THE MUSCLES
Functions of the skeleton include giving the body its shape and providing mobility, using bones as levers.
Without the aid of muscles, the body would be immobile. Although the skeleton determines the size of
the body’s framework, muscle and fat determine body shape. Functions of muscles are body movement,
blood circulation, and heat production. All muscle tissues have specific characteristics (contractility,
extensibility, elasticity, and irritability), as described later in this topic.
Voluntary Movement
♦ Enables walking, standing, sitting, and other movements
♦ Maintains body in upright position
♦ Participates in body balance
Involuntary Muscle Action
♦ Maintains heartbeat to pump blood
♦ Provides arterial blood flow
♦ Promotes lymphatic and venous blood return to heart
♦ Dilates and contracts blood vessels to control blood flow
♦ Maintains respiration
♦ Performs digestion processes
♦ Performs elimination processes
♦ Participates in reflexes
♦ Enables all other involuntary actions of body
Protection
♦ Protects body in emergency by reflex action
♦ Covers, surrounds, and protects internal organs (viscera)
♦ Supports internal organs
Other Functions
♦ Produces heat
♦ Assists in maintaining stable body temperature (in shivering, “goose flesh," muscles give off heat)
♦ Provides shape to body
Muscle Classification
Three types of muscle tissue by function: skeletal, smooth, and cardiac. Each is also identified according
to appearance. Skeletal and cardiac muscles are striated; they consist of fibers marked by bands crossing
them, giving them a striped appearance. Smooth muscle is nonstriated. Muscles are further determined
to be voluntary or involuntary. Muscle tissue is approximately 15% denser than adipose (fat) tissue.
Skeletal Muscles
Skeletal muscles, which control movements of the skeleton, are under voluntary (conscious) control.
There are more than 630 skeletal muscles in the human body, constituting approximately 40% of body
weight. Their functions include locomotion, facial expression, and posture. The two types of voluntary
muscles are fast-twitch—those which contract quickly and powerfully, but encounter rapid fatigue, and
slow-twitch—those which can sustain a contraction, but do not exert great force. It is believed that
people are born with a predisposition toward more fast-twitch or more slow-twitch muscles.
The masseter (jaw) muscle is considered to be the strongest muscle in the body by volume, because it is
able to exert great bite strength. The tongue is also a strong muscle, actually being made up of 16
muscles.
Smooth Muscles
Smooth muscle controls involuntary motions inside body organs (viscera). It is also known as involuntary
or visceral muscle. Smooth muscle is responsible for propelling urine through the urinary tract, moving
food in the digestive tract, dilating the pupils of the eyes, activating arrector pili in the skin, and dilating
or contracting blood vessels to assist in blood circulation and blood pressure maintenance. Smooth
muscles are capable of sustained or rhythmic contractions and can also respond to nervous stimulation
in emergencies.
Comparison of Different Types of Muscle Tissue
MUSCLES
SMOOTHSMOOTH
Location
Wall of hollow organs,
vessels, respiratory
passageways
CARDIAC
SKELETAL
Wall of heart
Attached to bones
Tapered at each end,
Cell
single nucleus,
characteristics
nonstriated
Long and cylindrical;
Branching networks, single
multinucleated; heavily
nucleus, lightly striated
striated
Control
Involuntary
Involuntary
Voluntary
Action
Produces peristalsis;
contracts and relaxes
slowly; may sustain
contraction; helps
maintain blood pressure
by regulating size of
arteries
Pumps blood out of heart;
self-excitatory, but
influenced by nervous
system and hormones
Produces movement at joints;
stimulated by nervous system;
contracts and relaxes rapidly;
produces heat through aerobic
production of energy; assists in
blood return to heart