MUSCLE SPINDLES AND THEIR MOTOR CONTROL

Effects of changing body position on dental occlusion

Graded changes in body position produced no noticeable changes in wax registrations of dental contact patterns resulting from voluntary jaw closure. Occlusal contact patterns that resulted from electrically stimulated jaw closures indicated an increase in closing force and a mesial shift of prime occlusal contact points as the body was raised to an upright position.

The electromyographic silent period produced by supramaximal electrical stimulation in normal man

The electromyographic silent period produced by supramaximal electrical stimulation of the median nerve was recorded in the abductor pollicis brevis muscle of four normal subjects during maximal isometric voluntary contraction. Except for an inconstant F response, electrical silence could usually be induced in the muscle from the twitch potential until the reappearance of uninterrupted voluntary activity. The silent period produced by stimulation at the wrist was approximately 25 msec longer than that produced by stimulation at the elbow and was independent of muscle tension. Further shortening of the muscle during the twitch contraction did not significantly alter the length of the silent period. A silent period in the abductor pollicis brevis muscle was also obtained after stimulation of the ulnar nerve, at the wrist and at the elbow. The onset of this period of silence was delayed, but it ended at the same time after the stimulus as the corresponding silent periods produced by median nerve stimulation. It is concluded that the end point of the silent period produced by supramaximal electrical stimulation of a mixed peripheral nerve is determined by an inhibitory spinal reflex, afferent impulses travelling in slowly-conducting fibres that are directly activated by the stimulus. Under these conditions the length of the silent period gives no indication of spindle activity in the muscle.

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.

Effect of procaine infiltration into biceps brachii muscle in man on supraspinal control of reflex transmission

Sensitivity of the tonic stretch reflex (TSR) of biceps brachii in normal man was measured at different levels of voluntary contraction before and after gamma efferent nerve fibres were selectively blocked by infiltration of dilute procaine. The procaine infiltration reduced TSR sensitivity to one half of its control value. It had no effect, however, on the slope of the relation between TSR gain and level of voluntary contraction. It was concluded that the increase in TSR sensitivity associated with voluntary contraction was not mediated by the gamma efferent fibres and control of reflex transmission is therefore not the main function of the fusimotor system.

The effect of muscle length and rate of fusimotor stimulation on the frequency of discharge in primary endings from muscle spindles in the cat

1. Responses from the primary endings of muscle spindles in the soleus muscle of the cat were recorded during repetitive fusimotor stimulation at a number of different muscle lengths.2. An increase in the rate of stimulation increased the size of both the peak and the plateau of the responses to stimulation of both static and dynamic fusimotor fibres.3. Responses, with the exception of the peak frequency of the discharge during dynamic fusimotor stimulation, increased in size on raising the muscle length up to maximum body length. The peak of the dynamic response reached its highest value at intermediate lengths.4. The effect of increasing stimulation rate and muscle length was to reduce both the latency and time to peak of fusimotor responses. The change in latency with muscle length was particularly dramatic at low stimulus rates.5. In an attempt to compare fusimotor responses with the behaviour of extrafusal muscle fibres, a model is proposed which consists of a mixture of extrafusal tension and rate of change of tension. This model could simulate the static fusimotor responses reported here.

The frequency response of frog muscle spindles under various conditions

1. Nerve impulses were recorded from afferents from non-contracting spindles from the isolated extensor longus dig. IV muscle of the frog during small sinusoidal changes in muscle length at frequencies from 0.001 to 100 Hz. A computer of average transients was used to average the spike distribution during a number of cycles, and hence to determine the spindle response in impulses/sec at different phases of the cycle.2. At any one frequency the response could be described by a sinusoid, whose amplitude was approximately proportional to the amplitude of the stretch and whose phase was approximately constant, together with a non-linearity dependent principally upon non-linearities in the static response.3. The frequency response was estimated from the sinusoid responses. In conventional terms, it consisted of a straight line of positive slope below 2 Hz and a maximum between 7 and 16 Hz.4. The slope of the frequency response was dependent on the mean length of the preparation, typically varying from zero to about 0.5 (3 db/octave) over the physiological range of the passive muscle. The shape of the peak appeared to depend on the mean firing frequency.5. The responses to ramp stretches of one second duration and up to 2 mm in amplitude were also measured. Responses predicted from the sinusoid measurements were the same shape as the measured responses, but were larger by a factor of about 1.4.6. The shapes of both the frequency responses and the responses to ramps were hardly affected by an operation that removed most of the polar parts of the spindles.7. The results are discussed in terms of internal spindle mechanisms.

Muscle afferent potential ('A-wave') in the surface electromyogram of a phasic stretch reflex in normal humans

The experiments reported in this paper tested the hypothesis that the afferent potential elicited by a tendon tap in an isometrically recorded phasic stretch reflex can be detected in the surface EMG of normal humans when appropriate techniques are used. These techniques involved (1) training the subjects to relax mentally and physically so that the EMG was silent before and immediately after the diphasic MAP which reflects a highly synchronous discharge of afferent impulses from low threshold muscle stretch receptors after a tendon tap, and (2) using a data retrieval computer to summate stimulus-locked potentials in the EMG over a series of 16 samples using taps of uniform peak force and duration on the Achilles tendon to elicit the tendon jerk in the calf muscles. A discrete, diphasic potential (`A-wave') was recorded from EMG electrodes placed on the surface of the skin over the medial gastrocnemius muscle. The `A-wave' afferent potential had the opposite polarity to the corresponding efferent MAP. Under control conditions of relaxation the `A-wave' had a latency after the onset of the tap of 2 msec, the peak to peak amplitude was of the order of 5 μV and the duration was in the range of 6 to 10 msec. Further experiments were conducted to show that the `A-wave' (1) was not an artefact of the instrumentation used, (2) had a threshold at low intensities of stimulation, and (3) could be reliably augmented by using a Jendrassik manoeuvre compared with the potential observed during control (relaxation) conditions. The results support the conclusion that the `A-wave' emanates from the pool of muscle spindles which discharges impulses along group Ia nerve fibres in response to the phasic stretch stimulus because the primary ending of the spindles is known to initiate the stretch reflex and the spindles can be sensitized by fusimotor impulses so that their threshold is lowered as a result of a Jendrassik manoeuvre. The finding has important implications for the investigation of the fusimotor system in intact man.

Muscle spindle response at the onset of isometric voluntary contractions in man. Time difference between fusimotor and skeletomotor effects

1. Impulses in single muscle afferents were recorded from the median nerves of waking human subjects with percutaneously inserted tungsten needle electrodes. During isometric voluntary contractions, unitary discharges were analysed from muscle spindle endings in the wrist and finger flexor muscles and the electromyographic activity from these muscles was recorded simultaneously.2. When the subject activated the muscle portion in which a spindle was located, the afferent discharge increased in spite of the mechanical unloading effects of the skeletomotor contraction indicating a concomitant fusimotor activation. This was valid for slowly rising contractions as well as small fast rising twitches.3. The time of onset of spindle acceleration was determined in relation to the time of onset of the electromyographic activity for thirty-one units studied altogether in more than seven hundred contractions. It was found that spindle acceleration regularly occurred after the onset of the electromyographic activity.4. There was a considerable variation from one test to the other, for the individual units, with regard to the exact time of onset of spindle acceleration, although spindle acceleration occurred mostly within 0.5 sec after the onset of the electromyographic activity in sustained contractions and within 0.1 sec in small fast rising twitches. It was not possible to assess to what extent this variation was accounted for by variations in the mechanical unloading effects of the skeletomotor contraction or variations in the timing of the fusimotor outflow.5. For many units, spindle acceleration did not occur until 10-50 msec after the onset of the skeletomotor contraction. This time is of the same order of magnitude as the time difference in latency from the spinal cord to the recording points in the two systems, as estimated from reasonable assumptions.6. It was concluded that the fusimotor system does not participate in the initiation of voluntary contractions in man, but that the skeletomotor activity is initiated by descending impulses from supraspinal structures and their effects on the neuronal organization within the spinal cord.7. The fact that fusimotor activation occurs also in very small and short lasting twitches, when spindle acceleration must have a negligible influence on the skeletomotor outflow, suggests that the fusimotor and the skeletomotor systems are rigidly co-activated in voluntary contractions.8. The finding that spindle acceleration does not occur until 10-50 msec after the onset of the electromyographic activity suggests that there is an approximately simultaneous onset of the fusimotor and the skeletomotor outflows from the spinal cord.

Muscle silent period in Parkinson's disease

The muscle silent period was measured in 11 patients with moderate to severe rigidity associated with Parkinson's disease. The determinations were made under conditions of maximum disability for each patient, since all medications had been withdrawn before testing. The duration of the EMG silence, produced by small and large electrical twitch contractions of the adductor pollicis muscle, fell within a range of values previously determined for normal individuals. Major alleviation of the rigidity and bradykinesia with chronic oral l-dopa therapy was not accompanied by any change in the silent period. It was concluded that in untreated Parkinsonism, and also after its treatment with l-dopa, the functioning of the muscle spindles and local inhibitory reflexes remains normal.

Projection from low-threshold muscle afferents of hand and forearm to area 3a of baboon's cortex

1. The precentral and postcentral banks of the Rolandic fissure of the arm area of the baboon's cortex have been probed to their depths with extracellular micro-electrodes under nitrous oxide and oxygen anaesthesia, supplemented by minimal intravenous pentobarbitone or chloralose.2. Afferent volleys were sent in from the deep (motor) radial nerve and the deep palmar (motor) branch of the ulnar nerve. Their entry into the central nervous system was timed at the dorsal root entry zone. The nerves were stimulated in continuity and the effects of stimuli below threshold for the motor axons were investigated.3. Area 3a, in the depths of the postcentral bank, which is cytoarchitectonically transitional between areas 3 and 4, is the receiving area for afferent impulses from muscle.4. Evoked potential waves and unitary discharges began 4 msec, and the majority of units discharged between 5 and 10 msec, after the afferent volley reached the dorsal root entry zone.5. Similar responses were elicited by a brief pull (70 mu in 1 msec) or brief vibration (50 mu at 250-400 Hz) applied to the tendons of m. extensor digitorum communis.6. No potential waves were evoked in area 4, even in the depths adjacent to area 3a, by muscle afferent volleys.