A.G. Brown

Direct observations of synapses between GABA-immunoreactive boutons and muscle afferent terminals in lamina VI of the cat's spinal cord.

Single group Ia muscle afferent fibres in the lumbar spinal cord of the cat were impaled with microelectrodes and labelled with horseradish peroxidase. Two collateral axons were prepared for combined light and electron microscopy. Arbors selected from lamina VI were processed by the postembedding immunogold technique with antiserum which specifically recognizes GABA in glutaraldehyde-fixed tissue. Twelve Ia boutons were examined through series of thin sections with the electron microscope and all of them were associated with presynaptic axon terminals which were positively labelled for GABA. Some Ia boutons received synaptic contacts from several GABAergic terminals. The present study establishes that a GABA-like substance is present in axon terminals presynaptic to Ia afferent boutons in lamina VI of the spinal cord. This evidence provides a morphological basis for presynaptic inhibition of Ia afferent input into lamina VI.

Ultrastructure of hair follicle afferent fibre terminations in the spinal cord of the cat

SummaryIn acute electrophysiological experiments on anaesthetized cats, single identified hair follicle afferent fibres were injected with horseradish peroxidase (HRP). The HRP was injected from an intra-axonal microelectrode in the lumbosacral spinal cord. One to six hours after injection the animals were perfused and the tissue prepared for light and electron microscopy (EM). Axon collateral arborizations containing HRP reaction product were identified in thick sections under the light microscope and the same tissue then cut on the ultramicrotome for EM study. The terminal branches of the collaterals kept their myelin sheaths until they were 0.45–l.0 μm in diameter, just before they formed synaptic boutons. Synaptic boutons (1.0–4.0 μm in diameter) were usually of theen passant variety and made contact with dendrites. The contacts were asymmetrical (Type I) and contained round, clear synaptic vesicles of 35–60 nm diameter. Both the non-myelinated portion of the terminal axon and the synaptic boutons received axo-axonic contacts. These axo-axonic boutons contained clear (agranular) vesicles irregular in profile.

Effects of activity in non-myelinated afferent fibres on the spinocervical tract

Microelectrode recordings were made from axons of the spinocervical tract (SCT) in unanaesthetized decerebrate-spinal cats. Pure volleys in non-myelinated (C) cutaneous nerve fibres were obtained by DC-polarization block of conduction in the myelinated (A) fibres, and SCT cells were classified according to whether or not they were excited by C fibres. The effects of conditioning C responses in the SCT with A and C fibre input were examined, as were the effects of conditioning A fibre responses in the tract with C fibre inputs. The effects of stimulating descending systems in the cervical spinal cord on the C fibre-evoked responses of SCT cells were determined. SCT units with a slowly-adapting pressure-sensitive component in their receptive fields responded to cutaneous C fibres in addition to A fibres. Units which responded only to hair movement when the receptive field was stimulated mechanically were not excited by C fibres. Responses in SCT neurones produced by C fibres were profoundly inhibited by conditioning volleys in A fibres of both ipsilateral and contralateral cutaneous nerves and by activity in several descending systems. Conditioning with C fibres failed to affect the responses of SCT cells to either A or C fibres. It is concluded that cutaneous C fibres excite some SCT cells but have no other effect on transmission through this system. It is suggested that there are common inhibitory interneurones in the paths from descending systems and cutaneous A and C fibres to the excitatory input to SCT cells.

Relationships between hair-follicle afferent axons and glycine-immunoreactive profiles in cat spinal dorsal horn

In order to identify synapses between hair-follicle afferent axons and glycine-containing structures in cat spinal cord, semithin sections containing physiologically identified primary afferent boutons which had been filled with horseradish peroxidase (HRP) were reacted with anti-glycine antiserum, while adjacent ultrathin sections were examined for synaptic contacts. Four axodendritic synapses between hair-follicle afferent boutons and glycine-immunoreactive dendrites and 4 axoaxonic synapses in which HRP-filled boutons were postsynaptic to immunoreactive axons were identified. These results suggest that glycine is involved in the spinal processing of input from A beta hair-follicle afferent axons.

Fine structure of spinocervical tract neurones and the synaptic boutons in contact with them

Acute electrophysiological experiments were performed on 7 adult cats and spinocervical tract (SCT) neurones were identified and injected intracellularly with horseradish peroxidase (HRP). At the conclusion of experiments regions of spinal cord were prepared for light and electron microscopy. SCT neurones were studied in thick sections and areas of interest were prepared from the same material for electron microscopy. The general ultrastructural features of SCT neurones were categorised and particular attention was paid to axon terminals contacting SCT dendrites. These terminals were found to be of two varieties: those containing clear circular vesicles and an occasional granular vesicle (63% of total) and those containing elongated agranular vesicles (37% of total).