The nervous systemIB MYP Biology: Revision notes
Section 1
The nervous system and the CNS
The nervous system lets the body detect changes and respond quickly. The central nervous system (CNS) is the brain and spinal cord. It processes information and decides what to do. Nerves are bundles of neurones that carry electrical impulses between the CNS and the rest of the body.
A change in the environment is a stimulus. The body's response follows this chain:
stimulus → receptor → coordinator → effector → response
- Receptors detect stimuli.
- The coordinator (brain or spinal cord) processes information.
- Effectors (muscles and glands) carry out the response.
Section 2
Sense organs and stimuli
Sense organs contain receptors that detect particular stimuli:
- Eye: light
- Ear: sound, and changes in position (balance)
- Nose: chemicals in the air (smell)
- Tongue: chemicals in food and drink (taste)
- Skin: touch, pressure, temperature and pain
Flavour combines taste and smell, which is why food seems bland with a blocked nose.
Section 3
Neurones
A neurone is a nerve cell that carries electrical impulses. There are three types:
- Sensory neurone: carries impulses from a receptor to the CNS
- Relay neurone: in the CNS, links sensory and motor neurones
- Motor neurone: carries impulses from the CNS to an effector
Neurones are long and thin, which helps them carry impulses over long distances.
Section 4
Synapses
Neurones do not touch. A small gap called a synapse separates them. When an impulse reaches the end of a neurone, it releases a chemical (neurotransmitter). The chemical diffuses across the gap and triggers a new impulse in the next neurone.
Synapses make sure that impulses travel in one direction only.
An electrical impulse does not jump the synapse. A chemical diffuses across it.
Section 5
Reflex arcs
A reflex is a fast, automatic response that does not involve conscious thought. The pathway is the reflex arc:
stimulus → receptor → sensory neurone → relay neurone (CNS) → motor neurone → effector → response
The brain is not involved in the decision, so the response is faster. Reflexes protect the body. Examples: pulling a hand away from a hot object, blinking when something comes near the eye, the pupil getting smaller in bright light, and the knee jerk.
Write the reflex arc in order and name the type of neurone between each part.
Section 6
Investigating reaction time
Reaction time is the time between a stimulus and the response. A simple test is the ruler drop: one person drops a ruler without warning, and the other catches it. The distance the ruler fell shows the reaction time: a shorter distance means a faster reaction.
- Independent variable: what you change (for example, which hand)
- Dependent variable: distance the ruler falls
- Control variables: same ruler, same drop position, same person dropping
Repeat several times, ignore any anomalous results, and calculate a mean.
Must Know
- CNS = brain and spinal cord; stimulus, receptor, coordinator, effector, response
- Sensory, relay and motor neurones
- A synapse is a gap; a chemical diffuses across it
- A reflex arc is fast and automatic
- Reaction time: shorter ruler distance = faster response
That's the notes covered.
Carry on to the next subtopic.
Exam questions on The nervous system
- A chef accidentally touches a hot pan with her hand. She pulls her hand away before she feels any pain, and only a moment later does she notice the burn.Explain why the reflex pulls the chef's hand away faster than a decision made by her brain.2 marks
- At a food festival in Mumbai, judges assess a new snack. They look at its colour, smell it, taste it, feel its texture in the mouth and listen to how loudly it crunches.Suggest why food seems to have less flavour when you have a blocked nose.2 marks
- A class investigates whether reaction time is different for the dominant hand and the non-dominant hand, using the ruler-drop test. One student holds a 30 cm ruler vertically above the open fingers of a partner and drops it without warning. The partner closes the fingers as fast as possible, and the distance the ruler fell is read from the top of the thumb. Each student does five drops with each hand. One student's distances for the dominant hand were 14, 12, 13, 25 and 13 cm. For the non-dominant hand they were 18, 20, 19, 17 and 21 cm.Identify the independent variable, the dependent variable and one control variable in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).