Transcript Neurones & the Action Potential

Neurones & the
Action
Potential
Objective: To understand how neurones conduct
impulses from one part of the body to another.
Write down anything you can remember from
GCSE about the nervous system:
Hints.. What are some of it’s roles? What are the
structures involved in it?
http://www.teachers.tv/video/21091
The Nervous System
STRUCTURE
They have three distinct
parts:
 (1) Cell body,
 (2) Dendrites, and
 (3) the Axon
The particular type of
neuron that stimulates
muscle tissue is called
a motor neuron.
Dendrites receive
impulses and conduct
them toward the cell
body.
Myelinated Axons
The axon is a single long, thin
extension that sends impulses
to another neuron.
They vary in length and are
surrounded by a manylayered lipid and protein
covering called the myelin
sheath, produced by the
schwann cells.
Schwann cell
Types of Neurons
Based on: Mader, S., Inquiry Into Life, McGraw-Hill
•Sensory neurons - carry impulses to the
relay of the brain
• Relay neurons - interpret impulses
•Motor neurons - carry impulses to the muscles
Reflex Arc
http://www.sumanasinc.com/webcontent/animations/content/reflexarcs.html
Resting Potential
In a resting neuron (one that is not conducting an impulse), there
is a difference in electrical charges on the outside and inside of the
plasma membrane. The outside has a positive charge and the
inside has a negative charge.
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html
Contribution of Active Transport
There are different numbers of potassium ions (K+) and
sodium ions (Na+) on either side of the membrane.
Even when a nerve cell is not conducting an impulse,
for each ATP molecule that’s hydrolysed, it is actively
transporting 3 molecules Na+ out of
the cell and 2 molecules
of K+ into the cell, at
the same time by
means of the
sodium-potassium pump.
Contribution of facilitated diffusion
The sodium-potassium
pump creates a
concentration and
electrical gradient for Na+
and K+, which means that
K+ tends to diffuse
(‘leak’) out of the cell and
Na+ tends
to diffuse in. BUT, the membrane is much more
permeable to K+, so K+ diffuses out along its
concentration gradient much more slowly.
RESULTS IN:
a net positive charge
outside & a net negative charge inside.
Such a membrane is POLARISED
Nerve Impulse
•Stimulation of neuron, opens channels so
that sodium (Na+) goes in, depolarizing the
membrane
•The adjacent section of membrane allows
Na+ to go in, depolarizing it
•This wave of depolarization continues at a
rapid rate down the neuron, resulting in a
nerve impulse traveling to the brain in
milliseconds
Nerve Impulse
++++++++
---------Neuron
Polarized membrane
of resting neuron
++++++++
- - - -Neuron
-----0000
Nerve
impulse
+++++
-Neuron
-----
Sodium (Na) moves
inside to depolarize
Nerve impulse is wave
of depolarization
moving down neuron
Action Potential
When the cell membranes
are stimulated, there is
a change in the
permeability of the
membrane to sodium
ions (Na+).
The membrane becomes
more permeable to Na+
and K+, therefore
sodium ions diffuse into the cell down a concentration
gradient. The entry of Na+ disturbs the resting
potential and causes the inside of the cell to become
more positive relative to the outside.
DEPOLARISATION
In order for the neuron to
generate an action
potential the membrane
potential must reach the
threshold of excitation.
As the outside of the cell has
become more positive than
the inside of the cell, the
membrane is now
DEPOLARISED.
When enough sodium ions
enter the cell to depolarise
the membrane to a critical
level (threshold level) an
action potential arises
which generates an
impulse.
Speed of Nerve Impulses
Impulses travel very rapidly along neurones.

The presence of a myelin sheath greatly increases the
velocity at which impulses are conducted along the
axon of a neuron.

Between the sheaths are the Nodes of Ranvier,
where sodium channels are
concentrated. Action potentials
jump from one to the next.
This is called
SALTATORY CONDUCTION
Speed of Nerve Impulses

Action potentials go faster along axons with
bigger diameters, because there’s less electrical
resistance.

They go faster as temperature increases, up to
around 40oC . After that, the proteins begin to
denature.
All-or-None Principle
Throughout depolarisation, the Na+ continues to rush
inside until the action potential reaches its peak and the
sodium gates close.
If the depolarisation is not great enough to reach
threshold, then an action potential and hence an
impulse are not produced.
This is called the All-or-None Principle.
A threshold stimulus must be applied to get an action
potential.
Refractory Period
Straight after an action potential has
been generated, the membrane
enters a refractory period when it
can’t be stimulated, because sodium
channels are recovering and can’t be
opened.