Biomedical subjects

G Macefield

Publications and source records attributed to G Macefield.

17 recordsLinked to original sources

PubMed1992

Peripheral and central delays in the cortical projections from human truncal muscles. Rapid central transmission of proprioceptive input from the hand but not the trunk.

In contrast to the cortical connections to and from the muscles of the hand, the transmission of an afferent volley from the intercostal muscles to the cerebral cortex takes approximately 10 ms longer than it takes a cortical motor volley to reach the muscle. This disparity in afferent and efferent cortical transmission times could be due to a slower peripheral conduction velocity of intercostal muscle afferents or a slower afferent conduction within the central nervous system. The present study derived peripheral and central conduction times for the truncal muscles from the onsets of the mechanically evoked intercostal and abdominal spinal reflexes and the onsets of the cortical sensory potentials. Mean latencies of the ipsilateral intercostal and abdominal reflexes (evoked and recorded in the mid-clavicular line) were 11.9 +/- 0.7 (SEM) ms and 13.7 +/- 0.9 ms, respectively; calculated peripheral conduction velocities were 69.4 +/- 4.1 m/s and 56.2 +/- 2.3 m/s (assuming equal velocities for the sensory and motor axons and an intraspinal delay of 1 ms). Central sensory conduction time (spinal cord to cortex) was calculated by subtracting the peripheral conduction times for the intercostal and abdominal afferents (5.5 +/- 0.3 ms and 6.4 +/- 0.4 ms) from the onsets of the cortical sensory potentials (19.4 +/- 0.8 ms and 25.3 +/- 12.3 ms); central sensory conduction times (14.2 +/- 1.7 ms and 18.6 +/- 2.3 ms) were 8-11 ms longer than central motor conduction times. These results demonstrate that peripheral conduction velocities of intercostal and abdominal afferents are not slow, and that, when compared with the extremities, there is a relatively long central conduction time for proprioceptive information from the trunk to the cerebral cortex.

Adult↗

PubMed1991

Long-lasting depression of central synaptic transmission following prolonged high-frequency stimulation of cutaneous afferents: a mechanism for post-vibratory hypaesthesia.

High-frequency vibration or electrical stimulation of cutaneous afferents may produce long-lasting hypaesthesia. Such stimulation alters the excitability of axons in the peripheral nerve but there is evidence that this does not completely explain the hypaesthesia. The present study was undertaken to determine whether a prolonged afferent barrage results in depression of synaptic transmission at a central site. Changes in central excitability to cutaneous inputs were examined in normal subjects by measuring the cerebral evoked potential at different stages after high-frequency conditioning stimulation of the digital nerves. Changes in peripheral excitability were eliminated by adjusting the stimulus intensity so that a constant afferent volley entered the central nervous system. Following the conditioning stimulation (4-5 T, 200 Hz, 10 min), the cortical potential evoked by constant submaximal test volleys was depressed by up to 50% for 25 min. The attenuation was less profound (10-20%) but more prolonged (greater than 45 min) when maximal test volleys were used, and occurred regardless of whether the high-frequency stimulation was applied to the test digit or to adjacent digits. It is concluded that prolonged activation of cutaneous afferents causes a depression in central excitability independent of and additional to peripheral changes, and it is suggested that this mechanism contributes to the associated perceptual disturbances. By analogy it is suggested that the hypaesthesia associated with prolonged vibration may be of central rather than peripheral origin.

Adult↗

PubMed1991

Paraesthesiae and tetany induced by voluntary hyperventilation. Increased excitability of human cutaneous and motor axons.

Anxiety can induce hyperventilation, and the resultant hypocapnia and hypocalcaemia can lead to paraesthesiae and tetany. To define the nature of the disturbance created in peripheral nerve, the excitability of cutaneous and motor axons was monitored in 6 normal subjects requested to hyperventilate until paraesthesiae developed in the hands, face and trunk. This occurred when alveolar PCO2 (PACO2) had declined on average by 20 mmHg. Spontaneous EMG activity developed when PACO2 had declined by a further 4 mmHg. Changes in the excitability of cutaneous and motor axons were measured from changes in the compound action potentials evoked by stimulation of the median nerve at the wrist and recorded over the digital nerves of the index finger and over the thenar muscles, respectively. As PACO2 declined, the size of the compound sensory and muscle potentials evoked by a constant stimulus progressively increased, indicating an increase in axonal excitability. These changes occurred before paraesthesiae or tetany developed. In each subject there was a statistically significant inverse correlation between PACO2 and axonal excitability. Independent of this increase in axonal excitability, there was no significant change in the supernormal phase of the recovery cycle of cutaneous axons. Microneurographic recordings from the median nerve in 2 subjects revealed spontaneous bursting activity of cutaneous axons, perceived as paraesthesiae. It is concluded that the paraesthesiae and tetany induced by hyperventilation result solely from changes in the excitability of cutaneous and motor axons in the peripheral nerve, presumably due to an alteration in the electrical properties of the axonal membrane resulting from a reduced plasma [Ca2+]. The supernormal phase may entrain the ectopic discharge and thereby determine the maximal discharge frequency of impulses in ectopically generated trains, but does not otherwise contribute to the physiological disturbance.

Action Potentials↗

PubMed1991

The cortical drive to human respiratory muscles in the awake state assessed by premotor cerebral potentials.

1. We investigated the possibility of a cortical contribution to human respiration by recording from the scalp of awake subjects the premotor cerebral potentials that are known to precede voluntary limb movements. 2. Electroencephalographic activity (EEG) was recorded from scalp electrodes and averaged for 1.8-2.0 s before the time at which airway pressure exceeded an inspiratory or expiratory threshold. Clear premotor cerebral potentials were recorded during brisk, self-paced nasal inhalations or exhalations. In ten subjects, a slow cortical negativity (Bereitschaftspotential) was apparent in the averaged EEG, commencing 1.2 +/- 0.3 s before the onset of inspiratory (scalene) or expiratory (abdominal) muscle activity (EMG). It was maximal at the vertex, with a mean slope of 12.3 +/- 5.8 microV/s, and was followed by a post-movement positivity. 3. In four subjects the inspiratory premotor potential culminated in a large negativity, the motor potential, which began 24 +/- 15 ms before the onset of scalene EMG. It is argued that such a short latency is consistent with a volitionally generated respiratory command which travels relatively directly to the respiratory muscles, having a total central delay which is no longer than that for voluntary finger movements. 4. That the respiratory premotor and motor potentials did not originate in subcortical structures was supported by their absence in a patient suffering from chronic reflexogenic hiccups, in whom cerebral activity was back-averaged from each brisk hiccup. 5. During quiet breathing, in which subjects were relaxed and distracted from thinking about their respiration, no premotor cerebral potentials preceding inspiration could be detected. This failure was not due to the slow rate of rise of inspiratory activity during quiet breathing as compared with a brisk sniff, because premotor potentials were detected when subjects intermittently generated slow active expiratory efforts. 6. These observations suggest that during quiet breathing the cerebral cortex does not contribute to respiratory drive on a breath-by-breath basis. Conversely, the presence of clear premotor cerebral potentials when subjects performed self-paced inspiratory or expiratory manoeuvres illustrates the powerful cortical projection to human respiratory muscles.

Adult↗

PubMed1991

Decline in spindle support to alpha-motoneurones during sustained voluntary contractions.

1. To address whether the muscle spindle support to alpha-motoneurones is maintained during prolonged isometric voluntary contractions, the discharge of eighteen muscle spindle afferents, originating in the dorsiflexors of the ankle or toes, was recorded from the common peroneal nerve in eight subjects. Isometric contractions were generally sustained for 1 min, usually below 30% of the maximal voluntary dorsiflexion force. 2. Once the afferent had been identified, subjects were instructed to dorsiflex the foot slowly to recruit the spindle ending, to continue the ramp contraction until a predetermined target force was reached, and then to hold that force until requested to relax. 3. Five muscle spindle afferents maintained a constant discharge frequency during the hold phase of the isometric contraction. Following relaxation of the contraction two spindle afferents from tibialis anterior, exhibited a post-contraction discharge despite the absence of detectable electromyographic activity (EMG). 4. The discharge frequency of most of the spindle afferents (72%) declined progressively during the isometric contraction. The mean firing rates had declined to two-thirds of those at the onset of the contraction by 30 s, and to half after 1 min. The decline in spindle firing rate commenced during the ramp phase of the contraction and was statistically significant by 10 s, when force was held constant. The extent of the decline was greater for those units with the higher initial firing rates and for those units studied after many preceding contractions. 5. In the same contractions a progressive increase in EMG was required to maintain force and consequently the change in EMG was inversely related to the change in spindle discharge. While many mechanisms may contribute to the decline in spindle discharge during a sustained isometric contraction, it is argued that the result will be a progressive disfacilitation of alpha-motoneurones, which may contribute to the decline in motor unit firing rates during a sustained contraction.

Adult↗

PubMed1990

Voluntary activation of human motor axons in the absence of muscle afferent feedback. The control of the deafferented hand.

The ability to activate human motoneurons supplying individual intrinsic muscles of the hand was examined during acute deafferentation of the muscles. Tungsten microelectrodes were inserted percutaneously into motor fascicles of the ulnar nerve of 5 subjects, which was then blocked distally with local anaesthetic. In 4 subjects unitary action potentials were recorded from 16 motor axons, which were identified with respect to their target muscles. In the complete absence of muscle afferent feedback, subjects could voluntarily recruit motoneurons, grade their discharge and sustain a constant level of activity. Significant facilitation of motor efforts was provided by cutaneous feedback from the digits via the median nerve. During attempted maximal voluntary efforts the mean discharge frequencies of single motor axons were significantly lower than those of normally-innervated motor units. This finding suggests that peripheral afferents have a net facilitatory influence on motoneurons. However, during prolonged (20-30 s) maximal voluntary efforts the deafferented motoneurons did not display the progressive decline in discharge frequency shown by normally-innervated motor units during contractile fatigue, a finding consistent with two possible explanations: disfacilitation or reflex inhibition of the motoneuron pool by peripheral afferents. The results also indicate that the otherwise intact nervous system can perform some simple motor tasks with no proprioceptive input other than knowledge of the motor commands. Other factors may contribute to the poor motor performance reported for patients with severe sensory deficits.

Adult↗

PubMed1990

Perceptual responses to microstimulation of single afferents innervating joints, muscles and skin of the human hand.

1. Microneurographic techniques were used to isolate single afferent axons within cutaneous and motor fascicles of the median and ulnar nerves at the wrist in thirteen subjects. Of the sixty-five identified afferents, eleven innervated the interphalangeal and metacarpophalangeal joints, sixteen innervated muscle spindles, three innervated Golgi tendon organs and thirty-five supplied the glabrous skin of the hand. 2. Intrafascicular stimulation through the recording microelectrode, using trains of constant-voltage positive pulses (0.3-0.8 V, 0.1-0.2 ms, 1-100 Hz) or constant-current biphasic pulses (0.4-13.0 microA, 0.2 ms, 1-100 Hz), evoked specific sensations from sites associated with some afferent species but not others. 3. Microstimulation of eight of the eleven joint afferent sites (73%) evoked specific sensations. With four, subjects reported innocuous deep sensations referred to the relevant joint. With the other four, the subjects reported a sensation of joint displacement that partially reflected the responsiveness of the afferents to joint rotation. 4. Microstimulation of fourteen of the sixteen muscle spindle afferent sites (88%) generated no perceptions when the stimuli did not produce overt movement. However, subjects could correctly detect the slight movements generated when the stimuli excited the motor axons to the parent muscle. 5. With seven of the nine rapidly adapting (type RA or FAI) cutaneous afferents (88%) microstimulation evoked sensations of 'flutter-vibration', and with two of eight slowly adapting (type SAI) afferents (25%) it evoked sensations of 'sustained pressure'. Of the eighteen SAII afferents, which were classified as such by their responses to planar skin stretch, the majority (83%) generated no perceptions, confirming previous work, but three evoked sensations of movements or pressure. 6. The present results suggest a relatively secure transmission of joint afferent traffic to perceptual levels, and it is concluded that the human brain may be able to synthesize meaningful information on joint displacement on the basis of impulses in a single joint afferent. This could partly compensate for the low responsiveness of individual joint afferents within the physiological range of joint displacements. Although single muscle spindle afferents can adequately encode joint position and movement, the results suggest that the brain needs the information from more than one muscle spindle afferent to perceive changes in joint angle.

Action Potentials↗

PubMed1989

The cortical distribution of muscle and cutaneous afferent projections from the human foot.

Somatosensory evoked potentials to electrical stimulation of muscle and cutaneous afferents from the foot were recorded in normal human subjects using multiple channels centred on the vertex and referenced to the contralateral earlobe. Low-threshold muscle afferents were selectively activated by an insulated microelectrode inserted percutaneously at the motor point of abductor hallucis. Low-threshold cutaneous and joint afferents of the hallux or second toe were stimulated with ring electrodes. The posterior tibial and sural nerves were stimulated at the ankle through surface electrodes. The cerebral distribution of the initial cortical response (N33-P40) to stimulation of muscle afferents largely paralleled that to stimulation of its parent nerve, the posterior tibial nerve (which contains afferents of muscle, cutaneous and joint origin). They were maximal slightly posterior and ipsilateral to the vertex. The cutaneous-joint afferent projection from the hallux paralleled that from the sural nerve and both were less lateralized than the tibial and abductor hallucis projections.

Adult↗

PubMed1989

Projection of low-threshold afferents from human intercostal muscles to the cerebral cortex.

Low-threshold afferents from human limb muscles are known to project to the sensorimotor cortex and to contribute to proprioception. However, there are few data on the cortical projection of afferents from human respiratory muscles. The present study employed evoked-potential techniques to determine whether low-threshold muscle afferents from the chest wall project to cortical levels in conscious human subjects. In four subjects intramuscular afferents of the second parasternal and fifth lateral intercostal muscles were selectively stimulated through an insulated microelectrode inserted percutaneously at the respective motor point. Evoked potentials were recorded and averaged from eight scalp sites. The initial cortical component of the cerebral response to intramuscular stimulation of the second and fifth interspaces was a negative potential commencing at 19.2 +/- 2.1 msec and 20.7 +/- 1.1 msec respectively. The dominant early cortical potential was largest at the vertex, and was comparable in amplitude (0.58 +/- 0.23 microV) to that for individual muscles of the upper and lower limbs. The cortical focus was distributed differently from that for cutaneous afferents of the chest wall and for both muscle and cutaneous afferents from the upper and lower limbs. This study provides direct evidence for a short-latency projection from intercostal muscle afferents (group I and/or II) to the human cerebral cortex.

Adult↗

PubMed1989

Conduction velocities of muscle and cutaneous afferents in the upper and lower limbs of human subjects.

In the cat and monkey the fastest axons in the peripheral nerve are group I afferents from muscle, but there are no definitive data on conduction velocity for these afferents in human subjects. Knowledge of the relative conduction velocities of muscle and cutaneous afferents is important for the interpretation of reflex studies, evoked potentials and other aspects of motor control. To rectify this deficiency, the conduction velocities of the fastest muscle and cutaneous afferents were determined for the median, ulnar and tibial nerves of normal subjects. Low-threshold muscle afferents innervating abductor pollicis brevis, abductor digiti minimi and abductor hallucis were stimulated selectively through a microelectrode inserted percutaneously at the motor point. Low-threshold cutaneous afferents were stimulated with ring electrodes around the proximal phalanx of digits II or V for the upper limb and digit II for the lower limb. Compound action potentials were recorded with bipolar near-nerve electrodes at two sites in the proximal limb segment and conduction velocities of the fastest afferents in the neural volley calculated. The mean conduction velocities of the muscle and cutaneous afferents were, respectively, 74.7 +/- 6.5 m.s-1 and 80.3 +/- 6.7 m.s-1 for the median nerve, 67.5 +/- 10.2 m.s-1 and 67.5 +/- 10.5 m.s-1 for the ulnar nerve, and 54.7 +/- 3.4 m.s-1 and 52.8 +/- 3.2 m.s-1 for the tibial nerve. For upper and lower limb nerves the conduction velocities of low-threshold muscle and cutaneous afferents were not significantly different when measured over the same proximal segment.

Adult↗

PubMed1988

Responses to passive movement of receptors in joint, skin and muscle of the human hand.

1. Microneurographic techniques were employed to record unitary activity from afferents associated with digital joints of six conscious human subjects. Of 120 single afferents sampled from the median and ulnar nerves at the wrist, eighteen (15%) were classified as joint afferents; the majority of the sample (72.5%) were of cutaneous origin, and 12.5% were from muscle spindles and tendon organs. 2. Of the eighteen joint afferents six were tonically active in the rest position of the hand. All except two were recruited or accelerated their background discharge during passive joint movement. Three tonically active afferents were responsive to passive movement throughout the physiological range. The majority of the afferents, including the other three tonically active units, responded only towards the limits of joint rotation. 3. As a group, the sample of joint afferents had a limited capacity to signal the direction of joint movement. Nine of the sixteen joint afferents sensitive to movement responded in two axes of angular displacement, and two responded in all three axes. In any one axis of rotation eight afferents were activated in both directions of movement. However, one afferent, associated with the interphalangeal joint of the thumb, responded uni-directionally throughout the physiological range of joint movement and was thereby capable of adequately encoding joint position and movement. 4. Twenty-one of twenty-nine slowly adapting and eleven of eighteen rapidly adapting cutaneous afferents tested were activated by joint movement, but only towards the limits of joint rotation; half of the thirty-two movement-sensitive afferents were bi-directionally responsive. Muscle spindle afferents responded to stresses applied to the joint only if the resulting passive movement stretched the parent muscle. 5. It is concluded that human joint afferents possess a very limited capacity to provide kinaesthetic information, and that this is likely to be of significance only when muscle spindle afferents cannot contribute to kinaesthesia.

Action Potentials↗

PubMed1987

Laryngeal motoneurone activity in the rabbit during asphyxic gasping.

Unitary activity was recorded from 44 recurrent laryngeal nerve (RLN) and 18 external laryngeal nerve (ELN) motoneurones in anaesthetized, paralysed rabbits during the development of periods of severe asphyxia caused by temporarily interrupting artificial ventilation. Ventilation was only recommenced when a simultaneously recorded phrenic neurogram showed the animal had attempted several gasps. Inspiratory-phased fibres (17 RLN, 14 ELN) displayed brief high frequency discharges with each gasp. Some inspiratory-phased ELN motoneurones (6/14) also displayed longer tonic discharges after each of the first few gasps. In the expiratory intervals immediately following each gasp many expiratory-phased fibres (13/21 RLN, 2/4 ELN) were active and 7 RLN fibres discharged a long train of impulses with a decrementing discharge pattern. In 3 anaesthetized rabbits, breathing spontaneously through a tracheostomy, a constant airflow was directed up through the larynx; successive gasps were associated with wide swings in translaryngeal pressure which were confirmed visually as reflecting rapid dilation of the glottis during gasps and intense constriction in the intervals between gasps.

Action Potentials↗

PubMed1987

Phrenic and external intercostal motoneuron activity during progressive asphyxia.

Phrenic and external intercostal motoneuron activities were compared during progressive asphyxia induced by the interruption of artificial ventilation in the pentobarbital-urethan-anesthetized, gallamine-paralysed rabbit. The relative augmentation of inspiratory activity of the T1-T4 external intercostal nerves was significantly greater than that of the phrenic nerve during asphyxic hyperpnea. This was associated with a greater recruitment of intercostal than of phrenic motoneurons, particularly late in the hyperpneic phase immediately before the period of asphyxic apnea. However, peak and average discharge frequencies developed by intercostal motoneurons (n = 20) were only approximately 60% of those of the phrenic motoneurons (n = 28). Gasping respiration terminated the apneic period and was associated with a further intense recruitment of intercostal though not of phrenic motoneurons, but discharge frequencies developed by the intercostal motoneurons remained approximately 60% of those of the phrenic motoneurons. The instantaneous frequency profiles generated by the motoneurons often exhibited progressive changes during the terminal stages of hyperpnea (reduction in inspiratory duration and duty cycle and increases in inspiratory slope and discharge frequencies) such that much of the character of gasping respiration became evident before the apnea. Such smooth transitional sequences do not obviate the existence of an "independent gasping center" but do require that such a proposed center at least possess the capacity for interaction with those sites responsible for the generation of eupneic and hyperpneic respiration.

Action Potentials↗

PubMed1986

Respiratory muscle activity during asphyxic apnoea and opisthotonus in the rabbit.

The behaviour of submandibular, cervical, thoracic and abdominal respiratory muscles was examined in the pentobarbitone-urethane-anaesthetized rabbit during progressive asphyxia induced by rebreathing. During asphyxic hyperpnoea the external intercostal, interchondral and scalene inspiratory activities augmented until succeeded by the apnoeic period, in which all were inhibited with the diaphragm. Likewise, the genioglossus, sternohyoid and thyrohyoid muscles exhibited inspiratory augmentation during asphyxic hyperpnoea until the onset of apnoeic inhibition. However, late in the apnoea these muscles, together with the sternothyroid, sternomastoid and digastric muscles, generated an augmenting tonic discharge associated with an intense abdominal constriction, and with the extension of the limbs characteristic of opisthotonus. This intense tonic activity, which was never expressed by the diaphragm and thoracic inspiratory muscles, was immediately interrupted or terminated by the subsequent inspiratory efforts of gasping respiration, during which the abdominal muscles were inhibited but all the submandibular, cervical and thoracic inspiratory muscles greatly participated. The mylohyoid muscles presented augmenting expiratory activity during asphyxic hyperpnoea which declined during the apnoea. These muscles, however, did not exhibit the intense tonic discharge expressed by the expiratory abdominal and inspiratory submandibular and cervical muscles in late apnoea and were not active in gasping.

Animals↗

PubMed1986

Inspiratory augmentation during asphyxic hyperpnoea and gasping: proprioceptive influences.

The relative inspiratory augmentations of sternomastoid, scalene, external intercostal, interchondral and diaphragmatic electromyographic activities were examined during the progressive asphyxia induced by rebreathing in pentobarbitone/urethane anaesthetized rabbits. Diaphragmatic activity augmented significantly less than that of the scalene, intercostal and interchondral muscles in response to the asphyxic increase in inspiratory drive (hyperpnoea). Cervical vagotomy significantly increased the levels of inspiratory activity during the asphyxic hyperpnoea but did not abolish these relations. Dorsal rhizotomy at levels appropriate for the respective recording sites significantly limited the intercostal and interchondral augmentations, and depressed diaphragmatic activity to a less pronounced extent, but did not affect scalene activity. The asphyxic apnoea succeeding the hyperpnoea was terminated by gasping respiration. Excepting the interchondrals each muscle exhibited significantly greater electromyographic amplitudes during gasping than during the terminal hyperpnoeic efforts. Only during gasping were the sternomastoid muscles recruited. The intercostal and scalene gasps were significantly greater than those exhibited by either the interchondrals or the diaphragm. Such disproportionate responses were not affected by vagotomy or dorsal rhizotomy. The disparate inspiratory contributions during progressive asphyxia can therefore only be partially accounted for by differences in proprioceptive control.

Animals↗

PubMed1985

Tachypnoeic response to amyl nitrite inhalation in the rabbit.

Intratracheal inhalation of amyl nitrite, a non-specific smooth muscle relaxant, in the pentobarbitone/urethane anaesthetized rabbit caused reductions in tidal volume and both inspiratory and expiratory times, without a preceding apnoea, that were independent of the associated hypotension and of reflex influences from the carotid sinus region but dependent on supra-abdominal vagal integrity. In artificially ventilated, paralyzed rabbits amyl nitrite caused a pronounced sensitization of pulmonary stretch receptors (PSRs) during the inflation phase, typically with a reduction in the level of activity during the deflation phase. The time course of the change in the pattern of PSR activity paralleled that of the tachypnoeic response. The sensitization of a small sample of rapidly adapting 'irritant' receptors was of a significantly shorter duration. A unitary analysis of non-myelinated vagal afferents was not attempted. The sensitization of these vagal afferents cannot be attributed to the smooth muscle relaxant properties of amyl nitrite since other relaxants (sodium nitrite, sodium nitroprusside, phentolamine) did not possess this capacity, and such properties would be expected to diminish their activities.

Afferent Pathways↗

PubMed1985

Diaphragmatic, thoracic and limb motor effects of amyl nitrite inhalation.

In the pentobarbitone/urethane anaesthetized rabbit and pentobarbitone anaesthetized cat intratracheal inhalation of amyl nitrite, a non-specific smooth muscle relaxant, caused a vagally dependent depression of patellar reflex and spontaneous (thermal shivering and external intercostal inspiratory) motor activities. A prolonged vagally-independent potentiation succeeded the initial inhibition. The potentiation of intercostal activity may account for the increase in tidal volume produced by amyl nitrite after vagotomy. The patellar reflex potentiation survived thoracolumbar spinal transection but could not be attributed to a direct facilitation of neuromuscular or muscular events. A correlation existed between the relative dependence of fusimotor support of a muscle and its susceptibility to the inhibitory and subsequent facilitatory affects of amyl nitrite inhalation. The diaphragm and interchondral muscles, essentially independent of the autogenetic facilitation provided by muscle spindles, demonstrated neither of these effects. Conversely, the intercostal muscles depend greatly on such facilitation, as do the pectoral and limb muscles during thermal shivering.

Amyl Nitrite↗