STRUCTURE AND FUNCTION OF THE NEUROLOGIC SYSTEM

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STRUCTURE AND FUNCTION OF THE NEUROLOGIC SYSTEM
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Organization

• Central nervous system (CNS)
• Anatomical structures
• Brain enclosed -- cranial vault
• Spinal cord enclosed -- bony spine
• Peripheral nervous system (PNS)
• Anatomical organization (Fig.12-26)
• Nerves
• Cranial 12 pairs
• Spinal -- 31 pairs
• Can be afferent or efferent

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• Functional organization
• Somatic nervous system
• Regulates voluntary, motor control
• Neurotransmitter acetyl choline (ACh)
• Autonomic nervous system
• Regulates internal environment

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• Most organs dually innervated
• Sympathetic neurons (Fig.12-24,23)
• From thoracic, lumbar spinal regions
• Important for fight or flight (incrd heart
  rate/respn, decrd digestion)
• Neurotransmitters ACh and epinephrine/norepi

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• Parasympathetic neurons (Fig.12-25,23)
• From other spinal regions
• Important to conserve energy and maintain
  homeostasis (decrd heart rate, incrd digestion)
• Neurotransmitter ACh

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Neural tissue

• Neuron primary cell of nervous system
• About 1011 neurons/body
• Each neuron adapted for specific function
• Functions of neurons
• Detect envl changes
• Initiate body response to changes
• Fuel source -- mostly glucose

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• Anatomic components (Fig.12-1)
• Cell body soma
• Most in CNS
• Those in PNS grouped together as ganglia
• Dendrites
• Extensions of cell body
• Carry information TOWARD cell body

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• Axon
• Usually one per neuron
• Long projection carries impulses AWAY from cell
  body
• Myelin insulating lipid covering
• Forms sheath
• Allows fast flow of ions in one direction ?
  proper impulse conduction (away from cell body)
• Interruptions in myelin coating nodes of
  Ranvier
• Nec for ions from ISF to enter axon for proper
  impulse

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• Supporting cells of neurological system
  (Fig.12-3,Table12-1)
• Schwann cells in PNS
• Form myelin sheath around axons
• Neuroglia -- nerve glue
• Support CNS neurons
• About ½ volume of the brain and spinal cord

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• Several types of neuroglial cells
• Astrocytes -- star shape
• Form contact between neurons, circulatory system
• Buffer zone between neurons (delicate) and
  molecules circulating in blood
• Oligodendroglia
• Deposit myelin in CNS (similar job as the Schwann
  cells in PNS)
• Microglia
• Phagocytic cells digest debris in CNS
• Ependymal cells
• Help produce cerebrospinal fluid (csf)

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• Nerve injury and regeneration (Fig.12-4)
• Mature neurons dont divide, proliferate
• Injury can ? permanent loss of function
• Regeneration of some PNS neurons is possible
• Axon of neuron (so only myelinated fibers)
  repaired
• Regeneration more optimistic if cell crushed
• If cut, scar tissue can form ? impede ion flux
  through cell membrane, so impede proper impulses

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• Regeneration more optimistic if injury further
  away from cell body
• With regeneration, see
• Swelling distal to injury
• Filaments hypertrophy
• Myelin sheath and axon begin to degenerate, BUT
• Proximal to injury, see projection of new
  neurofibriles
• Neurilemma (membrane that surrounds the myelin
  sheath) acts as guide
• Not in CNS, where myelin somewhat different
• Scar tissue forms, and decrd/no regeneration of
  neuronal tissue

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Nerve impulses

• Action potentials generated
• Neuron selectively changes electrical potential
  of its plasma membrane
• ? Influx of Na through selective channels (gated
  Na channels) at dendrite or soma
• In response to biochemical signal from a
  neurotransmitter released from an impinging
  neuron
• ? Changes electrical potential of membrane in
  that region

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• Neurons influence neighboring neurons (Fig.12-2)
• Release neurotransmitters (biochemicals signal an
  action potential in a neighboring neuron)
• Synapse region between two nearby neurons
• First neuron in a series presynaptic
• Second neuron postsynaptic
• Presynaptic impinges on postsynaptic
• Neurotransmitters synthd, stored in vesicles
  near end of presynaptic neuron

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• When action potential reaches end of presynaptic
  neuron
• Signals vesicle holding neurotransmitters to
  merge with neurons plasma membrane in
  presynaptic area
• Neurotransmitters released into synapse
• Neurotransmitters travel through synapse, where
  they encounter postsynaptic neuron
• On plasma membrane of postsynaptic neuron is a
  receptor specific for a particular
  neurotransmitter

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• Neurotransmitter binds the receptor on the
  postsynaptic neuron
• Signals opening of nearby Na channels
• ? Membrane potential changes in the postsynaptic
  neuron
• ? Generation of action potential
• Action potential travels through postsynaptic
  neurons dendrite, cell body and axon to axon
  ending (now presynaptic)
• Signals neurotransmitter release to next neuron
  or muscle fiber on which it impinges, and changes
  occur within that cell

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• Some widely studied neurotransmitters
• Norepinephrine, epinephrine, dopamine, ACh,
  serotonin (and MANY others)(Table12-2)
• Excitatory neurotransmitters cause Na to flood
  into neuron ? depolarization and action potential
• Inhibitory neurotransmitters dampen Na influx
  into neuron ? inhibition of depolarization, so no
  action potential
• Different neurotransmitters have different
  functions (some excitatory, some inhibitory)

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Central Nervous System (CNS)

• The brain
• Allows reasoning, intelligence, personality, mood
• Weighs about 3 lb. in average adult
• Receives about 20 of cardiac output
• Divisions (Table 12-3Fig.12-6)
• Different regions, each associated with different
  function (Fig.12-7)
• BUT some functions controlled by more than one
  region
• Ex cerebrum -- centers for sensory/motor,
  reasoning, memory, intelligence

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• Characteristics/Structures
• Gyri convolutions of tissue along brain surface
• Importance increase surface area of brain
• Sulci grooves between gyri
• Gray matter cerebral cortex
• Cell bodies of neurons (so not myelinated)
• White matter myelinated nerve fibers ( axons)
• Lies beneath cerebral cortex

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• Spinal cord (Fig.12-9,10,11)
• Long nerve cable
• Continuous with brain
• Lies in vertebral canal
• Surrounds, protects spinal cord
• Divided into 31 anatomical sections
• Gray matter (Fig.12-11)
• In center of spinal cord
• Butterfly shaped
• Divided ? 3 horns
• Composed of neuronal cell bodies

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• White matter
• Surrounds gray matter
• Myelinated tissue (so axons)
• Forms ascending, descending tracts
• Motor neurons (Fig.12-12,13)
• Directly influence the muscle cells
• Cell bodies of motor neurons lie in gray matter
  of spinal cord
• Axons extend out of spinal cord
• Regulate motor activity

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• Protective structures of the CNS
• Cranium
• 8 fused bones encloses and protects the brain
• Epidural space
• Lies between cranium and meninges
• Site of blood collection (? epidural hematoma) if
  trauma ? disruption of blood vessels of
  scalp/skull

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• Meninges 3 protective membranes (Fig.12-14)
• Dura mater 2 layers of tissue
• Arachnoid membrane named for appearance (spider
  web)
• Pia mater cells to produce cerebrospinal fluid
• Spaces between layers -- also sites where blood
  may collect if hemorrhage

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• Cerebrospinal fluid (csf)
• Clear, colorless fluid similar to ISF and plasma
  (Table 12-4)
• Helps cushion CNS
• Produced within pia mater (about 600 mL/day)
• Circulates within cranium in cavities,
  subarachnoid space
• Exerts pressure within brain, spinal cord
• Forms pressure gradient between arteries,
  cavities of CNS
• Reabsorbed into venous circulation
• Valves in arachnoid membrane move fluid into
  venous circulation (and opposite)

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• Vertebral column (Fig.12-15,16)
• Vertebrae
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• Intervertebral discs
• Between vertebrae
• Pulpy, absorb shock
• Prevent damage to nervous system structures
• If rupture ? back pain

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Vertebral circulation

• Arises from aortic arch ? internal carotid
  arteries and ? vertebral arteries (Fig.12-18
  Table 12-5)
• May be conducting (? brain surface), OR
• Penetrating (? structures below the cortex)
• Healthy brain can regulate its blood supply to
  maximize oxygen supply
• Can increase extraction of oxygen from blood when
  systemic bp decreases (for awhile)
• Can decrease resistance in cerebral vessels when
  systemic bp decreases (up to a point)

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• Blood-brain barrier
• Supporting neural cells and blood capillaries
  have rel. tight junctions
• Selectively allow partic blood components from
  blood ? brain
• Important in brain chemotherapy