Nerve physiology

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Nerve physiology
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Physiology of Nerves

• There are two major regulatory systems in the
  body, the nervous system and the endocrine
  system.
• The endocrine system regulates relatively slow,
  long-lived responses
• The nervous system regulates fast, short-term
  responses

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Neuron structure

• Neurons all have same basic structure, a cell
  body with a number of dendrites and one long axon.

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Types of neurons
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Divisions of the nervous system
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Non-excitable cells of the nervous system
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Structure of gray matter
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Signal transmission in neurons
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Resting potential
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Ionic basis of Em

• NaK-ATPase pumps 3Na out for 2 K pumped in.
• Some of the K leaks back out, making the
  interior of the cell negative

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Electrochemical Gradients
Figure 12.12
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Ion channels

• Remember Ohms Law IE/R
• When a channel opens, it has a fixed resistance.
• Thus, each channel has a fixed current.
• Using the patch-clamp technique, we can measure
  the current through individual channels

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Gated channels ligand-gated
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Gated channels voltage-gated
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Gated channels mechanically-gated
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Graded potential

• A change in potential that decreases with
  distance
• Localized depolarization or hyperpolarization

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Graded Potentials
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Graded Potentials
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Action Potential

• Appears when region of excitable membrane
  depolarizes to threshold
• Steps involved
• Membrane depolarization and sodium channel
  activation
• Sodium channel inactivation
• Potassium channel activation
• Return to normal permeability

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The Generation of an Action Potential
Figure 2.16.1
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Graded potentials vs Action Potential
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Characteristics of action potentials

• Generation of action potential follows
  all-or-none principle
• Refractory period lasts from time action
  potential begins until normal resting potential
  returns
• Continuous propagation
• spread of action potential across entire membrane
  in series of small steps
• salutatory propagation
• action potential spreads from node to node,
  skipping internodal membrane

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The Generation of an Action Potential
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Induction of an action potential I
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Induction of an action potential II
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Voltage-gated Na channels

• These channels have two voltage sensitive gates.
• At resting Em, one gate is closed and the other
  is open.
• When the membrane becomes depolarized enough, the
  second gate will open.
• After a short time, the second gate will then
  shut.

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Voltage-gated K channels

• Voltage-gated K channels have only one gate.
• This gate is also activated by depolarization.
• However, this gate is much slower to respond to
  the depolarization.

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Cycling of V-G channels
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Action potential propagation

• When the V-G Na channels open, they cause a
  depolarization of the neighboring membrane.
• This causes the Na and K channels in that piece
  of membrane to be activated

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AP propagation cont.

• The V_G chanels in the neighboring membrane then
  open, causing that membrane to depolarize.
• That depolarizes the next piece of membrane, etc.
• It takes a while for the Na channels to return
  to their voltage-sensitive state. Until then,
  they wont respond to a second depolarization.

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Propagation of an Action Potential along an
Unmyelinated Axon
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Saltatory Propagation along a Myelinated Axon
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Saltatory Propagation along a Myelinated Axon
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Schwann cells cont.

• In unmyelinated nerves, each Schwann cell can
  associate with several axons.
• These axons become embedded in the Schwann cell,
  which provides structural support and nutrients.

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Synaptic transmission
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g Aminobutyric Acid

• Also know as GABA
• Two know receptors for GABA
• Both initiate hyperpolarization in the
  post-synaptic membrane
• GABAA receptor allows an influx of Cl- ions
• GABAB receptors allow an efflux of K ions

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Transmitter effects on Em

• Most chemical stimuli result in an influx of
  cations
• This causes a depolarization of the membrane
  potential
• At least one transmitter opens an anion influx
• This results in a hyperpolarization.

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EPSPs and IPSPs

• If the transmitter opens a cation influx, the
  resulting depolarization is called an Excitatory
  Post Synaptic Potential (EPSP).
• These individual potentials are sub-threshold.
• If the transmitter opens an anion influx, the
  resulting hyperpolarization is called an
  Inhibitory Post Synaptic Potential (IPSP
• All these potentials are additive.

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Post-synaptic integration
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Signal integration
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Signal integration cont.
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Presynaptic inhibition
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Presynaptic facillitation
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Neural circuits I
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Neural circuits II
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Myelination I

• In the central nervous system, myelin is formed
  by the oligodendrocytes.
• One oligodendrocyte can contribute to the myelin
  sheath of several axons.

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Myelination II

• In the peripheral nervous system, myelin is
  formed by Schwann cells.
• Each Schwann cell associates with only one axon,
  when forming a myelinated internode.

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White and gray matter in the nervous system
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Structure of the spinal cord I

• The CNS is made up not only of the brain, but
  also the spinal cord.
• The spinal cord is a thick, hollow tube of nerves
  that runs down the back, through the spine.

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Structure of the spinal cord II
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Structure of the spinal cord III
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Structure of the spinal cord IV