Synapses

• is a physical and functional gap which occur between one neuron and the other and muscle or neuron and gland which serves the purpose of electrical or chemical transmission of impulses
• contains the end of presynaptic neuron + synaptic cleft + end of postsynaptic neuron

Mechanism of synaptic transmission

• action potential arrives at presynaptic neuron
• stimulates opening of voltage-gated channels for Ca2+
• Ca2+ diffuse into cytoplasm of presynaptic membrane
• Ca²⁺ cause vesicles containing acetylcholine (ACh – a type of neurotransmitter) move towards the presynaptic membrane
• Vesicle fuses with the membrane, Ach is released and diffuses across the synaptic cleft
• Ach temporarily binds to receptor proteins on the postsynaptic membrane; causes chemically-gated ion channels for Na+ to open
• Na+ diffuse through postsynaptic membrane à depolarizes membrane à generates action potential
• Recycling of Ach, catalyzed by acetylcholinsterase (stops continuous production of action potentials)
  • acetylcholine → acetate + choline
• choline moves back into the presynaptic neuron.
  • choline + acetyl coenzymeA → acetylcholine
• ACh transported back to presynaptic vesicles

Q/A Explain the mechanism of transmission of a nerve impulse across a cholinegic synapse

Logical sequence of events is very important when explaining a mechanism

• arrival of an impulse
• Depolarization of presynaptic membrane
• opening if calcium channels
• increase in membrane permeability to Ca²⁺
• rushing of Ca²⁺ into presynaptic knob
• synaptic vesicle fuse with presynaptic cleft(by exocytosis)
• acetylcholine released into synaptic cleft
• attaches to a receptor on the post synaptic membrane
• entry of Na⁺
• depolarization of membrane

Functions of synapses

• ensures one-way transmission: neurotransmitters are only released on one side of synapse;
  receptors on the other side of synapse
• allow connection of nerve pathways
• integration of impulses
• involved in memory making and learning
• Decreases the overload of information in the brain