Coordination and Response
Matthew Williams
||10 min read|CSEC BiologyHormonesNervous SystemReflexSection BTropisms
Stimulus and response, the nervous system, reflex arcs, brain regions, the eye, glaucoma, invertebrate responses, endocrine hormones, plant tropisms, and drug abuse.
Living organisms need to detect and respond to changes in their environment to survive. Rapid changes are handled by the nervous system; slower, longer-lasting changes are managed by hormones. Both systems work by linking a stimulus (a change) to a response (an action).
Stimulus and Response
A stimulus is any change in the environment, internal or external, that can be detected by an organism. A response is the reaction to that stimulus. The pathway from stimulus to response follows a consistent pattern:
stimulus → receptor → coordinator → effector → response
- Receptor — detects the stimulus (e.g. light receptor in the eye, temperature receptor in skin)
- Coordinator — processes the signal and decides on a response (brain or spinal cord)
- Effector — carries out the response (muscle or gland)
Responding to stimuli is important for survival: finding food, avoiding predators, maintaining body temperature, and escaping harmful conditions.
The Nervous System
The nervous system is divided into:
- Central nervous system (CNS) — brain and spinal cord; processes information and coordinates responses
- Peripheral nervous system (PNS) — all nerves that connect the CNS to the rest of the body
Neurones
Neurones are specialised cells that carry electrical impulses. There are three types:
| Type | Function | Location |
|---|---|---|
| Sensory | carries impulses from receptor to CNS | sense organs to brain/spinal cord |
| Relay (interneurone) | connects sensory and motor neurones within the CNS | brain and spinal cord |
| Motor | carries impulses from CNS to effector | brain/spinal cord to muscles or glands |
A typical neurone has:
- dendrites — short extensions that receive impulses from other neurones
- cell body — contains nucleus and cytoplasm
- axon — long fibre that carries the impulse away from the cell body
- myelin sheath — fatty insulation that speeds up impulse transmission
- synaptic knob — tip of the axon where neurotransmitters are released
Synapses
A synapse is the gap between two neurones. Electrical impulses cannot jump the gap directly. Instead:
- Impulse arrives at synaptic knob
- Neurotransmitter molecules are released into the gap
- Neurotransmitter diffuses across and binds to receptors on the next neurone
- A new impulse is generated in the next neurone
Synapses ensure impulses travel in one direction only.
Reflex Actions
A reflex is a rapid, automatic, involuntary response to a stimulus. Reflexes are important because they protect the body before the brain has time to process the situation consciously.
Reflex arc: receptor → sensory neurone → relay neurone (in spinal cord) → motor neurone → effector
Example: touching something hot → hand pulls away before conscious thought. The signal detours through the spinal cord without waiting for the brain, then the brain receives the signal afterwards.
The Brain
The brain is the main coordinator of the nervous system. Three key regions to know:
| Region | Main functions |
|---|---|
| Cerebrum | conscious thought, memory, learning, language, voluntary movement, interpreting sensory information |
| Cerebellum | coordination of movement, balance, posture, fine motor control |
| Medulla oblongata | automatic (involuntary) functions: heart rate, breathing rate, peristalsis, swallowing |
The Eye
The eye is the sense organ for light. Each part has a specific function:
| Structure | Function |
|---|---|
| Cornea | transparent covering; refracts (bends) most of the incoming light |
| Iris | controls pupil size; adjusts amount of light entering |
| Pupil | opening in the iris through which light passes |
| Lens | adjusts focus by changing shape (accommodation) |
| Retina | contains light-sensitive cells (rods and cones) |
| Rods | detect light and dark; work in dim light; no colour |
| Cones | detect colour; require bright light; concentrated at fovea |
| Optic nerve | carries impulses from retina to brain |
Accommodation
Accommodation is the process of changing the shape of the lens to focus on objects at different distances.
| Viewing | Ciliary muscles | Suspensory ligaments | Lens shape | Focal length |
|---|---|---|---|---|
| Near object | contract | relax (slack) | fat (more curved) | short |
| Distant object | relax | taut (pull lens) | thin (less curved) | long |
Eye Defects
| Defect | Cause | Correction |
|---|---|---|
| Myopia (short-sightedness) | eyeball too long; image forms in front of retina | concave (diverging) lens |
| Hyperopia (long-sightedness) | eyeball too short; image would form behind retina | convex (converging) lens |
| Glaucoma | fluid pressure builds up inside the eye, damaging the optic nerve | eye drops, laser treatment, or surgery to reduce pressure |
Exam Tip
Glaucoma is caused by increased fluid pressure inside the eye — it damages the optic nerve and can cause blindness if untreated. It is different from myopia/hyperopia, which involve the shape of the eyeball.
Skin as a Sense Organ
The skin contains several types of receptor that respond to different stimuli:
- touch and pressure receptors — detect contact and weight
- temperature receptors — separate receptors for heat and cold
- pain receptors — respond to damaging stimuli; trigger protective reflexes
Hormones and the Endocrine System
Hormones are chemical messengers produced by endocrine glands, released into the blood, and carried to target organs. Their effects are slower but longer-lasting than nerve impulses.
| Gland | Hormone(s) | Target | Effect |
|---|---|---|---|
| Pancreas (islets of Langerhans) | insulin | liver, muscle, fat cells | lowers blood glucose: promotes glucose uptake and glycogen storage |
| Pancreas | glucagon | liver | raises blood glucose: breaks down glycogen to glucose |
| Adrenal glands | adrenaline | heart, muscles, liver | prepares body for "fight or flight": raises heart rate, dilates pupils, releases glucose |
| Pituitary gland | ADH | kidney collecting duct | increases water reabsorption |
| Pituitary gland | FSH, LH | ovaries | control menstrual cycle and ovulation |
| Ovaries | oestrogen, progesterone | uterus and elsewhere | control female reproductive cycle |
| Testes | testosterone | body | male secondary sexual characteristics; sperm production |
| Thyroid | thyroxine | whole body | regulates metabolic rate |
Nervous vs Hormonal Control
| Feature | Nervous | Hormonal |
|---|---|---|
| Speed | very fast (milliseconds) | slow (seconds to minutes) |
| Duration | short-lived | longer-lasting |
| Transmission | electrical impulse along neurones | chemicals in blood |
| Specificity | precise: specific target | widespread: reaches all cells but only target cells respond |
| Example | reflex action | insulin controlling blood sugar |
Responses of Invertebrates
Invertebrates such as woodlice, earthworms, and millipedes show clear behavioural responses to environmental stimuli. These responses improve survival:
| Stimulus | Typical response | Survival advantage |
|---|---|---|
| Light intensity | move away from bright light (negative phototaxis) | avoid desiccation and predators; remain under leaf litter |
| Temperature | move toward moderate warmth; avoid extremes | prevent overheating or freezing |
| Moisture | move toward humid areas (positive hydrotropism) | prevent desiccation; maintain cell function |
These responses can be investigated in a choice chamber — a container divided into two regions with different conditions. The number of animals in each region after a set time indicates their preference.
Plant Tropisms
Plants respond to environmental stimuli by growing toward or away from them. These growth movements are called tropisms.
| Tropism | Stimulus | Response |
|---|---|---|
| Phototropism | light | shoot grows toward light (positive); root grows away (negative) |
| Geotropism | gravity | root grows downward (positive); shoot grows upward (negative) |
Tropisms result from unequal growth — one side of the shoot or root elongates faster than the other, causing a bend. The detailed hormonal mechanism is beyond the CSEC syllabus; focus on the direction of growth and its survival advantage.
Drug Abuse
Drugs are substances that alter the way the body or mind functions. They include legal drugs (alcohol, caffeine, prescription medicines) and illegal drugs (heroin, cocaine). Misuse of any drug — including the misuse of prescription drugs such as diet pills, tranquilisers, steroids, and analgesics — has physiological, social, and economic consequences.
Alcohol
Alcohol (ethanol) is a legal depressant — it slows the nervous system:
- Short term: slowed reactions, impaired judgement, loss of coordination, lowered inhibitions
- Long term: liver cirrhosis (scarring that prevents normal liver function), brain damage, addiction, heart disease
Heroin (an illegal depressant)
Heroin is a highly addictive opiate that mimics natural pain-relief chemicals in the brain:
- Physiological: severe dependence; withdrawal is painful; sharing needles spreads HIV and hepatitis
- Social: crime to fund habit; breakdown of family relationships
- Economic: costly treatment; lost productivity; law enforcement burden
General Effects of Drug Abuse
| Effect type | Examples |
|---|---|
| Physiological | addiction and dependence; organ damage (liver from alcohol, lungs from tobacco); brain chemistry changes; withdrawal symptoms |
| Social | strained family relationships; crime; poor decision-making; loss of employment; neglect of responsibilities |
| Economic | cost of healthcare; reduced productivity; law enforcement costs; burden on families |
Exam Tip
Exam questions on drug abuse often ask for effects under multiple headings. Make sure you can give distinct answers for physiological, social, and economic effects — the same point will not score in two categories.