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Biology 3201
Unit 1 – Maintaining Dynamic
Equilibrium II
Section 1 – Nervous System
“The Neuron”
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The Neuron


The basic functional
unit of the nervous
system.
Function: Send
impulses to and from
the CNS and PNS and
the effectors
(muscles/glands)
Neuron Structure






Dendrite  Fine hair-like extensions on the end of a
neuron.
 Function: receive incoming stimuli.
Cell Body or Soma  The control center of the neuron.
 Function: Directs impulses from the dendrites to the
axon.
Nucleus  Control center of the Soma.
 Function: Tells the soma what to do.
Axon Pathway for the nerve impulse (electrical message)
from the soma to the opposite end of the neuron.
Myelin Sheath  An insulating layer around an axon. Made
up of Schwann cells.
Nodes of Ranvier  Gaps between schwann cells.
 Function: Saltatory Conduction (Situation where speed
of an impulse is greatly increased by the message
‘jumping’ the gaps in an axon).
Types of Neurons

There are 3 types of neurons.
1.
Sensory Neurons 
2.
3.
Neurons located near receptor
organs (skin, eyes, ears).

Function:
receive incoming stimuli from the
environment.
Motor Neurons  Neurons located near effectors
(muscles
and glands)

Function:
Carry impules to effectors to initiate a
response.
Interneurons  Neurons that relay messages between
other neurons such as sensory and motor neurons. (found
most often in Brain and Spinal chord).
Types of Neurons
Nerves

Nerves  Collections of neurons that are
joined together by connective tissue.

Responsible for transferring impulses from
receptors to CNS and back to effectors.
How Do Neurons Operate?

Neuron at Rest  Resting Potential
 Occurs when the neuron is at rest.
 A condition where the outside of the membrane is positively(+)
charged compared to the inside which is negatively(-) charged.
 Neuron is said to be polarized.
 Neuron has a voltage difference of -70 mV
How is resting potential maintained?
Ion Distribution
How is resting potential maintained?




At rest, the sodium gates are closed.
Membrane is 50 times more permeable to K+ ions
causing them to “leak” out.
This causes outside of membrane to have an
abundance of + charges compared to inside. The
inside of the membrane is negative compared to
the outside. This is helped by the (-) proteins
etc.
The “sodium-potassium” pump pulls 2 K+ ions in
for 3 Na+ ions sent out. This further creates a
charge difference!!
Action Potential
The mechanism by which neurons send
impulses. They are comprised of
electrical signals generated at the soma
and moving along the axon toward the
end opposite the soma (motor neurons)
Action potentials occur in two stages:
 Depolarization
 Repolarization
Depolarization in an action potential

When the neuron is excited past its “Threshold” the
following events occur:
 Sodium ions (Na+) rush into the axon.
 This neutralizes the negative ions inside.
 The inside of the axon becomes temporarily (+) while
the outside becomes temporarily (-). The reversal of
charge is known as “depolarization”
 Nearby Sodium (Na+) channels open to continue the
depolarization.
Let’s Have a look.

http://www.mcgrawhill.ca/school/booksites/biology/student+reso
urces/toc/unit+4+homeostasis+maintaining+dynamic+equilibriu
m/chapter+12+the+nervous+system/cool+stuff+to+see+and+do
/movie+channel+behaviour.php
Repolarization

This is the restoring of the (+) charge on the outside
of the axon and (-) on the inside.

Potassium gates open and potassium floods out.



This generates positive charge on the outside of membrane.
Sodium Channels Close (no + charges can get inside)
The Sodium/Potassium pump rapidly moves Sodium out of
the cell.

Further creates the (+) charge outside with a (-) charge inside.
Refractory Period

Brief period of time between the triggering
of an impulse and when it is available for
another.

NO NEW action potentials can be created
during this time.
Saltatory Conduction


The “jumping” of an impulse between the “Nodes of
Ranvier” thus dramatically increasing it’s speed.
Only occurs in axons having Myelin.
 2m/s  120 m/s
All or None Response
If an axon is stimulated above its
threshold it will trigger an impulse down
its length.
 The strength of the response is not
dependent upon the stimulus.
 An axon cannot send a mild or strong
response. It either responds or does not!!!

Homework

View ILO 06 – Structure of a Neuron


View ILO 07 – Nerve Tansmission


Complete Activity 1 and 2 (Go to Work)
Answer Q. 2, 4, 5, 7
Explain the critical role Myelin plays to an
axon. Include at least two roles it plays.