1094 



HANDBOOK OF PHVSIOLOG^■ 



NEUROPHYSIOLOGY II 



nerve for a short part of its course. The main nerve 

 from the inferior obhque muscle starts by being 

 mixed, but a branch to the fifth ner\e leaves the 

 motor nerve as it winds round tiie lateral edge of the 

 inferior rectus. The proximal afferent branch from 

 the superior oijlique runs with the fourth nerve for a 

 short distance before leaving to run with other, 

 usually three, branches from the more distal parts of 

 the muscle to the fifth ner\e. A few \ery fine branches 

 have occasionally been .seen in the orbit of the cat 

 running from the fourth nerve to the fifth (148); 

 Cooper, as reported in Cooper et al. (40), was able 

 to lead from such a branch and obtained a sustained 

 discharge in response to stretching the superior 

 oblique muscle. Connections between the eye muscle 

 nerves and the fifth nerve are more commonly seen 

 in the cavernous sinus. They were reported in man 

 and described by Stibbe (129); they have also been 

 seen by Cooper (unpublished observations) in a 

 number of animals. 



So far no unequivocal afferent discharge has been 

 detected in the intracranial portions of the third and 

 fourth nerves, either in the goat (Daniel & 

 Whitteridge, unpublished observations) or in the 

 cat (Cooper, unpublished oljservations), although 

 those nerves certainly contained excitable motor 

 fibers while the existence of afferent fibers was being 

 tested. One might conclude that afferent fibers in 

 the goat and cat which may enter the central nervous 

 system i)y the motor nerves, if they exist at all, must 

 be of very small diameter; but the question is by no 

 means settled. Fine degenerating fibers in the medial 

 rectus of the monkey found by Tozer & Sherrington 

 (136) after section of the ophthalmic branch of the 

 fifth nerve are almost certainly due to afierent fibers 

 which leave the third nerve trunk to reach the fifth 

 nerve. On the other hand Tozer & Sherrington in- 

 ferred that the majority of afferent fibers from the 

 eye muscles run into the central nervous system by 

 the motor nerves, in view of the persistence without 

 degeneration of large numbers of fibers which run to 

 the musculotendinous junction and seem to be af- 

 ferent in nature. It is just possible that these fibers in 

 fact reach the motor root of the fifth nerve by fila- 

 ments which pass deep to the semilunar ganglion and 

 might therefore not have been cut with the oph- 

 thalmic division of the fifth. Except in the ungulates, 

 all the branches so far investigated running from the 

 eve muscles to the fifth nerve are small and it is 

 dithcult to see how they can supply all the sensory 

 endings in the muscles. Wilkinson (148) emphasizes 

 this point in the cat. There is a consideral)le histo- 



logical literature on fibers from the mesencephalic 

 root of the fifth nerve which are said to join the motor 

 nerves in their intracereljral course (132). 



.\ search of the brain stem of the goat with a 

 microelectrode gave good evidence of primary 

 neurons gi\ing rise to the afferent endings in the 

 extraocular muscles. These neurons were located in 

 the pons close to the point of entry of the fifth nerve 

 (37), as shown in figure 4. Enough responses were also 

 obtained from cells of the mesencephalic nucleus of 

 the fifth nerve in the midbrain to establish that these 

 primary afferent fibers have their cells of origin in this 

 nucleus, as do the fibers from the proprioceptors in 

 the jaw mu.scles (43). Secondary neurons excited by 

 proprioceptors in the eye muscles hav'e been found in 

 the central tegmental tract, the deeper layers of the 

 .stiperior coiliculus, the posterior commissure, path- 

 ways adjacent to the eye muscle nuclei and in the 

 .superior cerebellar peduncle (38, 39). Some similar 

 evidence of the distribution of afferent fibers from 

 the eye mirscles to the mesencephalic nucleus of the 

 fifth nerve and onwards is available for the cat (65). 



EFFECTS OF STRETCH 



No stretch reflex has been elicited from tlie extra- 

 ocular muscles. Pulling on the tendon of the superior 

 oblique in decerebrate goats produced either no 

 change or a fall in rate of discharge of about 10 per 

 cent in single motor units or motor ner\e fibers. In 

 the same animals, a brisk discharge in motoneurons 

 of the superior oblique followed rotation of the head, 

 and pulling on a .slip of the masseter produced a 

 reflex contraction of its muscle fibers. Pulling on the 

 tendon of the inferior oblique sometimes produced 

 a small increase in the discharge in fibers of the 

 superior ol)lique. No response to shocks applied to 

 the central end of the afferent nerve trunks has ijeen 

 observed in the motoneurons of the fourth nerve, 

 nor has tapping the tendon of an eye muscle had any 

 oijserved effect (Whitteridge, unpublished observa- 

 tions). These results agree with those of McCouch & 

 Adler (103) on the cat. Many authors have oijserved 

 that the motor discharges during nystagmus and 

 vestiiiular reflexes are unaffected by cocainization of 

 the eye muscles (112) or by peripheral section of the 

 motor ner\es (105). 



One can therefore conclude that a stretch reflex 

 is probably not a basic mechanism associated with 

 these muscles. Correspondingly, there is no increase 

 in the discharge of afferent fibers from muscle 



