BIOL1019 Chap.4 Special Senses: Eye and Ear
Special Senses: Eye and Ear
Vision, hearing and equilibrium depend on complex sense organs that turn light, sound and movement into action potentials. This chapter follows light through the cornea and lens to the retina's three layers of rods and cones, bipolar cells and ganglion cells, and on through the optic nerves to the occipital cortex.
It compares rods with cones, explains accommodation for near vision and common failures such as glaucoma, cataracts and myopia, and then follows sound from the tympanic membrane and ossicles to the organ of Corti and the auditory cortex of the temporal lobe. Equilibrium closes the chapter, separating static sensing in the vestibule from dynamic sensing in the semicircular ducts.
The course's own checklist asks you to identify the structures of the eye and ear, explain how light and sound become action potentials and describe each pathway, so every section here ends in a sequence you can write as a numbered short answer.
What this chapter covers
- 01
Accessory structures of the eye
- 02
Cornea, lens and accommodation
- 03
Rods, cones and visual pigments
- 04
The visual pathway to the occipital lobe
- 05
Outer, middle and inner ear
- 06
Organ of Corti and the hearing pathway
- 07
Static and dynamic equilibrium
- 08
Clinical failures: cataract, glaucoma, myopia
Worked example · free
Tracing light from the cornea to the cortex
- +1The cornea begins focusing the light and the lens completes focusing it onto the retina.
- +1Rods and cones in the retina absorb photons with their visual pigments, rhodopsin and photopsin.
- +1The signal passes from photoreceptor to bipolar cell to ganglion cell, whose axons form the optic nerve.
- +1The optic nerves cross before the signal reaches the visual cortex of the occipital lobe.
Key terms
- Accommodation
- The change in lens shape by the ciliary muscle that focuses near or distant objects on the retina.
- Rhodopsin
- The visual pigment of rods, made of opsin bound to retinal.
- Organ of Corti
- The structure on the basilar membrane whose hair cells convert sound vibration into nerve impulses.
- Perilymph
- The fluid in the scala vestibuli and scala tympani that carries pressure waves from the oval window.
- Glaucoma
- Raised pressure inside the eye from impaired drainage of aqueous humor, which can damage the optic nerve.
- Dynamic equilibrium
- The sense of movement, especially rotation, detected by the semicircular ducts.
- Otolith
- A dense crystal in the maculae of the saccule and utricle whose movement bends hair cells when the head tilts.
Special Senses: Eye and Ear FAQ
Why is the fovea where we see most clearly?
Cones are densely clustered in the fovea at the centre of the macula lutea, and cones provide colour and fine detail. The sharpest image is therefore formed there, which is why you look directly at text to read it. Rods, which are highly light-sensitive but give black and white vision, are more important away from the centre.
How does sound become a nerve impulse?
Sound waves vibrate the tympanic membrane and the malleus, incus and stapes. The stapes pushes on the oval window and creates pressure waves in the perilymph that distort the basilar membrane. Hair cells of the organ of Corti bend against the tectorial membrane, generating impulses in the cochlear branch of cranial nerve VIII.
What is the difference between static and dynamic equilibrium?
Static equilibrium concerns position while the body is stationary, sensed in the vestibule by the maculae of the saccule and utricle, where tilting moves the otoliths and bends hair cells. Dynamic equilibrium concerns movement, especially rotation, sensed by the semicircular ducts. Both send signals along the vestibular branch of cranial nerve VIII.
Why does the ciliary muscle contract for near vision?
To focus a close object the lens must bend light more strongly, so it needs a rounder shape. Contraction of the ciliary muscle lets the lens round up and increases its focusing power. The muscle is under parasympathetic control through M3 muscarinic receptors; for distant objects it relaxes and the lens flattens.
Exam move
Write the visual, hearing and equilibrium pathways as three numbered lists and recite them until each takes under thirty seconds. Add one clinical condition to the step it disrupts, such as glaucoma at the optic nerve or glue ear in the middle ear. Use the study-skill advice the lectorial gives: review the learning objectives first, star what the lecturer stresses and write your own likely exam questions.
Close each session by comparing rods and cones on pigment, location, structure and type of vision, because a side-by-side comparison is the most efficient way to bank marks in this chapter.
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