Applied mathematics and Human Anatomy and Physiology

Contraction and Relaxation of
Muscles on Molecular Level

, Contraction of muscle fibers at the molecular
level
Muscles contract when myofilaments (actin and myosin) in sarcomeres
contract the sarcomeres.
A muscle becomes depolarised when an action potential reaches it. This
releases calcium ions. They are crucial for giving myosin access to myosin
binding sites on actin. Myosin heads which are already bound to ATP can bind
to actin. The hydrolysis and release of ATP from the myosin heads causes
them to swivel and pull actin filaments inwards, contracting the sarcomere.
When a new ATP molecule binds to myosin heads, they release and the
sarcomere relaxes. The sequence of events that result in the
contraction of an individual muscle fiber begins with a signal—the
neurotransmitter, ACh—from the motor neuron innervating that
fiber. The local membrane of the fiber will depolarize as positively
charged sodium ions (Na+) enter, triggering an action potential that
spreads to the rest of the membrane will depolarize, including the T-
tubules.