This is the Coordination and response page.


  • Coordination in plants

  • Coordination in animals - the CNS - the synapse - senses (sight only)

  • Hormones in humans

Sensitivity is one of the characteristics of living organisms. Every organism, unicellular or multicellular, reacts to changes in the environment, either internal or external. Plants react to lack of water or light, humans feel goose bumps when cold, animals move away from very hot objects, etc.
Multicellular organisms respond in two ways: either using chemical messengers called hormones or using a central nervous system.

A stimulus is a change either in the internal or external environment.


Plants react to several environmental factors in different ways, using hormones.

When plants react to external changes in light or water, for example they show tropic responses. A tropic response is a growth movement towards or away from a stimulus.

Therefore, a growth towards a stimulus is a positive tropism and a growth away from a stimulus is called a negative tropism.

We will study two responses: to light and to gravity.

This is a short presentation summarizing these two responses.



Coordination and response


Main concepts to remember:

central nervous system (brain + spinal cord: the coordination centre) - neurones (types, structure and functions) - synapses - neurotransmitters - reflex arc - vision: eye structure, pupil reflex and accommodation.

We will study 3 types of neurones (cells in the nervous system): sensory, relay or interneurone and motor. Neurons have dendrites (cytoplasmic prolongations) to connect to other neurones and form synapses (where the information is passed from one neurone to the other).
Information travels along neurones as electrical impulses.
neuron structure.gif
structure of a neurone

3 types of neurones.gif

types of neurone structure.jpeg

Neuron101.jpg cp_neurophysio2_synapse.jpg

a link to an animation on neurones and synapses


A reflex is a very quick response, involuntary, that does not involve decision-making processes, usually to prevent us from harm or help us survive. An example is the knee cap reflex.

- stimulus (a change in the environment)
- receptor (a cell or a gland that detects the stimulus)
- sensoy neuron (neuron that sends the information from the receptor to the CNS
- relay neuron (connects sensory and motor neurones)
- motor neuron (neuron that sends the information from the CNS to the effector)
- effector (muscle or gland that carries out the response)
- response (action)


  1. A reflex arc starts with a stimulus detected by receptor.
  2. This information is sent as electrical impulses to the CNS via the sensory neuron.
  3. In the CNS, the relay neuron connects the sensory neuron with the motor neuron, which sends the information to the effector.
  4. This in turn causes a response.

An example: the blinking reflex
blink reflex.PNG

Senses: vision

  • The structure of the eye
  • The pupil reflex
  • Focusing

The eye

external structure of the eye-labelled.jpg

internal structure of the eye - labelled.jpg

The pupil reflex

pupil reflex 2.jpg

Accommodation or focusing
Refraction, in physics, is the bending of light rays when they pass from one medium to another. The degree of bending of light rays will be variable, depending on the density of the two media. Air for example has a low density, so it causes slight bending of light. In contrast, glass has a higher density, so it causes larger bending of light. Accommodation is the ability of the eye to change its focus.
There are four structures involved in accommodation or focusing:
  • the cornea
  • the ciliary muscles
  • the suspensory ligaments
  • the lens

The cornea is where most refraction takes place. The ciliary muscles and suspensory ligaments will change the shape of the lens to direct the light rays to the retina at the back of the eye. The fovea is where we have the sharpest focus. (see internal structure above).

As a person ages, the ability to accommodate diminishes until it is lost. The loss of accommodation is referred to as presbyopia when it leads to the loss of near vision. The loss of accommodation is due to normal, age-related optical and physical changes of the lens, ciliary muscle, and suspensory ligaments.
eye focusing complete.jpg
diagram 1 showing focusing according to distance

eye focusing with labels.png
diagram 2 showing how the lens changes shape

eye focusing with txt.gif
diagram 3 showing how the ciliary muscles and the suspensory ligaments work to change the shape of the lens

Some animations on this topic:

And what happens when our vision is blurry?
Common refraction errors and their correction with lenses.
A is a person who is far-sighted. Eyeball is too small. Corrective concave lens B brings rays closer to focus on the retina.
C is a person who is short-sighted. Eyeball too big. Corrective convex lens D brings rays farther to focus on the retina.

An animation to help you understand