IB Biology HLNerves & SignallingPaper 1 & 2~11 min read
Neurones in the Brain
Pain is not in your hand. Your hand only sends a signal — the pain itself is created in your brain, and that gap between detecting and feeling is where this whole topic ends up.
📚 What you need to know
Pain receptors, or nociceptors, are sensory receptors with free nerve endings that are unencapsulated.
Their exposed dendrites carry transient receptor potential (TRP) channels, which open in response to stimuli that suggest a risk of tissue damage.
Those stimuli include high temperature, acid, and chemicals such as capsaicin in chilli peppers.
Entry of positively charged ions causes the threshold potential to be reached, generating an action potential that travels along the sensory neurone to the CNS.
Impulses pass to the cerebral cortex, where pain is perceived and a protective response results. This gives a survival and reproductive advantage.
The cerebrum is the largest part of the human brain — about 80% of the total mass — and handles conscious activities.
It is divided into two hemispheres, each with a thin outer layer called the cerebral cortex or grey matter, made of cell bodies.
The cortex is highly folded, increasing surface area, so more neurones and more connections fit in — giving emergent properties and consciousness.
How pain is detected
Pain receptors are called nociceptors, and they are simply sensory receptors. What makes them different from most other receptors is what they are not wrapped in.
They have free nerve endings, which are unencapsulated — there is no capsule or specialised structure around them.
Because the dendrites are exposed, they sit right in the tissue and can respond to a whole range of different stimuli.
Those exposed dendrites carry transient receptor potential (TRP) channels, which open when a stimulus indicates a risk of damaged tissue.
Stimuli that open TRP channels include:
High temperature
Acid
Chemicals, for example capsaicin in chilli peppers
Capsaicin explains why chilli feels hot rather than just tasting strong. It opens the same channels that heat opens, so the brain receives exactly the signal it would get from something burning your mouth. Nothing is actually hot — the receptor has been fooled.
From stimulus to feeling
Notice that nothing is felt until stage four. The first three stages are just ions moving and impulses travelling — pain itself is made by the brain.
Nerve impulses pass through the neurones to the cerebral cortex in the brain, where pain is perceived and a protective response results — pulling your hand back, dropping something, moving away. An organism sensitive to these stimuli avoids damage, survives longer and is more likely to reproduce, so this sensitivity gives a survival and reproductive advantage.
Careful with reflexes. The protective movement can be a spinal reflex that happens before the impulse reaches the cortex. So you often move your hand away before you feel the pain. Detecting and perceiving are two different events.
The cerebrum
The cerebrum is the largest part of the brain in humans, accounting for about 80% of the total mass of the brain. It carries out a large variety of functions involved with conscious activities, including:
Vision
Hearing
Speech
Thinking
Memory
It is divided into two hemispheres. Each one has a thin outer layer known as the cerebral cortex, or grey matter, and this layer consists of the cell bodies of neurones.
The folds are the point. A flat sheet the size of the skull would hold a fraction of the neurones, and far fewer of the connections that complex behaviour depends on.
Why folding matters
This is a chain of consequences, and the exam wants the whole chain:
The cortex is highly folded, which increases its surface area.
A larger surface area allows it to contain a greater number of neurones.
With more neurones, more connections between neurones can be made.
The more connections there are, the greater the ability of the brain to carry out complex behaviours.
Behaviours that result from a combination of complex pathways are called emergent properties — you cannot predict them by looking at a single neurone.
It is in this part of the brain that interactions between neurones lead to consciousness. This idea of consciousness includes the qualitative perception of feelings associated with colour, temperature and sound, as well as communication, all of which results in a complex awareness of the environment.
🧠
The folding chain
Folds → surface area → neurones → connections → complex behaviour → consciousness. Six links. Write them in order and you will collect every mark in the question.
Worked examples
WE 1
Describe how a painful stimulus is detected and perceived
Describe how touching a very hot surface leads to the perception of pain. (4 marks)
Point 1: the receptor
Nociceptors in the skin have free, unencapsulated nerve endings with TRP channels in their exposed dendrites.
Point 2: the channels open
High temperature indicates a risk of tissue damage, so the TRP channels open and positively charged ions enter.
Point 3: the impulse
This brings the membrane to threshold potential, generating an action potential that travels along the axon of the sensory neurone to the CNS.
Point 4: perception
Impulses pass through neurones to the cerebral cortex, where pain is perceived and a protective response follows.
TRP channels open, threshold reached, impulse to the cortex, pain perceivedsay perceived in the cerebral cortex. "The message goes to the brain" is too vague for the final mark
WE 2
Explain the significance of a folded cortex
Explain how the highly folded structure of the cerebral cortex relates to the complex behaviours humans are capable of. (3 marks)
Step 1: the physical effect
Folding increases the surface area of the cortex without needing a larger skull.
Step 2: what fits in
A greater surface area holds a greater number of neurone cell bodies, so many more connections between neurones can form.
Step 3: the consequence
More connections mean more complex pathways, allowing more complex behaviours — these are emergent properties, and interactions between neurones here lead to consciousness.
More folds, more neurones, more connections, more complex behaviourthe term emergent properties is specifically named in the syllabus, so use it
💡 Exam tips
Learn the full name: transient receptor potential (TRP) channels.
Use the word unencapsulated when describing nociceptors — it is what distinguishes them.
Say positively charged ions enter, not specifically sodium, unless the question tells you which ion.
Learn the 80% figure for the cerebrum and the term grey matter for the cortex.
Write the folding chain in order. Each link tends to be worth a mark.
Keep detection (at the receptor) separate from perception (in the cortex).
⚠ Common mistakes
Saying pain is felt in the skin. The skin detects; the cerebral cortex perceives.
Saying nociceptors are encapsulated. They are the opposite — free and exposed.
Saying capsaicin burns the mouth. It opens the same channels heat does; there is no actual heat damage.
Saying the cortex is folded to fit more blood vessels. It is about surface area for cell bodies.
Describing grey matter as axons. Grey matter is cell bodies.
Using "emergent properties" without explaining it. Say it means behaviours arising from combinations of complex pathways.
That completes Nerves & Signalling. The nine notes tell one continuous story: a cell shaped to carry a signal, a charge held ready across its membrane, a spike that is all-or-nothing, a chemical handover at every junction, and finally billions of those junctions working together until something that started as ions moving across a membrane becomes a thought.
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