Transcript St. Francis Xavier University Action Potential

Biology 201
Dr. Edwin DeMont
Communication I: Nervous and
Sensory Systems
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Communication systems
Two forms of communication integrate body function
that helps to maintain homeostasis:
(1) Neurons which transmit electrical signals to
initiate a rapid response in a tissue.
(2) Hormones which are a slower chemical
signals that initiate a widespread and often
prolonged response and often in a variety of
tissues.
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Nervous system
All animals depend on information on their
surroundings, whether for finding food and mates or
escaping from predators. They must be able to rapidly
assess important quantities of the environment.
Most information is obtained through
specialized sense organs, i.e. for vision, hearing,
taste, smell and touch.
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Nervous system
The variety of stimuli and the receptors (sense organs)
is large, but the general sequence of events initiated by
the external stimulus is similar.
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Nervous system
The nerve impulses carried in the different sensory
nerves are all of the same fundamental nature.
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Neurons
Filmed live
The function unit of the
nervous system is the
neuron.
Neurons have two
important properties:
(1) excitability, the
ability to respond to
stimuli
(2) conductivity, the
ability to conduct a signal
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Ion Channels
Plasma membrane of
neuron contains channels
in which certain ions can
pass through. Some of
the channels are gated
and only open under
certain circumstances.
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Resting Membrane
Potential
The resting neuron (non-conducting) has a resting membrane
potential of approximately -70 mV. There is a higher
concentration of sodium ions (+) outside the plasma membrane
and potassium (+) and various proteins (-) and ions (chlorine
ions) are found inside the membrane.
Why?
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Resting Membrane
Potential
Plasma membrane contains open channels only for
potassium. This ion moves out of the cell down its
concentration gradient.
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Resting Membrane
Potential
As potassium moves out the inside of the cell becomes
more negatively charged and this tends to attract the
positively charged potassium back into the cell.
Diffusional force
Electrical force
Animation
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Action potential
Changes in the resting electrical potential across the plasma
membrane is the most important factor in the creation and
subsequent conduction of a nerve impulse.
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Action potential
When a stimulus is applied to a region on the resting plasma
membrane the permeability of sodium (Na+) increases at that
point (some sodium channels are opened).
Membrane potential becomes more positive.
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Action Potential
At a certain level of membrane potential special voltage
sensitive sodium channels open and the membrane potential
rapidly rises to around 30 mV.
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Action Potential
Shortly after the voltage sensitive Na+ channels open special
voltage sensitive potassium channels open and potassium
pours out and the voltage drops (Na+ channels close.)
Animation
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Action Potential
The sodium inside the cell can be removed by the
sodium-potassium pump.
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Action Potential
This series of changes in membrane potential trigger a similar
cycle in adjacent regions of the membrane and the wave of
depolarization moves down the axon.
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Action Potential
The action potential is caused by the concentration of
ions on the two sides of the membrane. Therefore the
stimulus causes a full strength action potential or
none at all. This is called an all-or-none law.
So if all the action potentials are the same, how
is the strength of the stimuli conveyed?
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Between Neurons
Information that travels in axons is transmitted
to other neurons at the synapse. The
transmission at the synapse is of two distinct
kinds: electrical or chemical.
Electrical transmission is not as widespread as
chemical transmission, but it has
characteristics that in certain situations confer
considerable biological advantage.
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Chemical
Chemical transmission across a synapse.
The neurotransmitter
(eg. acetylcholine)
binds to the receptor
site and causes a
depolarization in the
postsynaptic
membrane.
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Chemical
Post-synapse action
potentials can be generated
many ways:
(1) Input from many
different axons.
(2) Input from many
action potentials on
the same axon.
(3) Any combination
Thus information is
‘integrated’ at the
synapse.
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Sensory Reception
The human ear
Inner ear includes the
semicircular canal,
which is involved in
equilibrium and the
cochlea, which is
involved in hearing.
Biology 403 – Physiology of Sensations (Dr. Beye)
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Sensory Reception
The sense of equilibrium
Cilia of receptor hairs bend
and initiates a generator
potential
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Sensory Reception
1. Sound waves enter the mouth
The fish inner ear
2. Transmitted to gas in the
swim bladder
3. Vibrates at corresponding
amplitudes and frequencies
as the incoming sound
4. Weberian ossicles vibrate
5. Vibrations move forward to
inner ear
6. Response initiated –
example fast start
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Sensory Reception
My computer
apps on ‘caesar’ (S:)
Biomechanics
Programs
Demonstration of a
fast start in a fish
FastStart
Select Image Series
Accept Settings
Step >
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Neural Network
Image source
Fast start in fish: Mauthner Cells
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