Understand Neural Signalling - IB Biology

SECTION 1

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.

1791 • Galvani's Frog Legs

Luigi Galvani demonstrated that electrical stimulation could make a frog's leg muscle twitch, providing the first evidence that animal tissues generate electricity.

1852 • Helmholtz Measures Nerve Speed

Hermann von Helmholtz measured the speed of nerve impulses in frogs at roughly 27 m/s, proving that signals are not instantaneous but travel at a measurable velocity.

1906 • Cajal & Golgi Win the Nobel Prize

Santiago Ramón y Cajal and Camillo Golgi shared the Nobel Prize for their work on the structure of the nervous system, establishing the neuron doctrine — the idea that the nervous system is made of individual cells (neurons) rather than a continuous network.

1952 • Hodgkin & Huxley Model

Alan Hodgkin and Andrew Huxley used the giant axon of a squid to describe the ionic mechanisms behind the action potential, winning the Nobel Prize in 1963 for this groundbreaking work.

1970s • Neurotransmitter Receptors Identified

Researchers identified specific receptor proteins on postsynaptic membranes, revealing how chemical signals such as acetylcholine and dopamine are received by target cells.

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.

SECTION 2

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

A neuron at rest maintains a voltage of about −70 mV across its membrane, with the inside more negative than the outside. This is established primarily by the sodium-potassium pump (Na⁺/K⁺ ATPase) and potassium leak channels.

Action Potential

When a stimulus reaches threshold (about −55 mV), voltage-gated Na⁺ channels open rapidly, causing depolarisation to about +30 mV. This all-or-nothing electrical impulse then travels along the axon.

Synaptic Transmission

At the end of the axon, the electrical signal is converted to a chemical one. Neurotransmitters are released into the synaptic cleft and bind to receptors on the next cell.

Saltatory Conduction

In myelinated neurons, the action potential "jumps" between gaps in the myelin sheath called nodes of Ranvier, dramatically increasing the speed of transmission up to 120 m/s.

Refractory Period

After firing, a neuron enters a brief refractory period during which it cannot fire again. This ensures signals travel in one direction and limits the maximum firing rate.

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KEY TAKEAWAY

Think of neural signalling like a stadium "wave." Each fan (ion channel) stands up (opens) and sits down (closes) in sequence — the wave travels along the stands, but no individual fan actually moves from one seat to another. Similarly, the electrical signal travels along the axon, but the ions themselves only move across the membrane locally. The wave is fast, directional, and repeatable — just like an action potential.

SECTION 3

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.

A motor neuron receives signals through its dendrites, integrates them at the cell body (soma), and conducts the action potential along its myelinated axon to the synaptic terminals, where neurotransmitters are released.

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.

SECTION 4

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.

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All-or-Nothing Principle

An action potential either fires fully or does not fire at all — there is no "half" action potential. If threshold is not reached, no impulse is generated. Once threshold is reached, the action potential always has the same amplitude (~100 mV change). The intensity of a stimulus is encoded by the frequency of action potentials, not by their size.

RESTING MEMBRANE POTENTIAL (SIMPLIFIED)

V_rest ≈ −70 mV

V_rest = resting membrane potential, measured in millivolts (mV). This value arises from the unequal distribution of Na⁺ and K⁺ ions and the selective permeability of the membrane, primarily to K⁺.

SODIUM-POTASSIUM PUMP RATIO

3 Na⁺ out : 2 K⁺ in per ATP hydrolysed

For every molecule of ATP used, the pump moves 3 sodium ions out of the cell and 2 potassium ions in. This net export of positive charge contributes to the negative resting potential.

SECTION 5

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.

Synaptic transmission converts an electrical signal into a chemical one. Vesicles release neurotransmitter into the cleft, where it binds to receptors on the postsynaptic membrane.

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.

SECTION 6

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.

Tracing the Withdrawal Reflex Arc

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Step 1 — Stimulus Detected

Thermoreceptors (sensory receptors in the skin) detect the high temperature. The heat causes a generator potential — a local depolarisation in the receptor cell. If this is strong enough, it triggers an action potential in the sensory neuron.

Action potential initiated in sensory (afferent) neuron.
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Step 2 — Transmission Along the Sensory Neuron

The action potential propagates along the sensory neuron's axon towards the spinal cord. If the neuron is myelinated, the impulse travels via saltatory conduction. Each section of the axon undergoes the same sequence: Na⁺ channels open (depolarisation), then K⁺ channels open (repolarisation).

Signal arrives at the dorsal root of the spinal cord.
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Step 3 — Synapse with Relay Neuron

In the spinal cord, the sensory neuron synapses with a relay neuron (also called an interneuron). Synaptic vesicles release neurotransmitter into the cleft. The neurotransmitter binds to receptors on the relay neuron, generating a new action potential.

Chemical signal crosses synapse → new electrical signal in relay neuron.
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Step 4 — Synapse with Motor Neuron

The relay neuron then synapses with a motor neuron (efferent neuron). The same process of synaptic transmission occurs: Ca²⁺ influx → vesicle fusion → neurotransmitter release → receptor binding.

Motor neuron carries the action potential toward the effector (muscle).
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Step 5 — Effector Response

The motor neuron synapses with a muscle fibre at the neuromuscular junction. Acetylcholine (ACh) is released, binds to receptors on the muscle cell, and triggers muscle contraction. Your hand pulls away from the hot surface — all in a fraction of a second.

Muscle contracts → hand withdraws. Reflex completed.

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Why Reflexes Are Fast

Notice that this reflex bypasses the brain entirely — the signal only needs to travel to the spinal cord and back. This reduces the number of synapses (typically just two or three), minimising the delay. The brain is informed afterwards, which is why you feel the pain after you've already moved your hand.

SECTION 7

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

Comparison of nervous and endocrine communication

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KEY TAKEAWAY

Think of the nervous system as a phone call — fast, direct, and aimed at one recipient. The endocrine system is more like a social media post — it reaches many people but takes longer and the effects may linger. Your body uses both systems, and they often work together. For example, during a stressful event, the nervous system triggers the immediate fight-or-flight reaction, while the endocrine system sustains it by releasing adrenaline and cortisol.

SECTION 8

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.

From IB core concepts to advanced applications

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.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain what is meant by the "all-or-nothing" principle of the action potential. Why does a stronger stimulus not produce a larger action potential?

Show answer

The all-or-nothing principle states that if a stimulus reaches threshold (about −55 mV), the action potential fires at its full amplitude (from roughly −70 mV to +30 mV); if threshold is not reached, no action potential occurs. A stronger stimulus does not produce a bigger action potential because the same voltage-gated Na⁺ channels are involved, and they either open fully or stay closed. Instead, a stronger stimulus is encoded by a higher frequency of action potentials — more impulses per second — rather than by a greater amplitude of each individual impulse.

Question 2: BASIC CALCULATION

A myelinated motor neuron conducts impulses at 100 m/s. If the neuron is 1.2 m long (for example, from the spinal cord to the foot), how long does it take for the signal to travel the full length of the axon? Express your answer in milliseconds.

Show answer

Using the relationship time = distance ÷ speed: t = 1.2 m ÷ 100 m/s = 0.012 s = 12 ms. The signal reaches the foot in just 12 milliseconds. This remarkable speed is due to saltatory conduction, where the action potential jumps between nodes of Ranvier along the myelinated axon, rather than depolarising every segment of the membrane.

Question 3: INTERMEDIATE

During synaptic transmission, calcium ions (Ca²⁺) enter the presynaptic terminal when the action potential arrives. Explain the sequence of events from Ca²⁺ entry to the generation of a new electrical signal in the postsynaptic neuron.

Show answer

When the action potential reaches the presynaptic terminal, voltage-gated Ca²⁺ channels open and Ca²⁺ flows into the terminal down its concentration gradient. The influx of Ca²⁺ causes synaptic vesicles (which are filled with neurotransmitter) to move toward and fuse with the presynaptic membrane in a process called exocytosis. Neurotransmitter molecules are released into the synaptic cleft and diffuse across the ~20 nm gap. They bind to specific receptor proteins on the postsynaptic membrane, causing ligand-gated ion channels to open. If the synapse is excitatory, Na⁺ enters the postsynaptic cell, depolarising it and potentially triggering a new action potential if threshold is reached.

Question 4: APPLIED

Multiple sclerosis (MS) is a disease in which the immune system attacks and destroys the myelin sheath around neurons. Using your knowledge of saltatory conduction, predict and explain how this disease would affect neural signalling and the symptoms a patient might experience.

Show answer

Without myelin, the action potential can no longer jump between nodes of Ranvier via saltatory conduction. Instead, it must propagate continuously along the bare axon, which is much slower. In severely affected areas, the action potential may fail entirely because ion channels are sparsely distributed along the internodal regions that were previously insulated. This would lead to symptoms such as muscle weakness, loss of coordination, numbness, tingling, blurred vision, and fatigue, depending on which neurons are demyelinated. Motor neurons controlling voluntary movement and sensory neurons carrying information from the extremities are commonly affected, explaining why many MS patients experience difficulty walking and altered sensation.

Question 5: CRITICAL THINKING

A neuron receives 15 excitatory inputs and 10 inhibitory inputs simultaneously from different presynaptic neurons. Explain the concept of summation, and discuss the factors that would determine whether this neuron fires an action potential.

Show answer

Summation is the process by which a postsynaptic neuron integrates all incoming excitatory and inhibitory signals to determine whether the net depolarisation at the axon hillock reaches threshold. In this example, the neuron receives both excitatory postsynaptic potentials (EPSPs), which depolarise the membrane, and inhibitory postsynaptic potentials (IPSPs), which hyperpolarise it. The neuron will fire only if the sum of all EPSPs minus all IPSPs brings the membrane potential at the axon hillock to about −55 mV. Factors that influence the outcome include: the magnitude of each individual postsynaptic potential, the proximity of each synapse to the axon hillock (closer synapses have more influence because the signal decays with distance), and the timing of the inputs — if excitatory signals arrive simultaneously (spatial summation) or in rapid succession (temporal summation), they are more likely to bring the neuron to threshold. This integration allows the nervous system to perform complex processing, not simply relay signals.

SUMMARY

Lesson Summary

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.