The sensory organs: eye and ear

The sensory organs are specialised structures that receive information for vision, hearing, balance, smell and taste. The eye focuses light onto sensory cells, while the ear receives sound and detects head movement. Receptors, the cells that respond to these stimuli, communicate with the brain through specific nerves.

What are the three coats of the eyeball?

The eyeball lies within the orbit, the bony cavity that contains the eye and its supporting structures. Its wall has three coats, meaning layers that surround its internal contents. These coats provide protection, carry blood vessels and contain the sensory tissue for vision.

• Fibrous coat: the tough outer layer consists of the sclera and cornea. The sclera is the opaque white portion; the cornea is the transparent anterior portion through which light enters.
• Vascular coat: the middle layer contains many blood vessels and consists of the choroid, ciliary body and iris. The choroid lies between the sclera and the inner coat; the ciliary body supports the lens and produces aqueous humour, the clear fluid in the anterior eye; the iris surrounds the pupil, the opening through which light passes.
• Retina: the retina is the inner coat, with a light-sensitive part lining the posterior eyeball. Its photoreceptors, cells that detect light, include rods and cones.

Rods contribute to vision in dim light, while cones support colour vision and fine detail. The macula is the central retinal region concerned with detailed vision; its central depression, the fovea, provides the sharpest visual discrimination. The optic disc is where nerve fibres leave the retina and contains no photoreceptors.

Where are the lens and the eye’s chambers?

The lens is a transparent structure behind the iris that helps focus light onto the retina. The suspensory ligament of lens consists of fibres connecting its capsule, the enclosing membrane, to the ciliary body. The ciliary muscle changes tension in these fibres, altering lens shape during accommodation, the adjustment of focus for different viewing distances.

The spaces within the eyeball have distinct boundaries and contents:

• Anterior chamber: the space between the cornea and iris, containing aqueous humour.
• Posterior chamber: the narrow space behind the iris and in front of the lens and its supporting fibres, also containing aqueous humour.
• Vitreous chamber: the large space behind the lens, containing the vitreous body, a transparent gel.

Aqueous humour passes from the posterior chamber through the pupil into the anterior chamber. It leaves mainly through drainage structures at the angle between the cornea and iris. The vitreous body occupies most of the eyeball’s interior and helps maintain contact between the light-sensitive retina and its supporting layer.

The cornea provides much of the eye’s light-bending power, while the lens supplies adjustable focusing. The iris regulates the pupil’s size rather than changing the shape of the lens.

Which muscles move the eye, and where does the optic nerve go?

The extraocular muscles are muscles outside the eyeball that move it within the orbit. The rectus muscles run relatively directly towards the eyeball, while the oblique muscles approach it at an angle. Their combined actions position the eyes for coordinated viewing.

• Medial rectus: turns the eye towards the nose.
• Lateral rectus: turns the eye away from the nose.
• Superior rectus: mainly raises the eye.
• Inferior rectus: mainly lowers the eye.
• Superior oblique: turns the upper pole of the eye inward and helps lower the eye, particularly when it is turned towards the nose.
• Inferior oblique: turns the upper pole outward and helps raise the eye, particularly when it is turned towards the nose.

Most cranial nerves connect directly to the brain or brainstem; the spinal root of the accessory nerve arises from the upper spinal cord. The oculomotor nerve supplies most extraocular muscles; the trochlear nerve supplies superior oblique, and the abducens nerve supplies lateral rectus. A separate muscle, levator palpebrae superioris, raises the upper eyelid and also receives oculomotor nerve fibres.

The optic nerve carries visual signals from the retina towards the brain. Its fibres arise from retinal ganglion cells, the retinal nerve cells whose long processes leave the eyeball. At the optic chiasm, a junction beneath the brain, fibres from the nasal half of each retina cross to the opposite side, while fibres from the temporal half remain on the same side.

In the 3D model beside this article, you can rotate and zoom, select the eyeball, and isolate it or fade it to compare its position with surrounding structures.

What are the external and middle ear?

The external ear consists of the auricle, the visible projecting part, and the external acoustic meatus, the passage leading inward. This passage ends at the tympanic membrane, or eardrum, which separates it from the middle ear. Sound reaching the membrane makes it vibrate.

The middle ear is an air-filled cavity in the temporal bone, a skull bone that houses the middle and internal ear. It contains the auditory ossicles, small bones that transmit vibration from the tympanic membrane to the internal ear:

• Malleus: attaches to the tympanic membrane.
• Incus: connects the malleus to the stapes.
• Stapes: has a footplate that fits into the oval window, an opening leading to the internal ear.

The auditory tube connects the middle ear to the nasopharynx, the part of the throat behind the nasal cavity. It provides a route for air exchange. The round window, another opening between the middle and internal ear, is closed by a membrane that permits fluid displacement during sound transmission.

How does the internal ear detect sound and balance?

The internal ear contains the bony labyrinth, a system of cavities within the temporal bone. Inside it lies the membranous labyrinth, a connected system of fluid-filled sacs and ducts. Perilymph is the fluid around the membranous labyrinth; endolymph is the fluid within it.

The cochlea is the coiled part concerned with hearing. Its cochlear duct contains the spiral organ, also called the organ of Corti, which rests on the basilar membrane. Sensory hair cells in this organ respond to movement produced by sound vibrations.

The vestibular apparatus is the part concerned with balance and includes the utricle, saccule and semicircular ducts. The utricle and saccule are sacs whose receptor regions, called maculae, detect linear acceleration and head position relative to gravity. The semicircular ducts detect rotational acceleration through receptor ridges called cristae in their expanded ends, the ampullae.

The vestibulocochlear nerve carries information from the internal ear. Its cochlear division carries hearing signals, while its vestibular division carries information about head position and movement.

Where are the receptors for smell and taste?

Smell receptors lie in the olfactory epithelium, a specialised lining near the roof of the nasal cavity within the nose. Their nerve fibres pass through the cribriform plate, a perforated part of the ethmoid bone, to reach the olfactory bulb beneath the brain. These fibres form the olfactory nerve.

Taste receptors occur in taste buds, small sensory structures found mainly on the tongue, with others in the palate and throat. Many lie within papillae, small projections of the tongue’s surface, although the thread-like filiform papillae lack taste buds. Taste signals travel through the facial, glossopharyngeal and vagus nerves according to their region of origin.

Which cranial nerves carry the special senses?

The sensory pathways below carry information from receptor regions towards the brain. Nerves that move the eyes are distinct from the optic nerve, which carries vision.