A. G. Brown

A quantitative study of cutaneous receptors and afferent fibres in the cat and rabbit

1. The discharge in myelinated afferent fibres innervating hairs in anaesthetized cats and rabbits, dissected from the saphenous nerve, was recorded during controlled movements of the hairs.

The morphology of hair follicle afferent fibre collaterals in the spinal cord of the cat

1. The enzyme horseradish peroxidase (HRP) was injected into single axons that innervated hair follicle receptors to study the morphology of their collaterals in the dorsal horn of the cord. The axons were impaled near the dorsal root entrance zone in the lumbosacral spinal cord of anaesthetized cats and HRP injected by passing current through the intra‐axonal micro‐electrode. The morphology was revealed by subsequent histochemistry.

Responses of spinocervical tract neurones to natural stimulation of identified cutaneous receptors

SummaryMicroelectrode recordings were made from ascending fibres of the spinocervical tract in spinal, decerebrate and anaesthetized cats. Three types of unit were recognised in spinal cats on the basis of their response to mechanical stimulation; units excited by 1. hair movement; 2. hair movement and skin pressure; 3. pressure and pinch of the skin. Five types were recognised in decerebrate and anaesthetized cats; units excited by 1. movement of guard hairs and skin pressure; 2. movement of tylotrich hairs; 3. movement of all the hairs and skin pressure; 4. pressure and pinch of the skin; 5. units which could not be influenced from the periphery. The presence or absence of inhibitory fields and the mean rate of spontaneous discharges depended on the type of preparation and the type of unit. The differences between spinal and decerebrate or anaesthetized cats suggest that a descending neuronal system, intact in the decerebrate and anaesthetized animals, operates to control the input to the spinocervical tract.The mean frequency of response of units sensitive to hair movement was related to the velocity of hair movement by a power function. All units responded with an increased frequency of discharge to heating the skin to high temperatures, the degree of response depending on the preparation and type of unit. Some units responded with an increased discharge to low skin temperatures.

Effects of descending impulses on transmission through the spinocervical tract.

1. Micro‐electrode recordings were made from ascending axons of the spinocervical tract in unanaesthetized decerebrate cats before, during and after reversible cold block of impulse conduction in the spinal cord rostral to the recording site.

The density, distribution and topographical organization of spinocervical tract neurones in the cat.

1. In acute experiments, detailed grids of micro‐electrode recordings were made from spinocervical tract (s.c.t.) cells in the lumbosacral cord of anaesthetized cats. These grids provided electrophysiological data on the location, distribution, density and somatotopic organization of s.c.t. neurones.

Projections from Pacinian corpuscles and rapidly adapting mechanoreceptors of glabrous skin to the cat's spinal cord.

1. Single axons innervating Pacinian corpuscles and rapidly adapting mechanoreceptors of the foot and toe pads were injected with horseradish peroxidase near their entrance to the lumbosacral spinal cord in cats anaesthetized with chloralose and paralysed with gallamine triethiodide. Subsequent histochemistry revealed the morphology of the intra‐spinal parts of the axons. 2. All Pacinian corpuscle axons that could be traced into the dorsal root bifurcated upon entering the cord into ascending and descending branches. All Pacinian corpuscle axons gave rise to collaterals that entered the dorsal horn. 3. The collaterals of Pacinian corpuscle afferent fibres had a distinctive morphology. They provided two regions of termination, a larger dorsal region in laminae III and IV and a smaller ventral region in laminae V and VI. Within the dorsal region the terminal axons ran mainly in the longitudinal axis of the cord and carried many boutons en passant. Within the ventral region the axons ran dorso‐ventrally in the transverse plane of the cord and although carrying some boutons en passant also gave rise to clusters of boutons. 4. The collaterals of rapidly adapting afferent fibres had a distinctive morphology different from that of the Pacinian corpuscle afferent fibre collaterals. The termination region of rapidly adapting afferents was limited almost exclusively to lamina III, with only slight extension into lamina IV. Boutons were mainly of the en passant type and terminal axons were generally orientated within the longitudinal axis of the cord. 5. The morphology of the afferent fibre collaterals is discussed in relation to the physiology of the dorsal horn.

Cutaneous afferent fibre collaterals in the dorsal columns of the cat

SummaryMicroelectrode recordings were made from cutaneous afferent fibre collaterals in the lumbar dorsal columns of cats, which were antidromically excited from the dorsal column at C2. The following types of cutaneous afferent unit have axon collaterals which ascend the dorsal columns to the dorsal column nuclei; hair follicle units Types G (36.4%) and T (31.8%), slowly-adapting units Types I (8.3%) and II (5.3%) pad mechanoreceptor units (8.3%) and a few high threshold units. The conduction velocities of dorsal column collaterals were measured. All collaterals had slower velocities than corresponding peripheral nerve fibres and the degree of slowing was determined by the class of afferent unit, with the pad units slowing the least and Type T hair follicle units the most. The differential slowing was confirmed by recording compound action potentials from various parts of dorsal column-peripheral nerve pathways.

The morphology of spinocervical tract neurones revealed by intracellular injection of horseradish peroxidase (cat)

1. The morphology of physiologically identified spinocervical tract neurones was studied using the intracellular injection of horseradish peroxidase in anaesthetized cats.

Spinal cord collaterals from axons of type II slowly adapting units in the cat.

1. The morphology of single axons, and their collaterals, of Type II slowly adapting mechanoreceptors situated at the claw bases was studied. Intra‐axonal injections of horseradish peroxidase were made into the axons near their entrance to the lumbosacral spinal cord of anaesthetized cats. The morphology was revealed by subsequent histochemistry. 2. Nine Type II axons were stained. All but one bifurcated into ascending and descending branches upon entering the cord. Eighty‐nine collaterals arose from the axons at a mean spacing of about 570 micrometers. 3. The collaterals formed plate‐like arborizations usually about 500‐600 micrometers wide in the transverse plane but only 100‐300 micrometers thick in the longitudinal axis of the cord. The terminal arborizations were in laminae III‐VI. 4. Synaptic boutons in laminae III and IV were more numerous than in laminae V and VI. Boutons en passant were common in laminae III and IV and arranged in series of three to six, whereas in deeper laminae only two or three boutons formed a series de passage. 5. The morphology of the slowly adapting Type II collateral is discussed. 6. Some general principles of the organization of cutaneous afferent fibres in the lumbosacral cord are presented.

The morphology of spinocervical tract neurones in the cat.

1. The morphology of physiologically identified spinocervical tract (SCT) neurones was studied using the intracellular injection of Procion dyes in anesthetized and decerebrate cats. 2. Extracellular recordings were made from SCT neurones at depths between 1000 and 2850 mum from the cord surface but neurones were only stained at depths between 1100 and 2400 mum. 3. The dendritic trees of stained SCT neurones were reconstructed in the transverse plane of the spinal cord. All SCT neurones had well developed dorsal dendrites but despite this it is not possible to consider the twenty‐two SCT cells in out sample as consituting a morphologically homogenous population. 4. There was no correlation between the form of the dendritic trees and the depth of SCT neurones in the dorsal horn as determined both from measurements from the dorsal grey‐white border and the position of cells with respect to the border between Rexeds laminae II and III. 5. Six types of SCT neurones were identified on the basis of the form of their dendritic trees as viewed in the transverse plane: (1) radially symmetrical, (2) semicircular, (3) large elliptical, (4) bilobed, (5) triangular, (6) small elliptical. Each of these types was found only in a certain region across the dorsal horn although any one region could contain more than one type. 6. Spinocervical tract neurones with small elliptical dendritic trees always had receptive fields encompassing part of the hip or thigh and were unique in being located in the lateral portions of the horn. 7. There was no correlation between the morphology of SCT neurones and their excitatory cutaneous inputs, receptive field size, axonal conduction velocity or depth in the dorsal horn.