Muscle contraction is initiated by the release of ACh from the axon terminals of alpha motor
neurons. ACh produces a large EPSP in the postsynaptic membrane due to the activation of
ACh receptors which is sufficient to evoke an action potential in the muscle fiber.
By the process of excitation-contraction coupling, this action potential (excitation)
triggers the release of Ca2+from an organelle inside the muscle fiber, which leads to
contraction of the fiber.
Relaxation occurs when the Ca levels are lowered by reuptake into the organelle.
Muscle Fiber Structure – See Fig. 13.10
Fusion of myoblasts in early development leads to the formation of multinucleated
Muscle fibers are enclosed by an excitable cell membrane called the sarcolemma.
Within the muscle fiber are myofibrils which contract in response to an action potential
sweeping down the sarcolemma. 2+
o Myofibrils are surrounded by the sarcoplasmic reticulum (SR) (stores Ca ).
o Action potentials sweeping along the sarcolemma gain access to the SR by way
of tunnels called T tubules.
o When the T tubule comes in close apposition to the SR, there is a specialized
coupling of proteins in the two membranes.
o A voltage-sensitive cluster of four calcium channels (tetrad) in the T tubule
membrane is linked to a calcium release channel in the SR (see Fig. 13.11).
o The arrival of an action potential in the T tubule membrane causes a
conformational change in the tetrad, which opens the calcium release channel in
the SR membrane.
This leads to an increase in free Ca2+within the cytosol which causes the
myofibrils to contract
The Molecular Basis of Muscle Contraction – See Fig. 13.12
The myofibril is divided into segments called sarcomeres by disks called Z lines.
On each side of the Z lines is a series of bristles called thin filaments. Thin filaments
form adjacent Z lines face one another but do not come in contact.
Between the two sets of thin filaments are thick filaments
Muscle contraction occurs when the thin filaments slide along the thick filaments, bringing
adjacent Z lines toward one another. Thus, the sarcom