Response and Coordination
Senior 3 · Biology
Response and Coordination
Lesson 2 of 4
25 min
In football, the penalty spot is 11 metres from the goal line. A hard penalty can travel at about 25 metres per second (a typical figure), so it reaches the goal in less than half a second. In that time the goalkeeper's eyes must detect the ball, the brain must judge where it is going, and muscles in the legs and arms must contract in the right order. A message has to travel from the eye to the brain and from the brain to the legs, more than a metre away, very fast. That message is a nerve impulse, and the cells that carry it are neurones.
Definition
Coordination: the way an organism links what it detects to what it does, so that its parts work together at the right time.
Receptor: a cell or organ that detects a stimulus, e.g. light-sensitive cells in the eye. Effector: a muscle or gland that carries out the response.
Neurone (nerve cell): a cell specialised to carry electrical impulses. A nerve is a bundle of many neurones' fibres wrapped together, like the wires in a cable.
Central nervous system (CNS): the brain and the spinal cord. Peripheral nervous system (PNS): all the nerves that connect the CNS with the rest of the body: 12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord.
Synapse: the junction between two neurones, where a tiny gap separates them.
A plant can afford to respond slowly. An animal moves: it must find food, escape predators and keep its body working while conditions change around it. It therefore needs a system that collects information from receptors, decides what to do, and sends instructions to effectors. Animals have two such systems that work together: the nervous system, which sends fast electrical impulses along neurones, and the endocrine (hormonal) system, which sends chemical messages in the blood (Lesson 4). Figure 1 shows the pathway of every nervous response.
The CNS is the coordinator. The spinal cord, protected inside the vertebral column, carries impulses between the body and the brain and also handles many reflexes by itself (Lesson 3). The brain, protected by the skull, has several parts with different jobs:
| Part of the brain | Main functions | Example |
|---|---|---|
| Cerebrum (the two large cerebral hemispheres) | Thinking, learning, memory, speech and language; voluntary movements; interpreting information from the sense organs | Deciding which way to dive |
| Cerebellum | Balance, posture, and making movements smooth and accurate | Riding a bicycle without falling |
| Medulla oblongata | Automatic (involuntary) control of breathing rate, heart rate and blood pressure; reflexes such as swallowing, coughing and sneezing | Breathing faster during a match |
| Hypothalamus | Monitors body temperature and the water content of the blood; hunger and thirst; controls the pituitary gland | Feeling thirsty after sweating (Unit 10) |
| Pituitary gland (attached below the hypothalamus) | Releases several hormones, some of which control other glands | Releasing ADH (Lesson 4) |
All neurones have a cell body, which contains the nucleus, and long, thin extensions called nerve fibres. Fibres that carry impulses towards the cell body are dendrites (a single long one is a dendron); the fibre that carries impulses away from the cell body is the axon. Many fibres are wrapped in a fatty myelin sheath, which insulates them; the impulse jumps from one gap in the sheath, a node, to the next, which makes it much faster.
Figure 2: simplified drawings, not to scale. A real motor neurone's axon can be about a metre long. The yellow segments are the myelin sheath; the gaps between them are nodes.
| Feature | Sensory neurone | Relay neurone | Motor neurone |
|---|---|---|---|
| Carries impulses | From a receptor to the CNS | From a sensory to a motor neurone, inside the CNS | From the CNS to an effector |
| Position of the cell body | In a swelling (ganglion) on a nerve, just outside the spinal cord | Inside the CNS | Inside the CNS |
| Fibres | Long dendron, short axon | Short dendrites and a short axon | Many short dendrites, one long axon |
Neurones do not touch. Where one meets the next there is a synapse with a tiny gap, the synaptic cleft. An electrical impulse cannot jump this gap, so a chemical carries the message across (Figure 3):
Figure 3: not to scale. The cleft is typically about 20 nanometres wide, far narrower than drawn.
Because the vesicles are only on one side and the receptors only on the other, a synapse lets impulses pass in one direction only. Synapses also let one neurone pass its message to many others, or collect messages from many. Many drugs act at synapses: nicotine, for example, attaches to one type of receptor protein in place of the body's own neurotransmitter, which is part of why it is addictive (Senior 1, Unit 12).
Worked example 1: is the nerve fast enough?
A penalty kick travels 11 m at 25 m s⁻¹. An impulse travels from the goalkeeper's spinal cord to a calf muscle, a distance of 1.0 m, along a motor neurone at 100 m s⁻¹ (a typical speed for a large myelinated neurone). Compare the two times.
Worked example 2: measuring the speed of an impulse
In a laboratory, a nerve is stimulated at two points, A and B, 0.12 m apart, and the time at which each impulse reaches a recording electrode further along is measured. The impulse from A arrives 2.0 ms later than the impulse from B. (Illustrative data.)
Warning
Common mistakes. (1) Mixing up a neurone (one cell) and a nerve (a bundle of fibres from many neurones). (2) Writing that the impulse "jumps across" the synapse as electricity: a chemical, the neurotransmitter, diffuses across. (3) Labelling every long fibre an axon: in a sensory neurone the long fibre that carries impulses towards the cell body is the dendron. (4) Saying the brain controls everything consciously: breathing and heart rate are controlled automatically by the medulla.
What are Nerve Cells, Neurons and Synapses?, FuseSchool. Watch for the three kinds of neurone and the steps at a synapse, and match them with Figures 2 and 3.
Tip
Memory hook: "SRM" and "Den in, Ax out". Impulses go Sensory → Relay → Motor. In every neurone, dendrites bring impulses in to the cell body and the axon takes them out.
Practice: the nervous system
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 to 3, the tables, the worked examples (Worked example 2 uses illustrative data) and the practice questions.
