The Peripheral nervous system

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The Peripheral nervous system

• Anatomy Physiology/Nervous System

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Neurons

• Nerve cells are called neurons
• These cells are specialized to transmit
  electrical impulses from one part of the body to
  another.
• Although they can vary in structure and
  appearance, all neurons have the same basic
  features

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Diagram of a Neuron
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Nerve impulses

• Electrical signals move through the body along
  pathways of neurons that branch out from the
  brain and spinal cord.
• These pathways form complex networks that allow
  the brain to communicate almost instantly with
  any part of the body.

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Nerve Fibers

• Bundles of nerve fibers in the body are called
  nerves, while in the brain they are called
  tracts
• The corpus callosum is an example of a tract.
• In the brain, the white matter is made of
  myelinated tracts, while the grey matter is made
  of un-myelinated tracts

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Nerve Classification

• There are many types of nerves in the body and
  two main ways to group or classify them
• 1. Functional Classification grouping neurons
  based on the direction the impulse travels
• 2. Structural Classification grouping neurons
  based on the number of parts connected to the
  cell body

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Functional Groups

• There are three main functional groups of
  neurons
• Sensory (Afferent) Neurons neurons that carry
  information from receptors in the skin or organs
  towards the brain.
• Motor (Efferent) neurons that carry information
  away from the brain and to the muscles or glands.
• Association (Inter) Neurons neurons that
  connect motor and sensory neurons on long neural
  pathways.

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Structural Groups

• There are three main structural groups of
  neurons
• Multipolar neurons neurons with an axon and
  several dendrites attached to the cell body.
• These are the most common neurons
• Bipolar neurons neurons with one axon and one
  dendrite attached to the cell body.
• Unipolar neurons neurons with a short stem
  attached to the cell body.
• Generally an axon and several dendrites will
  extend out from that stem.

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Generating Impulses

• Many different types of stimuli excite neurons to
  become active and generate an impulse or
  electrical signal
• Light excites neurons in the eyes
• Sound excites neurons in the ears
• Pressure excites neurons in parts of the skin
• Most neurons in the body are excited by chemicals
  called neurotransmitters.

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neurotransmitters

• Neurotransmitters chemicals that are stored in
  the axon terminals of association neurons and
  used to transmit nerve impulses from one neuron
  to another.
• Some examples of common neurotransmitters
  include
• Dopamine
• Seratonin
• GABA
• Adrenaline

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polarization

• Regardless of how the neuron is stimulated, the
  result in always the same.
• A resting (non-stimulated) neuron is polarized,
  which means there are more positively charged
  ions outside the neuron membrane than there are
  inside.

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depolarization

• Normally, sodium (Na) ions cannot move back and
  forth freely across the neuron membrane.
• When the neuron becomes stimulated (by a
  neurotransmitter or any other stimulus), the
  sodium channels in the membrane open.
• As the channels open, sodium rushes in (following
  the rules of diffusion) and the charges even out,
  the neuron is said to be depolarized.

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Action potential

• Depolarization of the neuron generates a long
  distance electrical impulse called an action
  potential.
• This impulse is an all or nothing response,
  meaning that it either happens or it doesnt.
• An action potential never goes part way along a
  neuron or weakens with distance.

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Repolarization

• Almost immediately after the rush of sodium ions
  into the cell, the membrane again becomes
  impermeable to sodium ions.
• Sodium-Potassium pumps in the neuron membrane are
  activated and the original concentrations of
  sodium ions are restored.
• Until the neuron becomes repolarized it cannot
  conduct another impulse.

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Crossing the synapse

• When an action potential reaches the end of a
  neuron, it does not continue across the synapse,
  or space between neurons.
• Instead, the action potential triggers vesicles
  filled with neurotransmitters to bind to the
  neuron membrane and release the neurotransmitter
  into the synapse.
• The neurotransmitters cross the synapse and bind
  to specific receptors on the receiving neuron.
• This stimulates a new action potential.

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Graphing an action potential