<P> Henneman proposed that the mechanism underlying the Size Principle was that the smaller motor neurons had a smaller surface area and therefore a higher membrane resistance . He predicted that the current generated by an excitatory postsynaptic potential (EPSPs) would result in a higher voltage change (depolarization) across the neuronal membrane of the smaller motor neurons and therefore larger EPSPs in smaller motoneurons . Burke later demonstrated that there was a graded decrease of both EPSP and inhibitory post synaptic potential (IPSP) amplitudes from small to large motoneurons . This seemed to confirm Henneman's idea, but Burke disagreed, pointing out that larger neurons with a larger surface area had space for more synapses . Burke eventually showed (in a very small sample of neurons) that smaller motoneurons have a greater number of synaptic inputs from a single input source . The topic is probably still regarded as controversial . </P> <P> Under some circumstances, the normal order of motor unit recruitment may be altered, such that small motor units cease to fire and larger ones may be recruited . This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs . </P> <P> The force produced by a single motor unit is determined in part by the number of muscle fibers in the unit . Another important determinant of force is the frequency with which the muscle fibers are stimulated by their innervating axon . The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary from frequencies low enough to produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction . Generally, this allows a 2 to 4-fold change in force . In general, the motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached . This smooths out the incremental force changes which would otherwise occur as each additional unit was recruited . </P> <P> The distribution of motor unit size is such that there is an inverse relationship between the number of motor units and the force each generates (i.e., the number of muscle fibers per motor unit). Thus, there are many small motor units and progressively fewer larger motor units . This means that at low levels of recruitment, the force increment due to recruitment is small, whereas in forceful contractions, the force increment becomes much larger . Thus the ratio between the force increment produced by adding another motor unit and the force threshold at which that unit is recruited remains relatively constant . </P>

How motor unit composition and innervations affects the strength of muscle contractions