Pain perception and consciousness

Cells (Interaction and interdependence) · Neural signalling · note 7 of 7

Spec C2.2.15, C2.2.16

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Pain perception and consciousnessSpec C2.2.15, C2.2.16

In short

Pain is perceived when neurons with free nerve endings in the skin are stimulated. Their nerve endings have channels for positively charged ions that open in response to high temperature, acid or chemicals such as capsaicin in chilli peppers. Positive ions enter, the threshold is reached and impulses travel to the brain, where pain is perceived.

Perception of pain

Pain receptors are neurons with free nerve endings in the skin.
The nerve endings have channels for positively charged ions. They open in response to a stimulus such as high temperature, acid, or certain chemicals such as capsaicin in chilli peppers.
Positively charged ions enter the nerve ending and depolarize it until the threshold potential is reached.
Nerve impulses pass along the neurons to the brain.
Pain is perceived in the brain, not in the skin.

Capsaicin opens the same channels as high temperature, which is why chilli peppers feel hot even though they do not change the temperature of the mouth.

Consciousness as an emergent property

Consciousness is a property that emerges from the interaction of individual neurons in the brain. No single neuron is conscious. The property arises only from huge numbers of neurons (tens of billions in the human brain) interacting through their synapses.

Emergent properties are another example of the consequences of interaction: a whole system has properties that its parts do not have on their own, so they cannot be predicted by studying the parts separately.

Exam tip:

Say pain is perceived in the brain. The receptors in the skin only generate nerve impulses.

Quick check

What are the two types of fibre that project from the cell body of a neuron?
Show answer
One long axon and multiple shorter dendrites.
How many sodium and potassium ions does one cycle of the sodium–potassium pump move?
Show answer
Three Na⁺ out and two K⁺ in, using one ATP.
Why can a signal only pass one way across a synapse?
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Only the presynaptic neuron releases neurotransmitter, and only the postsynaptic membrane has receptors for it.
What ion enters the presynaptic neuron and triggers neurotransmitter release?
Show answer
Calcium ions (Ca²⁺).
HL only Where are ion pumps and channels clustered in a myelinated fibre?
Show answer
At the nodes of Ranvier.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why is the resting potential negative?

The resting potential is negative because sodium–potassium pumps move three Na⁺ out for every two K⁺ in, the membrane lets K⁺ leak out much faster than Na⁺ leaks in, and negatively charged proteins stay inside the cytoplasm. Together these leave the inside of the neuron about 70 mV more negative than the outside.

How does a nerve impulse cross the synapse?

The impulse depolarizes the presynaptic membrane, so calcium ions enter and cause vesicles to release neurotransmitter by exocytosis. The neurotransmitter, such as acetylcholine, diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. Positive ions enter and depolarize it; if threshold is reached, a new action potential starts.

Why do myelinated neurons conduct impulses faster?

HL only Myelin insulates the axon, so action potentials can only occur at the nodes of Ranvier, where ion pumps and channels are clustered. Local currents spread under the myelin to the next node, so the impulse jumps from node to node. This saltatory conduction is much faster than depolarizing every part of the membrane in turn.

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