HUMAN BIOLOGY

the nerVOus system 245

What is a neurotransmitter?

• Neurotransmitters are chemicals that carry signals between
  the cells at a chemical synapse.


• A neurotransmitter may excite or inhibit the activity of a
  target cell.


• By way of synaptic integration, nervous system messages can
  be reinforced or downplayed, sent onward or suppressed.

taKe-hoMe Message

chemical synapse A gap

between two neurons or

between a neuron and mus-

cle cell or gland cell.

neuromodulator A sub-

stance that can modify the

effects of a neurotransmitter.

neurotransmitter A chemi-

cal that carries neural mes-

sages across a chemical

synapse.

synaptic integration

Process in which the com-

peting signals arriving at

a neuron are summed up

before the neuron responds.

Dopamine acts in fine motor control and influences some
type of learning. GABA inhibits the release of other
neurotransmitters.
Serotonin acts on brain cells that govern emotional states,
sleeping, sensory perception, and regulation of body tem-
perature. Some neurons secrete nitric oxide (NO), a gas that
controls blood vessel dilation. It is not stored in synaptic
vesicles but instead is manufactured as needed. As an
example, a sexually aroused male has an erection when
NO calls on blood vessels in his penis to dilate, allowing
blood to rush in.
Neuromodulators can magnify or dampen the effects
of a neurotransmitter. These substances include natural
painkillers called endorphins. Endorphins inhibit nerves
from releasing substance P, which conveys information
about pain. In athletes who exercise beyond normal fatigue,
endorphins can produce a euphoric high.

Competing signals are “summed up”

At any moment, many signals are washing over the
input zones of a receiving neuron. All of them are graded
potentials (their magnitude can be large or small), and
they compete for control of the membrane potential at the
trigger zone. The ones called EPSPs (for excitatory post-
syn ap tic potentials) depolarize the membrane—they bring
it closer to threshold. On the other hand, IPSPs (inhibitory
post synaptic potentials) may hyperpolarize the membrane

(drive it away from threshold) or help keep the membrane

at its resting level.

Synaptic integration tallies up the competing signals

that reach an input zone of a neuron at the same time—a lit-

tle like adding up the pros and cons

of a certain course of action. This

process, called summation, is how sig-

nals arriving at a neuron are sup-

pressed, reinforced, or sent onward

to other cells in the body.

Integration occurs when neu-

rotransmitter molecules from more

than one presynaptic cell reach a

neuron’s input zone at the same

time. Signals also are integrated

after a neurotransmitter is released

repeatedly, over a short time period,

from a neuron that is responding to

a rapid series of action potentials.

neurotransmitter molecules must

be removed from the synapse

The flow of signals through the nervous system depends

on the rapid, controlled removal of neurotransmitter

molecules from synapses. Some of the neurotransmitter

molecules diffuse out of the gap. Enzymes cleave others

in the synapse, as when acetylcholinesterase breaks down

ACh. Also, membrane transport proteins actively pump the

neurotransmitter molecules back into pre synaptic cells or

into neighboring neuroglia.

Certain drugs can block the reuptake of particular neuro-

transmitters. For example, some anti depressant drugs elevate

a depressed person’s mood by blocking the reuptake of

serotonin. Others shift the balance of a combination of neu-

rotransmitters, such as serotonin and norepinephrine.

Neurotransmitter examples of effects

Acetylcholine (ACh) Causes skeletal muscle

contraction; affects mood

and memory

Epinephrine and Speed heart rate; dilate the

norepinephrine pupils and airways to lungs;

slow GI tract contractions;

increase anxiety

Dopamine Reduces excitatory effects of other

neurotransmitters; roles in memory,

learning, fine motor coordination

Serotonin Elevates mood; has a role in memory

and learning

GABA Inhibits the release of other

neurotransmitters

Table 13.1 Major Neurotransmitters and Their Effects

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