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Chapter 8

BPK 205 Chapter Notes - Chapter 8: Multiple Sclerosis, Myelin, Akon

Biomedical Physio & Kines
Course Code
BPK 205
Parveen Bawa

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26. Take an unmyelinated neuron, give all details to describe generation and conduction
of an action potential from the trigger zone down to the terminals. [2-3 pages with a
figure(s) 40 minutes]
The axon in an unmyelinated neuron has much more capacity to hold the charge
difference due to the small distance between the outside and the inside of the axon. This
large capacitance makes the required change in charge greater in order to generate an
action potential. Because more charge difference is required between outside and inside
of the neuron, the axon has less conductivity. Action potentials will be much slower.
Also, myelin sheath is there to protect from current leak. In its absence there would be
current leak further weakening the action potential signal. This can result in Multiple
Sclerosis. However, some neurons which have axons smaller than 1 micrometer in
diameter benefit from being unmyelinated. Conduction velocity for unmyelinated axons
are proportional to the square root of the diameter of the axon. At greater axon diameters,
myelinated axons conduct much faster.
27. Take an myelinated neuron, give all details to describe generation and conduction of
an action potential from the trigger zone down to the terminals.[2-3 pages with a
figure(s); 40 minutes]
Myelinated axons have nodes and internodes. The unmyelinated nodes are where action
potentials are created because that is where the Na+ ion channels are located. Action
potentials travel passively during the internodes which are myelinated. The myelin gives
the axons a greater diameter; hence, a lesser charge capacity. The small capacitance
allows action potentials to be created at a lower charge difference (the voltage threshold
will be the same, but it will be easier to get to that voltage threshold to create action
potentials). This makes for much faster conductivity of the axon. Myelinated axons
conduction velocity increases linearly according to axon diameter.
28. Question: What is the importance of capacitance in the generation of an action
potential in a myelinated and an unmyelinated axon? [~ 1 page]
Answer: Under resting conditions the membrane is charged to - 70 mV (-ve inside with
respect to outside). Q = CV, where Q is the charge, C the capacitance and V the
membrane voltage. The membrane acts like a capacitor with capacitance C. It holds
charge Q producing membrane potential V (at this potential only passive channels are
open). Then we reduce the charge Q on the membrane to make the membrane voltage V
= -60 mV. At this voltage, sufficient number of voltage gated Na+ channels open to start
the regenerative cycle. The voltage at which the regenerative action starts is called the
threshold for the generation of an action potential.
In an unmyelinated axon, C is very high all along the axon. When an action
potential is produced at the axon hillock, current from that action potential flows to
reduce the membrane potential of the neighbouring segment of the membrane (towards
the terminal). Since C is high, it takes some time to change V to reach threshold for an
action potential in the neighbouring segment.
In a myelinated axon, C is reduced considerably at the internodes. The reduction
in C is proportional to the number of myelin membranes surrounding the axon. With
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