Motoneurons of the Lateral and Medial Rectus Extraocular Muscles in Squirrel Monkey and Cat

Horn

Scherberger

et al. 2001

145

193

Eye muscle fibers can be divided into two categories: nontwitch, multiply innervated muscle fibers (MIFs), and twitch, singly innervated muscle fibers (SIFs). We investigated the location of motoneurons supplying SIFs and MIFs in the six extraocular muscles of monkeys. Injections of retrograde tracers into eye muscles were placed either centrally, within the central SIF endplate zone; in an intermediate zone, outside the SIF endplate zone, targeting MIF endplates along the length of muscle fiber; or distally, into the myotendinous junction containing palisade endings. Central injections labeled large motoneurons within the abducens, trochlear or oculomotor nucleus, and smaller motoneurons lying mainly around the periphery of the motor nuclei. Intermediate injections labeled some large motoneurons within the motor nuclei but also labeled many peripheral motoneurons. Distal injections labeled small and medium-large peripheral neurons strongly and almost exclusively. The peripheral neurons labeled from the lateral rectus muscle surround the medial half of the abducens nucleus: from superior oblique, they form a cap over the dorsal trochlear nucleus; from inferior oblique and superior rectus, they are scattered bilaterally around the midline, between the oculomotor nucleus; from both medial and inferior rectus, they lie mainly in the C-group, on the dorsomedial border of oculomotor nucleus. In the medial rectus distal injections, a "C-group extension" extended up to the Edinger-Westphal nucleus and labeled dendrites within the supraoculomotor area. We conclude that large motoneurons within the motor nuclei innervate twitch fibers, whereas smaller motoneurons around the periphery innervate nontwitch, MIF fibers. The peripheral subgroups also contain medium-large neurons which may be associated with the palisade endings of global MIFs. The role of MIFs in eye movements is unclear, but the concept of a final common pathway must now be reconsidered.

Section: Oculomotor Nucleusmentioning

confidence: 95%

Motoneurons of twitch and nontwitch extraocular muscle fibers in the abducens, trochlear, and oculomotor nuclei of monkeys

Horn

Scherberger

et al. 2001

145

193

“…1A). In agreement with other studies, we found that the A and B group motoneurons were nearly the same size, but the C group neurons were significantly smaller (A motoneurons: 356.3 Ϯ 94 m 2 mean area Ϯ SD, n ϭ 58; B motoneurons: 386.7 Ϯ 118.7 m 2 , n ϭ 54; C group: 221.5 Ϯ 58 m 2 , n ϭ 57; P ϭ 0.001, SPSS 7.5 two-tailed Student's t-test; Bü ttner-Ennever and Akert, 1981;McClung et al, 2001). …”

Section: Nontwitch Motoneuron Subgroups In the Oculomotor Nucleusmentioning

confidence: 99%

Twitch and nontwitch motoneuron subgroups in the oculomotor nucleus of monkeys receive different afferent projections

Wasicky

Horn

2004

Motoneurons in the primate oculomotor nucleus can be divided into two categories, those supplying twitch muscle fibers and those supplying nontwitch muscle fibers. Recent studies have shown that twitch motoneurons lie within the classical oculomotor nucleus (nIII), and nontwitch motoneurons lie around the borders. Nontwitch motoneurons of medial and inferior rectus are in the C group dorsomedial to nIII, whereas those of inferior oblique and superior rectus lie near the midline are in the S group. In this anatomical study, afferents to the twitch and nontwitch subgroups of nIII have been anterogradely labeled by injections of tritiated leucine into three areas and compared. 1) Abducens nucleus injections gave rise to silver grain deposits over all medial rectus subgroups, both twitch and nontwitch. 2) Laterally placed vestibular complex injections that included the central superior vestibular nucleus labeled projections only in twitch motoneuron subgroups. However, injections into the parvocellular medial vestibular nucleus (mvp), or Y group, resulted in labeled terminals over both twitch and nontwitch motoneurons. 3) Pretectal injections that included the nucleus of the optic tract (NOT), and the olivary pretectal nucleus (OLN), labeled terminals only over nontwitch motoneurons, in the contralateral C group and in the S group. Our study demonstrates that twitch and nontwitch motoneuron subgroups do not receive identical afferent inputs. They can be controlled either in parallel, or independently, suggesting that they have basically different functions. We propose that twitch motoneurons primarily drive eye movements and nontwitch motoneurons the tonic muscle activity, as in gaze holding and vergence, possibly involving a proprioceptive feedback system.

“…The most prominent group of MIF motoneurons is formed by those of the IR and MR, termed the C group . It lies at the dorsomedial border of nIII and can be identified in several species, such as cat, lesser galago, marmoset, and macaque (Büttner‐Ennever and Akert, ; Büttner‐Ennever et al, ; Clarke et al, ; McClung et al, ; Spencer and Porter, ; Sun and May, ). Recent tract‐tracing studies in monkey indicated that the peripheral MIF motoneuron groups including the C group may also contain the cell bodies of palisade endings (Lienbacher et al, ; Zimmermann et al, ).…”

mentioning

confidence: 99%

Internal organization of medial rectus and inferior rectus muscle neurons in the C group of the oculomotor nucleus in monkey

Tang

Mustari

et al. 2015

Summary Mammalian extraocular muscles contain singly-innervated twitch muscle fibers (SIF) and multiply-innervated non-twitch muscle fibers (MIF). In monkey, MIF motoneurons lie around the periphery of oculomotor nuclei and have different premotor inputs from the motoneurons inside the nuclei. The most prominent MIF motoneuron group is the C-group, which innervates the medial rectus (MR) and inferior rectus (IR) muscle. To explore the organization of both cell groups within the C-group we performed small injections of choleratoxin subunit B into the myotendinous junction of MR or IR in monkeys. In three animals the IR and MR myotendinous junction of one eye was injected simultaneously with different tracers (choleratoxin subunit B and wheatgerm agglutinin). It revealed that both muscles were supplied by two different non-overlapping populations in the C-group. The IR neurons lie adjacent to the dorsomedial border of the oculomotor nucleus, whereas MR neurons are located further medially. A striking feature was the differing pattern of dendrite distribution of both cell groups. Whereas the dendrites of IR neurons spread into the supraoculomotor area bilaterally, those of the MR neurons were restricted to the ipsilateral side and sent a focussed bundle dorsally to the preganglionic neurons of the Edinger-Westphal nucleus, which are involved in the ‘near response’. In conclusion, MR and IR are innervated by independent neuron populations from the C-group. Their dendritic branching pattern within the supraoculomotor area indicates a participation in the near response providing vergence, but also reflects their differing functional roles.