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.     

The responses of muscle spindles in sheep extraocular muscles.

1. Responses from de-efferented muscle spindles in sheep extraocular muscles have been recorded in the Gasserian ganglion using glass microelectrodes. 2. The afferent fibre conduction velocities ranged from 32 to 106 m/sec and most were intermediate. The distribution was unimodal. 3. The afferent responses to ramp stretches and longitudinal vibration of the muscle could not be divided into two groups, but both the dynamic indices and vibration sensitivities of the afferents increased with their conduction velocities. 4. Injection of suxamethonium did not help to classify afferents with intermediate conduction velocity. 5. It is concluded that the afferents from sheep extraocular muscle spindles form a single population whose properties are correlated with their nerve fibre diameters.     

Fusimotor Effects on Position and Velocity Sensitivity of Spindle Endings in the External Intercostal Muscle of the Gat

Position and velocity responses of spindle endings in the external intercostal muscle of the cat have been determined in the absence and in the presence of fusimotor fibre stimulation at a constant rate. The length input signal had triangular wave form. A close functional resemblance was found between intercostal and leg muscle spindles: primary and secondary endings could be separated among intercostal afferents with the same methods that have been used in leg muscles; dynamic and static fibres were discerned and the fusimotor effects on position and velocity sensitivity were the same as in the hind limb; the ratio between dynamic and static fibres was about 1: 3, i.e. the same as in the hind leg. Some dissimilarities existed: the number of ‘intermediate’ endings was larger in the intercostal than in the leg muscles; position and velocity sensitivities were generally larger in intercostal endings. These discrepancies are discussed. As a general conclusion it is stated that a spindle model elaborated from studies on leg spindles can easily be modified to account also for intercostal spindle behaviour.     

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.     

Classification and response characteristics of muscle spindle afferents in the primate.

A study was made of the response characteristics of spindle afferents in the baboon soleus muscle. Afferents were isolated from the dorsal roots, their conduction velocities were determined, and their responses were recorded to muscle stretch at rates of 2.5-45 mm/s and amplitudes of 2-10 mm. Spindle afferents could be classified as primary or secondary on the basis of two criteria. The first criterion was conduction velocity. The conduction velocity histogram was bimodal, with peaks at about 45 and 80 m/s and an intermediate region from 55 to 70 m/s. The second criterion was the pattern of adaptation following the peak of ramp stretch. This latter criterion has the advantage of allowing units with intermediate conduction velocities also to be confidently classified as primary or secondary. The velocity and position sensitivities of primate spindle afferents were determined. The mean dynamic index and mean dynamic sensitivity of secondary afferents were about 45% of the corresponding values for primary afferents. On the other hand, the position sensitivities of primary and secondary spindle afferents in the baboon were not significantly different.     

Neck muscle spindle activity in the decerebrate, unparalyzed cat: dynamics and influence of vestibular stimulation

1. Using floating electrodes, we recorded from neck-muscle spindle afferents in the C2 dorsal root ganglion of the decerebrate cat. Nerves to dorsal neck muscles were cut so that the afferents presumably originated mainly from ventral and ventrolateral perivertebral muscles and sternocleidomastoid. One goal of our experiments was to study possible vestibular influence exerted on these spindles via the fusimotor system. Unparalyzed preparations were therefore used. 2. Stimuli consisted of sinusoidal rotations in vertical planes. Neck tilt stretched neck muscles, whereas whole-body tilt stimulated vestibular receptors. 3. For each afferent we first determined the most effective direction of neck tilt, then used stimuli oriented close to this direction to study response dynamics, particularly gain of responses to stimuli of different amplitudes (0.5-7.5 degrees). 4. Three-quarters of the afferents failed to respond to 0.5 degrees, 0.2-Hz neck rotations. Stimuli that were effective usually elicited responses that had low gain and were linear over the whole range of amplitudes. Only a few afferents had behavior typical of spindle primary afferents: high-gain responses to small sinusoidal stimuli, gain decreasing as stimulus amplitude increases. This prevalence of static spindle responses in the unparalyzed cat is in striking contrast to results obtained on neck-muscle spindles in paralyzed, decerebrate cats, and on hindlimb extensor muscle spindles in decerebrate, unparalyzed cats. 5. Paralysis produced by injection of Flaxedil changed the behavior of 2/4 spindle afferents tested, causing the appearance of high-gain responses to 0.5 degrees stimuli and of nonlinear behavior.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Organization of group I activated cells in the main and external cuneate nuclei of the cat: Identification of muscle receptors

Summary Extracellular recording was made from 77 primary afferent fibres, 106 cells in the external cuneate nucleus, and 60 cells in the main cuneate nucleus, all activated by slowly adapting muscle stretch receptors. The nature of the muscle receptors responsible for the activation was determined by various types of receptor stimulation.Primary group I afferents from muscle spindles and tendon organs in distal forelimb muscles showed complete overlap of conduction velocities and thresholds to electrical stimulation. Both types of group I afferents as well as group II muscle spindle afferents were shown to ascend through the dorsal funiculus to the level of the cuneate nuclei.Three groups of cells were identified in the external cuneate nucleus, activated by group I muscle spindle afferents, tendon organ afferents and group II muscle spindle afferents, respectively.Almost all group I activated cells in the main cuneate nucleus, including all 34 cells identified as cuneo-thalamic relay cells, received their afferent input from muscle spindle afferents. Three cells were activated by tendon organ afferents.     

Properties of cat neck muscle spindles and their excitation by succinylcholine

Summary The sensitivity to sinusoidal stretching and small-amplitude vibration, and the variability of the resting discharge rate of de-efferented muscle spindles in the neck extensor muscle biventer cervicis (b.c.) of the cat have been studied. The effects of intra-arterial infusion of succinylcholine (SCh) on the response of the receptors to ramp stretches of the muscle were also determined. When activated by SCh, one group of afferents showed only a slow facilitation of their discharge rate, similar to that of spindle secondary sensory endings in hind-limb muscles. A second group of afferents developed a large dynamic response and a marked increase in their static discharge rate and were presumed to originate in normal b₁b₂c spindles in the b.c. muscle. A third group of afferents developed only a marked increase in static discharge, without potentiation of the dynamic response, suggesting an origin in the b₂c units of tandem spindles which are relatively common in the neck muscles. On the basis of their passive characteristics alone, afferents from b₁b₂c units could not be readily distinguished from those from b₂c units. The characteristics of these receptors, and their differences from the well-studied hind-limb spindle afferents, are discussed.     

Correlation of the dynamic behaviour of deefferented primary muscle spindle endings with their static behaviour

20 deefferented primary muscle spindle endings, 8 afferents from the extensor digitorum longus muscle (EDL) and 12 from the tibialis anterior muscle (TA), were investigated in 12 anesthetized cats. Dynamic responses of muscle spindles to ramp stretches were correlated with their static responses. The variation in the state and dynamic behaviour was found to be interdependent. The correlation coefficients were higher for EDL spindles than for TA spindles. The correlation coefficients depended upon the definition of the dynamic responses.     

Static γ-motoneurones couple group Ia and II afferents of single muscle spindles in anaesthetised and decerebrate cats

Ideas about the functions of static γ-motoneurones are based on the responses of primary and secondary endings to electrical stimulation of single static γ-axons, usually at high frequencies. We compared these effects with the actions of spontaneously active γ-motoneurones. In anaesthetised cats, afferents and efferents were recorded in intramuscular nerve branches to single muscle spindles. The occurrence of γ-spikes, identified by a spike shape recognition system, was linked to video-taped contractions of type-identified intrafusal fibres in the dissected muscle spindles. When some static γ-motoneurones were active at low frequency (< 15 Hz) they coupled the firing of group Ia and II afferents. Activity of other static γ-motoneurones which tensed the intrafusal fibres appeared to enhance this effect. Under these conditions the secondary ending responded at shorter latency than the primary ending. In another series of experiments on decerebrate cats, responses of primary and secondary endings of single muscle spindles to activation of γ-motoneurones by natural stimuli were compared with their responses to electrical stimulation of single γ-axons supplying the same spindle. Electrical stimulation mimicked the natural actions of γ-motoneurones on either the primary or the secondary ending, but not on both together. However, γ-activity evoked by natural stimuli coupled the firing of afferents with the muscle at constant length, and also when it was stretched. Analysis showed that the timing and tightness of this coupling determined the degree of summation of excitatory postsynaptic potentials (EPSPs) evoked by each afferent in α-motoneurones and interneurones contacted by terminals of both endings, and thus the degree of facilitation of reflex actions of group II afferents.     

Resting discharge of human muscle spindles is not modulated by increases in sympathetic drive

There is evidence in experimental animals that, in addition to receiving fusimotor drive, muscle spindles are subject to modulation by the sympathetic nervous system. We examined the validity of this idea in human subjects by recording from muscle spindles in the relaxed ankle and toe extensor muscles during a strong and sustained physiological activation of muscle sympathetic outflow. Unitary recordings were made from 20 primary and 17 secondary muscle spindle afferents via a tungsten microelectrode inserted percutaneously into the peroneal nerve in 10 awake, healthy subjects seated with the legs supported in the extended position. ECG, blood pressure, respiration and calf circumference were also recorded. The majority of the muscle spindles were spontaneously active at rest; a background discharge was induced in four silent spindles by vibrating the tendon. A sustained increase in muscle vasoconstrictor activity, an increase in calf volume and a fall in pulse pressure were produced by subjects performing a 30-40 s maximal inspiratory breath-hold. Despite this strong increase in muscle sympathetic outflow no significant changes occurred in the discharge of either primary or secondary muscle spindle afferents, measured as a change in mean frequency and variability over sequential 5 s epochs and compared with the preceding period of rest. Strong chemoreceptor-driven sympathetic bursts during sustained expiratory breath-holds also failed to modulate the firing of 14 spindle endings. We conclude that a sustained, physiological increase in muscle sympathetic activity causes no detectable change in muscle spindle firing, lending no support to the concept that the sympathetic nervous system can influence the sensitivity of human muscle spindles directly.     

Muscle spindle afferent studies in the baboon

1. The muscle spindle afferent conduction velocity and response to muscle twitch and stretch in young baboons has been recorded in order to find a conduction velocity that can be used to separate primary and secondary afferents.2. A number of the features of the response of the spindle afferents to stretch were examined. It was found that none could distinguish between primary and secondary afferents with greater certainty than the conduction velocity.3. Spindle afferents with conduction velocities below 50 m/sec can be classified as secondary and those with conduction velocities above 68 m/sec can be classified as primary with a reasonable degree of certainty.4. The spindle afferents with conduction velocities between 51 and 67 m/sec are most likely not a separate intermediate group but represent a region of overlap between the two groups.5. Comparisons of the difference between the response of primary and secondary afferents to overstretch suggested a mechanism to explain the saturation of the primary afferent response at frequencies far below those at which it is capable of firing.     

Activity patterns in individual hindlimb primary and secondary muscle spindle afferents during normal movements in unrestrained cats.

1. Chronically implanted microelectrode wires in the L7 and S1 dorsal root ganglia were used to record unit activity from cat hindlimb primary and secondary muscle spindle afferents. Units could be reliably recorded for several days, permitting comparison of their activity with homonymous muscle EMG and length during a variety of normal, unrestrained movements. 2. The general observation was that among both primary and secondary endings there was a broad range of different patterns of activity depending on the type of muscle involved and the type of movement performed. 3. During walking, the activity of a given spindle primary was usually consistent among similar step cycles. However, the activity was usually poorly correlated with absolute muscle length, apparently unrealted to velocity of muscle stretch, and could change markedly for similar movements performed under different conditions. 4. Spindle activity modulation not apparently related to muscle length changes was assumed to be influenced by fusimotor activity. In certain muscles, this presumption leads to the conclusion that gamma-motoneurons may be activated out of phase with homonymous alpha-motoneurons as well as by more conventional alpha-gamma-motoneuron coactivation. 5. Simultaneous recordings of two spindle primary afferents from extensor digitorum longus indicated that spindles within the same muscle may differ considerably with respect to this presumed gamma-motoneuron drive. 6. Spindle secondary endings appeared to be predominantly passive indicators of muscle length during walking, but could demonstrate apparently strong fusimotor modulation during other motor activities such as postural changes and paw shaking. 7. Both primary and secondary endings were observed to undergo very rapid modulation of firing rates in response to presumed reflexly induced intrafusal contractions. 8. It is suggested that the pattern of fusimotor control of spindles may be tailored to the specific muscle and task being performed, rather than necessarily dominated by rigid alpha-gamma coactivation.     

In-parallel and in-series behavior of human muscle spindle endings

1. The responses of 62 putative muscle spindle afferents innervating the pretibial flexor muscles of normal human subjects were studied during graded twitch contractions of the receptor-bearing muscle to search for possible in-series coupling between spindle endings and motor units. 2. The majority of afferents (n = 57) responded unequivocally in an in-parallel manner to the twitch contractions, regardless of contraction strength. There were two patterns of in-parallel response: afferents without background activity discharged during the relaxation phase of the twitch; afferents with a background discharge were transiently silenced during the contraction phase and resumed their discharge on the relaxation phase. 3. Evidence of in-series coupling was found for five afferents during submaximal twitch contractions, to which each afferent responded in a mixed "biphasic" manner, with increases in discharge during both the contraction and relaxation phases of the twitch. Background discharge, response to stretch, and response during isometric voluntary contractions suggested that four of the afferents innervated primary spindle endings and the fifth a secondary spindle ending. 4. It is argued that the five atypical spindle endings responded in an ambiguous manner during twitch contractions of the receptor-bearing muscle because there was an in-series mechanical coupling between motor units and the spindle. The incidence of demonstrable in-series responses has serious implications for the mechanisms of spindle activation during normal motor behavior, but has only minor implications for the use of the twitch test to identify muscle spindle endings.     

The preferred sensory direction of muscle spindle primary endings influences the velocity coding of two-dimensional limb movements in humans

The present study compares how accurately two different but close velocities of movement are discriminated by populations of muscle spindle primary afferents whether or not one takes into account the direction of the movement and the preferred sensory directions of the units (i.e., the direction of movement to which the afferents are the most sensitive). The activities of 26 muscle spindle primary endings originating from the tibialis anterior, the extensor digitorum longus, the extensor hallucis longus, and the peroneus lateralis muscles were recorded in the lateral peroneal nerve. Their responses to movements imposed at two velocities (12.5 and 18 mm/s) were analyzed. These movements were straight-line movements imposed in eight directions and circular movements in both clockwise and anticlockwise directions. The encoding of the movement velocity was analyzed in two ways. First, the mean frequencies of discharge of the muscle spindle afferents were compared for the two velocities. Second, the data were analyzed using a "neuronal population vector model." This model is based on the idea that such neuronal coding can be analyzed in terms of a series of population vectors (i.e., mean contribution of all the muscle spindle afferents within one directionally tuned muscle) and by finally calculating a sum vector. The results showed no clear and consistent difference in the response frequency of the muscle spindle afferents for the two velocities of movement compared. Rather, the most consistently significant differences between the two velocities were in the lengths of the sum vectors. It is concluded that the encoding of two-dimensional movement velocity relies on populations of muscle spindle afferents coming from the whole set of muscles surrounding a particular joint, each muscle making an instantaneous, oriented, and weighted contribution to the sensory coding of the kinematics parameters.     

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 temperature on the discharges of muscle spindles and tendon organs

1. In anaesthetized cats the effects of temperature on the nervous outflow from skeletal muscle via thick myelinated afferent fibres were studied. Single unit recordings were made from afferents of muscle spindles and tendon organs during slow and fast temperature changes of the medial gastrocnemius muscle which was deefferented by ventral root section and prestretched to a tension of 100 p.
2. Group I afferent units from muscle spindles were activated by warming and depressed by cooling, the effect of warming being much more pronounced than that of cooling. Afferents from secondary spindle endings with a high background discharge behaved similar to Ia fibres, whereas those with a low initial discharge rate showed an activation by cooling and a depression (mostly to cessation of firing) by warming. The discharges of group I afferents from tendon organs varied; an activation by warming was the most frequently observed reaction.
3. Some of the afferents from muscle spindles and tendon organs showed signs of a dynamic sensitivity to thermal stimulation, but in general the dynamic component in the responses to temperature changes was only small.
4. The results suggest that the afferent outflow via thick myelinated fibres from a resting, moderately prestretched muscle strongly depends on temperature. At raised intramuscular temperatures (about 42°C) the nervous outflow is characterized by an increased activity in all of the I a and many of the I b afferents, while the majority of group II spindle afferents will be depressed. In contrast, in a cold muscle (about 29°C) the nervous outflow via afferents from primary spindle endings will be reduced, while the net activity from secondary spindle endings will be increased and no marked changes are expected to occur in the discharges of I b fibres.

     

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.     

Dynamic and static contributions to the rhythmic γ activation of primary and secondary spindle endings in external intercostal muscle

1. In cats anaesthetized with sodium pentobarbital 160 external intercostal muscle spindle afferents were identified by their pause in response to ventral root stimulation; the internal intercostal muscle was denervated. 2. In order to assignate the afferents to either primary or secondary endings they were tested for their responsiveness to vibration (Bianconi & Van Der Meulen, 1963). The maximal frequency which they were able to follow regularly for at least four cycles, termed 'critical frequency', was determined. 3. The endings fell into two groups: low-f (frequency) sensitive endings with critical frequencies below 400 c/s and high-f sensitive endings with critical frequencies above 400 c/s. The latter were regarded as primary endings and the former as secondary ones. 4. The manner in which the spindle endings resumed activity after a pause produced by shocks to the ventral root, i.e. whether 'phasic' or 'tonic' (Granit & Van Der Meulen, 1962) was studied in all the spindle afferents. 5. All the secondary (low-f sensitive) endings were 'tonic' except three for which the determination of critical frequency was questionable. Both 'tonic' and 'phasic' properties were found among the primary (high-f sensitive) endings. 6. The majority of the secondary endings (74%) showed inspiratory ythmic fusimotor activation in parallel with the skeletomotor contracton as did the primary endings (79 %). 7. Fifty-seven spindle endings which all showed marked rhythmic inspiratory gamma activation were tested for respiratory variations in their dynamic responses to steady stretch and length changes introduced at low repetition rates. 8. The results indicate that both 'dynamic' and 'static' gamma fibres are represented among the rhythmic gamma fibres controlling primary muscle spindle endings, whereas rhythmic activation of secondary endings seems to be mediated only by 'static' fibres.