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2. BRAIN MECHANISMS: NEUROBIOLOGY AND NEUROANATOMY

membrane, and the membrane of the receiving neuron is known as the
postsynaptic membrane. The synaptic cleft contains extracellular fluid
through which chemical substances can diffuse to interact with a variety of
membrane proteins known as receptors.
Changes in the release or reuptake of neurotransmitters play an important
role in the mechanism of action of many psychoactive substances. Cocaine
and amphetamine, for example, block the reuptake of the neurotransmitters
dopamine and norepinephrine, thereby prolonging the actions of these
transmitters. These mechanisms will be examined in more detail in Chapter 4.

Neurotransmission
Action potential

Neurons communicate with each other through a highly specialized, precise
and rapid method. The action potential is a brief electrical impulse that travels
along an axon and allows one neuron to communicate with another through
the release of neurotransmitter. The action potential is possible because of
the selectively permeable membrane that maintains a chemical and electrical
gradient across the membrane known as the membrane potential. The
membrane at rest is polarized; however, it can become depolarized if diffusion

Fig.2.11 The action potential
During an action potential, voltage-sensitive sodium channels open causing
a rapid influx of sodium and resulting depolarization of the cell. The cell is
repolarized by the opening of potassium channels that permit the efflux of
potassium from the cell and restore the resting membrane potential.
Active ion pumps later exchange sodium for potassium within the cell.
50
30
Overshoot phase
mV
0
Sufficient depolarization of The membrane potential at any
the axon results in an action given time depends on how many
potential. –30 and which channels are open.
Threshold
–50 Undershoot phase
Resting
potential
–70
Open K + Closed Na + Open Na + Closed K + Inactivated Open K +
channel channels channel channel Na + channels channel

Open K + channels Na + channels open, Additional voltage- Na + channels are All gated channels
create resting depolarizing the cell gated Na + channels inactivated; gated K + close. The cell returns
potential to threshold open, causing rapid channels open, to its resting potential
change of polarity – repolarizing and even
the action potential hyperpolarizing the cell
(undershoot phase)
Source: Reproduced from Rosenzweig, Leiman, & Breedlove, 1999, with permission from the
publishers.
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