Page 109 - 84 human physiolofy part-1

The Muscular System

Skeletal muscle is further divided into several subtypes:

• Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in
mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry
more oxygen and sustain aerobic activity.
• Type II, fast twitch, muscle has three major kinds that are, in order of increasing contractile
speed:
• a) Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries
and appears red.
• b) Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin.
This is the fastest muscle type in humans. It can contract more quickly and with a greater
amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity
before muscle contraction becomes painful (often attributed to a build-up of lactic acid). N.B. in
some books and articles this muscle in humans was, confusingly, called type IIB
• c) Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in
mitochondria and myoglobin. In small animals like rodents or rabbits this is the major fast
muscle type, explaining the pale color of their meat.

For most muscles, contraction occurs as a result of conscious effort originating in the brain. The
brain sends signals, in the form of action potentials, through the nervous system to the motor neuron
that innervates the muscle fiber. However, some muscles (such as the heart) do not contract as a result
of conscious effort. These are said to be autonomic. Also, it is not always necessary for the signals to
originate from the brain. Reflexes are fast, unconscious muscular reactions that occur due to
unexpected physical stimuli. The action potentials for reflexes originate in the spinal cord instead of the
brain.

There are three general types of muscle contractions, skeletal muscle contractions, heart muscle
contractions, and smooth muscle contractions.

Muscular System Working With Other Body Systems

1. Homeostasis
2. Protection
3. Calcium Metabolism
4. Maintaining Body Temperature

Skeletal Muscle Contractions

Steps of a skeletal muscle contraction:

• An action potential reaches the axon of the motor neuron.
• The action potential activates voltage gated calcium ion channels on the axon, and calcium
rushes in.
• The calcium causes acetylcholine vesicles in the axon to fuse with the membrane, releasing
the acetylcholine into the cleft between the axon and the motor end plate of the muscle fiber.
• The skeletal muscle fiber is excited my large mylenated nerve fibers which attach to the

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