HUMAN BIOLOGY

244 Chapter 13

how neurons Communicate

Action potentials can stimulate neurons to release the

chemical signals called neurotransmitters. These molecules

diffuse across a chemical synapse, a narrow gap between

a neuron’s output zone and the input zone of a neighboring

cell. Some chemical synapses occur between neu-

rons, others between a neuron and a muscle cell

or gland cell.

At a chemical synapse, one of the two cells

stores neurotransmitter molecules in synaptic

vesicles in its cytoplasm. This is the presynap-

tic cell. The cell’s plasma membrane has gated

channels for calcium ions, and they open when

an action potential arrives. There are more cal-

cium ions outside the cell, and when they flow

in (down their gradient), synaptic vesicles fuse

with the plasma membrane, discharging their

content. Neurotransmitter molecules now pour

into the synapse, diffuse across it, and bind

with receptor proteins on the plasma membrane

of the postsynaptic, or receiving, cell. Binding

changes the shape of these proteins, so that a

channel opens up through them. Ions then dif-

fuse through the channels and move into the

receiving cell.

neurotransmitters can excite

or inhibit a receiving cell

How a receiving cell responds to a neurotrans-

mitter depends on the type and amount of a

neurotransmitter, the kinds of receptors the cell

has, and some other factors. Exciting signals help

drive the membrane toward an action potential.

Inhibiting signals have the opposite effect. Table

13.1 lists some common neurotransmitters and

their effects in the body.

One neurotransmitter, acetylcholine (ACh), can

excite or inhibit different target cells in the brain,

spinal cord, glands, and muscles. Figure 13.7

shows a neuromuscular junction chemi cal syn-

apse between a motor neuron and a muscle cell.

ACh released from the neuron diffuses across

the gap and binds to receptors on the muscle cell

membrane. It excites this kind of cell, triggering

the action potentials that cause skeletal muscle

contractions.

Epinephrine and norepinephrine prepare

the body to respond to stress or excitement.

n Action potentials may cause a neuron to release neuro‑

transmitter molecules that diffuse to a receiving cell.

This is one way that information flows from cell to cell.

n Link to Neuromuscular junctions 6.4

13.3

Figure 13.7 Animated! a neuromuscular junction forms between

axon endings of motor neurons and skeletal muscle fibers.

axon of

a motor

neuron

neuromuscular

junction

synaptic

vesicle

axon terminal of

motor neuron

plasma membrane

of muscle fiber

synapse

binding site for

neurotransmitter

ion channel closed

1 Action potentials
flow along the axon of a
motor neuron to neuro-
muscular junctions,
where an axon terminal
forms a synapse with a
muscle fiber.

(^2) The axon terminal
stores chemical signaling
molecules (green) called
neurotransmitters inside
synaptic vesicles.
(^3) Arrival of an action
potential causes exocytosis
of synaptic vesicles, and
neurotransmitter molecules
enter the synapse.
4 The plasma membrane of the
muscle fiber has receptors for
the neurotransmitter.
(^5) Binding of a neurotransmitter
opens a channel through the recep-
tor. The opening allows ions to flow
into the receiving cell.
neurotransmitter
ion flows through
now-open channel
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