How do neurons communicate at synapses, and how do neurotransmitters and drugs affect this?
The cells of the nervous system (neurons and glial cells), synaptic transmission by neurotransmitters, the removal of neurotransmitters, summation, the roles of endorphins and dopamine, and how recreational and therapeutic drugs affect neurotransmission (agonists, antagonists and effects on reuptake).
An SQA Higher Human Biology answer on the cells of the nervous system and neurotransmitters, covering neurons and glial cells, synaptic transmission and the removal of neurotransmitters, summation, the roles of endorphins and dopamine, and how agonist, antagonist and reuptake-affecting drugs change neurotransmission.
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What this dot point is asking
The SQA wants you to describe neurons and glial cells, explain synaptic transmission and the removal of neurotransmitters, explain summation, describe the roles of endorphins and dopamine, and explain how agonist, antagonist and reuptake-affecting drugs alter neurotransmission.
The cells of the nervous system
Neurons do the signalling, while glial cells outnumber them and keep the nervous system working. This division of labour links to the synapse, where one neuron passes its signal to the next.
Synaptic transmission
After it has acted, the neurotransmitter must be removed so the synapse can reset, either by being broken down by an enzyme or by being reabsorbed (reuptake) into the first neuron. Without removal, the receptors would be continually stimulated.
Summation lets the nervous system respond to several weak signals that individually would not be enough, increasing sensitivity, and links to the converging pathways seen elsewhere in the course.
Endorphins and dopamine
Two neurotransmitters are named specifically:
- Endorphins reduce the sensation of pain and produce a feeling of wellbeing. Their release increases during exercise, in response to injury and during certain pleasurable activities.
- Dopamine is the neurotransmitter of the brain's reward pathway, inducing feelings of pleasure and reinforcing the behaviours that triggered it. This reward role is central to understanding addiction.
How drugs affect neurotransmission
Drugs change neurotransmission by acting on receptors or on the removal of neurotransmitters:
- An agonist binds to and stimulates the same receptors as a neurotransmitter, so it mimics or enhances the normal response.
- An antagonist binds to the receptors but blocks them, preventing the neurotransmitter from acting and so reducing the response.
- Some drugs alter the rate of reuptake of a neurotransmitter. Slowing reuptake leaves more neurotransmitter in the cleft, prolonging its effect (the basis of some antidepressants and of stimulant drugs).
Repeated use of drugs that act on the dopamine reward pathway can lead to tolerance (the brain reduces its own receptors or neurotransmitter, so more drug is needed) and addiction, which is why these drugs are so harmful.
Examples in context
Example 1. Exercise and endorphins. During prolonged exercise the brain releases endorphins, which reduce pain and produce the feeling sometimes called runner's high. This shows endorphins acting as pain-reducing neurotransmitters.
Example 2. Addiction and dopamine. Many recreational drugs increase dopamine in the reward pathway, producing intense pleasure. With repeated use the brain adapts, so more drug is needed for the same effect (tolerance) and the user becomes dependent, illustrating how acting on neurotransmission drives addiction.
Try this
Q1. Name the gap between two neurons at a synapse. [1 mark]
- Cue. The synaptic cleft.
Q2. Explain how an antagonist drug reduces the response of the next neuron. [1 mark]
- Cue. It binds to and blocks the receptors, so the neurotransmitter cannot bind and stimulate the neuron.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksDescribe how an impulse is transmitted across a synapse from one neuron to the next.Show worked answer →
A 4-mark answer needs the sequence of synaptic transmission.
When a nerve impulse arrives at the end of the first neuron, it causes vesicles to release a chemical neurotransmitter into the synaptic cleft, the small gap between the two neurons. The neurotransmitter diffuses across the cleft.
The neurotransmitter then binds to specific receptor molecules on the membrane of the next neuron. If enough binds to reach the threshold, a new impulse is generated in the second neuron. The neurotransmitter is then removed from the cleft, either by being broken down by an enzyme or by being reabsorbed (reuptake) into the first neuron, so the synapse is ready for the next impulse.
Award (1) impulse causes release of neurotransmitter into the cleft, (2) neurotransmitter diffuses across, (3) binds to receptors and can generate a new impulse, and (4) neurotransmitter removed by enzyme breakdown or reuptake.
SQA Higher 20213 marksExplain the difference between an agonist and an antagonist drug, and describe the role of dopamine in the brain.Show worked answer →
This is a 3-mark question on drugs and neurotransmitters.
An agonist is a drug that binds to and stimulates the same receptors as a neurotransmitter, so it mimics or enhances the normal response. An antagonist binds to the receptors but blocks them, so it prevents the neurotransmitter from acting and reduces the response.
Dopamine is a neurotransmitter involved in the brain's reward pathway, inducing feelings of pleasure and reinforcing behaviours. Many recreational drugs act by increasing dopamine activity, which is linked to their addictive nature.
Award (1) an agonist mimics or stimulates the receptors, (2) an antagonist blocks the receptors, and (3) dopamine is a reward-pathway neurotransmitter linked to pleasure and addiction.
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