Chapter 12b

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Chapter 12b

• Neurophysiology

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• Processing of sensory information and
  communication
• Messages are conveyed as action potentials
• Communication depends on membrane potentials,
  graded potentials and action potentials

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5 Neural Membrane Processes

• Resting potential
• transmembrane potential (TMP) of resting cell
• Results from uneven distribution of ions across
  membrane
• Usually -70mV for average neuron

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• Graded potential
• temporary, localized change in TMP
• caused by stimulus
• Generated in soma or dendrite
• Action potential
• electrical impulse
• produced by graded potential
• moves along surface of axon to synapse

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• Synaptic activity
• releases neurotransmitters at presynaptic
  membrane
• produces graded potentials in postsynaptic
  membrane
• Information processing
• response (integration of stimuli) of postsynaptic
  cell

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Figure 127 (Navigator)
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How is resting potential created and maintained?
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• Concentration gradient of ions (Na, K)
• ECF has high concentration of Na Cl-
• Cytosol has high concentration of K
• Selectively permeable through channels
• Maintains charge difference across membrane (-70
  mV)

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Product of both passive and active forces
Figure 128 (Navigator)
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Passive Forces Across the Membrane

• Chemical gradients
• concentration gradients of ions (Na, K)
• Electrical gradients
• potential difference across membrane
• Slightly negative on inner surface
• Slightly positive charge on outer surface
• Electrochemical gradient
• Sum of chemical and electrical forces

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Electrical Currents and Resistance

• Electrical current
• movement of charges to eliminate potential
  difference
• Resistance
• the amount of current a membrane resists
• May be altered by opening/closing channels
  creating a current

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Electrochemical Gradients
Figure 129a, b
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Electrochemical Gradients
Figure 129c, d
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Active Forces Across the Membrane

• Sodiumpotassium ATPase (exchange pump)
• are powered by ATP
• carries 3 Na out and 2 K in
• balances passive forces of diffusion
• maintains resting potential (70 mV)

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Changes in Transmembrane Potential

• Transmembrane potential rises or falls
• in response to temporary changes in membrane
  permeability
• resulting from opening or closing specific
  membrane channels

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Sodium and Potassium Channels

• Membrane permeability to Na and K determines
  transmembrane potential
• Sodium and potassium channels are either passive
  or active

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Passive Channels

• Also called leak channels
• Are always open
• Permeability changes with conditions

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Active Channels

• Also called gated channels
• Open and close in response to stimuli
• At resting potential, most gated channels are
  closed

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Gated Channels
Figure 1210
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3 Classes of Gated Channels

• Chemically regulated channels
• open in presence of specific chemicals at a
  binding site
• found on neuron cell body and dendrites

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• Voltage-regulated channels
• respond to changes in transmembrane potential
• characteristic of excitable membrane
• found in neural axons, skeletal muscle
  sarcolemma, cardiac muscle

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• Mechanically regulated channels
• respond to membrane distortion
• found in sensory receptors (touch, pressure,
  vibration)

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Graded Potentials

• Any stimulus that opens a gated channel
• produces a graded potential
• Also called local potentials
• Changes in transmembrane potential
• cant spread far from site of stimulation

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• Opening sodium channel produces graded potential

Figure 1211 (Navigator)
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Graded Potentials Step 1
Figure 1211 (Step 1)
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Graded Potentials Step 2
Figure 1211 (Step 2)
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• Repolarization
• stimulus is removed, transmembrane potential
  returns to normal
• Hyperpolarization
• Increasing the negativity of the resting
  potential
• Result of opening a potassium channel

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Figure 1212
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Effects of Graded Potentials

• At cell dendrites or cell bodies
• trigger specific cell functions
• At motor end plate
• releases ACh into synaptic cleft

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What events are involved in the generation and
propagation of an action potential?
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Action Potentials

• Propagated changes in transmembrane potential
• Affect an entire excitable membrane
• Link graded potentials at cell body with motor
  end plate actions

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Initiating Action Potential

• Initial stimulus
• a graded depolarization to change resting
  potential to threshold level (60 to 55 mV)
• All or none principle
• stimulus exceeds threshold amount and action
  potential is triggered or it wont

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Generating the Action Potential
Figure 1213 (Navigator)
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Steps of A P Generation

• Depolarization to threshold
• Activation of Na channels and rapid
  depolarization
• Inactivation of Na channels, activation of K
  channels
• Return to normal permeability

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The Refractory Period

• time period
• from beginning of action potential
• to return to resting state
• during which membrane will not respond normally
  to additional stimuli
• Absolute vs. Relative

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Propagation of Action Potentials

• moves along entire length of axon
• series of repeated actions, not passive flow
• Continuous propagation
• unmyelinated axons
• Saltatory propagation
• myelinated axons

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Saltatory Propagation

• Faster and uses less energy than continuous
  propagation
• Myelin insulates axon, prevents continuous
  propagation
• Local current jumps from node to node
• Depolarization occurs only at nodes

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Comparison of graded and action potentials
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Graded Potential

• Depolarizes or hyperpolarizes
• No threshold value
• Dependent of intensity of stimuli
• Effect decreases with distance
• No refractory period
• Occurs in most cell types

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Action Potential

• Depolarizes only
• Distinct threshold value
• All or none phenomenon
• No decrease in strength along axon
• Refractory period occurs
• Occurs only in excitable cells

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What factors affect the propagation speed of
action potentials?
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Axon Diameter and Propagation Speed

• Ion movement is related to cytoplasm
  concentration
• Axon diameter affects action potential speed
• The larger diameter, the lower the resistance

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3 Groups of Axons

• Classified by
• diameter
• myelination
• speed of action potentials
• Type A, Type B, and Type C fibers

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• Information travels within the nervous system
  as propagated electrical signals (action
  potentials)
• The most important information (vision, balance,
  motor commands) is carried by large-diameter
  myelinated axons