HUMAN BIOLOGY

240 Chapter 13

neurons: the Communication specialists

Figure 13.2 shows where different events occur in a

motor neuron. The dendrites and cell body are an “input

zone” where the neuron receives arriving chemical signals

and converts them to electrical signals. The part of the axon

nearest the cell body (sometimes called an axon “hillock,”

meaning little hill) is a “trigger zone” where an arriving

electrical signal triggers signals along the conducting por-

tion of the axon to the axon terminals. These terminals are

the neuron’s “output zone”: They release signaling mol-

ecules that serve as messages to other cells.

Recall from Chapter 4 that roughly 90 percent of your

nervous system consists of cells called glia (neuroglia).

Glia help maintain the proper concentrations of vital ions

in the fluid around neurons and assist in the formation of

connections between brain neurons. Some physically sup-

port and protect neurons. Others provide insulation that

allows signals to move along sensory and motor neurons

with lightning speed.

properties of a neuron’s plasma membrane

allow it to carry signals

Neurons are suited for communication partly because

they are excitable—that is, a neuron can respond to certain

stimuli by producing an electrical signal.

Chapter 3 described how the plasma membrane’s lipid

bilayer prevents charged substances—such as ions of potas-

sium (K^1 ) and sodium (Na^1 )—from freely crossing it. Even

so, ions can cross the membrane through channel proteins

that span the bilayer (Figure 13.3). Some channels are always

open, so that ions can steadily “leak”—by diffusion—in or

out. Other channels open like gates under the proper circum-

stances. These controls mean that the concentrations of an ion

can be different on either side of the plasma membrane.

For example, in a resting neuron, the gated sodium

channels are closed and the plasma membrane allows

only a little sodium to leak inward. The membrane is more

permeable to K^1. As a result, each ion has its own con-

centration gradient across the membrane (Figure 13.3A).

Following the rules of diffusion, sodium tends to move in

and potassium tends to move out (Figure 13.3B).

For several reasons, on balance the cytoplasm next to the

membrane is more negative than the fluid just outside the

n    Three types of neurons are the nervous system’s

communication cells.

n    Links to Nervous tissue 4.4, The plasma membrane 3.4

The nervous system detects infor-

mation about external and inter-

nal conditions. It then integrates

those inputs and selects or controls

muscles and glands that carry out

responses.

neurons are the

communication lines

of the nervous system

Three types of neurons carry out

the functions of the nervous system.

Sensory neurons collect informa-

tion about stimuli such as light or

touch. They usually signal an inter-

neuron. interneurons, which are

located in the brain and spinal cord,

are go-betweens that receive signals

from and send signals to other neu-

rons. They may signal another inter-

neuron or a motor neuron. Motor

neurons control muscles and

glands. Muscles and glands are called effectors because

their response to the motor neuron’s signal produces the

final effect.

As you can see in Figure 13.1, neurons have a large cell

body that contains the cell’s nucleus and other organelles.

The figure also shows extensions of the cytoplasm, which

send out and receive signals. For example, all neurons

have an axon, an extension that transmits electrical sig-

nals along its length and releases chemical signals at its

terminals. Motor neurons and interneurons also have

dendrites, which are cytoplasmic extensions that receive

chemical signals from other neurons. Sensory neurons

don’t receive signals from other neurons, so they don’t

have dendrites. One end of their axon has receptor end-

ings that detect a particular type of stimulus; the other

end has signal-sending axon terminals.

Figure 13.1 Three types of neurons carry the nervous system’s messages. (© Cengage Learning)

13.1

receptor

endings

axon

axon terminal

cell

body

cell

body

cell

body

axon axon axon axon

terminals

dendrites

dendrites

C Motor neurons have many dendrites and a

long axon.

A Sensory neurons have a long axon with receptor
endings at one end and axon terminals at the other.

B Interneurons have many

dendrites and a short axon.

axon Extension of a neuron

that conducts signals away

from the neuron.

dendrite Short, branching

extension of a neuron that

receives incoming signals.

interneuron Neuron in the

spinal cord or brain that

receives sensory input, pro-

cesses it, and sends signals

to other neurons.

motor neuron Neuron that

relays signals from inter-

neurons to muscles and

glands—effectors that carry

out responses.

nervous system The body’s

neuron-based control sys-

tem; it works together with

the endocrine system.

sensory neuron Neuron that

collects information about a

stimulus and relays it to the

spinal cord and brain.

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