Cambridge O Level Biology · Syllabus 5090 · Coordination and Control
Receptor
What is Receptor?
A cell, or group of cells, that detects a stimulus and generates an electrical impulse in response. Receptors are selective: each type responds only to a particular kind of stimulus, so light receptors cannot detect sound and temperature receptors cannot detect light.
This definition is part of the Coordination and Control chapter in Cambridge O Level Biology.
Receptor in context
Coordination is how an organism detects a change and produces a useful response to it. Every example in this chapter — pulling a hand off a hot pan, narrowing a pupil, sweating on a hot afternoon, storing glucose after a meal — runs through the same five stages: stimulus → receptor → coordinator → effector → response. Mammals run that chain two ways. Nervous control sends electrical impulses along neurones: fast, precisely targeted, usually short-lived. Hormonal control sends chemicals dissolved in blood plasma: generally slower, more widespread, often longer-lasting. Homeostasis is the job both systems do together — holding internal conditions within narrow limits by negative feedback, in which every correction opposes the change that triggered it.
Common mistakes with Receptor
- Write it carefully. "The eye sees the object" will not earn a mark on a question about receptors. "Light receptors in the retina detect the light and impulses are sent along the optic nerve to the brain, where they are interpreted" earns several.
- Do not make the skin do the hypothalamus's job. Skin receptors detect the temperature of the environment at the surface, not the temperature of the core. If a question asks how the body detects that its own temperature has risen, the answer must include the hypothalamus monitoring the blood.
Questions students ask about Receptor
What is the order of a reflex arc?
Stimulus, receptor, sensory neurone, relay neurone in the central nervous system, motor neurone, effector, response. The effector is a muscle or a gland, and the impulse never travels backwards along this pathway.
What happens at a synapse?
An electrical impulse arrives at the end of the first neurone. Vesicles release a neurotransmitter into the synaptic gap. The neurotransmitter diffuses across the gap and binds to complementary receptor proteins on the next neurone, where it stimulates a new electrical impulse. No electrical impulse crosses the gap itself.
Why can an impulse cross a synapse in only one direction?
Because the synapse is structurally asymmetrical. Vesicles containing neurotransmitter are present only in the neurone on one side of the gap, and complementary receptor proteins are present only on the membrane of the neurone on the other side. Neurotransmitter can therefore be released from one side only and detected on the other side only.
What are the parts of the eye and what do they do?
The cornea refracts light entering the eye. The iris contains muscles that control the size of the pupil, which is the opening light passes through. The lens refracts light and focuses it on the retina, changing shape to do so. The ciliary muscles alter the tension in the suspensory ligaments, which are attached to the lens. The retina contains the light receptor cells, and the fovea is the region of the retina with the greatest density of them. The optic nerve carries impulses to the brain, and the blind spot is the point where it leaves, containing no light receptors.
What happens to the pupil in bright light?
The circular muscles of the iris contract and the radial muscles relax, so the pupil becomes smaller. Less light then reaches the retina, which reduces the risk of damage to the light receptors. In dim light the reverse happens: the radial muscles contract, the circular muscles relax, and the pupil becomes larger.
What is a hormone?
A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs. Although a hormone reaches every organ in the body, only cells carrying receptors complementary to that hormone respond to it.
What is the difference between nervous and hormonal control?
Nervous control uses electrical impulses travelling along neurones, is rapid, is usually short-lived, and acts on the effectors that those neurones connect to. Hormonal control uses chemical hormones carried in the blood, is generally slower to begin, is often longer-lasting, and acts on any cells carrying complementary receptors.