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One really, really long nerve!
• Dissection on a giraffe: investigating the
recurrent laryngeal nerve
Electrochemical Impulses
Electrochemical Impulses
• Nerve impulses are electrochemical messages
created by the movement of ions through the
neuron cell membrane
• Negative and positive ions are both found in
large numbers inside & outside neurons.
• Negative ions are large & tend to stay inside
the cell.
• Movement of positive ions involves diffusion,
active transport and a Na+/K+ pump.
• K+ found inside neurons tends to diffuse out of
the cell.
• Na+ found outside the cell tends to diffuse in.
Electrochemical Gradient
• Since the cell membrane is more permeable
to K+ than Na+ , K+ moves out of the cell faster
than Na+ moves in.
• This results in an electrochemical gradient
across the cell membrane, and an overall
external positive charge which is referred to as
resting potential.
Gradient that is
created is
referred to as
the neuron’s
resting potential
Action Potential
• When a message is received by the dendrites
of a neuron, the following changes, referred to
as an action potential occur:
1. The neuron cell membrane becomes more
permeable to Na+ than to K+ (Na+ gates open)
2. Na+ rushes into the neuron by diffusion.
Action Potential
3. A reversal of charge occurs, called
depolarization (this is the ‘firing’ of the neuron)
4. Once the inside of the neuron becomes positive,
the Na+ gates close.
5. A Na+ - K+ pump in the cell membrane moves
sodium out and potassium in, restoring the
resting potential (called re-polarization)
Movement of the Action Potential
• Depolarization occurs as a wave along the
membrane of the axon
• The wave of depolarization is followed by a
wave of re-polarization
• The action potential moves along the axon by
jumping from one node of Ranvier to another
Some Important Terminology
Refractory period: time it takes for the nerve cell
membrane to re-polarize and be able to
conduct another action potential
Threshold level: minimum level of stimulus
required for a neuron to respond (varies
between neurons)
Some Important Terminology
All-or-none response: neurons fire maximally or
not at all (means that increasing the stimulus
above threshold level does not increase the
response; see p. 422)
Speed of transmission: determined by intensity
of the stimulus, amount of myelin and
diameter of the axon