Anatomy of the Human Eye Quiz

Stimuli, Receptors, and Responses

• Organisms can detect and respond to stimuli (changes in their environment).
• Irritability is the ability of an organism to detect stimuli and respond accordingly.
• Stimuli can be internal or external.
• Receptors detect stimuli and convert them into nerve impulses.
• The nerve impulses travel to a control centre (usually the brain).
• The control centre interprets the impulses as a sensation and sends impulses to effectors (muscles or glands).
• Effectors produce a response.
• There are four main types of receptors in humans: photoreceptors, mechanoreceptors, chemoreceptors, and thermoreceptors.

Human Eyes as Sense Organs for Detecting Light

• Eyes are the sense organs for detecting light.
• The brain interprets the nerve impulses from the eyes to produce vision.
• Structures surrounding the eye include eyebrows, eyelashes, eyelids, and tear glands.
• Eyebrows prevent sweat from entering the eye.
• Eyelashes trap dirt.
• Eyelids close to protect the eye from foreign objects and strong light.
• Tear glands secrete tears which keep the eye moist and clean (containing lysozyme to kill bacteria).
• The eyeball is a spherical structure in a bony socket called the orbit.
• The eyeball is attached to the eye socket by three pairs of eye muscles that allow the eye to rotate.

Internal Structures of the Eye

• The wall of the eyeball has three layers: the outer (sclera and cornea), middle (choroid, ciliary body, and iris), and inner (retina).
• The sclera is the tough, white outer layer.
• The cornea is a transparent front part of the eye.
• The choroid is the pigmented middle layer rich in blood vessels to supply oxygen and nutrients to the retina.
• The iris contains circular muscles and radial muscles, controlling the size of the pupil.
• The pupil is the hole in the centre of the iris.
• The lens is a transparent, elastic, biconvex structure, which changes shape to focus light onto the retina.

Retina

• The retina is the inner layer of the eye containing photoreceptors (rod cells and cone cells).
• The yellow spot is the central region of the retina containing only cone cells.
• The blind spot is where the optic nerve leaves the eye. No photoreceptors are here.
• Rod cells are more sensitive to light and used for dim light.
• Cone cells are less sensitive, used for bright light and color vision.

Eye Defects and Corrections

• Short sight: Light rays from distant objects are focused in front of the retina, resulting in blurred vision. Correct with concave lenses.
• Long sight: Light rays from near objects are focused behind the retina, resulting in blurred vision. Correct with convex lenses.
• Colour blindness: Deficiencies in one or more types of cone cells. Difficulty distinguishing colours.

Human Ears as Sense Organs for Detecting Sound

• Ears are the sense organs for detecting sound.
• The brain interprets the nerve impulses from the ears to produce the sensation of hearing.
• The ear consists of the outer ear (pinna and auditory canal), middle ear (eardrum, ear bones), and inner ear (cochlea and semicircular canals).
• The pinna collects sound waves and directs them to the auditory canal.
• The eardrum vibrates when sound reaches it.
• Ear bones amplify the vibrations from the eardrum to the oval window.
• The oval window creates waves in the perilymph of the cochlea.
• Sensory hair cells in the cochlea convert vibrations into nerve impulses, which travel along the auditory nerve to the brain.
• The semicircular canals are in the inner ear and help maintain balance.

Phototropic Responses in Plants

• Plants show irritability (respond to stimuli).
• Tropic responses: Directional growth in response to a stimulus.
• Phototropism is directional growth in response to unilateral light.
• Shoots exhibit positive phototropism (grow towards light).
• Roots exhibit negative phototropism (grow away from light).
• Auxins (plant hormones) are produced in shoot and root tips and stimulate cells to elongate.
• Auxin is transported from the light side to the shaded side in shoots.
• High auxin concentration stimulates shoot growth (the shaded side grows faster).
• High auxin concentration inhibits root growth (the shaded side grows more slowly).