Ronald H. Baxendale

The influence of joint afferent discharge on locomotion, proprioception and activity in conscious cats.

Injection of local anaesthetic into the knee joint cavity in a series of 14 cats produced obvious abnormalities of posture and gait in half the animals. However, on employing a more sensitive testing procedure, 7 out of 8 animals exhibited reduced proprioceptive acuity after knee joint anaesthesia, and all animals tested showed marked reduction in motor activity after this procedure. Control procedures revealed that these effects were due to the local anaesthetic agent, and that this remained localized to the knee joint and did not diffuse out to block cutaneous and muscle afferents. Thus, joint afferents would appear to play a significant role in the regulation of posture and movement.

Evidence for a human spinal stepping generator

Flexion reflexes elicited by surface stimulation in spinal cord-injured man have been used to provide limb flexion in gait restoration. In this study, the pattern of the flexion reflexes observed in one subject supports the theory that a spinal stepping generator exists in man.

TWITCH INTERPOLATION IN THE ASSESSMENT OF THE MAXIMUM FORCE-GENERATING CAPACITY OF THE JAW-CLOSING MUSCLES IN MAN

The method of twitch interpolation was employed to study the maximum potential bite forces of humans. Transcutaneous electrical stimuli were applied to parts of one or both masseter muscles in eight volunteers while they bit with a variable but controlled isometric force on a unidirectional force transducer held between the anterior teeth. In all participants the twitch force produced by a single 1-ms pulse, of 25 50 mA intensity, was inversely and linearly related to the voluntary bite force. For each participant the slope of the regression between twitch force and bite force depended on the stimulus intensity and not on whether the stimulus was applied to one or both masseters. Extrapolation of the regression lines to zero twitch force showed that they converged towards a bite-force value that, for any given participant varied only a small amount between different stimulus intensities. For most participants this bite force lay above the maximum that they produced voluntarily: voluntary maximum bites ranged from 153 to 593 N, while the extrapolations predicted a narrower and higher potential range of 282-629 N. It was concluded that, for the masseters at least, there is often spare force-generating capacity which individuals are either unable or not prepared to utilize. This method is non-invasive and may help to define better the maximum bite-force potential of humans.

Responses of quadriceps motor units to mechanical stimulation of knee joint receptors in the decerebrate cat

Recordings were obtained from single quadriceps motor units in decerebrate cats. Tonic discharges were induced in these units by stretch applied to the patellar tendon. The activity of these motor units was examined during repetitive mechanical indentation of the posterior aspect of the knee joint capsule. The amplitude of indentation could be graded to regularly elicit a volley of one or more action potentials from afferents travelling in the posterior articular nerve (PAN). Either repetitive indentation at a fixed frequency or random indentation resulted in entrainment of motor unit discharge with a latency of 6.5-17 ms from the onset of the first PAN spike. This effect was achieved in one case by a single joint afferent. Usually two or more afferents were required to produce entrainment. This effect was mediated by joint receptors as its reversibly abolished by application of lignocaine to the joint nerve. The modal reflex latency suggests that group II joint afferents were responsible for this effect. Stimulation of some areas of the joint capsule readily produced motor unit entrainment whereas other areas did not although PAN afferents were also activated at the latter sites, suggesting inhomogeneity in the synaptic actions of joint afferents.

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 degrees and least intense when the joint was held in midrange positions between 90 degrees and 120 degrees. There was a modest increase in discharge as the elbow was held in more flexed positions and activity at 30 degrees was about half of that at 165 degrees. Alterations in the tone of muscles around the joint did not affect the adapted EJN discharge at any angle.

The effect of mechanical stimulation of knee joint afferents on quadriceps motor unit activity in the decerebrate cat.

The present experiments demonstrate that repetitive activation of as few as two joint afferents by an imposed mechanical stimulus results in entrainment of stretch-induced firing of quadriceps motor units. There is no significant input from extracapsular receptors as the effect is abolished during local anaesthetic block of the joint nerve. These results indicate that joint receptors can exert potent effects on motoneurone excitability which may be of considerable functional significance.

Dishabituation of the flexion reflex in spinal cord-injured man and its application in the restoration of gait

The flexion withdrawal reflex, evoked by surface electrical stimulation, has been used to provide hip flexion for the restoration of gait in paraplegics. A major limitation to its use has been the decrease in the magnitude of the response to repeated stimulation (habituation). In this study it was found that by using high intensity stimulation the response could be dishabituated. It was demonstrated that sufficient hip flexion for functional electrical stimulation-assisted gait was maintained using high intensity pulses in a one-step-ahead controller.

Modulation of transmission in flexion reflex pathways by knee joint afferent discharge in the decerebrate cat

In the intact hindlimb, flexion reflexes are most easily obtained when the knee is extended. Movement of the knee joint after the muscles have been tenotomized and held at fixed lengths still modulates flexion reflex excitability. After anaesthesia of the joint capsule movement of the limb no longer affects flexion reflex excitability.

Modulation of transmission in forelimb flexion reflex pathways by elbow joint afferent discharge in decerebrate cats.

In intact forelimbs in decerebrate cats, flexion reflexes are most easily elicited when the elbow joint is extended and extension reflexes are most easily elicited when the elbow joint is flexed. Abolition of sensory discharge arising from the elbow joint by intra-articular injection of local anaesthetic eliminates this modulation of reflex excitability. We conclude that elbow joint afferent discharge modulates the transmission in flexion reflex pathways to forelimb motoneurones.

Fourier analysis of the relation between the discharge of quadriceps motor units and periodic mechanical stimulation of cat knee joint receptors

It has been widely assumed that joint receptors contribute to the reflex regulation of movement and posture, although there have been few clear demonstrations of joint‐mediated reflex actions on alpha‐motoneurones other than those attributable to flexor reflex afferents. The present study extends our recent work on joint‐mediated reflexes by using Fourier analysis of spike train interactions to demonstrate that restricted mechanical activation of a small number (one to five) of knee joint receptors by localized indentation of the joint capsule can modify the firing pattern of quadriceps motoneurones in decerebrated cats. The modulation of this discharge pattern can be reversibly abolished by application of droplets of lignocaine directly to the joint nerve and consequently can be attributed unambiguously to articular afferents. Activation of single joint afferents could on occasion produce changes in motor unit firing patterns, but usually activation of two or more was required before secure effects were observed. Increasing the intensity of indentation (resulting in activation of greater numbers of joint receptors) increases the strength of coupling between joint afferent input and motor unit responses, although the relationship is not linear. The relation between the discharge patterns of pairs of motor units was also examined, and it was found that significant coupling occurred at the stimulus frequency, superimposed on a ‘background’ coupling from unidentified sources. The phase relationship between pairs of motor units was not affected by the periodic stimulus. However, it was observed that if two motor units were firing independently of one another in the absence of capsule probing, maintained capsular indentation produced a striking synchronization between the discharges of the two motor units. These experiments show strong joint‐mediated reflex effects on motor unit firing indicating that joint receptors may have an important role to play in motor control.