The Nervous System

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The Nervous System
Notes

• Chapter 35
• Section 2

2
The Nervous System
Notes

• Chapter 35
• Section 2

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Lecture Outline The Nervous System
PowerPoint Notes
textbook questions
Keys
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Nervous System Communication

• Function
• Receives Stimuli
• Interprets information
• Causes a response
• -Controls and coordinates
  bodily functions
• -Responds to stimuli (inside or
  outside the body)

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What is the basic functional unit of the nervous
system?

• Neuron!
• What does the neuron do?
• Carries messages throughoutthe body
• How does it carry the messages?
• By conducting electrical signals
• What are these signals called?
• Nerve impulses!

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What does a neuron look like?
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Neuron Anatomy

• Three parts to a NEURON
• Cell body
• Large, central portion of the neuron where all
  organelle are located
• What is its job?
• Interpret incoming signals

Cell Body
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Neuron Anatomy
Dendrites

• 2. Dendrites
• Short, highly branched fibers
• what is its job?
• Carries impulses toward the cell body
• Referred to as the afferent process

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Neuron Anatomy

• 3. Axon
• Long, slightly branched fiber
• What is its job?
• Carry impulses away from the cell body
• Referred to as the efferent process

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Neuron Anatomy

• Three parts of the Axon
• A. Axon hillock
• site of initiation of an action potential
• (point where axon and cell body meet)
• B. Axon fiber
• the main portion of the axon
• C. Axon terminal
• branched end of the axon
• point of communication with other cells

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The Axon
Axon Hillock
fiber
Axon Terminals
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Neuron Anatomy
Additional Parts of a Neuron
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Neuron anatomy
What do you remember?
dendrites
Myelinsheath
nucleus
Direction of impulse
Cell body
axon
Nodes ofRanvier
Axonterminals
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3 Types of Neurons

• I. Sensory Neurons
• Receive incoming stimuli
• Five types of sensory neurons
• Thermo-receptors
• Mechano-receptors
• Chemo-receptors
• Photo-receptors
• Pain-receptors

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Thermo-receptors

• Location
• Skin
• Hypothalamus
• Body Core
• Function
• Sensation of hot and cold
• Detects change in body core temp.

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Mechano-receptors

• Location
• Skin
• Skeletal muscle
• Inner ear
• Function
• Touch
• Pressure
• Muscle movement
• Motion
• Sound

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Chemo-receptors

• Location
• Nose
• Tongue (taste buds)
• Blood vessels
• Function
• Smell
• Taste
• Detects levels of CO2 in blood

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Photo-receptors

• Location
• Eyes
• Function
• Allow vision thru detection of light

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Pain-receptors

• Location
• Everywhere, except the brain
• Function
• Sensation of pain
• Detects chemicals released by damaged cells

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Three Types of Neurons

• II. Motor Neurons
• Carry impulses to muscles and glands
• Cause a response to some stimuli
• III. Interneurons
• Connect sensory and motor neurons
• Allow for quick response (reflex action)

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Three types of neurons
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Neurons

• What is grey matter?
• Collective cell bodies and dendrites of all
  neurons
• What is white matter?
• Myelinated nerve fibers
• Axons of all neurons
• Can be approximatelyone meter in length

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What is a Nerve?

• Bundle of axonsheld together by connective
  tissue.
• What color isa nerve?
• White!
• Why?
• Because axons are white matter and they
  composenerves

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How are nerves held together?

• Connective tissue
• What is this connective
• tissue called?
• Neuroglial cells (nerve glue)
• Approximately half of the volume of the brain is
  composed of neuroglial cells
• Most brain tumors develop in mesoglial cells
  NOT neurons

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What do neuroglial cells do?

• Support the axons
• Insulate the electrical impulses
• Like electrical tape insulates electric wires
  this prevents leakingof electric signals

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An example of a neuroglial cell

• Schwann cell
• This wraps around the axon in multiple layers
• It is composed of a fatty material called
• Myelin
• Regular breaks in the myelin sheath are called
• Node of Ranvier

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Impulse speed

• Some neurons are fast, good conductors of
  impulses
• Other neurons are slow, poor conductors of
  impulses
• What distinguishes thesetwo types?
• Diameter of the axon
• Myelination
• Fastest axons have a large diameter and are
  myelinated. How much faster?
• Up to 100 times faster! (Reflexes are this type)

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The Nerve Impulse

• How are messages (impulses)carried by the
  nervous system?
• As electrical and chemical signals.
• How does the impulse develop?
• Charged particles (ions) move across the cell
  membrane
• A neuron is ready to transmit an impulse when it
  is in the resting state.
• Resting membrane potential is -70 mV

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Resting Membrane Potential

• The inside of the cell has a negative charge as
  compared to the outside of the cell membrane.
• How does this charge difference develop?
• There are more Na (sodium) ions outside and
  fewer K (potassium) ions inside.

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The Nerve Impulse

• How does the impulse begin?
• The neuron is stimulated by another neuron or by
  stimuli from the environment.

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The Nerve Impulse

• If the stimulus is strong enough, it reaches
  threshold level.
• This stimulates an impulse.
• If the stimulus is not strong enough, no impulse
  occurs (all-or-none principal)

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The Nerve Impulse

• What happens if threshold is reached?
• An action potential (nerve impulse) begins
• What is an action potential?
• Rapid reversal of membrane potential in response
  to a stimulus
• How does this happen?
• Sodium channels open allowingNa to flood into
  the cell.
• The membrane potential rises to 30 mV (rising
  phase) as inside of cell becomes more positive

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The Nerve Impulse

• Please, please, tell us what happens next
• When the membrane potential reaches 30 mV, the
  sodium channels close.
• Potassium channels open and K flows out of the
  cell.
• This causes the membranepotential to become more
  negative again (falling phase).

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The Nerve Impulse
Na channels close
50
2
K channels open, K begins to leave cell
K leaves cell
0
Membrane potential (mV)
1
Na channels open, Na enters cell
Threshold of excitation
K channels close
3
-70
Excess K outside diffuses away
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The Nerve Impulse
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The Nerve Impulse
Action potential
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0
Depolarization
Repolarization
Voltage (mV)
Failed initiations
Threshold
-55
Resting state
-70
Refractory period
0 1 2 3
4 5
Time (ms)
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The Nerve Impulse

• What happens when the action potential passes
  down the axon?
• The resting potential is restored via the
  sodium/potassium pump.
• Now the membrane is ready to transmit another
  impulse.

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The Nerve Impulse

• An action potential only moves in one direction
  down the axon.
• From axon hillock, thru axon fiber to the axon
  terminal.
• The action potential is regenerated at each Node
  of Ranvier down the length of the axon.

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The Nerve Impulse

• What happens when the impulse reaches the axon
  terminal?
• It must pass thruthe synapse.
• What is a synapse?
• It is the gap between two neurons, or between a
  neuronand organ (effector).
• Why is there a gap?
• So the neurons dont short each other out!!

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The Synapse

• How do neurons pass the impulse across the
  synapse?
• Using chemicals called neurotransmitters
• What do neurotransmitters do?
• Stimulate the dendrites of other neurons or
  membranes of other cells.
• If a dendrite is stimulated, it sends its message
  to the cell body and the message is passed on
• If a muscle or gland is stimulated, a reaction
  occurs in that organ.

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The Synapse
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The Synapse
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The Synapse
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• Neuron Smear
• Identification Note distinctive shape of neuron,
  with long processes (dendrites and/or axons, 5)
  extending out from main cell body.
• Features to Know
• The large, irregularly shaped cell body (3)
  contains a darker nucleus (2), which contains an
  even darker-staining nucleolus (1).
• There are also numerous supporting
  
  glial cells, though only
  their small dark nuclei (4) are easily seen.

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