Grey Matter

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3 basic types of neurones 

SENSORY neurone 

  • Transmit impulses from receptor --> CNS

RELAY neurone

  • major component of the integration centre
  • connects sensory to motor

MOTOR neurone

  • Transmit impulses from CNS --> effectors
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Saltatory conduction

Schwann cells - Protect the nerves

      - Speed up transmission of nerve impulses

Saltatory conduction = Jumping of action potential from one  node of Ranvier to the next.

  • Myelin sheath act as an electrical insulator
  • Depolarisation only occers at the nodes of Ranvier
  • Action potential jumps from node to node
  • Increasing the conduction velocity

Speed of transmission influenced by

  • Diameter of axon (increase diameter,increase speed)
  • Myelin sheath
  • Temperature(increase temp,increase speed)
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Resting Potential

Resting potential = Difference in charge which exists between the inside and outside of the cell membrane when neurone is NOT transmitting an impulse

Maintained by 

  • Sodium-potassium pumps - 3 sodium out,2 potassium in
  • Axon membrane more permeable to pottasium ions (because more potassium channels)
  • More potassium can leak out back again faster tha sodium can diffuse in
  • In more -ve ----> polarised

All or nothing law :

  • Action potential is fixed in size
  • action potential produced in full as result of stimulus greater than threshold or not at all.
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Action Potential

  • Sodium gates open
  • Sodium ions diffuse into cytplasm down concentration gradient
  • Membrane depolarises
  • Threshold level reached --> action potential generated
  • once reached +40mC,sodium gates close ---> potassium gates open
  • Potassium diffuse out ---> repolarisation
  • Potassium gates slow to close so excess potassium leave the axon
  • Making inside more -ve ----> hyperpolarisation
  • Sodium gates close --> sodium-potassium pump restarts

How resting potential restored?

  • Potassium gates close
  • Potassium ions diffuse back into axon
  • Sodium potassium pump restarts
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  • Action potential arrives
  • Causing calcium channels to open ---> Ca ions enter the presynaptic neurone
  • Influx of calcium ions causes the synaptic vessicle containing neurotransmitter  to move to presynaptic membrane and fuse with the membrane.
  • Neurotransmitters released into synaptic cleft
  • By exocytosis
  • Neurotransmitter diffuse across the synaptic cleft.
  • Bind to receptors on postsynaptic membrane
  • Na channels open --->Na ions diffuse into postsynaptic neurone
  • Depolarisation of postsynaptic membrane
  • Threshold level reached --->action potential generated

P.s. Enzymes break down the neurotransmitters

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Summation - Simultaneous release of neurotransmitter from synapses of several neurones to cause enough depolarisation to generate action potential

  • Temporal - Different neurones arrive at one time
  • Spatial - One neurones in quick succession


  • Too many action potentials in rapid succession
  • A short rest to restore more vessicles


  • Constant stimulation of the receptor cell -->gradual decline in response
  • Concentrate on more important,new stimuli

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Rod Cells

In the dark

  • Sodium pumo establish ion gradient
  • High conc of Na+ outside rod cells
  • Na+ channels in the membrane of outer segment opens
  • Na+ constantly diffuse in
  • But move out again by sodium pump
  • Resting potential produced
  • Inhibitory neurotransmitter released
  • No depolarisation --> no action potential generated

In the light

  • Light energy absorbed by rhodopsin
  • Cis-retinal change to trans-retinal (bleaching)
  • Can no longer bind to opsin
  • Bleaching cause Na+ channel to close -->less Na+ ions diffuse in
  • Causing hyperpolarisation
  • No inhibitory neurotransmitter released
  • Bipolar cells depolarised
  • Action potential formed
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Iris reflex

  • Light falling on the sensory cells of the retina causes impulse to travel along neurones in the optic nerve to the brain
  • Impulses detected in control centre in the midbrain
  • Synapse with branches of parasympathetic cranial nerve (oculomotor)
  • Which transmit impulses to the iris
  • Which then stimulate the muscles in the iris
  • Circular muscles contract,radial muscles relax
  • So pupul constricts

If the converse way..

-sympathetic nerve

-circular relax,radial contract

-pupil widen

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