Biomedical Engineering Reference
In-Depth Information
CHAPTER 6
Synapses
Previous chapters have described how electrical activity propagates down the dendrites, integrates in the
soma, and propagates down the dendrites.Most of the action, however, occurs at the connections between
neurons. There are two primary ways that neurons connect. First are
gap junctions
which are proteins that
directly connect the intracellular space of two neurons. Typically, gap junctions are modeled as linear
resistor. In this chapter, we will focus primarily on the second type of connection, the synapse. Using
engineering terms, the electrical signal that reaches the axon terminal is
transduced
to a chemical signal
by the pre-synapse. This chemical signal then diffuses across the synaptic cleft and is transduced back to
an electrical signal by the post-synapse on the dendrite of another neuron. A molecule that can transduce
an electric signal across a synapse is called a
neurotransmitter
. In reality, these three steps are much more
complex and can be broken down in the list below and shown graphically in Fig. 6.1.
1. Neurotransmitters are packaged into vesicles in the soma.
2. Depolarizations in the dendrites charge the soma and cause the axon hillock to fire an action
potential.
3. The action potential propagates unattenuated down the axon.
4. The action potential reaches the pre-synaptic terminal.
5.
Ca
2
+
channels in the pre-synapse open and
Ca
2
+
enters the pre-synapse.
6.
Ca
2
+
in the pre-synapse triggers fusion of vesicles (with neurotransmitters inside) with the mem-
brane.
7. The neurotransmitter is released into the synaptic cleft by exocytosis.
8. The neurotransmitter diffuses across the synaptic cleft.
9. The neurotransmitter binds to receptors on the post synaptic membrane.
10. Post-synaptic ion channels open by either a direct or indirect (second messenger) mechanism.
11. The post-synaptic membrane potential changes (either depolarizes or hyperpolarizes).
12. The neurotransmitter is released by the post-synapse, inactivated and finally returned to the pre-
synapse.
We have already quantified steps 2-4 and we will not quantify 1 or 7. In this chapter, we will first consider
action at the pre-synapse (steps 5 and 6) and then turn to action at the post-synapse (steps 8-12).
Although it is typical to examine synapses that connect an axon of one neuron to the dendrite of another,
synapses also may form directly on the soma.
