Biology 315' Chapter 12' Nervous Tissue

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Biology 315. Chapter 12. Nervous Tissue
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Parts of the Nervous System

• 1. Central Nervous System CNS
• Brain
• Spinal Cord
• 2. Peripheral Nervous System PNS
• Afferent brings information from the periphery
  TO the CNS
• Efferent sends information from CNS out to
  periphery

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PNS

• Afferent Sensory
• Efferent Motor
• Somatic Nervous System generic motor nerves,
  voluntary control of muscles
• Autonomic Nervous System, Involuntary
• Parasympathetic vegetative functions
• Sympathetic fight or flight
• Enteric Nervous System the brain of the gut

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True Blue Words and Terms

• Nerve
• Tract
• Gray Matter
• White Matter
• Nucleus
• Ganglion
• Plexus
• Stimulus
• Action Potential

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Functions of the Nervous System

• I watch the tennis ball being served visual info
  carried to brain from retina
• Information about the angle, the speed, etc., of
  the serve is integrated in cerebrum and a
  response is formed
• Motor outputs through cerebrum and cerebellum
  stimulate muscles to move I return the ball

• Sensory
• Light shines in your eye impulse carried to
  brain from retina
• Integrative
• Vision processing area integrates all information
  and formulates a response
• Motor
• Pupillary constriction

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Cell types in the Nervous System

• Neurons the only cells that can carry the
  impulse
• Glial cells supportive
• PNS Glial cells Schwann cell (makes myelin)
• Satellite cells in ganglia
• CNS Glial cells oligodendrocytes
• astrocytes
• microglia
• ependymal

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Neurons vs. glial cells
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Ganglion
GANGLION
Neurons or Glial Cells?
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Types of Neurons

• Structure
• Multipolar
• Bipolar
• Pseudounipolar / unipolar
• Function
• Sensory afferent
• Association/ interneurons integrative
• Motor efferent

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Nerve fibers axons and dendrites

• Myelinated
• Myelin insulates axon increases speed of AP
  conduction
• Composed of plasma membranes of Schwann cells
  (PNS) or oligodendrocytes (CNS)
• Gaps nodes of Ranvier. These allow saltatory
  conduction
• Unmyelinated
• Slower conduction

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Olfactory Nerve carries sensation of smell to
the brain

• Sensory or motor?
• Axons or dendrites?

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Ulnar Nerve Axons or dendrites?

• Carries sensory information from some fingers and
  hand
• Carries motor information to muscles of the hands

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Electrical Signals in Neurons..It all starts
with the plasma membrane
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The plasma membrane is composed primarily of
_________

• Non-polarized substances can get through the
  lipid bilayer.
• Oil and water do not mix well. So
• How do polarized substances like Na K
  Ca 2 get across this lipid bilayer ?
• Answer
• Special ion channels in the plasma membrane

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• These ion channels allow for diffusion of small
  ions.
• Therefore, one would expect that the
• Nainside the cell Na outside the
  cell
• Kinside the cell K outside the cell
• This would mean that the charge across the
  membrane would be 0.

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Resting Membrane Potential
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BUTNO !!!!
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Electrical gradient different charge on either
side of the membrane

• Chemical gradient different concentrations of
  Na and K on either side of the membrane
• Hence, the Electrochemical gradient, via the Na
  K pump

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The Na/ K pump and the membrane potential
3 sodiums pumped out for every 2 K pumped in
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If this balance is disturbedi.e., when the
voltage changes, current flows
The membrane is said to be POLARIZED.
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What can upset the resting membrane?

• Activity or stimulus at the synapse
• When neurotransmitter binds to its receptor, the
  membrane potential changes

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Types of Electrical Signals I Graded Potentials

• Graded Potential a small deviation from the
  membrane potential caused by a stimulus.
• If it makes the membrane de-polarize a little ?
• the membrane is more EXCITABLE
• If it makes the membrane more polarized
  (hyperpolarization) ? the membrane is less
  excitable ( INHIBITED. )
• Action Potential

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Graded Potentials

• Are LOCAL.just a short distance on the plasma
  membrane
• Are GRADED.the potential varies dependent on the
  strength of the stimulus. They are not
  All-or-None
• Can be added together SUMMATION
• Can occur in dendrites/cell bodies
  Post-synaptic potential (PSP)
• Often occur in sensory receptors

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Types of Electrical Signals II Action Potentials

• AP Nerve Impulse
• AP a series of changes in the potential
  (voltage) of the membrane
• Depolarization membrane gets more
• Repolarization membrane goes back to -
• Refractory period another AP cannot fire for a
  while
• ALL or NONE It either fires or it doesnt.
  No in-between.

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What happens during Depolarization

• Na rushes in..the flood gates have been
  opened.
• K rushes out..
• The membrane potential rises towards 0 and even
  gets positive.

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What happens during Repolarization

• The Na /K pump kicks in again.
• Na is pumped out of the axon
• K is pumped into the axon.
• But not at equal rates..
• 3 Na out for each 2 K in

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Propagation of the action potential
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Depolarization

• Na rushes in..the flood gates have been
  opened.
• K rushes out..
• The membrane potential rises towards 0 and above
  (gets positive.)
• Eventually it gets so positive inside that
  further influx of positive ions is repelled.

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What happens during Repolarization

• The Na /K pump kicks in again.
• Na is pumped out of the axon
• K is pumped into the axon.
• The membrane potential is restored..
• A hyperpolarization phase may occur in which the
  membrane overshoots the membrane potential.

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The Refractory Period

• 1) Absolute no stimulus can provoke another AP.
  (Na channels inactivated resting)
• 2) Relative an AP can be initiated only by a
  greater than normal stimulus.
• (note the larger the diameter of the axon, the
  shorter the absolute refractory period)

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The Action Potential Propagates, beginning at
the trigger zone, the axon hillock

• A nerve impulse is self-propagating. At the
  leading edge of an action potential, sodium gates
  open, allowing sodium ions to flow into the cell.
  This flow of ions triggers more sodium gates to
  open, causing the action potential to move.
• At the trailing edge of an action potential,
  potassium gates open, allowing positive ions to
  flow out, and restoring the resting potential of
  the neuron.

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Questions to consider..

• Why does the impulse only propagate down the
  axon in one direction?
• Why does the impulse only get transmitted one-way
  at the synapse?

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Continuous conduction

• Relatively slow
• Step-by-step depolarization and repolarization
• Found in unmyelinated fibers

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Saltatory conduction

• Impulse jumps from node to node
• Faster
• Takes less energy because fewer regions of the
  membrane undergo depol/repol and Na pump doesnt
  have to work so hard.
• In myelinated nerves

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What affects the speed of propagation?

• Presence of myelin
• Diameter of axon
• Temperature
• (put ice on an injury)

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The ABCs of nerve fiber classification

• Papa Bear A largest shortest refractory
  period myelinated large sensory and motor
  fibers
• Mama Bear B middle diameter longer refractory
  period myelinated sensory from skin and viscera
  to CNS motor to some autonomics
• Baby Bear C smallest longest refractory
  period unmyelinated heat, cold, pain, plus some
  motor

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Signal Transmission at the Synapse
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Signal Transmission

• Usually axon of first neuron to dendrite of
  second neuron AXODENDRITIC
• Some axon-axon, and some axon/cell body synapse.
• Two types
• ELECTRICAL
• CHEMICAL

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The Electrical Synapse

• Impulse conducted directly between cells via gap
  junctions (no NT involved)
• Faster than chemical synapse (no synaptic delay)
• Better synchronization.e.g., all the muscle
  fibers in the ventricles of the heart must
  contract at once in order for the heart to work
  as a pump

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The Chemical Synapse(remember the NMJ )

• Neurons are separated by synaptic cleft
• Presynaptic neuron releases NT which diffuses
  across the
• Synaptic cleft and binds to the receptor on the
• Postsynaptic neuron, which produces some
  postsynaptic potential

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Transmission of the Impulse

• Nerve impulse arrives at presynaptic terminal
• The depolarization of the presynaptic axon opens
  voltage-gated Ca2 channels, and
• Ca2 ions flow into the cell
• (under resting conditions, Ca2outside of cell
  gt Ca2 inside the cell)

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And Then.in True Blue fashion

• Ca2 intracellular triggers exocytosis of
• synaptic vesicles, which release
• Neurotransmitters, which
• Diffuse across synaptic cleft and
• Bind to receptors on post-synaptic membrane

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• NT/receptor complex opens ion channels, and
• Ions flow in
• If Na channels are opened,
• Na rushes in? Depolarization
• If Cl- channels are opened,
• Cl- rushes in ? Hyperpolarization

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Will an Action Potential be triggered? Enquiring
minds want to know

• If the sum of the postsynaptic potentials reaches
  threshhold..
• YES!!!!!

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If the sum of the postsynaptic potentials does
not reach threshhold.. No

• But the membrane may be excited/more likely to
  fire (slightly depolarized) EPSP
• Or the membrane may be inhibited/ less likely
  to fire (hyperpolarized) IPSP

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How does the Synapse turn off?Remove that NT!

• NT diffuses away from the synaptic cleft
• NT is degraded by enzymes
• NT is taken up by presynaptic site.
• SSRIs

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Our friend, the Neurotransmitter

• Most are small molecules
• ACh
• excitatory or inhibitory
• NMJ some other PNS and CNS synapses
• inactivated by acetylcholinesterase

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More NTs

• Amino acids
• Glutamate excitatory inactivated by reuptake
• Aspartate
• Glycine - inhibitory
• GABA gamma amino butyric acid also inhib.
• VERY important in the brain
• Valium enhances the action of GAB
• (probably other amino acids as well)

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.

• Biogenic amines

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More on the biogenic amines

• NE
• E the catecholamines
• Dopamine Inactivated by catechol-O-
  methyl transferases, such as MAO
• Serotonin 5-OH-tryptamine
• SSRIs

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More small molecules ATP and other
purinese.g., adenosine excitatory in CNS and
PNS

• Nitric Oxide NO
• Synthesized as needed
• Lipid soluble
• Immediate action
• Highly reactive but short acting
• Relaxes vascular smooth muscle

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And the last of the NTs

• Neuropeptides chains of amino acids
• Endorphins
• endogenous morphine / opioids
• Enkephalins
• Substance P

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Neural Circuits

• Diverging from one neuron to many neurons
• Converging from many to one
• Reverberating a series of coordinated
  impulses
• Parallel after-discharge more complicated

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Regeneration/Repair of Nerve Tissue
Neuroplasticity

• PNS
• If cell body remains intact, the peripheral
  nerves may repair
• Research CNS
• EGF stimulates the brain cells of the adult mouse
  to proliferate into neurons and glial cells
• Some new neurons in the hippocampus
• Problems with scar tissue after injury
  interfere with repair

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Repair of fibers in the PNS

• 24-48 hours post-injury Nissl bodies break up
  chromatolysis
• 3-5 days post-injury distal to the injury axon
  swells, breaks into fragments, myelin
  deteriorates Wallerian degeneration
• Leftovers are phagocytized by
• If Schwann cell tube is present, the
  regenerating axon will sprout

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• Multiple Sclerosis
• Epilepsy
• Guillain-Barre
• Neuropathy
• Neuroblastoma
• Rabies