Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe the structure and function of the human ear and explain hearing.
• Describe the structure and function of the human eye and explain vision.
• Explain common disorders of the ear and eye.
• Understand the mechanism of accommodation in the eye.
• Compare the roles of rods and cones in the retina.

Lesson summary

The human nervous system is the body's control center, coordinating actions and transmitting signals between different parts of the body. Our senses – sight, hearing, smell, taste, and touch – are the gateways through which we perceive and interact with the world around us. Understanding how these systems function is crucial not only for comprehending basic human biology but also for recognizing and addressing potential health issues.

Lesson notes

2.1 The Human Ear: Structure and Function The ear is responsible for both hearing and balance.

It consists of three main parts: the outer ear, the middle ear, and the inner ear.

Outer Ear: The outer ear comprises the pinna (auricle) and the auditory canal (ear canal). The pinna collects sound waves and funnels them into the auditory canal. The auditory canal leads to the tympanic membrane (eardrum). The pinna is cartilaginous and shaped to efficiently capture sound.

Middle Ear: The middle ear is an air-filled cavity containing three tiny bones called ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). The tympanic membrane vibrates in response to sound waves, and these vibrations are transmitted and amplified by the ossicles. The stapes is attached to the oval window, an opening into the inner ear. The Eustachian tube connects the middle ear to the nasopharynx (upper part of the throat). It equalizes pressure between the middle ear and the outside environment, which is crucial for proper hearing.

Inner Ear: The inner ear contains the cochlea, responsible for hearing, and the vestibular apparatus, responsible for balance. The cochlea is a spiral-shaped, fluid-filled structure. Inside the cochlea is the organ of Corti, which contains hair cells that act as sensory receptors. These hair cells convert the mechanical vibrations into electrical signals. The vestibular apparatus consists of the semicircular canals and the vestibule, which contain sensory receptors for detecting head movements and maintaining balance.

Mechanism of Hearing: Sound waves enter the auditory canal and cause the tympanic membrane to vibrate. The vibrations are amplified by the ossicles (malleus, incus, stapes). The stapes transmits the vibrations to the oval window, which sets the fluid inside the cochlea in motion. The movement of the fluid causes the basilar membrane within the cochlea to vibrate. Different frequencies of sound cause different parts of the basilar membrane to vibrate maximally. The hair cells in the organ of Corti are stimulated by the vibrating basilar membrane. The stimulated hair cells generate nerve impulses that travel along the auditory nerve to the brain. The brain interprets these impulses as sound. 2.2 The Human Eye: Structure and Function The eye is the organ responsible for sight. It converts light into electrical signals that the brain can interpret.

Outer Layer: The outer layer consists of the sclera (the white of the eye) and the cornea (the transparent front part of the eye). The sclera provides protection and support for the eye. The cornea refracts (bends) light as it enters the eye.

Middle Layer: The middle layer, or uvea, consists of the choroid, the ciliary body, and the iris. The choroid is a layer of blood vessels that nourishes the retina. The ciliary body controls the shape of the lens. The iris is the colored part of the eye and contains muscles that control the size of the pupil, the opening in the center of the iris.

Inner Layer: The inner layer is the retina, which contains photoreceptor cells called rods and cones. Rods are sensitive to dim light and are responsible for black-and-white vision. Cones are responsible for color vision and function best in bright light. The retina also contains neurons that process visual information and transmit it to the brain via the optic nerve. The fovea is a small area in the center of the retina that contains a high concentration of cones and is responsible for sharp, detailed vision. The blind spot is the point where the optic nerve leaves the eye, and there are no photoreceptors in this area.

Mechanism of Vision: Light enters the eye and is refracted (bent) by the cornea and lens. The lens focuses the light onto the retina. The photoreceptor cells (rods and cones) in the retina convert the light into electrical signals. These signals are processed by other neurons in the retina. The signals are transmitted along the optic nerve to the brain. The brain interprets these signals as images.

Accommodation: The eye's ability to focus on objects at different distances is called accommodation. The ciliary muscles control the shape of the lens. When focusing on distant objects, the ciliary muscles relax, causing the lens to become thinner and flatter. When focusing on near objects, the ciliary muscles contract, causing the lens to become thicker and more rounded. This change in lens shape allows the eye to focus light from objects at varying distances onto the retina. Rods vs.