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SUMMARY1. To determine whether there is any specificity of regenerating nerves for their original muscles, the common lateral gastrocnemius soleus nerve (l.g.s.) innervating the fast-twitch lateral gastrocnemius (l.g.) and slow-twitch soleus muscles was sectioned in the hind limb of twenty adult rats. The proximal nerve stump was sutured to the dorsal surface of the l.g. muscle and 4-14 months later, the contractile properties of the reinnervated l.g. and soleus muscles and their single motor units were studied by dissection and stimulation of the ventral root filaments. Contractile properties of normal contralateral muscles were examined for comparison and motor units were isolated in l.g. and soleus muscles for study in a group of untreated animals.2. Measurement of time and rate parameters of maximal twitch and tetanic contractions showed that the rate of development of force increased significantly in reinnervated soleus muscles and approached the speed of l.g. muscles but rate of relaxation did not change appreciably. In reinnervated l.g. muscles, contraction speed was similar to normal l.g. muscles but relaxation rate declined toward the rates of relaxation in control soleus muscles.3. After reinnervation by the common l.g.s. nerve, the proportion of slow motor units in l.g. increased from 10 to 31 % and decreased in soleus from 80 to 31 %. The relative proportions of fast and slow motor units in each muscle were the same as the proportions of fast and slow units in the normal l.g. and soleus muscles combined. 4. It was concluded that fast and slow muscles do not show any preference for their former nerves and that the change in the force profile of the reinnervated muscles is indicative of the relative proportions of fast and slow motor units: fast units dominate the contraction phase and slow units the relaxation phase of twitch and tetanic contractions of the muscle.

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Neural Mechanisms of Sensori-Motor Integration New methods of investigation have enlarged understanding of the mechanisms underlying activity in the nervous system. Excitable cells transmit impulses by means of their special membrane properties and excitation is transmitted from cell to cell across specialized sites called synapses. In the nuclei of the central nervous system there are many small neurons that have no axon or only very short axons and dendrites. These are referred to as interneurons and the chemical transmitters they release may be excitatory or inhibitory to the cells with which they synapse. Cells with axons ending in a nucleus and cells with dendrites in the nucleus plus the interneurons which may intervene between the input of the message by an axon reaching the nucleus and its transmission onwards, form networks of cells that act as micro-circuits, affecting the nature of the neural signal. Control of transmission by these networks is the means by which sensory and motor impulses may be modified, enhanced, suppressed or facilitated. The integration of many sensory inputs, and the feedback during movement modulate and shape the motor response. An understanding of the mechanisms of inhibition and facilitation becomes increasingly important for therapists who use techniques based on “sensori-motor integration“.