The Nervous System

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The Nervous System
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The Nervous System

• Feeling, Thinking, Remembering, Moving, Being
  aware of the world,
• Coordination of body functions to maintain
  homeostasis,
• Response to changing conditions
• All require activity from the nervous system

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Nervous Tissue

• Masses of nerve cells (neurons)
• Neurons are the structural and functional unit of
  the nervous system
• Neurons are specialized to react to physical and
  chemical changes in their surroundings
• Neurons transmit information in the form of
  electrochemical changes (nerve impulses)
• Supporting cells neuroglial cells
• Functions as connective tissue of the nervous
  system

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Neurons

• Cell body contains the nucleus
• Dendrites receive electrochemical messages
• Axons send electrochemical messages
• Nerves bundles of axons

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Neuroglial Cells

• Fill spaces
• Provide structural frameworks
• Produce myelin
• Carry on phagocytosis
• In the CNS, neuroglial cells outnumber neurons

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Function and Structure

• Central Nervous System (CNS)
• Brain and spinal cord
• Peripheral Nervous System (PNS)
• Nerves that connect the central nervous system to
  other body parts
• The two divisions of the nervous system provide
  three functions
• Sensory
• Integrative
• Motor

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Sensory Function

• Sensory function derives from sensory receptors
  at the ends of peripheral neurons
• Receptors gather information by detecting changes
  inside and outside of the body
• Sensory receptors convert environmental
  information into nerve impulses which are
  transmitted over peripheral nerves to the central
  nervous system

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Integrative Function

• Signals from peripheral nerves are brought
  together
• Create sensations, add to memory, or help produce
  thoughts that translate sensations into
  perceptions
• As a result, we make conscious or subconscious
  decisions and then act on them

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Motor Function

• How we act on decisions
• Uses peripheral neurons which carry impulses from
  the CNS to effectors
• Effectors are outside of the nervous system
• Muscles
• glands
• Motor function can be divided into two categories
• Somatic Nervous System (Conscious Control)
• Skeletal muscle
• Autonomic Nervous System (Involuntary Control)
• Heart, smooth muscle, glands

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Homeostasis

• How does the nervous system help maintain
  homeostasis?

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Neuroglial Cells

• Important part of nervous tissue
• Fill in spaces, provide structural frameworks,
  produce myelin, and carry out pahgocytosis
• Greatly outnumber neurons

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Types of Neuroglial Cells

• Microglial Cells support neurons and
  phagocytize bacterial cells and cellular debris
• Oligodendrocytes occur in rows along nerve
  fibers and provide myelin around axons of CNS
  neurons
• Astrocytes provide structural support, help
  regulate concentrations of nutrients and ions,
  form scar tissue after injury to CNS
• Ependymal Cells form epithelial-like membrane
  that covers parts of brain and spinal cord
• Schwann Cells found in PNS forms myelin sheath
  around axons

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Neurons

• Vary in size and shape
• Terminology specific to neurons
• Neurofibrils microtubules and microfilaments of
  cytoskeleton
• Chromatophillic substance similar to
  endoplasmic reticulum
• Myelin composed of Schwann cells wrapped around
  the axon, layers have a high proportion of
  lipid, forms the myelin sheath
• Neurilemma outside portion of the Schwann cells
• Node of Ranvier narrow gap in the myelin sheath
  between Schwann cells

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Node of Ranvier
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About Myelin

• Myelin is an electrically insulating phospholipid
  layer that surrounds the axons of many neurons.
  It is an outgrowth of glial cells Schwann cells
  supply the myelin for peripheral neurons while
  oligodendrocytes supply it to those of the CNS.
• The main function of a myelin layer (or sheath)
  is to increase in the speed at which impulses
  travel along the fiber. Along unmyelinated
  fibers, impulses move continuously as waves, but,
  in myelinated fibers, they hop. Myelination also
  helps prevent the electrical current from leaving
  the axon.

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Note diameter and myelin sheath differences
between the axons in this picture.  Somatic motor
and "external" sensory neurons would be larger
diameter neurons autonomic motor and visceral
sensory would be smaller diameter neurons.
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Neuron Structure Cont.

• Axons with myelin sheaths are called myelinated
  and those without are called unmyelinated
• In the CNS, groups of myelinated axons appear
  white White Matter
• Unmyelinated axons and cell bodies appear gray
  Gray Matter

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Myelin Development

• Read Blue Box on pg 209
• What is the effect of proper nutrition on myelin
  formation in infants and children?
• Extra credit opportunity!
• Research the connection between the addition of
  DHA and ARA in infant formula
• or
• the importance of Omega-3 fatty acids in myelin
  production throughout a persons lifetime. What
  are some sources of Omega-3 fatty acids?

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Injury to Neurons

• Peripheral nerves when damaged axons often
  regenerate, aided by the Schwann cells
  (neurilemma)
• CNS nerves do not usually regenerate, axons are
  myelinated by oligodentrocytes which do not
  provide a neurilemma

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Neuron Regeneration

• Read Blue Box pg 210
• What is the significance of this finding for
  people who have sustained injury to the CNS?
• Do you think there is a way to use this finding
  to benefit those who have suffered from brain
  injury?

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Classification of Neurons

• Neurons differ in structure, size, shape, and
  function

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Classification of Neurons - Structure

• Bipolar Neurons
• Only two processes, (one on each end) one is the
  axon and the other is the dendrite
• found in eyes, nose, and ears
• Unipolar Neurons
• Single process which divides into two branches
• One branch is associated with dendricites near
  peripheral body part (peripheral process), the
  other branch (central process) enters the brain
  or spinal cord
• Masses of these cell bodies specialize into
  ganglia
• Multipolar Neurons
• Have many processes arising from cell body, only
  one process is the axon, the rest are dendrites
• Most neurons in brain and spinal cord are
  multipolar

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Classification Of Neurons - Function

• Sensory Neurons (Afferent neurons)
• Carry nerve impulses from peripheral body parts
  to the brain and spinal cord
• Most sensory neurons are unipolar
• Interneurons
• Found only in the brain and spinal cord
• Usually multipolar
• Transmit impulses from one part of the brain to
  another
• Motor Neurons (Efferent neurons)
• Carry nerve impulses out of the brain or spinal
  cord to effectors (muscles and glands)
• Usually multipolar

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Cell Membrane Potential

• The surface of a cell membrane is usually
  electrically charged (polarized) from unequal
  distribution of positive and negative ions
  between the two sides of the membrane
• This characteristic becomes important for
  conduction of nerve impulses.

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Distribution of Ions

• Remember
• Cell membranes contain pores (carrier and channel
  proteins)
• Some channels are always open, and others need to
  be opened or closed using energy (ATP)
• This makes cell membranes selective about what
  passes through the membrane

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Ion Distribution Cont.

• Potassium ions pass through cell membrane much
  easier than sodium ions
• This makes potassium ions important part of
  membrane polarization

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Resting Potential

• Sodium and potassium ions follow the laws of
  diffusion net movement of ions from high
  concentration to low concentration
• Resting cell membrane is more permeable to
  potassium ions than sodium ions, results in
  potassium diffusing out of cell quicker than
  sodium.
• Causes outside of cell to have more positive
  charge than inside of cell
• The cell expends energy to drive the Na/K pump
  that actively transports sodium and potassium
  ions in the opposite direction to maintain
  concentration gradient of Na and K

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What factors contribute to this membrane
potential?
                                            Sou
rce http//ifcsun1.ifisiol.unam.mx/Brain/mempot.h
tm The nerve cell membrane also contains special
passageways for these two ions that are
Two ions are responsible sodium (Na) and
potassium (K). An unequal distribution of these
two ions occurs on the two sides of a nerve cell
membrane because carriers actively transport
these two ions sodium from the inside to the
outside and potassium from the outside to the
inside. AS A RESULT of this active transport
mechanism (commonly referred to as the SODIUM -
POTASSIUM PUMP), there is a higher concentration
of sodium on the outside than the inside and a
higher concentration of potassium on the inside
than the outside.
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Resting Potential Cont.

• Potential difference - The difference in
  electrical charge between two regions
• In nerve cells the potential difference is called
  the Resting Potential
• The cell remains in this polarized state until
  the nerve cell is disturbed.

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Potential Changes

• Nerve cells are excitable they can respond to
  changes in their surroundings
• If the membranes resting potential decreases
  (inside becomes less negative than outside) we
  call it depolarization
• Changes in the potential is directly proportional
  to the intensity of stimulation
• If additional stimulation arrives before the
  previous stimulation subsides, the change in
  potential is still greater
• Summation additive phenomenon stimulation
• Reach threshold potential
• Causes an action potential

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Action Potential

• At threshold potential, permeability suddenly
  changes in the region being stimulated
• Channels for sodium ions open and allow sodium to
  diffuse inward
• As this happens, membrane loses negative charge
  and becomes depolarized
• At same time, potassium channels open and allow
  potassium ions () to pass outside of cell
  membrane, causing membrane to become negatively
  charged again (repolarization)

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Resting Potentials and Action Potentials

• Resting Potential refer to fig 9.8 pg 214
• Action Potential refer to fig 9.9 pg 215

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Nerve Impulse

• When an action potential occurs in one region of
  a nerve cell membrane, it causes a bioelectric
  current to flow to adjacent portion of the
  membrane
• Action potential in one region stimulates the
  adjacent region, and so on
• A wave of action potentials moves down the axon
  to the end
• Nerve impulse propagation of action potentials
  along a nerve axon
• Refer to fig 9.10 pg 215

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Events Leading to the Conduction of a Nerve
Impulse

• Neuron membrane maintains resting potential
• Threshold stimulus is received
• Sodium channels in a local region of the membrane
  open
• Sodium ions diffuse inward, depolarizing the
  membrane
• Potassium channels in the membrane open
• Potassium ions diffuse outward, repolarizing the
  membrane
• The resulting action potential causes a local
  bioelectric current that stimulates adjacent
  portion of the membrane
• Wave of action potentials travels the length of
  the axon as a nerve impulse

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• Like muscle fiber contraction, nerve impulse
  conduction is an all-or-none response
• If a neuron responds at all, it responds
  completely

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The Synapse

• Synapse junction between two communicating
  neurons
• Synaptic cleft gap between two neurons
• The impulse must have a way to cross the gap.

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The Synapse Cont.

• When a nerve impulse reaches the synaptic knob at
  the end of an axon, synaptic vesicles release a
  neurotransmitter that diffuses across the
  synaptic cleft. (just like we learned about in
  the muscular system)
• Read Topic of Interest (pg 218) Factors Affecting
  Synaptic Transmission