HUMAN BIOLOGY

the nerVOus system 241

What kinds of cells carry nervous system

signals?

• Sensory neurons, interneurons, and motor neurons all carry
  nervous system signals, or nerve impulses.


• In a resting neuron, differences in the concentrations of Na^1
  and K^1 across the plasma membrane produce a difference in
  electrical charge across the plasma membrane.


• This charge difference is called the resting membrane potential.
  It sets the stage for a neuron to do its physiological work of
  firing a nerve impulse.

taKe-hoMe Message

is more negative than than the outer side of the membrane.

This difference is called resting membrane potential.

The term means that the charge difference has the potential

to do physiological work in the body. That “work” is the

launching of a nerve impulse.

Various kinds of signals occur in the nervous system,

but not all of them spark nerve impulses. Only a signal

that is strong enough when it reaches a resting neuron’s

input zone may spread to a trigger zone. When a strong

enough signal does arrive, however, it can cause the volt-

age difference across the plasma

membrane to reverse, just for an

instant. In the following section,

we see how these reversals produce

nervous system signals.

resting membrane potential

Difference in the electrical

charge on either side of a

neuron’s plasma membrane.

Figure 13.3 Animated! ions produce an electrical gradient across a neuron’s plasma membrane.

a Gradients of sodium (Na^1 ) and potassium (K^1 ) ions across a neuron’s plasma membrane. B How ions

cross the plasma membrane of a neuron. They are selectively allowed to cross at protein channels and

pumps that span the membrane. (© Cengage Learning)

A

inside cell

outside cell

tissue fluid

cytoplasm

3 Na+ 2 K+

aTP ADP + Pi

B

Transport proteins actively

transport three Na^1 out of

a neuron for every two K^1

they pump in.

Passive transporters allow

K^1 ions to move across

the plasma membrane,

down their concentration

gradient.

Voltage-gated channels for

Na^1 or K^1 are closed in a

neuron at rest (left), but open

when it is excited (right).

Figure 13.2 Animated! a motor neuron has four main function
zones. (© Cengage Learning)

axon terminals

cell body

dendrite

axon

Conducting zone

1 Input zone

3

Trigger

zone

Output zone

2

4

1 Dendrites and the cell body are the neuron’s input zone. They

receive chemical signals from another neuron and convert them to

the electrical signals called nerve impulses.

2 Nerve impulses spread to a trigger zone at the start of an axon.

3 If the nerve impulses are strong enough, they are conducted

along the axon to its terminal endings.

4 Axon terminals are a signal output zone. They release chemical

signals that influence another cell. In the case of a motor neuron,

signals affect a muscle cell or gland cell.

membrane. Electrical charges may be measured in milli-
volts, and for many neurons, the steady charge difference
across the plasma membrane is about 2 70 millivolts. The
minus indicates that the cytoplasm side of the membrane

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