Unit 7 Part A: Nerve Physiology

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Unit 7 - Part A Nerve Physiology
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Illustrate And/Or Label The Following Parts Of A
Neuron And Identify Or Indicate Their
FunctionsCell Body, Dendrite, Axon, Nucleus,
Axon Hillock, Axon Terminals, Nodes Of Ranvier,
Schwann Cell, Myelin Sheath, Neurilemma, Receptor
Unit 7a - Objective 1
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Examine The Nerve Cell Diagram And Locate The
Following StructuresCell Body, Dendrite, Axon,
Nucleus, Axon Hillock, Axon Terminals, Nodes Of
Ranvier, Schwann Cell, Myelin Sheath, Neurilemma
and Receptor.
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Functions Of Nerve Cell Parts
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The Nerve Cell Body (Soma)

• Enlarged part of the nerve cell - contains
  cytoplasm and cell organelles.
• Receives information from dendrites, sends
  messages out through the axon.
• - Primary site for maintaining the life of the
  nerve cell supports the dendrites and axon.

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The Dendrite

• An incoming nerve cell process can act as a
  receptor or connect to separate specialized
  receptors.
• Conducts stimulus information to the nerve cell
  body.
• Produces voltage changes in response to various
  stimuli and assists in nerve impulse formation.

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The Axon Hillock

• Junction site between the nerve cell body and
  the axon.
• Processes voltage changes, or generator
  potentials (GPs), from cell body and dendrites
  assists formation of a transmittable nerve
  impulse.

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The Axon

• Conducts nerve impulses away from the nerve cell
  to the axon terminals.
• Very small in diameter, but can be very long
  (e.g. the length of a leg).
• Each nerve cell has only one axon.
• If cut, distal part degenerates due a disruption
  of the cytoplasm extending from the cell body.

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Axon Terminals

• Bulbous distal endings of the many branches that
  extend from the end of an axon. Also be called
  synaptic knobs, boutons or even end feet.
• Serves as a secretory component that releases
  neurotransmitters in response to nerve impulses.

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Nodes of Ranvier

• Space or gap on a nerve cell process (axon or
  dendrite) between the myelin sheaths formed by
  Schwann Cells.
• The exposed cell membrane in the node facilitates
  the formation and transmission of nerve impulses.

Nodes of Ranvier
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The Schwann Cell

• Specialized cell that supports and maintains the
  fibers (axons and dendrites) of nerve cells in
  the peripheral nervous system (PNS). Contains
  myelin material.
• Assists in repair and regeneration of fibers.
• Wraps around a section of a nerve fiber and
  creates a protective myelin sheath.

Neuron with and without Schwann cells
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The Myelin Sheath

• The Schwann Cell wraps around a section of nerve
  cell fiber in jellyrollfashion resulting in a
  tight coil of concentric membranes called the
  Myelin Sheath.
• The whitish, fatty myelin material
• insulates and protects the nerve
• cell fiber.

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The Neurilemma

• The most external portion of the plasma or cell
  membrane of the Schwann Cell.
• This specialized membrane surrounds the myelin
  sheath.
• The neurilemma is sometimes called the sheath of
  the Schwann Cell or a neuron husk.

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Receptor

• A specialized part of a nerve cell or the nervous
  system - detects stimuli and produces voltage
  changes that can lead to nerve impulses.
• Tips of dendrites, the nerve cell body, and
  sections of the axon can have receptors.
• The voltage produced by receptors are called
  graded or generator potentials.

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Identify Or Draw And Label A Spinal Cord
Cross-Section Depicting A Simple Reflex Arc And
Include The Followingsensory (afferent)
neuron, motor (efferent) neuron, synapse, central
canal, dorsal (posterior) root, ventral
(anterior) root, gray matter, white matter,
posterior horn, anterior horn and interneuron.
Unit 7a - Objective 2
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Spinal Cord Diagram
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Spinal Cord Diagram
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Spinal Cord Diagram
White Matter
Gray Matter
Dorsal Horn
Dorsal Root
Ventral Horn
Ventral Root
Dorsal Root Ganglion
Central Canal
Spinal Nerve
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Describe or recognize the sequence of events
which occur during the utilization of a reflex
arc.
Unit 7a - Objective 3
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In a simple reflex arc, such as the knee jerk, a
stimulus is detected by a receptor cell, which
synapses with a sensory neuron. The sensory
neuron carries the impulse from site of the
stimulus to the central nervous system (the brain
or spinal cord), where it synapses with an
interneuron. The interneuron synapses with a
motor neuron, which carries the nerve impulse out
to an effector, such as a muscle, which responds
by contracting.
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The Reflex Arc

• Key components of a reflex arc
• receptor, sensory neuron, interneuron (may be
  absent), motor neuron and effector (e.g. muscle).
• receptor detects stimuli ? produces graded
  potentials that cause the formation of nerve
  impulses in the neurons.
• nerve impulses then produce rapid responses in
  muscle (jump due to sound).

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Flow Chart of Reflex Arc
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Unit 7a - Objective 4
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Describe How A Nerve Impulse Is Initiated And
Propagated Along A Neuron. Include the Terms
Below. Be Able To Indicate Or Recognize The Role
Of Each In Neural Activity resting
membrane potential, stimulus, sodium ions,
potassium ions, calcium ions, Na and K pumps,
Na and K channels, depolarization and
repolarization, action potential, nodes of
Ranvier, myelin sheath, EPSP, IPSP,
neurotransmitters.
Objective 4 -
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Excitatory and Inhibitory Post-Synaptic
Potentials - (EPSP) and (IPSP)
(EPSP)
(IPSP)
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Before we describe the process that leads to the
formation of nerve impulse, we need to examine
some of the characteristics of this event.
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Characteristics of the Nerve Impulse

• electrochemical event
• occurs in nerve cells after proper
  stimulation
• all-or-none process
• fast acting and quick to recover
• described by a voltage curve called an action
  potential
• can be conducted the entire length of a nerve
  cell without diminishment (domino effect).

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Characteristics of a Nerve Impulse Continued

• serves as the primary information signal used by
  the nervous system to provide communication about
  stimuli, nerve cell activity, neurotransmitter
  release, and to generate various output responses
  (motor action, glandular secretion, etc.).
• initiated by graded or generator potentials from
  a stimulus.

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Initiation or Generation of a Nerve Impulse (
Action Potential )

• At rest, the voltage-sensitive sodium and
  potassium gates in the channels in the plasma
  membrane of a nerve cell are nearly closed and
  the Na/K pump moves 3 Na ions to the ECF and 2
  K ions to the ICF.
• This contributes to the formation of a resting
  membrane potential of -70 millivolts (mV).

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Voltage Gated Channels
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Initiation of the Nerve Impulse Continued

• The resting membrane potential is a
  trans-membrane voltage determined by measuring
  the voltage of the ICF compared to the ECF.
• The negative state (-70mV) of the ICF compared to
  a positive ECF is due to the unequal flow of ions
  across the membrane.

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Resting Membrane Voltage (RMV)
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Initiation of the Nerve Impulse Continued

• The sodium/potassium pump also contributes to the
  resting membrane potential by pumping out three
  sodium ions to the ECF and pumping in two
  potassium ions to the ICF.
• The effect of this process is to make the
  outside of the nerve cell positive compared to
  the inside of the cell which becomes negative
  (-70 mV).

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Initiation of the Nerve Cell Impulse Continued

• At rest, the ICF of an axon has a voltage of
  about -70 mV
• When the membrane of the axon is properly
  stimulated, Na ions begin to leak into the ICF.
  This causes the voltage to change to a less
  negative state.
• When ICF voltage reaches a threshold of about -
  55 mV, sodium gates open.

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Initiation of the Nerve Impulse Continued

• As sodium gates open, Na flow through sodium
  channels increases and quickly changes the
  voltage from a resting level of - 70 mV to
  30mV.
• This rapid shift from a negative to a positive
  state is called DEPOLARIZATION.
• At 30 mV, the sodium gates close.

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Initiation of the Nerve Impulse Continued

• When the sodium gates close at 30 mV, the
  depolarization process stops.
• Interestingly, the 30 mV condition causes the
  potassium gates to open and allows potassium to
  flow from the ICF to the ECF.
• The potassium flow quickly reverses the potential
  from 30 mV to about - 70mV. This is called
  REPOLARIZATION.

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Initiation of the Nerve Impulse Continued

• The rapid depolarization and repolarization
  process generates a a voltage pulse peak that is
  called the ACTION POTENTIAL or NERVE IMPULSE.
• The formation of a nerve impulse stimulates the
  formation of still another nerve impulse in the
  next section of the axon membrane following a
  domino-like effect.

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Nerve Impulse Diagram
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Red line rising phase depends on influx of Na
into the cell Green line Action Potential Blue
line falling phase depends on outflux of K
out of cell Pink line hyperpolarization too
much K out of cell, and resets itself.
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Action Potential The resting potential tells
about what happens when a neuron is at rest. An
action potential occurs when a neuron sends
information down an axon, away from the cell
body. Neuroscientists use other words, such as a
"spike" or an "impulse" for the action potential.
The action potential is an explosion of
electrical activity that is created by a
depolarizing current. This means that some event
(a stimulus) causes the resting potential to move
toward 0 mV. When the depolarization reaches
about -55 mV a neuron will fire an action
potential. This is the threshold. If the neuron
does not reach this critical threshold level,
then no action potential will fire. Also, when
the threshold level is reached, an action
potential of a fixed sized will always fire...for
any given neuron, the size of the action
potential is always the same. There are no big or
small action potentials in one nerve cell - all
action potentials are the same size. Therefore,
the neuron either does not reach the threshold or
a full action potential is fired - this is the
"ALL OR NONE" principle
Action potentials are caused by an exchange of
ions across the neuron membrane. A stimulus first
causes sodium channels to open. Because there are
many more sodium ions on the outside, and the
inside of the neuron is negative relative to the
outside, sodium ions rush into the neuron.
Remember, sodium has a positive charge, so the
neuron becomes more positive and becomes
depolarized. It takes longer for potassium
channels to open. When they do open, potassium
rushes out of the cell, reversing the
depolarization. Also at about this time, sodium
channels start to close. This causes the action
potential to go back toward -70 mV (a
repolarization). The action potential actually
goes past -70 mV (a hyperpolarization) because
the potassium channels stay open a bit too long.
Gradually, the ion concentrations go back to
resting levels and the cell returns to -70 mV.
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Initiation of a Nerve Impulse Continued

• If the axon is myelinated due to a Schwann Cell ,
  the nerve impulse forms only in the Nodes of
  Ranvier and skips over the insulating myelin
  sheath from node to node.
• At the conclusion of each repolarization event,
  the sodium/potassium pumps move the sodium and
  potassium ions back to their main storage areas
  and reset the membrane.

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Saltatory Conduction
Speeds conduction velocity
Myelinated axons conduct nerve impulse faster
than non-myelinated. Conduction velocity is
proportional to diameter of axon (larger ?
faster). Myelination allows small diameter axons
to conduct signals quickly. More axons can fit in
small volume.
Action potential jumps from one node to the next
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Fun Facts
Some nerve poisons (e.g., scorpion venom) open
Na channels and shut K channels, disrupting
action potentials.   Local anesthetic drugs
(Novocain, Xylocaine) block the Na channels and
prevent action potentials along sensory
neurons.   Some general anesthetics (ether,
chloroform) open some K channels in the brain a
bit wider than usual. This counter-acts the
effects of Na channels being opened and prevents
action potentials from propagating, too.
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Initiation of the Nerve Impulse Continued

• When the nerve impulse reaches the axon terminal,
  a set of events will be triggered which will
  release a certain amount of neurotransmitter
  (e.g. acetylcholine).
• The neurotransmitter then accumulates in a
  synapse and generates a postsynaptic voltage
  potential in the next cell of a nerve pathway
  sequence.

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Initiation of the Nerve Impulse Continued

• If the postsynaptic potential is postive, then it
  is called an Excitatory Postsynaptic Potential or
  EPSP.
• If the postsynaptic potential is negative, then
  it is called an Inhibitory Postsynaptic Potential
  or IPSP.
• EPSPs stimulate further nerve impulses, whereas,
  IPSPs inhibit nerve impulses.

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On A Graph of the Nerve Impulse, be able to label
resting membrane potential, depolarization,
repolarization, voltage change, ion flow sections
and locations and the nerve impulse.
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Recognize Or Describe The Following As They Apply
To Nerve Impulse Conductionsynapse, divergent
synapse, convergent synapse, presynaptic neuron,
postsynaptic neuron, acetylcholine, excitatory
neurotransmitter, inhibitory neurotransmitter,
summation, spatial summation, temporal summation,
saltatory transmission, proprioception,
anesthetic and acetylcholinesterase.
Unit 7A - Objective 5
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The Synapse

• The nerve synapse is a specialized junction that
  transfers nerve impulse information from a pre
  synaptic membrane to a postsynaptic membrane
  using neurotransmitters and enzymes
• The synapse operates as an on/off switch and as
  a filter for information flow.

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synapse
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Divergent Synapse

• A junction that occurs between a presynaptic
  neuron and two or more postsynaptic neurons
  (ratio of pre to post is less than one).
• The stimulation of the postsynaptic neurons
  depends on the rapid accumulation of
  neurotransmitter by the presynaptic neuron over
  time (e.g. Temporal Summation).

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Divergent Synapse
Postsynaptic neurons
Presynaptic neuron
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Convergent Synapse

• A junction between two or more presynaptic
  neurons and a postsynaptic neuron (the ratio of
  pre to post is greater than one).
• The stimulation of the postsynaptic neuron
  depends on the accumulation of neurotransmitter
  from the presynaptic neurons (e.g. Spatial
  Summation).

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Convergent Synapse
Presynaptic neurons
Postsynaptic neuron
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Presynaptic Neuron

• The nerve cell that conducts nerve impulses to
  the synaptic junction.
• Contains the presynaptic membrane that releases
  neurotransmitters in proportion to the incoming
  nerve impulses.
• The neurotransmitters from the presynaptic
  neuron(s) stimulate the postsynaptic neuron (s).

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Postsynaptic Neuron

• The nerve cell that conducts nerve impulses away
  from the synaptic junction.
• Contains the postsynaptic membrane that responds
  to neurotransmitters from the presynaptic neuron.
• The accumulation of neurotransmitter for the
  operation of the postsynaptic neuron is due to
  temporal or spatial summation.

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Acetylcholine

• A chemical that operates as a common excitatory
  neurotransmitter.
• Released from vesicles in the presynaptic
  membrane.
• Stimulates receptors in the postsynaptic
  membrane.
• Broken down by the enzyme Acetylcholinesterase.

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Excitatory Neurotransmitter

• A chemical that accumulates in the synapse from
  presynaptic neurons and stimulates the
  postsynaptic neuron to produce nerve impulses.
• A common example is Acetylcholine.
• Excitation is produced through the formation of
  excitatory postsynaptic potentials (EPSPs).

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Spatial Summation

• The accumulation of neurotransmitter in the
  synapse due the combined activity of several
  presynaptic neurons entering the Area (Space) of
  a Convergent Synapse.
• A space (spatial) dependent process.

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Temporal Summation

• The accumulation of neurotransmitters in a
  synapse due to the rapid activity of a
  presynaptic neuron over a given Time period.
• Occurs in a Divergent Synapse.
• Is a Time (Temporal) dependent process.

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Saltatory Transmission

• A rapid transmission process that involves the
  movement of nerve impulses from one node of the
  axon to another node.
• When nerve impulses travel from node to node, the
  myelin sheath between the nodes can be skipped or
  jumped over (saltatory).
• The skipping of nerve impulses from node to node
  speeds the conduction process.

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Proprioception

• A sense of movement within ones own body which
  contributes to balance.
• Depends on proprioceptors which constantly
  monitor movement in the body by detecting the
  degree of stretch in muscles, tendons, ligaments,
  joints and sheaths of connective tissue (e.g.
  muscle fascia).

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Anesthetic

• A chemical that alters nerve cells in such a way
  that nerve impulse formation and transmission are
  suppressed.
• When anesthetics operate in pain pathways, loss
  of sensation occurs.

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Acetylcholinesterase

• A specialized enzyme that occurs in the synapse
  and which breaks down acetylcholine.
• Operates as an off switchin the synapse and
  prevents the transmission of further information
  in a nerve cell pathway.

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Inhibitory Neurotransmitter

• A chemical released into a synapse from a
  presynaptic neuron that blocks the formation of
  nerve impulses in the postsynaptic neuron.
• A common example is gamma amino-butyric acid
  (GABA).
• Inhibition is achieved through the formation of
  inhibitory postsynaptic potentials (IPSPs).

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