) and contralateral hip joint (Hcontra, extension up). Locomotion of this mesencephalic cat (dorsal roots intact) was evoked by stimulation of locomotor region of the brain stem (pulses of 1 ms, 30/s, 20 V). D and E show the activity of a VSCT neuron during locomotion of a mesencephalic cat (both hindlimbs deafferented) and movements in ipsilateral hip joint (Hind, flexion up) and contralateral shoulder joint (Fore, flexion up). Two steps of hindlimb during period of 1 step of forelimb are marked by horizontal line in E. F shows another example of different periods of fore‐ and hindlimb movements, observed in a mesencephalic cat (both hindlimbs deafferented) during beginning of locomotion; horizontal line indicates 3 steps of forelimb during 4 steps of hindlimb (inkwriter recording). G shows activity of VSCT neuron during locomotion of thalamic cat (cerebellum removed, both hindlimbs deafferented) evoked by stimulation of posterior hypothalamus.

From Arshavsky et al.
Figure 28. Figure 28.

Vestibulo‐ and rubrospinal activity during locomotion. Left graphs summarize Orlovsky's material on identified vestibulospinal neurons. Activity (shaded) is correlated to step cycle. Majority (67%) are active in relation to period of extensor activity, but pattern may be subdivided (groups I‐IV). Overall vestibulospinal activity is shown below, before and after cerebellectomy. Corresponding data for rubrospinal neurons are at right.

From Orlovsky
Figure 29. Figure 29.

Section of brain stem (left) indicating location at which mesencephalic locomotor region (MLR) can be stimulated (black circle) and in which spasticity is evoked (shaded). Horsley‐Clarke coordinates are indicated in millimeters (anterior, a; posterior, p). CS and CI, colliculus superior and inferior. Lesions of MLR did not prevent locomotion on pyramidal tract stimulation, but if more ventral (shaded) region was damaged, locomotion was abolished. Right panel shows transections at P1 and P2 and the detailed anatomy around MLR, mostly located about P2. Cu, cuneiform nucleus; BC, brachium conjunctivum; LC, locus coeruleus; α, locus coeruleus α; LSC, locus subcoeruleus; Pbl, nucleus parabrachialis.

Left, modified from Shik et al. ; right from Sakai et al.
Figure 30. Figure 30.

Projections from nucleus cuneiformis. See text for details.

Figure 31. Figure 31.

Scheme of limb generator as mosaic of unit burst generators. It is assumed that each subunit by itself can produce a bursting output. Interconnections between unit generators decide the relative phase of different muscle groups and how they are used in locomotion. E, extensor; f, flexor; H, hip; K, knee; A, ankle; FE, foot extensor; FF, foot flexor; EDB, short toe flexor, extensor digitorum brevis. Scheme to left exemplifies connections that could produce locomotor output, during forward and backward locomotion (see text).

Figure 32. Figure 32.

Network in which oscillation is due to reciprocal interconnections between 2 cell groups.

Figure 33. Figure 33.

Miller‐Scott model. Connections between Ia‐interneurons (Ia‐IN) and Renshaw cells (RC) and α‐motoneurons are indicated, as well as excitation of α‐motoneurons (arrows). Recurrent effects on α‐motoneurons were not incorporated into this scheme.

From Miller and Scott
Figure 34. Figure 34.

Left: ring model of 3 neurons. Right: inhibitory trough of 1 neuron (e.g., C) coincides with spike train (F) of another neuron (e.g., A), which will disinhibit neuron B. Duration of recovery time (R) is influenced by excitability of neurons. P, period length.

From Friesen and Stent reproduced, with permission, from the Annual Review of Biophysics and Bioeng., vol. 7. © 1978 by Annual Reviews Inc
Figure 35. Figure 35.

Off‐switch model. The i activity builds up slowly and drives c‐neurons, which build up activity in the “off‐cells,” which have a very high threshold. Off‐cells will, when activated, block activity in i‐cells. A new cycle starts when i‐cells have recovered. Vagus input in the Hering‐Breuer reflex is integrated into control at c‐cell level. The i‐cells drive the inspiratory motoneurons (insp. mn.) and indirectly inhibit the expiratory motoneurons (exp. mn.).

Figure 36. Figure 36.

Model for burst generation. Model consists of 4 currents in addition to those responsible for action potential: 1) a steady, time‐ and voltage‐independent gNa. (sodium conductance) that tends to pull the membrane potential in the depolarizing direction; 2) an early gK (potassium conductance); 3) a slow gNa (in Helix the function of this current is served by gCa); and 4) a slow gK. Burst is postulated to be turned off by 2 processes: buildup of the slow gK and decay of the slow gNa. These 2 factors drive the membrane potential in the hyperpolarized direction into the range where the early (anomalously rectifying) K+ conductance is reactivated. As the slow gNa decays with time, the constant gNa again moves the membrane potential in the depolarizing direction. The slow depolarization produces a decrease in the early (anomalously rectifying) gK, which tends to keep the membrane going in depolarizing direction, thereby turning off gK even more. As gK decreases, the Na+ current becomes even more effective in depolarizing the membrane. The positive interaction of a decreasing gK and a consequently more effective Na+ current have a regenerative depolarizing tendency. This produces the pacemaker depolarization that turns on the slow gNa and then terminates in the burst. As the early gK inactivates further, it leads to an increase in spike height and rate of firing.

From Cellular Basis of Behavior: An Introduction to Behavioral Neurobiology, by Eric R. Kandel. W. H. Freeman and Co. Copyright © 1976
Figure 37. Figure 37.

Plateau potentials in a crustacean stomatogastric neuron. A depolarizing pulse (lower trace) generates a long‐lasting plateau on top of which spikes can be recorded (upper traces, left). Plateau terminates by itself but can be made to stop by a hyper‐polarizing pulse (right). Sweep corresponds to 3 s.

From Russell and Hartline . © 1978 by The American Association for the Advancement of Science
Figure 38. Figure 38.

Evolution of locomotor movements in human. Walking and running movements progressively develop from unstable gait of a child that has just started to walk. This is a gradual development, and different dynamic biomechanical features evolve. Gait matures in late adolescence.

From Bernstein
Figure 39. Figure 39.

Summary of section NEURAL GENERATION OF “BASIC LOCOMOTOR SYNERGY,” p. 1194. Central pattern generating networks (CPGs) are core of locomotor control system. A significant part of these networks is located in the spinal cord, but their activity is normally controlled from higher centers (volition). The CPGs drive the motoneurons. Potent feedback acts on CPGs but also directly on motoneurons and cerebellum (via DSCT, dorsal spinocerebellar tract). Cerebellum also receives efference copy feedback from the CPGs (ventral spinocerebellar tract and spinoreticulocerebellar path). Cerebellum influences motoneurons via vestibulospinal (VES), reticulospinal (RET), and rubrospinal (RUB) systems. In volition square, the possible different structures and pathways involved are indicated. SLR, subthalamic locomotor region; MLR, mesencephalic locomotor region; PLR, pontine locomotor region; DLF, dorsolateral funicle of spinal cord; RET SP, reticulospinal; VES SP, vestibulospinal; RUB SP, rubrospinal; COER SP, coeruleospinal; VSCT, ventral spinocerebellar tract; SRCP, spinoreticulocerebellar path; MOTON, motoneurons.

Figure 40. Figure 40.

Electromyographical (EMG) responses to electrical stimulation of the dorsum of the hindfoot at different phases of locomotion in a chronic spinal cat. Electrical stimulation (2 mA, single pulse, 5 ms) is delivered through 2 1‐cm2 silver plates taped on the shaved dorsum of the foot. A: stimulus occurs during 2nd semitendinosus (St) locomotor burst and evokes a discharge in St but not in quadriceps (Q). B: 4 steps later, same stimulus occurs during stance, and a response is seen in Q. Time calibrations are shown for A and B (500 ms) and C and D (10 ms). St and Q responses in A and B are at higher time resolution in C and D. int., Rectified and filtered. (From Forssberg, H. Phasic control of reflexes during locomotion in vertebrates, p. 647–674 in ref. .)

Figure 41. Figure 41.

Different phasic gating mechanisms during rhythmic activity. Upper series shows membrane‐potential oscillations occurring during step cycle. In most depolarized part (above threshold, thr), cell is spiking (cf. Fig. ). In middle and lower series, spikes have been omitted from the drawings (horizontal bars indicate spiking period). Middle record shows that any additional input (add. input), being smaller than the step‐cycle modulations, will be gated by the step‐cycle oscillations. Thus an excitatory postsynaptic potential occurring (cf. arrows). in hyperpolarized phase will not reach threshold for generating an action potential and will thus have no behavioral effect. If it instead occurs in the depolarized phase, it will affect motoneuron activity. This must be a general mechanism, causing gating in the CNS whenever there is a rhythmic process (cf. other rhythmic behaviors, EEG). This mechanism is sufficient to explain most of the findings of phase‐dependent reflex effects (cf. refs. and ). In addition, input to the motoneuron may also be gated to diminish the synaptic effect in a certain phase. It has been established that the CPG has such a phasic gate control of the reflex transmission. MOTON, motoneuron.

Figure 42. Figure 42.

Organization of locomotor control. Central pattern generator (CPG) excites α‐γ‐motoneurons (moton) to flexor (F), extensor (E), and special motor nuclei (Sp) and inhibits the antagonists via Ia inhibitory interneurons. The reciprocally coupled Ia‐interneurons are in 3rd panel, OUTPUT CPG. Renshaw cells are also indicated in this panel. •, Inhibitory synapses; , excitatory synapses. Additional input to motoneurons and output interneurons are from cortical and cerebellar efferent channels. Peripheral feedback acts on these neurons. Inputs to CPG (left panel) are from brain; other locomotor CPGs, e.g., for interlimb coordination; feedback from limbs; and nonspecific excitation from afferents. The CPG also exerts phasic control of reflex transmission. CORT, corticospinal; RU, rubrospinal; RET, reticulospinal; VES‐SPIN, vestibulospinal.



Figure 1.

Step cycle. Joint angles in ankle and hip are plotted throughout 1 step in a slow walk (left) and a fast gallop (right). Below graphs period of foot contact is indicated by hatched region. Note striking difference in duration of contact period (stance phase) between walk and gallop, whereas swing phase remains constant. Different terms used for different phases of step cycle are listed below left graph. Under right graph approximate position of limb is drawn for the different phases as indicated by interrupted lines. Drawings do not take into account movement of lower spine.

Adapted from Goslow et al. ; see also ref.


Figure 2.

Duration of step cycle (left) and support phase (middle) at different velocities of walking and running in human. Each line indicates observations on one individual. The right graph shows the duration of flexion and extension vs. cycle duration.

From Grillner et al.


Figure 3.

The support length (ssup, given in meters in A) and the support phase (Tsup, given in seconds in B) vs. velocity of running (m/s). Solid line (alt in A and B) indicates one extreme hypothetical case in which the velocity was only changed by varying the support length; dotted line (alt 2) shows the other extreme case where only Tsup is modified. The dashed line indicates the normal linear relation between ssup and velocity of running in humans and the related changes in Tsup. See text.

Part A from Grillner et al.


Figure 4.

Vertical and longitudinal shear forces during walking in humans. Note the initial breaking (decelerative) force and the late accelerative force (left). The peak values of both the decelerative and accelerative forces increase with walking speed (right).

From Herman et al.


Figure 5.

Position, force vectors, and torques during support phase in walking cat. Cat was walking over a force plate showing amplitude and direction of different force vectors and origin of mechanical axis; position of leg was recorded simultaneously. The mass of the limb segments were obtained postmortem. Each drawing depicts the hindlimb and represents the calculation from 1 “frame.” The frames, represented from left to right, were recorded at 160/s, the 1st 2 after contact is represented and then every 10th frame following, i.e., no. 10, 20, 30, 40, and so forth. Filled columns at ankle, knee, and hip joints represent the torque, and the dotted envelope the maximal positive and negative values obtained (calibration in lower right corner). Note difference between ankle and knee joint in relative torque, and net braking torque in the hip between frames 1 and 40. The contact forces are shown below with their resultant and computed forces acting at the hip above the hip joint.

From Zomlefer, unpublished data


Figure 6.

Simplified scheme of electromyographical activity in cat hindlimb. Most extensors (gen. ext, hip‐toe) show a similar pattern, but definite small differences exist. The toe flexor (toe fl) represents extensor digitorum brevis, whereas extensor digitorum longus behaves like the ankle flexor tibialis anterior. Some differences also exist between the different flexors (gen. fl). The knee flexors (knee fl), e.g., semitendinosus, have a short early flexor burst and a more variable burst during Te1 and Te2. Some muscles like sartorius or rectus femoris may also display double bursts.



Figure 7.

Interlimb coordination during walk and trot in cat. Periods of foot contact are plotted for 1 step cycle starting with foot strike of left hindlimb (LH). Bars indicate period of contact. LF, RH, and RF indicate left forelimb, right hindlimb, and right forelimb. Below contact of each limb is plotted phase in which limb is put down in relation to step cycle of LH, i.e., time from onset of step cycle to foot contact divided by duration of entire step cycle.

Adapted from Stuart et al.


Figure 8.

Pattern of foot contacts during different types of gallop. LH, left hindlimb; LF, left forelimb; RH, right hindlimb; RF, right forelimb.

Adapted from Stuart et al.


Figure 9.

Intersegmental coordination in fishes—phase coupling. Graph shows lag between activation of 2 consecutive segments vs. duration of 1 cycle (i.e., from onset of 1 electromyogram burst on 1 side to onset of next burst). Lag between activation of 2 segments thus varies with cycle length; i.e., at fast speeds of swimming, the intersegmental lag becomes shorter. Lag is always a constant fraction of cycle length; i.e., it is a phase coupling. Graph is plotted from results from spinal dogfish. On right is shown an eel‐like fish, with phase lag of 0.5. Phase lag remains constant at different velocities of swimming.

From Grillner


Figure 10.

Forward velocity, frequency, and amplitude of lateral displacement of the body in fishes. A: frequency (ordinate, Hz, triangles) of alternation (tailbeat) of an eel swimming at different forward velocities (abscissa) and also the corresponding cycle duration (ms, circles). Fishes were swimming in a closed chamber, 0.15 m × 0.15 m × 0.70 m, in which the water flow could be varied up to velocities of 3.3 m/s by a motor‐driven propeller in a closed system. The wires of electromyogram (EMG) electrodes were passed through an aperture in the swimming chamber and the fishes, on which spots of light‐reflecting material were sewn, were filmed (80 frames/s) through transparent chamber with synchronization between EMG and film. B: lateral displacement of tip of tail fin of a 280‐mm long trout swimming at different speeds of forward velocity. C: amplitude of lateral displacement at different points of an eel (distance from tip of head is shown to right). Amplitude of lateral displacement is increasing from head to tail. Scale (ordinate in mm) given for lowest curve applies to all. Bars below record for points corresponding to 25 mm, 129 mm, 183 mm, and 239 mm indicate the period of EMG activity on 1 side at same level as point where lateral displacement was measured. Crossing points at the different levels are connected by thin lines to emphasize the propagated nature of the wave.

From Grillner et al.


Figure 11.

Control of lower trunk during walking (cat). Movement of different points on back and hindlimb has been recorded together with the electromyographical (EMG) activity represented in its rectified and filtered version. A: movement in ankle joint (degrees) and activity

ipsi‐ (i) and contralateral (co)] in knee extensor, vastus lateralis (VL), and ipsilateral erector spinae muscles, multifidus (MfL) and longissimus (LoL) at 4th lumbar segment. Note the 2 bursts in the back muscles. B: the 2 bursts are well correlated to onset of EMG activity in the ipsi‐ and contralateral extensors at different period lengths (k). C: simultaneous lateral movements at 9th thoracic (Th) and 3rd and 5th lumbar (L) segments and, below, the changes in angle between these points. E: corresponding sagittal (up and down) displacement. Horizontal (side) displacement is largest at lower velocities (long cycle duration, part D), whereas sagittal displacement changes less (F). A, C, and E are the same step cycles; vertical lines indicate touchdown. [From Carlson et al.


Figure 12.

Longitudinal body movement of horse and cheetah during gallop and lateral movements of newt. Note flexion and extension of lower spine for horse and cheetah. Bottom, period of foot contact for galloping cheetah. Right, lateral body movements of walking newt. Note how these movements prolong step.

Data for cheetah from Hildebrand ; data for newt from Gray


Figure 13.

Generation of movement according to a chain reflex scheme and a central pattern generation scheme; mu., muscle.



Figure 14.

Effect of dorsal root transection on the electromyographical (EMG) pattern during walking for mesencephalic cat. Periods of EMG activity in 12–15 consecutive step cycles are compared before and after ipsilateral and then contralateral transection of all dorsal roots (3rd lumbar‐4th sacral). Bars indicate main period of EMG activity during successive cycles. Each cycle starts with the onset of activity in the ankle extensor (lateral gastrocnemius); dot to right shows termination of cycle. Hip flexor is iliopsoas; knee flexor is semitendinosus. In addition to its main activity in flexion, this muscle has a smaller and more variable burst during the extension phase, which is indicated by two dots. Lowermost muscle is lateral sartorius (hip/knee), which is a flexor of the hip but also has an extensor function at the knee. Data were obtained in a high mesencephalic cat stimulated in locomotor region. Treadmill speed was 1.6 m/s in left panel and 2.0 m/s in center and right panels. Note that characteristic short period of activity of semitendinosus remains after transections and extensor bursts, as in the intact cat.

Adapted from Grillner and Zangger


Figure 15.

Mesencephalic cat on treadmill. Drawings show arrangements for fixation and recordings including lateral tilt conditions.

From Orlovsky and Pavlova


Figure 16.

Location of subthalamic (SLR), mesencephalic (MLR), and pontine (PLR) locomotor regions in cat brain stem. Brain stem is drawn against Horsley‐Clarke coordinates (in millimeters) in rostrocaudal plane (ANT, anterior, POST, posterior). CS and CI, colliculus superior and inferior; NR, nucleus ruber. Lines I, II, and III refer to brain stem transections referred to in text (see ref. ).



Figure 17.

Averaged EMGs from spinal and intact cats. Rectified and filtered EMGs from muscles indicated are averaged after normalization of step‐cycle duration. Mean is indicated as a continuous line and standard deviations as a dotted line. Recordings are from different cats (chronic spinal, n = 2; intact, n = 2) and experimental sessions but may still be comparable, since the cycle durations are chosen to be about 800 ms (± 20 ms) and the onset of the step cycle is triggered by movement in the ankle where flexion (F) and first extension phases (E1, E2) have been used as trigger points. Different phases are marked as lines. Number of averaged locomotor bursts varied between 8 and 12. IP, iliopsoas; SART, sartorius; GM, gluteus medius; ST, semitendinosus; Q, quadriceps femoris; TA, tibialis anterior; FDL, flexor digitorum longus; LG, lateral gastrocnemius; SOL, soleus; EDB, extensor digitorum brevis.

From Forssberg, Grillner, and Halbertsma


Figure 18.

Comparison of electromyographical (EMG) activity of flexors and extensors under intact and spinal conditions in the turtle. Duration of EMG activity is plotted as function of cycle period during swimming movements elicited by electrical stimulation of dorsolateral funicle. Circles represent data obtained from hip retractor and knee flexor. Squares represent data obtained from knee extensor. Closed symbols indicate data obtained in intact condition. Open symbols indicate data obtained in spinal condition. Cycle periods were grouped into bins of 200 ms each, and each point indicates the average of EMG durations for each bin. Vertical bars indicate ± 1 SD of the EMG duration in each bin.

From Lennard and Stein


Figure 19.

Phase lag and the effect of dorsal root (DR) transection in the swimming spinal dogfish. A: time lag between onset of electromyographical (EMG) activity between 2 electrodes 25 segments apart in red lateral muscle (normalized to delay per segment) vs. time duration of burst (equivalent to period length) in a swimming spinal dogfish. r, correlation coefficient. B and C show same type of representation of EMG activity and cycle length as in Figure at 6 different levels (segment number indicated to left). In C, dorsal roots of segment numbers 30–50 have been transected bilaterally (spinal dogfish).

Data in A modified from Grillner ; data in B and C modified from Grillner et al.


Figure 20.

Abolition of burst activity in ankle flexor motoneurons by stretch of ipsilateral ankle extensor muscles (iE) in a mesencephalic cat walking on a treadmill. Top traces, force in ankle extensors (calibration = 2.6 kg); middle traces, electromyogram (EMG) recorded from ankle extensor muscles (triceps surae); bottom traces, EMG recorded from ipsilateral ankle flexor muscle (tibialis anterior; iF). B is continuous with A. Ankle extensor muscles were initially stretched so that inhibitory pauses in extensor activity occasionally did not occur. Note that abolition of inhibitory pause in extensor activity was always associated with abolition of a flexor burst. Near end of B, extensor muscle length was reduced and there was a corresponding marked decrease in extensor EMG.

From Pearson and Duysens


Figure 21.

Locomotion in spinal cat. Reflex effects in support phase. Electromyogram (EMG) and hip movements during lift‐off responses. A: EMG and hip angle. Ipsilateral (i) and contralateral (co) gastrocnemius (G) and semitendinosus (St) recordings. During period where ipsilateral limb is held, iG is tonically active; it is silent while contralateral leg walks. Continuous measurement of hip angle was retraced with help of a computer to transform the cosine output to an angle output. B: average hip angle and EMG during 40 normal walking cycles normalized to 1 from the onset of iG activity. Vertical lines indicate ± SD of the mean; dotted horizontal lines indicate SD of mean EMG duration. C: hip angle at initiation of swing and lift‐off responses. Hip angles corresponding to onset of flexor EMG St were measured in 100 normal walking cycles and 48 lift‐off responses. These angles are plotted as histograms (open areas, swing of normal locomotion; hatched areas, lift‐off responses) as percentage of relative frequency. Mean angle at onset of St during walking was set at 0 (±SD of 4.8°). Angle for lift‐off responses is − 5.6° (±SD of 10.9°).

From Grillner and Rossignol


Figure 22.

Peripheral effects on centrally generated activity in a curarized low spinal cat with fictive locomotion (dopa and nialamide). Artificially applied movements of femur (hip), as during locomotion, drive efferent burst activity. Neurograms (Pb, posterior biceps; TA, tibialis anterior; Sol, soleus; co, contralateral) are rectified and filtered. All branches to sciatic, obturator, and femoral nerves are cut. Hip joint and muscles around hip are innervated; iliopsoas may be transected.

Unpublished data cited in Andersson, Grillner, et al.


Figure 23.

Summary of reflex effects acting on central pattern generators for locomotion. Left panel shows effects that may be important during different parts of hip flexion; right panel shows corresponding information during hip extension.

From Grillner


Figure 24.

Single motor unit activity during locomotion in mesencephalic cat. Units are recorded in ventral root filaments. A and B both show pattern of activity in 3 different cycles correlated to lateral gastrocnemius (LG) electromyographical activity. A and B are at different treadmill velocities and consequently at different cycle durations. Note initial doublets in A and B, clearly distinguished at higher time resolution in a and b.

From Zajac and Young


Figure 25.

Ia‐interneurons recorded during fictive locomotion in mesencephalic cat. A: interneuron response to activation of homonymous muscle nerve during resting conditions at low (continuous recording) and high (vertical record) time resolutions. Corresponding data are shown in B during an episode of locomotion. Note that there are bursts of activity and that the neuron responds to nerve stimulation only during burst periods.

From Fel'dman and Orlovsky


Figure 26.

Interneuron with asymmetric activity during fictive locomotion in spinal dopa preparation. Note that activity is related to that of ipsilateral flexor, but large increase in activity occurs only in last part of burst. At bottom is schematic representation (as in Fig. ) of periods of activity in several successive cycles of filaments and of interneuron. In left graph, cycle starts with onset of activity in interneuron. In right graph, zero point is moved to end of interneuron burst. It can be seen that termination of activity in the interneuron is related tightly to termination of flexor burst. IN, interneuron; fl., flexor; ext., extensor; i., ipsilateral; c, contralateral; int., rectified and filtered. For comparison both total duration of interneuronal activity in each cycle and duration of main burst are given.

From Edgerton, Grillner, et al.


Figure 27.

Activity of ventral spinocerebellar tract (VSCT) neurons. Antidromic invasion (from the ipsilateral vermis) of the VSCT neurons is shown in A (single stimulus) and B (400/s). C shows activity of a VSCT neuron together with movements in both ipsilateral (Hipsi, flexion up) and contralateral hip joint (Hcontra, extension up). Locomotion of this mesencephalic cat (dorsal roots intact) was evoked by stimulation of locomotor region of the brain stem (pulses of 1 ms, 30/s, 20 V). D and E show the activity of a VSCT neuron during locomotion of a mesencephalic cat (both hindlimbs deafferented) and movements in ipsilateral hip joint (Hind, flexion up) and contralateral shoulder joint (Fore, flexion up). Two steps of hindlimb during period of 1 step of forelimb are marked by horizontal line in E. F shows another example of different periods of fore‐ and hindlimb movements, observed in a mesencephalic cat (both hindlimbs deafferented) during beginning of locomotion; horizontal line indicates 3 steps of forelimb during 4 steps of hindlimb (inkwriter recording). G shows activity of VSCT neuron during locomotion of thalamic cat (cerebellum removed, both hindlimbs deafferented) evoked by stimulation of posterior hypothalamus.

From Arshavsky et al.


Figure 28.

Vestibulo‐ and rubrospinal activity during locomotion. Left graphs summarize Orlovsky's material on identified vestibulospinal neurons. Activity (shaded) is correlated to step cycle. Majority (67%) are active in relation to period of extensor activity, but pattern may be subdivided (groups I‐IV). Overall vestibulospinal activity is shown below, before and after cerebellectomy. Corresponding data for rubrospinal neurons are at right.

From Orlovsky


Figure 29.

Section of brain stem (left) indicating location at which mesencephalic locomotor region (MLR) can be stimulated (black circle) and in which spasticity is evoked (shaded). Horsley‐Clarke coordinates are indicated in millimeters (anterior, a; posterior, p). CS and CI, colliculus superior and inferior. Lesions of MLR did not prevent locomotion on pyramidal tract stimulation, but if more ventral (shaded) region was damaged, locomotion was abolished. Right panel shows transections at P1 and P2 and the detailed anatomy around MLR, mostly located about P2. Cu, cuneiform nucleus; BC, brachium conjunctivum; LC, locus coeruleus; α, locus coeruleus α; LSC, locus subcoeruleus; Pbl, nucleus parabrachialis.

Left, modified from Shik et al. ; right from Sakai et al.


Figure 30.

Projections from nucleus cuneiformis. See text for details.



Figure 31.

Scheme of limb generator as mosaic of unit burst generators. It is assumed that each subunit by itself can produce a bursting output. Interconnections between unit generators decide the relative phase of different muscle groups and how they are used in locomotion. E, extensor; f, flexor; H, hip; K, knee; A, ankle; FE, foot extensor; FF, foot flexor; EDB, short toe flexor, extensor digitorum brevis. Scheme to left exemplifies connections that could produce locomotor output, during forward and backward locomotion (see text).



Figure 32.

Network in which oscillation is due to reciprocal interconnections between 2 cell groups.



Figure 33.

Miller‐Scott model. Connections between Ia‐interneurons (Ia‐IN) and Renshaw cells (RC) and α‐motoneurons are indicated, as well as excitation of α‐motoneurons (arrows). Recurrent effects on α‐motoneurons were not incorporated into this scheme.

From Miller and Scott


Figure 34.

Left: ring model of 3 neurons. Right: inhibitory trough of 1 neuron (e.g., C) coincides with spike train (F) of another neuron (e.g., A), which will disinhibit neuron B. Duration of recovery time (R) is influenced by excitability of neurons. P, period length.

From Friesen and Stent reproduced, with permission, from the Annual Review of Biophysics and Bioeng., vol. 7. © 1978 by Annual Reviews Inc


Figure 35.

Off‐switch model. The i activity builds up slowly and drives c‐neurons, which build up activity in the “off‐cells,” which have a very high threshold. Off‐cells will, when activated, block activity in i‐cells. A new cycle starts when i‐cells have recovered. Vagus input in the Hering‐Breuer reflex is integrated into control at c‐cell level. The i‐cells drive the inspiratory motoneurons (insp. mn.) and indirectly inhibit the expiratory motoneurons (exp. mn.).



Figure 36.

Model for burst generation. Model consists of 4 currents in addition to those responsible for action potential: 1) a steady, time‐ and voltage‐independent gNa. (sodium conductance) that tends to pull the membrane potential in the depolarizing direction; 2) an early gK (potassium conductance); 3) a slow gNa (in Helix the function of this current is served by gCa); and 4) a slow gK. Burst is postulated to be turned off by 2 processes: buildup of the slow gK and decay of the slow gNa. These 2 factors drive the membrane potential in the hyperpolarized direction into the range where the early (anomalously rectifying) K+ conductance is reactivated. As the slow gNa decays with time, the constant gNa again moves the membrane potential in the depolarizing direction. The slow depolarization produces a decrease in the early (anomalously rectifying) gK, which tends to keep the membrane going in depolarizing direction, thereby turning off gK even more. As gK decreases, the Na+ current becomes even more effective in depolarizing the membrane. The positive interaction of a decreasing gK and a consequently more effective Na+ current have a regenerative depolarizing tendency. This produces the pacemaker depolarization that turns on the slow gNa and then terminates in the burst. As the early gK inactivates further, it leads to an increase in spike height and rate of firing.

From Cellular Basis of Behavior: An Introduction to Behavioral Neurobiology, by Eric R. Kandel. W. H. Freeman and Co. Copyright © 1976


Figure 37.

Plateau potentials in a crustacean stomatogastric neuron. A depolarizing pulse (lower trace) generates a long‐lasting plateau on top of which spikes can be recorded (upper traces, left). Plateau terminates by itself but can be made to stop by a hyper‐polarizing pulse (right). Sweep corresponds to 3 s.

From Russell and Hartline . © 1978 by The American Association for the Advancement of Science


Figure 38.

Evolution of locomotor movements in human. Walking and running movements progressively develop from unstable gait of a child that has just started to walk. This is a gradual development, and different dynamic biomechanical features evolve. Gait matures in late adolescence.

From Bernstein


Figure 39.

Summary of section NEURAL GENERATION OF “BASIC LOCOMOTOR SYNERGY,” p. 1194. Central pattern generating networks (CPGs) are core of locomotor control system. A significant part of these networks is located in the spinal cord, but their activity is normally controlled from higher centers (volition). The CPGs drive the motoneurons. Potent feedback acts on CPGs but also directly on motoneurons and cerebellum (via DSCT, dorsal spinocerebellar tract). Cerebellum also receives efference copy feedback from the CPGs (ventral spinocerebellar tract and spinoreticulocerebellar path). Cerebellum influences motoneurons via vestibulospinal (VES), reticulospinal (RET), and rubrospinal (RUB) systems. In volition square, the possible different structures and pathways involved are indicated. SLR, subthalamic locomotor region; MLR, mesencephalic locomotor region; PLR, pontine locomotor region; DLF, dorsolateral funicle of spinal cord; RET SP, reticulospinal; VES SP, vestibulospinal; RUB SP, rubrospinal; COER SP, coeruleospinal; VSCT, ventral spinocerebellar tract; SRCP, spinoreticulocerebellar path; MOTON, motoneurons.



Figure 40.

Electromyographical (EMG) responses to electrical stimulation of the dorsum of the hindfoot at different phases of locomotion in a chronic spinal cat. Electrical stimulation (2 mA, single pulse, 5 ms) is delivered through 2 1‐cm2 silver plates taped on the shaved dorsum of the foot. A: stimulus occurs during 2nd semitendinosus (St) locomotor burst and evokes a discharge in St but not in quadriceps (Q). B: 4 steps later, same stimulus occurs during stance, and a response is seen in Q. Time calibrations are shown for A and B (500 ms) and C and D (10 ms). St and Q responses in A and B are at higher time resolution in C and D. int., Rectified and filtered. (From Forssberg, H. Phasic control of reflexes during locomotion in vertebrates, p. 647–674 in ref. .)



Figure 41.

Different phasic gating mechanisms during rhythmic activity. Upper series shows membrane‐potential oscillations occurring during step cycle. In most depolarized part (above threshold, thr), cell is spiking (cf. Fig. ). In middle and lower series, spikes have been omitted from the drawings (horizontal bars indicate spiking period). Middle record shows that any additional input (add. input), being smaller than the step‐cycle modulations, will be gated by the step‐cycle oscillations. Thus an excitatory postsynaptic potential occurring (cf. arrows). in hyperpolarized phase will not reach threshold for generating an action potential and will thus have no behavioral effect. If it instead occurs in the depolarized phase, it will affect motoneuron activity. This must be a general mechanism, causing gating in the CNS whenever there is a rhythmic process (cf. other rhythmic behaviors, EEG). This mechanism is sufficient to explain most of the findings of phase‐dependent reflex effects (cf. refs. and ). In addition, input to the motoneuron may also be gated to diminish the synaptic effect in a certain phase. It has been established that the CPG has such a phasic gate control of the reflex transmission. MOTON, motoneuron.



Figure 42.

Organization of locomotor control. Central pattern generator (CPG) excites α‐γ‐motoneurons (moton) to flexor (F), extensor (E), and special motor nuclei (Sp) and inhibits the antagonists via Ia inhibitory interneurons. The reciprocally coupled Ia‐interneurons are in 3rd panel, OUTPUT CPG. Renshaw cells are also indicated in this panel. •, Inhibitory synapses; , excitatory synapses. Additional input to motoneurons and output interneurons are from cortical and cerebellar efferent channels. Peripheral feedback acts on these neurons. Inputs to CPG (left panel) are from brain; other locomotor CPGs, e.g., for interlimb coordination; feedback from limbs; and nonspecific excitation from afferents. The CPG also exerts phasic control of reflex transmission. CORT, corticospinal; RU, rubrospinal; RET, reticulospinal; VES‐SPIN, vestibulospinal.

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Sten Grillner. Control of Locomotion in Bipeds, Tetrapods, and Fish. Compr Physiol 2011, Supplement 2: Handbook of Physiology, The Nervous System, Motor Control: 1179-1236. First published in print 1981. doi: 10.1002/cphy.cp010226