4.2.2 - Neurons and synaptic transmission

Neurons as communication cells

Neurons receive, carry and pass on information using specialised structures.

The basic job of a neuron

The nervous system works because specialised cells can carry information quickly from one place to another. These cells are neurons. A neuron can receive information, pass an electrical impulse along its length, and communicate with another cell at the end.

The main structures each contribute to communication:

• Dendrites receive signals from other neurons or from sensory receptors.
• The cell body, or soma, contains the nucleus and keeps the neuron alive.
• The axon carries the electrical impulse away from the cell body.
• The myelin sheath is a fatty insulating layer around many axons that helps impulses travel faster.
• Nodes of Ranvier are gaps in the myelin sheath. In myelinated neurons, the impulse effectively jumps from node to node.
• Axon terminals or terminal boutons are the endings of the axon. They form synapses with the next cell.

Why structure matters

Imagine touching a sharp pin. The receptor in the skin detects the stimulus. A neuron must carry that information towards the spinal cord or brain, another neuron may process or relay it inside the central nervous system, and another neuron may carry a command to a muscle. The structure of each neuron helps it do its particular job in that pathway.

An impulse is a brief change in voltage across the cell membrane, produced by the movement of charged particles called ions. The voltage change is regenerated along the axon, rather than a parcel of electricity being used up as it travels. Myelin reduces current loss between nodes; the action potential is regenerated at the gaps, making transmission faster.

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The sensory pathway points upwards from receptor to CNS; the relay and motor examples point downwards from their inputs to their terminals. Compare the sensory cell body, positioned on a side branch, with the motor cell body at one end of a long axon. These are typical simplified shapes; function and destination identify the type more reliably than shape alone.

Sensory, relay and motor neurons

The three functional types differ in the direction and destination of the information they carry.

Three functional types

Sensory neurons are sometimes called afferent neurons because they carry information arriving at the CNS. Their job is input. They respond to physical or chemical stimuli such as heat, pressure, light, sound or chemicals involved in taste and smell. Many sensory neurons have a cell body positioned off to the side of the main axon, which is why their shape can look different from the textbook "tree" shape.

Relay neurons are also called interneurons in many biology sources. “Interneuron” is an equivalent term; recognise both. Their job is processing and connection inside the brain or spinal cord. They often have many dendrites because they may receive information from several neurons and pass it on within a neural circuit.

Motor neurons are sometimes called efferent neurons because they carry commands exiting the CNS. Their job is output. A motor neuron may have a cell body at one end, a long axon, and terminals that connect to an effector. If the effector is a muscle, the result may be contraction.

Applying the pathway

In a simple withdrawal reflex, Aisha touches a very hot baking tray:

1. Receptors in her skin detect heat and tissue damage.
2. A sensory neuron carries an impulse from the receptor towards the spinal cord.
3. A relay neuron in the spinal cord passes the message across the CNS pathway.
4. A motor neuron carries an impulse from the spinal cord to muscles in her arm.
5. The muscles contract, pulling her hand away.

This is a simplified withdrawal circuit, not a claim that every response contains exactly three neurons. Complex responses involve many relay neurons, while some reflexes have a more direct sensory-to-motor connection. The distinction remains input towards the CNS, connections within it, and output towards an effector.

How structure supports function

The shape and connections of a neuron support its role in a communication pathway.

Comparing the shapes

All three neuron types use the same basic principles, but their structure is adapted to their role.

Sensory neurons need to carry information from receptors over potentially long distances towards the CNS. In the common textbook sensory neuron, a long dendron carries the impulse from the receptor towards the cell body, which lies to one side; a shorter axon carries it towards the CNS. Real sensory neurons vary in shape, so these relative lengths describe a typical example rather than every sensory neuron.

Relay neurons are usually found in the brain and spinal cord. They tend to be shorter and highly branched, which suits their role in connecting many neurons inside the CNS. The many dendrites and synaptic connections allow a relay neuron to integrate information before passing it on.

Motor neurons carry commands away from the CNS to effectors. A long axon lets a command travel from the spinal cord towards a muscle or gland. Branching terminals allow one motor neuron to communicate with muscle fibres or other effector cells.

Deciding from clues

If a scenario says a neuron carries an impulse from a receptor to the spinal cord, it is sensory. If it says the neuron is inside the brain or spinal cord and links other neurons, it is relay. If it says the neuron carries an impulse from the CNS to a muscle or gland, it is motor.

That is usually more reliable than trying to memorise one fixed drawing. Real neurons vary widely, so exam answers should prioritise the function and the direction of travel.

A useful but simplified classification

Suppose one nerve carries a signal from skin to spinal cord and another carries a signal from spinal cord to a leg muscle. Both may have long, myelinated fibres. Length and myelin alone cannot distinguish them: the first is sensory because it brings input to the CNS; the second is motor because it carries output to an effector. A short branching neuron entirely within the cord could connect them as a relay neuron.

Synaptic transmission

At a chemical synapse, neurotransmitter links the electrical activity of one neuron to a response in the next cell.

What happens at a synapse

Neurons do not usually touch end-to-end. The whole junction is a synapse; its tiny gap is the synaptic cleft. The signal travels electrically along the axon, but it crosses most synapses chemically.

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At an individual chemical synapse, the sequence is:

1. An action potential reaches the axon terminal of the presynaptic neuron.
2. The action potential triggers small membrane-bound sacs, vesicles, to fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.
3. Neurotransmitter diffuses across the synaptic cleft.
4. Neurotransmitter binds to specific receptor sites on the postsynaptic neuron. This changes ion movement and therefore the electrical state of that cell.
5. The postsynaptic neuron is affected. The effect may be excitatory or inhibitory.
6. Neurotransmitter is removed from the synaptic cleft, for example by breakdown or reuptake, so the signal is brief and controlled.

The electrical impulse itself does not jump the cleft. Chemical transmission normally proceeds in one direction at this junction: transmitter is released from presynaptic vesicles and detected by receptors on the postsynaptic membrane. This is different from claiming that a whole brain circuit can never feed information back.

Active pathway example

A drug that blocks postsynaptic receptors can prevent the normal effect even if the presynaptic impulse arrives and transmitter is released. Releasing more transmitter would not necessarily restore transmission if the receptors remain blocked. This example shows why release, diffusion and receptor binding are separate necessary links, rather than interchangeable descriptions.

Excitation and inhibition

Postsynaptic signals change the likelihood of an action potential, rather than guaranteeing a response.

Two possible postsynaptic effects

At rest, the inside of a neuron is electrically negative relative to the outside. A sufficiently large depolarisation reaches a threshold, triggering an action potential. Below threshold, there is no action potential; above it, the impulse is not made larger by a stronger input.

An excitatory effect usually involves depolarisation: the postsynaptic membrane becomes less negative and moves closer to the threshold needed to fire. An excitatory neurotransmitter does not guarantee that the next neuron fires. It increases the chance.

An inhibitory effect may involve hyperpolarisation: the postsynaptic membrane becomes more negative and moves further from threshold. Inhibition can also reduce the effect of excitatory inputs without making the membrane more negative. The essential distinction is a decreased likelihood of firing, rather than compulsory hyperpolarisation at every inhibitory synapse.

The postsynaptic neuron often receives many inputs at once. Their combined effect is called summation; the strength and timing of the inputs matter, not just the number of excitatory and inhibitory synapses. Some are excitatory and some are inhibitory. Whether it fires depends on the balance of these inputs. This is why neural communication is not just an on/off chain. It is an integrated system of signals.

Worked example: why counting inputs is insufficient

In a deliberately simplified model, a neuron needs a net depolarising change of 7 units to reach threshold. Two excitatory inputs each contribute +4 units and one inhibitory input contributes −3. The net change is $+4 + 4 - 3 = +5$ units, below threshold, so no action potential occurs. “Two excitatory signals beat one inhibitory signal” would predict the wrong answer.

Real inputs vary in strength, timing and position on the neuron. Summation combines their effects; a later impulse or reduced inhibition might allow threshold to be reached. Excitation increases firing likelihood, while inhibition reduces it: neither term alone guarantees what the whole neuron will do.

A transmitter's effect also depends on the receptor and circuit. It is therefore too strong to infer a specific behaviour simply from the presence of one neurotransmitter.

Quick recap

Sensory neurons carry input to the CNS, relay neurons connect cells within it, and motor neurons carry output to effectors. Structures such as dendrites, axons and myelin support these functions. At a chemical synapse, released transmitter crosses the cleft and binds to receptors. Summation of excitatory and inhibitory effects determines whether threshold is reached.