Biomedical Engineering Reference
In-Depth Information
Dendrites
Nucleus
Axon
Node of Ranvier
Myelin sheath
Schwann cell nucleus
Schwann cell
Schwann cell
Axon
Neurolemma
Myelin sheath layers
Axon terminal
FIGURE 5.10
The composition of the myelin insulation for the motor neuron axon is
shown here. The gaps between Schwann cells for the uninsulated parts
of the axon are known as the nodes of Ranvier. (Courtesy of http://
www.apptechnc.net/ windelspecht/nervous/pages/motor neuron jpg.htm.)
At rest, this membrane is relatively impermeable to sodium ions (Na
+
). There
is also a sodium/potassium (Na
+
/K
+
) pump, which actively pumps sodium
outside the axonal body in exchange for potassium ions resulting in a concen-
tration gradient across the membrane. At equilibrium, the resting membrane
potential is in the range of
−
70 to
−
90 mV with the axonal body more nega-
tively charged.
The cell membrane has sodium gates composed of molecular pores that
open and close due to changes in the cell-membrane potential. When cur-
rent is injected into the axon, it causes depolarization, which opens the gates
allowing sodium ions to rush into the axon along the concentration gradient.
If the depolarization exceeds 10-30 mV above the resting potential, it gen-
erates an action potential. A positive feedback system then comes into play
whereby a single depolarization causes successive depolarizations to occur and
progressively more sodium gates open. Action potentials are all or no events
and propagate in both directions from the point of origin along the axon. This
means that if an action potential begins, it will continue to travel along the
axon until it reaches both ends of the nerve. The opening of the sodium gate
is limited to about 1-2 ms before a secondary inactivation gate causes the cell
membrane to become impermeable to sodium ions and the sodium flow stops.
This flow of sodium ions into the axonal body has reversed the concentration
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