Biomedical Engineering Reference
In-Depth Information
Dendrites
Axon Hillock
Axon
Cell Body
Presynaptic Terminals
Node of Ranvier
Myelin Sheath
FIGURE 12.1
A typical neuron.
since the primary focus here is to better understand the signaling properties of a neuron.
Overall, the complex abilities of the brain are best described by virtue of a neuron's intercon-
nections with other neurons or the periphery and not a function of the individual differences
among neurons.
A typical neuron, as shown in Figure 12.1, is defined with four major regions: cell body
(also referred to as the soma), dendrites, axon, and presynaptic terminals. The cell body of a
neuron contains the nucleus and other organelles needed to nourish the cell and is similar
to other cells. Unlike other cells, however, the neuron's cell body is connected to a number
of branches called dendrites and a long tube called the axon that connects the cell body to
the presynaptic terminals. Some neurons have multiple axons.
Dendrites are the receptive surfaces of the neuron that receive signals from thousands of
other neurons passively and without amplification. Located on the dendrite and cell body
are receptor sites that receive input from presynaptic terminals from adjacent neurons.
Neurons typically have 10
4
to 10
5
synapses. Communication between neurons is
through chemical synapses. Chemical synapses, as described in Chapter 3, involve the
use of a neurotransmitter that changes the membrane potential of an adjacent neuron. Other
cells, such as muscle and cardiac cells, use electrical synapses. An electrical synapse involves
the use of a gap junction that directly connects the two cells together through a pore.
Also connected to the neuron cell body is a single axon that ranges in length from 1
meter in the human spinal cord to a few millimeters in the brain. The diameter of the axon
also varies from less than 1 to 500
m. In general, the larger the diameter of the axon, the
faster the signal travels. Signals traveling in the axon range from 0.5 m/s to 120 m/s. The
purpose of an axon is to serve as a transmission line to move information from one neuron
to another at great speeds. Some axons are surrounded by a fatty insulating material called
the myelin sheath and have regular gaps, called the nodes of Ranvier, that allow the action
potential to jump from one node to the next. The action potential is most easily envisioned
as a pulse that travels the length of the axon without decreasing in amplitude.
Most of the remainder of this chapter is devoted to understanding this process. At the end
of the axon is a network of up to 10,000 branches with endings called the presynaptic term-
inals. A diagram of the presynaptic terminal is shown in Figure 3.28. All action potentials that
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