eventually prevents the loss of more K+ions, and an equilibrium is reached; the cell
becomes polarized and the transmembrane potential (resting potential) stabilizes.
The difference between an ordinary cell and an excitable cell becomes evident when
a depolarizing current is applied. In an ordinary cell, such as an erythrocyte, the trans-
membrane potential is equal to zero; in a neuron, however, an explosive, self-limiting
process allows the potential to overshoot zero and become about 30 mV more positive
within the cell than outside it. This depolarization is called an action potential, and is
carried first by sodium ions and then by potassium ions (see figure 4.3). Spread of the
action potential along a neuron is the means by which information is transmitted in the
CNS. The neuron is the fundamental anatomical unit of the brain; the action potential
is the fundamental physiological (functional) unit of the brain. An action potential lasts
only about a millisecond, during which time sodium rushes in and potassium rushes out
through ion channel proteins opened by conformational change. The original ionic dis-
equilibrium is then re-established through the rapid elimination of Na+ions. In myelin-
ated nerves, such ion exchange can occur only at the nodes of Ranvier, and the action
potential jumps very rapidly from node to node without a loss of potential. This wave
of depolarization passes along the axon to the nerve ending and can be repeated several
hundred times per second.
4.1.3 Synaptic TransmissionSynaptic transmission is the process whereby neurons communicate with each other
and with the target organs whose physiology they are influencing; synaptic transmis-
sion permits the action potential to jump from one neuron to the next. It is imperative
196 MEDICINAL CHEMISTRY
Figure 4.3 The neuron is the fundamental structural (anatomical) unit of the brain. The action
potential is the fundamental functional (physiological) unit of the brain and is the means of trans-
mitting information within the nervous system. An action potential is generated by changes in the
transmembrane voltage gradient across the neuronal membrane. The action potential is initiated by
the opening of voltage-gated Na+channels. The resulting wave of depolarization travels along the
neuron as an electrical signal, transmitting information.