Transcript Differential Permeability of the Membrane

Explaining behavior at the
level of the neuron
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Your brain and the rest of your nervous
system is made up of neurons.
Neurons are brain cells
All neurons are separated from one
another, but communicate
electrochemically.
The neuron
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The neuron consists of three parts
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The cell body - contains the nucleus and
much of the machinery that keeps a
neuron alive and working.
The dendrites - widely branching structures
that receive transmissions from other
neurons.
The axon - a single, long, thin, straight
fiber with branches near its tip
Myelin sheath
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The axon is coated in an insulating substance known
as Myelin.
Myelin allows for faster transmission of impulses
along an axon.
Myelin has breaks in it known as the Nodes of
Ranvier
Once an impulse reaches the end of an axon (the
terminal buttons), molecules are released that can
either excite or inhibit the receiving cell.
Resting Potential
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Normally there is an electrical polarization
across the membrane of an axon.
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This means that there is a negative charge on the
inside of the cell and a positive charge on the
outside.
At resting potential the inside of the neuron is
at -70 millivolts.
Four Factors Determine the Ionic
Distribution That Underlies the Resting
Potential
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Differential Permeability of the Membrane
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The Sodium/Potassium Pump
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Diffusion
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Electrostatic Pressure
Differential Permeability
of the Membrane
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Ions pass through membrane at special pores
called ion channels
When neurons are at rest, the membrane is:
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extremely resistant to the passage of Sodium
(Na+) ions
only slightly resistant to the passage of Potassium
(K+) ions and Chloride (Cl-) ions
The sodium potassium pump
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There are little pumps that pump Sodium (NA+) out
of the cell, and potassium (K+) in.
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The pumps move 3 Sodium molecules out for every 2
potassium molecules they move in
Sodium and potassium both have a +1 charge
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more Sodium is being moved out than potassium is being
moved in
the build up of Sodium on the outside of the membrane
makes it positive and the inside negative.
Diffusion also known as Random Motion
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Ions in solution are in random motion
Thus, any time that there is an accumulation
of a particular class of ions in one area,
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the probability is increased that random motion
will move ions out of this area (because there are
more ions available to leave)
the probability is decreased that random motion
will move more ions into the area (because there
are fewer ions available to come in)
Electrostatic Pressure
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Like charges repel and opposite
charges attract
Therefore electrostatic pressure
disperses any accumulation of positive
or negative charges in an area
Generation of Action
Potentials
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action potentials (APs; neuron firing)
are triggered at the axon hillock when
a neuron is depolarized to the point that
the membrane potential at the axon
hillock reaches about -65 mV
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this is the threshold of excitation for
many neurons
they are all-or-none (they occur full
blown or not at all)
How does an impulse travel
down an axon?
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The action potential is an excitation that
travels along an axon at a constant
strength, no matter how far it must
travel.
It is slower then a straight electrical
impulse, but has the advantage of
maintaining it’s strength no matter how
far it must travel.
Travel of the action potential
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When a neuron fires, certain gates open up
that allow Sodium to flow in.
When sodium flows in the electrical charge is
neutralized across the membrane.
Then the sodium channels close, and
potassium channels open, allowing potassium
to leave the cell.
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This returns the cell to it’s resting potential (-70
mv).
Travel of the action potential.
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The axon only has these Sodium gates at
breaks in the myelin sheath called the Nodes
of Ranvier.
The sodium gates are voltage dependent that is they open up when the voltage across
the membrane drops
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Thus, the action potential moves like a wave
jumping from one Node of Ranvier to the next
down to the end of the axon.
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Saltatory Conduction
What happens when an action potential
reaches the end of an axon?
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The end of an axon has several
branching areas called the terminal
buttons
Each edge of the terminal button is
called the presynaptic membrane.
The presynaptic membrane is separated
from the other neuron by what is called
the synaptic cleft.
The Synapse
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The axon that has fired releases a chemical
into the synaptic cleft.
This chemical crosses the gap and binds to
what is called the postsynaptic membrane.
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The chemicals are called neurotransmitters.
They bind at the postsynaptic membrane at
what are called receptors.
Neurotransmitters
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There are many types of
neurotransmitters
Acetylcholine
Serotonin
Dopamine
Dopamine
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Dopamine is one neurotransmitter that
has been associated with many
neurological disorders
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Parkinson’s disease.
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Muhammed Ali
Schizophrenia
Sometimes these diseases can be treated
by increasing dopamine levels in the brain
Behavior and the Nervous
System
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Psychologists distinguish between the
central nervous system and the
peripheral nervous system.
Central nervous system consists of the
brain and the spinal cord
Peripheral nervous system is composed
of bundles of axons between the spinal
cord and the rest of the body.
Peripheral nervous system
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The peripheral system can be further
divided
Somatic nervous system = nerves that
communicate with the skin and
muscles.
Autonomic nervous system = nerves
that communicate with the heart,
stomach, and other organs.
Autonomic nervous system
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The autonomic nervous system is a
system that we do not have as much
control over.
It largely controls things we wouldn’t
want to have to think about
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breathing
heart rate
Divisions of the autonomic
nervous system
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The sympathetic system - controls fight
or flight - increases heart and breathing
rate.
The parasympathetic system decreases heart rate, controls digestion,
basically runs the body during normal
functioning.
Organization and functioning
of the brain
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The cerebral cortex - the outer surface
of the brain. The wrinkled area.
Right and left hemisphere
crosses over - communicates via the
corpus callosum
The four lobes of the cerebral
cortex
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Frontal Lobe - thought to be involved in
planning and working memory
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Parietal Lobe - body sensations
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primary motor cortex
primary somatosensory cortex
Occipital lobe - vision
Temporal lobe - hearing - advanced
visual processing - emotion
What if we cut the corpus
callosum?
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Travel from the eye to the brain is
divided.
Information from the left side of each
eye travels to the left hemisphere of the
brain
Information from the right side of each
eye travels to the right hemisphere of
the brain.
Hemisphere division continued
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The left hemisphere of the brain
controls speech for most people
Most people are only able to describe
information that reaches the left
hemisphere.
However information that reaches the
right hemisphere quickly crosses the
corpus callosum to the left hemisphere,
so that it can be described verbally.
What if the corpus callosum is
cut
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In one experiment researchers showed
a woman with a severed corpus
callosum a picture of a naked woman to
the left field of vision = to the right of
her face
When asked what she had seen, she
laughed and said a nude picture.
Continued.
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When the nude picture was shown only in her
right field of vision (left of her face), she
laughed and turned a little red in the face.
When asked why she was laughing, she said I
don’t know, oh that silly machine.
The right side of the brain knew what it saw
and caused her to laugh, the left side of the
brain heard the laughter, and tried to
interpret why it occurred.
continued
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If she had been allowed to point at
several alternatives with her left hand
(controlled by the right hemisphere),
she would have been able to correctly
point at the picture she had seen, even
though she would say that she didn’t
know what she had seen.
Unified consciousness?
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We all experience a unified consciousness.
That is, we experience a single self.
The split brain experiments show that that
unified consciousness depends on the two
hemispheres being able to communicate.
If the corpus callosum is severed then each
hemisphere begins to act and experience
things independently of the other.