Programme 15: Responding to Stimuli

Programme Outline

This programme shows three different aspects of living things' sensitivity to stimuli. Dissection of a horse's eye, an eye similar in structure to a human eye, provides students with the opportunity to learn its anatomy, whilst simple blow-up lenses show how the eye works. Diagrams and film are then used to show how the human nervous system takes messages in through its sensory organs, and transmits them to the central nervous system which, in turn, sends messages back initiating a response. How plants show sensitivity to light and gravity, and respond to it, is shown in experiments that are easy to replicate in a school laboratory.

The programme is divided into three parts:

15.1 The Eye
15.2 The Nervous System
15.3 Plant Tropisms

15.1 The Eye

A horse's eye is similar to a human eye and is dissected to show all its parts. The white, tough, protective, outside layer is called the sclera, which is colourless at the front. The clear part is called the cornea, behind which you can see the eyeball. The sclera is cut at the back to reveal the blood vessels lying in the inner back wall of the eye – the retina. A camera, looking through the retina shows how a hand, viewed in front of it, is seen upside down.

Cutting into the sclera releases the jelly-like vitreous humour, which gives the eyeball its shape.

When the eye is cut in half you can see the firm, elastic lens held in place by a circle of ligaments. A disc of coloured tissue, called the iris, determines the colour of an eye.

A diagram shows the position of the lens, iris and pupil and how the iris expands and contracts to regulate the amount of light entering the eye. The lens bends the light entering the eye.

A model lens shows how increasing the width of a lens changes the image seen through it.

Nerves in the retina transmit the light signals to the brain via the optic nerve.

15.2 The Nervous System

The nervous system is your body's communication network and determines your every move. A diagram shows how the central nervous system – the brain and spinal cord are connected to the peripheral nervous system – the complex system of nerves spreading throughout the body.

Sensory organs, like the ear, receive information and send it to the brain. The brain analyses the information and sends out messages to the appropriate part of the body, eg muscles, to make it respond. Nerves send these messages as tiny pulses of electricity, which can be monitored by a sensitive oscilloscope.

Reflex actions bypass the brain and they travel along a reflex arc. The knee jerk is as an example of this.

15.3 Plant Tropisms

Plants' responses to stimuli are called tropisms. Phototropism is movement towards light. This is demonstrated by putting seedlings into a box, into which light enters through a small hole. In just three hours, a camera records how the seedlings move towards the light. The light makes the cells on each side of the stem grow at different rates, causing the plant to bend towards the light.

To find out how cress seedlings respond to light coming from two different directions, a box with two holes is placed over them.

How do you predict the cress seedlings will respond?
Answer: The seedlings do not grow towards either hole, but in between.

What do you think would happen if the intensity of one of the lights were decreased by sticking translucent tape over one hole?
Answer: The seedlings would grow towards the more intense light.

Geotropism or gravitropism is the plants' response to gravity. Seedlings grow away from gravity. We see seedlings grown on their side. Half are stationary and grow upwards. The other half are rotating and grow straight.

How do you think plants will grow in a space capsule?
Answer: In the absence of gravity, the plants would rotate while growing.