Neuron Structure and Function

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Neuron Structure and Function

• Chapter 4

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General Nervous System Functions

• Control of the internal environment
• Nervous system works with endocrine system
• Difference?
• Voluntary control of movement
• Spinal cord reflexes
• Assimilation of experiences necessary for memory
  and learning

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Components of the nervous system
CNS
Motor neurons
Sensory neurons
Action
Integration
Perception
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Organization of the Nervous System

• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• Neurons outside the CNS
• Sensory division
• Afferent fibers transmit impulses from receptors
  to CNS
• Motor division
• Efferent fibers transmit impulses from CNS to
  effector organs
• Somatic system to muscle
• Autonomic system

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Neurons

• Vary in structure and properties
• Use same basic mechanisms to send signals

Figure 4.1
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Structure of a Neuron Fig 4.2

• Cell body
• Dendrites
• Conduct impulses toward cell body
• Axon
• Carries electrical impulse away from cell body
• May be covered by Schwann cells
• Forms discontinuous myelin sheath along length of
  axon
• Synapse
• Contact points between axon of one neuron and
  dendrite of another neuron

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Electrical Activity in Neurons

• Resting membrane potential
• At rest, neurons are negatively charged
• Action potential
• Potential across membrane changes

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Squid Giant axon
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Membrane permeability

• Small molecules and those less strongly
  associated with water will pass across membrane
• Ions will not cross membrane

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Sodium potassium dependant ATPase

• Ouabain

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Resting Potential

• Resting potential
• Donnan Equilibrium
• Approx -70mV

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Resting Potential
Negative charge
Positive charge
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Nearnst Equation

• EPotential across membrane
• RUniversal gas constant
• T Absolute temperature oK
• zCharge
• FFaradays constant
• At 18oC, RT/ZF 0.058
• During Resting potential
• KI 400mM, Ko 20mM
• Calculate Resting Potential?

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Nearnst Equation

• EPotential across membrane
• RUniversal gas constant
• T Absolute temperature oK
• zCharge
• FFaradays constant
• At 18oC, RT/ZF 0.058
• During Resting potential
• KI 400mM, Ko 20mM
• Calculate Resting Potential?

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log
0.058
E
400
E
E -0.0754V
E -75.4mV
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How do we manipulate potential experimentally?
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Goldman Equation I

• EPotential across membrane
• RUniversal gas constant
• T Absolute temperature oK
• zCharge
• FFaradays constant
• At 18oC, RT/ZF 0.058
• During Resting potential
• PK1 (K i400mM K o20)
• PNa0.0013 (Na i50 Na o440)
• PCl 0.0017 (Cl- i90 Cl- o560)
• So what is the Resting potential?

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Electrical Activity in Neurons

• Resting membrane potential
• At rest, the neurons are negatively charged
• Determined by concentrations of ions (Na, K,
  Cl-) across membrane
• Action potential
• Depolarization
• Permeability of the membrane changes-leads to
  sodium influx
• Repolarization
• Change in membrane permeability, restores resting
  potential

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Goldman Equation II

• During Action Potential
• PK1 (K i400mM K o20)
• PNa100 (Na i50 Na o440)
• PCl 0.0017 (Cl- i90 Cl- o560)
• What is the potential across the membrane?

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So what happens during an action potential?
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Sodium is pumped out during resting potential
Dinitrophenol
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Sodium enters nerve during AP
200,000 impulses!
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Ion Channels
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Several types of ion channel
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Potassium channel
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Potassium channel
Bacterial Potassium channel
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Lysine residues on S4
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Sodium channel
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Structure of a bacterial K channel
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Specificity of the selectivity filter in a
bacterial K channel
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Model for the gating of a bacterial K channel
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Sodium Channel Blockers
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Potassium Channel Blockers
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Hodgkin Cycle
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Action Potential
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Patch clamping
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• Sodium channels are either open or closed
• Sensitivity varies

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Matters of Threshold!

• Threshold is voltage at which sodium channel
  opening is subject to positive feedback
• Variable depending on nerve and portion of nerve.

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Effect of ion channel density on membrane
threshold

• To generate an AP, a certain number of sodium
  ions must cross membrane
• Determined by number of open sodium ion channels
• Ion channels have varying voltage sensitivities
  (normal distribution)
• Assume we need to open 100 channels to generate
  AP

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Effect of ion channel density on membrane
threshold, part deux

• Cell One (Blue)
• 1,000 channels per mm2
• Must generate enough potential to open 1/10th
• Cell Two (Red)
• 10,000 channels per mm2
• Must generate enough potential to open 1/100th

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Refractory period

• Ensures discrete impulses
• Absolute
• Relative
• 2 factors involved
• State of gate
• Duration of opening

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Nature of refractoriness

• Absolute
• Sodium channels in closed/inactive state
• Therefore no sodium can enter to depolarize the
  cell
• Relative
• Potassium channels
• Different Na channels stay open for different
  times

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Accommodation

• Gradually changing sub-threshold stimulus
  strength
• Time-dependent response to depolarization

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• Strong accommodation
• Weak accommodation
• Increasing interspike interval
• Adaptation

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Summary
Sodium and potassium flux during AP