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

183

141

SUMMARY1. The structural relationship between the afferent projection and the dendrites of the interneurones was examined in the cercal-to-giant interneurone system of the cricket using intracellular recording and dye injection techniques.2. The physiology of the sensory neurones beneath the cereal filiform hairs was investigated by placing a recording pipette over the end of a cut hair and using movements of the pipette to characterize the directionality of the receptor. Most of the filiform receptors could be classified as belonging to one of four major types. Each type is sensitive to a different wind direction and is confined to particular regions of the cercus.3. The location of the terminal arborizations of each type of sensory cell was revealed by staining with cobalt chloride. Single cells were stained reliably by placing a dye-filled pipette over a cut hair. Each physiological receptor type arborizes in a different region of the central nervous system. Therefore the neuropile is functionally divided according to wind direction.4. The dendrites of three identified interneurones were examined in the context of this afferent projection. It was found that each of these neurones has dendrites in regions of neuropile corresponding to different wind directions.5. By searching for unitary synaptic potentials in identified interneurones, it was possible to show a strong correlation between anatomical overlap of primary afferent and interneurone and the existence of a synaptic connexion. Further, when there was no overlap, no synaptic potentials were seen. 6. Therefore the over-all excitatory receptive field of an interneurone could be predicted by examining its dendritic structure. Each of the three identified interneurones examined in this study was found to have a directional response that matched the response predicted on the basis of its anatomy.

Section: The Interneuronesmentioning

confidence: 85%

Receptive fields of cricket giant interneurones are related to their dendritic structure.

Bacon¹,

Murphey²

1984

183

141

“…Fifty to seventy-five neurones represents about 6-10o% of the population of s.c.t. neurones subserving the hind limb (Brown, Fyffe, Noble, Rose & Snow, 1980). It is concluded that a single action potential in a single hair follicle afferent fibre may be a significant event as far as the central nervous system is concerned.…”

Section: Discussionmentioning

confidence: 99%

Excitatory actions of single impulses in single hair follicle afferent fibres on spinocervical tract neurones in the cat.

Brown¹,

Koerber²,

Noble³

1987

SUMMARY1. In cats under chloralose anaesthesia single dorsal root ganglion cells with axons innervating hair follicles were stimulated intracellularly to produce single impulses. At the same time single spinocervical tract (s.c.t.) neurones were recorded extracellularly, from their axons in the upper lumbar cord.2. When the receptive field of the afferent fibre was contained within the impulse firing zone of the s.c.t. cell, a single afferent impulse increased the probability of firing of the neurone. In thirty-nine pairs of units, where the afferent fibre had a group II conduction velocity, coupling was very efficient and for seventeen pairs the single afferent impulse produced one or more impulses in the s.c.t. cell in at least 90 % of trials. The mean number of impulses evoked in s.c.t. cells by a single group II afferent impulse was 1-47. The latencies of the impulses ranged from 1-5 to 14-0 ms, with times to peak and total durations of 25--17'5 ms and 4'5-28'0 ms respectively. For two pairs of units where the afferent fibre had a group III conduction velocity the effectiveness of single afferent impulses was much less and the latencies, but not the durations, of the impulses were longer (12 and 17 ms).3. When the receptive field of the hair follicle afferent fibre was outside, but close to, the firing zone of the s.c.t. neurone there was no indication that single afferent impulses affected the probability of neuronal discharge for thirteen of fifteen pairs of units. Weak excitation was observed in two pairs and this was clear only when two or more afferent impulses were employed.4. There was a tendency for hair follicle afferent fibres with their receptive fields at or near the centre of the s.c.t. cell's firing zone to be most effective, producing shorter latency responses with more impulses at higher frequencies. When the afferent's field was peripherally located in the s.c.t. neurone's firing zone there was a wide range of responses but these included those with the longest latencies and very few impulses.5. The results are discussed with reference too previous work on the spinocervical tract and to the known actions of single impulses on other neuronal types.

“…The spot-like or round to oval cutaneous receptive field that is essentially two-dimensional is transformed into a three-dimensional column (continuous or interrupted) enormously elongated in comparison with its width -by a factor of at least 20. This arrangement of the cutaneous input forms the basis of the somatotopic organization ofdorsal horn neurones, which also show a sagittal columnar distribution of their receptive fields (Brown, Fyffe, Noble, Rose & Snow, 1980b).…”

Section: Sa Type II Cutaneous Axon Collaterals Discussionmentioning

confidence: 99%

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

Brown¹,

Fyffe²,

Rose³

et al. 1981

SUMMARY1. 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 jLm.3. The collaterals formed plate-like arborizations usually about 500-600 ,um wide in the transverse plane but only 100-300 jsm 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.