Neurons

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Neurons
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Neurons

• Definition An individual nerve cell
• 100 billion neurons make up the brain
• Neurons link to each other in tight clusters
  and long chains
• Millions of neurons must send messages at the
  same time to produce a single thought.
• Each neuron in the brain is linked to as many
  as 15,000 other neurons.

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• Neurons have 4 basic parts
• Dendrites neuron fibers that receive incoming
  messages
• Soma (cell body) main body of a neuron
• Axon thin fiber that carries information away
  from the cell body of a neuron
• Axon terminals, or terminal buttons branching
  fibers at the end of axons that link to the
  dendrites and somas of other neurons so
  information can be passed from neuron to neuron

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The Nerve Impulse

• Electrically charged chemical molecules, called
  ions, are found in and around neurons
• Some ions are negatively charged, and some are
  positively charged
• The inside of a human neuron has an electrical
  charge of about -70 millivolts
• (Inside the neuron is a negative charge, outside
  the neuron is a positive charge.)
• Resting potential the electrical charge of a
  neuron at rest

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The Nerve Impulse contd

• Messages received from other neurons alter the
  resting potential.
• If the resting potential changes enough, the
  cell reaches its threshold, or trigger point, for
  firing.
• The threshold for human neurons is approx -50
  millivolts
• When a neuron reaches its threshold, an action
  potential, or nerve impulse, moves down the axon
  at up to 200 mph.

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The Action Potential

• The axon membrane is pierced by tiny molecular
  channels called ion channels
• These channels are blocked when the neuron is
  at its resting potential.
• During the action potential, the channels open,
  allowing sodium ions (Na) to rush into the axon,
  beginning at the soma and moving toward the axon
  terminals, briefly creating a positive charge
  within the axon.
• This is an all-or-nothing event the impulse
  occurs completely, or not at all

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Definitions

• Synapse microscopic space between 2 neurons
  over which messages pass
• Neurotransmitters any chemical released by a
  neuron that alters activity in other neurons
• Synaptic vesicles tiny sacs in the axon
  terminal that store neurotransmitters
• Receptor sites tiny areas on the surface of
  neurons and other cells that are sensitive to
  neurotransmitters or hormones

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Definitions contd

• Acetylcholine neurotransmitter released by
  neurons to activate the muscles
• Neuropeptides brain chemicals that regulate the
  activity of neurons
• Enkephalins opiate-like brain chemicals that
  regulate reactions to pain and stress
• Endorphins chemicals that are similar in
  structure and pain-killing effect to opiate drugs
  such as morphine released by the pituitary gland
• Dopamine neurotransmitter involved in the
  control of bodily movements (involved in
  Parkinsons Disease and Alzheimers)
• Serotonin mood control

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Communication Between Neurons

• Communication between neurons is chemical.
• When an action potential reaches the tip of the
  axon terminal, the synaptic vesicles move to the
  surface and release neurotransmitters into the
  synaptic gap
• Neurotransmitter molecules cross the synaptic
  gap and attach to receptor sites on the receiving
  neuron.
• Neurotransmitters either excite or inhibit the
  receiving neuron
• A neuron needs to receive several exciting
  messages at about the same time to fire the
  action potential.

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Receptor sites
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Nerves

• Nerves are large bundles of axons and dendrites
• Nerves have a whitish color b/c they contain
  axons coated with a fatty layer of tissue called
  myelin.
• Neurilemma thin layer of cells that encases
  most axons outside of the brain and spinal cord
• creates a tunnel damaged fibers can follow to
  repair themselves

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