The Chemical Context of Life

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Ca2+ ATPase pump

Synaptic terminal of motor neuron

Synaptic cleft

T Tubule

Plasma membrane

Ca2+

Ca2+

CYTOSOL

SR

ATP

ADP

P i

ACh

Slide 64

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.

Slide 65

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.

Slide 66

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.

Slide 67

Motor units in a vertebrate skeletal muscle

Spinal cord

Motor neuron cell body

Motor neuron axon

Nerve

Muscle

Muscle fibers

Synaptic terminals

Tendon

Motor unit 1

Motor unit 2

Slide 68

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.

Slide 69

Summation of twitches

Summation of two twitches

Tetanus

Single twitch

Time

Tension

Pair of action potentials

Action potential

Series of action potentials at high frequency

Slide 70

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.