Structure and function of synapses

• The synapse is the place where the axon of one neurone connects with the dendrite of another or with an effector
• Passing of an impulse across a cholinergic synapse

  1. Impulses causes calcium ion channels to open and calcium ions to diffuse into the synaptic knob
  2. Vesicles move towards and fuse with the presynaptic membrane
  3. Acetylcholine is released via exocytosis and diffuses across the synaptic cleft
  4. Acetylcholine binds with receptors on sodium ion channels in the postsynaptic membrane
  5. Sodium ion channels open and sodium ions diffuse into the postsynaptic neurone
  6. The postsynaptic membrane is depolarised and if this is above the threshold value, an action potential is produced
• Calcium ions are removed from the synaptic knob by active transport, ensuring the concentration of calcium ions is always higher outside the membrane
• Acetylcholine is removed from the receptors on the voltage-gated sodium ion channels by the enzyme acetylcholinesterase. This enzyme catalyses the hydrolysis of acetylcholine into acetyl and choline which diffuses back into the synaptic knob
• The acetyl and choline are combined to synthesise acetylcholine so that it can be reused in the vesicles
• Summation causes a build-up of neurotransmitters in the synapse by either spatial summation or temporal summation
• Temporal summation is when a single presynaptic neurone releases neurotransmitters many times in a short period of time
• Spatial summation is when several presynaptic neurones together release enough neurotransmitter to exceed the threshold value
• Summation avoids the nervous system being overloaded
• Inhibition

  • some synapses make it less likely that a new action potential will be created on the postsynaptic neurone
  • they prevent action potentials forming by using another neurotransmitter/molecule
• Inhibition Example 1

  • Presynaptic neurone releases a type of neurotransmitter that causes the opening of potassium ion protein channels on the postsynaptic neurone
  • The opening of potassium channels increases the diffusion of potassium ions out of the postsynaptic cell body
  • There is a greater potential difference across the membrane because the inside is more negative
  • The sodium ions diffusing in are not enough to reach the threshold value
• Inhibition Example 2

  • Presynaptic neurone releases a type of neurotransmitter that binds to chloride ion protein channels on the postsynaptic neurone
  • This opens the chloride ion channels which increases the diffusion of chloride ions into the postsynaptic cell body
  • There is a greater potential difference across the membrane because the inside is more negative
  • The sodium ions diffusing in are not enough to reach the threshold