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Coordination

12 min read

Nervous coordination

The body coordinates its activities using the nervous system, which sends fast electrical impulses, and the endocrine system, which sends slower chemical messages called hormones. The nervous system has the central nervous system (brain and spinal cord) and the nerves that run to every part of the body.

A motor neuroneA nerve cell with a cell body containing the nucleus and several short branches called dendrites that receive signals. A long axon, wrapped in a fatty myelin sheath with gaps called nodes of Ranvier, carries impulses away. At the end, axon terminals pass the signal on to a muscle or another cell.cell body (nucleus)dendritesmyelin sheathnode of Ranvieraxonaxon terminalsimpulse

There are three kinds of neurone. Sensory neurones carry impulses from receptors to the central nervous system. Relay neurones link neurones inside the spinal cord and brain. Motor neurones carry impulses from the central nervous system to effectors, which are muscles and glands.

Reflex action

A reflex action is a fast, automatic response to a stimulus. It does not wait for a decision by the brain, so it protects the body, for example pulling the hand away from a hot object.

A reflex arcThe path of a quick, automatic response. A receptor in the skin detects a stimulus such as heat. A sensory neurone carries the impulse to the spinal cord, where a relay neurone passes it to a motor neurone. The motor neurone carries the impulse to a muscle, the effector, which contracts and pulls the hand away.spinalcordreceptor (skin)effector (muscle)sensory neuronerelay neuronemotor neurone

Sense organs

The eye detects light. Light is focused by the cornea and lens onto the retina, which has light-sensitive cells that send impulses to the brain along the optic nerve. The ear detects sound and balance.

The human eyeA cross-section of the eye with its main parts: the cornea at the front which bends light most, the iris with the pupil in the centre that controls how much light enters, the lens that changes shape to focus, the retina at the back where the image forms with rods and cones, and the optic nerve that carries signals to the brain.corneairislensaqueous and vitreous humourretina (rods and cones)optic nervepupil
The human earSound waves enter the outer ear and make the eardrum vibrate. Three small bones, the hammer, anvil and stirrup, pass and increase the vibrations to the oval window of the cochlea, which is filled with fluid and lined with hair cells that turn the vibrations into nerve impulses. The semicircular canals sense balance, and the Eustachian tube equalises pressure.outer ear (pinna)eardrumhammer, anvil, stirrupcochleasemicircular canals (balance)auditory nerveEustachian tube

Measuring reaction time

In the ruler-drop test one person drops a ruler and another catches it. The distance it falls before it is caught gives the reaction time, using the equation of free fall.

d = distance the ruler fell, g = 9.8 m/s².
Try itRuler-drop reaction time

Choose how far the ruler fell before it was caught.

Check yourself Which neurone carries impulses from the spinal cord to a muscle?
Check yourself Why is a reflex action quick?
Check yourself Where does light form an image in the eye?

Key terms

Stimulus
A change that is detected by a receptor.
Effector
A muscle or gland that responds.
Reflex action
A fast automatic response to a stimulus.
Retina
The light-sensitive layer at the back of the eye.

Summary

  • The nervous system sends fast impulses; the endocrine system sends slower hormones.
  • Sensory, relay and motor neurones form a reflex arc.
  • The eye detects light and the ear detects sound and balance.
  • Reaction time can be found from the ruler-drop test.
Movement and Locomotion Excretion