Twitch relaxation of the cat soleus muscle at different lengths and temperatures

We recorded isometric and isotonic twitches, in situ, from the cat soleus at various muscle lengths and temperatures. At a given temperature the duration of isometric twitches increased approximately 60% for each 10% increase in muscle length, which was primarily owing to decreases in the rate of relaxation. For the relaxation of isometric twitches recorded at different muscle lengths, the equivalent activation energies determined were the same (13.2 +/- 0.3 kcal/M). The duration of isotonic twitch contractions increased only 20% for each 10% increase in muscle length. Even a small amount of shortening (3%) diminished the dependence of twitch duration on muscle length. In this case, twitch duration increased approximately 30% for every 10% increase in muscle length. Hence, even small changes in internal and/or external compliance (eg, changes in the tendon-fiber continuity) can greatly influence twitch duration. Our findings are consistent with the hypotheses that in the cat soleus, Ca2+ sequestration is primarily governed by a single energy dependent process and that the Ca2+ sensitivity of the contractile apparatus increases with increasing sarcomere length.     

Post-tetanic depression of twitch tension in the cat soleus muscle

Changes produced by a preceding tetanus in the isometric twitch myogram were examined in cat soleus (slow twitch) muscle in situ, at 37°C, and with supramaximal nerve stimulation. A tetanus produced a depression in the twitch tension (P) accompanied by a shortening of time to peak (TP) and time to half-relaxation (HR), and a slight depression in the maximum rate of tension rise (MR). Recovery of P, TP, and HR, after a standard tetanus of 100 Hz for 300 ms, showed an early-fast-phase (time constant of 0.8 to 0.9 s) and a late-slow-phase (time constant of 70 to 80 s), whereas that of MR showed only the late phase of recovery. Analysis of the data indicated the occurrence of two post-tetanic influences which produce different maxima of tension depression at zero test interval and have different rates of recovery. The twitch tension depression, when examined at the same test interval (0.7 s), increased with increase in the number of stimuli in the preceding tetanus. The relationship between the post-tetanic twitch tension and the number of stimuli in the tetanus was approximately logarithmic and was similar for tetanic frequencies of 50, 100, and 200 Hz. Unlike in the fast-twitch muscles, there was no clear evidence of a post-tetanic influence which may potentiate the twitch tension: the most prominent post-tetanic effect, in this muscle, was an acceleration of the tension relaxation in the twitch.     

Classification of muscle spindle receptors

The conduction velocities of muscle spindle afferent fibers have a bimodal distribution, and classifications of spindle receptors based on afferent fiber diameter have therefore divided these receptors into two groups, the well known primary and secondary endings. However, measures of spindle function that are likely to be important for kinesthetic sensibility such as dynamic response, adaptation and linear directionality (hysteresis) are distributed rather uniformly. Therefore, from this functional perspective it might be argued that muscle spindle receptors should not be subdivided at all. On the other hand, different receptors demonstrate these properties to varying degrees, and there are simple, linear correlations among log (dynamic response), log (adaptation), linear directionality and conduction velocity. Thus, the receptors can be divided into as many as 5–10 different subpopulations that differ significantly in one or more of these properties.     

Electrical 'tuning' in muscle spindle receptors

A variety of mechanical and electrical mechanisms have been shown to tune sensory receptors to selected stimuli. This study demonstrates the presence of electrical tuning in the mammalian muscle spindle. Apamin and tetraethyl ammonium ions (TEA), blockers of Ca2+-activated K+ channels, and ZnCl2, a Ca2+ channel blocker, were shown to change the dynamic behavior of the muscle spindle. We conclude that a Ca2+-activated K+ channel in the sensory nerve of the spindle provides negative feedback which alters its dynamic behavior tending to compensate for muscle dynamics. This mechanism is similar to that found in sensory hair cells in the cochlea and vestibule and may represent a general strategy for tuning in sensory receptors.     

Classification of muscle spindle afferents in the peroneus brevis muscle of the cat

Muscle-spindle afferents are commonly classified according to their conduction velocity. Under certain conditions such classifications may not be feasible and another form of identification is required. In this study 5 tests, comprising either quantitative or qualitative criteria, have been evaluated as a means of classifying spindle afferents. The choice of these tests was made on the basis of predicted physiological differences arising from the structural variations in the endings. Prior conditioning of the spindles was found to enhance the distinction between the two types of afferent. All the tests generated similar identifications with a maximum of 10% of afferents being classified differently by any two tests.     

Joint angle signaling by muscle spindle receptors

Nerve impulses were recorded from sensory fibers supplying the tibialis anterior and soleus muscles of anesthetized cats as the ankle joint was moved from one end of the flexion-extension axis to the other and back again in steps of 6-7 degrees. The rate of movement from one position to the next was 40 deg/s and each position was held for 16-18 s. Plots were made of receptor discharge frequency as a function of ankle joint angle during joint movement (dynamic input-output (I-O) functions) as well as 2 and 15 s after movement terminated (2 and 15 s static I-O functions). Only receptors with a sustained (5s) static response within the physiological range were studied. A total of 229 tibialis anterior receptors met this criterion, of which 11 were identified as tendon organs. One hundred and five soleus receptors were studied, of which 6 were tendon organs. Thus tendon organ activity accounted for only a small part of the muscle afferent signal under passive conditions. The spindle receptors in soleus and tibialis anterior divided the ankle flexion-extension range about equally between them, those in soleus signaling over the flexion half of the range and those in tibialis anterior over the extension half. At angles where the receptors in a particular muscle did not signal joint angle, the tendon of the muscle was observed to be slack. Thus the total muscle afferent discharge in a relaxed animal is high at one end of the range, declines progressively as the ankle is displaced to an intermediate position, and then increases again as the joint moves toward the opposite end of the range. The spindle receptors within an individual muscle were recruited rather early as the muscle came under tension so that over most of a muscle's signaling range joint angle could have been coded by changes in receptor discharge frequency but not by which spindle receptors were active. To evaluate the information signaled by individual muscle spindle receptors, the following measurements were made from plots of impulse frequency vs joint angle: dynamic response, defined as the frequency difference between the dynamic and 2 s static I-O functions during muscle lengthening; adaptation, defined as the frequency difference between the 2 and 15 s static I-O functions during muscle lengthening.(ABSTRACT TRUNCATED AT 400 WORDS)     

Enzyme and size profiles in chronically inactive cat soleus muscle fibers.

Spinal isolation (SI), i.e., the isolation of the lumbar spinal cord via a rostral and a caudal cord transection and bilateral dorsal rhizotomy, was used to determine the effects of chronic (6 months) inactivity on the size and metabolic properties of fibers in the cat soleus. Fibers were classified as dark or light, based on their staining reactions to myosin ATPase, alkaline preincubation, and immunohistochemically as expressing fast and/or slow myosin heavy chains (MHC). Succinate dehydrogenase (SDH) and alpha-glycerophosphate dehydrogenase (GPD) activities were assessed histochemically. Following SI, both the light and the dark ATPase fibers in the SI cats were significantly smaller than the light ATPase fibers in the controls. Normally 100% of the fibers were light ATPase and reacted exclusively with the slow antibody. After SI, approximately 45% of the fibers were dark ATPase fibers, many reacting with both fast and slow MHC antibodies. The total amount and concentration of GPD were higher in the light and dark ATPase fibers in SI compared with light ATPase fibers in controls. In contrast, although the total amount of SDH per fiber was decreased, reflecting the decrease in fiber size, the mean SDH concentration per fiber was unchanged following SI. These data indicate that there is a close coordination in the regulation of GPD activity and the type of myosin. SDH activity, on the other hand, appears to be resistant to decreased levels of activity and unloading, i.e., there seems to be a minimum level of oxidative potential in the soleus that is independent of activity level. Fiber sizes, however, are very sensitive to less-than-normal amounts of neuromuscular activity and/or loading.     

Specific tension measurements in single soleus and medial gastrocnemius muscle fibers of the cat

Direct measurements of the sizes of and forces produced by single fibers of the cat soleus and medial gastrocnemius muscles were made to determine whether or not different fiber types have characteristically distinct specific tensions. Single fibers (5-mm lengths), whose sarcolemmas had been chemically removed using a 5-mM EGTA "skinning" solution, were attached to a photodiode force transducer. Each single fiber was first placed in "relaxing" solution (22 +/- 1 degrees C, pH 7.0, pCa 8), its sarcomere length set at 2.7 micron using its laser diffraction pattern, and its diameter measured using the calibrated graticule of a microscope eyepiece (+/- 2 micron). Subsequently, each fiber was transferred to an activating bathing solution (pCa 3.6) in which the fiber produced its maximum tension. The specific tension values for single soleus muscle fibers displayed a threefold range (1.19 to 3.53 kg/cm2) with a mean value of 2.30 +/- 0.61 (SD) kg/cm2 (N = 42). The medial gastrocnemius fibers studied had a fourfold range in specific tensions (1.05 to 4.47 kg/cm2) and a mean value of 2.42 +/- 0.61 (SD) kg/cm2 (N = 104). Many medial gastrocnemius fibers (N = 64) were type-identified using a standard actomyosin ATPase histochemical assay. Type I medial gastrocnemius fibers had mean specific tension values of 2.45 +/- 0.47 kg/cm2 (N = 18), whereas, type II single fibers had mean specific tension values of 2.43 +/- 0.67 kg/cm2 (N = 46). Our results suggest that there is no significant difference between the specific tensions of the different muscle fiber types within the cat medial gastrocnemius muscle.     

Discharge characteristics of the elbow joint nerve of the cat

Direct recordings were made from intact elbow joint nerves (EJN) in decerebrate cats. These multiunit recordings indicate that the EJN is a relatively pure articular nerve and does not contain significant numbers of muscle afferent fibres. In every cat the EJN exhibited a tonic discharge throughout a full range of elbow joint positions. The discharge was most intense when the elbow was extended to 165° and least intense when the joint was held in midrange positions between 90° and 120°. There was a modest increase in discharge as the elbow was held in more flexed positions and activity at 30° was about half of that at 165°. Alterations in the tone of muscles around the joint did not affect the adapted EJN discharge at anyangle.     

Monosynaptic projections of single muscle spindle afferents to neck motoneurons in the cat

Monosynaptic connections to dorsal neck motoneurons of the cat from single afferents supplying primary endings of neck muscle spindles were studied using spike-triggered averaging techniques. Single-fiber EPSPs were detected in only 11 of the 112 afferent-motoneuron pairs examined. The average amplitude of single-fiber EPSPs recorded in motoneurons with a membrane potential of greater than -40 mV was 49 microV. Motoneurons receiving functional contacts from a single afferent were confined to a small rostrocaudal zone within the motor nucleus. The low frequency of single-fiber EPSPs in neck motoneurons could not be attributed to the absence of projections to the ventral horn or to damage to either the afferents or motoneurons. Our results suggest, therefore, that single afferents from neck muscle spindles make functional contacts with a small fraction of neck motoneurons, unlike the arrangement seen in more commonly studied hindlimb muscle systems (Henneman and Mendell, 1981).     

Discharge characteristics of axotomized abducens internuclear neurons in the adult cat

The aim of the present work was to characterize the axotomy-induced changes in the discharge properties of central nervous system neurons recorded in the alert behaving animal. The abducens internuclear neurons of the adult cat were the chosen model. The axons of these neurons course through the contralateral medial longitudinal fascicle and contact the medial rectus motoneurons of the oculomotor nucleus. Axotomy was carried out by the unilateral transection of this fascicle (right side) and produced immediate oculomotor deficits, mainly the incapacity of the right eye to adduct across the midline. Extracellular single-unit recording of abducens neurons was carried out simultaneously with eye movements. The main alteration observed in the firing of these axotomized neurons was the overall decrease in firing rate. During eye fixations, the tonic signal was reduced, and, on occasion, a progressive decay in firing rate was observed. On-directed saccades were not accompanied by the high-frequency spike burst typical of controls; instead, there was a moderate increase in firing. Similarly, during the vestibular nystagmus, neurons hardly modulated during both the slow and the fast phases. Linear regression analysis between firing rate and eye movement parameters showed a significant reduction in eye position and velocity sensitivities with respect to controls, during both spontaneous and vestibularly induced eye movements. These firing alterations were observed during the 3 month period of study after lesion, with no sign of recovery. Conversely, abducens motoneurons showed no significant alteration in their firing pattern. Therefore, axotomy produced long-lasting changes in the discharge characteristics of abducens internuclear neurons that presumably reflected the loss of afferent oculomotor signals. These alterations might be due to the absence of trophic influences derived from the target.     

Ultrastructure of muscle spindle afferent terminations in lamina VI of the cat spinal cord

Two group Ia muscle spindle afferents were impaled in the lumbosacral enlargement of the cat's spinal cord and intra-axonally labeled with horseradish peroxidase. Terminations in lamina VI were examined with the electron microscope. Boutons formed synaptic associations with somata and large dendritic shafts in the lamina VI neuropile and received axo-axonic contacts. It is concluded that group Ia terminals synapse with the proximal regions of lamina VI neurons and are under strong presynaptic control.