Comprehensive Physiology Wiley Online Library

The Brain Stem Reticular Core and Sensory Function

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Abstract

The sections in this article are:

1 Terminology
2 Historical Development of Concepts
3 Organization of Reticular Core
3.1 Cell Groups
3.2 Dendritic Patterns
3.3 Afferent Systems
3.4 Axonal Outflow: General Picture
3.5 Systems With Specific Chemical Signature
4 Thalamic Nonspecific System
4.1 Cell Groups
4.2 Dendritic Patterns
4.3 Afferent Systems
4.4 Axonal Outflow
4.5 Nucleus Reticularis Thalami
5 Overview
Figure 1. Figure 1.

Equally spaced transverse Nissl‐stained sections through brain stem of cat show grouping of cells in reticular core. Dots on right indicate specific cell bodies, while dashed lines on left indicate approximate boundaries of major reticular nuclei. Terminology and nuclear areas are based on studies by Olszewski of the rabbit, with minor modifications. The following list of abbreviations refers only to structures making up reticular core. a, Accessory group of paramedian reticular nucleus; d, dorsal group of paramedian reticular nucleus; h, region poor in cells of Meesen and Olszewski surrounding motor trigeminal nucleus; k, cell group k of Meesen and Olszewski; m, cell group m of Meesen and Olszewki; N. ic., nucleus intercalatus; N.r.l., lateral reticular nucleus; N.r.t., nucleus reticularis tegmenti pontis; N.t.d., dorsal tegmental nucleus; N.t.v., ventral tegmental nucleus; P.g., periaqueductal gray; P.h., nucleus prepositus hypoglossi; R. gc., nucleus reticularis gigantocellularis; R.l., nucleus reticularis lateralis (of Meesen and Olszewski); R. mes., reticular formation of mesencephalon; R.n., nucleus of the raphe; R.pc., nucleus reticularis parvocellularis; R.p.o., nucleus reticularis pontis oralis; R.p.c., nucleus reticularis pontis caudalis; R.v., nucleus reticularis ventralis; v, ventral group of paramedian reticular nucleus.

From Brodal
Figure 2. Figure 2.

Equally spaced transverse thionine‐stained sections through brain stem of adult cat. Outlines, made by means of a projection apparatus, were checked under microscope and details entered. Nuclei of the raphe are indicated by dots; density and sizes of dots serve to give an approximate impression of architecture of various nuclei, a, Accessory group of paramedian reticular nucleus; B.c., brachium conjunctivum cerebelli; B.p., brachium pontis; Coll.i., inferior colliculus; Coll.s., superior colliculus; C.r., restiform body; C.s., nucleus centralis superior; Cun., nucleus cuneiformis; d, dorsal group of paramedian reticular nucleus; d, descending (spinal) vestibular nucleus; D.t., dorsal tegmental nucleus (Gudden); E.W., Edinger‐Westphal nucleus; F.l.m., medial longitudinal fasciculus; i.c., nucleus intercalatus; Ip., interpeduncular nucleus; L., lateral vestibular nucleus (Deiters'); L.c., locus coeruleus; L.i., nucleus linearis intermedius; L.r., nucleus linearis rostralis; M, medial vestibular nucleus; N.c.t., nuclei of trapezoid body; N.cu.e., external cuneate nucleus; N.f.c., cuneate nucleus; N.f.g., gracile nucleus; N.int., nucleus interstitialis (Cajal); N.l.l., nuclei of lateral lemniscus; N.m.d.X, dorsal motor vagus nucleus; N.mes. V, mesencephalic trigeminal nucleus; N.r., red nucleus (n. ruber); N.r.l., nucleus reticularis lateralis (nucleus funiculi lateralis); N.r.t., nucleus reticularis tegmenti pontis (Bechterew); N.tr.sp.V, nucleus of spinal trigeminal tract; N.III, V, VI, VII, XII, cranial nerves; Ol.i., inferior olive; Ol.s., superior olive; P, griseum pontis; P.c., cerebral peduncle; p.h., nucleus prepositus hypoglossi; P.s., nucleus parasolitarius; Py., pyramid; R.d., nucleus raphe dorsalis; R.gc., nucleus reticularis gigantocellularis medullae oblongatae; R.m., nucleus raphe magnus; R.o., nucleus raphe obscurus; R.p., nucleus raphe pontis; R.pa., nucleus raphe pallidus; R.p.c., nucleus reticularis pontis caudalis; S, superior vestibular nucleus; S.n., substantia nigra; T, trapezoid body; Tr.s., solitary tract; Tr.sp.V., spinal trigeminal tract; Ts., ventral tegmental nucleus of Tsai; V.t., ventral tegmental nucleus (Gudden); x, group × of Brodal and Pompeiano; III, IV, VII, X, XII, cranial motor nerve nuclei.

From Taber
Figure 3. Figure 3.

Dendritic patterns in brain stem reticular core (based on Golgi‐stained sections): g, nucleus gigantocellularis; r, nucleus of the raphe; l, lateral reticular nucleus, which has many of the features of sensory relay nuclear cells and probably serves, in part, that function for the adjacent spinothalamic tract; i, inferior cerebellar penduncle; tr, descending root and nucleus of the trigeminal nerve; a, nucleus ambiguus.

Figure 4. Figure 4.

Sagittal section through lower half of brain stem of 10‐day‐old rat. Most of dendrite mass of reticular‐core cells is organized along dorsoventral axis as seen in this type of section, with marked compression along rostrocaudal axis. This orientation places dendrites parallel to terminal presynaptic components, which in this case arise from pyramidal tract (Tr. Pyr.) and from a single axon of a magnocellular reticular neuron (n. retic. mag). This type of dendrite organization, which is especially characteristic of reticular cells of medial two‐thirds of core, produces sets of 2‐dimensional modular neuropil fields leading to stack‐of‐chips analogy (see inset, lower left). This is contrasted with dendritic patterns in adjacent hypoglossal nucleus (n. XII); n. inf. ol., inferior olive; n. pontis, the pons.

From Scheibel and Scheibel
Figure 5. Figure 5.

Cross sections through same level of medulla of newborn and adult cats showing apparent loss of dendritic spines and apparent regrouping of reticular cell dendrites into bundles. Neurons include those of most rostral part of n. reticularis parvocellularis (A) and of n. reticularis gigantocellularis (B). C, medial longitudinal fasciculus; D, n. prepositus hypoglossi; E, medial vestibular nucleus. Rapid Golgi variant. Original magnification X 160.

From Scheibel, Davies, and Scheibel
Figure 6. Figure 6.

Dendrite system a is oriented toward axons from medial longitudinal fasciculus (5); dendrite system b is oriented toward axons from vestibular nuclear complex (1); c corresponds to descending trigeminal system (2); d corresponds to spinothalamics (3); e is oriented toward axons from corticospinal fibers (4).

Figure 7. Figure 7.

Reticular neuron from gigantocellular nucleus in a 10‐day‐old cat. A single terminating afferent establishes a series of terminal boutons along proximal portion of dendrite and cell body. Golgi modification X 440.

Figure 8. Figure 8.

Convergence of heterogeneous afferents on single elements of brain stem reticular core, demonstrated physiologically and histologically. Strips A‐N (left) and C‐G (bottom right) illustrate patterns of spike discharge of 2 elements of bulboreticular formation. At left, A is firing spontaneously; B is inhibited by cerebellar polarization; C rebounds following cessation of polarization; D returns to more normal discharge pattern; E is stimulated by nose pressure; F and G are stimulated by patellar tendon taps administered bilaterally; H and I are unaffected by short trains of vagal stimulation; J and K are unaffected by auditory clicks; L is driven by repetitive cortical stimulation; M and N show, with aid of expanded time base, that latency of the corticifugal discharge to the bulboreticular unit is very short. Strips C‐F (bottom right) show that another bulboreticular unit that is sensitive to pressure to nose (G) can also be driven by auditory clicks (C and D). This rather minimal effect is unmasked (E and F) when spontaneous activity of unit is inhibited by cerebellar polarization. A‐C top right, bulboreticular cells, lying within several hundred μm of each other, in a 10‐day‐old kitten. Axons from a number of fiber systems were traced to these cells, although only terminal portions are shown. Horizontally running fibers such as A1‐A7 and B7‐B8 appear to belong to spinoreticular and long reticuloreticular components, while B1‐B4, approached from dorsal and lateral aspects of bulb, represent sensory collaterals and cerebelloreticular collaterals. All records from locally anesthetized, paralyzed (Flaxedil) cat.

From Scheibel and Scheibel
Figure 9. Figure 9.

Contrast in habituation patterns of bulboreticular neuron exposed to repetitive sciatic stimulation at 2 V, 1/s, 0.5‐ms pulse width. Solid line, first series of stimuli; dashed line, second series of stimuli, delivered following a 3‐min rest period after habituation had been achieved; dotted vertical line, time of onset of stimuli. Oscillatory variations in number of spike discharges per stimulus during period of habituation seem characteristic of this process. All records from locally anesthetized, paralyzed (Flaxedil) cat.

Adapted from Scheibel and Scheibel
Figure 10. Figure 10.

Cyclic response of single medullary reticular neuron whose activity was continuously monitored for 9 h. Nearly 3 complete cycles of exogenously driven and endogenously driven activity are charted here. Periods of sensitivity to exogenous inputs are almost twice as long as those to slow endogenous rhythms. Line‐bar notation above curves indicates no clear‐cut relationship between these swings and states of consciousness of animal. Recent reevaluation of all of these data, however, throws this interpretation open to question. All records from locally anesthetized, paralyzed (Flaxedil) cats.

Adapted from Scheibel and Scheibel
Figure 11. Figure 11.

Sagittal section of entire brain of 7‐day‐old mouse showing 2 reticular cells in gigantocellular nucleus of rostral medulla. Both cells emit axons that bifurcate and course rostrad and caudad. A number of collaterals are given off by each axon, some of which reach cranial nerve nuclei, such as Deiters' component of vestibular complex (n.D.); both inferior and superior colliculi (I.C. and S.C.); and pretectum (Pt). CM, centre médian; LP, lateral posterior; LG, lateral geniculate; LD, lateral dorsal; CL, central lateral; AV, anterior ventral; V, ventral complex; VA, ventral anterior; R, nucleus reticularis thalami; ZI, zona incerta; and SN, substantia nigra.

From Scheibel and Scheibel
Figure 12. Figure 12.

Several possible conduction circuits through reticular core of the brain stem: a, type of chaining of short‐axoned cells hypothesized by Moruzzi and Magoun and by a number of other workers to explain conduction characteristics marked by slow transmission, long latency, and recruiting; b, single, long‐axoned cell, reaching from bulb (dashed line at left) to diencephalon, illustrating type of conductor found in large numbers in reticular formation; c, the many collaterals of long conductors, as in b, may provide for more circuitous paths through reticular core, producing greater lateral dispersion and increasingly longer conduction times and longer latencies.

From Scheibel and Scheibel
Figure 13. Figure 13.

Semischematic sagittal sections of the brain stem of the cat showing arrangement and distribution of reticular cells sending axons rostrally (left) and caudally (right). Despite considerable overlap caudally directed axons appear to arise somewhat more rostrad than do rostrally directed fibers. Arrows at sides of figures indicate that axon systems are both crossed and uncrossed except for fibers descending from the pons, which are uncrossed.

From Brodal
Figure 14. Figure 14.

Division of medial pontomedullary reticular cells into functional zones (1–5) on basis of somatomotor connections. V, n. ventralis; Gc, n. gigantocellularis; PoC, m. pontis caudalis; PoO, n. pontis oralis; io, inferior olive; tb, trapezoid body; nrtp, n. reticularis tegmenti pontis; p, pons; h, hypoglossal; g, genu; ab, abducens.

Adapted from Petersen
Figure 15. Figure 15.

Radioautographic analysis of projections from pontile tegmentum to abducens nucleus and nucleus prepositus hypoglossi in cat. Injection site is shown in heavy black in section E. Distribution of silver grain overlying efferent pathways traced from deposit is shown by dots. Of special interest is labeling in the ipsilateral abducens nucleus (VI) in section G and the nucleus prepositus hypoglossi (PH) in sections H and J. Considerable labeling also occurs throughout medial magnocellular portion of tegmentum, both at level of injection site and more caudally. Most intensive labeling appears in rostral part of nucleus gigantocellularis, (Ngc). Rostral projections are more sparse and are found mainly in mesencephalic tegmentum, ventral half of pretectal region, and in accessory oculomotor nuclei including interstitial nucleus of Cajal (NIC), and nucleus of posterior commissure (NPC). In thalamus, discrete accumulations appear at juncture of central lateral (Cl) and medial dorsal (MD) nuclei. NR, nucleus ruber; SN, substantia nigra; IP, interpeduncular nucleus; LTN, lateral terminal nucleus; MLF, medial longitudinal fasciculus; NPp, nucleus papilliformis; BC, brachium conjunctivum; Npo, nucleus pontis oralis; NSG, nucleus supragenualis; SV, superior vestibular nucleus; BP, brachium pontis; PH, nucleus prepositus hypogloss; MV, medial vestibular nucleus; Dr, descending vestibular nucleus; NI, nucleus intercalatus; cuE, external cuneate nucleus; NG, nucleus gracilis; NC, nucleus cuneatus; f, cell groups f of Brodal.

Adapted from Graybiel
Figure 16. Figure 16.

Drawing of generalized small mammalian brain, showing some elements of ascending cholinergic reticular system of pons and mesencephalon and their projections throughout the neuraxis. Dashed lines represent probable but not fully established pathways. cd, Caudate nucleus; cun, n. cuneiformis; d, dorsal leaf of ascending reticular projection; gp, globus pallidus; h, hypothalamus; nrt, n. reticular thalami; nrtp, n. reticularis tegmenti pontis; ot, optic tract; sn, substantia nigra; sub, subthalamus; th, thalamus; v, ventral leaf of ascending reticular formation; vt, ventral tegmental nucleus.

Adapted from Shute and Lewis
Figure 17. Figure 17.

Some major projections of noradrenergic systems of reticular core and their apparent terminal areas. Major pathways are ascending ones, using both dorsal leaf (d) and ventral leaf (v) of familiar reticular core pathways. Cell group A6 (locus coeruleus) undoubtedly projects to cerebellum, tectum, lower brain stem, and spinal cord; medullary cell groups A1 and A5 may do so. h, Hypothalamus; st, stria terminalis. Most data from rat brain.

Adapted from Ungerstedt
Figure 18. Figure 18.

Distribution of cell bodies containing catecholamine (norepinephrine) in dorsolateral tegmentum of pons of cat. Rostrally the cells appear grouped about brachium conjunctivum cerebelli (br) and extend somewhat beyond confines of locus coeruleus (coe). Further caudad some cells are located near ventricular surface. bp, Brachium pontis; ci, inferior colliculus; cu, nucleus cuneiformis; mlf, medial longitudinal fasciculus; so, superior olive; m5, main sensory nucleus of trigeminal nerve; vs, superior vestibular nucleus. A 2.5, A 3.5, and A 4.5 indicate stereotaxic transverse plane of section.

Based on data of Chu and Bloom
Figure 19. Figure 19.

Projection system of single cell in locus coeruleus as reconstructed from 2 adjacent Golgi‐stained sections of 14‐day‐old mouse, bf, Basal forebrain; cb, cerebellum; h, hypothalamus; lc, locus coeruleus; nv, vestibular nuclear complex; p, pons; rp, nucleus raphe pontis; sc, superior colliculus; sn, substantia nigra; vm, ventromedial thalamus; zi, zona incerta.

Adapted from Scheibel and Scheibel ; used with permission of the New York Academy of Sciences
Figure 20. Figure 20.

Drawing of Golgi‐stained cross section of 10‐day‐old kitten through pontile tegmentum showing how neurons of nucleus raphe pontis (Rp) climb along raphe blood vessels that are lightly impregnated (a) and unimpregnated (b). Vertical arrow indicates midline. Dendrites of some neurons bridge across midline and make contact with raphe vessels on both sides. Inset, part of the raphe vascular system at this level; square, area depicted in drawing. Rpc, nucleus reticularis pontis caudalis; p, pons; mlf, medial longitudinal fasciculus; g, genu of nerve VII. X 186.

From Scheibel, Tomiyasu, and Scheibel
Figure 21. Figure 21.

Diagrammatic map of feline thalamic nonspecific system at four levels along the rostrocaudal axis, as determined by areas giving rise to recruiting responses. AD, anterodorsal nucleus; aHd, dorsal hypothalamic area; AL, ansa lenticularis; AM, anteromedial nucleus; AV, anteroventral nucleus; BCI, brachium of inferior colliculus; CC, corpus callosum; Cd, caudate nucleus; Cl, claustrum; CL, central lateral nucleus; CM, central medial nucleus; CP, posterior commissure; Da, nucleus of Darkschewitch; En, entopeduncular nucleus; Fil, filiform nucleus, fsc, subcallosal fasciculus; FT, thalamic fasciculus; Fx, fornix; GC, central gray matter; GM, medial geniculate body; HbL, lateral habenular nucleus; HbM, medial habenular nucleus; H1, field of Forel; HL, lateral hypothalamus; Hp, posterior hypothalamus; IAM, interanteromedial nucleus; IP, interpeduncular nucleus; Is, interstitial nucleus; LD, lateral dorsal nucleus; Lim, nucleus limitans; LM, medial lemniscus; LME, external medullary lamina; LP, lateral posterior nucleus; mc, pars magnocellularis; MD, medial dorsal nucleus; MFB, medial forebrain bundle; NCM, central medial nucleus; NCP, posterior commissural nucleus; NR, red nucleus; P, posterior nucleus; Pc, paracentral nucleus; Ped, cerebral peduncle; Prt, pretectum; Pt, parataenial nucleus; Pul, pulvinar; PVA, anterior periventricular nucleus; PVH, periventricular hypothalamic nucleus; R, reticular nucleus; RE, nucleus reuniens; S, medullary stria; SG, suprageniculate nucleus; Sm, submedian nucleus; SN, substantia nigra; Spf, subparafascicular nucleus; ST, terminal stria; THP, habenulopeduncular tract; TMT, mammillothalamic tract; TO, optic tract; TTC, central tegmental tract; VA, ventral anterior nucleus, VL, ventral lateral nucleus; VM, ventral medial nucleus; VPL, ventral posterolateral nucleus; VPM, ventral posteromedial nucleus; ZI, zona incerta. Arrows in upper left diagram indicate 2 major directions of information flow at rostral pole of thalamus. Dotted areas represent zones from which recruitment responses were obtained.

Adapted from Jasper
Figure 22. Figure 22.

Dendritic and axonal organization in posterior portion of medial thalamus in area of parafascicular nucleus of 12‐day‐old rat. Dendrites arranged primarily in mediolateral orientation. They are crossed by rostrocaudally oriented periventricular fiber bundles (Pv) and accompanied by collaterals from descending intralaminar fibers (a, a1) and ascending reticular fibers (c). Axon (b), which may have come from nucleus reticularis thalami, bears a number of small terminal synaptic structures resembling axosomatic boutons. Modified rapid Golgi‐impregnated section cut in horizontal plane. Inset: slightly schematized drawing of horizontal section through diencephalon and forebrain of young rat, showing location (rectangle) of area involved on each side of midline (vertical dashed line). Arrows, paths of converging afferents from more rostral areas. Other abbreviations: ld, lateral dendrite mass; md, medial dendrite mass; pfd, perifascicular dendrites; fr, fasciculus retroflexus (of Meynert). X 143.

From Scheibel and Scheibel
Figure 23. Figure 23.

Some axonal pathways of intralaminar and medial thalamic neurons of 14‐day‐old rat. Axon a from an anteromedial nuclear cell (Am) projects rostrally, both ipsilaterally and contralaterally. Axon b is the contralateral projection of an anteromedial neuron with both local and distant connections, while axons c and d, of similar derivation, project both rostrally and caudally on contralateral side. Axons e and f are from paracentral neurons (Pc), and both project rostrally and caudally. Aq, aqueduct of Sylvius; AV, anterior ventral nucleus; Cl, central lateral nucleus; Pf, parafascicular nucleus; VA, ventral anterior nucleus; and fr, fasciculus retroflexus. Inset: slightly schematized drawing of horizontal section through diencephalon and forebrain of young rat. Rectangle: area involved in figure. Modified rapid Golgi‐stained section cut in horizontal plane. X 120.

From Scheibel and Scheibel
Figure 24. Figure 24.

Electron photomicrograph of glomerulus similar to those discussed in text. Surrounding glial sheaths (GL) encapsulate several presynaptic axonal processes (A) and a preterminal process (P) entering the complex. Midline thalamus, adult cat. X 16,000.

From Pappas et al.
Figure 25. Figure 25.

Comparison between organizations of axonal elements of thalamic specific and nonspecific systems. Drawing based on a number of Golgi‐stained sections of rat and mouse. Top: thalamofugal elements. A neuron of the ventrobasal complex (10) projects a virtually uncollateralized axon (SP) toward cortex. A neuron of the nonspecific system (NSP) generates an axon that bifurcates into rostral and caudal running branches, both of which are richly collateralized, ipsilaterally and contralaterally. 1, Parataenial nucleus; 2, anterior ventral nucleus; 3, interanteromedial nucleus; 4, anterior medial nucleus; 5, paracentral nucleus; 6, central lateral nucelus; 7, central medial nucleus; 8, centre médian‐parafascicular complex; 9, n. reticularis thalami; 10, ventrobasal complex; and 11, posterior thalamic complex. Bottom: descending thalamopetal elements (A and B) are axons running from cortex to parts of specific ventral nuclear complex. A is a characteristic 3‐dimensional terminal while B is a 2‐dimensional discoid arbor. C is an axon descending from cortex to nonspecific fields. Diffuse collateral system includes branches to area of nucleus reticularis (D) and area of ventral anterior nucleus (D) to contralateral nonspecific fields (E and F), to the posterior nuclear complex (G) and to the mesodiencephalic junction (H).

From Scheibel and Scheibel
Figure 26. Figure 26.

Rostral projection of thalamic nonspecific system through inferior thalamic peduncle on orbitofrontal cortex (C of) of 50‐day‐old, partially demyelinated mouse. Axons from anterior thalamic nuclei, including paracentral (PC) and medial portion of ventral anterior (VA), project rostrally via inferior thalamic peduncle as nonspecific projection (nsp) past striatum (str) and branch widely in subgriseal white matter. Here some branches reach the nucleus accumbens septi (nAcSp). nR, nucleus reticularis thalami; fxc, columns of fornix; sp, septum; and rZ, radiation of Zuckerkandl. Ependymal neuroglial cells (g) line lateral ventricles. Inset: slightly schematized drawing of horizontal section through diencephalon and forebrain of young rat. Rectangle: area involved in figure. Arrows: position and course of thalamocortical nonspecific projection. Modified rapid Golgi‐impregnated section cut in horizontal‐oblique plane. X 113.

From Scheibel and Scheibel
Figure 27. Figure 27.

Reciprocal connections between mesencephalic tegmental cells of the nucleus cuneiformis (Cun) and the nucleus reticularis thalami (nR), which may constitute part of thalamic gating system mentioned in text. CM, centre médian; Cm, central medial; Am, anterior medial nuclei; and fr, fasciculus retroflexus. Elements selected from several sections of 10‐to‐14‐day‐old rat. Fine arrow, mesodiencephalic interface. X 101.

Figure 28. Figure 28.

Schematic drawing contrasting rostral course of axon systems from brain stem reticular core and nonspecific thalamic systems. Brain stem reticular axons (1) ascending through tegmentum (Teg); bifurcation occurs just caudal to thalamus, forming both the dorsal leaf (2), which terminates in thalamic intralaminar (Th) and dorsomedial fields, and the ventral leaf (3), which runs ventrolaterally through subthalamus (Sb) and hypothalamus (H) thereby swinging ventrally to nucleus reticularis thalami (nR). Axons of thalamic nonspecific system send caudally directed projection (5) back to tegmental level, while rostral component (4) perforates nucleus reticularis thalami and continues rostrally via inferior thalamic peduncle.

From Scheibel and Scheibel


Figure 1.

Equally spaced transverse Nissl‐stained sections through brain stem of cat show grouping of cells in reticular core. Dots on right indicate specific cell bodies, while dashed lines on left indicate approximate boundaries of major reticular nuclei. Terminology and nuclear areas are based on studies by Olszewski of the rabbit, with minor modifications. The following list of abbreviations refers only to structures making up reticular core. a, Accessory group of paramedian reticular nucleus; d, dorsal group of paramedian reticular nucleus; h, region poor in cells of Meesen and Olszewski surrounding motor trigeminal nucleus; k, cell group k of Meesen and Olszewski; m, cell group m of Meesen and Olszewki; N. ic., nucleus intercalatus; N.r.l., lateral reticular nucleus; N.r.t., nucleus reticularis tegmenti pontis; N.t.d., dorsal tegmental nucleus; N.t.v., ventral tegmental nucleus; P.g., periaqueductal gray; P.h., nucleus prepositus hypoglossi; R. gc., nucleus reticularis gigantocellularis; R.l., nucleus reticularis lateralis (of Meesen and Olszewski); R. mes., reticular formation of mesencephalon; R.n., nucleus of the raphe; R.pc., nucleus reticularis parvocellularis; R.p.o., nucleus reticularis pontis oralis; R.p.c., nucleus reticularis pontis caudalis; R.v., nucleus reticularis ventralis; v, ventral group of paramedian reticular nucleus.

From Brodal


Figure 2.

Equally spaced transverse thionine‐stained sections through brain stem of adult cat. Outlines, made by means of a projection apparatus, were checked under microscope and details entered. Nuclei of the raphe are indicated by dots; density and sizes of dots serve to give an approximate impression of architecture of various nuclei, a, Accessory group of paramedian reticular nucleus; B.c., brachium conjunctivum cerebelli; B.p., brachium pontis; Coll.i., inferior colliculus; Coll.s., superior colliculus; C.r., restiform body; C.s., nucleus centralis superior; Cun., nucleus cuneiformis; d, dorsal group of paramedian reticular nucleus; d, descending (spinal) vestibular nucleus; D.t., dorsal tegmental nucleus (Gudden); E.W., Edinger‐Westphal nucleus; F.l.m., medial longitudinal fasciculus; i.c., nucleus intercalatus; Ip., interpeduncular nucleus; L., lateral vestibular nucleus (Deiters'); L.c., locus coeruleus; L.i., nucleus linearis intermedius; L.r., nucleus linearis rostralis; M, medial vestibular nucleus; N.c.t., nuclei of trapezoid body; N.cu.e., external cuneate nucleus; N.f.c., cuneate nucleus; N.f.g., gracile nucleus; N.int., nucleus interstitialis (Cajal); N.l.l., nuclei of lateral lemniscus; N.m.d.X, dorsal motor vagus nucleus; N.mes. V, mesencephalic trigeminal nucleus; N.r., red nucleus (n. ruber); N.r.l., nucleus reticularis lateralis (nucleus funiculi lateralis); N.r.t., nucleus reticularis tegmenti pontis (Bechterew); N.tr.sp.V, nucleus of spinal trigeminal tract; N.III, V, VI, VII, XII, cranial nerves; Ol.i., inferior olive; Ol.s., superior olive; P, griseum pontis; P.c., cerebral peduncle; p.h., nucleus prepositus hypoglossi; P.s., nucleus parasolitarius; Py., pyramid; R.d., nucleus raphe dorsalis; R.gc., nucleus reticularis gigantocellularis medullae oblongatae; R.m., nucleus raphe magnus; R.o., nucleus raphe obscurus; R.p., nucleus raphe pontis; R.pa., nucleus raphe pallidus; R.p.c., nucleus reticularis pontis caudalis; S, superior vestibular nucleus; S.n., substantia nigra; T, trapezoid body; Tr.s., solitary tract; Tr.sp.V., spinal trigeminal tract; Ts., ventral tegmental nucleus of Tsai; V.t., ventral tegmental nucleus (Gudden); x, group × of Brodal and Pompeiano; III, IV, VII, X, XII, cranial motor nerve nuclei.

From Taber


Figure 3.

Dendritic patterns in brain stem reticular core (based on Golgi‐stained sections): g, nucleus gigantocellularis; r, nucleus of the raphe; l, lateral reticular nucleus, which has many of the features of sensory relay nuclear cells and probably serves, in part, that function for the adjacent spinothalamic tract; i, inferior cerebellar penduncle; tr, descending root and nucleus of the trigeminal nerve; a, nucleus ambiguus.



Figure 4.

Sagittal section through lower half of brain stem of 10‐day‐old rat. Most of dendrite mass of reticular‐core cells is organized along dorsoventral axis as seen in this type of section, with marked compression along rostrocaudal axis. This orientation places dendrites parallel to terminal presynaptic components, which in this case arise from pyramidal tract (Tr. Pyr.) and from a single axon of a magnocellular reticular neuron (n. retic. mag). This type of dendrite organization, which is especially characteristic of reticular cells of medial two‐thirds of core, produces sets of 2‐dimensional modular neuropil fields leading to stack‐of‐chips analogy (see inset, lower left). This is contrasted with dendritic patterns in adjacent hypoglossal nucleus (n. XII); n. inf. ol., inferior olive; n. pontis, the pons.

From Scheibel and Scheibel


Figure 5.

Cross sections through same level of medulla of newborn and adult cats showing apparent loss of dendritic spines and apparent regrouping of reticular cell dendrites into bundles. Neurons include those of most rostral part of n. reticularis parvocellularis (A) and of n. reticularis gigantocellularis (B). C, medial longitudinal fasciculus; D, n. prepositus hypoglossi; E, medial vestibular nucleus. Rapid Golgi variant. Original magnification X 160.

From Scheibel, Davies, and Scheibel


Figure 6.

Dendrite system a is oriented toward axons from medial longitudinal fasciculus (5); dendrite system b is oriented toward axons from vestibular nuclear complex (1); c corresponds to descending trigeminal system (2); d corresponds to spinothalamics (3); e is oriented toward axons from corticospinal fibers (4).



Figure 7.

Reticular neuron from gigantocellular nucleus in a 10‐day‐old cat. A single terminating afferent establishes a series of terminal boutons along proximal portion of dendrite and cell body. Golgi modification X 440.



Figure 8.

Convergence of heterogeneous afferents on single elements of brain stem reticular core, demonstrated physiologically and histologically. Strips A‐N (left) and C‐G (bottom right) illustrate patterns of spike discharge of 2 elements of bulboreticular formation. At left, A is firing spontaneously; B is inhibited by cerebellar polarization; C rebounds following cessation of polarization; D returns to more normal discharge pattern; E is stimulated by nose pressure; F and G are stimulated by patellar tendon taps administered bilaterally; H and I are unaffected by short trains of vagal stimulation; J and K are unaffected by auditory clicks; L is driven by repetitive cortical stimulation; M and N show, with aid of expanded time base, that latency of the corticifugal discharge to the bulboreticular unit is very short. Strips C‐F (bottom right) show that another bulboreticular unit that is sensitive to pressure to nose (G) can also be driven by auditory clicks (C and D). This rather minimal effect is unmasked (E and F) when spontaneous activity of unit is inhibited by cerebellar polarization. A‐C top right, bulboreticular cells, lying within several hundred μm of each other, in a 10‐day‐old kitten. Axons from a number of fiber systems were traced to these cells, although only terminal portions are shown. Horizontally running fibers such as A1‐A7 and B7‐B8 appear to belong to spinoreticular and long reticuloreticular components, while B1‐B4, approached from dorsal and lateral aspects of bulb, represent sensory collaterals and cerebelloreticular collaterals. All records from locally anesthetized, paralyzed (Flaxedil) cat.

From Scheibel and Scheibel


Figure 9.

Contrast in habituation patterns of bulboreticular neuron exposed to repetitive sciatic stimulation at 2 V, 1/s, 0.5‐ms pulse width. Solid line, first series of stimuli; dashed line, second series of stimuli, delivered following a 3‐min rest period after habituation had been achieved; dotted vertical line, time of onset of stimuli. Oscillatory variations in number of spike discharges per stimulus during period of habituation seem characteristic of this process. All records from locally anesthetized, paralyzed (Flaxedil) cat.

Adapted from Scheibel and Scheibel


Figure 10.

Cyclic response of single medullary reticular neuron whose activity was continuously monitored for 9 h. Nearly 3 complete cycles of exogenously driven and endogenously driven activity are charted here. Periods of sensitivity to exogenous inputs are almost twice as long as those to slow endogenous rhythms. Line‐bar notation above curves indicates no clear‐cut relationship between these swings and states of consciousness of animal. Recent reevaluation of all of these data, however, throws this interpretation open to question. All records from locally anesthetized, paralyzed (Flaxedil) cats.

Adapted from Scheibel and Scheibel


Figure 11.

Sagittal section of entire brain of 7‐day‐old mouse showing 2 reticular cells in gigantocellular nucleus of rostral medulla. Both cells emit axons that bifurcate and course rostrad and caudad. A number of collaterals are given off by each axon, some of which reach cranial nerve nuclei, such as Deiters' component of vestibular complex (n.D.); both inferior and superior colliculi (I.C. and S.C.); and pretectum (Pt). CM, centre médian; LP, lateral posterior; LG, lateral geniculate; LD, lateral dorsal; CL, central lateral; AV, anterior ventral; V, ventral complex; VA, ventral anterior; R, nucleus reticularis thalami; ZI, zona incerta; and SN, substantia nigra.

From Scheibel and Scheibel


Figure 12.

Several possible conduction circuits through reticular core of the brain stem: a, type of chaining of short‐axoned cells hypothesized by Moruzzi and Magoun and by a number of other workers to explain conduction characteristics marked by slow transmission, long latency, and recruiting; b, single, long‐axoned cell, reaching from bulb (dashed line at left) to diencephalon, illustrating type of conductor found in large numbers in reticular formation; c, the many collaterals of long conductors, as in b, may provide for more circuitous paths through reticular core, producing greater lateral dispersion and increasingly longer conduction times and longer latencies.

From Scheibel and Scheibel


Figure 13.

Semischematic sagittal sections of the brain stem of the cat showing arrangement and distribution of reticular cells sending axons rostrally (left) and caudally (right). Despite considerable overlap caudally directed axons appear to arise somewhat more rostrad than do rostrally directed fibers. Arrows at sides of figures indicate that axon systems are both crossed and uncrossed except for fibers descending from the pons, which are uncrossed.

From Brodal


Figure 14.

Division of medial pontomedullary reticular cells into functional zones (1–5) on basis of somatomotor connections. V, n. ventralis; Gc, n. gigantocellularis; PoC, m. pontis caudalis; PoO, n. pontis oralis; io, inferior olive; tb, trapezoid body; nrtp, n. reticularis tegmenti pontis; p, pons; h, hypoglossal; g, genu; ab, abducens.

Adapted from Petersen


Figure 15.

Radioautographic analysis of projections from pontile tegmentum to abducens nucleus and nucleus prepositus hypoglossi in cat. Injection site is shown in heavy black in section E. Distribution of silver grain overlying efferent pathways traced from deposit is shown by dots. Of special interest is labeling in the ipsilateral abducens nucleus (VI) in section G and the nucleus prepositus hypoglossi (PH) in sections H and J. Considerable labeling also occurs throughout medial magnocellular portion of tegmentum, both at level of injection site and more caudally. Most intensive labeling appears in rostral part of nucleus gigantocellularis, (Ngc). Rostral projections are more sparse and are found mainly in mesencephalic tegmentum, ventral half of pretectal region, and in accessory oculomotor nuclei including interstitial nucleus of Cajal (NIC), and nucleus of posterior commissure (NPC). In thalamus, discrete accumulations appear at juncture of central lateral (Cl) and medial dorsal (MD) nuclei. NR, nucleus ruber; SN, substantia nigra; IP, interpeduncular nucleus; LTN, lateral terminal nucleus; MLF, medial longitudinal fasciculus; NPp, nucleus papilliformis; BC, brachium conjunctivum; Npo, nucleus pontis oralis; NSG, nucleus supragenualis; SV, superior vestibular nucleus; BP, brachium pontis; PH, nucleus prepositus hypogloss; MV, medial vestibular nucleus; Dr, descending vestibular nucleus; NI, nucleus intercalatus; cuE, external cuneate nucleus; NG, nucleus gracilis; NC, nucleus cuneatus; f, cell groups f of Brodal.

Adapted from Graybiel


Figure 16.

Drawing of generalized small mammalian brain, showing some elements of ascending cholinergic reticular system of pons and mesencephalon and their projections throughout the neuraxis. Dashed lines represent probable but not fully established pathways. cd, Caudate nucleus; cun, n. cuneiformis; d, dorsal leaf of ascending reticular projection; gp, globus pallidus; h, hypothalamus; nrt, n. reticular thalami; nrtp, n. reticularis tegmenti pontis; ot, optic tract; sn, substantia nigra; sub, subthalamus; th, thalamus; v, ventral leaf of ascending reticular formation; vt, ventral tegmental nucleus.

Adapted from Shute and Lewis


Figure 17.

Some major projections of noradrenergic systems of reticular core and their apparent terminal areas. Major pathways are ascending ones, using both dorsal leaf (d) and ventral leaf (v) of familiar reticular core pathways. Cell group A6 (locus coeruleus) undoubtedly projects to cerebellum, tectum, lower brain stem, and spinal cord; medullary cell groups A1 and A5 may do so. h, Hypothalamus; st, stria terminalis. Most data from rat brain.

Adapted from Ungerstedt


Figure 18.

Distribution of cell bodies containing catecholamine (norepinephrine) in dorsolateral tegmentum of pons of cat. Rostrally the cells appear grouped about brachium conjunctivum cerebelli (br) and extend somewhat beyond confines of locus coeruleus (coe). Further caudad some cells are located near ventricular surface. bp, Brachium pontis; ci, inferior colliculus; cu, nucleus cuneiformis; mlf, medial longitudinal fasciculus; so, superior olive; m5, main sensory nucleus of trigeminal nerve; vs, superior vestibular nucleus. A 2.5, A 3.5, and A 4.5 indicate stereotaxic transverse plane of section.

Based on data of Chu and Bloom


Figure 19.

Projection system of single cell in locus coeruleus as reconstructed from 2 adjacent Golgi‐stained sections of 14‐day‐old mouse, bf, Basal forebrain; cb, cerebellum; h, hypothalamus; lc, locus coeruleus; nv, vestibular nuclear complex; p, pons; rp, nucleus raphe pontis; sc, superior colliculus; sn, substantia nigra; vm, ventromedial thalamus; zi, zona incerta.

Adapted from Scheibel and Scheibel ; used with permission of the New York Academy of Sciences


Figure 20.

Drawing of Golgi‐stained cross section of 10‐day‐old kitten through pontile tegmentum showing how neurons of nucleus raphe pontis (Rp) climb along raphe blood vessels that are lightly impregnated (a) and unimpregnated (b). Vertical arrow indicates midline. Dendrites of some neurons bridge across midline and make contact with raphe vessels on both sides. Inset, part of the raphe vascular system at this level; square, area depicted in drawing. Rpc, nucleus reticularis pontis caudalis; p, pons; mlf, medial longitudinal fasciculus; g, genu of nerve VII. X 186.

From Scheibel, Tomiyasu, and Scheibel


Figure 21.

Diagrammatic map of feline thalamic nonspecific system at four levels along the rostrocaudal axis, as determined by areas giving rise to recruiting responses. AD, anterodorsal nucleus; aHd, dorsal hypothalamic area; AL, ansa lenticularis; AM, anteromedial nucleus; AV, anteroventral nucleus; BCI, brachium of inferior colliculus; CC, corpus callosum; Cd, caudate nucleus; Cl, claustrum; CL, central lateral nucleus; CM, central medial nucleus; CP, posterior commissure; Da, nucleus of Darkschewitch; En, entopeduncular nucleus; Fil, filiform nucleus, fsc, subcallosal fasciculus; FT, thalamic fasciculus; Fx, fornix; GC, central gray matter; GM, medial geniculate body; HbL, lateral habenular nucleus; HbM, medial habenular nucleus; H1, field of Forel; HL, lateral hypothalamus; Hp, posterior hypothalamus; IAM, interanteromedial nucleus; IP, interpeduncular nucleus; Is, interstitial nucleus; LD, lateral dorsal nucleus; Lim, nucleus limitans; LM, medial lemniscus; LME, external medullary lamina; LP, lateral posterior nucleus; mc, pars magnocellularis; MD, medial dorsal nucleus; MFB, medial forebrain bundle; NCM, central medial nucleus; NCP, posterior commissural nucleus; NR, red nucleus; P, posterior nucleus; Pc, paracentral nucleus; Ped, cerebral peduncle; Prt, pretectum; Pt, parataenial nucleus; Pul, pulvinar; PVA, anterior periventricular nucleus; PVH, periventricular hypothalamic nucleus; R, reticular nucleus; RE, nucleus reuniens; S, medullary stria; SG, suprageniculate nucleus; Sm, submedian nucleus; SN, substantia nigra; Spf, subparafascicular nucleus; ST, terminal stria; THP, habenulopeduncular tract; TMT, mammillothalamic tract; TO, optic tract; TTC, central tegmental tract; VA, ventral anterior nucleus, VL, ventral lateral nucleus; VM, ventral medial nucleus; VPL, ventral posterolateral nucleus; VPM, ventral posteromedial nucleus; ZI, zona incerta. Arrows in upper left diagram indicate 2 major directions of information flow at rostral pole of thalamus. Dotted areas represent zones from which recruitment responses were obtained.

Adapted from Jasper


Figure 22.

Dendritic and axonal organization in posterior portion of medial thalamus in area of parafascicular nucleus of 12‐day‐old rat. Dendrites arranged primarily in mediolateral orientation. They are crossed by rostrocaudally oriented periventricular fiber bundles (Pv) and accompanied by collaterals from descending intralaminar fibers (a, a1) and ascending reticular fibers (c). Axon (b), which may have come from nucleus reticularis thalami, bears a number of small terminal synaptic structures resembling axosomatic boutons. Modified rapid Golgi‐impregnated section cut in horizontal plane. Inset: slightly schematized drawing of horizontal section through diencephalon and forebrain of young rat, showing location (rectangle) of area involved on each side of midline (vertical dashed line). Arrows, paths of converging afferents from more rostral areas. Other abbreviations: ld, lateral dendrite mass; md, medial dendrite mass; pfd, perifascicular dendrites; fr, fasciculus retroflexus (of Meynert). X 143.

From Scheibel and Scheibel


Figure 23.

Some axonal pathways of intralaminar and medial thalamic neurons of 14‐day‐old rat. Axon a from an anteromedial nuclear cell (Am) projects rostrally, both ipsilaterally and contralaterally. Axon b is the contralateral projection of an anteromedial neuron with both local and distant connections, while axons c and d, of similar derivation, project both rostrally and caudally on contralateral side. Axons e and f are from paracentral neurons (Pc), and both project rostrally and caudally. Aq, aqueduct of Sylvius; AV, anterior ventral nucleus; Cl, central lateral nucleus; Pf, parafascicular nucleus; VA, ventral anterior nucleus; and fr, fasciculus retroflexus. Inset: slightly schematized drawing of horizontal section through diencephalon and forebrain of young rat. Rectangle: area involved in figure. Modified rapid Golgi‐stained section cut in horizontal plane. X 120.

From Scheibel and Scheibel


Figure 24.

Electron photomicrograph of glomerulus similar to those discussed in text. Surrounding glial sheaths (GL) encapsulate several presynaptic axonal processes (A) and a preterminal process (P) entering the complex. Midline thalamus, adult cat. X 16,000.

From Pappas et al.


Figure 25.

Comparison between organizations of axonal elements of thalamic specific and nonspecific systems. Drawing based on a number of Golgi‐stained sections of rat and mouse. Top: thalamofugal elements. A neuron of the ventrobasal complex (10) projects a virtually uncollateralized axon (SP) toward cortex. A neuron of the nonspecific system (NSP) generates an axon that bifurcates into rostral and caudal running branches, both of which are richly collateralized, ipsilaterally and contralaterally. 1, Parataenial nucleus; 2, anterior ventral nucleus; 3, interanteromedial nucleus; 4, anterior medial nucleus; 5, paracentral nucleus; 6, central lateral nucelus; 7, central medial nucleus; 8, centre médian‐parafascicular complex; 9, n. reticularis thalami; 10, ventrobasal complex; and 11, posterior thalamic complex. Bottom: descending thalamopetal elements (A and B) are axons running from cortex to parts of specific ventral nuclear complex. A is a characteristic 3‐dimensional terminal while B is a 2‐dimensional discoid arbor. C is an axon descending from cortex to nonspecific fields. Diffuse collateral system includes branches to area of nucleus reticularis (D) and area of ventral anterior nucleus (D) to contralateral nonspecific fields (E and F), to the posterior nuclear complex (G) and to the mesodiencephalic junction (H).

From Scheibel and Scheibel


Figure 26.

Rostral projection of thalamic nonspecific system through inferior thalamic peduncle on orbitofrontal cortex (C of) of 50‐day‐old, partially demyelinated mouse. Axons from anterior thalamic nuclei, including paracentral (PC) and medial portion of ventral anterior (VA), project rostrally via inferior thalamic peduncle as nonspecific projection (nsp) past striatum (str) and branch widely in subgriseal white matter. Here some branches reach the nucleus accumbens septi (nAcSp). nR, nucleus reticularis thalami; fxc, columns of fornix; sp, septum; and rZ, radiation of Zuckerkandl. Ependymal neuroglial cells (g) line lateral ventricles. Inset: slightly schematized drawing of horizontal section through diencephalon and forebrain of young rat. Rectangle: area involved in figure. Arrows: position and course of thalamocortical nonspecific projection. Modified rapid Golgi‐impregnated section cut in horizontal‐oblique plane. X 113.

From Scheibel and Scheibel


Figure 27.

Reciprocal connections between mesencephalic tegmental cells of the nucleus cuneiformis (Cun) and the nucleus reticularis thalami (nR), which may constitute part of thalamic gating system mentioned in text. CM, centre médian; Cm, central medial; Am, anterior medial nuclei; and fr, fasciculus retroflexus. Elements selected from several sections of 10‐to‐14‐day‐old rat. Fine arrow, mesodiencephalic interface. X 101.



Figure 28.

Schematic drawing contrasting rostral course of axon systems from brain stem reticular core and nonspecific thalamic systems. Brain stem reticular axons (1) ascending through tegmentum (Teg); bifurcation occurs just caudal to thalamus, forming both the dorsal leaf (2), which terminates in thalamic intralaminar (Th) and dorsomedial fields, and the ventral leaf (3), which runs ventrolaterally through subthalamus (Sb) and hypothalamus (H) thereby swinging ventrally to nucleus reticularis thalami (nR). Axons of thalamic nonspecific system send caudally directed projection (5) back to tegmental level, while rostral component (4) perforates nucleus reticularis thalami and continues rostrally via inferior thalamic peduncle.

From Scheibel and Scheibel
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Arnold B. Scheibel. The Brain Stem Reticular Core and Sensory Function. Compr Physiol 2011, Supplement 3: Handbook of Physiology, The Nervous System, Sensory Processes: 213-256. First published in print 1984. doi: 10.1002/cphy.cp010306