132 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”Fig. 7.4. Schematic cross-section of a typical synapse. The upper portion (pre-synaptic
component) is the nerve ending; the lower portion (post-synaptic component) is a den-
dritic spine. Between the two is a narrow space called the synaptic cleft. The flow of
information is from the pre-synaptic to the post-synaptic neuron. When a nerve impulse
arrives at the axonal terminal, calcium influx triggers the release of neurotransmitters
from the vesicles. Four things can happen to the released transmitter. It can (1) hit the
post-synaptic receptors for further propagation of message; (2) be degraded by enzymes;
(3) be taken up into the pre-synaptic terminal for storage; (4) hit the pre-synaptic
auto-receptors for negative feedback. [Courtesy Michael W. King.]
which is one of two types: excitatory post-synaptic potential (EPSP) or
inhibitory post-synaptic potential (IPSP), depending on whether the
receptor responds by depolarization (becoming less negative inside) or
hyperpolarization (becoming more negative inside), respectively. When
depolarization reaches a threshold it triggers the initiation of an action
potential in the axon hillock (origin of the axon), thereby propagating the
nerve impulse along the axon toward the terminals. As a rule, membrane
depolarization excites a neuron, while hyperpolarization inhibits it.^3
The simplest neuronal circuit consists of an afferent (sensory) and
an efferent (motor) neuron connected by a single synapse, as in a simple