Nervous coordination

• The myelin sheath provides electrical insulation made of lipids.
• nodes of ranvier enable exchange of ions with extracellular fluid.
• Schwann cells produce the myelin sheath by wrapping around the axon many times.
• the membrane has a resting potential of -70mV and is said to be polarised.
• in a resting neurone there are more positive ions on the outside of the axon than the inside.

  • so the inside of the axon is negatively charged compared to the inside.
• maintaining the resting potential:

  1. the sodium-potassium pump actively transports 3 sodium ions out of the cell and 2 potassium ions into the cell.
  2. the sodium-potassium leakage channels are open in a resting membrane.
  3. there are about 100x more potassium channels tha n sodium channels so the membrane is more permeable to potassium.
  4. this maintains a resting potential of -70mV.
• the membrane of a neurone contains voltage gated sodium ion channels and voltage gated potassium ion channels.

  • in resting these gates are closed.
• depolarisation:

  1. when a stimulus reaches a resting neurone the sodium gated channels open & Na+ flow into the neurone.
  2. if enough Na+ move in it causes more voltage gated Na+ channels to open.
  3. the Na+ diffuse along an electrochemical gradient.
  4. depolarisation occurs due to a reduction in potential difference.
  5. this causes the inside of the axon to become positively charged at +40mV.
• repolarisation:

  1. once the inside of the axon is positively charged the Na+ gated channels close and the K+ gated channels open.
  2. K+ diffuse out of the axon along an electrochemical gradient.
  3. the membrane is repolarisd and returns to its resting potential of -70mV.
• hyperpolarisation:

  1. the K+ channels stay open slightly too long after repolarisation.
  2. causing a breif period where the potential is more negative than the resting potential.
  3. a breif refractory period occurs after an action potential.
  4. during this the membrane cannot conduct another action potential.
  5. this is because the sodium-potassium pump needs to restore the balance of ions for resting potential.
• what is the importance of the refractory period?

  • ensures action potentials are unidirectioinal.
  • ensures action potentials are seperate from each other.
  • limits the number of action potentials.
• synapses are junctions betwen neurones which use a neurotrasmitter to transmit information across a synaptic cleft from one neurone to the next.
• Chloinergic synapses use acetylcholine (ACh) as a neurotransmitter
• Action potential across a synapse

  1. Action potential arrives at the pre-synaptic bulb causing Na+ channels open & depolarise the axon terminal

  2. Voltage gated Ca2+ channels open & allow Ca2+ to enter the synaptic bulb

  3. Ca2+ causes vesicles containing acetylcholine to fuse with the pre-synaptic membrane & release ACh across the synaptic cleft

  4. ACh binds to receptors on the post-synaptic membrane causing chemical gated Na+ channels to open so Na+ enter & depolarise the post-synaptic membrane

  5. If the excitatory potential exceeds a threshold, an action potential is generated

  6. Acetylcholine is broken down by acetylcholinesterase & leaves the receptor

  7. The components of ACh diffuse back to the pre-synaptic cell & are used to reform acetylcholine using ATP
• how is the pre-synaptic bulb adapted to its function?

  • many mitochondria - to supply ATP for ACh reformation.
  • many ACh vesicles - to be able to trigger an action potential.
  • many SER - to produce lipids as part of vesicles for fusion with the membrane.
  • voltage gated Ca2+ channels - so Ca2+ can enter and cause fusion of ACh vesicles.
• synapses ensure nerve impulses are unidirectional because the receptor for acetylchloine are only on the post synaptic cell & vesicles containing ACh are only in the pre synaptic neurone.
• a neurone that synapses to multiple post-synaptic neurones is known as divergense.
• when many pre-synaptic neurones synapse with one post-synaptic neurone its known as convergense.
• what are the types of summation?

  • temporal
  • spatial
• temporal summation is when several potentials are received in a row, generating high enough to exceed a threshold and cause an action potential
• spatial summation is when several presynaptic neurones contribute to exceeding a threshold and causing an action potential.
• synapses can be excitatory or inhibitory
• cholinergic synapses (use ACh) or adrenergic synapses (use noradrenaline) are excitatory synapses.
• inhibitory synapses:

  • may act by opening Cl- channels allowing Cl- to diffuse into the post-synaptic membrane.
  • leading to hyperpolarisation (-90mV)
  • this ensures muscles work in antagonistic pairs (one at a time)