The Synapse And The Presynaptic And Postsynaptic Terminals
As we learned when we talked about parts of
the neuron, neurons have specialized parts
called dendrites and axons. Dendrites bring
information into the neuron and axons take
information away from the neuron.
Neurons don’t operate in isolation. They
communicate with each other in networks. If
you were to think about yourself and all your
friends as connected by strings so that all of
you would be part of a network of kids, then
that’s a bit like neuron networks work, only the
neurons are in much larger groups than a
group of you and your friends. Instead of
strings, the neurons are connected by the
axons and dendrites.
However, unlike you and your friends connected by
strings, there is a gap between the place where an
axon or a dendrite connects with a cell
body, axon, or dendrite in the network. This gap is
called the synapse. There are three major parts of
1. a presynaptic terminal containing
neurotransmitter, mitochondria (responsible for energy
production in the cell) and other cell bodies, called
2. a postsynaptic terminal containing receptor sites for
3. a synaptic gap or space between the presynaptic
terminal and the postsynaptic terminal
When an action potential occurs, an electrical
signal or impulse travels down the axon to the
presynaptic terminal. When it reaches the
presynaptic terminal, the electrical impulse causes
vesicles (little packets of neurotransmitter) to
move to the presynaptic membrane. There the
vesicles fuse with the presynaptic membrane and
release the neurotransmitters they contain into the
The neurotransmitter molecules travel across the
synaptic gap and bind on the other side with
receptors on the postsynaptic terminal. This
causes a change in the excitability of the
postsynaptic cell. The postsynaptic cell will then be
either more or less likely to fire an action potential,
depending on whether the impulse is excitatory or
inhibitory. If the number of excitatory postsynaptic
events is large enough, the message will then be
sent along to the next neuron in the network.
Axon and dendrites connecting at the presynaptic/postsynaptic terminals, showing the neurotransmitter being
released into the synaptic gap.
During the process that allows synaptic transmission of
neurotransmitter to occur, a serious of things have to happen:
1. the neuron itself has to make the chemicals that will be used as
neurotransmitter. These are made either in the axon’s presynaptic
terminal or in the neuron’s cell body.
2. the neurotransmitter has to be transported to the presynaptic
terminal if the neurotransmitter has been made in the cell body.
3. the action potential (the electrical part of the electrochemical event
that causes a message to be sent) has to travel down the axon and
cause calcium to enter the cell at the presynaptic terminal. Calcium
causes neurotransmitter to be released from the presynaptic terminal
into the synaptic gap.
4. the released neurotransmitter have to cross the synaptic gap and
attach to receptor sites on the postsynaptic terminal of the
5. Left over neurotransmitter not used to send the message on
through the neighboring neuron needs to either be taken back up by
the presynaptic terminal, broken down and recycled so it can be
used again, or disposed of.