8 SYNAPTIC TRANSMISSION AND NEURAL INTEGRATION
Electrical synapses: operate by allowing electrical signals to be transmitted from one
neuron to another through gap junctions.
Chemical synapses: operate through the release of neurotransmitters that activate
signal transduction mechanisms.
Exist between neurons, and neurons and glial cells.
The plasma membrane of adjacent cells are linked together by gap junctions (electrical
signal directly transferred to the adjacent cell by ions flowing).
Allows rapid communication (usually bidirectional) between adjacent neurons that
synchronizes the electrical activity in these cells.
Communication can be excitatory or inhibitory at the same synapse, as either a
depolarizing or a hyperpolarizing current can spread through the junctions.
One neuron secretes a neurotransmitter into the extracellular fluid in response to an
action potential arriving at its axon terminal. The neurotransmitter than binds to
receptors on the plasma membrane of a second cell, triggering an electrical signal that
may or may not initiate an action potential.
Effector organs: muscles and glands.
Neuroeffector junction: a synapse between a neuron and an effector cell
FUNCTIONAL ANATOMY OF CHEMICAL SYNAPSES
Presynaptic neuron: transmits signals to the second; Postsynaptic neuron: receives
signals from the first; Synaptic cleft: the narrow space between the presynaptic and
postsynaptic neurons (30-50nm wide).
Signaling across a synapse is unidirectional, presynaptic neuron to the dendrite or cell
body of the postsynaptic neuron (axodendritic or axosomatic synapses, respectively).
In some cases, the presynaptic neurons axon terminal forms a synapse with the
postsynaptic neurons axon terminal (axoaxonic synapse). Vital for modulating
communication at axodendrite and axosomatic synapses.
The axon terminal of the presynaptic neuron releases neurotransmitters into the
synaptic cleft. Then the neurotransmitters diffuse rapidly across the cleft and bind to
receptors on the postsynaptic neuron. The ligand binding produces a response in the
postsynaptic neuron by signal transduction mechanisms.
The axon terminal of the presynaptic neuron contains numerous small, membrane-
bound compartments called synaptic vesicles, which store neurotransmitter molecules.
Enzymes in the cytosol of the axon terminal synthesize neurotransmitters. After they
are transported into synaptic vesicles.
Cytosolic Ca triggers the release of neurotransmitter by exocytosis. The axon terminal
has mostly voltage-gated Ca channels; open when depolarized. This allows Ca to go
down its electrochemical gradient, increasing the [Ca ]in the axon terminal. Ca then
causes the membranes of the synaptic vesicles to fuse with vesicle attachment sites on
the inner surface of the axon terminal membrane and undergo exocytosis.
Amount of neurotransmitters released depends on [Ca ] in the cytosol, which depends
on the frequency of action potentials in the presynaptic neuron.8 SYNAPTIC TRANSMISSION AND NEURAL INTEGRATION
Milliseconds after an action potential, neurotransmitter release stops because the
voltage-gated Ca channels close rapidly, and because Ca ions are actively transported
out of the axon terminal on a continual basis, hence cytosolic [Ca] back to resting level.
If a 2 action potential arrives while there are neurotransmitters in the synaptic cleft,
[Ca] increase and release more neurotransmitters. Thus the [neurotransmitter] in the
synaptic cleft increases as the frequency of action potential increases.
The neurotransmitter binds to receptors of the postsynaptic neuron (a brief and
Continual binding of neurotransmitter to receptor doesnt happen because many
processes quickly clear the neurotransmitter in the synaptic cleft: molecules degraded
by nearby enzymes, molecules can be actively transported back into the presynaptic
neuron (reuptake) to be recycled, or can be diffused out of the cleft.
It takes 0.5-5 msec from the time an action potential arrives at the axon terminal before
a response occurs in the postsynaptic cell (synaptic delay). Mainly due to the time
required for Ca to trigger the exocytosis of neurotransmitter. Diffusion of
neurotransmitter to the receptor is so rapid.
SIGNAL TRANSDUCTION MECHANISMS AT CHEMICAL SYNAPSES
The fast response in the postsynaptic neuron occurs whenever a neurotransmitter
binds to a channel-linked receptor (ionotropic receptor; ligand-gated channels).
Postsynaptic potential (PSP): occurs very rapidly and turns off rapidly (milliseconds)
because the channel closes as soon as the neurotransmitter leaves the receptor.