lag, latent period

The latent period is the small moment of time it takes for the body to begin contracting a muscle in response to a stimulus. Suppose the stimulus is the sight of a soccer ball flying through the air and heading your direction at a very fast speed; giving you only enough time to bring you hands up to protect your head.

The sight of the stimulus begins the latent period. Upon seeing the ball an action potential is generated and the information is sent to the brain. The brain then sends an action potential on the descending nerve tracks and will eventually reach multiple motor units on the various arm muscles.

When the action potential reaches the axon terminal of the motor unit it will draw in Ca2+. Increased Ca2+ levels will cause the release of Acetylcholine (ACh) in the synaptic cleft. Once ACh has crossed the synaptic cleft, it binds to sodium potassium pumps and generates an action potential in the sacrolemma of the muscle. This action potential will reach the sarcoplasmic reticulum and calcium will then also be dumped into the muscle fibers. The Ca2+ will activate the tropomyosin complex and allow the myosin heads to attach to actin and begin the muscle contraction.

Lots was left out but the general idea is there. All of this happens in milliseconds and occurs before any muscle contraction begins, this is why it is referred to as the latent or lag period of muscle contraction.