Search

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.

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.

The present experiments demonstrated that labyrinth, neck and elbow joint afferents interact to influence the excitability of crossed extensor reflex pathways projecting to triceps brachii in decerebrate cats. Reflex excitability was dependent on head, neck and elbow position such that it was increased when the elbow was held at full flexion and either the head maintained side down or the neck rotated side up. Such interaction between these afferents is likely to be important in postural control.

The present experiments were performed to investigate the influence of elbow joint afferent discharge on the excitability of reflex arcs mediating flexion withdrawal and crossed extensor reflexes in the forelimb muscles of decerebrate cats. The excitability of flexion withdrawal and crossed extensor reflexes in forelimb muscles was shown to be modulated by elbow joint position. Flexion withdrawal reflexes were most easily elicited when the elbow was extended and crossed extensor reflexes were most easily elicited when the elbow was flexed. This modulation of transmission was not confined to reflex pathways projecting to muscles acting at the elbow but also included pathways to muscles acting at the wrist and shoulder in addition to muscles in the contralateral forelimb. The changing excitability of reflex pathways caused by elbow movement was unaltered by degloving the skin covering the join and tenotomy of the medial head of triceps and the long head biceps. However, modulation of reflex excitability by joint movement was totally abolished by local anaesthesia of the joint in an otherwise intact limb. Thus, the present experiments indicate that transmission in forelimb flexion reflex pathways can be powerfully influenced by elbow joint afferent discharge.

In decerebrate cats with intact innervation of the fore- and hindlimbs, flexion reflexes are most easily elicited in forelimb muscles when the hindlimb is extended, and hindlimb flexion reflexes are most easily elicited when the forelimbs are extended. After intra-articular injection of local anaesthetic, this modulation of reflex excitability is abolished. Thus, in addition to their known segmental effects, joint afferents also exert significant ascending and descending effects on motoneurone excitability.

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.

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 were due to the local anaesthetic agent, and that this remained localized 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.

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.

In intact forelimbs in decebrate 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.

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.

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.