Muscle Twitch and Muscle Tension - Motor unit size and force

• A motor unit is a group of muscle cells controlled by a single neuron. • A stimulus of sufficient intensity will cause all the cells in the motor unit to contract. • A single contraction, caused by a single action potential, is called a muscle twitch. • Latent period: A brief period immediately following the stimulation where no contraction is observed. • During the latent period, calcium ions are flowing out of the sarcoplasmic reticulum, crossbridges are forming and elastic components of the muscle are being stretched. • Contraction period: Muscle contracts. • Relaxation period: Muscle relaxes. • Muscles involved in fine control have small motor units. • Muscles that move large body parts have large motor units, • Twitch contraction force depends on motor unit size. • Large motor units produce more tension and a greater contraction force than small motor units. Before we examine how postural muscles maintain a contraction, we need to first look at a single muscle twitch, which is a single contraction in response to a brief threshold stimulation. Think of a threshold as being like a minimum amount of pressure needed to activate a key on your computer. Just resting your fingers on the keyboard has no effect. You need to apply a slight pressure in order to get a response. Likewise, a minimum amount of stimulation is needed to induce a muscular contraction.

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