Discharge activity of spindle afferents from the gastrocnemius-soleus muscle during head rotation in the decerebrate cat

The activity of spindle afferents originating from both primary and secondary endings of the isometrically extended (6-8 mm) gastrocnemius-soleus (GS) muscle was recorded in precollicular decerebrate cats during sinusoidal head rotation about the longitudinal axis above a stationary body. In the first group of experiments to test the influence of vestibular volleys on fusimotor neurons, an acute bilateral neck deafferentation at C1-C3 was performed to eliminate possible influences arising from neck receptors; head rotation (0.026 Hz, +/- 15 degrees) induced a weak periodic rate modulation in 6/38 (15.8%) of the tested spindle afferents; the average response gain was 0.18 +/- 0.12, SD imp./s/deg (mean firing rate, 18.9 +/- 2.8 imp./s), and the average phase angle was -43.2 +/- 47.0 degrees, SD lag with respect to ipsilateral side-down displacement of the head (alpha-response pattern). In a second group of experiments head rotation studied after acute bilateral section of VIII cranial nerve, thereby stimulating only neck receptors, failed to influence in a reliable manner the firing rate of 38 additional spindle afferents. In a third group of experiments in which both VIII nerves and cervical dorsal roots were left intact, head rotation induced a response in 7/45 (15.6%) of the tested spindle afferents similar to that observed after cervical deafferentiation and thus depended on stimulation of labyrinth receptors alone. Over the examined frequency range of head rotation from 0.015 to 0.325 Hz (+/- 15 degrees), the response gain of spindle afferents was relatively stable during sinusoidal labyrinth stimulation. For most of the spindle afferents the phase angle of the response elicited at the lower frequencies was related to the direction of head orientation towards the ipsilateral sidedown, thus being attributed to labyrinth volleys originating from macular receptors; at 0325 Hz the stimulus was less effective and some units showed a phase advance relative to head position which was attributed to costimulation of canal receptors. Displacement of the muscle under study obtained by either rotation of the whole animal or body alone beneath a stationary head elicited a periodic modulation of spindle afferent discharge, independent of head orientation or type of preparation, in 51/73 (70%) of the muscle spindles tested; the average response gain was 0.20 +/- 0.19, SD imp./s/deg, and an average phase lead of +14.1 +/- 20.5 degrees, SD with respect to the peak of the ipsilateral side-down displacement of the body or of the animal was observed.(ABSTRACT TRUNCATED AT 400 WORDS)     

A characterisation of spindle afferents from the intertransverse tail muscles in the rat

Summary The conduction velocities of myelinated afferent axons from the intersegmental muscles at the tail-base in rat range from 5.0–43.0 m · sec^–1, and have a bimodal distribution with a boundary between the groups at 25.0 m · sec^–1. Calculations of the dynamic indices of spindle endings, observed when stretching the muscle, show that axons from primary endings conduct at velocities above 28.0 m · sec^–1 and those from secondary endings at below 24.0 m · sec^–1. These characteristics resemble those of sipindles in the rat hind-limb muscles, and do not compare with thsoe in the more distally situated segmental tail muscles.     

Activity of spindle afferents from cat anterior thigh muscles. II. Effects of fusimotor blockade

Chronically implanted electrodes and nerve cuff catheters were used to record the activity of individual muscle spindle afferents during treadmill walking as low doses of lidocaine were infused around the femoral nerve to progressively block gamma motoneuron activity. Both primary and secondary endings from both the monarticular knee extensors and the biarticular hip/knee muscles of the anterior thigh showed large decreases in afferent activity, usually well before changes in the electromyographic activity, force output, or length and velocity were seen in the parent muscles. This decline in the proprioceptive signal feeding back onto the spinal cord, which we presume to have involved most of the spindles supplied by the femoral nerve, did not cause noticeable irregularities or instability of the walking gait. At the peak of the fusimotor blockade, spindle afferents from knee extensor muscles lost about half of their usually brisk activity during the near-isometric contraction of the stance phase. Significant decreases in the response to passive stretch during the flexion phase also occurred. At the peak of the fusimotor blockade, spindle afferents from the biarticular muscles lost all of their activity during the rapidly shortening swing phase and about half of their activity during the rapidly lengthening stance phase. For both monarticular and biarticular muscle spindles, the activity decreases in stance and swing phase often occurred at distinctly different stages of the progressive fusimotor blockade, indicating several different sources of fusimotor control. From these data, we infer that the sensitivity of most spindle afferents is substantially influenced by fusimotor activity during phases of both extrafusal activity and extrafusal silence. At least some of this influence appears to come from fusimotor neurons whose recruitment is independent of the extrafusal recruitment. The extent and type of fusimotor effects on spindle afferent sensitivity (dynamic or static) appear to be specialized for the mechanical events that tend to occur during those phases.     

Reflex pathways from large muscle spindle afferents and recurrent axon collaterals to motoneurones of wrist and digit muscles: a comparison in cats, monkeys and humans

Whereas a large body of data is available on the control of hand motoneurones from the brain, not much is known about the contribution of the spinal interneuronal apparatus to the differentiated movement repertoire of the hand. This review summarises recent data on the excitatory Ia pathways and on recurrent inhibition for cats, monkeys and humans. The basic principles of organisation have, in general, been preserved in the different species. Thus the motoneurones to cat and human long and short digit muscles seem not to possess a recurrent axon collateral system. With regard to the Ia pathways, specialised connectivity patterns have developed in the long digit extensor and wrist extensor muscles. They allow the former group to support the differentiated movement repertoire of the digits, and the latter group to be part of a general extensor or flexor synergism. Modifications between the species are present, however, with regard to the proximodistal connectivity across the elbow. Whereas they are regularly present in the cat, they are less developed in the monkey and absent in man, which frees the human hand from the elbow position. Received: 28 September 1998 / Accepted: 22 January 1999     

Responses of cat peroneus brevis muscle spindle afferents during recovery from nerve-crush injury

The responses of regenerated muscle spindle afferents to ramp-and-hold stretch of the peroneus brevis muscle in the cat were recorded at periods from 26 to 140 days after crushing the common peroneal nerve. During the early stages of recovery a number of abnormally responding afferents were observed. The most marked abnormality was the absence or rapid failure of firing during the held phase of the stretch. The proportion of abnormal afferents became less as recovery progressed. Electrical stimulation of isolated static and dynamic gamma-axons increased the firing rates of the afferents during the ramp-and-hold stretch such that a gamma static axon would restore the response of an abnormal afferent to the held phase of the stretch. The regenerated afferents have been classified according to the degree of abnormality displayed. These abnormalities can be accounted for by assuming a subtractive reduction in the firing frequency of the regenerated afferents. This is attributed to an increase in the pacemaker threshold.     

Analysis of activity of muscle spindles of the jaw-closing muscles during normal movements in the cat.

Recordings have been made of afferent activity from spindles of the jaw-closing muscles, together with jaw movement and e.m.g. from temporalis and masseter in conscious, unrestrained cats. 2. In the twenty-nine units studied, the pattern of spindle behaviour observed during eating and lapping was generally what might be expected of stretch receptors. Maximal firing frequencies were found during opening of the mouth (lengthening), while during active closing the discharge was progressively reduced or abolished. Nevertheless, changes in the relation of stretch to firing frequency in different movements indicated that fusimotor drive was not constant. 3. spindle afferents could be divided into two groups on the basis of their maximal firing frequency during eating. "High-frequency" units (range 240-600 impulses/sec) showed pronounced velocity sensitivity, which supports the proposal that they correspond to spindle primaries. 'Low-frequency" units (range 80-200 impulses/sec) showed predominantly length sensitivity and probably correspond to secondary endings. 4. Length sensitivity of low-frequency units was considerably greater in lapping movements than in eating, indicating increased static fusimotor drive in the former. Sensitivity in the opening phase of eating was indistinguishable from that recorded in deeply anaesthetized animals. 5. High-frequency units were generally silenced immediately active shortening commenced. 6. No simple relationship existed between temporalis or masseter e.m.g. and spindle firing. 7. These results imply that normal masticatory movements are not initiated or driven to any appreciable extent via the fusimotor route. Close alpha-gamma co-activation is not a feature of this situation. On the other hand, in some other movements, such as licking the lips, fusimotor drive could fluctuate so as largely to cancel the unloading effects of active muscle shortening.     

A novel myosin present in cat jaw-closing muscles

Summary Cat jaw-closing muscles (masseter, posterior and anterior temporalis) are physiologically much faster than fast-twitch muscles of the hind limb. They have a highly oxidative metabolism and contain an alkali-stable ATPase as shown by histochemical staining. These observations have been extended here using immunocytochemical, enzymatic and biochemical analyses of myosin from posterior temporalis muscles. Antisera against posterior temporalis myosin do not precipitate gastrocnemius or soleus muscle actomyosin in Ouchterlony double diffusion. Using enzyme-linked immunoadsorbant, antisera to posterior temporalis myosin do not cross-react with heavy chains from flexor hallucis longus (FHL) or soleus myosin at dilutions where the reaction with posterior temporalis heavy chain is easily detected. Myosin isolated from posterior temporalis has an ATPase activity in the presence of EDTA (K⁺-ATPase) or calcium ions, that is, more than twice the corresponding value for FHL myosin. The myosin is unusual in its high degree of lability on storage at 0° C: it loses over 25% of the K⁺-ATPase per day and aggregates readily. Although the myosin is activated over 80-fold by actin, the maximum velocity obtained by extrapolation to saturating actin concentrations is considerably below that obtained for FHL myosin. This probably reflects the marked instability of the myosin rather than its true actin-activated ATPase. An alternative method of comparing actomyosin ATPase activity is by using myofibril preparations. Using conventional procedures it has not been possible to prepare myofibrils from posterior temporalis because of the extensive network of connective tissue surrounding the fibres. More drastic techniques, for example, the mini cell disruption chamber, will disrupt the fibres, but the myofibrils so produced are very small and often highly aggregated. Analysis of the myosin by gel electrophoresis shows that the LC1 and LC2 light chains of posterior temporalis myosin can be distinguished from those of soleus and FHL myosins. Polypeptide mapping of the various heavy chains also shows that posterior temporalis heavy chains are chemically different from those of the hind-limb muscle myosins.Fast-twitch oxidative fibres have been identified in hind limb and body muscles of the rat, cat and chicken; posterior temporalis myosin differs from these myosins also on the basis of its histochemical properties. We conclude that the myosin of jaw-closing muscles is phenotypically distinct from both fast-twitch oxidative and fast-twitch glycolytic muscles of the body and limbs, though similar super-fast fibres may occur elsewhere than in the jaw.     

Bilateral experimental muscle pain changes electromyographic activity of human jaw-closing muscles during mastication

 The effects of bilateral experimental muscle pain on human masticatory patterns were studied. Jaw movements and electromyographic (EMG) recordings of the jaw-closing muscles were divided into multiple single masticatory cycles and analyzed on a cycle-by-cycle basis. In ten men simultaneous bilateral injections of hypertonic saline (5%) into the masseter muscles caused strong pain (mean±SE: 7.5±0.4 on a 0–10 scale), significantly reduced EMG activity of jaw-closing muscles in the agonist phase, and significantly increased EMG activity in the antagonist phase. Nine of the subjects reported a sensation of less intense mastication during pain. Injections of isotonic saline (0.9%) did not cause pain or significant changes in masticatory patterns. The influence of higher brain centers on conscious human mastication can not be discarded but the observed phase-dependent modulation could be controlled by local neural circuits and/or a central pattern generator in the brain stem which are capable of integrating bilateral nociceptive afferent activity. Received: 19 February 1997 / Accepted: 14 April 1997     

Response properties of vestibular afferents in alert cats during optokinetic and vestibular stimulation

Unlike secondary vestibular neurons primary vestibular afferents of the alert cat do not respond to rotation of large-field visual patterns. The phases and gains of the responses of horizontal canal afferents to head rotation as well as the resting discharges found in the alert animal do not significantly differ from those measured in animals anesthetized with barbiturates.     

Dynamics of vestibular neurons during rotational motion in alert rhesus monkeys

The temporal processing in the encoding of head rotation was investigated by comparing the dynamics of vestibular nuclei neurons with those of the regularly and irregularly firing semicircular canal afferents in alert rhesus monkeys. During earth-vertical axis rotations, neurons without eye movement sensitivity differed in their response dynamics from both regularly and irregularly firing semicircular canal afferents. At high frequencies, central responses increased in sensitivity and maintained phase leads of nearly 30° relative to head velocity. These persistent high-frequency phase leads resembled those of irregularly firing (but not regularly firing) semicircular canal afferents. However, at low frequencies, central responses exhibited significantly smaller phase leads than those of irregularly firing semicircular canal afferents, and dynamics resembled more those of the regularly firing afferents. The response dynamics of central non-eye movement cells were significantly different from those of position-vestibular-pause and eye-head neurons (collectively referred to as eye movement cells). In contrast to the persistent phase leads of non-eye movement neurons, all eye movement cells modulated closely in phase with head velocity at all frequencies down to 0.05 Hz during visual suppression tasks. Vertical canal non-eye movement neurons that were insensitive to both translations and static head tilts led head velocity by approximately 5–30° during high-frequency earth-horizontal axis rotations. Unlike the earth-vertical axis responses that led head velocity at low frequencies by as much as 20–40°, vertical canal neurons only slightly led or even lagged behind head velocity during low-frequency earth-horizontal axis rotations. Posterior canal central non-eye movement cells lagged behind head velocity significantly more than anterior canal neurons. These frequency dependencies of central vestibular neurons in comparison with those of the afferents suggest that both low- and high-pass filtering might be necessary to convert primary semicircular canal afferent response dynamics to central neuron ones.     

Behavior of horizontal semicircular canal afferents in alert monkey during vestibular and optokinetic stimulation

Summary The behavior of single vestibular nerve fibers from the lateral semicircular canal was recorded during sinusoidal oscillations of the head, during optokinetic stimulation with the head stationary, and during spontaneous oculomotor behavior in the alert monkey. The response of similar fibers to adequate vestibular stimulation was also studied in some of the animals under deeply anesthetized conditions. In the alert animals all units were spontaneously active and their discharge was modulated only by adequate vestibular stimulation. Ipsilateral horizontal rotations of the head were excitatory for all units. No modification of this basic vestibular response by visual stimulation including full-field striped drum rotation was observed. Furthermore no correlation of unit activity with oculomotor function including voluntary saccadic and pursuit eye movements was found in any of the units. The regularity of spontaneous discharge was the most consistent characteristic that differentiated the unit response into types. Most units were very regular in discharge, but a few were very irregular. The averaging of unit discharge over several cycles of oscillatory head rotation showed that the irregular type units were also consistently modulated by adequate vestibular stimulation. Both regular and irregular type units were found in the anesthetized animals. Unimodal distributions of the quantitative values for unit resting discharge rate, sensitivity, and phase relationship were found. The distributions for these three parameters were similar in the units recorded in the anesthetized animals. Thus at least these characteristics of semicircular canal response seem not to be affected by the vestibular efferent system which should be altered or eliminated in the case of the anesthetized animals.Research supported by NIH Grant EY0995-04.     

Automatic detection and measurement of EMG silent periods in masticatory muscles during chewing in man.

Silent periods are transient stops of muscle activity that are induced by mechanical or electrical stimulus. The current report describes a new algorithm which enables automatic detection and measurement of silent periods that occur in the EMG of human masticatory muscles during chewing efforts together with jaw positions in space at the occurrence of the silent periods. The EMG signal for detection of the silent period is modeled based on the observation of the EMG records induced from the jaw-closing muscles during chewing efforts in seven volunteer subjects. The algorithm for automatic detection and measurement of the EMG silent period has been applied successfully to real EMG data and the performance of the software was confirmed to be sufficiently reliable. The results of the analysis are stored in a biosignal database for possible clinical use.