Page 108 - 84 human physiolofy part-1

Chapter 6

body). Contrary to popular belief, the number of muscle fibers cannot be increased through exercise;
instead the muscle cells simply get bigger. It is however believed that myofibrils have a limited
capacity for growth through hypertrophy and will split if subject to increased demand.

The ten types of muscle have significant differences. However, all but three use the movement of
actin against myosin to create Muscle contraction|contraction and relaxation. In skeletal muscle,
contraction is stimulated by action potential|electrical impulses transmitted by the nerves, the motor
nerves and motoneurons in particular. All skeletal muscle and many smooth muscle contractions are
facilitated by the neurotransmitter acetylcholine.

Muscular activity accounts for most of the body's energy consumption. Muscles store energy for
their own use in the form of glycogen, which represents about 1% of their mass. This can be rapidly
converted to glucose when more energy is necessary.

Types

There are three types of muscle:

• Smooth muscle or "involuntary muscle" is a spindle shaped muscle found within the walls
of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, ureters,
bladder, and blood vessels. Smooth muscle contains only one nucleus, and no striations.

• Cardiac muscle is also an "involuntary muscle" but it is striated in structure. Like Smooth
muscle, cardiac muscle contains only one nucleus. Cardiac muscle is found only within the
heart.

• Skeletal muscle or "voluntary muscle" is anchored by tendons to the bone and is used to
affect skeletal movement such as locomotion. Skeletal muscle is multinucleated the nucleus is
peripherally located, and the skeletal muscle is striated. Functions of the skeletal muscle
include:
• Support of the body
• Aids in bone movement
• Helps maintain a constant temperature throughout the body
• Assists with the movement of cardiovascular and lymphatic vessels through contractions
• Protection of internal organs and helps to stabilize joints

Cardiac and skeletal muscle are striated in that they contain sarcomere and are packed into highly-
regular arrangements of bundles; smooth muscle has neither. Striated muscle is often used in short,
intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.

108 | Human Physiology