SECTION C SUMMARY
I. An excitatory synapse brings the membrane of the postsynaptic
cell closer to threshold. An inhibitory synapse hyperpolarizes
the postsynaptic cell or stabilizes it at its resting level.
II. Whether a postsynaptic cell ﬁ res action potentials depends on
the number of synapses that are active and whether they are
excitatory or inhibitory.
III. Neurotransmitters are chemical messengers that pass from
one neuron to another and modify the electrical or metabolic
function of the recipient cell.
Functional Anatomy of Synapses
I. A neurotransmitter, which is stored in synaptic vesicles in the
presynaptic axon terminal, carries the signal from a pre- to a
Mechanisms of Neurotransmitter Release
I. Depolarization of the axon terminal raises the calcium
concentration within the terminal, which causes the release of
neurotransmitter into the synaptic cleft.
II. The neurotransmitter diffuses across the synaptic cleft and
binds to receptors on the postsynaptic cell; the activated
receptors usually open ion channels.
Activation of the Postsynaptic Cell
I. At an excitatory synapse, the electrical response in the
postsynaptic cell is called an excitatory postsynaptic potential
(EPSP). At an inhibitory synapse, it is an inhibitory
postsynaptic potential (IPSP).
II. Usually at an excitatory synapse, channels in the postsynaptic
cell that are permeable to sodium, potassium, and other small
positive ions open; at inhibitory synapses, channels to chloride
and/or potassium open.
I. The postsynaptic cell’s membrane potential is the result of
temporal and spatial summation of the EPSPs and IPSPs at the
many active excitatory and inhibitory synapses on the cell.
II. Action potentials are generally initiated by the temporal and
spatial summation of many EPSPs.
I. Synaptic effects are inﬂ uenced by pre- and postsynaptic events,
drugs, and diseases (Table 6–5).
Neurotransmitters and Neuromodulators
I. In general, neurotransmitters cause EPSPs and IPSPs, and
neuromodulators cause, via second messengers, more complex
metabolic effects in the postsynaptic cell.
II. The actions of neurotransmitters are usually faster than those
III. A substance can act as a neurotransmitter at one type of
receptor and as a neuromodulator at another.
IV. The major classes of known or suspected neurotransmitters
and neuromodulators are listed in Table 6–6.
I. The junction between a neuron and an effector cell is called a
II. The events at a neuroeffector junction (release of
neurotransmitter into an extracellular space, diffusion of
neurotransmitter to the effector cell, and binding with a
receptor on the effector cell) are similar to those at a synapse.
Additional Clinical Examples
I. Ethanol alters brain function by targeting proteins involved
in synaptic transmission throughout the brain. By inhibiting
glutamate and enhancing GABA signaling, it has a global
depressant effect on the nervous system. Its effects on
dopaminergic and endogenous opioid signaling result in
euphoria, mood elevation, and occasionally addiction. High
doses are fatal due to suppression of cardiovascular and
respiratory centers in the brainstem.
SECTION C KEY TERMS
SECTION C CLINICAL TERMS
excitatory amino acid
excitatory postsynaptic potential
inhibitory postsynaptic potential