When a noxious stimulus is applied to a limb, the signal is first transmitted via sensory fibres to the interneurone(s) in the spinal cord and then onto the a motor neurones. This is an example of a polysynaptic withdrawal reflex (Figure NE.22). The neuronal circuitry involves not only activation of muscle(s) which carry out the withdrawal, but also the inhibition of the antagonist muscles. Other muscle groups may also be stimulated or inhibited so that the withdrawal of the limb and movement of the rest of the body are coordinated to move safely away from the noxious stimulus.
Transection of the spinal cord is accompanied by various changes that demonstrate important physiological principles. In man, cord transection is followed by a variable period during which all spinal reflexes are profoundly depressed or absent. All muscles innervated by spinal nerves below the level of the cord lesion become paralysed and their voluntary control is lost forever. Anaesthesia of the parts of body innervated by spinal nerves below the level of the transection will be seen. During the period of spinal shock the resting membrane potential of the spinal motor neurones becomes temporarily greater than usual by 2-5 mV. The initial phase of spinal shock is followed by recovery of reflex function.
It is thought that the return of spinal reflexes may be due to a degree of denervation hypersensitivity although there is a suggestion that collateral sprouting from adjacent neurones may play a part. The time-course of the recovery of reflex function is highly variable in man, ranging from 24H (rarely) to 6 weeks. The most frequent interval is about 2 weeks from initial injury. The first reflexes to return are flexor responses to touch and ano-genital reflex responses. Reflex responses become hyperactive in the early recovery phase although this reduces with time. Tendon reflexes are the slowest to recover and become hyperactive, usually accompanied by a degree of permanent clonus.
In a chronic spinal state, seen in paraplegic patients, a mass reflex response will be seen after even minor noxious stimulus applied to the skin (such as pinching). The mass reflex involves the rapid spreading of reflex activity from one centre to another, accompanied by autonomic involvement. Evacuation of bladder and rectum, sweating, pallor and swings of arterial pressure will be seen. This is applied to some effect in chronic spinal patients to give a degree of continence control with variable success.
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