Essentials of Anatomy and Physiology

trolled fall against gravity. Without such control
(which we do not have to think about) a downhill walk
would quickly become a roll. These various kinds of
contractions are needed for even the simplest activi-
ties. (With respect to increasing muscle strength, see
Box 7–1: Anabolic Steroids.)

MUSCLE SENSE

When you walk up a flight of stairs, do you have to
look at your feet to be sure each will get to the next
step? Most of us don’t (an occasional stumble doesn’t
count), and for this freedom we can thank our
muscle sense. Muscle sense(proprioception) is the
brain’s ability to know where our muscles are and what
they are doing, without our having to consciously
look at them.
Within muscles are receptors called stretch recep-
tors(proprioceptors or muscle spindles). The general
function of all sensory receptors is to detect changes.
The function of stretch receptors is to detect changes
in the length of a muscle as it is stretched. The sen-
sory impulses generated by these receptors are inter-
preted by the brain as a mental “picture” of where the
muscle is.
We can be aware of muscle sense if we choose to be,
but usually we can safely take it for granted. In fact,
that is what we are meant to do. Imagine what life
would be like if we had to watch every move to be sure
that a hand or foot performed its intended action.
Even simple activities such as walking or eating would
require our constant attention.
At times, we may become aware of our muscle
sense. Learning a skill such as typing or playing the

guitar involves very precise movements of the fingers,

and beginners will often watch their fingers to be sure

they are moving properly. With practice, however,

the movements simply “feel” right, which means that

the brain has formed a very good mental picture of the

task. Muscle sense again becomes unconscious, and

the experienced typist or guitarist need not watch

every movement.

All sensation is a function of brain activity, and

muscle sense is no exception. The impulses for muscle

sense are integrated in the parietal lobesof the cere-

brum (conscious muscle sense) and in the cerebellum

(unconscious muscle sense) to be used to promote

coordination.

ENERGY SOURCES FOR

MUSCLE CONTRACTION

Before discussing the contraction process itself, let us

look first at how muscle fibers obtain the energy they

need to contract. The direct source of energy for mus-

cle contraction is ATP. ATP, however, is not stored in

large amounts in muscle fibers and is depleted in a few

seconds.

The secondary energy sources are creatine phos-

phate and glycogen. Creatine phosphate is, like

ATP, an energy-transferring molecule. When it is bro-

ken down (by an enzyme) to creatine, phosphate, and

energy, the energy is used to synthesize more ATP.

Most of the creatine formed is used to resynthesize

creatine phosphate, but some is converted to creati-

nine, a nitrogenous waste product that is excreted by

the kidneys.

The most abundant energy source in muscle fibers

The Muscular System 141

BOX7–1 ANABOLIC STEROIDS

will increase muscle size, but there are hazards,

some of them very serious. Side effects of such

self-medication include liver damage, kidney dam-

age, disruption of reproductive cycles, and

mental changes such as irritability and aggres-

siveness.

Female athletes may develop increased growth

of facial and body hair and may become sterile as a

result of the effects of a male hormone on their own

hormonal cycles.

Anabolic steroidsare synthetic drugs very similar

in structure and action to the male hormone testos-

terone. Normal secretion of testosterone, begin-

ning in males at puberty, increases muscle size and

is the reason men usually have larger muscles than

do women.

Some athletes, both male and female, both

amateur and professional, take anabolic steroids to

build muscle mass and to increase muscle strength.

There is no doubt that the use of anabolic steroids