Propagation of Nerve Impulses
Introduction
1. Why must action potentials be rapidly conducted over long distances?
In order for the nervous system to communicate with other cells.
Conduction of Action Potentials (AP)
2. A positive feedback cycle begins when a graded potential reaches
threshold. Describe what happens to each of the following.
a. Gated Sodium Channels –
When these open, sodium ions flood into the cell, this segment of the membrane depolarizes until its
voltage is reversed.
b. Adjacent membrane section when polarity is reversed At the peak of the action potential the influx of sodium causes sodium gates in the adjacent segment of
membrane to open.
c. Repolarization of first membrane section: Gated Sodium Channels and Potassium Channels –
At the same time gated potassium channels are opening and sodium channels are closing,
repolarizing the original segment of the membrane and placing it in a refractory period.
d. What is the refractory period?
It is a period of time where a second action potential cannot be generated in this portion of
the membrane.
e. Describe how the action potential “moves” down the axon.
The action potential generated in one segment of the membrane can
depolarize the adjacent membrane segment to threshold which would
generate an action potential in that segment. in the segment in front,
thus generating another action potential.
Conduction Rates
3. Investigate the effects of myelination.
a. Describe continuous conduction along an unmyelinated
sheath.
The step- by- step depolarization of an axon and generation
of action potentials sequentially along the entire length of a
nerve fiber is called continuous conduction.
b. What is the function of Schwann cells?
The membrane of the Schwann cell spirals around the axon forming
the sheath and electrically insulating that portion of the axon. The
insulation prevents voltage loss due to ionic current across the membrane.
c. What are the Nodes of Ranvier?
These are gaps in the myelin that represent exposed patches of membrane. This causes the
action potential to appear to leap from node to node in salutatory conduction.
d. Describe saltatory conduction. In myelinated neurons the depolarization and the propagation of
action potentials occur only at the Nodes of Ranvier
In myelinated neurons the only uninsulate, exposed portion of the neuron
is the
Nodes of Ranvier. These are the only sites where the membrane can
depolarize to threshold and generate action potentials. Thus, conduction
along myelinated axons appear to jump from to jump
from node to node, a process called saltatory conduction.
e. Compare the speeds of saltatory and continuous conduction.
Saltatory conduction is more efficient and action potentials only need to be generated from one node to
the next, resulting in a much more rapid conduction when compared to continuous conduction.
4. Investigate the effects of axon diameter.
a. Explain why larger axons conduct action potentials faster than smaller axons.
Axons with larger diameters conduct action potentials more rapidly
than those with smaller diameters because larger axons have less
resistance to current flow.
5.Investigate the effects of temperature.
a. How does temperature affect conduction speed?
In comparing cooled axons vs. warmer axons the cooled axons will
conduct potentials less rapidly than warm axons.
Stimulus Intensity & AP Frequency
6. Mechanical or chemical stimuli will generate graded potentials.
Explain what determines if a graded potential is adequate to generate
an action potential.
Stimuli that do not produce a graded potential that reaches threshold will not result
in the generation of an action potential. When a stimulus is large enough to
depolarize the membrane to threshold, an action potential is generated.