Chapter 10 Nerve Tissue

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Chapter 10 Nerve Tissue
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• The fxn of the Nervous Sx. is communication and
  control. All other systems in the body are
  subject to its directions and exist in support
  of its continued function.
• There are two types of cells in the nervous
  system
• Neurons cells that conduct nerve impulses (aka
  action potentials)
• Neuroglia these cells function in support of
  the neurons, making them capable of doing their
  job

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• A neuron has several parts. Each will have a
  cell body (soma), dendrites (cell processes that
  receive information), and an axon (a cell process
  that sends information).
• Neurons send messages to each other across small
  spaces known as synapses (similar to synapses at
  NMJs). They send neurotransmitters across the
  synapse to conduct nerve impulses and communicate
  with the next neuron in line.

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• The nervous system has two basic divisions
• Central Nervous System consists of the brain
  and spinal cord.
• Peripheral Nervous System all other nerve
  tissue in the rest of the body comprises this
  sub-system and these are connected to the above
  sub-system.

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• Neurons function in different ways
• Sensory send information to the CNS
• Motor send information to the PNS
• Sensory receptors pick up info and send it up to
  the CNS via sensory nerves
• Effectors receive info from the CNS and put it
  into action.

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• Neurons can be classified in the following ways
• Structural based on physical make up
• Bipolar has only two basic cellular processes
• Unipolar has only one basic cellular process
• Multipolar has many cellular processes
• Functional based on direction of nerve flow
• Sensory send info to the CNS from the PNS
• Interneurons perform the job of interpretation
  of info, and decide what to do with the info
• Motor Send info to the PNS from the CNS

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• There are 5 different types of neuroglia
• Astrocytes these are the most numerous of all
  neuroglia. These small, star shaped cells hold
  the neurons in their spatial relationships with
  each other. They also play a role in the
  Blood/Brain Barrier.
• Schwann Cells/Oligiodendrocytes these cells
  produce myelin. They are called Schwann cells in
  the PNS and Oligiodendrocytes in the CNS
• Microglia these cells are macrophages and act
  to clean up cellular debris as well as protect
  against invaders.
• Ependymal Cells these cells secrete
  cerebrospinal fluid (CSF). They line the hollow
  spaces in the brain (ventricles).
• Satellite Cells exist only in the PNS and
  support clusters of nerve cell bodies called
  ganglia.

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• Regeneration of Nerve Axons nerves in the PNS
  are capable of regenerating their axons such that
  old connections may be re-established after an
  injury where an axon was severed. This type of
  regeneration is unlikely in the CNS due to the
  fact that oligodendrocytes do not proliferate
  following an injury.

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• The Synapse
• Nerve impulses pass from one another across the
  synapse. A presynaptic neuron is the neuron that
  brings the impulse to the synapse, releases the
  neurotransmitter or electrical signal and
  ultimately stimulates the postsynaptic neuron.
  Exocytosis is used to cause neurotransmitters to
  be released from synaptic vesicles in the
  synaptic knob. The neurotransmitter then
  diffuses across the synapse to interact with the
  dendrites of the postsynaptic neuron.

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• Membrane Potential
• A cell membrane is usually said to be charged or
  polarized. This means that a charge exists at
  the cell membrane due to charged particles on
  either side of the phospholipid bilayer. This
  charge is referred to as Membrane Potential.
• The interior of the cell is charged negatively
  with respect to the outside.

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• Sodium is the chief extracellular cation,
  Potassium is the chief intracellular cation. The
  chief intracellular anion is Chloride. The
  action of the Sodium/Potassium Pump is largely
  responsible for maintaining the correct
  proportion of cations to anions.
• The Resting Membrane Potential of a neuron is -70
  mV.

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• A local potential change is a situation where a
  small portion of the cell membrane is subjected
  to a reversal of the charge inside the cell due
  to an incoming stimulus. A small portion of the
  interior of the cell becomes depolarized (more
  positive than previously stated). This
  depolarization quickly returns to its standard
  polarized state quickly, without affecting the
  entire cell membrane of the axon.

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• In order for an action potential to take place,
  the depolarization of the membrane must reach
  Threshold Stimulus.
• Threshold Stimulus is generally at around -55 mV
  in the average neuron. At the peak of the action
  potential the charge inside the cell reaches
  about 25-30 mV.
• Nerve impulses follow the All or None Principle.
  In an action potential, neurons will fire an
  action potential completely or not at all.

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• Refractory Periods
• In Absolute Refractory Period no further action
  potential is possible, no matter how strong the
  stimulus. The inner charge on the cell is far
  too depolarized to allow for any further net
  change in charge.
• Relative Refractory Period is a time when a
  neuron will produce another action potential only
  if a stimulus is strong enough to force the
  already depolarized neuron into a greater state
  of depolarization. This can only happen if the
  stimulus in question is stronger than the
  original stimulus that started the current action
  potential.

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• Saltatory Conduction is a situation where the
  transmission of neural impulse appears to jump
  from one Node of Ranvier to the next node in
  sequence.
• The diameter of a fiber also plays a part in the
  conduction of signal, with larger fibers carrying
  signal faster due to decreased resistance on part
  of the neuron.

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• Neurotransmitters
• There are at least thirty different kinds of
  neurotransmitters that have been identified as
  such. The best studied of these is Ach.
• Recall that neurotransmitters are released from
  synaptic vesicles in the synaptic knobs of axons
  via exocytosis. The synaptic vesicles, once they
  release their product become one with the cell
  membrane as they are constructed of the same
  phospholipid bilayer as the cell membrane.

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• Neurotransmitters are either broken down or
  inactivated by enzymes at the site of binding on
  the postsynaptic neuron. This is to prevent
  continuous stimulation of the postsynaptic
  neuron. It also allows for the recycling of the
  neurotransmitter in question.
• Acetylcholinesterase is an enzyme that breaks
  acetylchonine down into an acetyl group and a
  choline group. The individual two parts of the
  original neurotransmitter are then transported
  back into they synaptic knob where they are
  reformed into acetylcholine and reused as a
  neurotransmitter again.

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• Other neurotransmitters include
• Endorphins and Enkephalins these
  neurotransmitters are known to have an
  opiate-like effect on the body and act as pain
  killers. Endorphins are thought to have a longer
  lasting effect than enkephalins. These
  substances are considered neuropeptides.
• Dopamine and Seratonin are well known
  neurotransmitters that are associated with
  depression like symptoms.

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• Impulse Processing
• Convergence vs. Divergence
• Convergence is exhibited when a single neuron or
  neuron pool receives input or stimulus from one
  or more neurons. Convergence makes summation of
  separate signals possible. Thus sub-threshold
  signal can be added together ultimately produce a
  signal from several sources.
• Divergence is exhibited when one neuron
  stimulates more than one neuron with its signal
  and those in turn do the same. In this way many
  neurons may be stimulated by one single signal.