Essentials of Anatomy and Physiology

CARDIAC MUSCLE

The cells of the heart, cardiac muscle, are shown in
Fig. 4–6. They are branched, have one nucleus each,
and have faint striations. The cell membranes at the
ends of these cells are somewhat folded and fit into
matching folds of the membranes of the next cells.
(Interlock the fingers of both hands to get an idea of
what these adjacent membranes look like.) These
interlocking folds are called intercalated discs, and
permit the electrical impulses of muscle contraction to
pass swiftly from cell to cell. This enables the heart to
beat in a very precise wave of contraction from the
upper chambers to the lower chambers. Cardiac mus-
cle as a whole is called the myocardium, and forms
the walls of the four chambers of the heart. Its func-
tion, therefore, is the function of the heart, to pump
blood. The contractions of the myocardium create
blood pressure and keep blood circulating throughout
the body, so that the blood can carry out its many
functions.
Cardiac muscle cells have the ability to contract by
themselves. Thus the heart maintains its own beat.
The role of nerve impulses is to increase or decrease
the heart rate, depending upon whatever is needed
by the body in a particular situation. We will return to
the heart in Chapter 12.

NERVE TISSUE

Nerve tissueconsists of nerve cells called neurons
and some specialized cells found only in the nervous

system. The nervous system has two divisions: the

central nervous system (CNS) and the peripheral

nervous system (PNS). The brain and spinal cord are

the organs of the CNS. They are made of neurons and

specialized cells called neuroglia. The CNS and the

neuroglia are discussed in detail in Chapter 8. The

PNS consists of all of the nerves that emerge from

the CNS and supply the rest of the body. These nerves

are made of neurons and specialized cells called

Schwann cells. The Schwann cells form the myelin

sheath to electrically insulate neurons.

Neurons are capable of generating and transmit-

ting electrochemical impulses. There are many differ-

ent kinds of neurons, but they all have the same basic

structure (Fig. 4–7). The cell body contains the

nucleus and is essential for the continuing life of the

neuron. An axonis a process (the term “process” here

means “something that sticks out,” a cellular exten-

sion) that carries impulses away from the cell body; a

neuron has only one axon. Dendritesare processes

that carry impulses toward the cell body; a neuron

may have several dendrites. A nerve impulse travels

along the cell membrane of a neuron, and is electrical,

but where neurons meet there is a small space called a

synapse, which an electrical impulse cannot cross. At

a synapse, between the axon of one neuron and the

dendrite or cell body of the next neuron, impulse

transmission depends upon chemicals called neuro-

transmitters. A summary of nerve tissue is found in

Table 4–4, and each of these aspects of nerve tissue is

covered in more detail in Chapter 8.

Nerve tissue makes up the brain, spinal cord, and

Tissues and Membranes 81

Table 4–4 NERVE TISSUE

Part Structure Function

Neuron (nerve cell)

Cell body

Axon

Dendrites

Synapse

Neurotransmitters

Neuroglia

Schwann cells

Contains the nucleus

Cellular process (extension)

Cellular process (extension)

Space between axon of one neuron and the

dendrite or cell body of the next neuron

Chemicals released by axons

Specialized cells in the central nervous

system

Specialized cells in the peripheral nervous

system

• Regulates the functioning of the neuron


• Carries impulses away from the cell body


• Carry impulses toward the cell body


• Transmits impulses from one neuron
  to others


• Transmit impulses across synapses


• Form myelin sheaths and other functions


• Form the myelin sheaths around neurons