Control and coordination: nervous system, endocrine system and sense organs
The nervous system sends fast electrical messages along nerves; the endocrine system sends chemical messages, hormones, through the blood. Together they control and coordinate the body.
Why the body needs control
Your body works like a team. Each organ must work at the right time and in step with the others. This is coordination. Think of eating a meal. Your eyes see the food, and the brain registers it. Nerves tell the muscles of your hand to pick up the food and put it in your mouth. In the stomach and intestine, digestive juices flow only while food is there, helped by hormones. When you have eaten enough, a hunger centre in the brain gives a feeling of fullness, and you stop. Many jobs, such as breathing and digesting, go on without our thinking about them.
Nervous system vs endocrine system
The two systems work as a team. For example, digestive juices flow when food arrives, under the control of both nerves and hormones.
Stimulus, receptor and effector
- A stimulus is any change, inside or outside the body, that can start a response. It may be touch, a prick or pressure, sound, a chemical (a smell or a taste), light, heat or cold, or electricity.
- A receptor detects the stimulus. Receptors are found in the sense organs.
- Sensory nerves carry the message to the brain or spinal cord, where it is sorted out (integrated).
- Motor nerves carry the order to an effector: a muscle, which contracts, or a gland, which secretes.
- The action that follows is the response.
Sense organs and what they detect
| Sense organ | Stimulus it detects | Sense |
|---|---|---|
| Eyes | Light | Sight |
| Ears | Sound | Hearing |
| Nose | Chemicals in the air | Smell |
| Tongue | Chemicals in food | Taste |
| Skin | Touch, pressure, pain, heat and cold | Touch |
From stimulus to response
Example: you pass a garbage dump and smell it (stimulus). Receptors in the nose detect the smell, the brain sorts it out, and your hand covers your nose (response).
A map of the nervous system
- Central nervous system (CNS): the brain and the spinal cord. It is the body's information processor.
- Brain: the cerebrum (thinking, memory, voluntary actions), the cerebellum (balance) and the medulla oblongata (involuntary actions such as heartbeat and breathing).
- Spinal cord: controls reflex actions and carries messages to and from the brain.
- Peripheral nervous system (PNS): the nerves that arise from the brain and spinal cord and link them with the rest of the body.
- The PNS has two parts: the somatic system (12 pairs of cranial nerves and 31 pairs of spinal nerves) and the autonomic system (sympathetic and parasympathetic).
When you smell jalebis frying at a sweet shop, receptors in your nose send messages to your brain, and your mouth may water. Riding a bicycle through a crowded bazaar needs your eyes, ears, brain and leg muscles to work together.
1 marks: “What is a stimulus?” Any change inside or outside the body, such as touch, light or heat, that can start a response.
2 marks: “Define receptor and effector. Give one example of each.” Receptor: the part of a sense organ that detects a stimulus, such as smell receptors in the nose. Effector: the muscle or gland that carries out the response, such as the hand muscles or the salivary glands.
2 marks: “Give two differences between control by the nervous system and control by the endocrine system.” Nervous: messages are electrical impulses along nerves; the response is fast. Endocrine: messages are hormones carried by the blood; they control slower, longer changes such as growth.
Every response follows Stimulus, Receptor, Sensory nerve, CNS, Motor nerve, Effector: think Some Really Smart Children Make Excuses.