Anatomy and Physiology

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Anatomy and Physiology

• Chapter 9
• The Nervous System
• Part I

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Introduction

• Organs of the nervous system are divided into
• Central nervous system (brain and spinal cord)
• Peripheral nervous system (nerves that connect
  the CNS to other body parts).

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• Nervous tissue includes
• Neurons structural and functional units of the
  nervous system
• Neuroglial cells specialized cells that provide
  psyiological requirements and also function
  similar to connective tissue

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Functions of the Nervous System

• The nervous system provides sensory, integrative,
  and motor functions.
• Coordinates all other body functions to maintain
  homeostasis
• Enables the body to respond to changing
  conditions

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Sensory Functions

• Employs receptors that detect
• Internal changes ex. temperature and oxygen
  concentration
• External changes ex. light and sound intensities
• Sensory receptors convert environmental
  information into nerve impulses which are
  transmitted over peripheral nerves to the CNS

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Integrative Functions

• Collect sensory information and make conscious or
  subconscious decisions that motor functions carry
  out.
• Signals are brought together, creating
  sensations, adding to memory and helping to
  produce thoughts that translate sensation into
  perceptions

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Motor Functions

• Employ peripheral neurons to stimulate effectors
  to respond.
• Somatic Nervous System consciously controls
  skeletal muscles
• Autonomic Nervous System controls involuntary
  effectors such as the heart, smooth muscle in
  blood vessels, and various glands

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Neuron Structure

• A neuron includes
• Cell body
• Axons / Dendrites tubular, cytoplasm-filled
  nerve fibers which conduct nerve impulses to or
  from the cell body
• Organelles mitochondria, lysosomes, golgi
  apparatus, cell membrane, nucleus, nucleolus,
  Nissl bodies (ER) ribosomes, and neurofibrils.

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Dendrites

• Provide receptive surfaces along with cell body.
  Carry impulses to the cell body
• Short and highly branched fibers with which
  fibers from other neurons communicate
• A neuron may have many dendrites.

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Axon

• A single axon arises from the cell body
• May give off side branches.
• Larger axons of the peripheral nervous system are
  enclosed in a myelin sheaths composed of Schwann
  cells.
• Conducts impulses away from the cell body.
• Communicates with receptor surfaces of other cells

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Schwann Cells

• Specialized neuroglial cells that are wrapped
  around the axon like a bandage forming a myelin
  sheath
• Composed mainly of cell membranes of lipid and
  protein (lipoprotein)
• Neurilemma is the portion of the Schwann cell
  that contains most of the cytoplasm and the
  nucleus and is the outer layer of the myelin
  sheath.
• Nodes of Ranvier narrow gaps in the myelin
  sheath between Schwann cells.

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Myelin Development

• Begins to form on nerve fibers during the 14th
  week of prenatal development.
• Many nerve fibers in newborns are not completely
  myelinated. Infants nervous system is unable to
  function as effectively as older children and
  adults.
• Responses to stimuli by infants are coarse and
  undifferentiated and may involve the whole body.
• Fibers develop sheaths by the time a child begins
  walking and development continues into
  adolescence.
• Interference with supply of essential nutrients
  during developmental years may limit myelin
  formation and impair nervous system function
  later in life.

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• White matter groups of myelinated fibers of the
  CNS that appear white.
• Gray matter unmyelinated nerve fibers and neuron
  cell bodies within the CNS

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• Mature neurons of the CNS do not divide.
• Neural stem cells can divide and give rise to
  neurons or neuroglia.
• Found in the hippocampus, ventricles, and in the
  peripheral nervous system
• When peripheral nerves are damaged, their axons
  often regenerate with the help of the neurilemma
  of the Schwann cells. Can also form new synapses.
• CNS axons are myelinated by oligodendrites which
  do not have a neurolemma and do not usually
  regenerate

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• To picture relative sizes of a typical neurons
  parts, imagine that the cell body is the size of
  a tennis ball. The axon would then be a mile long
  and half an inch thick. The dendrites would fill
  a large bedroom.

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Classification of Neurons On the Basis of
Structure
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Classification of Neurons On the Basis of
Function
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Ischemic Cell Change

• Neurons deprived of oxygen irreversibly change
  shape, have shrunken nuclei, and eventually
  disintegrate.
• Ischemia oxygen deficiency resulting from lack
  of blood flow through tissue.
• Hypoxemia Abnormally low blood oxygen
  concentration
• Toxins can block aerobic respiration

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Neuroglial Cells of the CNS

• Astrocytes Commonly found between neurons and
  blood vessels. Provide support, regulate nutrient
  and ion concentrations. Forms scar tissue that
  fills spaces following CNS injury.
• Oligodendrocytes Occur in rows along nerve
  fibers, form myelin within brain and spinal cord
  but do not form neurilemmal sheaths.
• Microglial Cells Scattered throughout CNS.
  Support neurons and phagocytize bacterial cells
  and cellular debris.
• Ependymal Cells Form epithelial-like membrane
  that covers part of the brain and inner linings
  of the brain and spinal cord.
• Schwann cells in the peripheral nervous system,
  form the covering called a myelin sheath around
  axons

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Cell Membrane Potential

• A cell membrane is usually electrically charged
  or polarized.
• Polarization arises from unequal ion distribution
  between sides of the membrane.
• Important in the conduction of muscle and nerve
  impulses.

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Distribution of Ions

• Ion distribution is due to pores and channels in
  the membranes that allow passage of some ions but
  not others.
• Potassium ions pass more easily through cell
  membranes than do sodium ions making potassium
  ions a major contribution to membrane
  polarization. (Calcium ions are even less
  permeable)

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Resting Potential

• A high concentration of sodium ions is on the
  outside of a membrane, and a high concentration
  of potassium ions is on the inside due to active
  transport.
• In a resting cell, more positive ions leave than
  enter, so the outside of the cell membrane
  develops a positive charge while the inside
  develops a negative charge.
• This difference in electrical charge between the
  two regions is called a potential difference.

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Potential Changes

• Stimulation of a membrane affects the membranes
  resting potential.
• When its resting potential becomes more positive
  (or less negative compared to the outside), a
  membrane becomes depolarized.
• Potential changes are subject to summation.
• The amount of change in potential is directly
  proportional to the intensity of stimulation.
  Graded.
• Summation is the additive phenomenon in which
  additional stimulation will further change the
  potential if it arrives before the effects of the
  previous stimulation subsides.
• Achieving threshold potential triggers an action
  potential.

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Action Potential

• At threshold, sodium channels open, and sodium
  ions diffuse inward, depolarizing the membrane.
• About the same time, potassium channels open and
  potassium ions diffuse outward, repolarizing the
  membrane.
• This rapid change in potential is an action
  potential.
• Rapid sequence of depolarization and
  repolarization takes about 1/1000 of a second.
• Many action potentials can occur before active
  transport reestablishes the resting potential.

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Nerve Impulse

• A wave of action potentials
• An action potential in one region stimulates the
  adjacent region and a wave of action potentials
  moves along the fiber as a bioelectric current.

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Impulse Conduction

• Unmyelinated fibers conduct impulses over their
  entire surfaces.
• Myelinated fibers conduct impulses more rapidly.
• The Schwann cells insulate the nerve fiber but
  the Nodes of Ravier interrupt the sheath and
  allow action potentials to occur at the exposed
  areas. The nerve impulse jumps from node to node.

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• Nerves with larger diameters conduct impulses
  faster than those with smaller diameters.
• Thick motor fiber associated with a skeletal
  muscle 120 m/s
• Thin unmylinated sensory fiber in the skin 0.5 m/s

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All-or-none Response

• A nerve impulse is conducted in an all-or-none
  manner whenever a stimulus of threshold intensity
  is applied to a fiber. (like in a muscle fiber
  contraction)
• All the impulses conducted on a fiber are of the
  same strength.
• A greater intensity of stimulation does not
  produce a stronger impulse, but rather more
  impulse per second.

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Local Anesthetic Drugs

• Decrease membrane permeability to sodium ions and
  prevent impulses from passing through affected
  regions.
• Prevent impulses of touch and pain from reaching
  the brain.

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The Synapse

• A junction between two communicating neurons in a
  nerve pathway
• Synaptic cleft the gap between two nerves
• Can be axon to dendrite or axon to cell body

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Synaptic Transmission

• Impulses usually travel from a dendrite to a cell
  body, then along the axon to a synapse.
• Axons have synaptic knobs at their distal ends,
  which secrete neurotransmitters.

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• A neurotransmitter is released when a nerve
  impulse reaches the end of an axon.
• A neurotransmitter reaching the nerve fiber on
  the distal side of the synaptic cleft triggers a
  nerve impulse.

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Excitatory and Inhibitory Actions

• Excitatory neurotransmitters that increase
  postsynaptic permeability to sodium ions and
  trigger nerve impulses.
• Inhibitory neurotransmitters that decrease
  membrane permeability to sodium ions making it
  less likely that threshold will be reached.

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• The net effect of synaptic knobs communicating
  with a neuron depends on which knobs are
  activated from moment to moment and whether or
  not the post-synaptic neurons threshold is
  reached.
• The synaptic knobs of 1000 or more neurons may
  communicate with the dendrites and cell body of a
  single post synaptic neuron.

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Neurotransmitters

• The nervous system produces about 50 different
  neurotransmitters.
• Some neurons release only one type of
  neurotransmitter while others produce 2 or 3
  kinds.
• Neurotransmitters include acetylcholine,
  monoamines, amino acids, and neuropeptides.
• Acetylcholine stimulates skeletal muscle
  contractions. Excitatory.
• Monoamines epinephrine, norepinephrine,
  dopamine, serotonin, and histamine.
• See Table 9.2 p218

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• Usually synthesized in the cytoplasm of the
  synaptic knobs and stored in synaptic vesicles.
• A synaptic knob releases neurotransmitters when
  an action potential increases membrane
  permeability to calcium ions.
• After being released, neurotransmitters are
  decomposed or removed from synaptic clefts by
  being transported back into the synaptic knob and
  recycled.
• Decomposition or removal of neurotransmitters
  prevents continuous stimulation of postsynaptic
  neurons.

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Factors affecting Synaptic Transmission

• Nerve impulses reaching synaptic knobs at too
  rapid a rate can exhaust neurotransmitter
  supplies. Impulse conduction ceases until more
  neurotransmitters are synthesized.
• Epileptic seizures Abnormal and too rapid
  impulses originate from certain brain cells and
  reach skeletal muscle fibers stimulating violent
  contractions until neurotransmitters run out and
  seizure subsides.
• Dilantin drug used to treat seizure disorders by
  increasing the effectiveness of the sodium active
  transport mechanism to stabilize membrane
  thresholds against too intense stimulation

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• Caffeine stimulates nervous system activity by
  lowering the threshold at synapses so that
  neurons are more easily excited.
• Cocaine blocks the reuptake of dopamine and
  serotonin at synapses, enhancing the inhibitory
  effect of these substances on postsynaptic cells.

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Impulse Processing

• How the nervous system responds to nerve impulses
  reflects the organization of neurons in the brain
  and spinal cord

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Neuronal Pools

• Neurons are organized into pools within the
  central nervous system and work to perform a
  common function
• Each pool receives impulses from input fibers,
  processes them, and conducts impulses away on
  output fibers.
• May have excitatory or inhibitory effects on
  other pools or on peripheral effectors.

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Facilitation

• Each neuron in a pool may receive excitatory and
  inhibitory stimuli.
• A neuron is facilitated when it receives
  subthreshold stimuli and becomes more excitable.

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Convergence

• Impulses from two or more incoming fibers may
  converge on a single neuron.
• Convergence enables impulses from different
  sources to have an additive effect on a neuron.

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Divergence

• Impulses leaving a pool may diverge by passing
  into several output fibers.
• Divergence amplifies impulses.