Comparative analysis of the facilitating effects induced on muscular periphery by the microstimulation of various cerebellar nuclei

The facilitatory effects evoked on the motor periphery by the activation of neuronal pools in cerebellar nuclei were analized in 13 cats. The aim of the work was to compare the frequency and the characteristics of the motor facilitations induced on the ipsilateral forelimb by the microstimulation of cerebellar foci in the fastigial (CBM or in the interposital (NIA) nucleus. CBM or NIA sites, previously identified for the motor effects, were microstimulated, together with the contralateral motor cortex, to give evidence of the facilitations. It was observed that 51% of the NIA motor sites, 46% of the rostral and 33% of the caudal CBM ones, were able, when activated, to evoke facilitatory effects on at least one muscle. The most frequent motor pattern observed following NIA microstimulation was the contraction of a proximal muscle and simultaneously the facilitation of a distal one. Similar responses were detected upon activation of neuronal pools in both zones of CBM. A good number of CBM foci (39% in the rostral division and 33% in the caudal one), however, was unable to induce facilitation, eliciting, upon stimulation, only massive axial movements. Distal muscles were involved by facilitatory effects in a higher number of cases following NIA stimulation (61% of all the facilitatory responses) than CBM rostral (39%) or caudal (43%) one. Furthermore, a particular characteristic of a good percentage of CBM facilitating foci (36% in rostral and 28% in caudal CBM) was the capability to elicit motor activity in the contralateral side and simultaneously facilitation in the ipsilateral one.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in evoked cortico-motor responses in the rat during aging and after partial decortication

Some changes of the organization of cortical motor representations, which were revealed by means of the intracortical microstimulation (ICMS) in aged rats after unilateral partial decortication, were true consequences of the decortication, but had no significant relationship to the aging. Distributions of latent periods of contralateral hind-leg EMG-responses to the ICMS had no significant shifts both during aging and after the surgery. Values of short-latency responses of ipsilateral proximal and axial muscles to the ICMS were in late time periods, from 8 to 16 months after surgery, significantly lower than ones of contralateral homonymous muscles. It is supposed, that ipsi- and bilateral short-latency responses to the ICMS in proximal and axial muscles of operated rats during late time periods are relayed through some pathways from the brain stem to the spinal cord.     

Difference in properties of spontaneous electric activities of visceral nociceptive neurons in bilateral anterior cingulate gyrus of cats

The aim of the present study is to explore the role of anterior cingulate gyrus (ACG) in bilateral cerebral cortex in visceral nociceptive sensation. Electrical stimulation of greater splanchnic nerve (GSN) was used as visceral nociceptive stimulus, and intracellular recording techniques in vivo was used to record and analyze the responses to stimuli and spontaneous electric activities of the neurons in the bilateral ACG. According to the responses to electrical stimulation of GSN, the neurons in the bilateral ACG were divided into GSN-stimulus-relative neurons (GSRNs) and GSN-stimulus-irrelative ones. According to the characteristics of the evoked responses to electrical stimulation of the GSN, GSRNs could be further classified into visceral nociceptive neurons (VNNs) and non-visceral nociceptive neurons (NVNNs). VNNs included specific visceral nociceptive neurons (SVNNs) and non-specific visceral nociceptive neurons (NSVNNs). The results showed that the percentage of GSRNs in the contralateral ACG (38.18%) was significantly higher than that in the ipsilateral ACG (29.49%, P<0.01), suggesting although GSN afferent fibers project to bilateral ACG, they mainly project to the contralateral ACG. Compared with ipsilateral ACG, contralateral ACG possessed lower proportion of SVNNs and higher proportion of NSVNNs (P<0.01). The absolute values of resting potentials (RP) of GSRNs, VNNs, NVNNs and SVNNs in ipsilateral ACG were less than those of corresponding neurons in contralateral ACG. However, there were no significant differences in the absolute values of RP of NSVNNs between ipsilateral and contralateral ACG. There were no significant differences in modes, frequencies and amplitudes of spontaneous electric activities of VNNs and NVNNs between ipsilateral and contralateral ACG. Additionally, the percentage of neurons having spontaneous electric activities from VNNs was significantly higher than that from NVNNs, which indicated that the excitability of VNNs was higher than that of the NVNNs in bilateral ACG. These results suggest that the patterns and degrees of the responses to nociceptive GSN-stimulation of the ipsilateral and contralateral ACG are different, thus providing new experimental data for the asymmetry of functions of the bilateral brain.     

Interlimb Reflexes Induced by Electrical Stimulation of Cutaneous Nerves after Spinal Cord Injury

Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.     

Are region-specific changes in fibre types attributable to nonuniform muscle hypertrophy by overloading?

Muscle fibre composition was compared among the proximal (25%), middle (50%) and distal (75%) regions of the muscle length to investigate whether compensatory overload by removal of synergists induces region-specific changes of fibre types in rat soleus and plantaris muscles. In addition, we evaluated fibre cross-sectional area in each region to examine whether fibre recruitment pattern against functional overload is nonuniform in different regions. Increases in muscle mass and fibre area confirmed a significant hypertrophic response in the overloaded soleus and plantaris muscles. Overloading increased the percentage of type I fibres in both muscles and that of type IIA fibres in the plantaris muscle, with the greater changes being found in the middle and distal regions. The percentage of type I fibres in the proximal region was higher than that of the other regions in the control soleus muscle. In the control plantaris muscle, the percentage of type I and IIA fibres in the middle region were higher than that of the proximal and distal regions. With regard to fibre size, type IIB fibre area of the middle and distal regions in the plantaris increased by 51% and 57%, respectively, with the greater changes than that of the proximal region (37%) after overloading. These findings suggest that compensatory overload promoted transformation of type II fibres into type I fibres in rat soleus and plantaris muscles, with the greater changes being found in the middle and distal regions of the plantaris muscle.     

Visual response properties of neck motor neurons in the honeybee

Recent behavioural studies have demonstrated that honeybees use visual feedback to stabilize their gaze. However, little is known about the neural circuits that perform the visual motor computations that underlie this ability. We investigated the motor neurons that innervate two neck muscles (m44 and m51), which produce stabilizing yaw movements of the head. Intracellular recordings were made from five (out of eight) identified neuron types in the first cervical nerve (IK1) of honeybees. Two motor neurons that innervate muscle 51 were found to be direction-selective, with a preference for horizontal image motion from the contralateral to the ipsilateral side of the head. Three neurons that innervate muscle 44 were tuned to detect motion in the opposite direction (from ipsilateral to contralateral). These cells were binocularly sensitive and responded optimally to frontal stimulation. By combining the directional tuning of the motor neurons in an opponent manner, the neck motor system would be able to mediate reflexive optomotor head turns in the direction of image motion, thus stabilising the retinal image. When the dorsal ocelli were covered, the spontaneous activity of neck motor neurons increased and visual responses were modified, suggesting an ocellar input in addition to that from the compound eyes.     

Effects of peripheral inputs from hindlimb on the monosynaptic reflex of motoneurons innervating tail muscles.

The effects of group II muscle (PBSt, GS) and cutaneous afferent (Sur, SPc, Tib) inputs from the hindlimb on the monosynaptic reflexes of motoneurons innervating tail muscles were studied in lower spinalized cats. Stimulation of the cutaneous nerves at the conditioning-test stimulus interval of about 10-20 ms facilitated and inhibited the monosynaptic reflexes of ipsilateral and contralateral tail muscles, respectively. The effects of the muscle nerve stimulation were not so prominent as those elicited by cutaneous nerve stimulation. The monosynaptic reflex was also inhibited by muscle nerve stimulation at 10-50 ms intervals. The effects of conditioning stimulation of the hindlimb peripheral nerves at short intervals were depressed or blocked by section of the ipsilateral lateral funiculus at S1 spinal segment. These findings show that the neuronal pathway from hindlimb afferents to tail muscle motoneurons passed the lateral funiculus of the spinal cord and modulates the motoneuronal activity of tail muscles.     

Local innervation patterns of the metathoracic flexor and extensor tibiae motor neurons in the cricket Gryllus bimaculatus

To elucidate neural mechanisms underlying walking and jumping in insects, motor neurons supplying femoral muscles have been identified mainly in locusts and katydids, but not in crickets. In this study, the motor innervation patterns of the metathoracic flexor and extensor tibiae muscles in the cricket, Gryllus bimaculatus were investigated by differential back-fills and nerve recordings. Whereas the extensor tibiae muscle has an innervation pattern similar to that of other orthopterans, the flexor has an innervation unique to this species. The main body of the flexor muscle is divided into the proximal, middle and distal regions, which receive morphologically unique terminations from almost non-overlapping sets of motor neurons. The proximal region is innervated by about 12 moderate-sized excitatory motor neurons and two inhibitory neurons while the middle and distal regions are innervated by three and four large excitatory motor neurons, respectively. The most-distally located accessory flexor muscle, inserting on a common flexor apodeme with the main muscle, is innervated by at least four small excitatory (slow-type) and two common inhibitory motor neurons. The two excitatory and two inhibitory motor neurons that innervate the accessory flexor muscle also innervate the proximal bundles of the main flexor muscle. This suggests that the most proximal and distal parts of the flexor muscle participate synergistically in fine motor control while the rest participates in powerful drive of tibial flexion movement.     

Effect of lung volume on the respiratory action of the canine pectoral muscles.

Lung volume influences the mechanical action of the primary inspiratory and expiratory muscles by affecting their precontraction length, alignment with the rib cage, and mechanical coupling to agonistic and antagonistic muscles. We have previously shown that the canine pectoral muscles exert an expiratory action on the rib cage when the forelimbs are at the torso's side and an inspiratory action when the forelimbs are held elevated. To determine the effect of lung volume on intrathoracic pressure changes produced by the canine pectoral muscles, we performed isolated bilateral supramaximal electrical stimulation of the deep pectoral and superficial pectoralis (descending and transverse heads) muscles in 15 adult supine anesthetized dogs during hyperventilation-induced apnea. Lung volume was altered by application of a negative or positive pressure (+/- 30 cmH2O) to the airway. In all animals, selective electrical stimulation of the descending, transverse, and deep pectoral muscles with the forelimbs held elevated produced negative intrathoracic pressure changes (i.e., an inspiratory action). Moreover, with the forelimbs elevated, increasing lung volume decreased both pectoral muscle fiber precontraction length and the negative intrathoracic pressure changes generated by contraction of each of these muscles. Conversely, with the forelimbs along the torso, increasing lung volume lengthened pectoral muscle precontraction length and augmented the positive intrathoracic pressure changes produced by muscle contraction (i.e., an expiratory action). These results indicate that lung volume significantly affects the length of the canine pectoral muscles and their mechanical actions on the rib cage.     

Significance of the corpus callosum in the paired activity of the respiratory center

The asymmetrical reactions of respiratory neurons of the right and left halves of the respiratory center and varied changes in bioelectrical activity of external intercostal muscles on both sides of the chest were discovered in experiments on anesthetized cats in response to successive electrical stimulation of the symmetrical cortical areas of the right and left cerebral hemispheres before and after callosotomy. It was demonstrated that callosotomy increased on both sides of the respiratory center the quantity of neurons responsive to ipsilateral cortical stimulation and determined the character of the asymmetrical reactions of right and left respiratory neurons and intercostal muscles. On the basis of the data obtained it is concluded that the corpus callosum contributes to the functional integration of both halves of the respiratory center.     

The pattern of innervation in serially duplicated axolotl limbs: further evidence for the existence of local pathway cues?

The innervation of the biceps muscle was examined in regenerated and vitamin A-induced serially duplicated axolotl forelimbs using retrograde transport of horseradish peroxidase. The regenerated biceps muscle becomes innervated by motor neurones in the same position in the spinal cord as the normal biceps motor pool. In previous experiments in which the innervation of a second copy of a proximal limb muscle was examined in serially duplicated limbs (Stephens, Holder & Maden, 1985), the duplicate muscle was found to become innervated by motor neurones that would normally have innervated distal muscles. In the present study, the innervation of the second copy of biceps was examined under conditions designed to encourage nerve sprouting from 'correct' biceps axons. Following either partial limb denervation or denervation coupled with removal of the proximal biceps, the second copy of the muscle was still innervated by inappropriate motor neurones, which again would normally innervate distal limb muscles. These results are interpreted as evidence for the necessity for an appropriate local environment for axonal growth to allow reformation of a correct pattern of motor innervation in the regenerated limb.     

The oculomotor system of Daphnia magna

Summary The highly mobile cyclopic compound eye of Daphnia magna is rotated by six muscles arranged as three bilateral pairs. The three muscles on each side of the head share a common origin on the carapace and insert dorsally, laterally and ventrally on the eye. The dorsal and ventral muscles are each composed of two muscle fibers and the lateral muscle is composed of from two to five fibers, with three the most common number. Individual muscle fibers are spindle-shaped mononucleated cells with organized bundles of myofilaments. Lateral eye-muscle fibers are thinner than those of the other muscles but are otherwise similar in ultrastructure. Two motor neurons innervate each dorsal and each ventral muscle and one motor neuron innervates each lateral muscle. The cell bodies of the motor neurons are situated dorsally in the supraesophageal ganglion (SEG) and are ipsilateral to the muscles they innervate. The dendritic fields of the dorsal-muscle motor neurons are ipsilateral to their cell bodies; those of the ventral-muscle motor neurons are bilateral though predominantly contralateral. The central projections of the lateral-muscle motor neurons are unknown. In the dorsal and ventral muscles one motor axon synapses principally with one muscle fiber; in each lateral muscle the single motor axon branches to, and forms synapses with, all the fibers. The neuromuscular junctions, characterized by pre- and postsynaptic densities and clear vesicles, are similar in all the eye muscles.     

The effect of ambient temperature on infrared thermographic images of joints in the distal forelimbs of healthy racehorses

The aim of the study was to describe the dependence on ambient temperature of distal joint temperature at the forelimbs of racehorses. The study also investigated the influence of differing ambient temperatures on the temperature difference between joints: this was measured ipsilaterally (i.e. between the carpal and fetlock joints along each forelimb) and contralaterally (i.e. between the same joints of the left and right forelimbs). Sixty-four healthy racehorses were monitored over 10 months. At each session, three thermographic images were taken of the dorsal, lateral and medial aspects of the distal forelimbs. Temperature measurements were made from regions of interest (ROIs) covering the carpal and fetlock joints. There was a strong correlation between ambient temperature and absolute joint temperature at all ROIs. The study also observed a moderate correlation between ambient temperature and the ipsilateral temperature differences between joints when measured from the medial and lateral aspects. No significant correlation was noted when measured dorsally. The mean contralateral temperature differences between joints were all close to 0 °C. The data support previous reports that the temperature distribution between the forelimbs of the healthy equine is generally symmetric, although some horses differ markedly from the average findings.     

Reflection of space characteristics of an acoustic signal in the neuronal activity of the caudate nucleus

In chronic experiment, responses of single neurons of the caudate nucleus (CN) to spatial characteristics of acoustic signal have been investigated in dogs. It is shown that 92% of neurons of the caudate nucleus' head responding to sound stimulation asymmetrically react to contra- and ipsilateral monoaural, with a greater efficacy of a contralateral stimulation. For 50% of the CN neurons simultaneous sound inputs appear more effective in comparison with contralateral stimulation. 77% of the caudate neurons responding to sound have shown sensitivity to change of the value and sign of the interaural delay.     

Responses of efferent octopaminergic thoracic unpaired median neurons in the locust to visual and mechanosensory signals

Insect thoracic ganglia contain efferent octopaminergic unpaired median neurons (UM neurons) located in the midline, projecting bilaterally and modulating neuromuscular transmission, muscle contraction kinetics, sensory sensitivity and muscle metabolism. In locusts, these neurons are located dorsally or ventrally (DUM- or VUM-neurons) and divided into functionally different sub-populations activated during different motor tasks. This study addresses the responsiveness of locust thoracic DUM neurons to various sensory stimuli. Two classes of sense organs, cuticular exteroreceptor mechanosensilla (tactile hairs and campaniform sensilla), and photoreceptors (compound eyes and ocelli) elicited excitatory reflex responses. Chordotonal organ joint receptors caused no responses. The tympanal organ (Müller's organ) elicited weak excitatory responses most likely via generally increased network activity due to increased arousal. Vibratory stimuli to the hind leg subgenual organ never elicited responses. Whereas DUM neurons innervating wing muscles are not very responsive to sensory stimulation, those innervating leg and other muscles are very responsive to stimulation of exteroreceptors and hardly responsive to stimulation of proprioceptors. After cutting both cervical connectives all mechanosensory excitation is lost, even for sensory inputs from the abdomen. This suggests that, in contrast to motor neurons, the sensory inputs to octopaminergic efferent neuromodulatory cells are pre-processed in the suboesophageal ganglion.     

Sprouting of aberrant neuropeptide Y-immunoreactive sympathetic nerves into neonatally denervated smooth muscle.

Sympathetic nerves normally project ipsilaterally to lateral cranial targets. Following unilateral superior cervical ganglionectomy in neonatal rats, however, neurons from the contralateral superior cervical ganglion sprout into the denervated region. In the present study we examined neuropeptide Y immunoreactivity (NPY-ir) of neurons comprising ipsilateral (control) and denervation-induced contralateral pathways to the superior tarsal smooth muscle of the eyelid. Fluoro-Gold injection of the control muscle retrogradely labelled 133 +/- 18 neurons in the ipsilateral superior cervical ganglion; of these, 21 +/- 3% displayed detectable NPY-ir. Fluoro-Gold injections of the reinnervated muscle labelled 20 +/- 4 neurons in the contralateral superior cervical ganglion, of which 85 +/- 3% contained detectable NPY-ir. Examination of the control tarsal muscle revealed DBH-ir noradrenergic nerves throughout the muscle and vasculature, while NPY-ir nerves were present primarily around blood vessels. In the reinnervated preparation, NPY-ir fibers innervated both blood vessels and tarsal muscle in a pattern similar to that of DBH-ir innervation. Acute excision of the remaining superior cervical ganglion eliminated all DBH-ir fibers bilaterally; NPY-ir was reduced markedly in the reinnervated preparations, though some fibers remained. We conclude that, following neonatal denervation, the tarsal muscle is reinnervated by a subpopulation of sympathetic neurons that differs in neuropeptide phenotype from that of the normal ipsilateral innervation.     

Tonic inhibitory influences of locus coeruleus on the response gain of limb extensors to sinusoidal labyrinth and neck stimulations

Previous experiments had shown that in decerebrate cats activation of limb extensor motoneurons during side-down roll tilt of the animal or side-up neck rotation depends on both an increased discharge of excitatory vestibulospinal (VS) neurons and a reduced discharge of inhibitory reticulospinal (RS) neurons of the medulla, thus leading to disinhibition of limb extensor motoneurons. The present experiments were performed to find out whether the locus coeruleus (LC) complex keeps under its tonic inhibitory control the medullary inhibitory RS neurons and, if so, whether this structure intervenes in the gain regulation of the vestibular and neck reflexes acting on the limb extensor musculature. In precollicular decerebrate cats with good postural rigidity of the four limbs, the amplitude of modulation and thus the response gain of the first harmonic component of multiunit EMG responses of limb extensors to sinusoidal stimulation of labyrinth and neck receptors (at 0.15 Hz, +/- 10 degrees) were quite small in forelimb muscles (triceps brachii) and almost negligible or absent in hindlimb muscles (triceps surae). Electrolytic lesion limited to the LC complex decreased the tonic contraction of limb extensors, but greatly increased in the forelimbs (and brought to the light in the hindlimbs) the response modulation of extensor muscles to the same parameters of labyrinth or neck stimulation. Correspondingly, the response gain increased, but no change in the phase angle of the responses was observed. Both changes in posture, as well as in response gain of the limb extensors to labyrinth and neck stimulation, fully developed some time after the LC lesion. This increase in response gain of the vestibular and neck reflexes acting on the limb extensor muscles did not depend on the decrease in postural activity following the LC lesion, since it was still obtained when an increased static stretch of the extensor muscle following passive flexion of the limb compensated for the reduced EMG activity. Moreover, the slope of the regression line relating the gain of the multiunit EMG response of the triceps brachii to animal tilt with the base frequency greatly increased following lesioning of the LC, thus indicating that for the same background discharge of the muscle the amplitude of modulation, and thus the response gain, increased significantly. The effects described above involved mainly, but not exclusively, the limbs ipsilateral to the side of the lesion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Development of the gin trap reflex inManduca sexta: a comparison of larval and pupal motor responses

1. Responses of motor neurons in larvae and pupae of Manduca sexta to stimulation of tactile sensory neurons were measured in both semi-intact, and isolated nerve cord preparations. These motor neurons innervate abdominal intersegmental muscles which are involved in the production of a general flexion reflex in the larva, and the closure reflex of the pupal gin traps. 2. Larval motor neurons respond to stimulation of sensory neurons innervating abdominal mechanosensory hairs with prolonged, tonic excitation ipsilaterally, and either weak excitation or inhibition contralaterally (Figs. 4A, 6). 3. Pupae respond to tactile stimulation of mechanosensory hairs within the gin traps with a rapid closure reflex. Motor neurons which innervate muscles ipsilateral to the stimulus exhibit a large depolarization, high frequency firing, and abrupt termination (Figs. 2, 4B). Generally, contralateral motor neurons fire antiphasically to the ipsilateral motor neurons, producing a characteristic triphasic firing pattern (Figs. 7, 8) which is not seen in the larva. 4. Pupal motor neurons can also respond to sensory stimulation with other types of patterns, including rotational responses (Fig. 3A), gin trap opening reflexes (Fig. 3B), and 'flip-flop' responses (Fig. 9). 5. Pupal motor neurons, like larval motor neurons, do not show oscillatory responses to tonic current injection, nor do motor neurons of either stage appear to interact synaptically with one another. Most pupal motor neurons also exhibit i-V properties similar to those of larval motor neurons (Table 1; Fig. 10). Some pupal motor neurons, however, show a marked non-linear response to depolarizing current injection (Fig. 11).     

The morphology,physiology and function of suboesophageal neck motor neurons in the honeybee

We report some of the neural and muscular circuitry that allows honeybees to control head movements. We studied neck motor neurons with cell bodies in the suboesophageal ganglion, axons in the first cervical nerve (IK1) and terminals in neck muscles 44 and 51 (muscle classification: Snodgrass in Smithsonian Misc Coll 103:1-120, 1942). We show that muscle 44 actually comprises five separate bundles of muscle fibres (subunits), while muscle 51 is split into two subunits. Eight motor neurons innervate muscles 44 and 51. Two motor neurons have cell bodies in the ventral-median cell body group (one innervates a subunit in muscle 44, the other a subunit in muscle 51). One motor neuron has a ventrally located contralateral cell body (innervating a subunit in muscle 44) and five have laterally located ipsilateral cell bodies. Of the five lateral cells, one innervates a subunit in muscle 51, three selectively innervate subunits in muscle 44 and one co-innervates a subunit in muscle 44 with the contralateral cell. Extracellular recordings revealed three types of visually driven, direction-selective cell-types in each IK1 tuned for leftward, rightward and downward motion over the eyes. The spatiotemporal tuning of the units is similar to that of other visual interneurons in the bee brain.     

The number of sarcomeres and architecture of the M. gastrocnemius of the rat

The aim of this study was to investigate the number of sarcomeres of different regions (proximal, intermediate and distal third) of the M. gastrocnemius of the rat and compare them with in vivo measurements of the length of the most proximal and distal muscle bundles. These lengths were measured with the aid of dividers at the muscle resting length. The number of sarcomeres was calculated from the length of fibres (measured at 20 times enlargement) tested from HNO3-treated muscle and the average sarcomere length (determined from 80 microns samples taken along the fibres every 800 microns). Ten fibres were isolated from each of three regions of six muscles. All muscles showed the smallest number of sarcomeres in the proximal region of the muscle and increasingly higher numbers in the intermediate and distal parts. The number of sarcomeres in the proximal region is significantly (p less than 0.01) smaller than that of the distal region. These results agree with the results of in vivo length measurements of the most proximal and distal bundles (resp. 31 and 36% of the muscle resting length), the former being significantly (p less than .01) smaller. As there is no significant difference (p less than 0.01) in the length of the treated fibres of the three regions it is concluded that HNO3 treatment does affect the fibres of the muscle in the different regions in a non uniform fashion.