Invitation to Psychology

126 Chapter 4 Neurons, Hormones, and the Brain

decreases. Inhibition in the nervous system is vi-

tal. Without it, we could not sleep or coordinate

our movements. Excitation of the nervous system

would be overwhelming, producing convulsions.

What any given neuron does at any given mo-

ment depends on the net effect of all the messages

being received from other neurons. Only when

the cell’s voltage reaches a certain threshold will it

fire. Thousands of messages, both excitatory and

inhibitory, may be coming into the cell, and the re-

ceiving neuron must essentially average them. The

message that reaches a final destination depends

on the rate at which individual neurons are firing,

how many are firing, what types of neurons are fir-

ing, where the neurons are located, and the degree

of synchrony among different neurons. It does not

depend on how strongly the individual neurons are

firing, however, because a neuron always either fires

or doesn’t. Like the turning on of a light switch, the

firing of a neuron is an all-or-none event.

Watch the Video The Basics: How the Brain

Works, Part 1 at mypsychlab

Chemical Messengers in the

Nervous System Lo 4.6, Lo 4.7

The nervous system “house” would remain for-

ever dark and lifeless without chemical couriers

to carry messages from room to room. These

chemicals include neurotransmitters, hormones,

and neuromodulators.

Neurotransmitters: Versatile Couriers. As

we have seen, neurotransmitters make it pos-

sible for one neuron to excite or inhibit another.

Neurotransmitters exist not only in the brain but

also in the spinal cord, the peripheral nerves, and

certain glands. Through their effects on specific

nerve circuits, these substances control everything

your brain does. The nature of the effect depends

on the level of the neurotransmitter, its location,

and the type of receptor it binds with. Here we

will describe just a few of the better-understood

neurotransmitters and some of their known or

suspected effects.

Four neurotransmitters travel particular paths

through parts of the brain, like buses following a

route:

•   Serotonin affects neurons involved in sleep,

appetite, sensory perception, temperature regu-

lation, pain suppression, and mood.

•   Dopamine affects neurons involved in volun-

tary movement, learning, memory, emotion,

pleasure and reward, and, possibly response to

novelty.

breaks (nodes) between the myelin’s “sausages.”

Instead, the action potential “hops” from one node

to the next. (More precisely, the action potential

regenerates at each node.) This arrangement al-

lows the impulse to travel faster than it could if

the action potential had to be regenerated at every

point along the axon. Nerve impulses travel more

slowly in babies than in older children and adults

because when babies are born, the myelin sheaths

on their axons are not yet fully developed.

When a neural impulse reaches the axon

terminal’s buttonlike tip, it must get its mes-

sage across the synaptic cleft to another cell. At

this point, synaptic vesicles, tiny sacs in the tip of

the axon terminal, open and release a few thou-

sand molecules of a chemical substance called a

neurotransmitter. Like sailors carrying a message

from one island to another, these molecules then

diffuse across the synaptic cleft (see Figure 4.5).

When they reach the other side, the neu-

rotransmitter molecules bind briefly with receptor

sites, special molecules in the membrane of the

receiving neuron’s dendrites (or sometimes cell

body), fitting these sites much as a key fits a lock.

Electrical changes occur in the receiving neuron’s

membrane, and the ultimate effect is either an

excitatory effect (a voltage shift in a positive direc-

tion) or an inhibitory effect (a voltage shift in a

negative direction), depending on which receptor

sites have been activated. If the effect is excitatory,

the probability that the receiving neuron will

fire increases; if it is inhibitory, the probability

neurotransmitter A
chemical substance that
is released by a trans-
mitting neuron at the
synapse and that alters
the activity of a receiving
neuron.

Neural impulse

Axon terminal

Synaptic

cleft

Receptor

site

Receiving

neuron

Synaptic vesicles

(with neurotrans-

mitter molecules

inside)

Neurotransmitter

molecules

Figure 4.5 Neurotransmitter Crossing a Synapse

Neurotransmitter molecules are released into the syn-

aptic cleft between two neurons from vesicles (cham-

bers) in the transmitting neuron’s axon terminal. The

molecules then bind to receptor sites on the receiving

neuron. As a result, the electrical state of the receiving

neuron changes and the neuron becomes either more

likely to fire an impulse or less so, depending on the type

of neurotransmitter.