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8.1.6Unit 6: Practical Skills in Biology II

Synapses and Neurotransmitters — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 40 of 48 Unit quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Part 1 of 5Structure of a Synapse

8.1.6 Synapses and Neurotransmitters

A synapse is the junction where one neurone communicates with another cell, such as a neurone or a muscle fibre.

Synaptic cleft
  • A narrow fluid-filled gap separates the two cells, so the electrical impulse cannot pass straight across it.
Presynaptic neurone
  • The cell before the gap ends in a swollen synaptic knob that contains many mitochondria and vesicles.
Vesicles
  • These small membrane-bound sacs store neurotransmitter such as acetylcholine until an action potential arrives.
Postsynaptic membrane
  • The membrane after the gap carries specific receptor proteins linked to sodium ion channels.
Neurotransmitter
  • A chemical messenger released by exocytosis that diffuses across the cleft to carry the signal.

Part 2 of 5Transmission Across a Cholinergic Synapse

The sequence below follows one action potential arriving at a synapse that uses acetylcholine (ACh).

1
An action potential reaches the synaptic knob and depolarises its membrane, which opens voltage-gated calcium ion channels.
2
Calcium ions diffuse into the knob and cause vesicles to move to the presynaptic membrane and fuse with it.
3
Acetylcholine is released into the cleft by exocytosis and diffuses across to the postsynaptic membrane.
4
ACh binds to receptors, which opens sodium ion channels so that sodium ions diffuse into the postsynaptic cell.
5
If the threshold potential is reached, an action potential is generated in the postsynaptic neurone.
6
Acetylcholinesterase breaks ACh down, so the receptors are not stimulated continuously and the synapse can reset.

Part 3 of 5Roles of Synapses

The chemical nature of synapses gives the nervous system useful properties.

Roles of Synapses
Unidirectional
  • Neurotransmitter is released only on one side and receptors are only on the other, so impulses travel one way.
Divergence
  • One presynaptic neurone can connect to several others, sending a signal to many pathways at once.
Summation
  • Several small stimuli can add together to reach threshold when a single impulse would release too little transmitter.
Convergence
  • Several presynaptic neurones meeting one postsynaptic neurone combine their effects to bring it to threshold.
Rapid repeats
  • Many action potentials in quick succession release more neurotransmitter and open more sodium channels.
Type of summationWhat happens
SpatialImpulses from several presynaptic neurones arrive together at one postsynaptic neurone
TemporalImpulses arrive in quick succession along a single presynaptic neurone

Part 4 of 5The Pupil Reflex

Synapses link receptor, sensory, CNS and motor neurones in this protective reflex.

1
Light receptors in the retina detect high light intensity and pass impulses along sensory neurones to the CNS.
2
Impulses cross synapses in the CNS and are sent along motor neurones to the muscles of the iris.
3
The circular muscles contract, the radial muscles relax, and the pupil constricts, protecting the retina from damage.
4
In dim light the radial muscles contract and the circular muscles relax, so the pupil dilates and more light enters.

Part 5 of 5Exam Checklist & Tips

8.1.6 – Key Points for the Exam

  • Synapse A junction between two neurones, or between a neurone and an effector cell.
  • Neurotransmitter A chemical released from vesicles that diffuses across the synaptic cleft to stimulate the next cell.
  • Acetylcholinesterase The enzyme that hydrolyses acetylcholine in the cleft so stimulation of the postsynaptic cell stops.
  • Summation The adding together of the effects of several impulses so that threshold potential is reached.
  • Threshold potential The membrane potential that must be reached for an action potential to be generated.

Common Mistakes

  • Saying the electrical impulse jumps across the cleft; in fact a chemical neurotransmitter diffuses across it.
  • Forgetting that calcium ions enter the presynaptic knob first, before vesicles fuse with the membrane.
  • Writing that sodium channels open in the presynaptic membrane; they open in the postsynaptic membrane.
  • Confusing the muscles of the iriscircular muscles contract to constrict the pupil, radial muscles to dilate it.

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