Unilateral suppression of pharyngeal motor cortex to repetitive transcranial magnetic stimulation reveals functional asymmetry in the hemispheric projections to human swallowing

Age-related physiological and morphological changes of muscle spindles were examined in rats (male Fischer 344/DuCrj: young, 4–13 months; middle-aged, 20–22 months; old, 28–31 months). Single afferent discharges of the muscle spindles in gastrocnemius muscles were recorded from a finely split dorsal root during ramp-and-hold (amplitude, 2.0 mm; velocity, 2–20 mm s⁻¹) or sinusoidal stretch (amplitude, 0.05–1.0 mm; frequency, 0.5–2 Hz). Respective conduction velocities (CVs) were then measured. After electrophysiological experimentation, the muscles were dissected. The silver-impregnated muscle spindles were teased and then analysed using a light microscope. The CV and dynamic response to ramp-and-hold stretch of many endings were widely overlapped in old rats because of the decreased CV and dynamic response of primary endings. Many units in old rats showed slowing of discharge during the release phase under ramp-and-hold stretch and continuous discharge under sinusoidal stretch, similarly to secondary endings in young and middle-aged rats. Morphological studies revealed that primary endings of aged rat muscle spindles were less spiral or non-spiral in appearance, but secondary endings appeared unchanged. These results suggest first that primary muscle spindles in old rats are indistinguishable from secondary endings when determined solely by previously used physiological criteria. Secondly, these physiological results reflect drastic age-related morphological changes in spindle primary endings.     

An analysis of muscle spindle behavior using randomly applied stretches

The behavior of mammalian muscle spindles to relatively large amplitude randomly applied stretches is compared to the linear behavior elicited by small amplitude sinusoidal stretches. The results disclose two apparently independent sources of nonlinear behavior. One is a static nonlinearity affecting both primary and secondary endings in which the sensitivity decreases with stretch amplitude up to about 1% stretch. The other is a ‘dynamic’ nonlinearity affecting only primary endings that is manifest as a charge in the ratio of rate to proportional sensitivity depending on the duration of an applied stretch.     

The effects of muscle cooling and stretch on muscle spindle secondary endings in the cat.

1. The objectives of the investigation were to identify the muscle spindle endings which respond to cooling of the relaxed muscle and to study their response to stretch. 2. The discharge of single afferents from 162 de-efferented muscle spindles in the relaxed medial gastrocnemius muscle of the anaesthetized cat was studied in vivo during cooling of the muscle from 37 to 24 degrees C. Temperature measurements were made at the inner surface of the muscle, while cooling (never below 15 degrees C) was applied at the skin over the muscle. 3. The endings were classified as primary or secondary endings on the basis of their conduction velocity, the dividing line being set at 70 m/sec. A response to cooling was obtained only from endings with afferents conducting at velocities of 20-70 m/sec. These fifty-six endings (CR) represented 65% of the secondary endings studied; the remaining secondary endings (NCR) and the primary endings showed no activity during cooling of the relaxed muscle. 4. During maintained stretches of 4-12 mm, activity of the NCR and primary endings decreased when the muscle was cooled. Cooling affected the CR endings in the same way, but only if the muscle was stretched 6 mm or more. During a smaller maintained muscle stretch, cooling caused an increase in CR activity, superimposed on the response to stretch. 5. The response to a 10 mm stretch at velocities of 10-70 mm/sec was studied in twenty-six CR, eleven NCR and twenty-one primary endings. 6. The dynamic responses of CR endings were intermediate between those of the primary endings and NCR endings. For any velocity of stretch the mean dynamic index of the CR endings was significantly greater than that of the NCR endings but significantly less than that of the primary endings. 7. The mean static responses of the CR and primary endings, measured 0-5 sec after the end of ramp stretch, were the same and significantly greater than that of the NCR endings. 8. The results indicate that cooling of the relaxed mammalian muscle may be used to differentiate between primary endings and about two-thirds of the secondary endings. The remaining secondary endings can be recognized by their small dynamic and static response to stretch.     

Initial burst of primary endings of isolated mammalian muscle spindles.

The initial burst has been studied in primary endings of isolated mammalian muscle spindles subject to controlled ramp-and-hold stretch. Near the onset of ramp stretch the primary ending discharges at a frequency dependent on stretch velocity. The initial burst is reduced or abolished by repetitive stretch. After block of impulse activity by tetrodotoxin, the receptor potential of primary endings shows an initial component, a rapid depolarization which occurs near the onset of ramp stretch at the same time as the initial burst. This initial component depends, in rate of rise and amplitude, on stretch velocity. It is also reduced or abolished by repetitive stretch. Recording of tension development by the isolated spindle in response to ramp-and-hold stretch shows an early rise in tension associated with the initial burst and the initial component of the receptor potential. This tension rise is also dependent on stretch velocity and is reduced or abolished by repetitive stretch. The results provide direct evidence that the initial burst results from mechanical factors, probably from cross bridge formation between thick and thin filaments as has been suggested (3).     

The sensory reinnervation of hind limb muscles of the cat following denervation and de-efferentation.

The sensory reinnervation of muscle spindles following lesions of the peripheral nerve was studied in hind limb muscles of the cat. Earlier results reporting complete redevelopment of both primary and secondary endings were confirmed.However, after section of the ventral roots reinnervation of muscle spindles was impaired in that many primary endings did not develop the spiral-like structures and their appearance remained abnormal for up to 120 days. The response to stretch in two-thirds of such de-efferented regenerated primary endings was also abnormal. Although the phasic and vibration responses were present, the slowly adapting part of the response to maintained stretch was defective or absent in many of the primary endings.From these results it appears that motor innervation of the muscle is important for the normal redevelopment of the complex structure and function of the primary ending of the muscle spindle during reinnervation. The results do not indicate whether de-efferentation causes a permanent impairment or only a delay in redevelopment.     

After-effects of stretch on the responses of cat soleus muscle spindles to static fusimotor stimulation

Summary Mammalian muscle spindles show persistent after-effects following conditioning stretch or fusimotor stimulation. Most previous observations have been carried out on primary endings of spindles and using dynamic fusimotor stimulation. We report here observations on after-effects produced either by conditioning stretch or by static fusimotor stimulation on the responses of primary and secondary endings to a slow test stretch during which a brief burst of static fusimotor stimulation is applied. We find that the response to the test burst is large if the muscle is kept short after conditioning but it becomes depressed if the muscle is held stretched for 3 s following conditioning. We attribute these effects to the presence of stable cross-bridges between actin and myosin filaments in intrafusal fibres. We conclude that, qualitatively, after-effects using static fusimotor testing are the same as with dynamic fusimotor testing and this must be taken into account when providing an explanation for the phenomenon.     

The response to stretch of human intercostal muscle spindles studied in vitro.

1. The discharge properties of human muscle spindles have been studied in vitro in a preparation based on the biopsied external intercostal muscle. 2. The static and dynamic responsiveness of thirty-six endings in twenty visualized and histologically identified spindles have been investigated using amplitudes and velocities of stretch likely to encompass those occurring in vivo. 3. The dynamic index, measured at a stretch velocity of 3 mm/sec, ranged from 3 to 40 impulses/sec and was distributed bimodally, consitent with the presence of primary and secondary endings. 4. The relationship between the dynamic index and the velocity of stretch was approximately linear both for primary and secondary endings up to the maximum velocity tested (10 mm/sec). 5. The frequency/extension relationship was approximately linear for both primary and secondary endings. The mean values of the slope for primary and secondary endings were 16-1 +/- 8-3 S.D. of the observation and 12-1 +/- 6-5 impulses/sec per five per cent extension. 6. The slopes of the frequency/extension relationship for endings lying in the same spindle were positively correlated, significant at the 10% level. 7. It was estimated from the results in vitro that the position sensitivity of human intercostal spindles in vivo ranges from 2 to 21 impulses/sec per millimetre.     

Effects of fine-grained diet on annulospiral endings of muscle spindles in the masseter muscle in developing and adult mice

In the masticatory muscles, neuromuscular spindles have a very important role in controlling the jaw movement since they act as stretch receptors in skeletal muscles. The continuous intake of fine-grained diet which is easily chewable leads to degeneration of the sensory endings of Ia fibers in many muscle spindles of the mouse masseter muscle in only 120 d after birth.     

Modulating mechanosensory afferent excitability by an atypical mGluR

Mechanotransduction by proprioceptive sensory organs is poorly understood. Evidence was recently shown that muscle spindle and hair follicle primary afferents (lanceolates) constantly release glutamate from synaptic‐like vesicles (SLVs) within the terminals. The secreted glutamate activates a highly unusual metabotropic glutamate receptor (mGluR) to modulate the firing rate (spindles) and SLV recycling (lanceolates). This receptor has yet to be isolated and sequenced. To further investigate this receptor's pharmacology, ligands selective for classical mGluRs have been recently characterised for their ability to alter stretch‐evoked spindle firing and SLV endocytosis in these different endings. Here, it is described how the results of these screens facilitated the development of novel compounds to be used in the process of isolating and sequencing of this non‐canonical mGluR. This study shows how the compounds were tested for their ability to alter stretch‐evoked afferent firing in muscle spindles and SLV endocytosis in the lanceolate endings of hair follicles to ensure they maintained their ability to bind to the receptor. For the development of novel compounds, kainate was chosen as the parent ligand due to its potency and ease of chemical modification. Novel kainate derivatives were then synthesised and tested to find potent analogues suitable for ‘click‐chemistry’, an established technique for relatively quick, cheap, stereospecific and high‐yield chemical modifications (Angewandte Chemie (International ed. in English), 40, 2001, pp2004). Of the novel kainate analogues developed, unfortunately ZCZ49 and ZCZ50 lost the ability to produce a significant change in spindle stretch‐evoked firing. However, ZCZ90 was as potent as kainate, increasing firing by a similar margin at 1 μm (n = 8; P < 0.001). The addition of either a biotin or a fluorescein side group to ZCZ90, using the click‐chemistry technique, did not affect the potency and hence these compounds will be used in further studies of the receptor. As well as the development of these compounds, the study found not only many similarities, but also some key differences between the two types of primary mechanosensory endings investigated. These differences must be taken into account in further study. However, they also present an intriguing opportunity for these receptors to be targeted selectively to modulate ending sensitivity as treatments for muscle spasm in multiple sclerosis and spinal cord injury, and possibly even baroreceptor firing to treat hypertension.     

A comparative analysis of the encapsulated end-organs of mammalian skeletal muscles and of their sensory nerve endings

The encapsulated sensory endings of mammalian skeletal muscles are all mechanoreceptors. At the most basic functional level they serve as length sensors (muscle spindle primary and secondary endings), tension sensors (tendon organs), and pressure or vibration sensors (lamellated corpuscles). At a higher functional level, the differing roles of individual muscles in, for example, postural adjustment and locomotion might be expected to be reflected in characteristic complements of the various end‐organs, their sensory endings and afferent nerve fibres. This has previously been demonstrated with regard to the number of muscle‐spindle capsules; however, information on the other types of end‐organ, as well as the complements of primary and secondary endings of the spindles themselves, is sporadic and inconclusive regarding their comparative provision in different muscles. Our general conclusion that muscle‐specific variability in the provision of encapsulated sensory endings does exist demonstrates the necessity for the acquisition of more data of this type if we are to understand the underlying adaptive relationships between motor control and the structure and function of skeletal muscle. The present quantitative and comparative analysis of encapsulated muscle afferents is based on teased, silver‐impregnated preparations. We begin with a statistical analysis of the number and distribution of muscle‐spindle afferents in hind‐limb muscles of the cat, particularly tenuissimus. We show that: (i) taking account of the necessity for at least one primary ending to be present, muscles differ significantly in the mean number of additional afferents per spindle capsule; (ii) the frequency of occurrence of spindles with different sensory complements is consistent with a stochastic, rather than deterministic, developmental process; and (iii) notwithstanding the previous finding, there is a differential distribution of spindles intramuscularly such that the more complex ones tend to be located closer to the main divisions of the nerve. Next, based on a sample of tendon organs from several hind‐foot muscles of the cat, we demonstrate the existence in at least a large proportion of tendon organs of a structural substrate to account for multiple spike‐initiation sites and pacemaker switching, namely the distribution of sensory terminals supplied by the different first‐order branches of the Ib afferent to separate, parallel, tendinous compartments of individual tendon organs. We then show that the numbers of spindles, tendon organs and paciniform corpuscles vary independently in a sample of (mainly) hind‐foot muscles of the cat. Grouping muscles by anatomical region in the cat indicated the existence of a gradual proximo‐distal decline in the overall average size of the afferent complement of muscle spindles from axial through hind limb to intrinsic foot muscles, but with considerable muscle‐specific variability. Finally, we present some comparative data on muscle‐spindle afferent complements of rat, rabbit and guinea pig, one particularly notable feature being the high incidence of multiple primary endings in the rat.     

Afferent innervation of extraocular muscles in the rat studied by retrograde and anterograde horseradish peroxidase transport

After injection of horseradish peroxidase (HRP) into extraocular muscles of rat perikarya were labeled mainly along the medial edge of the ophthalmic subdivision of the trigeminal ganglion but not in the mesencephalic nucleus of the trigeminal nerve. Injections of HRP into the trigeminal ganglion labeled simple as well as branching and meandering free fiber endings in extraocular muscles. No evidence for muscle spindles was found, but the meandering endings may be considered as candidates for stretch receptors.     

Physiological properties of muscle spindles in dorsal neck muscles of the cat.

1. Single-fiber recording was used to examine the properties of 107 spindle endings in cat biventer cervicis (BC) and complexus (CM) muscles. Responses of receptors were examined following muscle contraction and ramp and hold stretch. Twenty-two endings in splenius (SP) were also examined, but their responses could not be quantitated because the anatomy of SP prevented the application of appropriate stretches. 2. Conduction velocitites of spindle afferents ranged from 13 to 90 m/s. Endings with primary response patterns usually had faster conduction velocities than secondary endings, but there was overlap in the conduction velocity ranges of the two subgroups. 3. Most neck spindle afferents could be classified as either primary or secondary by a constellation of physiological criteria including dynamic response pattern, dynamic index, and variability of resting discharge frequency. However, 22 of 107 endings from BC and CM had responses with characteristics intermediate between primary and secondary responses. The possible sources of these characteristics are discussed. 4. Despite the similarity in properties between spindles of different neck muscles, the length sensitivities of CM spindles were high compared to those of BC spindles. CM spindles showed length-related modulation of firing frequency over a more restricted range of initial muscle lengths than did BC spindles. 5. Eight Golgi tendon organs (GTO) were identified by their characteristics responses. Conduction velocities obtained for five GTO afferent nerves ranged from 50 to 67 m/s. Recordings were also made from receptros in deep muscles surrounding the vertebrae. These receptors had properties characteristic of muscle spindles.     

A `late supernormal period' in the recovery of excitability following an action potential in muscle spindle and tendon organ receptors

1. Discharge patterns have been recorded from five types of stretch receptor; frog muscle spindles, lizard tendon organs, cat soleus tendon organs and primary and secondary endings of cat soleus muscle spindles.2. The fully adapted discharge of each type of receptor is irregular, especially for frog spindles and primary endings of cat spindles as compared with the other three types (the ;regularly firing' receptors). Frog spindles and some cat spindle primary endings would maintain a discharge at very low mean rates (1/sec or less) while the remaining receptors would stop suddenly, as soon as their rate of discharge fell below a critical value characteristic for each individual ending.3. This pattern of discharge suggests that there is a peak in the excitability of ;regularly firing' receptors at a time following a preceding impulse, which corresponds to the intervals between impulses at each particular receptor's slowest rate of maintained firing, and that the excitability subsequently falls again. Primary endings of cat muscle spindles also showed some evidence of such a ;late supernormal period', but frog spindles did not.4. Direct evidence for the ;late supernormal period' was obtained from experiments in which a maintained discharge was restarted by an antidromic action potential in a receptor which had stopped firing, and to which had been applied a stretch just too small to restart the discharge.5. It is shown in an Appendix that a model receptor in which the recovery of excitability following an impulse has a hyperbolic time course, and in which Gaussian distributed noise is superimposed on the generator potential, can have a discharge pattern very closely resembling that of a frog spindle (cf. Buller, 1965).6. After addition of a late supernormal period to the model, its discharge pattern could mimic closely that of a lizard or cat tendon organ, or of a secondary ending of a cat spindle.     

Receptor potential and spike initiation in two varieties of snake muscle spindles.

1. Receptor potentials, in response to ramp-and-hold stretch, have been recorded from two varieties of snake muscle spindles. 2. The two types of spindles have a similar sensitivity of impulse discharge to amplitude of receptor potential during the static phase of stretch. 3. Receptor potentials from short-capsule spindles show a high dynamic sensitivity to velocity of stretch. Amplitude of dynamic receptor potentials is well related to frequency of dynamic discharge except beyond a certain velocity of stretch where the frequency deviates progressively more than expected from linearity. 4. Receptor potentials from long-capsule spindles show a low dynamic sensitivity to velocity of stretch and amplitude of dynamic receptor potentials is well correlated with dynamic firing frequency. 5. The threshold level of receptor potential for initiating spike discharge varies with the velocity of stretch, the relation being similar for the two types of spindles. 6. It is concluded that the basis for functional differentiation of snake spindles may lie in the mechanism by which deformation of sensory endings is transformed into receptor potential. 7. Late adaptation of impulse discharge, a characteristic feature of the response of the short-capsule spindle to maintained stretch, has been related to length changes of the sensory region measured directly with Nomarski optics. The linear relation found between the slow adaptive fall of impulse discharge and the simultaneous shortening of the sensory region strongly suggests a mechanical basis for the late adaptation.     

Ultrastructural identification of the primitive muscle spindle in the Xenopus laevis larvae

Summary The initial formation of muscle spindles was studied with electron microscopy using the toe muscle of Xenopus laevis. At the larval stage 57 (Nieuwkoop and Faber 1967), muscle spindles were first identified primarily by the presence of sensory endings associated with a thin bundle of myotubes, e.g. intrafusal (IF) myotubes which were partly invested by a single cellular layer. The number of IF myotubes per spindle was 5 to 6; the adult complement. IF-and extrafusal (EF) myotubes were almost identical in their size and structure. A few thinner IF myotubes with scaree myofibrils were also present. The reticular zone had been undeveloped. Sensory endings were smaller in size and in number per spindle than those in the adult, forming irregular beaded chains with occasional tubular expansions. The endings and IF myotubes were rarely in direct contact, being frequently interposed by a satellite cell and its process. Incipient fusimotor endings were widely distributed from the juxta-equatorial to the polar region. Large cored vesicles resembling the neurosecretory vesicles occurred in sensory and motor endings as well as in intramuscular nerve fibers. The vesicles may be involved in the neuronal influence upon the spindle differentiation.The results were compared with the formative process of mammalian spindles.     

Structures in sensory region of snake spindles and their displacment during stretch.

1. Structures within the sensory region of short- and long-capsule snake muscle spindles have been visualized using differential interference contrast microscopy. Profiles seen with Nomarski microscopy have been identified by electron microscopy of the same preparations. 2. Sensory nerve terminals, nuclei and other cytoplasmic inclusions in the intrafusal fiber, collagen bands, and capsular cells may be seen in the living preparation. 3. The length changes of various elements within the sensory region in response to stretch of the spindle have been measured using high-speed ciné photomicrography. This has been corrleated with the impulse response from sensory endings in short-and long-capsule spindles. 4. Short-capsule spindles, which have a high dynamic sensitivity, show length changes in the sensory region in response to ramp-and-hold stretch, which are not parallel to the changes in impulse frequency. The implications for mechanical models of spindle behavior are discussed.     

A model of spindle afferent response to muscle stretch

1. A unified model of the properties of stretch responses of mammalian spindle endings is proposed. This model encompasses the disparity between sensitivity of spindle endings to small and to large stretch of the muscle as well as the disparity in their dynamic responsiveness for different amplitudes of stretch. 2. In the model the mechanical properties of intrafusal fibers include a property akin to friction, which is hypothesized on the basis of reported observations on amphibian muscle. Transducer and encoder processes are modeled in the light of recent observations on isolated spindles. The model involves five unknown parameters whose values are selected by reference to certain reported observations on deefferented primary and secondary endings. The model can be used to predict responses to length changes of arbitrary time course. 3. Predicted responses to large ramp-and-hold stretch are quantitatively comparable to observations over a wide range of stretch velocities. The quantities compared include the increment in response during ramp stretch as well as the dynamic index, which is a measure of adaptation at stretch plateau. 4. At a fixed frequency of sinusoidal stretch, the relation between amplitudes of stretch and response is predicted in quantitative agreement with measurements. As the frequency of stretch is decreased, the predicted phase lead decreases and then increases, while the sensitivity decreases monotonically, in accord with observations. 5. In the model the high sensitivity for small stretch is not specific to any particular length of the muscle. When stretch is large, the region of high sensitivity is gradually reestablished at the new length, a phenomenon referred to as resetting. The dynamic response to a large stretch can be seen as arising, for the most part, from the dynamic process of resetting. 6. The influences of static or dynamic fusimotor activation on stretch responses of the primary ending are simulated by modifying the parameter values in the model. The modifications are such that static (dynamic) fusimotor activity speeds up (slows down) the resetting of the high-sensitivity region. The predictions mimic qualitatively the observed fusimotor effects not only on the response to large ramp stretch but also the contrasting effects seen with smaller, sinusoidal stretch.     

The responses of secondary endings of cat soleus muscle spindles to succinyl choline

This report describes the effects of succinylcholine (SCh) on the secondary endings of cat soleus muscle spindles and attempts to explain them in terms of the action of the drug on intrafusal fibres. All but 2 of 41 secondary endings studied in detail showed a significant response to a single intravenous injection of 200 g kg^-1 SCh. This consisted of a rise in the resting rate or development of a resting discharge if the spindle had previously been silent and an increase in the response to stretch. The increases in the responses to stretch were weaker than those observed for primary endings of spindles, but were much larger than those of tendon organs, which showed very little effect with this concentration of drug. The response to SCh showed two features consistent with its action being mediated via an intrafusal muscle fibre contraction rather than a direct depolarising action on the afferent nerve ending. In the presence of SCh, secondary endings were able to maintain a discharge during muscle shortening at rates, on average, more than 5 times greater than under control conditions. Secondly, the increase in spindle discharge produced by SCh showed a length dependence similar to that for fusimotor stimulation. Further support for the action of SCh being principally via an intrafusal fibre contraction was provided by the observation that its effects were abolished by the neuromuscular blocker gallamine triethiodide. The time course of recovery of SCh responses, following their blockade by gallamine, was much slower than recovery of extrafusal tension and closely paralleled that for the recovery of fusimotor responses. In three separate experiments on the medial gastrocnemius muscle the possibility that SCh may exert an excitatory action on spindle sensory endings through the liberation of potassium ions from the muscle was tested by tetanic stimulation of the muscle. This had no detectable excitatory effect. Several observations were made on the effect of SCh on responses of cutaneous receptors. SCh did not change levels of spontaneous activity or responses to mechanical stimulation of either slowly or rapidly adapting mechanoreceptors. It was argued for both tendon organs and cutaneous receptors that if SCh had a direct action on the nerve ending at the concentrations used here, some responses of these receptors to the drug might have been expected. All of the above supports the view that secondary endings of spindles are able to respond to SCh by the development of an intrafusal fibre contracture. The question of the intrafusal fibre types involved is discussed.     

Sympathetic innervation of human muscle spindles

The aim of the present study was to investigate the presence of sympathetic innervation in human muscle spindles, using antibodies against neuropeptide Y (NPY), NPY receptors and tyrosine hydroxylase (TH). A total of 232 muscle spindles were immunohistochemically examined. NPY and NPY receptors were found on the intrafusal fibers, on the blood vessels supplying muscle spindles and on free nerve endings in the periaxial space. TH‐immunoreactivity was present mainly in the spindle nerve and vessel. This is, to our knowledge, the first morphological study concerning the sympathetic innervation of the human muscle spindles. The results provide anatomical evidence for direct sympathetic innervation of the intrafusal fibers and show that sympathetic innervation is not restricted to the blood vessels supplying spindles. Knowledge about direct sympathetic innervation of the muscle spindle might expand our understanding of motor and proprioceptive dysfunction under stress conditions, for example, chronic muscle pain syndromes.