Question

Section 3: Short answer – Answer each question IN YOUR OWN WORDS. TYPE your responses and staple the printed copy to the rest of your assignment.

11) In the correct order, explain the neural mechanisms used to increase muscle force production using the concepts of summation, rate coding, recruitment, and the size principle. DO NOT simply write the definition of each concept. (3 points)

Answer 1

Force summation describes the addition of individual contractions to increase the intensity of overall muscle contractions.This can be achieved by increasing the number and size of contractile units simultaneously or by increasing the frequency at which action potentials are sent to muscle fibres.

The nervous system controls muscle force by controlling motor units.The first is motor unit recruitment ,the process of varying the number of activated motor units.The second is rate coding,the process of varying the rate at which each active motor unit generates action potentials.

To recruit motor units is to activate them .They are activated and deactivated in aset sequence.The size principle explains how this orderly recruitment is controlled.Accordingly the sequence depends on the size of motor neurons.Motor units with the smaller motor neurons are recruited first and deactivated last.Motor units with larger neurons are recruited last and deactivated first.According to the size principle ,the order of recruitment is predetermined by size and is not directly controlled by brain.The Henneman principle (size principle )states that the recruitment of motor units within a muscle proceeds from small to larger motor units as small motor units have low thresholds of activation and are recruited first.

Muscle force not only depends on the number of active motor units but also on the rate at which motor units discharge action potentials.There are two types of rate coding, one is by the CNS is to vary overall frequency of action potentials and another is to vary the temporal pattern of action potentials between different motor units.

Muscle force increases as discharge rate increases,but the relationship between muscle force and discharge rate is different for two types of motor units-tonic and phasic.Tonic motor units increase discharge rate as muscle force increases at low levels.Their discharge rate remains constant as muscle force continues to increase at high levels.In contrast is for phasic motor units.Tonic units are recruited first and less fatigable than phasic units.

Another form of rate coding is the discharge of two or more units at the same time.These syncronous discharge have been observed in muscles that were developed through strength training.