We have seen that skeletal muscle fibres are supplied by ramifications of somatic efferent neurons. We have also seen that axonal branches arising from one neuron may innervate a variable number of muscle fibres (that constitute a motor unit).
Each skeletal muscle fibre receives its own direct innervation. The site where the nerve ending comes into intimate contact with the muscle fibre is a neuromuscular (or myoneural) junction. Details of these junctions vary in different skeletal muscle fibres as follows.
In most neuromuscular junctions the nerve terminal comes in contact with a specialised area near the middle of the muscle fibre. This area is roughly oval or circular, and is referred to as the soleplate. The sole plate plus the axonterminal constitute the motor end plate. Motor end plates are considered in detail below.
On reaching a muscle fibre some axon terminals divide into a number of small ramifications each ending in an expansion applied to the surface of the muscle fibre. These are referred to as ‘en grappe’ endings.
In some cases the nerve fibre runs for some distance along the length of the muscle fibre giving off several ramifications that come in contact with the latter.
‘En grappe’ and trail endings are seen mainly in relation to intrafusal muscle fibres (present in muscle spindles)(Fig. 4.4).