Coordination and control – the nervous system
What you'll learn
- How the nervous system is organised into the brain, spinal cord, neurones, receptors and effectors.
- How nerve impulses travel through reflex arcs to protect you quickly.
- How the eye focuses light and how common eye defects can be corrected.
- How brain regions link to functions, and why brain research and treatment can be difficult.
Start from the big picture
A multicellular organism is built in levels: cells form tissues, tissues form organs, organs work together in organ systems, and all the systems make the whole organism.
The nervous system is one of these organ systems. It helps your body detect changes and respond to them quickly.
Nervous system
The nervous system is the organ system made of the brain, spinal cord, nerves and sensory receptors that coordinates the body using electrical nerve impulses.
A stimulus is a change in the environment, such as heat, light, sound or pressure. A sensory receptor is a cell or group of cells that detects a stimulus. A response is what the body does afterwards. An effector is the muscle or gland that produces the response; a gland is a structure that releases a substance.
The main parts of the nervous system
The central nervous system, or CNS, is the brain and spinal cord. The CNS processes information and coordinates responses.
A neurone is a specialised nerve cell that carries electrical impulses. A nerve impulse is an electrical signal travelling along a neurone.
| Component | Main job | Useful structure detail |
|---|---|---|
| Sensory neurone | Carries impulses from receptors to the CNS | Has a long fibre and a cell body part-way along it |
| Relay neurone | Connects neurones inside the CNS | Found in the brain or spinal cord, often with many branches |
| Motor neurone | Carries impulses from the CNS to effectors | Has a cell body in the CNS and a long axon going to a muscle or gland |
An axon is the long part of a neurone that carries impulses away from the cell body. Dendrites are branched endings that receive impulses from other neurones or receptors. Some neurones have a myelin sheath, a fatty insulating layer that helps impulses travel faster.
A synapse is the tiny gap between two neurones. When an impulse reaches a synapse, a chemical messenger is released, diffuses across the gap, and starts an impulse in the next neurone.
Coordinated responses
The usual pattern is: stimulus detected by receptor, impulse travels along a sensory neurone, CNS processes the information, impulse travels along a motor neurone, and an effector produces the response.
The nervous system can coordinate the whole body because it reaches many parts of the body, has many links between neurones, and uses different receptors for different types of stimulus.
Reflex arcs: fast protective responses
A reflex is a rapid, automatic response to a stimulus. Reflexes are usually protective, such as pulling your hand away from something hot.
A reflex arc is the nerve pathway followed during a reflex. It often goes through the spinal cord rather than waiting for a conscious decision by the brain.
Explaining a withdrawal reflex
- The heat stimulus is detected by sensory receptors in the skin, which start nerve impulses.
- A sensory neurone carries the impulses towards the spinal cord, which is part of the CNS.
- At synapses in the spinal cord, the impulse passes from the sensory neurone to a relay neurone, then to a motor neurone.
- The motor neurone carries impulses to an effector muscle in the arm.
- The muscle contracts, pulling the hand away before you consciously decide to move it.
Reflexes are not just “fast thoughts”
In a reflex, the initial response is automatic and coordinated through the CNS without a conscious decision. The brain may receive information afterwards, which is why you then become aware of pain.
Investigating reaction time
A reaction time is the time between detecting a stimulus and making a response. In a ruler-drop test, one person drops a ruler and another catches it. The distance fallen can be used to compare reaction times.
To make the investigation more reliable, repeat the test, calculate a mean, and keep control variables the same, such as the same hand, same starting position and same instructions.
Interpreting reaction time data
A student measures reaction time before and after drinking caffeine. The reaction times are:
Before: 0.24 s, 0.22 s, 0.23 s
After: 0.19 s, 0.20 s, 0.18 s
- Calculate the mean before caffeine: 0.24+0.22+0.233=0.23 s\frac{0.24 + 0.22 + 0.23}{3} = 0.23\ \text{s}30.24+0.22+0.23=0.23 s.
- Calculate the mean after caffeine: 0.19+0.20+0.183=0.19 s\frac{0.19 + 0.20 + 0.18}{3} = 0.19\ \text{s}30.19+0.20+0.18=0.19 s.
- Compare the means: 0.19 s is lower than 0.23 s, so the student’s reaction time was faster after caffeine in this set of results.
The eye: detecting light
This eye content is for separate Biology, J247. It is not normally assessed in OCR Gateway Combined Science.
Your eyes do not “send out” anything to see. Light reflected from objects enters the eye, is focused onto the retina, and the brain interprets the nerve impulses. Different people may perceive the same image slightly differently because the brain is involved in interpretation.
| Eye structure | Function |
|---|---|
| Cornea | Transparent front surface; refracts, or bends, incoming light |
| Iris | Coloured muscle that controls pupil size |
| Pupil | Hole that lets light into the eye |
| Lens | Transparent structure that changes shape to focus light |
| Retina | Light-sensitive layer containing receptor cells |
| Optic nerve | Carries impulses from the retina to the brain |
| Ciliary body | Contains muscles that change lens shape |
| Suspensory ligaments | Hold the lens and transmit tension to it |
To focus on a near object, the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes thicker and more curved. This refracts light more strongly.
To focus on a distant object, the ciliary muscles relax, the suspensory ligaments become tight, and the lens becomes thinner and less curved.
Common eye defects
Short-sightedness means distant objects look blurred. The image focuses in front of the retina. It can be corrected using a concave, or diverging, lens.
Long-sightedness means near objects look blurred. The image focuses behind the retina. It can be corrected using a convex, or converging, lens.
Colour blindness is difficulty distinguishing some colours, often red and green. It is usually caused by problems with cone cells in the retina. It is not normally “fixed” by ordinary glasses, but filters, tinted lenses or practical adaptations can help some people.
Choosing the correct lens
- If distant objects are blurred and the image forms in front of the retina, the person is short-sighted.
- A concave lens is needed because it diverges light rays before they enter the eye.
- This shifts the focus backwards so the image forms on the retina instead of in front of it.
The brain: control and coordination
This brain content is also separate Biology, J247. The brain is part of the CNS and contains different regions with different functions.
- The cerebrum is involved in conscious thought, memory and voluntary actions.
- The cerebellum coordinates movement and balance.
- The medulla controls automatic processes such as breathing and heart rate.
- The hypothalamus helps with homeostasis, which means keeping internal conditions stable, including temperature and water balance.
- The pituitary gland releases hormones. A hormone is a chemical messenger carried in the blood.
If you are taking Higher Tier, you also need to explain why investigating and treating the brain can be difficult.
A case study is a detailed study of one person or a small number of people. Case studies of brain damage can be useful, but they are hard to interpret because each person is different and damage may affect several brain areas at once. There are also ethical issues, such as consent, privacy, and avoiding harm to patients.
Treating the brain and nervous system is difficult because nervous tissue has a limited ability to repair itself. Damage may be irreversible, surrounding tissues can also be damaged, and some parts of the nervous system are hard to access safely.
Explaining why nervous system damage is hard to treat
- If a pathway in the CNS is damaged, the neurones may not regrow or reconnect properly, so impulses may not travel as before.
- Trying to reach the damaged area can be risky because the brain and spinal cord are delicate and closely packed with important tissues.
- This means treatment may reduce symptoms or prevent further damage, but it may not fully restore the original function.
In the exam
- For nerve pathway questions, write the structures in order: receptor, sensory neurone, CNS, motor neurone, effector.
- For “explain” questions, link structure to function, such as “long axon carries impulses over a long distance”.
- For reaction-time data, calculate or compare means and quote figures with units.
- For eye defects, state where the image focuses and name the correcting lens.
Check yourself
- Can you trace the pathway of a reflex from stimulus to response?
- How do the ciliary muscles and suspensory ligaments change the lens for near and distant objects?
- Why are brain case studies useful but difficult to interpret?
