Mechanism of interaction between motoneurons and muscles

Possible ways in which muscle fibres can affect motoneuron properties and motoneurons can affect muscle properties are discussed. A more detailed analysis of the local control by muscle of motor nerve terminal growth is then given. Experimental results from the gluteus maximus muscle of the mouse show that nerve terminal growth occurs very quickly if the muscle under the endplate is killed and that the growth can still be elicited after all communication with the cell body of the motoneuron has been eliminated. These observations show that all the materials needed for axon growth are normally present in axon terminals and that the control of terminal growth by the muscle is exercised at a local level. It is argued that this inhibitory control works by preventing the incorporation of these growth materials rather than by continual degradation of a persistently growing terminal. One reason for taking this view is that recent evidence suggests that the axonal cytoskeleton consists of a largely stationary array of discontinuous neurofilaments and microtubules.