You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Neurons are the fundamental units of the nervous system: specialised cells that transmit electrical impulses and communicate with one another via chemical signals at junctions called synapses. Understanding neuronal structure, electrical transmission along the axon and chemical transmission across the synapse is essential for the biological approach, because every explanation it offers — from the effect of a recreational drug to the neurochemistry of aggression — ultimately rests on how signals pass between cells. This lesson moves systematically from the cell to the synapse, ending with how excitation and inhibition are integrated through summation to determine whether the next neuron fires.
Key Definition: A neuron is a specialised nerve cell that receives, processes and transmits information through electrical and chemical signals.
This lesson addresses the Edexcel 9PS0 — Paper 1, Topic 3: Biological Psychology content on the structure and function of neurons and the process of synaptic transmission: the three functional types of neuron (sensory, relay and motor), the structure of a neuron, the electrical action potential, the sequence of synaptic transmission, and the distinction between excitatory and inhibitory neurotransmitters integrated through summation. It provides the cellular mechanism that underpins the whole topic. In assessment-objective terms, you should be able to describe neuron types, the action potential and the stages of synaptic transmission (AO1), apply this knowledge to scenarios such as a described drug effect or a clinical deficit (AO2), and evaluate the account — its evidential base, its applications, and the charge of reductionism (AO3).
Connects to…
There are three main types of neuron 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 types map directly onto the reflex arc: a sensory neuron carries information in from a 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 deliberate action. The difference between a reflex and a considered decision is largely a matter of how much processing the relay neurons perform between input and output: a reflex involves minimal spinal processing, whereas a deliberate action involves extensive processing across many relay neurons in the brain before a motor command is issued. The classification therefore provides the vocabulary for describing every pathway in the nervous system.
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 around 2 metres per second. The clinical importance of myelin is illustrated by multiple sclerosis, in which the immune system destroys myelin, slowing or blocking 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 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 threshold is reached, an action potential is initiated — and then travels along the axon to the terminal buttons, where it is passed on. 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 describe a motor neuron, include the cell body with nucleus, dendrites, axon, myelin sheath, nodes of Ranvier and terminal buttons, 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 involves the movement of ions (charged particles) across the neuronal membrane.
When a neuron is not transmitting an impulse it is at its resting potential. The inside 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 pump consumes ATP continuously), but it is precisely this poised, "ready-to-fire" state that lets the neuron 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 — 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 threshold (approximately −55 mV) a full action potential is generated; below threshold, none 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, intensity is communicated by the frequency of firing (a stronger stimulus makes a neuron fire more often per second) and by the number of neurons recruited. 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 preserved. The refractory period also imposes a ceiling on firing rate, which is why even the most intense stimulus produces a finite maximum sensation.
In myelinated neurons the action potential does not travel continuously; instead it "jumps" from one node of Ranvier to the next — saltatory conduction — greatly increasing speed. The myelin sheath insulates the axon, preventing ion exchange except at the unmyelinated nodes, so the action potential is regenerated only at the nodes, effectively skipping the insulated stretches rather than crawling along every point of the membrane. This is far faster and more energy-efficient, because fewer ions cross the membrane and so the pump has less work to do. The clinical mirror image is multiple sclerosis: when myelin is destroyed, saltatory conduction breaks down, signals slow or fail entirely, and movement, sensation and coordination are impaired — demonstrating how much the nervous system depends on intact myelin.
Hodgkin and Huxley (1952) used the giant squid axon to establish the ionic basis of the action potential through voltage-clamp experiments, demonstrating the sequential opening and closing of Na⁺ and K⁺ channels. Their work earned the Nobel Prize in Physiology or Medicine in 1963.
Exam Tip: Hodgkin and Huxley (1952) is a key reference for action-potential questions. Squid axons are very large (up to 1 mm across), making them easy to insert electrodes into — but generalising from squid to human neurons is a limitation you can turn into AO3.
Neurons do not physically touch. The junction between two neurons is the synapse, and the tiny gap between them is the synaptic cleft (approximately 20 nm wide). Communication across the synapse is 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 topic.
It is reasonable to ask why the nervous system uses 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 pass a signal on unchanged, whereas a chemical synapse can transform it. It can amplify or damp the signal; it can convert an excitatory message into an inhibitory one (depending on 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. It is also, as the next lesson shows, the point at which recreational and therapeutic drugs act.
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
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.