Role of Neurotransmitters at Synapses

Section 1 of 7What a synapse is

A gap between neurones

• A gap between neurones Neurones in a pathway do not touch one another.
• The tiny gap that separates the end of one neurone from the start of the next is a synapse.

A problem to solve

• A problem to solve An electrical impulse cannot jump across the gap by itself.
• A chemical signal is used instead to carry the message from one cell to the next.

Neurotransmitters

• Neurotransmitters Neurotransmitters are the chemical messengers that carry the signal across the gap.

Section 2 of 7Crossing the synapse

1

An electrical impulse travels along the first neurone and arrives at its end.

2

The arrival of the impulse triggers the release of neurotransmitter molecules from the end of the neurone into the gap.

3

The neurotransmitter molecules diffuse across the synapse, because the gap is very narrow and their concentration is higher at the releasing side.

4

The molecules bind to receptors on the membrane of the next neurone.

5

This binding causes a new electrical impulse to start in the second neurone, so the message continues.

Section 3 of 7Signal types along the pathway

Stage	Type of signal	How it moves
Along the first neurone	Electrical	Impulse travels along the cell
Across the synapse	Chemical	Neurotransmitter diffuses across the gap
Along the second neurone	Electrical	New impulse is generated

Section 4 of 7Why it matters

Synapse

Direction

• Direction Neurotransmitter is released from one side only, so the impulse passes in one direction through the pathway.

Passing on the message

• Passing on the message Synapses allow a signal to pass from a sensory neurone to the next neurone and eventually to an effector.

Receptors are specific

• Receptors are specific Each neurotransmitter binds only to receptors with a matching shape, which keeps the signal precise.

Section 5 of 7Why synapses are important

Linking neurones in a reflex

• Linking neurones in a reflex In a reflex arc, an impulse passes from a sensory neurone to a relay neurone and then to a motor neurone.
• Each of these links is a synapse, so every reflex pathway includes at least two of them.

A small delay

• A small delay Diffusion of neurotransmitter across the gap takes a little time, so impulses travel more slowly across synapses than along a neurone.
• The gap is so small that the delay is tiny and the response still seems instant.

Drugs and synapses

• Drugs and synapses Some drugs and poisons interfere with neurotransmitters or their receptors,
  • which is why they can change how the nervous system works.

Section 6 of 7Key terms

Term	Meaning
Synapse	The tiny gap between two neurones.
Neurotransmitter	A chemical released at a synapse that carries a signal to the next neurone.
Diffusion	The net movement of particles from a region of higher concentration to lower concentration.
Receptor molecule	A protein on the membrane that a specific chemical binds to.

Section 7 of 7Exam technique

1

Saying that the electrical impulse jumps across the synapse; it is a chemical that crosses.

2

Writing that neurotransmitters travel in the blood — they only cross the short gap.

3

Forgetting to state that a new impulse is generated in the next neurone after binding.

4

Thinking neurones are physically joined together.