The transduction properties of diaphragmatic mechanoreceptors

The transduction properties of diaphragmatic mechanoreceptors were studied using an isolated organ preparation. Following localization of their receptive field and receptor characterization, controlled diaphragmatic stretch in 2 mm increments was performed while recording the steady-state firing frequency of these afferents. Of 31 receptors recorded, 14 could be categorized into one of 3 types: (1) muscle spindles, (2) Golgi tendon organs, and (3) pressure-sensitive mechanoreceptors. These receptors were also found to be widely distributed in the diaphragm. Four of the muscle spindles examined were shown to possess a length sensitivity of 4-8 mm with a wide range of maximal discharge. The results of this study suggest that the diaphragm contains mechanoreceptors that transduce muscle length and projects this information regarding respiratory proprioception to the CNS.     

Immunohistochemical localization of neurocalcin in human sensory neurons and mechanoreceptors

The localization of neurocalcin in the developing and adult human peripheral nervous system (dorsal root and sympathetic ganglia (DRG, SG), and enteric nervous system (ENS)) was investigated using immunohistochemistry. A subpopulation of large-sized neurons in DRG of 9 and 12 weeks old embryos showed immunoreactivity (IR), whereas the sympathetic ganglia or enteric neurons did not. In adults, neurocalcin IR was restricted to a subpopulation of large (13%) and intermediate (15%) sized neurons in DRG. The protein was also found in muscular (67%) and cutaneous (12%) nerve fibers, as well as in the axons supplying muscular (muscle spindles, Golgi's tendon organs, and perimysial Pacinian corpuscles) and cutaneous (Meissner's but not Pacinian corpuscles) mechanoreceptors, as well as motor end-plates. Present results demonstrate that neurocalcin in both developing and adult humans can be used as a specific marker for a subpopulation of sensory neurons coupled to proprioception and touch, and for axons of motoneurons forming motor end-plates.     

Topography of muscle spindles and Golgi tendon organs in shoulder muscles of "Monodelphis domestica".

The topography of muscle spindles and Golgi tendon organs in the rotator cuff and surrounding shoulder muscles of a small laboratory marsupial (monodelphis domestica) were studied using light microscopy of serial sections. The shoulder joint of monodelphis has a large degree of freedom of movement allowing this animal to use the upper extremities for a wide range of activities like climbing and manipulating food. Thus, similar to the situation in man the shoulder joint is mainly secured by muscles. Silver stained serial paraffin sections were examined under the light microscope and the distribution of muscle spindles and Golgi tendon organs was reconstructed using three-dimensional image processing. In the two animals examined 113 and 131 muscle spindles respectively were found within the 4 rotator cuff muscles. In addition, 76 and 40 Golgi tendon organs respectively were seen at the musculo-tendinous junctions of these muscles preferentially close to the insertion at the humerus head. Also the surrounding shoulder muscles contain both muscle spindles and Golgi tendon organs in large numbers, but the ratio of Golgi tendon organs per muscle spindle appears to be lower. Number and localization of muscle spindles and Golgi tendon organs suggest, that these receptors are important for both reflex control of shoulder muscle tone as well as monitoring of static position and movement in the shoulder joint.     

The Sensorimotor System, Part II: The Role of Proprioception in Motor Control and Functional Joint Stability

Objective: To discuss the role of proprioception in motor control and in activation of the dynamic restraints for functional joint stability.Data Sources: Information was drawn from an extensive MEDLINE search of the scientific literature conducted in the areas of proprioception, motor control, neuromuscular control, and mechanisms of functional joint stability for the years 1970-1999.Data Synthesis: Proprioception is conveyed to all levels of the central nervous system. It serves fundamental roles for optimal motor control and sensorimotor control over the dynamic restraints.Conclusions/Applications: Although controversy remains over the precise contributions of specific mechanoreceptors, proprioception as a whole is an essential component to controlling activation of the dynamic restraints and motor control. Enhanced muscle stiffness, of which muscle spindles are a crucial element, is argued to be an important characteristic for dynamic joint stability. Articular mechanoreceptors are attributed instrumental influence over gamma motor neuron activation, and therefore, serve to indirectly influence muscle stiffness. In addition, articular mechanoreceptors appear to influence higher motor center control over the dynamic restraints. Further research conducted in these areas will continue to assist in providing a scientific basis to the selection and development of clinical procedures.     

Afferent fibres from muscle receptors in the posterior nerve of the cat's knee joint

Summary The properties of some receptors with afferent fibres in the cat's posterior knee joint nerve have been examined, especially those discharging tonically with the joint in intermediate positions between full flexion and extension. Some of these receptors behave like muscle spindles, and respond to manoeuvres which stretch popliteus muscle. Both in single unit and whole nerve recordings their discharge pauses during a popliteus twitch, and can be strikingly augmented by tetanic stimulation of a number of popliteus fusimotor fibres isolated from ventral root filaments. The action of succinylcholine on these receptors closely resembles its effect on popliteus spindle units with fibres sited normally in the popliteus nerve. Other units with properties suggesting origin from popliteus tendon organs were also observed; their fibres and those of the spindle units conducted at Group I velocity. It is concluded that some afferent fibres from popliteus spindles and possibly tendon organs commonly pursue an aberrant course in the posterior articular nerve of the knee joint.     

Innervation of muscle receptors in the cross-reinnervated soleus muscle of the cat

It has recently been reported (Gregory et al., J. Physiol., 331:367-383, 1982) that cutting a muscle nerve and letting it grow back into the muscle or cross-uniting the muscle with a foreign nerve results in major disruption of the normal response patterns of muscle spindles and tendon organs. Here we report observations on the structure of muscle receptors in cross-reinnervated and self-reinnervated soleus muscles in an attempt to detect abnormalities that might account for their disturbed function. Eight soleus muscles were reinnervated with the extensor digitorum longus nerve for periods up to 449 days and two were self-reinnervated. Following the physiological investigation, the muscle was fixed and stained according to the method of Barker and Ip (J. Physiol., 69:73P-74P, 1963). Spindles and tendon organs were teased from the muscle and photographed. In one cross-reinnervated muscle an attempt was made to isolate all receptors. About two-thirds of the normal number of spindles and tendon organs were found. Three categories of receptor were identified: normal, abnormal, and those having no visible nerve endings. There appeared to be little difference in degree of abnormality of receptors in self- and cross-reinnervated muscles. Of the 180 spindles, 3% were normal, 43% had no visible endings, and 54% had abnormal endings. Of 80 tendon organs, 38% were normally innervated, 33% were without visible innervation, and 29% had abnormal endings. We conclude that following long-term cross-reinnervation and self-reinnervation of soleus there is extensive disruption of the normal innervation pattern of both spindles and tendon organs which could account for their functional abnormalities.     

The distribution of Golgi tendon organs and muscle spindles in masseter and temporalis muscles of the cat.

Golgi tendon organs and muscle spindles were identified in serial sections of the temporalis and masseter muscles of kitten and cats. In the kitten, the position of each receptor was plotted in three-dimensional reconstitutions of the muscles. Seventy-four spindles and twenty tendon organs were identified in the temporalis, all in the region of the insertion into the mandible. Thirty-four spindles and six organs were located at the origin of the masseter muscle. The receptors were in deep portions of both muscles. All tendon organs were found to form complexes with one or more spindles.     

The Sensory Innervation of the Human Pharynx: Searching for Mechanoreceptors

The coordinate neural regulation of the upper airways muscles is basic to control airway size and resistance. The superior constrictor pharyngeal muscle (SCPM) forms the main part of the lateral and posterior walls of the pharynx and typically is devoid of muscle spindles, the main type of proprioceptor. Because proprioception arising from SCPM is potentially important in the physiology of the upper airways, we have investigated if there are mechanical sensory nerve endings substitute for the muscle spindles. Samples of human pharynx were analyzed using immunohistochemistry associated to general axonic and Schwann cells markers (NSE, PGP 9.5, RT‐97, and S100P), intrafusal muscle fiber markers, and putative mechanical sense proteins (TRPV4 and ASIC2). Different kinds of sensory corpuscles were observed in the pharynx walls (Pacini‐like corpuscles, Ruffini‐like corpuscles, spiral‐wharves nerve structures, and others) which are supplied by sensory nerves and express putative mechanoproteins. No evidence of muscle spindles was observed. The present results demonstrate the occurrence of numerous and different morphotypes of sensory corpuscles/mechanoreceptors in human pharynx that presumably detect mechanical changes in the upper airways and replace muscle spindles for proprioception. Present findings are of potential interest for the knowledge of pathologies of the upper airways with supposed sensory pathogenesis. Anat Rec, 296:1735–1746, 2013. © 2013 Wiley Periodicals, Inc.     

Changes in PAD patterns of group I muscle afferents after a peripheral nerve crush

In the anesthetized cat we have analyzed the changes in primary afferent depolarization (PAD) evoked in single muscle spindle and tendon organ afferents at different times after their axons were crushed in the periphery and allowed to regenerate. Medial gastrocnemius (MG) afferents were depolarized by stimulation of group I fibers in the posterior biceps and semitendinosus nerve (PBSt), as soon as 2 weeks after crushing their axons in the periphery, in some cases before they could be activated by physiological stimulation of muscle receptors. Two to twelve weeks after crushing the MG nerve, stimulation of the PBSt produced PAD in all MG fibers reconnected with presumed muscle spindles and tendon organs. The mean amplitude of the PAD elicited in afferent fibers reconnected with muscle spindles was increased relative to values obtained from Ia fibers in intact (control) preparations, but remained essentially the same in fibers reconnected with tendon organs. Quite unexpectedly, we found that, between 2 and 12 weeks after crushing the MG nerve, stimulation of the bulbar reticular formation (RF) produced PAD in most afferent fibers reconnected with muscle spindle afferents. The mean amplitude of the PAD elicited in these fibers was significantly increased relative to the PAD elicited in muscle spindle afferents from intact preparations (from 0.08–0.4 to 0.47-0.34 mV). A substantial recovery was observed between 6 months and 2.5 years after the peripheral nerve injury. Stimulation of the sural (SU) nerve produced practically no PAD in muscle spindles from intact preparations, and this remained so in those afferents reconnected with muscle spindles impaled 2–12 weeks after the nerve crush. The mean amplitude of the PAD produced in afferent fibers reconnected with tendon organs by stimulation of the PBSt nerve and of the bulbar RF remained essentially the same as the PAD elicited in intact afferents. However, SU nerve stimulation produced a larger PAD in afferents reconnected with tendon organs 2–12 weeks after the nerve crush (mean PAD changed from 0.05-0.04 to 0.32-0.17 mV). The results obtained indicate that the PAD patterns of the afferent fibers reconnected with muscle spindle and tendon organ afferents are changed after crushing their axons in the periphery: stimulation of the bulbar RF appears to produce larger PAD in fibers reconnected with muscle spindles, and stimulation of cutaneous afferents produces larger PAD in fibers reconnected with tendon organs. It is suggested that these alterations in the patterns of PAD of muscle afferents result from central changes in the balance of excitatory and inhibitory influences acting on the segmental pathways mediating the PAD. Although the functional role of these changes has not been established, they may reflect compensatory changes aimed to adjust information arising from damaged afferents.     

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.     

Quantitative analysis of static strain sensitivity in human mechanoreceptors from hairy skin.

1. Microelectrode recordings from 15 slowly adapting (SA) cutaneous mechanoreceptor afferents originating in hairy skin were obtained from the radial nerve in humans. 2. Controlled skin stretch was applied to the back of the hand that encompassed the physiological range of skin stretch during movements at the metacarpophalangeal (MCP) joints. 3. Both SA Group I and II afferents showed exquisite dynamic and static sensitivity to skin stretch. The median static strain sensitivity was 1.0 imp.s-1 per percent skin stretch for SAI units and 1.8 for SAII units. 4. Translated into sensitivity to movements at the MCP joint, both SAI and SAII afferents in the skin of the back of the hand displayed a positional sensitivity that was comparable with that reported for muscle spindle afferents. 5. These data give quantitative support to suggestions that skin receptors in the human hairy skin provide information on nearby joint configurations and therefore may play a specific role in proprioception, kinesthesia, and motor control.     

Proportions of slow-twitch and fast-twitch extrafusal fibers in receptive fields of tendon organs in chicken leg muscles

Golgi tendon organs are mechanoreceptors that monitor the contractile force produced by motor units. Receptors are most responsive to contractions of extrafusal muscle fibers that terminate closest to them and on them. Three anterior and four posterior chicken leg muscles were examined. Proportions of immunohistochemically identified slow-twitch extrafusal fibers and fast-twitch extrafusal fibers were calculated for 374 tendon organ receptive fields. Tendon organs were observed in muscle regions occupied either by slow-twitch fibers or fast-twitch fibers only, but most were found in regions that contained both slow-twitch and fast-twitch extrafusal fibers. The frequency with which each fiber type occurred near tendon organs approached the frequency with which it occurred in more inclusive regions. In receptive fields with mixed fiber populations, fast-twitch fibers were the predominant type, especially in the anterior leg muscles. Distribution patterns of extrafusal fiber types adjacent to and farther removed from tendon organs suggest that afferent discharges from tendon organs are by and large unbiased measures of the contractile activity of the extrafusal fiber population of the muscle portion in which the tendon organs are located. In mixed muscle regions, slow-twitch fibers and fast-twitch fibers attach on given tendon organs, enabling them to monitor forces produced by slow motor units and by fast motor units. Most tendon organs are situated in mixed extrafusal fiber fields with high fast-twitch fiber content, indicating that in chicken leg muscles sensory feedback from tendon organs is largely one from fast motor units. Anat. Rec. 252:34–40, 1998. © 1998 Wiley-Liss, Inc.     

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.     

Segmental and supraspinal control of synaptic effectiveness of functionally identified muscle afferents in the cat

The present investigation documents the patterns of primary afferent depolarization (PAD) of single, functionally identified muscle afferents from the medial gastrocnemius nerve in the intact, anesthetized cat. Classification of the impaled muscle afferents as from muscle spindles or from tendon organs was made according to several criteria, which comprised measurement of conduction velocity and electrical threshold of the peripheral axons, and the maximal frequency followed by the afferent fibers during vibration, as well as the changes in discharge frequency during longitudinal stretch, the projection of the afferent fiber to the motor pool, and, in unparalyzed preparations, the changes in afferent activity during a muscle twitch. In confirmation of a previous study, we found that most muscle spindle afferents (46.1–66.6%, depending on the combination of criteria utilized for receptor classification) had a type A PAD pattern. That is, they were depolarized by stimulation of group I fibers of the posterior biceps and semitendinosus (PBSt) nerve, but not by stimulation of cutaneous nerves (sural and superficial peroneus) or the bulbar reticular formation (RF), which in many cases inhibited the PBSt-induced PAD. In addition, we found a significant fraction of muscle spindle primaries that were depolarized by stimulation of group I PBSt fibers and also by stimulation of the bulbar RF. Stimulation of cutaneous nerves produced PAD in 9.1–31.2% of these fibers (type B PAD pattern) and no PAD in 8.2–15.4% (type C PAD pattern). In contrast to muscle spindle afferents, only the 7.7–15.4% of fibers from tendon organs had a type A PAD pattern, 23–46.1% had a type B and 50–61.5% a type C PAD pattern. These observations suggest that the neuronal circuitry involved in the control of the synaptic effectiveness of muscle spindles and tendon organs is subjected to excitatory as well as to inhibitory influences from cutaneous and reticulospinal fibers. As shown in the accompanying paper, the balance between excitation and inhibition is not fixed, but can be changed by crushing the afferent axons in the peripheral nerve and allowing subsequent reconnection of these afferent fibers with muscle receptors.     

The effect of high-frequency cutaneous vibration on different inputs subserving detection of joint movement

Stimuli that preferentially activate rapidly adapting cutaneous receptors impair proprioception in the fingers. These experiments assessed potential mechanisms. The ability to detect passive movements about interphalangeal joints of the fingers was measured when vibrotactile stimuli were applied to the moving digit or to an adjacent digit at a high frequency (300 Hz) and low amplitude (50 μm peak-to-peak) that favours activation of Pacinian corpuscle (PC) afferents. Detection of movement was significantly impaired when vibration was applied to either digit. However, vibration applied to an anaesthetized adjacent digit caused no impairment. Impairment of proprioception was still observed when only skin and joint (but not muscle) afferents could contribute to detection, but was not significant with only muscle afferents intact during anaesthesia of the moving digit. We suggest that activation of PC afferents, either in or near the moving digit, impairs movement detection through an interaction predominantly between the classes of cutaneous afferents.     

A quantitative study of muscle spindles and tendon organs in some intrinsic muscles of the hand in the bonnet monkey (Macaca radiata)

The number and density of muscle spindles and tendon organs have been determined in the following intrinsic muscles of the hand of bonnet monkeys: I lumbrical, II lumbrical, abductor digiti minimi, adductor pollicis, and I dorsal interosseous. All these muscles were found to be very rich in muscle spindles (17.6 to 42.31 per gram wet weight) but relatively poor in tendon organs (0.606 to 10.06 per gram wet weight). The lumbricals have very few tendon organs. The possible functional significance of these findings has been discussed.     

The role of muscle receptors in the detection of movements

This review discusses the role of muscle receptors, in particular, that of muscle spindles, in the detection of movements, both passive and active. Emphasis is placed on the importance of conditioning the muscles acting at a joint before making measurements of thresholds to passive movements, to take into account muscle's thixotropic property. The detection threshold:movement velocity relation is discussed and described for a number of different joints. Implications for muscle spindles are considered from the generalisation that, when expressed in terms of proportion of muscle fascicle length change, detection thresholds are about the same at different joints. It is concluded that the available data supports the view that muscle spindles lie in parallel with only a portion of a muscle fascicle and not the whole fascicle. At the elbow joint, where it has been tested, movement detection threshold is lower during passive movements than during contraction of elbow muscles. Both peripheral mechanisms and mechanisms operating within the central nervous system may be responsible for the rise in threshold. The signalling of movements by spindles during a contraction raises the question of how the central nervous system is able to extract the length signal under such circumstances, given that there is likely to be co-activation of alpha and gamma motoneurones. The evidence for a central subtraction of fusimotor-evoked impulses and some recent experiments relevant to this idea are described. In conclusion, a number of points of uncertainly have been revealed in this area and these should be the subject of future experiments.     

Characterization of sensory deficits in TrkB knockout mice

The sensory deficit in TrkB deficient mice was evaluated by counting the neuronal loss in lumbar dorsal root ganglia (DRG), the absence of sensory receptors (cutaneous—associated to the hairy and glabrous skin - muscular and articular), and the percentage and size of the neurocalcin-positive DRG neurons (a calcium-binding protein which labels proprioceptive and mechanoceptive neurons). Mice lacking TrkB lost 32% of neurons, corresponding to the intermediate-sized and neurocalcin-positive ones. This neuronal lost was accomplished by the absence of Meissner corpuscles, and reduction of hair follicle-associated sensory nerve endings and Merkel cells. The mutation was without effect on Pacinian corpuscles, Golgi's organs and muscle spindles. Present results further characterize the sensory deficit of the TrkB−/− mice demonstrating that the intermediate-sized neurons in lumbar DRG, as well as the cutaneous rapidly and slowly adapting sensory receptors connected to them, are under the control of TrkB for survival and differentiation. This study might serve as a baseline for future studies in experimentally induced neuropathies affecting TrkB positive DRG neurons and their peripheral targets, and to use TrkB ligands in the treatment of neuropathies in which cutaneous mechanoreceptors are primarily involved.     

Muscle spindle discharge in response to contraction of single motor units

1. In human subjects, microelectrode recordings were made from 25 muscle spindle afferents and two tendon organ afferents coming from muscles innervated by the peroneal nerve. 2. Stimulation at low intensity through the recording microelectrode activated efferent axons innervating motor units in close proximity to the muscle spindle or tendon organ. There was a clear alteration in the discharge of 17 afferents (15 muscle spindle, 2 tendon organ) in response to twitch contractions that involved only one, two, or three motor units. With three other afferents there was a less overt but statistically significant alteration in discharge rate by the twitch contraction of a single motor unit. 3. The sensitivity of 21 receptors (20 spindles, 1 tendon organ) to twitch contractions of anatomically close motor units was contrasted with their sensitivity to twitches of more remote motor units in the muscle. In no instance was the sensitivity to the contraction of remote motor units greater than that to the contraction of local motor units stimulated through the microelectrode; with remote stimulation many units usually had to be activated before the resulting twitch contraction altered the discharge of an afferent. 4. It is concluded that muscle spindles as well as tendon organs can play a role in monitoring the activity of motor units anatomically close to the receptor.     

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.