Electrophysiological identification of two types of fibres in rat extraocular muscles

Quantification of saccadic fatigability and diagnostic efficacy for myasthenia gravis

BACKGROUND AND OBJECTIVES

The diagnostic challenge of myasthenia gravis (MG) is exacerbated by the variable efficacy of current testing methodologies, necessitating innovative approaches to accurately identify the condition. This study aimed to assess ocular muscle fatigue in patients with MG using video-oculography (VOG) by examining repetitive saccadic eye movements and comparing these metrics to those of healthy control participants.

METHODS

This prospective, cross-sectional study was conducted at a tertiary care center and involved 62 patients diagnosed with MG (48 with ocular MG and 14 with generalized MG) and a control group of 31 healthy individuals, matched for age and sex. The assessment involved recording saccadic eye movements within a ± 15° range, both horizontally and vertically, at a rate of 15 saccades per minute over a 5-min period, resulting in 75 cycles. Participants were afforded a 3-min rest interval between each set to mitigate cumulative fatigue. The primary outcome was the detection of oculomotor fatigue, assessed through changes in saccadic waveforms, range, peak velocity, latency, and the duration from onset to target, with a focus on comparing the second saccade against the average of the last five saccades.

RESULTS

In the evaluation of repetitive saccadic movements, patients with MG exhibited a reduced saccadic range and a prolonged duration to reach the target, compared to healthy subjects. Furthermore, a significant elevation in the frequency of multistep saccades was observed among MG patients, with a marked rise observed over consecutive trials. Receiver operating characteristic (ROC) analysis revealed the discriminative performance of multistep saccade frequency, in conjunction with variations in saccadic range and duration from onset to target achievement between the second saccade and the mean of the final five saccades, as effective in distinguishing MG patients from healthy subjects. Although alterations in peak saccadic velocity and latency were less pronounced, they were nevertheless detectable.

DISCUSSION

The utilization of VOG for repetitive saccadic testing in the diagnosis of MG has demonstrated considerable diagnostic precision. This methodology affords significant accuracy in evaluating ocular muscle fatigue in MG patients, providing class III evidence supportive of its clinical application.

Extreme Tolerance of Extraocular Muscles to Diseases and Aging: Why and How?

The extraocular muscles (EOMs) possess unique characteristics that set them apart from other skeletal muscles. These muscles, responsible for eye movements, exhibit remarkable resistance to various muscular dystrophies and aging, presenting a significant contrast to the vulnerability of skeletal muscles to these conditions. In this review, we delve into the cellular and molecular underpinnings of the distinct properties of EOMs. We explore their structural complexity, highlighting differences in fiber types, innervation patterns, and developmental origins. Notably, EOM fibers express a diverse array of myosin heavy-chain isoforms, retaining embryonic forms into adulthood. Moreover, their motor innervation is characterized by a high ratio of nerve fibers to muscle fibers and the presence of unique neuromuscular junctions. These features contribute to the specialized functions of EOMs, including rapid and precise eye movements. Understanding the mechanisms behind the resilience of EOMs to disease and aging may offer insights into potential therapeutic strategies for treating muscular dystrophies and myopathies affecting other skeletal muscles.

Distribution and postnatal development of chondroitin sulfate proteoglycans in the perineuronal nets of cholinergic motoneurons innervating extraocular muscles

Fine control of extraocular muscle fibers derives from two subpopulations of cholinergic motoneurons in the oculomotor-, trochlear- and abducens nuclei. Singly- (SIF) and multiply innervated muscle fibers (MIF) are supplied by the SIF- and MIF motoneurons, respectively, representing different physiological properties and afferentation. SIF motoneurons, as seen in earlier studies, are coated with chondroitin sulfate proteoglycan rich perineuronal nets (PNN), whereas MIF motoneurons lack those. Fine distribution of individual lecticans in the composition of PNNs and adjacent neuropil, as well as the pace of their postnatal accumulation is, however, still unknown. Therefore, the present study aims, by using double immunofluorescent identification and subsequent morphometry, to describe local deposition of lecticans in the perineuronal nets and neuropil of the three eye movement nuclei. In each nucleus PNNs were consequently positive only with WFA and aggrecan reactions, suggesting the dominating role of aggrecan is PNN establishment. Brevican, neurocan and versican however, did not accumulate at all in PNNs but were evenly and moderately present throughout the neuropils. The proportion of PNN bearing motoneurons appeared 76% in oculomotor-, 72.2% in trochlear- and 78.3% in the abducens nucleus. We also identified two morphological subsets of PNNs, the focal and diffuse nets of SIF motoneurons. The process of CSPG accumulation begins just after birth, although considerable PNNs occur at week 1 age around less than half of the motoneurons, which ratio doubles until 2-month age. These findings may be related to the postnatal establishment of the oculokinetic network, performing different repertoires of voluntary eye movements in functionally afoveolate and foveolate animals.

Is the central mesencephalic reticular formation a purely horizontal gaze center?

Historically, the central mesencephalic reticular formation has been regarded as a purely horizontal gaze center based on the fact that electrical stimulation of this region produces horizontal saccades, it provides monosynaptic input to medial rectus motoneurons, and cells recorded in this region often display a peak in firing when horizontal saccades are made. We tested the proposition that the central mesencephalic reticular formation is purely a horizontal gaze center by examining whether this region also supplies terminals to superior rectus and levator palpebrae superioris motoneurons, both of which fire when making vertical eye movements. The experiments were carried out using dual tracer techniques at the light and electron microscopic level in macaque monkeys. Injections of biotinylated dextran amine or Phaseolus vulgaris leukoagglutinin into the central mesencephalic reticular formation produced anterogradely labeled terminals that were in synaptic contact with superior rectus and levator palpebrae superioris motoneurons that had been retrogradely labeled. These results indicate that this region is not purely connected with horizontal gaze motoneurons. In addition, we found that the number of contacts on vertical gaze motoneurons increased with more rostral injections involving the mesencephalic reticular formation adjacent to the interstitial nucleus of Cajal. This suggests that there is a caudal to rostral gradient for horizontal to vertical saccades, respectively, represented within the midbrain reticular formation. Finally, we utilized post-embedding immunohistochemistry to show that a portion of the labeled terminals were GABAergic, indicating they likely originate from downgaze premotor neurons.

Extraocular Motoneurons and Neurotrophism

Extraocular motoneurons are located in three brainstem nuclei: the abducens, trochlear and oculomotor. They control all types of eye movements by innervating three pairs of agonistic/antagonistic extraocular muscles. They exhibit a tonic-phasic discharge pattern, demonstrating sensitivity to eye position and sensitivity to eye velocity. According to their innervation pattern, extraocular muscle fibers can be classified as singly innervated muscle fiber (SIF), or the peculiar multiply innervated muscle fiber (MIF). SIF motoneurons show anatomical and physiological differences with MIF motoneurons. The latter are smaller and display lower eye position and velocity sensitivities as compared with SIF motoneurons.

Superior colliculus projections to target populations in the supraoculomotor area of the macaque monkey

A projection by the superior colliculus to the supraoculomotor area (SOA) located dorsal to the oculomotor complex was first described in 1978. This projection’s targets have yet to be identified, although the initial study suggested that vertical gaze motoneuron dendrites might receive this input. Defining the tectal targets is complicated by the fact the SOA contains a number of different cell populations. In the present study, we used anterograde tracers to characterize collicular axonal arbors and retrograde tracers to label prospective SOA target populations in macaque monkeys. Close associations were not found with either superior or medial rectus motoneurons whose axons supply singly innervated muscle fibers. S-group motoneurons, which supply superior rectus multiply innervated muscle fibers, appeared to receive a very minor input, but C-group motoneurons, which supply medial rectus multiply innervated muscle fibers, received no input. A number of labeled boutons were observed in close association with SOA neurons projecting to the spinal cord, or the reticular formation in the pons and medulla. These descending output neurons are presumed to be peptidergic cells within the centrally projecting Edinger–Westphal population. It is possible the collicular input provides a signaling function for neurons in this population that serve roles in either stress responses, or in eating and drinking behavior. Finally, a number of close associations were observed between tectal terminals and levator palpebrae superioris motoneurons, suggesting the possibility that the superior colliculus provides a modest direct input for raising the eyelids during upward saccades.

Functional Organization of Extraocular Motoneurons and Eye Muscles

Eye movements are indispensable for visual image stabilization during self-generated and passive head and body motion and for visual orientation. Eye muscles and neuronal control elements are evolutionarily conserved, with novel behavioral repertoires emerging during the evolution of frontal eyes and foveae. The precise execution of eye movements with different dynamics is ensured by morphologically diverse yet complementary sets of extraocular muscle fibers and associated motoneurons. Singly and multiply innervated muscle fibers are controlled by motoneuronal subpopulations with largely selective premotor inputs from task-specific ocular motor control centers. The morphological duality of the neuromuscular interface is matched by complementary biochemical and molecular features that collectively assign different physiological properties to the motor entities. In contrast, the functionality represents a continuum where most motor elements contribute to any type of eye movement, although within preferential dynamic ranges, suggesting that signal transmission and muscle contractions occur within bands of frequency-selective pathways.

Cerebellar projections to the macaque midbrain tegmentum: Possible near response connections

Since most gaze shifts are to targets that lie at a different distance from the viewer than the current target, gaze changes commonly require a change in the angle between the eyes. As part of this response, lens curvature must also be adjusted with respect to target distance by the ciliary muscle. It has been suggested that projections by the cerebellar fastigial and posterior interposed nuclei to the supraoculomotor area (SOA), which lies immediately dorsal to the oculomotor nucleus and contains near response neurons, support this behavior. However, the SOA also contains motoneurons that supply multiply innervated muscle fibers (MIFs) and the dendrites of levator palpebrae superioris motoneurons. To better determine the targets of the fastigial nucleus in the SOA, we placed an anterograde tracer into this cerebellar nucleus in Macaca fascicularis monkeys and a retrograde tracer into their contralateral medial rectus, superior rectus, and levator palpebrae muscles. We only observed close associations between anterogradely labeled boutons and the dendrites of medial rectus MIF and levator palpebrae motoneurons. However, relatively few of these associations were present, suggesting these are not the main cerebellar targets. In contrast, labeled boutons in SOA, and in the adjacent central mesencephalic reticular formation (cMRF), densely innervated a subpopulation of neurons. Based on their location, these cells may represent premotor near response neurons that supply medial rectus and preganglionic Edinger-Westphal motoneurons. We also identified lens accommodation-related cerebellar afferent neurons via retrograde trans-synaptic transport of the N2c rabies virus from the ciliary muscle. They were found bilaterally in the fastigial and posterior interposed nuclei, in a distribution which mirrored that of neurons retrogradely labeled from the SOA and cMRF. Our results suggest these cerebellar neurons coordinate elements of the near response during symmetric vergence and disjunctive saccades by targeting cMRF and SOA premotor neurons.

The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle

The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.

Neuroanatomical Structures in Human Extraocular Muscles and Their Potential Implication in the Development of Oculomotor Disorders

The potential role of sensory feedback from human extraocular muscles has been subjected to considerable speculation in the ophthalmic literature. Extraocular muscles pull against a fairly even load and do not initiate a stretch reflex, even when the eyes are directed toward the boundaries of their respective field of action. These unique working conditions and physiological properties have led to the notion that the sensory signal arising from receptors in extraocular muscles differs from the conventional proprioceptive signal arising from their somatic counterparts. The interest in the receptors at the myotendinous junction of human extraocular muscles has been renewed due to their alleged role in the development of binocular vision and their potential implication in the etiology of binocular vision anomalies. The idea that extraocular muscles provide knowledge of eye position and whether this function can be affected by surgical intervention has initiated several clinical and neuroanatomical studies. Many of these studies support this concept and suggest that surgical procedures that impose only minimal interference with the proprioceptive signal will give a better postoperative result. However, other studies contradict this view because the afferent capacity of the receptors can be questioned and some uncertainties remain. The purpose of this study was to review the related literature and discuss the possible role of ocular proprioceptors in relation to binocular vision and the development of eye motility disorders. [J Pediatr Ophthalmol Strabismus. 2018;55(1):14-22.].

A central mesencephalic reticular formation projection to medial rectus motoneurons supplying singly and multiply innervated extraocular muscle fibers

We recently demonstrated a bilateral projection to the supraoculomotor area from the central mesencephalic reticular formation (cMRF), a region implicated in horizontal gaze changes. C-group motoneurons, which supply multiply innervated fibers in the medial rectus muscle, are located within the primate supraoculomotor area, but their inputs and function are poorly understood. Here, we tested whether C-group motoneurons in Macaca fascicularis monkeys receive a direct cMRF input by injecting this portion of the reticular formation with anterograde tracers in combination with injection of retrograde tracer into the medial rectus muscle. The results indicate that the cMRF provides a dense, bilateral projection to the region of the medial rectus C-group motoneurons. Numerous close associations between labeled terminals and each multiply innervated fiber motoneuron were present. Within the oculomotor nucleus, a much sparser ipsilateral projection onto some of the A- and B- group medial rectus motoneurons that supply singly innervated fibers was observed. Ultrastructural analysis demonstrated a direct synaptic linkage between anterogradely labeled reticular terminals and retrogradely labeled medial rectus motoneurons in all three groups. These findings reinforce the notion that the cMRF is a critical hub for oculomotility by proving that it contains premotor neurons supplying horizontal extraocular muscle motoneurons. The differences between the cMRF input patterns for C-group versus A- and B-group motoneurons suggest the C-group motoneurons serve a different oculomotor role than the others. The similar patterns of cMRF input to C-group motoneurons and preganglionic Edinger-Westphal motoneurons suggest that medial rectus C-group motoneurons may play a role in accommodation-related vergence.

"Slow" skeletal muscles across vertebrate species

Skeletal muscle fibers are generally classified into two groups: slow (type I) and fast (type II). Fibers in each group are uniquely designed for specific locomotory needs based on their intrinsic cellular properties and the types of motor neurons that innervate them. In this review, we will focus on the current concept of slow muscle fibers which, unlike the originally proposed version based purely on amphibian muscles, varies widely depending on the animal model system studied. We will discuss recent findings from zebrafish neuromuscular junction synapses that may provide the framework for establishing a more unified view of slow muscles across mammalian and non-mammalian species.

Adaptability of the Immature Ocular Motor Control System: Unilateral IGF-1 Medial Rectus Treatment

PURPOSE

Unilateral treatment with sustained release IGF-1 to one medial rectus muscle in infant monkeys was performed to test the hypothesis that strabismus would develop as a result of changes in extraocular muscles during the critical period of development of binocularity.

METHODS

Sustained release IGF-1 pellets were implanted unilaterally on one medial rectus muscle in normal infant monkeys during the first 2 weeks of life. Eye position was monitored using standard photographic methods. After 3 months of treatment, myofiber and neuromuscular size, myosin composition, and innervation density were quantified in all rectus muscles and compared to those in age-matched controls.

RESULTS

Sustained unilateral IGF-1 treatments resulted in strabismus for all treated subjects; 3 of the 4 subjects had a clinically significant strabismus of more than 10°. Both the treated medial rectus and the untreated ipsilateral antagonist lateral rectus muscles had significantly larger myofibers. No adaptation in myofiber size occurred in the contralateral functionally yoked lateral rectus or in myosin composition, neuromuscular junction size, or nerve density.

CONCLUSIONS

Sustained unilateral IGF-1 treatment to extraocular muscles during the sensitive period of development of orthotropic eye alignment and binocularity was sufficient to disturb ocular motor development, resulting in strabismus in infant monkeys. This could be due to altering fusion of gaze during the early sensitive period. Serial measurements of eye alignment suggested the IGF-1-treated infants received insufficient coordinated binocular experience, preventing the establishment of normal eye alignment. Our results uniquely suggest that abnormal signaling by the extraocular muscles may be a cause of strabismus.

Adaptation of slow myofibers: the effect of sustained BDNF treatment of extraocular muscles in infant nonhuman primates

PURPOSE

We evaluated promising new treatment options for strabismus. Neurotrophic factors have emerged as a potential treatment for oculomotor disorders because of diverse roles in signaling to muscles and motor neurons. Unilateral treatment with sustained release brain-derived neurotrophic factor (BDNF) to a single lateral rectus muscle in infant monkeys was performed to test the hypothesis that strabismus would develop in correlation with extraocular muscle (EOM) changes during the critical period for development of binocularity.

METHODS

The lateral rectus muscles of one eye in two infant macaques were treated with sustained delivery of BDNF for 3 months. Eye alignment was assessed using standard photographic methods. Muscle specimens were analyzed to examine the effects of BDNF on the density, morphology, and size of neuromuscular junctions, as well as myofiber size. Counts were compared to age-matched controls.

RESULTS

No change in eye alignment occurred with BDNF treatment. Compared to control muscle, neuromuscular junctions on myofibers expressing slow myosins had a larger area. Myofibers expressing slow myosin had larger diameters, and the percentage of myofibers expressing slow myosins increased in the proximal end of the muscle. Expression of BDNF was examined in control EOM, and observed to have strongest immunoreactivity outside the endplate zone.

CONCLUSIONS

We hypothesize that the oculomotor system adapted to sustained BDNF treatment to preserve normal alignment. Our results suggest that BDNF treatment preferentially altered myofibers expressing slow myosins. This implicates BDNF signaling as influencing the slow twitch properties of EOM.

Transmitter inputs to different motoneuron subgroups in the oculomotor and trochlear nucleus in monkey

In all vertebrates the eyes are moved by six pairs of extraocular muscles enabling horizontal, vertical and rotatory movements. Recent work showed that each extraocular muscle is controlled by two motoneuronal groups: (1) Motoneurons of singly-innervated muscle fibers (SIF) that lie within the boundaries of motonuclei mediating a fast muscle contraction; and (2) motoneurons of multiply-innervated muscle fibers (MIF) in the periphery of motonuclei mediating a tonic muscle contraction. Currently only limited data about the transmitter inputs to the SIF and MIF motoneurons are available. Here we performed a quantitative study on the transmitter inputs to SIF and MIF motoneurons of individual muscles in the oculomotor and trochlear nucleus in monkey. Pre-labeled motoneurons were immunostained for GABA, glutamate decarboxylase, GABA-A receptor, glycine transporter 2, glycine receptor 1, and vesicular glutamate transporters 1 and 2. The main findings were: (1) the inhibitory control of SIF motoneurons for horizontal and vertical eye movements differs. Unlike in previous primate studies a considerable GABAergic input was found to all SIF motoneuronal groups, whereas a glycinergic input was confined to motoneurons of the medial rectus (MR) muscle mediating horizontal eye movements and to those of the levator palpebrae (LP) muscle elevating the upper eyelid. Whereas SIF and MIF motoneurons of individual eye muscles do not differ numerically in their GABAergic, glycinergic and vGlut2 input, vGlut1 containing terminals densely covered the supraoculomotor area (SOA) targeting MR MIF motoneurons. It is reasonable to assume that the vGlut1 input affects the near response system in the SOA, which houses the preganglionic neurons mediating pupillary constriction and accommodation and the MR MIF motoneurones involved in vergence.

Mechanics of mouse ocular motor plant quantified by optogenetic techniques

Rigorous descriptions of ocular motor mechanics are often needed for models of ocular motor circuits. The mouse has become an important tool for ocular motor studies, yet most mechanical data come from larger species. Recordings of mouse abducens neurons indicate the mouse mechanics share basic viscoelastic properties with larger species but have considerably longer time constants. Time constants can also be extracted from the rate at which the eye re-centers when released from an eccentric position. The displacement can be accomplished by electrically stimulating ocular motor nuclei, but electrical stimulation may also activate nearby ocular motor circuitry. We achieved specific activation of abducens motoneurons through photostimulation in transgenic mice expressing channelrhodopsin in cholinergic neurons. Histology confirmed strong channelrhodopsin expression in the abducens nucleus with relatively little expression in nearby ocular motor structures. Stimulation was delivered as 20- to 1,000-ms pulses and 40-Hz trains. Relaxations were modeled best by a two-element viscoelastic system. Time constants were sensitive to stimulus duration. Analysis of isometric relaxation of isolated mouse extraocular muscles suggest the dependence is attributable to noninstantaneous decay of active forces in non-twitch fibers following stimulus offset. Time constants were several times longer than those obtained in primates, confirming that the mouse ocular motor mechanics are relatively sluggish. Finally, we explored the effects of 0.1- to 20-Hz sinusoidal photostimuli and demonstrated their potential usefulness in characterizing ocular motor mechanics, although this application will require further data on the temporal relationship between photostimulation and neuronal firing in extraocular motoneurons.

A single mutation in the acetylcholine receptor δ-subunit causes distinct effects in two types of neuromuscular synapses

Mutations in AChR subunits, expressed as pentamers in neuromuscular junctions (NMJs), cause various types of congenital myasthenic syndromes. In AChR pentamers, the adult ε subunit gradually replaces the embryonic γ subunit as the animal develops. Because of this switch in subunit composition, mutations in specific subunits result in synaptic phenotypes that change with developmental age. However, a mutation in any AChR subunit is considered to affect the NMJs of all muscle fibers equally. Here, we report a zebrafish mutant of the AChR δ subunit that exhibits two distinct NMJ phenotypes specific to two muscle fiber types: slow or fast. Homozygous fish harboring a point mutation in the δ subunit form functional AChRs in slow muscles, whereas receptors in fast muscles are nonfunctional. To test the hypothesis that different subunit compositions in slow and fast muscles underlie distinct phenotypes, we examined the presence of ε/γ subunits in NMJs using specific antibodies. Both wild-type and mutant larvae lacked ε/γ subunits in slow muscle synapses. These findings in zebrafish suggest that some mutations in human congenital myasthenic syndromes may affect slow and fast muscle fibers differently.

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.

Analysis of spontaneous and nerve-evoked calcium transients in intact extraocular muscles in vitro

Extraocular muscles (EOMs) have unique calcium handling properties, yet little is known about the dynamics of calcium events underlying ultrafast and tonic contractions in myofibers of intact EOMs. Superior oblique EOMs of juvenile chickens were dissected with their nerve attached, maintained in oxygenated Krebs buffer, and loaded with fluo-4. Spontaneous and nerve stimulation-evoked calcium transients were recorded and, following calcium imaging, some EOMs were double-labeled with rhodamine-conjugated alpha-bungarotoxin (rhBTX) to identify EOM myofiber types. EOMs showed two main types of spontaneous calcium transients, one slow type (calcium waves with 1/2(max) duration of 2-12 s, velocity of 25-50 μm/s) and two fast "flash-like" types (Type 1, 30-90 ms; Type 2, 90-150 ms 1/2(max) duration). Single pulse nerve stimulation evoked fast calcium transients identical to the fast (Type 1) calcium transients. Calcium waves were accompanied by a local myofiber contraction that followed the calcium transient wavefront. The magnitude of calcium-wave induced myofiber contraction far exceeded those of movement induced by nerve stimulation and associated fast calcium transients. Tetrodotoxin eliminated nerve-evoked transients, but not spontaneous transients. Alpha-bungarotoxin eliminated both spontaneous and nerve-evoked fast calcium transients, but not calcium waves, and caffeine increased wave activity. Calcium waves were observed in myofibers lacking spontaneous or evoked fast transients, suggestive of multiply-innervated myofibers, and this was confirmed by double-labeling with rhBTX. We propose that the abundant spontaneous calcium transients and calcium waves with localized contractions that do not depend on innervation may contribute to intrinsic generation of tonic functions of EOMs.

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.

Identification of motoneurons supplying multiply- or singly-innervated extraocular muscle fibers in the rat

In mammals, the extraocular muscle fibers can be categorized in singly-innervated and multiply-innervated muscle fibers. In the monkey oculomotor, trochlear and abducens nucleus the motoneurons of multiply-innervated muscle fibers lie separated from those innervating singly-innervated muscle fibers and show different histochemical properties. In order to discover, if this organization is a general feature of the oculomotor system, we investigated the location of singly-innervated muscle fiber and multiply-innervated muscle fiber motoneurons in the rat using combined tract-tracing and immunohistochemical techniques. The singly-innervated muscle fiber and multiply-innervated muscle fiber motoneurons of the medial and lateral rectus muscle were identified by retrograde tracer injections into the muscle belly or the distal myotendinous junction. The belly injections labeled the medial rectus muscle subgroup of the oculomotor nucleus or the greatest part of abducens nucleus, including some cells outside the medial border of abducens nucleus. In contrast, the distal injections labeled only a subset of the medial rectus muscle motoneurons and exclusively cells outside the medial border of abducens nucleus. The tracer detection was combined with immunolabeling using antibodies for perineuronal nets (chondroitin sulfate proteoglycan) and non-phosphorylated neurofilaments. In monkeys both antibodies permit a distinction between singly-innervated muscle fiber and multiply-innervated muscle fiber motoneurons. The experiments revealed that neurons labeled from a distal injection lack both markers and are assumed to represent multiply-innervated muscle fiber motoneurons, whereas those labeled from a belly injection are chondroitin sulfate proteoglycan- and non-phosphorylated neurofilament-immunopositive and assumed to represent singly-innervated muscle fiber motoneurons. The overall identification of multiply-innervated muscle fiber and singly-innervated muscle fiber motoneurons within the rat oculomotor nucleus, trochlear nucleus, and abducens nucleus revealed that the smaller multiply-innervated muscle fiber motoneurons tend to lie separate from the larger diameter singly-innervated muscle fiber motoneurons. Our data provide evidence that rat extraocular muscles are innervated by two sets of motoneurons that differ in their molecular, morphological, and anatomical properties.

Biological organization of the extraocular muscles

Extraocular muscle is fundamentally distinct from other skeletal muscles. Here, we review the biological organization of the extraocular muscles with the intent of understanding this novel muscle group in the context of oculomotor system function. The specific objectives of this review are threefold. The first objective is to understand the anatomic arrangement of the extraocular muscles and their compartmental or layered organization in the context of a new concept of orbital mechanics, the active pulley hypothesis. The second objective is to present an integrated view of the morphologic, cellular, and molecular differences between extraocular and the more traditional skeletal muscles. The third objective is to relate recent data from functional and molecular biology studies to the established extraocular muscle fiber types. Developmental mechanisms that may be responsible for the divergence of the eye muscles from a skeletal muscle prototype also are considered. Taken together, a multidisciplinary understanding of extraocular muscle biology in health and disease provides insights into oculomotor system function and malfunction. Moreover, because the eye muscles are selectively involved or spared in a variety of neuromuscular diseases, knowledge of their biology may improve current pathogenic models of and treatments for devastating systemic diseases.

Histochemical differences between motoneurons supplying multiply and singly innervated extraocular muscle fibers

The extraocular muscle fibers of vertebrates can be classified into two categories: singly innervated fibers (SIFs) and multiply innervated fibers (MIFs). In monkeys, the motoneurons of SIFs lie within the oculomotor, trochlear, and abducens nucleus, whereas the motoneurons of MIFs appear in separate subgroups in the periphery of the classical nuclei borders. In the present study, we investigated the histochemical properties of SIF and MIF motoneurons by using combined tract-tracing and immunofluorescence techniques. In monkeys, SIF and MIF motoneurons of extraocular muscles were identified by tracer injections into the belly or the distal myotendinous junction of the medial or lateral rectus muscle. Alternatively, the motoneurons were identified by choline acetyltransferase immunostaining. These techniques were combined with the detection of histochemical markers for perineuronal nets, nonphosphorylated neurofilaments, parvalbumin, or cytochrome oxidase. The experiments revealed that the MIF motoneurons in the periphery of the motonuclei do not contain nonphosphorylated neurofilaments or parvalbumin and lack perineuronal nets. In contrast, SIF motoneurons express all markers at high intensity. Cytochrome oxidase immunostaining was found in both motoneuron populations. An additional population of motoneurons with "MIF properties" was identified within the boundaries of the abducens nucleus, which could represent the motoneurons innervating MIFs in the orbital layer of lateral rectus muscle. Our data provide evidence that SIF and MIF motoneurons, which can be correlated with twitch motoneurons and presumed non-twitch motoneurons, differ in their histochemical properties. The absence of perineuronal nets, nonphosphorylated neurofilaments, and parvalbumin may help to identify the homologous MIF motoneurons in other species, including humans.

Extraocular muscles have fundamentally distinct properties that make them selectively vulnerable to certain disorders

While skeletal muscles generally perform specific limited roles, extraocular muscles (EOMs) have to be responsive over a wider dynamic range. As a result, EOMs have fundamentally distinct structural, functional, biochemical and immunological properties compared to other skeletal muscles. While these properties enable high fatigue resistance and the rapid and precise control of extraocular motility, they might also explain why EOMs are selectively involved in certain disorders, such as chronic progressive external ophthalmoplegia (CPEO), myasthenia gravis and Graves' ophthalmopathy. This review first gives an overview of the novel myofibre classification in EOMs and then focuses on those properties that might explain why ophthalmoplegia should be so prominent in these disorders.

Presence and structure of innervated myotendinous cylinders in rabbit extraocular muscle

Innervated myotendinous cylinders (IMCs) in rabbit extraocular muscles (EOMs) were identified for the first time. The nature of IMC nerve terminals was demonstrated by means of electron microscopy and double fluorescent staining. The distal EOM portions of four rabbits of different age and sex were prepared for transmission electron microscopy and for double-fluorescent labelling. Antibody against neurofilament H and alpha-bungarotoxin were applied on longitudinal cryostat sections of distal myotendinous junction. IMCs were consistently and frequently observed at the distal myotendons of each EOM. More than 30 IMCs were counted in two medial recti of a 6 month and 3 year old rabbit. IMCs were enveloped by two to three layered capsules of fibrocytes. Each IMC contained the terminal portion of one multiply-innervated muscle fibre and its corresponding tendon. The tendon compartment of an IMC was entered by a single myelinated nerve fibre (2-3 microm in diameter). Inside the IMC, this nerve fibre ramified into up to four preterminal branches. Nerve terminals exclusively established contacts with the muscle fibre at its junction with the tendon fibrils. Nerve terminals contained mitochondria and a multitude of clear vesicles. Within the synaptic cleft a basal lamina was always present. alpha-Bungarotoxin labelled the muscle side of these myoneural contacts. Nerve terminals exhibited neither age nor sex differences. Among all species so far investigated, rabbit IMCs are unique by exhibiting exclusively myoneural terminal contacts. Based on fine structure and alpha-bungarotoxin binding, myoneural contacts in rabbit IMCs are almost certainly motor, as previously observed only in human IMCs. Thus, rabbit IMCs are supposed to have a predominant effector function.

Fibre types and myosin heavy chain expression in the ocular medial rectus muscle of the adult rat

Myosin heavy chain (MHC) expression was determined immunohistochemically in individual muscle fibre types characterised by activities of ATPase and the key oxidative and glycolytic enzymes in rat ocular medial rectus (MR) muscles. In the global layer (GL), glycolytic activity of muscle fibres was higher and oxidative activity lower, than in the orbital layer (OL). Muscle fibres in the former displayed rosette-like organisation with a slow fibre surrounded by several fast fibres, which expressed either MHCIIa or MHCIIb, but many co-expressed both isoforms. In the OL some slow fibres co-expressed MHCIIa. Extraocular MHC isoform (MHCeom) could not be determined immunohistochemically and no pure MHCIIx/d containing fibres were found, suggesting that these isoforms, demonstrated electrophoretically, are co-expressed with others. Slow muscle fibres in both layers co-expressed MHCbeta slow, MHCalpha cardiac and MHC-slow tonic. Neonatal isoform (MHCneo) was co-expressed in several fast and slow muscle fibres in the orbital, but not global layer. Slow fibres in the GL displayed very low oxidative activity. Electrophoretic analysis of ocular MR muscle homogenates revealed that about 50% of total MHC was MHCIIb, MHCeom was quite prominent (25%), and MHCIIa, MHCIIx/d and MHCI contributed each about 8%. MHCneo, MHCslow tonic and MHCalpha cardiac could not be identified as separate bands.

Identification of nerve endings in cat extraocular muscles

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 microm in length on striated muscle fibres 10-15 microm in diameter; and (3) "complex" endings on muscle fibres 15-20 microm in diameter. 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 microm. 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.

Lateral rectus EMG and contractile responses elicited by cat abducens motoneurons

Stimulation of 41 single, abducens nucleus motoneurons in the cat evoked electromyographic (EMG) and contractile responses in the ipsilateral lateral rectus muscle. Separate, bipolar, fine wire EMG recording electrodes in the global and orbital muscle layers showed that 22 muscle units were confined to the global layer, 8 to the orbital layer, and 11 units were contained in both ("bilayer") muscle layers. "Bilayer" units demonstrated significantly greater twitch (P < or = 0.002) and maximum tetanic (P < or = 0.001) tensions as well as faster fusion frequencies (P < or = 0.022) than either global or orbital units. "Bilayer" units also showed the lowest average kt values (the slope of the linear relationship between motoneuron stimulation frequency and isometric tetanic tension). "Bilayer" units were predominantly fast fatigable (FF). Global units displayed all muscle unit types including all the nontwitch (NT) units. Orbital units were identified as slow fatigable (SF) and fast fatigue resistant (FR).

Extraocular muscles: basic and clinical aspects of structure and function

Although extraocular muscle is perhaps the least understood component of the oculomotor system, these muscles represent the most common site of surgical intervention in the treatment of strabismus and other ocular motility disorders. This review synthesizes information derived from both basic and clinical studies in order to develop a better understanding of how these muscles may respond to surgical or pharmacological interventions and in disease states. In addition, a detailed knowledge of the structural and functional properties of extraocular muscle, that would allow some degree of prediction of the adaptive responses of these muscles, is vital as a basis to guide the development of new treatments for eye movement disorders.

Immunologic evidence for three isoforms of AMP deaminase (AMPD) in mature skeletal muscle

Four rabbit polyclonal antisera to purified AMP deaminase (AMPD) isozymes were used to precipitate homogenate AMPD activity from dissected gracilis, soleus and gastrocnemius muscles of the cat, rabbit, rat, mouse, Rhesus monkey, human and toad. The antisera were also tested against other unusual muscles: autonomically innervated striated muscle of the upper esophagus (UEM), skeletal muscle of patients with myo-AMPD deficiency and extraocular muscles (EOM) of humans and Rhesus monkeys. The reference antiserum, M, prepared against human psoas muscle AMPD, precipitated > 90% AMPD from all primate skeletal muscles tested, and from type-2 muscles of all mammals tested, but < 75% from cat and rodent soleus, toad gastrocnemius and primate UEM, EOM and myo-AMPD deficient muscles. Thus, a second isozyme was clearly indicated. Antibody B, against rat liver and kidney AMPD, had no effect with any muscle specimen. Antibody C, against rat heart AMPD, produced additive precipitation of AMPD from soleus of rat and mouse, while antibody E1, against human red cell (and heart) AMPD, produced additive AMPD precipitation from toad gastrocnemius, cat soleus and muscles of several AMPD-deficient humans. A second AMPD isozyme thus accounted for as much as 25% of total activity in some animal red muscles, but no more than 5% in human mixed muscles. At least one more isozyme is needed to account for muscle AMPD unreactive with all antibodies tested in rabbit soleus, toad gastrocnemius and primate UEM and EOM. A list is appended of the approximate AMPD activity in various human cells and tissues.

Extraocular motor units: type classification and motoneuron stimulation frequency-muscle unit force relationships

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat. Using a combination of two motor unit properties, the fusion frequency and an index of fatigability, the population of twitch motor units could be separated into 4 subgroups: fast fatigable (FF), fast fatigue resistant (FR), slow fatigable (SF) and slow fatigue resistant (S). Nontwitch motor units, a fifth subgroup (NT), formed 10% of the total studied population. The twitch tension and the maximum tetanic tension of the FF motor unit type were significantly stronger than all other motor unit types. The use of frequency varying stimulation patterns did not further differentiate the motor unit types. The relation between a series of single motoneuron stimulation frequencies and the resultant single muscle unit forces generated a slope defined as a motor unit's kt value. Motor units with low kt values had higher twitch tensions, higher maximum tetanic tensions, higher fusion frequencies and lower fatigue indices than motor units with high kt values. Motoneuron recruitment was tested by electrical stimulation of the medial rectus subdivision of the contralateral oculomotor nucleus. No correlations were seen between recruitment order and the mechanical parameters of the single abducens motor units.

Systematic variation in myosin expression along extraocular muscle fibres of the adult rat

Monoclonal antibodies (McAB) specific for fast (C14) and slow (S58) myosin, and a myosin antigenically similar to neonatal/embryonic myosin in mammals (ALD180), were used to characterize the myosin distribution in orbital layer fibres of rat extraocular muscles (EOM) in relation to innervation patterns. The orbital layer is composed of both singly-innervated (SIF) and multiply-innervated (MIF) fibres. The SIFs have the characteristics of twitch fibres, while the MIFs, in addition to possessing many small endings characteristic of tonic fibres, also have an en-plaque-like innervation in the endplate band resembling that of the adjacent SIFs. Myosin expression in MIFs and SIFs is unusual and varies systematically along the length of the fibres. Both SIFs and MIFs label with ALD180, but this labelling is absent in both fibre types in the endplate band region, where all fibres label with C14. Distally and also proximally to the endplate band, SIFs label with both ALD180 and C14, while the MIFs, innervated by many small, superficial endings in these regions, label with ALD180 only. This pattern of myosin expression could also be demonstrated in isolated fibres. The results are discussed in relation to the hypothesis that both populations of orbital layer fibres express constitutively both fast and the neonatal-like myosin, and that superimposed on this constitutive expression twitch or tonic innervation acts locally to selectively suppress either neonatal-like or fast myosin, respectively.

Motor innervation and acetylcholine receptor distribution of human extraocular muscle fibres

Motor innervation and acetylcholine receptor distribution of human extraocular muscle (EOM) fibres were investigated by using combined silver and acetylcholinesterase (AChE) staining method and [125I]alpha-bungarotoxin autoradiography. Three types of motor endplates were distinguished; (A) a large compact ending on a large-diameter fibre, (B) several endings regularly spaced on an intermediate-diameter fibre, each of which consisted of small stained particles and (C) numerous small endings scattered as a chain of beads on a small-diameter fibre. Type A fibre was singly innervated, while type B and C had multiterminal (possibly polyneuronal) innervation. These results indicate that the organization of human EOM endplates is quite different from that of limb muscle endplates and may provide some implications to understand the pathophysiology of the neuromuscular diseases, for example, myasthenia gravis.

Induction of action potentials by denervation of tonic fibres in rat extraocular muscles

The effects of denervation by nerve section on the electrical properties of tonic and twitch fibres of rat extraocular muscles were examined. Normally innervated tonic fibres lack action potentials. Upon direct stimulation they generate graded, voltage-dependent responses or slow peak potentials (s.p.p.s). However, one week after denervation the s.p.p.s are transformed into action potentials which are slower and broader than those of twitch fibres. The action potentials are Na dependent and partially resistant to blockade with 10(-5) M-tetrodotoxin and 10(-6) M-saxitoxin. Changing the holding potential of the fibres from -80 mV to more negative levels increases the maximal rate of rise of the action potential. This effect is not observed on the s.p.p.s of normally innervated fibres. Following denervation the resting potential of tonic and twitch fibres becomes about 10-15 mV less negative. In denervated muscles stimulation with pulses of hyperpolarizing current evokes graded responses in tonic fibres and action potentials in twitch fibres. In normally innervated muscles, these anodal break responses are never observed in tonic fibres and are very rare in twitch fibres. By two weeks after nerve section, reinnervation is present. The action potentials of tonic fibres are still present but stronger stimulation is needed to evoke anodal break responses. By three weeks, direct stimulation of tonic fibres evokes normal s.p.p.s in about 25% of the studied fibres and action potentials in the rest. By four weeks, most tonic fibres have lost the action potential but small anodal break responses can be evoked in most. It is suggested that following denervation a new population of Na channels appears in tonic fibres. The properties of these channels are different from those of the channels normally present in innervated tonic fibres but they are in some ways similar to those of the channels which appear in twitch fibres following denervation.

Reduction of miniature end-plate potential amplitude in extraocular and limb muscles in an animal model of myasthenia gravis

Miniature end-plate potential (MEPP) amplitudes of the superior oblique extraocular muscle (EOM) and forelimb flexor digitorum longus muscle (FDL) of rats with experimental autoimmune myasthenia gravis (EAMG) were expressed as reductions from mean MEPP amplitudes of the respective muscles of pair-matched control animals. Postjunctional blockade of neuromuscular transmission at EOM end-plates was no greater than at limb muscle end-plates. In contrast, the amount of reduction measured in EAMG-affected FDLs (52.3%) was marginally, but significantly, greater than that in EOMs (38.3%). A weak correlation was present between the MEPP reduction observed in an animal's FDL and that recorded in its EOM.

Potassium and caffeine contractures in fast and slow muscles of the chicken

1 K+ contractures, caffeine contractures and electrical properties were studied in slow (posterior latissimus dorsi; p.l.d.) and fast (anterior latissimus dorsi; a.l.d.) chicken muscles. 2. P.l.d. K+ contractures show a transient increase of tension that relaxes spontaneously. Contractures in a.l.d. show an initial component followed by a maintained tension. 3. A.l.d. K+ contractures of similar amplitude and time course were reproduced at 4 min intervals. In p.l.d., the interval needed for full recovery is about 30 min. In Cl-free saline p.l.d. and a.l.d. K+ contractures can be reproduced at 4 min intervals. 4. The time course of repolarization after a short exposure to 160 mM-KCl was much slower in p.l.d. than in a.l.d. In Cl-free saline the time course of repolarization becomes faster in p.l.d. 5. The membrane resistance was not modified in a.l.d. and was increased in p.l.d. by Cl-free saline. The calculated Cl- conductance in p.l.d. was about 70% of the total membrane conductance. 6. In a.l.d., Mn2+, D600 and external Ca2+ reduction greatly diminishes the maintained phase of the K+ contracture leaving the initial phase almost unmodified. Under similar conditions p.l.d. K+ contractures were slightly reduced. 7. P.l.d. caffeine contractures (10-40 mM) were not maintained and they were not modified by Ca-free saline, Cd2+, Co2+, Mn2+ and D600. 8. A.l.d. caffeine contractures (2-15 mM) were maintained and were highly dependent on external Ca2+. In addition they were greatly reduced by Cd2+, Co2+, Mn2 and D600. 9. It is suggested that caffeine contractures of a.l.d. are elicited by a Ca2+ entry into the muscle from the external fluid.

A histochemical study of fibre types in rat extraocular muscles

The histochemical fibre type composition of the rat superior and medial rectus (SR and MR), superior oblique (SO) and levator palpebrae superioris (LPS) muscles was studied using the myofibrillar ATPase and NADH-tetrazolium reductase (NADH-TR) techniques. In the SR, MR and SO a peripheral zone containing small fibres and a central zone containing both small and large fibres could be identified. Four fibre types were present in the central zone of these muscles and were categorised as Type 1, Type 2a. Type 2a'. and Type 2b. Four fibre types were also identified in the peripheral zone--Type 1, Type 2a, Type 2a' and Type 2c. In the LPS Type 1, Type 2, Type 2a', Type 2b and Type 2c fibres were evenly intermixed without a zonal arrangement. The Type 1, Type 2a, Type 2b and Type 2c fibres correspond to accepted fibres types in the rat limb muscles. The type 2a' and Type 2a" fibres were differentiated from the Type 2a fibres on the basis of size and pattern and intensity of staining with the NADH-TR technique. The Type 2c fibres, which possess both acid-stable and alkali-stable myofibrillar ATPase, are considered to represent intermediate fibres in the process of transformation from Type 2 to Type 1, rather than fibres with a dual innervation as has been suggested in the past. The implications of these histochemical findings are considered in relationship to the physiology of individual motor units and to the function of the extraocular muscles.

Ionic basis for electrical properties of tonic fibres in rat extraocular muscles

1. The ionic conductances underlying some of the electrophysiological properties of multiply innervated or tonic fibres of rat extraocular muscles were examined in vitro with double-barrelled micro-electrodes.2. Exposure of the muscle to a Cl-free saline did not change the effective resistance (R(eff)) of tonic fibres which was 5.14 +/- 0.45 MOmega (n = 7) in control saline and 4.78 +/- 0.45 MOmega (n = 12) in Cl-free saline (P > 0.1). In contrast, in singly innervated or twitch fibres Cl removal increased R(eff) from 1.77 +/- 0.21 MOmega (n = 19) to 2.69 +/- 0.12 MOmega (n = 22) (P < 0.001).3. Tonic fibres with membrane potentials restored to - 80 mV by injecting current responded to intracellular depolarizing pulses with a brief, slow response (slow peak potential) which added to the rising phase of the electrotonic potential. The slow peak potential began at a membrane potential of - 40 to - 35 mV and was graded. Increasing depolarizations evoked faster and larger responses which did not over-shoot the zero level of membrane potential.4. The slow peak potential was not blocked by 10 muM-D-600 hydrochloride but was markedly reduced by the absence of Na and by 10 muM-tetrodotoxin. The response was broadened about five times by 25 mM-tetraethylammonium.5. Raising bath temperature from 21-25 degrees C to 37 degrees C reversibly depressed and shortened the slow peak potential but did not transform it into an action potential.6. It is concluded that the characteristic high R(eff) of tonic fibres results from a lack of a membrane conductance to Cl and that the slow peak potential involves the transient activation of Na and K channels which are pharmacologically similar to the respective channels of twitch fibres.