Unit 8
Human Biology
Chapter 8.6
Sense OrgansClass 12 Biology – Sense Organs Notes PDF
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Introduction
Important human sense organs include the eye for vision and the ear for hearing and balance. Receptors for taste, smell, touch, temperature and pain provide additional sensory information.
1. Structure of Human Eye
Fig. 1 – Labelled Human Eye
| Part | Function |
|---|---|
| Cornea | Transparent front surface; provides major refraction of incoming light. |
| Iris | Controls pupil size and therefore amount of light entering the eye. |
| Lens | Fine-focuses light on the retina by changing curvature. |
| Retina | Contains photoreceptors that convert light into neural signals. |
| Optic nerve | Carries visual information toward the brain. |
2. Mechanism of Vision and Accommodation
Light is refracted mainly by the cornea and then by the lens so that an image is focused on the retina. Rods and cones convert light energy into electrical signals that pass through retinal neurons to the optic nerve.
Fig. 2 – Accommodation for Near and Distant Vision
3. Common Defects of Vision
| Defect | Problem | Typical Optical Correction |
|---|---|---|
| Myopia | Distant objects are focused in front of retina. | Concave/diverging lens. |
| Hypermetropia | Near objects tend to focus behind retina. | Convex/converging lens. |
| Presbyopia | Age-related reduction in accommodation. | Suitable reading/bifocal/progressive correction. |
| Astigmatism | Unequal curvature causes uneven focusing. | Cylindrical/toric correction. |
Fig. 3 – Myopia and Hypermetropia
4. Structure of Human Ear
Fig. 4 – Human Ear
Outer Ear
Pinna and auditory canal collect sound and direct it to the tympanic membrane.
Middle Ear
Contains ossicles that transmit and amplify vibrations from tympanum toward inner ear.
Inner Ear
Cochlea is concerned with hearing; vestibular apparatus contributes to balance.
Eustachian Tube
Helps equalize air pressure across the tympanic membrane.
5. Hearing and Balance
Sound waves vibrate the tympanic membrane. Ossicles transmit these vibrations to the inner ear, producing fluid movements in the cochlea. Sensory hair cells convert mechanical movements into neural signals.
Fig. 5 – Sound Transmission Path
The semicircular canals, utricle and saccule detect head movement and position, contributing to equilibrium.
6. Other Sensory Receptors
Smell
Olfactory receptors in the nasal cavity detect airborne chemicals.
Taste
Taste buds contain chemoreceptors that respond to dissolved chemicals.
Touch & Pressure
Mechanoreceptors in skin respond to deformation and contact.
Temperature & Pain
Specialized sensory endings detect thermal changes and potentially damaging stimuli.
7. Quick Revision & Exam Points
- Draw and label the human eye.
- Explain accommodation.
- Differentiate myopia and hypermetropia.
- Draw and label the human ear.
- Explain the mechanism of hearing.
- Write the role of semicircular canals in balance.
- List other major sensory receptors.
- Retina contains photoreceptors.
- Lens helps focus light.
- Myopia → distant vision blurred.
- Hypermetropia → near vision difficult.
- Cochlea → hearing.
- Semicircular canals → balance.
- Olfactory receptors → smell.
- Taste buds → taste.
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