5.3.9 Transmission Across a Cholinergic Synapse

• There are over 40 different known neurotransmitters
• One of the key neurotransmitters used throughout the nervous system is acetylcholine (ACh)
• Synapses that use the neurotransmitter ACh are known as cholinergic synapses
• The detailed process of synaptic transmission using ACh is as follows:

  1. The arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane
  2. This stimulates voltage-gated calcium ion channel proteins to open
  3. Calcium ions (Ca²⁺) diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse (high concentration of calcium ions) into the synaptic knob (low concentration of calcium ions)
  4. This stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft by exocytosis
  5. The ACh molecules diffuse across the synaptic cleft and temporarily bind to cholinergic receptors in the postsynaptic membrane
  6. This causes sodium ion channels to open
  7. Sodium ions to diffuse down an electrochemical gradient into the cytoplasm of the postsynaptic neurone
  8. The sodium ions cause depolarisation of the postsynaptic membrane, re-starting the electrical impulse once the threshold is reached
  9. The ACh molecules are broken down and recycled
     • This prevents the sodium ion channels staying permanently open and stops permanent depolarisation of the postsynaptic membrane,
     • The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline

  10. The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives

Synaptic transmission using acetylcholine (ACh)

Key features of synapses

Unidirectionality

• Synapses ensure the one-way transmission of impulses
• Impulses can only pass in one direction at synapses because neurotransmitter is released on one side and its receptors are on the other – chemical transmission cannot occur in the opposite direction
• This prevents impulses from travelling the wrong way, back to where they were initiated

Summation

• Sometimes, a single impulse that arrives at a synaptic knob is insufficient to generate an action potential in the post-synaptic neurone because:
  • Only a small amount of acetylcholine is released into the synaptic cleft
  • This means only a small number of the gated ion channels are opened in the axon membrane
  • Therefore an insufficient number of sodium ions pass through the membrane
  • So the threshold potential is not reached
  • The small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase

• To overcome this, the effect of multiple impulses can be added together in a process known as summation 
• There are several benefits of summation
  • It allows for the effect of a stimulus to be magnified
  • A combination of different stimuli can trigger a response
  • It avoids the nervous system being overwhelmed by impulses
    • Synapses act as a barrier and slow down the rate of transmission of a nerve impulse that has to travel along two or more neurones
    • They only allow the impulses to pass on if there has been input from other neurones and receptors

• There are two types of summation:
  • Temporal
  • Spatial

Temporal summation

• If multiple impulses arrive within quick succession the effect of the impulses can be added together to generate an action potential
  • A large amount of acetylcholine is released into the synaptic cleft
  • Therefore a large number of the gated ion channels open
  • So a sufficient number of sodium ions pass through the membrane

Spatial summation

• Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body can also generate an action potential through spatial summation
• The multiple impulses result in a large amount of acetylcholine being released into the synaptic cleft which results in the generation of an action potential

Temporal summation involves only one synaptic knob whereas spatial summation involves multiple synaptic knobs. The different types of summation produce different shaped graphs.

Inhibitory and excitatory synapses

• Excitatory neurotransmitters can stimulate the generation of an action potential in a postsynaptic neurone
  • This is done by opening sodium ion channels in the postsynaptic membrane which causes depolarisation if a threshold is reached

• inhibitory neurotransmitters can prevent the generation of an action potential in a postsynaptic neurone
  • They do this by opening potassium ion channels in the postsynaptic membrane which causes hyperpolarisation of the membrane

• If a neurone is subject to both excitatory and inhibitory synapses the following happens:
  • Sodium ions enter the cell body following stimulation by the excitatory synapse
  • The stimulation of the inhibitory synapse causes potassium ions to diffuse out of the cell body
  • This cancels out the effect of the sodium ions entering
  • The threshold potential is not reached so no action potential is generated

The inhibitory synapse (Y) causes the membrane potential to decrease, cancelling out the effect of the excitatory synapse (X) so that the threshold is not reached and no action potential is generated