Terminals Of Afferent Fibers Research Articles

Comparison of primary afferent and glutamate excitation of neurons in the mammalian spinal dorsal horn

The actions of L-glutamate and agonists, agents blocking their membrane receptors and dorsal root afferent volleys, were compared on intracellularly recorded neuronal activity in an in vitro horizontal slice preparation of the hamster spinal dorsal horn. Bath-applied L-glutamate or L-aspartate (less than or equal to 1 mM) rapidly depolarized and excited less than a third of the dorsal horn neurons sampled. Bathing solutions containing low Ca2+ eliminated synaptic transmission in the slices but failed to block the excitatory effects of L-glutamate for the majority of the neurons tested. N-Acetylaspartylglutamate had no effect on dorsal horn neurons at concentrations up to 1 mM. Neurons excited by L-glutamate were most commonly located in the superficial dorsal horn (laminae I and II). Neurons insensitive to L-glutamate were more broadly distributed, with a number being located in laminae III-V. Kynurenic acid, 2-amino-4-phosphonobutyric acid, and 2,3-piperidine dicarboxylic acid selectively antagonized rapid, short-lasting synaptic components of the dorsal cord potentials. Kynurenic acid reversibly antagonized intracellularly recorded L-glutamate-induced excitation, spontaneous synaptic potentials, and fast synaptic potentials evoked by dorsal root volleys. Compounds with strong antagonist actions at the NMDA receptor, 2-amino-5-phosphonovaleric acid and D-alpha-aminoadipic acid, were much less effective in suppressing the effects of L-glutamate or in blocking synaptic potentials. We conclude that a subset of spinal neurons directly excited by dorsal root fibers have excitatory membrane receptors activated by L-glutamate. This conclusion is consistent with the concept that L-glutamate or a substance binding to the receptors it activates is released from the central terminals of some primary afferent fibers and mediates fast synaptic transmission from them to certain spinal neurons in the dorsal horn.

Phase-linked modulation of excitability of presynaptic terminals of low-threshold afferent fibers in the inferior alveolar nerve during cortically induced fictive mastication in the guinea pig

Excitability of presynaptic terminals of low-threshold primary afferent fibers in the inferior alveolar nerve was tested in the trigeminal spinal nucleus of the ketamine-anesthetized, paralyzed guinea pig, by Wall's method. Fictive mastication was induced by repetitive stimulation of the cortical masticatory area, and was monitored by rhythmical burst activity in the jaw-opening anterior digastric motoneuron pool. The excitability was rhythmically modulated in a phase-linked manner during the masticatory cycle: it was decreased coincidentally with the digastric burst activity (jaw-opening phase) and increased during the middle and late periods of the interburst phase (jaw-closing phase) of the masticatory cycle. The results imply that presynaptic modulation of synaptic transmission of peripheral inputs from primary afferents to interneurons in the jaw-opening reflex pathway may contribute to the rhythmical modulation of the jaw-opening reflex evoked by innocuous stimulation of the intraoral structures during mastication; presynaptic inhibition contributing to the depression of the jaw-opening reflex during the jaw-closing phase and presynaptic facilitation to its enhancement during the jaw-opening phase.

The distribution and ontogenesis of [ 3H]nicotine binding sites in cat visual cortex

In vitro autoradiographic techniques using [ 3H]nicotine were used to characterise nicotine binding sites in developing kitten visual cortex. These binding sites in adult animals have a B max of 3.91 fmol/mg protein and a K d of 4.40 nM. Displacement experiments indicat that [ 3H]nicotine binds to a nicotinic receptor site that is similar to central nicotinic sites described by investigators in other mammals. The number of binding sites increases during postnatal development, peaking near 60 days of age and levelling-off thereafter. There is no evidence for large changes in affinity during postnatal development for this binding site. [ 3H]Nicotine binding sites are densely concentrated in layer IV in the visual cortex of adult animals, with sharply reduced binding outside of cortical areas 17 and 18. This laminar pattern does not change during postnatal development, but an increase in the number of binding sites in layer IV as well as in layers I and VI occurs during early postnatal life. These binding sites disappear when extrinsic cortical inputs are severed. However, they survive when neurons in the visual cortex are selectively destroyed with a cell-specific neurotoxin. Unilateral destruction of the lateral geniculate nucleus eliminates [ 3H]nicotine binding sites in the visual cortex ipsilateral to the lesion, suggesting that they are located presynaptically on the terminals of lateral geniculate nucleus afferent fibres. The laminar pattern of binding of [ 3H]nicotine during early development of the visual cortex is complimentary to that for muscarinic acetylcholine receptors. These latter receptors redistribute during postnatal development becoming less prominent in layer IV at the same time as the [ 3H]nicotine binding sites are increasing in number in this layer. For a short period of time at the height of the critical period for cortical plasticity, both populations of binding sites are located in layer IV.

Attenuation of the reflex pressor response to muscular contraction by an antagonist to somatostatin.

Although group III and IV fibers are known to compose the afferent pathway of the reflex arc causing the pressor response to static muscular contraction, little is known about the neurotransmitters released by these muscle afferents. Somatostatin might be one of these neurotransmitters because this peptide is found in the terminals of fine afferent fibers ending in the dorsal horn of the lumbar spinal cord. Therefore, in chloralose-anesthetized cats, the reflex pressor response to static contraction was examined before and after subarachnoid injections onto the lumbosacral cord of a peptide antagonist to somatostatin. We found that before giving the antagonist, the pressor response to contraction of the triceps surae muscles in 12 cats averaged 33 +/- 4 mm Hg, while 37 +/- 7 minutes after giving the antagonist, the pressor response averaged only 18 +/- 3 mm Hg (p less than 0.001). In contrast, the antagonist to somatostatin had no effect on either the pressor response to electrical stimulation of the cut central end of the sciatic nerve or the pressor response to stimulation of the posterior diencephalon. Furthermore, subarachnoid injection of a peptide antagonist to luteinizing hormone-releasing hormone had no effect on the reflex pressor response to static contraction. Our findings are consistent with the hypothesis that somatostatin plays a role in the spinal transmission of the contraction-induced pressor reflex arising from hind limb skeletal muscle.

Depolarizing effects of dopamine on the primary afferent fibers of a segment isolated from the spinal cord of newborn rats

Effects of dopamine on dorsal root potentials were investigated during experiments on a segment of spinal cord isolated from 12- to 18-day-old rats. Applying dopamine to the brain was found to produce a slow, reversible, dose-dependent depolarization at primary afferent fiber terminals. This dopamine-induced depolarization was retained during complete blockade of synaptic transmission brought about by exchanging calcium ions in the perfusing fluid by magnesium or manganese ions. Minimum dopamine concentration required to produce this effect was 1·10−10–1·10−9 M. Peak amplitude of depolarization equaled 1.5 mV. Duration of this reaction ranged from 5.5 to 36.7 min, depending on the duration and concentration of dopamine application. Depolarizing response to dopamine differed considerably from GABA-induced dorsal root depolarization in amplitude and rate of rise. Haloperidol, a dopamine antagonist, reduced dopamine-induced dorsal root depolarization. Findings indicate that dopamine acts directly on the membrane of primary afferent fiber terminals, shifting membrane potential toward depolarization. This raises the possibility that dopaminergic brainstem-spinal pathways may exert an effect on sensory information transmission in segmental reflex arcs already traveling to the spinal cord.

High susceptibility to hypoxia of afferent synaptic transmission in the goldfish sacculus.

1. The effect of hypoxia on synaptic transmission between hair cells and afferent fibers was examined in the sacculus of goldfish. For this, we recorded potentials, intracellularly, from large afferent fibers. Anoxia was introduced by perfusing the gill with water deprived of oxygen or by halting the water flow to the gill. 2. The ear of the goldfish is most sensitive to hypoxia. Sound-evoked afferent activities were profoundly depressed within several minutes after the introduction of hypoxia. 3. The depressed afferent activity was attributed to a reduction in the amplitude of sound-evoked excitatory postsynaptic potentials (EPSPs) generated at afferent fiber terminals, since no significant change was detected in the resting and action potentials of afferent fibers or in intensity of the threshold current required to set up an action potential. Also, there was no marked change in the electrical activity of hair cells, determined by the finding that the amplitude of intramacularly recorded microphonic potentials and that of the coupling potentials was not altered. 4. A statistical analysis of the amplitude of sound-evoked EPSPs revealed that the binomial parameter n decreased during hypoxia, in parallel with a reduction in the amplitude of EPSPs, while the binomial parameter p either remained unaltered or was augmented. No change was found in the quantal size, thereby indicating that the sensitivity of the postsynaptic membrane remained unchanged. These results indicate that presynaptic mechanisms within hair cells, especially those playing a role in transmitter release or in replenishment of the latter, are suppressed during hypoxia.

Divalent cations reduce depolarization of primary afferent terminations by GABA

Divalent metal cations, including zinc, cadmium, cobalt, nickel, strontium, manganese, magnesium and calcium, reduced the depolarization by microelectrophoretic γ-aminobutyric acid (GABA) and piperidine-4-sulphonic acid of the central terminations of muscle group Ia primary afferent fibres in the cat spinal cord without affecting the inhibition by GABA of the firing of spinal interneurones. There thus appears to be a difference in either the interaction of GABA with recognition sites, or in the mechanism by which such interaction activates chloride ionophores, at GABA-mediated bicuculline-sensitive synapses on the central terminals of peripheral primary afferent neurones and those on neurones located within the central nervous system.

Contribution of the Nervous System to the Pathophysiology of Rheumatoid Arthritis and Other Polyarthritides

Some clinical features of rheumatoid arthritis (RA) (for example, preferential joint involvement and bilateral symmetry), taken together with the strong evidence of neurogenic inflammatory processes, suggest that the nervous system contributes to the inflammatory component of RA and other polyarthritides. The authors propose that the increased risk and severity of disease in particular joints reflects a greater innervation of those joints by unmyelinated afferent and sympathetic efferent fibers. Release of the proinflammatory peptide, substance P, from the peripheral terminals of nociceptive joint afferent fibers, through interactions with many nonneural cells, exacerbates the inflammatory process. Release of mediators from sympathetic efferents (including norepinephrine) also contributes to the inflammation, either through an independent mechanism or by acting in concert with the nociceptive afferent-derived substances. Therapies directed at interruption of the nervous system contribution to the pathophysiology of these diseases should offer a new direction to treatment.

Nerve growth factor receptors in rat spinal cord: An autoradiographic and immunohistochemical study

The distribution of nerve growth factor receptors in the lumbar spinal cord of the rat was studied with autoradiographic and immunohistochemical techniques: [ 125I]nerve growth factor and specific monoclonal antibody (Mab 192) against nerve growth factor receptor were used to localize nerve growth factor binding sites. The distributions of nerve growth factor binding sites with highest density within the superficial layers (laminae I and II) of the dorsal gray matter were virtually identical as demonstrated by these two ligands; this suggests that Mab 192 can be used as a specific probe to identify nerve growth factor receptors in rat nervous system. Nerve growth factor receptor binding sites, as demonstrated by autoradiography, were also found in longitudinal bundles of fibers running dorsolaterally in the lateral funiculus. However, no immunoreactivity was detected in these areas by immunohistochemistry. No specific binding was found in the dorsal horn when [ 125I]nerve growth factor was co-injected with unlabeled nerve growth factor or after incubation with nonspecific monoclonal antibody. Dorsal root section produced a complete loss of nerve growth factor-specific labeling pattern throughout laminae I–II of the spinal cord. This suggests that nerve growth factor receptors are localized on the nerve terminals of primary afferent fibers which synapse in the region of the spinal cord. The presence of nerve growth factor binding sites in the dorsal horn of the spinal cord is consistent with the possibility that nerve growth factor, or a nerve growth factor-like substance, derived from the central nervous system, may have a role in trophic support of dorsal root ganglion neurons.

Neuropeptide Y-immunoreactive perikarya and nerve terminals in the rat medulla oblongata: relationship to cytoarchitecture and catecholaminergic cell groups.

The aim of this study was to examine details of the distribution of neuropeptide Y (NPY)-immunoreactive perikarya and nerve terminals in the medulla oblongata in relation to cytoarchitectonically and functionally distinct catecholaminergic regions. The immunoperoxidase method was combined with Nissl staining to determine nuclear boundaries of transmitter-identified nerve cell bodies and to examine the relationship between populations of NPY-immunoreactive neurons and catecholaminergic cell groups (A1, A2, C1, C2, and C3) in serial sections. Previous studies using immunofluorescence have described the existence of NPY catecholaminergic immunoreactive nerve cell bodies in the brainstem. No information is currently available with regard to details of the distribution of these peptidergic neurons and nerve terminals in the functional subnuclear units of the medulla oblongata. In this study we have delineated the anatomical association of NPY immunoreactivity with cardiovascular function. Neuropeptide Y-immunoreactive neurons were found located in close association with noradrenergic neurons of the A1 cell group in the caudal ventrolateral medulla oblongata, where they were usually found located dorsal to the lateral reticular nucleus (LRt). A second population of NPY-immunoreactive neurons was found located medial to the A1 cell group in the ventral subdivision of the reticular nucleus of the medulla (MdV). Neuropeptide Y-immunoreactive neurons in the rostral medulla were found located in regions corresponding to the principal distribution of adrenergic neurons in the C1, C2, and C3 cell groups. In the dorsomedial medulla (A2 region) NPY-immunoreactive neurons were localized in the area postrema (ap) and in a number of subnuclei of the nucleus of the tractus solitarius (nTS), i.e., the dorsal parasolitary region (dPSR), the dorsal strip (ds), the periventricular region (PVR), and the ventral parasolitary region (vPSR). The location of NPY-immunoreactive perikarya and nerve terminals in the dorsal subnuclei of the nTS, i.e., the dPSR and ds, is of particular significance, since this distribution corresponds with the location of small adrenergic neurons as well as with the site of termination of aortic and carotid sinus nerve afferent fibers. NPY-immunoreactive neurons in the dorsomedial medulla are ideally situated for receiving monosynaptic input from baroreceptor afferents and could play a key role in the central integration of cardiovascular reflexes.

Prevention of degradation of endogenous enkephalins produces inhibition of nociceptive neurones in rat spinal cord

The enkephalins, found in high levels in the superficial dorsal horn where they are associated with the terminals of afferent nociceptive fibres, are rapidly degraded by at least 3 peptidases. The use of inhibitors of these peptidases allows effects mediated by endogenous enkephalins to be observed. We report here the inhibitory effects on spinal nociceptive transmission of bestatin, a non-specific aminopeptidase inhibitor, thiorphan, an inhibitor of enkephalinase and kelatorphan, a mixed inhibitor of aminopeptidases, particularly aminopeptidase M, enkephalinase and dipeptidylaminopeptidase. The agents were applied intrathecally, directly onto the spinal cord, in halothane-anaesthetized intact rats. Bestatin ( n = 23neurones) produced weak inhibitions of C-fibre-evoked activity (maximum 17% inhibition) whereas thiorphan ( n = 20) produced maximal 25% inhibitions. Kelatorphan ( n = 32) produced maximal 46% inhibitions which were naloxone reversible. All 3 agents caused dose-dependent effects which were relatively selective for C-fibre evoked responses, sparing Aβ-fibre inputs. The results are discussed in relation to roles of the enkephalins in sensory modulation and the potential of these compounds as novel therapeutic agents.

Projections of visceral and somatic primary afferents to the sacral spinal cord of the domestic fowl revealed by transganglionic transport of horseradish peroxidase.

The termination of visceral and somatic primary afferent fibers in the sacral spinal cord of the domestic fowl was studied using transganglionic transport of horseradish peroxidase. Nerve terminals of visceral afferent fibers were found in the lateral edge and base of the dorsal horn and in the dorsal gray commissure in close proximity to parasympathetic preganglionic neurons. Somatic afferents terminated in laminae 2 and 3. The results demonstrate that visceral and somatic afferent fibers in this avian species terminate in different areas of the dorsal horn.

Mystacial vibrissae representation within the trigeminal sensory nuclei of the cat

Somatotopic arrangements of axon terminals of primary afferent fibers innervating follicles of the mystacial vibrissae were examined in the cat by the transganglionic horseradish peroxidase (HRP) method. Forty to 60 hours after injecting HRP into a single or a group of vibrissal follicles, transported HRP was visualized by the tetramethylbenzidine technique. HRP-labeled axon terminals were distributed in the ventral subnucleus of the principal sensory trigeminal nucleus (ventral Vp), in the oral and interpolar spinal trigeminal nuclei (Vo and Vi), and in the caudal spinal trigeminal nucleus (Vc) (layer I, deep part of layer II, layers III-V) with its spinal extension into the dorsal horn of the first cervical cord segment (rostral C1). In cross sections through the caudal parts of the ventral Vp, Vi, and layer IV of the Vc and rostral C1, a single mystacial vibrissa was represented in a one-to-one fashion by a patch of dense terminal arbors of primary afferent fibers. The more dorsally a horizontal row of the mystacial vibrissae was located, the more ventrally was it represented in the ventral Vp, the more ventrolaterally in the Vi, and the more ventrally in layer IV of the Vc and the rostral C1. In addition, the more anteriorly a vibrissa was located in a horizontal row of the mystacial vibrissae, the more medially was it represented in the ventral Vp, the more ventromedially in the Vi, and the more laterally in layer IV of the Vc and rostral C1; the most posteriorly located vibrissae in the horizontal rows of the mystacial vibrissae were represented along the lateral border of the ventral Vp and Vi, and most medially in layer IV of the Vc and rostral C1. Thus, the representation pattern in the ventral Vp was rotated clockwise at about 45 degrees angle in the Vi, and projected as a mirror image in layer IV of the Vc and rostral C1. It was also indicated that the anterior-posterior arrangement of the mystacial vibrissae was represented in a rostral-caudal organization within layer IV of the Vc and rostral C1. It was also indicated that the anterior-posterior arrangement of the mystacial vibrissae was represented in a rostral-caudal organization within layer IV of the Vc and rostral C1. Patchy patterns probably replicating the distribution of the vibrissae on the face of the cat were also revealed by the cytochrome oxidase histochemical staining in cross sections through the caudal parts of the ventral Vp, Vi, and layer IV of the Vc and rostral C1.

Distribution of activity in the spinal terminations of single hair follicle afferent fibers to somatotopically identified regions of the cat spinal cord

Single-unit spike-triggered averaging has been used to study the distribution of activity in the central processes of 11 single hair follicle afferent (HFA) fibers in relation to the somatotopic organization of dorsal horn neurons (DHNs). Central responses could be recorded from all but one HFA, and the waveforms of these responses were similar to the biphasic, monophasic positive and compound terminal potentials (TPs), the triphasic, positive-negative-positive axonal (preterminal) potentials (APs), and the focal synaptic potentials (FSPs) described by other workers in different preparations. No central responses could be recorded from one HFA even though this axon was shown to be intact throughout the experiment and noise levels in the averaged records were below 3.1 microV. The spatial organization of TPs and APs mirrored the anatomical organization determined using intraaxonal staining only in that the region containing these potentials was a longitudinally orientated narrow (mean width = 405 microns) strip of dorsal horn. Within this strip large TPs, APs, and FSPs were usually found only in those regions in which the receptive field (RF) of the HFA was relatively central to the RFs of DHNs. The region in which the RFs of DHNs encompassed the RF of the HFA, the somatotopically appropriate region, was also organized into a longitudinally orientated strip of dorsal horn of approximately the same width as the strip containing TPs and APs. In any single experiment the strips formed by the somatotopically appropriate region and the TP-AP region occupied the same mediolateral position, but in contrast, the rostral and/or caudal boundaries of these strips often occurred at different levels along the dorsal horn. In some cases the TP-AP strip extended rostrally and/or caudally beyond the somatotopically appropriate region or ended at the same rostrocaudal level. In other cases the somatotopically appropriate region extended rostrally and/or caudally beyond the TP-AP strip. These results are discussed in relation to the rostrocaudal spread of the dendritic trees and the RF organization of DHNs. The results show clearly that in intact cats, anesthetized with alpha-chloralose, some HFAs give rise to collaterals in somatotopically inappropriate regions of the dorsal horn and that at least some parts of these collaterals are invaded by incoming action potentials. The question of whether some HFAs give rise to collaterals that are either infrequently invaded or not invaded at all is discussed.

Exocytosis from large dense cored vesicles outside the active synaptic zones of terminals within the trigeminal subnucleus caudalis: A possible mechanism for neuropeptide release

It has been hypothesized that chemical interactions between neurons in the central nervous system can occur in the absence of well defined synaptic complexes, but morphological correlates have been difficult to find. The present study demonstrates exocytotic release from large (70–130nm) dense cored vesicles at structurally nonspecialized areas along the plasmalemma of structurally different categories of terminals and occasionally from dendrites and axons within the neuropil of the trigeminal subnucleus caudalis. In rats, the marginal (lamina I) and substantia gelatinosa (lamina II) layers contain the central terminals of primary afferent fibers from the infraorbital nerve that supply the skin and whiskers (vibrissae). Different types of interneurons are also present and may modify the input being relayed to higher centers. While exocytotic profiles were present in control animals, they increased significantly (P < 0.01) on the ipsilateral side 1–24 h after a unilateral skin lesion in the vibrissae area. A second increase (P < 0.001) occurred 14–15 days after the lesion. Virtually all examples of large vesicle exocytosis were observed at structurally nonspecialized sites while those at the active synaptic zones involved small clear vesicles. Substance P-like immunofluorescence, present in controls and on the ipsilateral side during the first 6 days, subsequently declined until 4 weeks after surgery when some recovery was noted. The increase in large vesicle exocytosis and the decrease in substance P are interpreted to reflect functional adjustments of different neurons in response to the lesion. The exocytosis involving large dense cored vesicles may serve to deliver transmitters and/or neuropeptide modulators to appropriate receptors in a wider area than release into a specialized synaptic cleft would allow.

A pharmacological study of group I muscle afferent terminals and synaptic excitation in the intermediate nucleus and Clarke's column of the cat spinal cord.

When administered microelectrophoretically GABA and piperidine-4-sulphonic acid depolarized the central terminations of muscle group Ia and Ib afferent fibres in the lumbar intermediate nucleus and Clarke's column of cats anaesthetised with pentobarbitone sodium. Both this depolarization, and primary afferent depolarization, generated by impulses in other primary afferent fibres which produce prolonged bicuculline-sensitive inhibition of the firing of group I afferent fibre-excited interneurones in the intermediate nucleus and cells in Clarke's column, are reduced by microelectrophoretic bicuculline methochloride. Systemically administered (+/-)-baclofen hydrochloride (maximum dose 8 mg kg-1) depressed the monosynaptic excitation of Clarke's column neurones by impulses in muscle and cutaneous afferent fibres. Microelectrophoretically administered (-)-baclofen reduced the bicuculline-sensitive primary afferent depolarization of group I terminations without, however, reducing the depolarizing action of GABA or piperidine-4-sulphonic acid. The depression by (-)-baclofen of the group I monosynaptic excitation of intermediate nucleus neurones is not reduced by concentrations of bicuculline methochloride adequate to suppress prolonged inhibition of these neurones.

On the probable absence of GABA receptors on the terminations of motor axon collaterals in the cat spinal cord.

When administered microelectrophoretically, GABA and the GABA-mimetic piperidine-4-sulphonic acid (P4S) appear to have no direct hyperpolarizing or depolarizing effect on the terminations of motor axon collaterals excited electrically in the ventral horn of the lumbar spinal cord of the cat. This lack of effect on axon terminals of motoneurones, which contrasts with the bicuculline-sensitive depolarization by P4S of the spinal terminals of primary afferent fibres, is consistent with previous reports of the probable absence of pharmacologically detectable GABA receptors on the spinal terminals of other central excitatory neurones, namely those of the red and lateral vestibular nuclei.

E-type prostaglandins depolarize primary afferent neurons of the neonatal rat

The mechanism of action of prostanglandins (PGs) to sensitize sensory terminals to noxious stimuli was studied in the isolated spinal cord-tail preparation of the newborn rat. Application of a small amount of capsaicin to the tail induced a nociceptive reflex that was recorded extracellularly from the lumbar ventral root. Pretreatment of the tail with PGE 1 or E 2 (0.8–4 μM) markedly potentiated the capsaicin-induced nociceptive reflex. In the isolated spinal cord preparation of the newborn rat, application of PGE 1 or E 2 (10 nM-1 μM) induced a depolarization of the dorsal root. Based on these results we propose a hypothesis that PGs regulate the resting potential of the peripheral terminals of nociceptive primary afferent fibers and that the depolarization is associated with lowering of threshold for various noxious stimuli.

Extent of the ipsilateral representation in the ventral posterior medial nucleus of the monkey thalamus.

Single and multiunit mapping was used to determine the extent of the representation of ipsilateral structures in the ventral posterior medial (VPM) nucleus of the thalamus in cynomolgus monkeys. The extent of the VPM occupied by terminations of afferent fibers arising in the ipsilateral principal trigeminal nucleus was also determined by anterograde transport of horseradish peroxidase. Both methods indicate that most of the medial half of VPM is occupied by the ipsilateral representation. This is much larger than previously suspected. Units in the medial half of VPM have small, well localized receptive fields on the ipsilateral side of the lower lip, tongue and palate, in the ipsilateral cheek pouch and on the ipsilateral teeth. The representation is largest for the ipsilateral side of the tongue and the cheek pouch. Most units in the lateral half of VPM have small, contralateral receptive fields. Few units in VPM have bilateral receptive fields. VPM is clearly distinguishable by cytochrome oxidase (CO) staining. Anteroposteriorly elongated, CO-positive aggregations correspond to elongated aggregations of units with the same or closely similar receptive fields, especially in the medial, ipsilateral representation.

Brainstem terminations of extraocular muscle primary afferent neurons in the monkey

The central terminations of afferent nerve fibers from the extraocular muscles of the monkey were investigated by means of transganglionic transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA/HRP). Following injections of selected extraocular muscles with WGA/HRP, terminal labeling was apparent in the ipsilateral trigeminal sensory and cuneate nuclei. The density of trigeminal projections varied markedly from one rostrocaudal level to the next, being heaviest within the ventrolateral portion of pars interpolaris of the spinal trigeminal nucleus. A second extraocular muscle afferent representation was noted in ventrolateral portions of the cuneate nucleus. This projection was restricted to rostral portions of pars triangularis of the cuneate nucleus, partially overlapping the afferent termination from dorsal neck muscles. It is likely that some of the problems encountered in formulating conclusions regarding the functional role of extraocular muscle proprioception are due to a lack of detailed information of the central termination pattern of muscle afferents. Taken together, the present findings should provide a basis for further anatomical and physiological studies designed to elucidate the role played by extraocular muscle proprioceptors in vision and oculomotor control.