Ca2+ ATPase pump
Synaptic terminal of motor neuron
Action potentials travel to the interior of the muscle fiber along transverse (T) tubules.
The action potential along T tubules causes the sarcoplasmic reticulum (SR) to release Ca2+
The Ca2+ binds to the troponin complex on the thin filaments.
This binding exposes myosin-binding sites and allows the cross-bridge cycle to proceed.
Nervous Control of Muscle Tension
Contraction of a whole muscle is graded, which means that the extent and strength of its contraction can be voluntarily altered.
There are two basic mechanisms by which the nervous system produces graded contractions:
Varying the number of fibers that contract
Varying the rate at which fibers are stimulated.
In a vertebrate skeletal muscle, each branched muscle fiber is innervated by one motor neuron.
Each motor neuron may synapse with multiple muscle fibers.
A motor unit consists of a single motor neuron and all the muscle fibers it controls.
Motor units in a vertebrate skeletal muscle
Motor neuron cell body
Motor neuron axon
Motor unit 1
Motor unit 2
Recruitment of multiple motor neurons results in stronger contractions.
A twitch results from a single action potential in a motor neuron.
More rapidly delivered action potentials produce a graded contraction by summation.
Tetanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials.
Summation of twitches
Summation of two twitches
Pair of action potentials
Series of action potentials at high frequency
Slow-twitch fibers contract more slowly, but sustain longer contractions. All slow twitch fibers are oxidative.
Fast-twitch fibers contract more rapidly, but sustain shorter contractions. Fast-twitch fibers can be either glycolytic or oxidative.
Most skeletal muscles contain both slow-twitch and fast-twitch muscles in varying ratios.