Neurones and neural pathways

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Neurones and neural pathways

• Unit 3
• Behaviour, populations and Environment

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Lesson Aims

• To examine the functions of dendrites, the cell
  body, axons and myelin sheath
• To learn about the post-natal development of
  myelination
• To examine the function of neurotransmitters

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Neurone Structure

• Dendrites receive nerve impulses and pass
  towards cell body
• Cell Body located in CNS. Contain nucleus, most
  of the cytoplasm and ribosomes required for
  protein synthesis
• Axons single fibre which carries impulses away
  from cell body

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Sensory Neurone

• take messages from sensory receptors in the skin
  and the specialised sense organs - the nose,
  tongue and ears, to the brain

Cultured sensory neurone
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Motor Neurone

• has many dendrites protruding from the cell body
  which is end-on to the axon
• Axon can be very long

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Myelinated Axon

• Fatty material
• Insulating jacket around nerve fibre
• Increases transmission rate of nerve impulses

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Myelination

• Development takes about 2 years after birth to
  complete
• Hypothalamus in infants not fully myelinated so
  cannot control temperature
• In Multiple Sclerosis (MS), there is patchy loss
  of myelin and this leads to delayed or blocked
  conduction of nervous impulses.

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Synapse

• Gap between an axon ending of one neurone
  (pre-synaptic) and dendrite or cell body of the
  next neurone (post-synaptic)
• Act like switches and may allow or prevent
  signals passing from one neurone to the next
• Many mitochondria provide the energy for active
  transport which helps maintain electrical
  potential and reabsorb neurotransmitters.

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Neurotransmitters

• Protein messengers which "jump across synapse
  between one neurone and the next to communicate
  impulses to the next neurone.
• An impulse arriving at a synaptic terminal
  stimulates the synaptic vesicles to secrete their
  contents into the synapse.
• If the number of excitatory signals exceeds the
  number of inhibitory signals, the neurone will
  "fire", carrying an impulse to the next synapse,
  but this only occurs if a certain threshold is
  reached

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Acetylcholine

• Neurotransmitter
• Acetylcholine from a parasympathetic nerve in the
  heart will inhibit heart rate and volume.
  However, it promotes peristalsis in the
  alimentary canal.
• Some acetylcholine receptors also respond to
  nicotine from cigarettes, often leading to
  tobacco addiction in those who smoke. They can
  also be blocked temporarily by anaesthetics
  called muscle blockers (relaxants)
• To avoid repeated contractions from the same
  stimulus, acetylcholine is immediately destroyed
  by an enzyme called acetylcholinesterase

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Noradrenaline

• Neurotransmitter
• normally produces effects such as increased heart
  rate, increased blood pressure, dilation of
  pupils, dilation of air passages in the lungs and
  narrowing of blood vessels in non-essential
  organs. This enables the body to perform well in
  stressful situations.
• Is removed by being reabsorbed back into the
  pre-synaptic ending.

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Converging pathway
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Diverging pathway
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The Facts You Need to Know

• p17
• from nerves are bundles of nerve fibres to
• the same transmitter molecule .