The Control of Muscles in Speech

This section summarises some of the available literature concerning the operation of muscles and nerves. All skeletal muscles comprise groups or bundles of smaller muscle fibres; these muscle fibres are of many different shapes and sizes depending on the muscle in question. Cylindrical gamma shaped muscle fibres are comparatively short compared with, say, flagelliform or laneolate muscle fibres. In addition there are spindle-shaped muscle fibres, so named because they have a thick central region with pointed ends. Sometimes a muscle fibre may extend the whole length of the muscle; other fibres are shorter, overlapping with other short fibres (Buchthal and Lindhard 1939). Several researchers give figures of the order of 10µ for the diameter of a muscle fibre, but several centimetres for its length (Fromkin and Ladefoged 1966). Muscles are attached at both ends, sometimes to bones or membranes attached to bones, sometimes (as with m. orbicularis oris — used in rounding and protruding the lips) to themselves. It is by contraction that a muscle causes. movement in whatever it is attached to, or can enable a greater load to be borne by tensing. Individual fibres composing the muscle contract or tense, causing the muscle as a whole to contract or tense. Liddel and Sherrington (1925) consider the muscle as comprising motor units. A motor unit may be regarded as "a motor cell, its axon process and the groups of muscle fibres innervated by this one nerve cell" (Fromkin and Ladefoged op. cit.; see also Cooper 1965). According to Fromkin and Ladefoged no estimate has been made of the number f fibres in the tongue and lip muscle motor units; however it is known that in general the larger the muscle as a whole the greater the number of fibres to each motor unit. Contraction occurs within the fibres of the motor unit when impulses arrive causing a chemical change at the junction between nerve and muscle, followed by a muscle action potential. The electrical changes in the muscle precede the actual contraction. If the attachment of the fibres is moveable then actual shortening will occur, if not then tensing will occur. The contraction of any one fibre or the fibres associated with a single motor nerve (or motor neuron) lasts only a few milliseconds; contraction of the entire muscle may of course last very much longer depending on the non-simultaneous contraction of different motor units. After contraction of the …