The vertebrate motor unit•muscles performing powerful, coarsely controlledmovements have larger number of fibers per motor unit example: a single calf musclemotor neuron mayconnect to ~2000 muscle fibers•muscles that produce more precise, delicate movementscontain fewer fibers per motor unitexample: extraocular muscleshave only ~12 musclefibers per motor neuronA single motor unit consists of a single motor neuron and allthe muscle fibers that it controls. The numbers of muscle fibers per motor unit and numbers ofmotor units per muscle vary widely.
Motor units in extraocular musclesExtraocular muscles(which control the direction of gaze) produce very precise, delicate movements. Each motor neuron connects to only ~12 muscle fibers.
The vertebrate motor unitsIncreasing the number of active motor units is recruitment.Asynchronous recruitmentof motor units delays fatigue:alternating motor activities gives motor units that have beenactive the opportunity to rest while others take over.
Action potentials and muscle fiber contractions A single action potential in the muscle fiber causes an all-or-none muscle fiber contraction (a twitch). The length and the strength of a sustainedcontraction (tetanus) depends on the frequency of action potentials.
Treppe and summationThe frequency of action potentials determines the tension.The graduated series of increasingly vigorous contractionsthat results when a series of identical stimuli are applied to a rested muscle is called treppe. It does not result in tetanus, and is important in cardiac muscle activity.When a muscle is stimulated by the motor neuron more thanonce in a short period (more frequently than in treppe),the successive twitches add to each other so the forcegenerated is greater. This is called summation.
SummationTreppeTreppe and summation
High frequency stimulation can cause a smoothly fusedcontraction called tetanus. This extreme shortening is because all of the elastic muscle fibers of the musclebecome completely shortened.•incomplete tetanus: there are brief relaxation phases betweenstimulations but the muscle does not relax completely•complete tetanus: relaxation phases are eliminated Tetanus
Incomplete tetanusComplete tetanusTetanus
FatigueIn skeletal muscles, repetitive stimulation leads to fatiguewhen the fibers run out of enough energy, evident as a dropin tension. Rest overcomes fatigue, but it will reoccur soonerif inadequate recovery time is allowed.