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Neurons are the fundamental units of the nervous system. They are specialised cells that transmit electrical impulses and communicate with one another via chemical signals at junctions called synapses. Understanding neuronal structure, electrical transmission, and synaptic transmission is essential for explaining how information flows through the nervous system and how drugs, neurotransmitters, and mental disorders affect behaviour. This lesson moves from the cell to the synapse, ending with how excitation and inhibition are integrated through summation.
Key Definition: A neuron is a specialised nerve cell that receives, processes, and transmits information through electrical and chemical signals.
This lesson addresses the following points in AQA A-Level Psychology (7182), Paper 2, Section A (Biopsychology):
Assessment objectives engaged: AO1 (the types and structure of neurons; electrical transmission; the sequence of synaptic transmission; excitation, inhibition, and summation), and AO3 (evaluation of the synaptic-transmission account — its evidential support, applications, and the charge of reductionism).
There are three main types of neurons in the human nervous system, each with a distinct function:
| Neuron Type | Function | Direction of Impulse | Location |
|---|---|---|---|
| Sensory neuron | Carries nerve impulses from sensory receptors to the CNS | From receptors → CNS | PNS (connecting receptors to spinal cord/brain) |
| Relay neuron (interneuron) | Connects sensory and motor neurons within the CNS | Within the CNS | Brain and spinal cord |
| Motor neuron | Carries nerve impulses from the CNS to effectors (muscles or glands) | From CNS → effectors | CNS to PNS |
Key Definition: An effector is a muscle or gland that carries out a response when stimulated by a motor neuron.
These three neuron types map directly onto the reflex arc: a sensory neuron carries information in from the receptor, a relay neuron processes it within the CNS, and a motor neuron carries the command out to the effector. This "in–process–out" sequence is the basic template for all nervous-system activity, from the simplest spinal reflex to the most complex deliberate action. The difference between a reflex and a considered decision is largely a matter of how much processing the relay neurons (and the brain) perform between input and output: a reflex involves minimal processing within the spinal cord, whereas a deliberate action involves extensive processing across many relay neurons in the brain before a motor command is issued. Understanding the three neuron types therefore provides the vocabulary for describing every pathway in the nervous system, which is why this classification is foundational to the whole of biopsychology.
A typical motor neuron has the following components:
| Structure | Function |
|---|---|
| Cell body (soma) | Contains the nucleus and most organelles; metabolic centre of the neuron |
| Dendrites | Short, branching extensions that receive signals from other neurons |
| Axon | A long, thin fibre that carries the nerve impulse (action potential) away from the cell body |
| Myelin sheath | A fatty, insulating layer formed by Schwann cells; speeds up electrical transmission |
| Nodes of Ranvier | Gaps between sections of myelin sheath where ion exchange occurs |
| Terminal buttons (synaptic knobs) | Swollen endings at the axon terminal that contain vesicles of neurotransmitter |
The myelin sheath is composed of layers of the cell membrane of Schwann cells wrapped around the axon. Myelinated neurons can transmit impulses at speeds of up to 120 metres per second, whereas unmyelinated neurons transmit at approximately 2 metres per second. The clinical importance of myelin is illustrated by multiple sclerosis, in which the immune system destroys myelin, slowing or blocking neural transmission and producing motor and sensory deficits.
It is worth being precise about the direction of information flow within a single neuron, because examiners reward accurate use of terminology. Signals are received at the dendrites, which carry them toward the cell body (soma). The cell body contains the nucleus and the metabolic machinery (mitochondria, ribosomes) that keep the neuron alive and manufacture the proteins and neurotransmitters it needs. From the cell body, the signal passes to the axon hillock — the trigger zone where, if the threshold is reached, an action potential is initiated — and then travels along the axon to the terminal buttons, where it is passed on to the next neuron. A useful distinction is that a dendron is a single long process carrying impulses toward the cell body (prominent in sensory neurons), whereas an axon carries impulses away from it; dendrites are the smaller branching projections that receive inputs. Keeping these terms straight is the difference between a vague and a precise description of neuronal structure.
Exam Tip: If asked to draw and label a motor neuron, include: cell body with nucleus, dendrites, axon, myelin sheath, nodes of Ranvier, and terminal buttons. Label each clearly and indicate the direction of impulse transmission (dendrites → cell body → axon → terminal buttons).
Neurons communicate over long distances by transmitting electrical impulses called action potentials along the axon. This process involves the movement of ions (charged particles) across the neuronal membrane.
When a neuron is not transmitting an impulse, it is said to be at its resting potential. The inside of the neuron is negatively charged relative to the outside, with a potential difference of approximately −70 mV. This is maintained by:
The result is a polarised membrane: negative inside, positive outside. The resting potential is best thought of as a stored electrical and chemical gradient — a kind of charged battery — that the neuron holds in readiness. Maintaining it costs energy (the sodium-potassium pump consumes ATP continuously), but it is precisely this poised, "ready-to-fire" state that allows the neuron to respond almost instantly when stimulated. Without the resting potential, there would be no gradient for sodium to rush down, and therefore no action potential.
Key Definition: The resting potential is the electrical charge across the neuronal membrane when the neuron is not firing, typically around −70 mV.
When a neuron receives a stimulus above a critical threshold, an action potential is generated. This involves a rapid reversal of the resting potential:
Key Definition: The all-or-nothing principle states that a neuron either fires a full action potential or does not fire at all. There is no partial firing.
If the stimulus reaches the threshold (approximately −55 mV), a full action potential is generated. If the stimulus is below threshold, no action potential occurs. The strength of the stimulus does not affect the size of the action potential — instead, a stronger stimulus increases the frequency of action potentials (rate coding). A brief refractory period after each action potential, during which Na⁺ channels are temporarily inactivated, ensures the impulse travels in one direction only and limits the maximum firing rate.
This has an important consequence for how the nervous system encodes information. Because every action potential is the same size, intensity cannot be signalled by the size of the impulse. A bright light and a dim light, or a gentle touch and a firm press, all produce identical individual action potentials. Instead, stimulus intensity is communicated in two ways: by the frequency of firing (a stronger stimulus makes a neuron fire more often per second) and by the number of neurons recruited (a stronger stimulus activates more neurons). This is why the all-or-nothing principle and rate coding must be understood together — the all-or-nothing law explains the uniform quality of each impulse, while rate coding explains how quantity of information is nevertheless preserved. The refractory period also imposes a ceiling on firing rate, which is why even the most intense stimulus produces a finite maximum sensation rather than an unbounded one.
In myelinated neurons, the action potential does not travel continuously along the axon. Instead, it "jumps" from one node of Ranvier to the next. This is called saltatory conduction and greatly increases the speed of transmission. The reason is that the myelin sheath insulates the axon, preventing ion exchange except at the unmyelinated nodes; the action potential is therefore regenerated only at the nodes, effectively skipping the insulated stretches in between rather than crawling along every point of the membrane. This is far faster and also more energy-efficient, because fewer ions cross the membrane and so the sodium-potassium pump has less work to do to restore the resting potential. The clinical mirror image of this benefit is multiple sclerosis: when myelin is destroyed, saltatory conduction breaks down, signals slow or fail entirely, and movement, sensation, and coordination are impaired — vividly demonstrating just how much the nervous system depends on intact myelin for rapid communication.
Hodgkin and Huxley (1952) — using the giant squid axon, they established the ionic basis of the action potential through voltage-clamp experiments. They demonstrated the sequential opening and closing of Na⁺ and K⁺ channels. Their work earned them the Nobel Prize in Physiology or Medicine in 1963.
Exam Tip: The Hodgkin and Huxley (1952) study is a key reference for action potential questions. Their use of squid axons was advantageous because squid axons are very large (up to 1 mm in diameter), making them easy to insert electrodes into. However, generalising from squid to human neurons is a limitation.
Neurons do not physically touch one another. The junction between two neurons is called a synapse — specifically, the tiny gap between them is the synaptic cleft (approximately 20 nm wide). Communication across the synapse is primarily chemical. This conversion from an electrical signal (along the axon) to a chemical signal (across the cleft) and back to an electrical signal (in the next neuron) is one of the most important processes in the whole specification.
It is reasonable to ask why the nervous system bothers with a chemical step at all, when an unbroken electrical wire would be faster. The answer reveals the purpose of the synapse: a direct electrical connection could only ever pass a signal on unchanged, whereas a chemical synapse can transform the signal. It can amplify it or damp it down; it can convert an excitatory message into an inhibitory one (depending on the neurotransmitter and receptor); it can integrate inputs from thousands of other neurons; and, critically, it can change over time through learning, strengthening with use (long-term potentiation). The synapse is therefore the point at which the nervous system becomes flexible, adaptable, and capable of learning — the chemical step is not a limitation but the very feature that makes complex behaviour and memory possible.
The sequence below, and the diagram that follows, trace one complete cycle from arriving action potential to neurotransmitter clearance.
graph TD
A[Action potential arrives at terminal button] --> B[Voltage-gated Ca2+ channels open<br/>Ca2+ enters the presynaptic neuron]
B --> C[Synaptic vesicles move to and fuse<br/>with the presynaptic membrane]
C --> D[Neurotransmitter released into<br/>the synaptic cleft by exocytosis]
D --> E[Neurotransmitter diffuses across cleft<br/>and binds to postsynaptic receptors]
E --> F{Excitatory or<br/>inhibitory?}
F -->|Excitatory| G[Postsynaptic membrane depolarised<br/>EPSP - more likely to fire]
F -->|Inhibitory| H[Postsynaptic membrane hyperpolarised<br/>IPSP - less likely to fire]
G --> I[Neurotransmitter cleared:<br/>reuptake / enzyme breakdown / diffusion]
H --> I
Key Definition: A neurotransmitter is a chemical substance released at the synapse that crosses the synaptic cleft and binds to receptors on the postsynaptic neuron, either exciting or inhibiting it.
Key Definition: Reuptake is the process by which neurotransmitter molecules are reabsorbed back into the presynaptic neuron after release, terminating the signal and recycling the neurotransmitter.
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