Sense Organ – SS3 Biology Lesson Note (First Term)

Sense organs are specialised structures in the body that detect stimuli and send signals to the nervous system.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Identify the primary sense organs and state what each detects.
2. Describe the main parts of the eye, ear, tongue and skin and their functions.
3. Describe the mechanism of smelling with the help of olfactory chemoreceptors.
4. Discuss how sniffing helps a dog to survive.
5. State ways of caring for the sense organs.

What is a stimulus and a receptor?

A stimulus is any change in the environment that a living thing can detect and respond to.

• Stimulus: a change such as light, sound, heat, pressure or a chemical, for example the smell of cooking rice.
• Receptor: a cell or group of cells that detects a stimulus and starts a nerve signal.
• Sensory neuron: a nerve cell that carries the signal from the receptor to the brain or spinal cord.
• Brain: the organ that interprets the signal and decides on a response.

What are the primary sense organs?

There are five primary sense organs: the eyes, ears, nose, tongue and skin.

What are the main parts of the eye and their functions?

The eye is the organ of sight, and it detects light and colour.

• Cornea: the clear front layer that lets light in and bends it.
• Iris: the coloured part that controls the size of the pupil.
• Pupil: the opening in the iris through which light enters.
• Lens: focuses light onto the retina.
• Retina: the inner layer that contains the light-sensitive cells.
• Optic nerve: carries signals from the retina to the brain.

What are the parts of the ear and their functions?

The ear is the organ of hearing and balance, and it has three parts: the outer ear, middle ear and inner ear.

Outer ear

• Pinna: collects sound waves and directs them into the ear canal.
• Ear canal: carries sound waves to the eardrum.

Middle ear

• Eardrum: a thin membrane that vibrates when sound waves hit it.
• Ear ossicles: three small bones that pass the vibrations on to the inner ear.

Inner ear

• Cochlea: changes vibrations into nerve signals for hearing.
• Semicircular canals: help the body to keep its balance.
• Auditory nerve: carries signals from the ear to the brain.

How does the tongue detect taste?

The tongue detects taste through taste buds, which are groups of receptor cells found mostly on its upper surface.

• Sweet: for example sugar and ripe mango.
• Sour: for example lime and unripe orange.
• Salty: for example table salt.
• Bitter: for example bitter leaf and bitter kola.
• Umami: the savoury taste of foods such as meat and fish.

• Food must dissolve in saliva before the taste buds can detect it.
• Smell and taste work together, so a blocked nose makes food seem tasteless.

What does the skin detect?

The skin is the sense organ of touch, and it contains receptors spread across the whole body.

• Touch receptors: detect light contact, such as a fly landing on the arm.
• Pressure receptors: detect firm pressure, such as holding a heavy bag.
• Heat and cold receptors: detect high and low temperatures.
• Pain receptors: detect harmful stimuli, such as a sharp thorn or a hot pot.

How does the mechanism of smelling work?

Smelling is the detection of airborne chemicals by olfactory chemoreceptors, which are sensory cells in the olfactory epithelium of the nasal cavity.

The steps below give a simplified explanation of how the olfactory system works.

1. Detection of odorants: When we inhale, airborne molecules (odorants) enter the nasal cavity.
2. Olfactory epithelium: The olfactory epithelium, located on the roof of the nasal cavity, contains specialised sensory cells called olfactory receptor neurons.
3. Olfactory receptor neurons: Each olfactory receptor neuron has hair-like structures called cilia, which extend into the mucus lining the nasal cavity.
4. Binding of odour molecules: Odorants dissolved in the mucus bind to receptor proteins on the cilia of the olfactory receptor neurons.
5. Generation of action potential: The binding of odour molecules to the receptor proteins triggers a series of biochemical reactions within the olfactory receptor neuron. These reactions lead to the generation of an action potential.
6. Transmission to the olfactory bulb: The action potentials travel along the axons of the olfactory receptor neurons, which bundle together to form the olfactory nerve. The olfactory nerve transmits these signals to the olfactory bulb, located at the base of the brain.
7. Processing in the olfactory bulb: The olfactory bulb processes the incoming signals, helping to identify the specific qualities of the odour.
8. Transmission to the brain: Information about the odour is then transmitted to higher brain regions, including the olfactory cortex and the limbic system, which are associated with memory and emotion.
9. Perception of smell: Finally, the brain interprets the signals and creates the perception of smell, allowing us to recognise and respond to various odours.

How does sniffing help a dog to survive?

Sniffing in animals, such as dogs during hunting, is a good example of an adaptation for survival.

• Enhanced detection of odours: Dogs have an acute sense of smell, and sniffing allows them to detect and analyse a wide range of odours. This is due to adaptations in their nasal anatomy and a large number of olfactory receptors.
• Tracking and hunting: Sniffing helps dogs track and locate prey. Following scent trails lets them find food efficiently, making them effective hunters.
• Identification of surroundings: Dogs sniff to gather information about their surroundings. They can detect other animals, identify territories, and assess threats or resources.
• Communication: Dogs use scent marking and sniffing to communicate with other animals. This helps in establishing social hierarchies, identifying individuals, and conveying their presence.
• Survival instinct: The use of smell for survival is deeply ingrained in dogs’ instincts. It helps them move about and make decisions in their environment.
• Avoidance of danger: Dogs can sniff out predators or hazardous substances. This helps them avoid threats and move through their environment safely.

How can we care for the sense organs?

Caring for the sense organs means protecting them from injury, infection and damage.

Evaluation

1. What are sense organs?
2. Name the five primary sense organs and state what each detects.
3. Describe the mechanism of smelling with the help of olfactory chemoreceptors.
4. Describe the adaptation for survival in dogs.
5. State the function of the retina and the cochlea.

Assignment

Draw a table of the five primary sense organs. For each, state the stimulus it detects and two ways of caring for it.

Instructional techniques and materials

Instructional techniques

• Identification
• Explanation
• Questions and answers
• Demonstration
• Videos from source

Instructional materials

• Videos
• Loudspeaker
• Textbook
• Pictures

Closing the lesson

• The teacher commends the students positively.