Neurons and Synaptic Transmission

The nervous system communicates using neurons — specialised cells that transmit signals electrically along their length and chemically to other neurons. The human nervous system contains roughly 86 billion of them, and understanding their structure is the foundation for everything in this topic.

Although neurons vary in shape, they share the same basic parts:

• Cell body — contains the nucleus and the genetic material of the cell.
• Dendrites — branch-like extensions that receive signals from other neurons and carry them towards the cell body.
• Axon — the long fibre that carries the electrical impulse away from the cell body.
• Myelin sheath — a fatty layer that insulates the axon and speeds up transmission.
• Nodes of Ranvier — gaps in the myelin sheath that force the impulse to jump along the axon, speeding it up further.
• Terminal buttons — the end of the axon, where the signal is passed on to the next neuron.

A neuron works in two ways at once: the signal travels electrically along the axon within a single neuron, but passes chemically between one neuron and the next.

The specification names three types of neuron, each with a distinct job and a distinct structure. The differences in the length of their dendrites and axons reflect the distances the signal has to travel.

Neuron	Direction of impulse	Location	Dendrites	Axons
Sensory	From sensory receptors to the CNS	Cell body in the PNS	Long	Short
Relay	Between neurons (connects sensory to motor)	Within the CNS	Short	Short
Motor	From the CNS to effectors (muscles/glands)	Cell body in the CNS	Short	Long

Sensory neurons carry impulses from the sensory receptors (in the eyes, skin, ears and so on) of the peripheral nervous system (PNS) towards the central nervous system (CNS). They have long dendrites and short axons.

Relay neurons connect sensory neurons to motor neurons, or to other relay neurons. They lie entirely within the CNS and have short dendrites and short axons.

Motor neurons carry impulses from the CNS to effectors — the muscles and glands that produce a response. They have short dendrites and long axons that reach out to the effector.

A quick memory aid: sensory neurons sense the world and carry the message in; motor neurons put the body in motion by carrying the message out; relay neurons sit in the middle and pass it along.

Before a neuron can pass a message to its neighbour, the signal has to travel along its own length. This happens electrically, and it depends on the charge inside the neuron changing.

When a neuron is not firing, it is at its resting potential: the inside of the cell is negatively charged relative to the outside. The neuron is switched on but idle, ready to respond.

When the neuron is activated by a stimulus of sufficient strength, the charge inside the cell briefly reverses and becomes positive. This momentary reversal is called an action potential — the electrical impulse itself.

The action potential then travels as a wave down the length of the axon, from the cell body towards the terminal buttons at the end. The myelin sheath and nodes of Ranvier speed this journey up.

Key sequence within one neuron: resting potential (inside negative) → stimulus → action potential (charge reverses) → impulse travels down the axon to the terminal buttons.

Once the action potential reaches the terminal buttons, it has run out of neuron to travel along. To reach the next cell, the signal must cross a gap — and here it stops being electrical.

Neurons do not physically touch. Where the terminal button of one neuron meets the next there is a tiny gap called the synapse, and the space itself is the synaptic cleft. The electrical impulse cannot cross this gap on its own, so the signal is passed across chemically.

The process happens in a fixed order:

1. The action potential arrives at the presynaptic terminal (the terminal button of the first neuron).
2. This triggers synaptic vesicles — tiny sacs — to release neurotransmitters into the synaptic cleft.
3. The neurotransmitter diffuses across the gap.
4. It binds to specialised receptors on the membrane of the postsynaptic neuron.
5. This is converted back into a new electrical impulse (or influences whether one is fired).
6. The neurotransmitter is then cleared away, either by reuptake into the presynaptic neuron or by being broken down.

Because vesicles that release neurotransmitters are only on the presynaptic side and receptors are only on the postsynaptic side, synaptic transmission runs in one direction only — from the presynaptic to the postsynaptic neuron.

When a neurotransmitter binds to the postsynaptic receptors, it changes the charge of the postsynaptic neuron. Whether this makes the next neuron more or less likely to fire depends on the type of neurotransmitter.

Effect	What it does to the postsynaptic neuron	Result	Examples
Excitatory	Makes it more positively charged	More likely to fire	Adrenaline, dopamine
Inhibitory	Makes it more negatively charged	Less likely to fire	Serotonin, GABA

Excitatory neurotransmitters create an excitatory postsynaptic potential (EPSP): they increase the positive charge of the postsynaptic neuron, pushing it closer to firing an action potential of its own.

Inhibitory neurotransmitters create an inhibitory postsynaptic potential (IPSP): they increase the negative charge, pushing the neuron away from firing.

Most neurotransmitters are not simply one or the other in every situation, so a single postsynaptic neuron is receiving many signals at once — some excitatory, some inhibitory.

Whether the postsynaptic neuron actually fires depends on summation: the net sum of all the excitatory and inhibitory influences it receives. If the excitatory signals outweigh the inhibitory ones and the combined effect passes the threshold, an action potential is triggered; if not, the neuron stays at rest.

This adding-up is what allows the nervous system to make fine decisions rather than firing every time it receives any signal at all.

1. Saying the electrical impulse "jumps" the synapse

The signal does not cross the synapse electrically. The impulse is electrical within a neuron, but at the synapse it stops; a chemical neurotransmitter is released, diffuses across the cleft, and binds to receptors to start a new impulse. Describing it as an electrical signal leaping the gap loses marks.

2. Mixing up the direction of sensory, relay and motor neurons

Sensory neurons carry impulses towards the CNS; motor neurons carry them away from the CNS to effectors; relay neurons connect them within the CNS. A reliable check: sensory carries in, motor carries out to put the body in motion.

3. Forgetting that synaptic transmission is one-directional

Neurotransmitters are released only from the presynaptic neuron and receptors sit only on the postsynaptic neuron, so signals travel one way across the synapse. Do not describe neurotransmitters crossing back the other way.

4. Treating all neurotransmitters as excitatory

Not every neurotransmitter makes a neuron fire. Inhibitory neurotransmitters such as serotonin and GABA make the postsynaptic neuron less likely to fire. A full answer names both excitation and inhibition and explains that the outcome depends on summation.

5. Confusing the synapse structure

The synapse is the whole junction; the synaptic cleft is the gap itself. The presynaptic neuron sends the signal and the postsynaptic neuron receives it. Vesicles and neurotransmitters are on the presynaptic side; receptors are on the postsynaptic side.