Identification of nerve endings in cat extraocular muscles

Abstract

Background The aim of the present study was to identify the varieties of sensory and motor nerve endings in cat extraocular muscles. Methods Sensory terminals were identify by injecting neuronal tracers (fast blue, biocytin, or peroxidase) into the trigeminal ganglion, which contains the sensory cells innervating the eye muscles. Motor terminals were identified by injections of horseradish peroxidase or DiI, a fluorescent carbocyanin dye, into either the oculomotor nerve or the IIIrd nuclei. Results Injections into the trigeminal ganglion anterogradely labelled three types of sensory nerve endings for each neuronal tracer used: (1) the well‐known “palisade” endings at the myotendinous junction of each extraocular muscle; (2) “compact” endings consisting of a dense terminal arborization extending up to 60 μm in length on striated muscle fibres 10‐15 μm in diameter; and (3) “complex” endings on muscle fibres 15‐20 μm in diamter. The complex ending issued from multiple collateral branches of the parent nerve fibre, which stretched and turned around the muscle fibre and gave off numerous terminal varicosities over a distance of about 140 μm. The sensory complex and compact endings presented strong similarities with some “atypical muscle spindles” previously described. In addition to the classic motor “plate” and “grape,” we found evidence for the existence of motor “spiral” endings with each tracer. Conclusions The sensory nature of the palisade endings was demonstrated, and two other types of sensory terminals were identified and described. The spiral nerve terminals were demonstrated to be motor in nature, and a possible function in the microsaccadic movements associated with fixation is suggested. Anat. Rec. 248:566‐575, 1997. © 1997 Wiley‐Liss, Inc.

Peripheral Muscle Targets and Central Projections of the Mesencephalic Trigeminal Nucleus in Macaque Monkeys

“…While such a correlation does not represent proof of this connection, it may explain the divergent findings across studies; i.e., a small and variable number of spindles may lead to variable labeling of a small number of MesV cells. While palisade endings are the only type of fiber specialization consistently found in vertebrate extraocular muscles, a number of other types of putative proprioceptors have been observed (Billig et al, 1997; Blumer et al, 2006; see Büttner-Ennever et al, 2006 for review). In fact, the quantity, distribution and subtypes of extraocular muscle endings vary between species, and perhaps even between individuals (Maier, 2000; Büttner-Ennever et al, 2003).…”

Molecular Characteristics Suggest an Effector Function of Palisade Endings in Extraocular Muscles

“…6,32,44,45 Because classic proprioceptors such as muscle spindles and Golgi tendon organs are absent in cat EOMs, Porter and Spencer 32 concluded that the labeled neurons in the trigeminal ganglion innervate the palisade endings. In anterograde tracing experiments Billig et al 23 injected neuronal tracers into the trigeminal ganglion of cats and found three different kinds of labeled nerve endings in the EOMs, one type resembling palisade endings.…”

“…Several groups 6,16,[21][22][23]26,[32][33][34] have suggested that palisade endings could be the source of afferent signals, but there are still conflicting reports on the functional nature of palisade endings, whether they are sensory 6,16,[21][22][23]26,[32][33][34] or motor 28,35 structures, or both. 30 Fine structural analyses on palisade endings in cats, rhesus monkeys, and sheep have indicated a sensory function.…”

“…A recent study showed that the cell bodies of the palisade endings are in the trigeminal ganglion, which implies a sensory function. 23 In summary, the functional properties of palisade endings are, at present, not fully understood but a sensory role is favored.…”

Ultrastructural changes in myotendinous nerve endings induced by injection of botulinum toxin into the extraocular muscle

“…In this study we used cats because this species has palisade endings in the myotendinous nerve similar to those of humans [28,29]. The dose of botulinum toxin used in experimental studies varies, though it is typically in the range of 2-10 IU [17,25,30,31].…”