Palisade endings and proprioception in extraocular muscles: a comparison with skeletal muscles.

This article describes current views on motor and sensory control of extraocular muscles (EOMs) based on anatomical data. The special morphology of EOMs, including their motor innervation, is described in comparison to classical skeletal limb and trunk muscles. The presence of proprioceptive organs is reviewed with emphasis on the palisade endings (PEs), which are unique to EOMs, but the function of which is still debated. In consideration of the current new anatomical data about the location of cell bodies of PEs, a hypothesis on the function of PEs in EOMs and the multiply innervated muscle fibres they are attached to is put forward.

Saccadic premotor burst neurons and histochemical correlates of their firing patterns in rhesus monkey.

Bursting behavior of brainstem premotor burst neurons (BNs) is essential for initiation of saccades and calibrating their metrics. Several ion channel families such as voltage-gated potassium (Kv) channels, low-voltage-activated calcium (Cav3) channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are major regulators of the bursting in neurons. Therefore, it was speculated that ion channels with rapid kinematics are essential for characteristic firing patterns of the BNs and rapid saccade velocities. However, the expression patterns of ion channels are yet to be confirmed. Confirmation would not only support the neuromimetic model predictions for saccade generation in brainstem, but also contemporary views that channelopathies can cause saccade disorders in humans. As proof of concept, we examined excitatory BNs in the rostral interstitial nucleus of medial longitudinal fasciculus (RIMLF, vertical saccades) and inhibitory BNs in nucleus paragigantocellularis dorsalis (PGD, horizontal saccades) histochemically in macaque monkeys. We found strong expression of Kv channels, which enable rapid-firing, as well as HCN1&2 and Cav3.2&3.3, which enable post-inhibitory rebound bursting, in both BN populations. Moreover, PGD was found to host multiple neuron groups in terms of calretinin immunoreactivity. Our results provide histochemical evidence that supports models proposing post-inhibitory rebound facilitates bursting in BNs. Furthermore, our findings support the notion that deductions can be made about electrophysiological firing properties by histochemical examination of functional groups within the brainstem saccadic circuitry. This development is an important building block supporting the concept of channelopathies in saccadic disorders. Future histological studies in humans will confirm this approach for saccadic disorders.

Potassium channels in omnipause neurons.

Potassium (K+) channels are major contributors to fast and precise action potential generation. The aim of this study was to establish the immunoreactivity profile of several potassium channels in omnipause neurons (OPNs), which play a central role in premotor saccadic circuitry. To accomplish this, we histochemically examined monkey and human brainstem sections using antibodies against the voltage gated K+-channels KV1.1, KV3.1b and K+-Cl- cotransporter (KCC2). We found that OPNs of both species were positive for all three K+-antibodies and that the staining patterns were similar for both species. In individual OPNs, KV3.1b was detected on the somatic membrane and proximal dendrites, while KV1.1 was mainly confined to soma. Further, KCC2 immunoreactivity was strong in distal dendrites, but was weak in the somatic membrane. Our findings allow the speculation that the alterations in K+-channel expression in OPNs could be the underlying mechanism for several saccadic disorders through neuronal and circuit-level malfunction.

Extraocular muscles involved in convergence are innervated by an additional set of palisade endings that may differ in their excitability: A human study.

Palisade endings are located at the myotendinous junction of extraocular muscles in most mammals. Irrespective of their unclarified function as motor or sensory nerve endings, a specialized role in convergence is proposed, based on their high number in the medial rectus muscle (MR). Further support comes from a study in monkey demonstrating that only the MR and inferior rectus muscle (IR) contain an additional population of palisade endings that express the calcium-binding protein calretinin (CR) in addition to choline acetyltransferase (ChAT). Here we studied, whether CR-positive palisade endings are present in human as well and confined to extraocular muscles most active during convergence. The systematic analysis of all eye muscles of 17 human specimen revealed that only the MR and IR contain an additional population of CR-positive palisade endings and multiple en-grappe endings, which target non-twitch muscle fibers along their whole length. Approximately 80% of all palisade endings in the MR expressed CR. Furthermore, the intrafusal muscle fibers of some muscle spindles in the MR were innervated by CR-positive annulospiral nerve endings that transmit the signals of muscle length changes to the brain. All extraocular muscles contained few thin CR-positive, but ChAT-negative nerve fibers, possibly representing free sensory or autonomic endings arising from the trigeminal ganglion. As in monkey, in the medial periphery of the human oculomotor nucleus ChAT-positive neurons were found to co-express CR. Therefore these neurons most likely represent the cell bodies of CR-positive palisade endings in the MR. Unlike in monkey, these neurons do not lie within a compact cell group, but are more scattered. In conclusion, the MR and IR in human contain two histochemically different populations of palisade and multiple endings that may contribute to ocular alignment and convergence in a different way.

A Subset of Palisade Endings Only in the Medial and Inferior Rectus Muscle in Monkey Contain Calretinin.

Purpose: To further chemically characterize palisade endings in extraocular muscles in rhesus monkeys. Methods: Extraocular muscles of three rhesus monkeys were studied for expression of the calcium-binding protein calretinin (CR) in palisade endings and multiple endings. The complete innervation was visualized with antibodies against the synaptosomal-associated protein of 25 kDa and combined with immunofluorescence for CR. Six rhesus monkeys received tracer injections of choleratoxin subunit B or wheat germ agglutinin into either the belly or distal myotendinous junction of the medial or inferior rectus muscle to allow retrograde tracing in the C-group of the oculomotor nucleus. Double-immunofluorescence methods were used to study the CR content in retrogradely labeled neurons in the C-group. Results: A subgroup of palisade and multiple endings was found to express CR, only in the medial and inferior rectus muscle. In contrast, the en plaque endings lacked CR. Accordingly, within the tracer-labeled neurons of the C-group, a subgroup expressed CR. Conclusions: The study indicates that two different neuron populations targeting nontwitch muscle fibers are present within the C-group for inferior rectus and medial rectus, respectively, one expressing CR, one lacking CR. It is possible that the CR-negative neurons represent the basic population for all extraocular muscles, whereas the CR-positive neurons giving rise to CR-positive palisade endings represent a specialized, perhaps more excitable type of nerve ending in the medial and inferior rectus muscles, being more active in vergence. The malfunction of this CR-positive population of neurons that target nontwitch muscle fibers could play a significant role in strabismus.

Is there any sense in the Palisade endings of eye muscles?

Palisade endings (PEs), which are unique to the eye muscles, are associated with multiply innervated muscle fibers. They lie at the myotendinous junctions and form a cap around the muscle fiber tip. They are found in all animals investigated so far, but their function is not known. Recently, we demonstrated that cell bodies of PEs and tendon organs lie around the periphery of the oculomotor nucleus in the C- and S-groups. A morphological analysis of these peripheral neurons revealed the existence of different populations within the C-group. We propose that a small group of round or spindle-shaped cells gives rise to PEs, and another group of multipolar neurons provide the multiple motor endings. If PEs have a sensory function, then their cell body location close to motor neurons would be in an ideal location to control tension in extraocular muscles; in the case of the C-group, its proximity to the preganglionic neurons of the Edinger-Westphal nucleus would permit its participation in the near response. Despite their unusual properties, PEs may have a sensory function.

Do palisade endings in extraocular muscles arise from neurons in the motor nuclei?

PURPOSE: The purpose of this study was to localize the cell bodies of palisade endings that are associated with the myotendinous junctions of the extraocular muscles. METHODS: Rhesus monkeys received tract-tracer injections (tetramethylrhodamine dextran [TMR-DA] or choleratoxin subunit B [CTB]) into the oculomotor and trochlear nuclei, which contain the motoneurons of extraocular muscles. All extraocular muscles were processed for the combined immunocytochemical detection of the tracer and SNAP-25 or synaptophysin for the visualization of the complete muscle innervation. RESULTS: In all muscles--except the lateral rectus--en plaque and en grappe motor endings, but also palisade endings, were anterogradely labeled. In addition a few tracer-labeled tendon organs were found. One group of tracer-negative nerve fibers was identified as thin tyrosine hydroxylase-positive sympathetic fibers, and a second less numerous group of tracer-negative fibers may originate from the trigeminal ganglia. No cellular or terminal tracer labeling was present within the mesencephalic trigeminal nucleus or the trigeminal ganglia. CONCLUSIONS: These results confirm those of earlier studies and furthermore suggest that the somata of palisade endings are located close to the extraocular motor nuclei--in this case, probably within the C and S groups around the periphery of the oculomotor nucleus. The multiple en grappe endings have also been shown to arise from these cells groups, but it is not possible to distinguish different populations in these experiments.