Apply Neural Signalling - IB Biology

SECTION 1

Historical Context & Motivation

For centuries, humans wondered how the brain communicates with the rest of the body. Ancient Greeks believed that spirits flowed through hollow nerves, while Renaissance thinkers proposed mechanical explanations. The true story of neural signalling began to unfold in the late 18th century, when scientists discovered that electricity plays a central role in nerve function. Each breakthrough brought us closer to understanding the remarkable electrochemical language that neurons use to transmit information at speeds up to 120 metres per second.

1791 • Galvani's Frog Legs

Luigi Galvani demonstrated that electrical stimulation could cause a frog's leg muscles to contract, providing the first evidence that nerves transmit electrical signals — a concept he called "animal electricity."

1852 • Helmholtz Measures Nerve Speed

Hermann von Helmholtz measured the speed of nerve impulses in frogs at roughly 27 m/s, disproving the idea that nerve signals travel instantaneously and showing they are physical, measurable events.

1897 • Sherrington Names the Synapse

Charles Sherrington coined the term "synapse" to describe the tiny gap between neurons where signals pass from one cell to the next, laying the foundation for understanding chemical neurotransmission.

1952 • Hodgkin & Huxley Model

Alan Hodgkin and Andrew Huxley used the giant squid axon to produce a mathematical model of the action potential, explaining how voltage-gated ion channels generate electrical impulses. They won the Nobel Prize in 1963.

2000s • Optogenetics & Modern Neuroscience

Scientists developed optogenetics, a technique that uses light to activate or silence specific neurons, allowing researchers to map neural circuits with unprecedented precision.

These discoveries raised a fundamental question that remains at the heart of IB Biology: How do neurons generate, transmit, and pass on electrical signals to coordinate the body's responses? Understanding neural signalling means understanding the electrochemical mechanisms that underpin every thought, reflex, and sensation you experience.

SECTION 2

Core Principles of Neural Signalling

Neural signalling relies on a series of elegant electrochemical events. Neurons are specialised cells that carry information in the form of electrical impulses along their membranes and then convert those impulses into chemical messages at junctions called synapses. To understand how this works, you need to grasp several foundational ideas.

Resting Membrane Potential

A neuron at rest maintains a voltage difference of about −70 mV across its membrane, with the inside more negative than the outside. This is established by the sodium-potassium pump (Na⁺/K⁺-ATPase), which actively pumps 3 Na⁺ out and 2 K⁺ in per cycle, and by 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. K⁺ channels then open to repolarise the membrane. This all-or-nothing spike is the action potential.

Propagation Along the Axon

The action potential travels along the axon like a wave. In myelinated neurons, the signal jumps between gaps called nodes of Ranvier in a process known as saltatory conduction, greatly increasing speed.

Synaptic Transmission

At the synapse, the electrical signal is converted into a chemical one. Neurotransmitters are released from vesicles in the presynaptic neuron, diffuse across the synaptic cleft, and bind to receptors on the postsynaptic membrane.

Excitation & Inhibition

Neurotransmitters can be excitatory (making the postsynaptic neuron more likely to fire) or inhibitory (making it less likely). The postsynaptic neuron integrates these signals to decide whether to fire.

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KEY TAKEAWAY

Think of neural signalling like a relay race with a twist. The action potential is a runner sprinting along the axon (the track), but at the synapse the runner can't jump to the next track. Instead, they throw a baton (neurotransmitter) across a gap, and the next runner catches it to start their own sprint. This two-part system — electrical along the axon, chemical across the synapse — gives the nervous system both speed and fine-tuned control.

SECTION 3

Visualising the Action Potential

The action potential is the core electrical event in neural signalling. The diagram below shows how the membrane potential of a neuron changes over time during an action potential, from the resting state through depolarisation, repolarisation, and the refractory period.

The action potential graph shows the characteristic spike in membrane potential. Starting at the resting potential (−70 mV), a stimulus pushes the voltage past threshold (−55 mV), triggering rapid depolarisation to +30 mV, followed by repolarisation and a brief hyperpolarisation before returning to rest.

Notice that the action potential is an all-or-nothing event. If the stimulus reaches threshold, the neuron fires a full-sized action potential regardless of how strong the stimulus is. Stronger stimuli do not produce bigger action potentials; instead, they increase the frequency of action potentials. During the brief refractory period that follows, the neuron cannot fire again immediately. This prevents the signal from travelling backwards and ensures one-way propagation along the axon.

SECTION 4

The Mechanism Step by Step

Ion Movements During the Action Potential

The action potential depends on the selective opening and closing of voltage-gated ion channels in the neuron's membrane. At rest, the sodium-potassium pump maintains a concentration gradient: Na⁺ is concentrated outside the cell and K⁺ is concentrated inside. Potassium leak channels allow some K⁺ to diffuse outward, contributing to the negative resting potential.

RESTING POTENTIAL MAINTAINED BY

Na⁺/K⁺-ATPase: 3 Na⁺ out, 2 K⁺ in (per ATP)

This active pump uses ATP to move Na⁺ and K⁺ against their concentration gradients. The net export of one positive charge per cycle contributes to the inside being more negative than the outside.

Depolarisation, Repolarisation, and Hyperpolarisation

Depolarisation: A stimulus causes some Na⁺ channels to open. If enough Na⁺ enters to reach threshold (−55 mV), many more voltage-gated Na⁺ channels open in a positive-feedback loop. Na⁺ floods inward, driving the membrane potential to about +30 mV.
Repolarisation: After roughly 1 ms, the Na⁺ channels inactivate (close) and voltage-gated K⁺ channels open. K⁺ rushes out, restoring the negative charge inside the cell.
Hyperpolarisation: K⁺ channels close slowly, so slightly too much K⁺ leaves. The membrane potential briefly dips below −70 mV (to about −80 mV) before the Na⁺/K⁺ pump restores resting conditions.

Synaptic Transmission — Chemical Signalling

When the action potential reaches the axon terminal (also called the synaptic knob), it triggers a sequence of chemical events. Voltage-gated Ca²⁺ channels open, allowing calcium ions to enter the terminal. The influx of Ca²⁺ causes synaptic vesicles to fuse with the presynaptic membrane through a process called exocytosis. Neurotransmitter molecules are released into the synaptic cleft (a gap of about 20 nm), diffuse across, and bind to specific receptors on the postsynaptic membrane. Depending on the type of neurotransmitter and receptor, the postsynaptic cell may be excited or inhibited.

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IB Exam Tip

You must be able to describe the sequence of events at a synapse in the correct order: action potential arrival → Ca²⁺ influx → vesicle fusion (exocytosis) → neurotransmitter diffusion across cleft → binding to postsynaptic receptors → ion channels open/close on postsynaptic cell. Marks are often lost for getting this sequence wrong or skipping a step.

SECTION 5

Synaptic Structure and Signal Integration

The synapse is where the nervous system gains its remarkable flexibility. Unlike the all-or-nothing action potential, synaptic transmission can be modulated — strengthened, weakened, or blocked. This is the basis of learning, memory, and the action of many drugs.

This diagram shows the key structures of a chemical synapse. Vesicles in the presynaptic terminal contain neurotransmitter molecules. When Ca²⁺ enters through calcium channels, vesicles fuse with the membrane and release neurotransmitters into the synaptic cleft. These bind to receptors on the postsynaptic membrane, triggering an excitatory or inhibitory response.

Signal Integration: Summation

A single postsynaptic neuron may receive signals from thousands of other neurons simultaneously. The postsynaptic cell adds up all the excitatory and inhibitory inputs through a process called summation. There are two types. Spatial summation occurs when signals from multiple presynaptic neurons arrive at the same time and combine their effects. Temporal summation occurs when a single presynaptic neuron fires repeatedly in quick succession, and the effects of each signal build up before the previous one fades. If the combined excitatory signals outweigh the inhibitory ones and push the postsynaptic membrane past threshold, an action potential is triggered in the postsynaptic neuron.

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Neurotransmitter Removal

After a neurotransmitter has delivered its message, it must be removed from the synaptic cleft to prevent continuous stimulation. Three mechanisms accomplish this: (1) enzymatic breakdown — for example, acetylcholinesterase breaks down acetylcholine; (2) reuptake — the presynaptic neuron reabsorbs the neurotransmitter; and (3) diffusion — the molecule simply drifts away from the cleft.

SECTION 6

Worked Example: Tracing a Neural Signal

Let's trace the complete path of a signal from a pain receptor in your finger to a response in your arm muscle. This example will tie together every concept covered so far.

From Pain Stimulus to Muscle Response

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Step 1 — Stimulus and Receptor Activation

You prick your finger on a pin. Nociceptors (pain receptors) in your skin detect the mechanical damage. The stimulus causes Na⁺ channels in the receptor membrane to open, generating a small depolarisation called a receptor potential.

Receptor potential generated at the sensory neuron ending
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Step 2 — Threshold Reached and Action Potential Initiated

If the receptor potential is strong enough, it depolarises the sensory neuron's membrane past the threshold of −55 mV. Voltage-gated Na⁺ channels open in a positive-feedback cascade, producing a full action potential that peaks at about +30 mV.

Action potential fires (all-or-nothing) at +30 mV
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Step 3 — Propagation Along the Axon

The action potential propagates along the myelinated axon of the sensory neuron via saltatory conduction, jumping from one node of Ranvier to the next. This dramatically increases the speed of transmission, potentially reaching 120 m/s in heavily myelinated fibres.

Signal travels rapidly via saltatory conduction toward the spinal cord
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Step 4 — Synaptic Transmission to Relay Neuron

At the spinal cord, the action potential reaches the axon terminal of the sensory neuron. Ca²⁺ enters through voltage-gated calcium channels, causing vesicles to fuse with the presynaptic membrane via exocytosis. Neurotransmitter (e.g., glutamate) diffuses across the 20 nm synaptic cleft and binds to receptors on a relay (interneuron) neuron, opening Na⁺ channels and potentially triggering a new action potential in the relay neuron.

Chemical signal crosses synapse → new action potential in relay neuron
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Step 5 — Motor Neuron Activation and Muscle Response

The relay neuron synapses with a motor neuron. The same process of Ca²⁺-triggered neurotransmitter release occurs again. At the neuromuscular junction (the synapse between motor neuron and muscle fibre), acetylcholine is released. It binds to receptors on the muscle fibre, opening Na⁺ channels and initiating muscle contraction. You pull your finger away from the pin.

Acetylcholine triggers muscle contraction → withdrawal reflex

SECTION 7

Factors Affecting Neural Signalling & Drug Action

Neural signalling can be modified by a variety of biological factors and pharmacological agents. Understanding how drugs and diseases alter synaptic function is an important application of the neural signalling concepts you have learned. The table below compares key factors that influence the speed and effectiveness of neural signalling.

Factor	Effect on Neural Signalling	Example / Application
Myelination	Increases speed via saltatory conduction; signals jump between nodes of Ranvier	Multiple sclerosis destroys myelin, slowing or blocking signal transmission
Axon diameter	Larger axons have lower resistance to ion flow, increasing conduction speed	Giant squid axon (up to 1 mm diameter) used in Hodgkin–Huxley experiments
Temperature	Higher temperature increases rate of ion diffusion and enzyme activity, speeding transmission up to a limit	Hypothermia slows neural processing, which is why cold limbs feel numb
Drugs — agonists	Mimic neurotransmitters and bind to receptors, activating the postsynaptic cell	Nicotine mimics acetylcholine at certain receptors in the brain
Drugs — antagonists	Block receptors without activating them, preventing the neurotransmitter's effect	Curare blocks acetylcholine receptors at neuromuscular junctions, causing paralysis
Reuptake inhibitors	Block reuptake of neurotransmitter into the presynaptic neuron, prolonging its effect in the cleft	SSRIs (e.g., fluoxetine) block serotonin reuptake, used to treat depression

Factors and drugs that modify neural signalling

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KEY TAKEAWAY

Think of a synapse like a phone conversation. An agonist drug is like someone who sounds exactly like your friend and tricks you into responding. An antagonist drug is like putting the phone on airplane mode — the call comes in but you can't hear it. A reuptake inhibitor is like leaving the voicemail on speaker so the message keeps replaying over and over. These real-world drug mechanisms show why understanding synapses matters for medicine and pharmacology.

SECTION 8

Connections to Advanced Topics

The neural signalling principles you have studied form the foundation for more advanced topics in neuroscience and medicine. The table below shows how key concepts in this lesson connect to higher-level ideas you may encounter in university biology, psychology, or medical studies.

IB Biology Concept	Advanced Extension
Action potential (all-or-nothing)	Hodgkin–Huxley equations mathematically model Na⁺ and K⁺ conductances using differential equations
Saltatory conduction in myelinated neurons	Cable theory explains how passive electrical spread between nodes enables the speed boost of myelination
Excitatory and inhibitory neurotransmitters	Synaptic plasticity (LTP and LTD) — the strengthening or weakening of synapses that underlies learning and memory
Drug effects on synapses (agonists/antagonists)	Psychopharmacology — how psychiatric medications target specific receptor subtypes and neurotransmitter pathways
Summation of excitatory and inhibitory signals	Neural networks and computational neuroscience — artificial neural networks are inspired by biological summation

How IB Biology neural signalling connects to advanced study

If you continue studying neuroscience, you will see how these foundational mechanisms scale up to explain everything from reflexes and voluntary movements to consciousness, emotion, and even artificial intelligence. The basic principles of ion channel gating, electrochemical gradients, and chemical neurotransmission remain the same at every level of complexity.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain why the action potential is described as an "all-or-nothing" event. If a neuron receives a stimulus that is twice as strong as threshold, how does the nervous system encode the increased intensity?

Show answer

The action potential is all-or-nothing because once the membrane potential reaches threshold (approximately −55 mV), a full action potential of the same magnitude (+30 mV) is always generated regardless of stimulus strength. If the stimulus is below threshold, no action potential occurs at all. The nervous system encodes greater stimulus intensity not by making bigger action potentials but by increasing the frequency of action potentials fired per unit time. A stronger pain stimulus, for example, causes the sensory neuron to fire many action potentials in rapid succession rather than producing a single larger spike.

Question 2: BASIC CALCULATION

A myelinated sensory neuron conducts an action potential at 100 m/s. If the distance from a receptor in the fingertip to the spinal cord is 0.8 m, calculate the time it takes for the signal to travel this distance.

Show answer

Using the relationship time = distance ÷ speed, we get: time = 0.8 m ÷ 100 m/s = 0.008 s, or 8 milliseconds. This calculation illustrates why myelinated neurons enable such rapid reflexes. Even though the distance seems short, the signal must still travel physically along the axon, and faster conduction speed means faster reaction times. Note that the total reflex time would be longer because synaptic transmission at each synapse adds approximately 0.5–1 ms of delay.

Question 3: INTERMEDIATE

A patient is diagnosed with a disease that destroys the myelin sheath around motor neurons. Explain, using your knowledge of saltatory conduction and the action potential, how this disease would affect the patient's ability to move their limbs.

Show answer

Without the myelin sheath, saltatory conduction cannot occur. In a healthy myelinated neuron, the action potential jumps from one node of Ranvier to the next, which greatly increases conduction speed and reduces the energy cost of signalling. When myelin is destroyed, the action potential must propagate continuously along the entire length of the axon membrane, which is much slower. Additionally, the exposed axon membrane may not have enough voltage-gated Na⁺ channels between the former nodes to sustain the action potential, so the signal may weaken and fail to reach the axon terminal. This means the motor neuron cannot reliably transmit signals to muscles, leading to muscle weakness, loss of coordination, and eventually paralysis — symptoms characteristic of diseases such as multiple sclerosis.

Question 4: APPLIED

A certain insecticide works by inhibiting the enzyme acetylcholinesterase at the neuromuscular junction. Predict and explain the effect this insecticide would have on an insect's muscles, and explain why it is lethal.

Show answer

Acetylcholinesterase normally breaks down acetylcholine (ACh) in the synaptic cleft after it has bound to postsynaptic receptors. If this enzyme is inhibited, ACh is not removed from the cleft and continues to stimulate the postsynaptic receptors on the muscle fibre repeatedly. This leads to continuous, uncontrolled muscle contraction (sustained depolarisation of the muscle cell membrane). The muscles cannot relax between contractions, which is known as spastic paralysis. This is lethal because the insect cannot perform essential functions like breathing or locomotion. The muscles that control ventilation remain permanently contracted, leading to suffocation. This is the mechanism of organophosphate insecticides, and similar nerve agents affect mammals through the same pathway.

Question 5: CRITICAL THINKING

A neuroscientist records the activity of a single postsynaptic neuron. She finds that the neuron receives input from three presynaptic neurons: Neuron A releases an excitatory neurotransmitter, Neuron B also releases an excitatory neurotransmitter, and Neuron C releases an inhibitory neurotransmitter. When only Neuron A fires, the postsynaptic neuron does not reach threshold. When Neurons A and B fire simultaneously, the postsynaptic neuron fires an action potential. When Neurons A, B, and C all fire simultaneously, the postsynaptic neuron does not fire. Using the concepts of EPSPs, IPSPs, and spatial summation, explain all three observations.

Show answer

This scenario demonstrates spatial summation and the integration of excitatory and inhibitory postsynaptic potentials. When only Neuron A fires, it produces an excitatory postsynaptic potential (EPSP) — a small depolarisation of the postsynaptic membrane. However, this single EPSP is not large enough to bring the membrane to threshold (−55 mV), so no action potential is generated. When Neurons A and B fire together, two EPSPs are produced simultaneously at different synaptic inputs on the postsynaptic cell. These EPSPs summate (spatial summation), and their combined depolarisation is enough to reach threshold, so the postsynaptic neuron fires. When all three neurons fire simultaneously, the EPSPs from A and B are counteracted by the inhibitory postsynaptic potential (IPSP) from Neuron C. The IPSP is a hyperpolarisation that makes the membrane more negative, effectively subtracting from the excitatory effect. The net change in membrane potential is now below threshold, so the postsynaptic neuron does not fire. This example shows how the nervous system performs "decision-making" at the cellular level — the postsynaptic neuron integrates all incoming signals and only fires if excitation outweighs inhibition sufficiently to reach threshold.

SUMMARY

Summary — Apply Neural Signalling

Summary

Neural signalling is a two-part electrochemical process. At rest, the sodium-potassium pump maintains a resting membrane potential of −70 mV. When a stimulus reaches threshold (−55 mV), voltage-gated Na⁺ channels open, causing depolarisation to +30 mV in an all-or-nothing action potential. K⁺ channels then open for repolarisation, followed by brief hyperpolarisation. The signal propagates along the axon, and in myelinated neurons it uses saltatory conduction to jump between nodes of Ranvier for maximum speed.

At the synapse, the electrical signal converts to a chemical one: Ca²⁺ influx triggers exocytosis of vesicles containing neurotransmitters, which diffuse across the synaptic cleft and bind to postsynaptic receptors. The postsynaptic cell may receive excitatory (EPSP) or inhibitory (IPSP) signals, which are integrated through spatial and temporal summation. Drugs such as agonists, antagonists, and reuptake inhibitors modify synaptic transmission, forming the basis of modern pharmacology.