Chapter 12

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Chapter 12 Introduction to the Nervous System

• Organization
• Cell Types

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Review

• What 3 parts make up the nervous system?
• Brain
• Spinal cord
• Nerves

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Functions of the Nervous System

• Detect changes (stimuli) in the internal or
  external environment
• Evaluate the information
• Initiate a change in muscles or glands
• Goal maintain homeostasis
• What does this remind you of??

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Organization of the Nervous System

• Central nervous system (CNS)
• Brain and spinal cord
• Peripheral nervous system (PNS)
• Nervous tissue in the outer regions of the
  nervous system
• Cranial nerves originates in the brain
• Spinal nerves originates from the spinal cord
• Central fibers extend from cell body towards the
  CNS
• Peripheral fibers extend from cell body away
  from CNS

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Afferent vs Efferent

• Nervous pathways are organized into division
  based on the direction they carry information
• Afferent division incoming information (sensory)
• Efferent division outgoing information (motor)
• (Efferent Exit)

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Somatic Autonomic Nervous Systems

• Nervous pathways are also organized according to
  the type of effectors (organs) they regulate
• Somatic nervous system (SNS)
• Somatic sensory division (afferent)
• Somatic motor division (efferent)

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Somatic Autonomic Nervous Systems cont

• Autonomic nervous system (ANS) Carry information
  to the autonomic or visceral effectors (smooth
  cardiac muscles and glands)
• Visceral sensory division (afferent)
• Efferent pathways
• Sympathetic division fight or flight
• Parasympathic division rest and repair

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• Figure 12-2

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Warm Up 1/5

• Complete the sentences
• Afferent pathways carry
• Efferent pathways carry.
• The PNS can be subdivided into the.
• These divisions are based upon.

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Review

• What are the two main cell types in the nervous
  system?
• (Hint we talked about this when we covered
  tissue types)
• Answer neurons and glia

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Cells of the Nervous System

• Neurons excitable cells that conduct information
• Glia (also neuroglia or glial cells) support
  cells, do not conduct information
• Most numerous
• Glia glue

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Types of Glia

• Five major types
• Astrocytes
• Microglia
• Ependymal cells
• Oligodendrocytes
• Schwann cells

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Astrocytes (12-3A)

• Star-shaped, largest, most numerous
• Cell extension connect neurons and capillaries
• Transfer nutrients from blood to neuron
• Help form blood-brain barrier (BBB)

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Blood-Brain Barrier

• Helps maintain stable environment for normal
  brain function
• feet of astrocytes wrap around capillaries in
  brain
• Regulates passage of ions
• Water, oxygen, CO2, glucose and alcohol pass
  freely
• Important for drug research
• Parkinsons Disease

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Microglia (12-3B)

• Engulf and destroy cellular debris (phagocytosis)
• Enlarge during times of inflammation and
  degeneration

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Ependymal cells (12-3C)

• Similar to epithelial cells
• Forms thin sheets that line the fluid-filled
  cavities of the brain and spinal cord
• Some cells help produce the fluid that fills
  these cavities (cerebral spinal fluid - CSF)
• Cilia may be present to help circulate fluid

http//www.lab.anhb.uwa.edu.au/mb140/corepages/ner
vous/Images/epen100he.jpg
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Oligodendrocytes (12-3D)

• Hold nerve fibers together
• Produce myelin sheaths in CNS

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Multiple Sclerosis (MS)

• Most common myelin disorder
• Characterized by
• myelin loss and destruction ? injury and death ?
  plaque like lesions
• Impaired nerve conduction ? weakness, loss of
  coordination, vision and speech problems
• Remissions relapses
• Autoimmune or viral infection
• Women 20-40 yrs
• No known cure

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Multiple Sclerosis (MS)
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Schwann cells (12-3E)

• Only in PNS
• Support nerve fibers form myelin sheaths
• Satellite cells (12-3G)
• Types of schwann cell that covers a neurons cell
  body

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Neurons

• All neurons have 3 parts
• Cell body (soma)
• Axon
• One or more dendrites

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Neuron Anatomy

• Soma resembles other cells
• Nissl bodies part of rough ER contain proteins
  necessary for nerve signal transmission nerve
  regeneration
• Dendrites branch out from soma receptors
  conduct impulse towards soma
• Axon process that extends from the soma at a
  tapered portion called the axon hillock
• Axon collaterals side branches
• Telodendria distal branches of axon
• Synaptic knob ends of telodendria

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Neuron Anatomy

• Myelin sheaths areas of insulation produced by
  Schwann cells increases speed of nerve impulse
• Myelinated white matter
• Unmyelinated gray matter
• Nodes of Ravier breaks in myelin sheath btwn
  Schwann cells
• Synapse junction btwn two neurons or btwn a
  neuron and an effector

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Structural Classification of Neurons

• Multipolar
• One axon, several dendrites
• Most numerous
• Bipolar
• One axon, one dendrite
• Least numerous
• Retina, inner ear, olfactory pathway
• Unipolar
• Axon is a single process that branches into a
  central process (towards CNS) and a peripheral
  process (towards PNS)
• Dendrites at distal end of peripheral process
• Always sensory neurons

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Functional Classification of Neurons

• Afferent
• Sensory
• Towards CNS
• Efferent
• Motor
• Towards muscles glands
• Interneurons
• Connect afferent efferent neurons
• Lie within CNS

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Nerves vs Tracts

• Nerves bundles of parallel neurons held
  together by fibrous CT in the PNS
• Tracts bundles of parallel neurons in the CNS

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Warm Up 1/7

• List the 5 types of glial cells and a key
  word/phase for each.
• Study for your quiz! ?

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Warm Up 1/10

• Describe the following structures
• Nissl bodies
• Myelin sheaths
• Axon hillock
• Axon collateral
• Telodendria
• Synaptic knobs

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Reflex Arc
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Examples of Reflex Arcs

• Ipsilateral
• Contralateral
• intersegmental

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Nerve Fibers

• Remember the difference between nerves and
  tracts?
• Endoneurium surrounds each nerve fiber
• Perineurium surrounds fascicles (bundles of
  nerve fibers
• Epineurium surrounds a complete nerve (PNS) or
  tract (CNS)

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Review Gray vs White Matter

• White matter myelinated nerve fibers
• Myelin sheaths help increase the speed of an
  action potential
• Gray matter unmyelinated nerve fibers cell
  bodies
• Ganglia regions of gray matter in PNS

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Nerve Fiber Repair

• Nervous tissue has a limited repair capacity b/c
  mature neurons are incapable of cell division
• Repair can take place if soma and neurilemma
  remain intact

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Steps of Nerve Fiber Repair

• Injury
• Distal axon and myelin sheaths degenerates
• Remaining neurilemma endoneurium forms a
  tunnel from the injury to the effector
• Proteins produced in the nissl bodies help extend
  a new axon down the tunnel to the effector

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Nerve Impulses

• Neurons are specialized to initiate and conduct
  signals ? nerve impulses
• Exhibit excitability conductivity
• Nerve impulse ? wave of electrical fluctuation
  that travels along the plasma membrane

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Membrane Potentials

• Difference in charges across the plasma membrane
• Inside slightly negative
• Outside slightly positive
• Result in a difference in electrical charges ?
  membrane potential
• Stored potential energy
• Analogy water behind a dam

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Membrane Potentials

• Membrane potential creates a polarized membrane
• Membrane as pole pole
• Potential difference of a polarized membrane is
  measured in millivolts (mV)
• The sign indicates the charge of the inside of a
  polarized membrane

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Resting Membrane Potential (RMP)

• When not conducting electrical signals, a
  membrane is resting
• -70mV
• RMP maintained by ionic imbalance across membrane
• Sodium-Potassium Pump
• Pumps 3 Na out for every 2 K pumps in
• Creates an electrical gradient (more positive on
  outside)

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Resting Membrane Potential (RMP)
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Local Potential

• Local potential - The slight shift away from the
  RMP
• Isolated to a particular region of the plasma
  membrane
• Stimulus-gated Na channels open ? Na enters ?
  membrane potential to moves closer to zero
  (depolarization)
• Stimulus-gated K channels open ? K exits ?
  membrane potential away from zero
  (hyperpolarization)
• Local potentials do not spread to the end of
  the axon

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Local Potentials
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Warm Up 1/11

1. What two structures must remain intact if nerve
   repair is to take place?
2. In a resting membrane potential, which side of
   the membrane is slightly positive? slightly
   negative?
3. Typically, a neurons resting membrane potential
   is.?
4. What helps maintain a resting membrane potential?

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Action Potentials

• Definitions
• Membrane potential of an active neuron (one that
  is conducting an impulse
• Action potential nerve impulse
• An electrical fluctuation that travels along the
  plasma membrane

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Steps of Producing an Action Potential (table
12-1)

1. A stimulus triggers stimulus-gated Na channels
   to open ? Na diffuses inside the cell ?
   depolarization
2. Threshold potential is reached (-59mV) ?
   voltage-gated Na channels open ? depolarization
   continues
3. Action potential peaks at 30mV, voltage-gated
   Na channels close
4. Voltage-gated K channels open ? K diffuses
   outward ? repolarization
5. Brief period of hyperpolarization (below -70mV) ?
   RMP is restored by Na/K pump

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Refractory Period
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Refractory Period

• Period of time where the neuron resists
  restimulation (AP cannot fire)
• Absolute refractory period half a millisecond
  after membrane reaches threshold potential
• Will not respond to ANY stimulus
• Relative refractory period few milliseconds
  after absolute refractory period (during
  repolarization)
• Only respond to VERY strong stimulus

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Refractory Period What does this mean?

• Greater stimulus quicker another action
  potential can take place
• The magnitude of the stimulus does not affect the
  magnitude of the AP
• b/c APs are all or nothing
• Does cause proportional increase in frequencies
  of impulses

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Conduction of an Action Potential

• During the peak of an AP, the polarity reverses
• Negative outside, positive inside
• Causes impulse to travel from site of AP to
  adjacent plasma membrane
• No fluctuation in AP due to all or nothing
  principle
• AP cannot travel backwards on axon due to
  refractory periods

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Conduction of an Action Potential

• How does myelin sheaths affect the speed of an
  action potential?
• Sheaths prevent movement of ions
• Electrical changes can only take place at Nodes
  of Ranvier
• APs leap from node to node (current flows under
  sheaths)
• Saltatory conduction

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Random Facts

• In nerve fibers that innervate skeletal muscle,
  impulses travel up to 130 m/s (300 mph)
• Sensory pathways from skin ? 0.5 m/s (lt1 mph)
• Many anesthetics block the sensation of pain by
  inhibiting opening of Na channels

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Warm Up 1/12-1/13

• What is threshold potential?
• What happens when threshold potential is reached?
• What happens at the peak of an action potential?
• What maintains the RMP after an action potential?
• True or false the magnitude of a stimulus
  directly affects the magnitude of an action
  potential.
• Suggested reading for action potentials (p.
  355-358)

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Types of Synapses

• Electrical synapses two cells joined end to end
  by gap junctions
• Ex btwn cardiac muscle cells, smooth muscles
  cells

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Types of Synapses

• Chemical synapses use neurotransmitter to send a
  signal from a presynaptic cell to postsynaptic
  cell

• 3 Parts
• Synaptic knob
• Synaptic cleft
• Plasma membrane of postsynaptic neuron

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Mechanisms of Synaptic Transmission

1. AP depolarizes synaptic knob
2. Voltage-gated Ca2 channels open ? Ca2 diffuses
   inside the cell
3. Ca2 triggers exocytosis of neurotransmitter
   vesicles
4. NTs diffuses across synaptic cleft ? bind w/
   receptors on postsynaptic cell

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Postsynaptic Potentials (Fig 12-22)

• Excitatory NTs cause Na and K channels to open
  ? depolarization ? excitatory postsynaptic
  potential (EPSP)
• Inhibitory NTs cause K and Cl- channels to open
  ? hyperpolarization ? inhibitory postsynaptic
  potential (IPSP)

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Summation

• For every postsynaptic cell there are usually
  1K-100K synaptic knobs
• Both excitatory inhibitory NTs are released
• Summation of local potentials (EPSP IPSP) occur
  at axon hillock
• EPSP gt IPSP ? reach threshold ? action potential
• EPSP lt IPSP ? threshold not reached ? no AP

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Neurotransmitters

• Small-Molecule Transmitters
• Acetylcholine
• Amines
• Serotonin
• Dopamine
• Epinephrine
• Norepinephrine
• Amino Acids
• Glutamate
• GABA
• Glycine
• Large-Molecule Transmitters
• Neuropeptide
• Endorphins