The Human Nervous System¶

Part of Homeostasis and Response.

The nervous system coordinates rapid responses to change. It is best understood as an information system that detects stimuli, processes them and sends impulses to effectors.

What You Need to Learn¶

Follow a nerve signal from a stimulus to a response, including the protective shortcuts used by reflexes. Connect the parts of the brain and eye to their jobs, explain focusing and vision correction, and interpret reaction-time results and explain body-temperature control. Higher Tier study also examines how evidence about brain function is gathered and why treatment is difficult.

Overview¶

The nervous system allows humans to react to their surroundings and coordinate behaviour. It works alongside the hormonal (endocrine) system:

• Nervous responses — fast (milliseconds), short-lived, electrical in nature.
• Hormonal responses — slower (seconds to minutes), longer-lasting, chemical in nature.

The nervous system is divided into:

• Central nervous system (CNS) — brain and spinal cord.
• Peripheral nervous system (PNS) — all nerves that run to and from the CNS, including sensory and motor nerves.

Actions can be voluntary (requiring conscious decision, e.g. picking up a cup) or involuntary/reflex (automatic and unconscious, e.g. withdrawing from pain).

Neurones¶

Electrical signals called nerve impulses travel along specialised cells called neurones (or nerve cells). A nerve contains bundles of nerve fibres from many neurones.

Types of Neurone¶

Structure of a Neurone¶

• Axon — long projection that carries the impulse.
• Myelin sheath — insulating fat layer around the axon; speeds up impulse transmission and prevents electrical short circuits.
• Cell body — contains the nucleus; its position differs between neurone types.
• Dendrons and dendrites — shorter extensions from the cell body that receive signals from other neurones.

Sensory Receptors¶

Receptors are groups of specialised cells that detect a specific stimulus and convert it into an electrical signal. Different receptors respond to different stimuli: temperature, pressure, light, chemicals, etc.

Synapses¶

A synapse is the junction between two neurones (or between a neurone and an effector). Impulses cannot jump the gap directly — they are transmitted chemically:

1. The electrical impulse reaches the end of the first axon (pre-synaptic membrane).
2. The impulse triggers the release of neurotransmitters from vesicles into the synaptic cleft (the gap).
3. Neurotransmitters diffuse across the cleft and bind to complementary receptors on the second neurone (post-synaptic membrane).
4. Binding triggers a new electrical impulse in the second neurone.

At a typical chemical synapse, neurotransmitter is released from the first neurone and the matching receptors are on the receiving membrane. This gives transmission across the synapse a direction. Some drugs (e.g. heroin) act by interfering with neurotransmitter receptors at synapses.

The Reflex Arc¶

A reflex is an involuntary, automatic, rapid response to a stimulus. Reflexes happen without a conscious decision. Many protective reflexes are coordinated in the spinal cord, while others, such as the pupil reflex, involve the brain. Avoiding conscious decision-making allows a rapid response that can limit injury.

Pathway (in order):

1. Stimulus detected by receptor.
2. Impulse travels along sensory neurone to the spinal cord (CNS).
3. Impulse passes across a synapse to a relay neurone in the spinal cord.
4. Impulse travels along motor neurone from spinal cord to effector.
5. Effector (muscle or gland) produces the protective response.

Withdrawing a hand from a hot surface follows this pathway. Some other reflexes, such as the knee-jerk reflex, have a simpler pathway without a relay neurone.

Investigating Reaction Time¶

Reaction time can be measured using the ruler-drop test. The distance fallen before the ruler is caught is converted to a reaction time using a reference table. It measures the whole response, including detection, processing and muscle action, rather than nerve-impulse speed alone. Repeat trials, keep the starting position and method consistent, and calculate a mean to compare conditions. For a graph, put the tested condition on the horizontal axis and mean reaction time, with its unit, on the vertical axis. A lower time means a faster response; differences smaller than the spread of repeated results provide weak evidence of an effect.

The Brain¶

The brain is the main coordination centre of the CNS. It contains billions of interconnected neurones organised into functional regions.

Investigating the Brain — Higher Tier¶

Scientists map brain regions to functions using several methods:

• MRI scans — magnetic fields and electromagnetic waves show brain structure and activity. No ionising radiation; safer for repeated use.
• CT scans — X-rays produce structural images; useful for identifying brain damage (e.g. stroke damage).
• PET scans — radioactive tracers highlight areas of high metabolic activity; can identify regions more or less active than normal.
• Electrical stimulation — applying small electrical currents to specific regions and recording responses; EEGs monitor brain electrical activity.
• Studying brain damage — patients with lesions in specific brain regions show loss of specific functions, revealing what those regions do.

Studying the brain is difficult because its interconnected regions may contribute to several functions, so damage rarely gives a simple one-region, one-function result. Surgery or electrical stimulation can damage delicate tissue. Treatment must avoid harming healthy regions, and damaged neurones are difficult to replace.

The Eye¶

The eye is a sensory organ containing receptors that detect light. It converts light signals into electrical impulses sent to the brain via the optic nerve.

Structure and Functions¶

Rods and Cones¶

The Pupil Reflex¶

The iris contains two sets of antagonistic muscles:

• In bright light: circular muscles contract, radial muscles relax → pupil constricts (gets smaller) → less light enters → protects retina.
• In dim light: radial muscles contract, circular muscles relax → pupil dilates (gets larger) → more light enters → improves vision.

Accommodation (Focusing)¶

The lens changes shape to focus on objects at different distances:

Focusing on near objects:

1. Ciliary muscles contract.
2. Suspensory ligaments go slack.
3. Lens becomes fatter and more curved → greater refraction → light focuses on retina.

Focusing on distant objects:

1. Ciliary muscles relax.
2. Suspensory ligaments become taut.
3. Lens becomes thinner and flatter → less refraction → light focuses on retina.

Eye Defects and Corrections¶

Myopia (short-sightedness):

• Cannot see distant objects clearly; focal point falls in front of the retina.
• Cause: eyeball too long or lens too curved.
• Correction: concave (diverging) lens — spreads light rays before they reach the eye.

Hyperopia (long-sightedness):

• Cannot see near objects clearly; focal point would fall behind the retina.
• Cause: eyeball too short or lens too flat.
• Correction: convex (converging) lens — brings light rays closer together before they reach the eye.

Treatments:

• Glasses or contact lenses (hard or soft).
• Laser surgery — reshapes the cornea permanently (mainly for myopia; some risk of infection or visual impairment).
• Lens replacement — artificial lens implanted surgically.

Cataracts: cloudy patches form on the lens, causing blurred vision. Treated by replacing the lens with an artificial one.

Colour blindness: faulty cone cells (usually red or green cones) prevent normal colour discrimination. Currently no cure.

Control of Body Temperature¶

The thermoregulatory centre in the brain monitors blood temperature using its own receptors. It also receives impulses from temperature receptors in the skin. These signals trigger responses that oppose a change in body temperature.

When the body is too hot, vasodilation widens the blood vessels supplying surface capillaries, increasing blood flow near the skin. More energy transfers to the surroundings. Sweat glands produce sweat; its evaporation transfers energy away from the skin and cools it.

When the body is too cold, vasoconstriction reduces blood flow near the surface and therefore reduces heat loss. Sweating stops, while skeletal muscles contract repeatedly in shivering. The extra muscle activity increases respiration, transferring energy that warms the body. Blood vessels change width; they do not move nearer to or further from the skin.

For Higher Tier, connect each response to energy transfer in a given situation: sweating cools only when the sweat evaporates, while shivering generates warming through increased respiration.

Common Confusions¶

• Reflex arc vs conscious response: a reflex does not require conscious thought, but some reflexes are coordinated in the brain. A withdrawal reflex is coordinated in the spinal cord.
• Relay neurone placement: relay neurones are in the CNS (spinal cord), not in the periphery.
• Accommodation muscle logic: ciliary muscles contracting makes the lens fatter (for near vision). Many students assume contracting pulls the lens tighter — but it is the suspensory ligaments that hold the lens taut, and when they go slack (because ciliary muscles contract inward), the elastic lens bulges.
• Pupil vs iris: the pupil is the gap; the iris is the muscle ring around it. The iris muscles change the pupil size, not vice versa.
• Myopia correction: myopia needs a concave lens (not convex). Long-sightedness needs convex.

Key Terms¶

• Reflex arc: the pathway taken by impulses in a rapid automatic response.
• Sensory neurone: a neurone that carries impulses from receptors to the central nervous system.
• Relay neurone: a neurone within the CNS that connects sensory and motor neurones.
• Motor neurone: a neurone that carries impulses from the central nervous system to effectors.
• Synapse: the junction between two neurones; impulses cross by neurotransmitter diffusion.
• Neurotransmitter: a chemical released at a synapse that carries the signal across the synaptic cleft.
• Accommodation: the process by which the lens changes shape to focus on near or distant objects.
• Vasodilation: widening of blood vessels near the skin surface to increase heat loss.
• Vasoconstriction: narrowing of blood vessels near the skin surface to reduce heat loss.
• Myelin sheath: the insulating fatty layer around a neurone's axon that speeds impulse transmission.
• Retina: the layer at the back of the eye containing rod and cone cells that detect light.
• Fovea: the region of the retina with the highest concentration of cone cells; gives the sharpest colour vision.
• Ciliary muscles: muscles in the eye that change the shape of the lens for accommodation.
• Suspensory ligaments: fibres that hold the lens in position in the eye.
• Myopia: short-sightedness; corrected by a concave lens.
• Hyperopia: long-sightedness; corrected by a convex lens.
• Cerebral cortex: the region of the brain responsible for conscious thought, intelligence, memory and language.
• Cerebellum: the region of the brain that coordinates movement and balance.
• Medulla oblongata: the region of the brain that controls unconscious processes such as heart rate and breathing.

Connected Pages¶

• Homeostasis
• Hormonal Coordination in Humans
• Biology Key Terms Glossary
• Required Practical Reference