Nerves

• Resting Potential

  • -70 mv in the cytoplasm of the neurone
  • Cytoplasm contains large protein anions and organic phosphates such as ATP4- which have negative charge.
  • Na+/K+ pump = pumps out of the cell 3Na+ and pumps in 2K+
  • High concentration of Na+ on outside of the cell
  • The membrane is impermeable to Na+ (Na+ gated channel is closed)
  • K+ can diffuse out of the cell through the K+ leakage channel
  • The membrane is more permeable to K+ (than Na+)
• Depolarisation

  • Stimulus causes Na+ gated channel to open
  • Na+ rapidly diffuse into the cell/neurone
  • If enough Na+ diffuse in due to the stimulus the threshold potential is reached
  • Na+ diffuse into the cytoplasm from high to low concentration increasing the charge from -70mv to -55mv
  • The change in charge causes Na+ voltage gated channels to open, therefore more Na+ can rapidly diffuse into the cell
  • Inside cell = +40mv outside cell = -40mv
  • Membrane has depolarised and it is self perpetuating
• Repolarisation

  • At +40mv the Na+ voltage gated channel closes
  • The K+ voltage gated channels open - membrane becomes very permeable to K+
  • K+ rapidly diffuse out of the cell
  • membrane repolarises
  • If too many K+ diffuse out of the cell, the membrane hyperpolarises to -90mv
  • To restore the resting potential, at -90mv the k+ voltage gated channels close, Na+/K+ pump restores resting potential
• Hydra


  • Demonstrates radial symmetry
  • Does not contain CNS
  • Contain many ganglion cells - provide connections in many directions
• Nerve Nets

  • Axons are non-myelinated
  • Slow conduction speed - 5 m/s
  • Made of only one type of nerve cell with short extensions joined to each other and branching into many different directions
  • Sense receptors respond to a limited number of stimuli
  • Nerve cells allow it to sense light, chemicals and physical contact allowing to sense its environment and act appropriately.
• Neurone structure


  Structure of a neurone

  A) Dendrites

  B) Nucleus

  C) Cell body

  D) Axon

  E) Node of Ranvier

  F) Schwann Cells

  G) Axon terminal

  H) Myelin Sheath

  
• Structure of Spinal chord

  Inner = Pia matter

  Middle = Arachnoid matter

  Outer = Dura matter

  A) White matter

  B) Grey matter

  C) Dorsal root

  D) Dorsal root ganglion

  E) Ventral root

  F) Central canal

  
• Synapses - Part 1

  • Action potential arrives at the axon terminal
  • Change in axon terminal membrane permeability to calcium ions - calcium voltage gated channels open
  • Calcium ions more concentrated on the outside due to calcium ion pump, therefore they rapidly diffuse in
  • Calcium ions cause synaptic ventricles to move toward presynaptic membrane
  • Synaptic ventricles fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft - exocytosis
  • Acetylcholine molecules diffuse across the cleft to the post synaptic membrane
• Synapses - Part 2

  • Acetylcholine molecules bind to chemically-gated sodium ion protein channels causing them to open
  • Sodium ions diffuse into dendrites of postsynaptic membrane
  • Threshold potential reached so more sodium ion voltage-gated channels open causing more sodium ions to diffuse in
  • Action potential reached in postsynaptic membrane - depolarisation
• Resetting the Synapse

  • Cholinesterase brake down acetylcholine in protein receptor to give choline and acetate
  • Sodium ion channels in the post synaptic membrane close
  • Choline and acetate diffuse back to the presynaptic membrane
  • acetyl co-enzyme A used to re-synthesise acetylcholine in synaptic knob
  • Energy from mitochondria used to repackage acetylcholine into vesicles - ATP needed
• Oscilloscope Trace


  1. Resting potential = polarised = -70mv
  2. Threshold potential = -55mv = all or nothing
  3. Depolarisation = up to +40mv
  4. Repolarisation = reset the membrane = from +40mv to -70mv
  5. Hyperpolarisation = go below resting potential = -90mv