Historical Context & Motivation
For centuries, scientists wondered how the body could respond so quickly to stimuli — how you can pull your hand away from a hot surface in a fraction of a second. Early thinkers believed that "animal spirits" flowed through hollow nerves, but the real story turned out to be far more interesting. The discovery of neural signalling — the process by which nerve cells generate and transmit electrical and chemical messages — is one of the great triumphs of biology. Understanding these signals explains everything from reflexes to memory, and it remains central to modern neuroscience and medicine.
These discoveries raised a central question that still drives neuroscience today: how does a single neuron convert a stimulus into an electrical impulse, pass that impulse along its length, and then communicate it chemically to the next cell? This lesson will walk you through each stage of that process.
Core Principles of Neural Signalling
Neural signalling depends on a handful of fundamental ideas that work together. A neuron must maintain a charge difference across its membrane when at rest, respond to stimuli by rapidly changing that charge, propagate the change along its length, and then pass the message to the next cell using chemical messengers. Each of these steps relies on the behaviour of ions — charged atoms — and the proteins embedded in the neuron's membrane.
Resting Membrane Potential
Action Potential
Synaptic Transmission
Saltatory Conduction
Refractory Period
Anatomy of a Neuron
Before we can understand how signals travel, we need to know the parts of a neuron. The diagram below shows a typical motor neuron — the type that carries signals from the central nervous system to muscles. Each region of the neuron plays a specific role in generating, conducting, or transmitting the nerve impulse.
Notice how the neuron has a clear directional flow: signals enter through the dendrites, are processed at the cell body, travel along the axon, and leave through the synaptic terminals. The myelin sheath — a fatty insulating layer produced by Schwann cells in the peripheral nervous system — wraps around the axon in segments, leaving tiny gaps called nodes of Ranvier exposed. These gaps are essential for saltatory conduction, which we will explore further in Section 5.
The Action Potential — Step by Step
The action potential is the electrical impulse that travels along a neuron's axon. It occurs because ions — mainly sodium (Na⁺) and potassium (K⁺) — move across the membrane through voltage-gated ion channels. The entire event lasts only about 1–2 milliseconds at any given point, but its effects propagate rapidly along the axon.
Phases of the Action Potential
- Resting state (−70 mV): The Na⁺/K⁺ ATPase pump maintains the resting potential by actively transporting 3 Na⁺ out and 2 K⁺ in per cycle. K⁺ leak channels allow some potassium to diffuse out, keeping the inside negative.
- Depolarisation to threshold (−55 mV): A stimulus (from a sensory receptor or another neuron) causes some Na⁺ channels to open. If the membrane reaches about −55 mV, threshold is reached and an action potential is triggered.
- Rapid depolarisation (−55 → +30 mV): Voltage-gated Na⁺ channels open in a positive feedback loop. Na⁺ rushes into the cell, making the inside rapidly positive.
- Repolarisation (+30 → −70 mV): Na⁺ channels inactivate and voltage-gated K⁺ channels open. K⁺ flows out, restoring the negative charge inside.
- Hyperpolarisation (−70 → −80 mV): K⁺ channels are slow to close, so potassium continues leaving briefly, overshooting the resting potential. The Na⁺/K⁺ pump then restores normal ion concentrations.
Synaptic Transmission & Saltatory Conduction
Once the action potential reaches the end of the axon, it must cross the synapse — a tiny gap (about 20 nm wide) between the presynaptic neuron and the postsynaptic cell. Because the electrical impulse cannot jump across this gap, the signal is converted from electrical to chemical. This process is called synaptic transmission.
Excitatory vs. Inhibitory Synapses
Not all synapses excite the next neuron. Excitatory neurotransmitters (such as acetylcholine and glutamate) make the postsynaptic membrane more likely to fire by causing depolarisation. Inhibitory neurotransmitters (such as GABA) make the postsynaptic membrane less likely to fire by causing hyperpolarisation. The postsynaptic neuron effectively "adds up" all the excitatory and inhibitory inputs it receives — a process called summation — to determine whether or not to fire its own action potential.
Saltatory Conduction
In myelinated neurons, the action potential does not travel continuously along the axon. Instead, it "jumps" from one node of Ranvier to the next, a process called saltatory conduction (from the Latin "saltare," meaning to jump). This is much faster than continuous conduction in unmyelinated neurons because the depolarisation only needs to occur at the nodes, where voltage-gated Na⁺ channels are concentrated. Myelinated neurons can conduct impulses at speeds up to 120 m/s, compared to about 1–2 m/s in thin, unmyelinated fibres.
Worked Example — Tracing a Reflex Arc
Let's trace the neural signalling pathway involved in a simple reflex — pulling your hand away from a hot pan. This is called the withdrawal reflex, and it illustrates every concept we have covered so far.
Nervous vs. Hormonal Communication
The nervous system is not the only way the body coordinates responses. The endocrine system uses hormones carried in the blood to communicate between organs. Understanding the differences — and similarities — between these two systems is essential for IB Biology.
| Feature | Nervous System | Endocrine System |
|---|---|---|
| Signal type | Electrical impulses (action potentials) + neurotransmitters | Chemical signals (hormones) in the blood |
| Speed | Very fast (milliseconds) | Slower (seconds to hours) |
| Duration | Short-lived; stops when impulse stops | Long-lasting; effects can persist |
| Target | Specific cells via synapses (localised) | Any cell with the appropriate receptor (widespread) |
| Transmission path | Along neurons (point-to-point wiring) | Through the bloodstream |
| Example | Withdrawal reflex (hand from hot surface) | Adrenaline release during a fight-or-flight response |
Connections to Advanced Neurobiology
The principles of neural signalling that you've learned form the foundation for several advanced topics in biology and medicine. While you won't need to master these for your IB exam, understanding how they connect will deepen your appreciation of why this topic matters.
| IB Core Concept | Advanced Extension |
|---|---|
| Action potential propagation | The Hodgkin-Huxley model uses differential equations to predict the exact timing and shape of action potentials based on ion conductances. |
| Neurotransmitter binding to receptors | Pharmacology studies how drugs can mimic (agonists), block (antagonists), or modulate neurotransmitter action — the basis for antidepressants, anaesthetics, and painkillers. |
| Myelin and saltatory conduction | Demyelinating diseases such as multiple sclerosis (MS) result from damage to the myelin sheath, slowing or blocking nerve impulses and causing loss of motor and sensory function. |
| Summation of excitatory and inhibitory signals | Neural networks and artificial intelligence (AI) are modelled on the way biological neurons integrate inputs. Machine learning algorithms use simplified "neurons" that sum weighted inputs. |
If you continue studying biology or medicine at university, the neural signalling mechanisms you learn here will expand into topics such as synaptic plasticity (the basis of learning and memory), neuroimaging techniques like fMRI, and the molecular mechanisms of neurological disorders. Every one of those advanced fields begins with the same fundamental events: ion channels opening, action potentials firing, and neurotransmitters crossing the synapse.
Practice Problems
Lesson Summary
Neural signalling is the process by which neurons generate and transmit information using electrical and chemical signals. At rest, a neuron maintains a resting membrane potential of about −70 mV, established by the Na⁺/K⁺ ATPase pump and K⁺ leak channels. When a stimulus brings the membrane to threshold (−55 mV), voltage-gated Na⁺ channels trigger an action potential — a rapid depolarisation to +30 mV followed by repolarisation. This follows the all-or-nothing principle, where signal strength is encoded by frequency, not amplitude. In myelinated neurons, saltatory conduction speeds up transmission by allowing the impulse to jump between nodes of Ranvier.
At the synapse, the electrical signal is converted to a chemical one: Ca²⁺ entry triggers exocytosis of neurotransmitters into the synaptic cleft, which then bind to receptors on the postsynaptic membrane. Excitatory and inhibitory signals are integrated through summation to determine whether the next neuron fires. The nervous system provides rapid, short-lived, and targeted responses, complementing the slower but longer-lasting effects of the endocrine system.