Programme 15: Responding to Stimuli

Answers to Activity Sheet 2

Speedy Reflexes

The activity has four main parts:

Stimulus Activity
A situation describes a reflex action. This could be read out or pupils could be asked for their own examples. Descriptions of several reflex actions could be mounted on card and discussed as a circus activity.

Matching
Pupils work in pairs or groups of three to match the words and their definitions.

Sequencing
Pupils order the sequence to create a reflex arc.

Plenary
1. Share the completed reflex arc.
Use a diagram to show where the stages of the arc are located.
Use the questions for further class discussion.
Display diagram, words and definitions in the classroom.

Words	Definitions
Stimulus	Information from the environment, eg light, sound, heat, chemicals, texture, pressure.
Receptor	Cells or organs which can detect a stimulus and pass information to a sensory nerve, eg light receptors in the retina of the eye, chemoreceptors in the nose and on the tongue.
Sensory Nerve	A nerve cell that transmits a nerve impulse from a receptor to the central nervous system.
Relay Nerve	A nerve cell in the central nervous system that transmits a nerve impulse from a sensory nerve cell to a motor nerve cell.
Motor Nerve	A nerve cell that transmits a nerve impulse from a relay nerve cell to an effector, eg a muscle.
Effector	An organ that responds to a stimulus. Usually a muscle, eg salivary gland.
Response	The action that an organism makes after receiving a stimulus. eg a movement, secreting saliva.

2. They help animals to survive by allowing a rapid response to a threatening situation.

3. It is possible to keep hold of a hot object, but some reflexes cannot be controlled easily. For example:

• salivating when you smell a favourite food
• blinking when something moves quickly towards your eye
• the iris constricting your pupil when a bright light shines on it
• your heart beating faster when you are in danger – or in love!

This tells us something about the nerve connections between these reflex arcs and the cerebral cortex of the brain.

4. The brain must be linked to the relay nerve cell through the central nervous system.

5. The ability to perform a reflex is present at birth, but some reflexes appear at certain times in development, for example, the clutching reflex of small babies when they are dropped. It is possible to learn to perform a reflex to a new stimulus by conditioning. This is the basis of Pavlov’s classic experiment in which dogs were taught to salivate in response to a bell ringing.

6. The relay nerve cell is wholly within the central nervous system. The cell body of the motor nerve is within the spinal cord but the fibre itself forms part of the peripheral nervous system.