Biomedical Engineering Reference
In-Depth Information
membrane potential. Additional neurons can be added using the same basic neuron, inter-
acting with each other using the current from the adjacent neuron (presynaptic terminal) to
stimulate the next neuron.
For illustration purposes, the interaction between two adjacent neurons is modeled using
SIMULINK, shown in Figure 12.34, and the results shown in Figure 12.35. Three voltage-
dependent channels for
Ca
þ2
, and also a leakage channel are used for the
axon. We use a myelinated axon with four passive compartments between each node of
Ranvier. The total axon consists of three active compartments and two myelinated passive
segments. The dendrite consists of five passive compartments, and the soma is a passive
spherical compartment. The stimulus is applied at the terminal end of the dendrite of the
first neuron. It is modeled as an active electrode compartment. The size of each axon com-
partment is the same but different than the dendrite compartment. The input to the first
neuron is shown in Figure 12.36.
Na
þ
,
K
þ
, and
12.8 CHEMICAL SYNAPSES
The previous section describes the movement of a signal through a change in membrane
potential from the dendrite, soma, and axon to the presynaptic terminal. In this section, we
examine the process that occurs at the presynaptic terminal, called the presynaptic neuron,
and the interaction with the postsynaptic terminal on an adjacent neuron's dendrite, called
the postsynaptic neuron. The major action in the communication between two adjacent neu-
rons is the movement of a neurotransmitter from the presynaptic terminal to the postsynap-
tic terminal via diffusion, transferring electrical energy to chemical energy, and then back
into electrical energy.
Another type of synapse is the gap junction that connects two neurons. Under this
arrangement, ions move directly from one cell to the other. Since this form of communica-
tion is rather rare among neurons, we will not cover it any further here. Gap junctions are
typically observed in signaling among smooth muscle fibers.
More than 40 neurotransmitters have been discovered, some that are excitatory and others
that are inhibitionary. An example of an excitatory neurotransmitter is acetylcholine (ACh),
which depolarizes the postsynaptic neuron's dendrite by opening sodium channels. An
example of an inhibitionary neurotransmitter is gamma-aminobutyric acid (GABA), which
opens the chloride channels that then hyperpolarizes the postsynaptic neuron's dendrite.
Figure 12.3 shows a set of converging presynaptic terminals on the postsynaptic neuron's
dendritic membrane. The space between the two is called the synaptic gap. The distance
between the two neurons is quite small and ranges from 200 to 500 angstroms. Important
features in this form of communication is that it is one-way and analog, which allow for
the summing of all asynchronous inputs. This one-way communication allows a precise
communication and control among neurons with signals going in one direction only. Keep
in mind that the gap junction is not a one-way communication channel, as it allows ions to
move in either direction. It should also be noted that a small percentage of presynaptic
terminals also converge on the soma, which eliminates the change in membrane potential
as a function of distance that occurs on the dendrite.
Figure 12.37 depicts a single presynaptic terminal (top) and the postsynaptic terminal
(bottom). The neurotransmitter is stored in numerous vesicles in the presynaptic neuron,
