Neck muscle and trigeminal input to the upper cervical cord and lower medulla of the cat.

Experiments on chloralose-anaesthetized cats have shown that low-threshold neck muscle afferents project to laminae IV and V in the dorsal horn of the upper cervical cord, to lamina VI including the region which encompasses the central cervical nucleus, as well as to extensive regions of the ventral horn. At posterior medullary levels projections also exist to laminae IV, V, and VI of the spinal nucleus of V (although those to lamina IV are circumscribed), to the deep layers and lateral margin of the cuneate nucleus, and to the inferior olive. These projections are both from low- and high-threshold afferents. Evidence of a functional relationship between the trigeminal and neck muscle afferent system was found both in the upper cervical cord and lower medulla. About 40% of units in both regions receive a convergent input and when convergence could not be demonstrated, prior stimulation of one modality in some instances affected the responsiveness of the unit to the other modality. A motor role was found for some trigeminal afferent projections to the upper cervical cord. Trigeminal afferents consistently activated antidromically identified motoneurons of splenius, biventer cervicis, and complexus.

Unit activity in the superior colliculus of the cat following passive eye movements.

Unit response in the superior colliculus and underlying structures has been examined in the choralose-anaesthetized cat following passive movement of an occluded eye. One group of units was sensitive to small saccadic movements, responded regardless of the initial postion of the eye, and in most instances responded to movements in opposit directions. A second numerically smaller group also responded when they eye was moved at saccadic velocity but only when the eye passed a fixed point. Such units with fixed positional thresholds were found following movements in both nasal and temporal directions as well as to both upward and downward movement. Both types of unit response were found after transection of the optic nerve and were also recorded when individual extraocular muscles were subjected to controlled stretch. It is assumed that most unit activity seen after passive movement of the occluded eye is due to activity in extraocular muscle receptors. In the deep layers of the superior colliculus responses to small eye movements were found to be due to the activation of very low threshold receptors sensitive to vibration in the facial area.

Motor and sensory fibres of neck muscle nerves in the cat.

The numbers and sizes of nerve fibres to the dorsal neck muscles, splenius, complexus and biventer cervicis have been examined in the cat. The total number of fibres is unusually high as is the content of sensory fibres (estimated as the loss of fibres after ganglionectomy). The fibre spectra of these sensory nerves has an unusually large number of fibres in the group II and III range (3-7 mum) and differs markedly in this way from other muscle nerves. The motor fibres contain a high proportion (64-99%) in the gamma fibre size range. Large motor fibres are absent in the nerves to biventer cervicis (a slow muscle). The ratio of unmyelinated to myelinated fibres in neck muscle nerves is similar to that in hind legs at about 2.5:1.