Transcript Membrane potentials

Neurobiology, Part 1
Overview of the nervous system
Structure of neurons and
associated cells
Examples of neurons
Glial cells
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10:1  50:1 glial cell:neuron ratio
Known functions
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Creates myelin sheath (in the vertebrates only)
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Why important?
Creates a matrix that connects neurons
Helps guide development of neural pathways
Blood-brain-barrier (tight junctions)
Provides metabolic support for neurons
NEW: appear to communicate chemically with
other glial cells and neurons
Membrane potentials:
the key to electrical signals
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What is meant by the membrane potential?
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Charge difference between the inside and ouside of
the membrane
An “electrical signal” of the nervous system is a
change in the memrane potential.
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Example: Action potential…
What determines the value of the
membrane potential?
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Chemical gradients
Electrical gradients
Selective permeability of the membrane
Chemical and electrical gradients
 Chemical gradients (“chalk talk”)
Na+-K+-pump:
maintains the gradients
 Think of it as operating in the background. It is no
way involved in individual action potentials!
Chemical and electrical gradients
 Electrical gradients (“chalk talk”)
Selective permeability
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The membrane potential at any time
is based on the permeability of the
membrane to particular ions.
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Impermeable ions cannot move and thus
cannot influence the membrane potential.
Neuron at rest: ~ -70 mV
 Why is the resting membrane potential
negative?
Resting potential
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At rest, the membrane is ~25 times
more permeable to K+ than Na+, thus
K+ is nearly solely responsible for the
RP of neurons.
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Relatively large number of K+ channels open
at rest
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These are “resting” or “passive” channels (always
open)
Very few Na+ channels open at rest.
Action potentials
 A rapid, transient change in the
membrane potential from negative to
positive and back again!
 This is the nerve impulse!
 What allows the changes in membrane
potential to occur during an AP?
 Changes in membrane permeability due to
opening and closing of voltage-gated
channels
 Resultant movement of ions.
Action potentials
 Follow the bouncing professor and
make your own custom drawing!
Action potentials (cont.)
 Know what is meant by the threshold
 For each stage of the AP: Ask yourself:
 What change in permeability occurred?
 What type of channel opened or closed and why?
 Which ion moved, and in which direction (in or
out of cell?)
 Understand why you see the direction you do.
 What change in membrane potential occurred
as a result of the ion movement?
 What stopped the movement of the ion?
Propagation of
the action
potential
 Passive spread
of positive
charge 
 Depolarization
of next
segment of the
axon
 Threshold
reached
 AP in next
section
Myelin sheath
 Cell type: Schwann cell
Saltatory conduction
 Ions only able to move at the Nodes of
Ranvier
 AP “jumps” from node to node
The synapse:
Write out steps in your own words!
 NOTE: See diagram in text: Newer diagram clearly
shows the voltage-gated calcium channels!
How is neurotransmitter activity
stopped?
 Three different ways… Know them!
Neural integration
Neural integration
 Summation: graded potentials (EPSPs and
IPSPs) are summed to either depolarize or
hyperpolarize a post-synaptic neuron