Asynchronous Activation ofMotor Units During Maintained Contraction

As anyone who has done prolonged physical labor or exercised vigorously can attest, muscle contractions cannot be maintained indefinitely; muscles fatigue and must be rested. Thus, the state of tetanus in Figure 11-6 could not be maintained in a single motor unit for very long without allowing the muscle fibers in the motor unit to relax. However, some muscles such as those involved in maintaining body posture are required to contract for prolonged periods. What mechanism helps prevent muscle fatigue during such prolonged contractions? During maintained tension in a muscle, all the motor neurons to the muscle are not active at the same time. The activity of the motor units occurs in bursts separated by quiet periods, and the activity of different motor units is

-

Tetanus

'en

-

Summation of

f

£

two twitch responses

j

_ Single

twitch

j

V\

AV

A

I I Time

Muscle

<•-„-^

action

Pair of

Series of muscle action potentials

potential

muscle action

at high frequency

potentials

Figure 11-6 Isometric tension in response to a series of action potentials in a muscle. The dashed lines show the expected response if only the first action potential of a series occurred.

Figure 11-7 Summation of muscle tension during asynchronous activation of three motor units in a muscle.

Muscle tension

Action potentials in motor neuron ©

Time

Muscle tension

Action potentials in motor neuron

LLLLI_LLLLL

Summation of tension from

Muscle tension

Action potentials in motor neuron

Figure 11-7 Summation of muscle tension during asynchronous activation of three motor units in a muscle.

staggered in time. An example of this kind of asynchronous activity during steady contraction is shown in Figure 11-7. Notice that the summation of the tensions produced by the activity of only three motor units, each active only halfthe time, can produce a reasonably smooth, steady tension. With hundreds of motor units available in many muscles, a much smoother and larger steady tension could be maintained with less effort on the part of any one motor unit. Thus, asynchronous activation of motor neurons to a muscle allows a prolonged contraction with reduced fatigue of individual motor units in the muscle.

0 0

Post a comment