HUMAN BIOLOGY

246 Chapter 13

information pathways

The stretch reflex contracts a muscle after gravity or

some other load has stretched the muscle. Suppose you

steadily hold out a bowl as someone loads peaches into it,

adding weight to the bowl. When your hand starts to drop,

the biceps muscle in your arm is stretched. This stretching

activates receptors in muscle spindles. These are sensory

organs in which specialized cells are enclosed in a sheath

that runs parallel with the muscle. The receptor endings

are the input zones of sensory neurons whose axons syn-

apse with motor neurons in the spinal cord (Figure 13.9).

Axons of the motor neurons lead back to the stretched mus-

cle. Action potentials that reach the axon endings trigger

the release of ACh, which triggers contraction. As long as

receptors continue to send messages, the motor neurons are

excited. This allows them to send signals to muscles that

maintain your hand’s position. This type of reflex is often

called a spinal reflex. The pathway neural signals travel to

generate a reflex is called a reflex arc.

n    Once a message is sent in the nervous system, where it

goes depends on how neurons are organized in the body.

nerves are long-

distance lines

Nerves are commu nication lines

between the brain or spinal cord

and the rest of the body. A nerve

consists of nerve fibers, which are

the long axons of sensory neurons,

motor neurons, or both. Connective

tissue encloses most of the axons

like electrical cords inside a tube

(Table 13.2 and Figure 13.8A). In the

central nervous system (the brain

and spinal cord) nerves are called

nerve tracts.

Each axon has an insulating

myelin sheath, which allows action

potentials to propagate faster than

they would otherwise. The sheath

consists of glial cells that wrap around the long axons

like jelly rolls. As you can see in Figure 13.8B, an exposed

node, or gap, separates each cell from the next one.

There, voltage-sensitive, gated sodium channels pepper the

plasma membrane. In a manner of speaking, action poten-

tials jump from node to node (a phenomenon that

sometimes is called saltatory conduction, after a

Latin word meaning “to jump”). The sheathed areas

between nodes hamper the movement of ions across

the plasma membrane, so stimulation tends to travel

along the membrane until the next node in line. At

each node, however, the flow of ions can produce

a new action potential. In large sheathed axons,

action potentials propagate at a remarkable 120 meters

(nearly 400 feet) per second!

In the central nervous system, glial cells called oli-

godendrocytes form the myelin sheath. In the rest of

the nervous system, glial cells called Schwann cells

form the sheath.

reflexes are the simplest

nerve pathways

Sensory and motor neurons of certain nerves take

part in automatic responses called reflexes. A reflex

is a simple, programmed movement in response to

a stimulus. Reflexes are always the same and do

not involve the brain. In the simplest ones, sensory

neurons synapse directly on motor neurons. In most

reflex pathways, however, the sensory neurons also interact

with several interneurons. These excite or inhibit motor

neurons as needed for a coordinated response.

axon of one neuron

many neurons bundled together

inside a connective tissue sheath

blood vessels

outer connective tissue of one nerve

axon

myelin sheath

formed by

glial cells

unsheathed node

containing gated

Na+ channels

A

B

Figure 13.8 Animated! Nerves are bundled long axons of

neurons. a Structure of a nerve. B Structure of a sheathed axon.

A myelin sheath formed by Schwann cells blocks the flow of ions

except at nodes between Schwann cells. (© Cengage Learning)

Neuron Nervous system cell specialized for

communication

Nerve fiber Long axon of one neuron

Nerve Long axons of several neurons enclosed

by connective tissue

Table 13.2 The Parts of Nerves

13.4

myelin sheath Wrapping of
glial cells around the axon
of a neuron. The sheath pro-
vides insulation that allows
nerve impulses to propa-
gate faster than they would
otherwise.

nerve Bundle of neuron
axons.

nerve tract Nerves in the
central nervous system (the
brain and spinal cord).

reflex Simple, stereotyped
movement in response to a
stimulus.

Schwann cells Type of glial
cells that sheathe axons
outside the central nervous
system.

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