This laboratory emphasizes the important structural elements of the eye and ear and briefly covers the taste buds and olfactory epithelium. All of these structures are complicated, and it is therefore important to focus on the essential elements of each. In the case of the eye, you must understand the structure of the retina and the system by which the aqueous humor is formed and drained. For the ear, you should understand the middle ear but focus more the organization of the cochlea and organ of Corti. You should also be able to name the important cells of the taste and olfactory epithelia.

The eye is highly specialized in converting light energy into nerve action potentials in order to transmit visual information to the brain. The key photoreceptors, known as the rod cells and cone cells, and downstream neurons lie within the retina. The rest of the structures in the eye are important for focusing images onto the retina and to support the retina.

The wall of the eye is composed of three layers:

The corneo-scleral layer supports the structure of the eye with its tough fibroelastic composition. This layer has two components:

The junction between cornea and sclera is called the limbus. The surface of the eye is covered by conjunctiva, which reflect into the eyelids.

The uveal layer is highly vascular. This layer has three components:

The retinal layer contains photoreceptors (rod cells and cone cells) as well as neurons that transmit the information to the optic nerve. This layer is divided into two sections by a line near the ciliary body known as the ora serrata. Posterior to the ora serrata, the layer is photosensitive. Anterior to the ora serrata, the retinal layer becomes a non-photosensitive epithelium. The retinal layer has ten sub-layers, which will be discussed later. For now, it is important to realize that the photosensitive cells are actually located at the most posterior portion in this layer; light must actually traverse downstream neurons and support cells before hitting the photoreceptors.

There are three chambers within the eyeball:

Both anterior and posterior chambers are filled with a clear watery aqueous humor . The aqueous humor is secreted into the posterior chamber by the ciliary body. It then circulates through the pupil and finally drains in to the canal of Schlemm at the angle of the anterior chamber. The vitreal cavity is filled with a gel-like, transparent substance called the vitreous body.

When light hits the eye, it is first roughly focused on the retina by the curvature of the cornea. The lens then provides fine focus of the corneal image upon the retina. Note that the curvature of the lens can be adjusted, depending on the tone of the ciliary body. The visual axis of the eye then passes through a region of retina called fovea. The fovea can be easily identified by a depression in the retina in the cross section. This region is the area of highest visual acuity. Surrounding the fovea is a yellow-pigmented zone called the macula lutea. As the light reaches the retina, the rod cells and cone cells transmit the information to neurons. The afferent nerve fibers eventually converge into the optic nerve and exit the eye. The location where optic nerve converges is known as the optic disc. Since there are not photoreceptors in this region, the area constitutes a blind spot.

The different cell types of the retina is categorized as follows:

Histologically, the retina is divided into ten layers. For this lab, simply understand the general organization of retina. You will study these layers in greater detail in the Neuroanatomy course. The layers are as follows (starting from the outermost layer):

The ear, or vestibulo-cochlear apparatus, is responsible for both hearing and the maintenance of equilibrium. It is divided into three regions: the external, middle, and internal ear. The first two of these regions receive, transmit, and amplify sound waves; the third region maintains balance and allows for the transduction of vibrations into neural signals.

The external ear receives sound waves and transmits them to the middle ear. The major structures of this region are the pinna and external auditory meatus . The pinnae and the first third of the meatus are composed of elastic cartilage, while the latter two thirds of the meatus are located within the petrous portion of the temporal bone. The skin lining the meatus contains hair follicles, sebaceous glands, and apocrine glands that secrete cerumen, better known as earwax.

The middle ear is the air-filled tympanic cavity that begins at the tympanic membrane, or eardrum, and continues to the oval window . It is lined by a simple squamous epithelium.

The tympanic membrane converts sound waves from the external ear into vibrations, which are amplified by the ossicles, and then transmitted to the inner ear. The middle ear also contain two small muscles, the tensor tympani and stapedius, which contract to dampen excessive vibrations. The Eustachian (auditory) tube connects the middle ear with the oropharynx in order to allow equilibration of pressure with the external environment.

The internal ear is a system of canals and cavities within the petrous portion of the temporal bone. These cavities make up the osseous (bony) labyrinth, which contains canals and sacs that form the membranous labyrinth. The membranous labyrinth is filled with fluid called endolymph, while areas outside the membranous labyrinth but within the bony labyrinth are surrounded by fluid known as perilymph.

The bony labyrinth includes three major cavities:

The basilar membrane supporting the organ of Corti extends from the spiral lamina to the outer wall of the cochlea. The vestibular membrane runs from the spiral lamina to the outer wall above the basilar membrane. These two membranes divide the bony labyrinth into three cavities:

The scala media, also known as the cochlear duct, contains the organ of Corti . This is a highly specialized sensory epithelium located on top of the basilar membrane. It contains two cell types:

The stereocilia of the hair cells touch the tectorial membane . When sound waves cause the vibration of the endolymph in the scala media, these vibrations are transmitted to the basilar membrane and cause the stereocilia to bend. This, in turn, leads to the depolarization of the hair cells, the excitement of nerve endings at their base, and the transmission of the stimulus through the auditory ganglia and nerve.

Because the basilar membrane varies in dimension along its length, different frequency sound waves are sensed at different locations along the organ of Corti. The organ of Corti is thus characterized as possessing a tonotopic map.

The oral mucosa of the tongue contains three types of papillae:

Taste buds may be found on the fungiform and cirvumvallate papillae. They are round or oval groups of cells lying in the epithelium that surround an opening known as the taste pore . The taste buds contain two types of cells:

The five taste modalities (sweet, sour, salty, bitter, and umami) are perceived in different regions of the tongue. Despite this fact, no structural differences between taste buds in these areas exist.

The upper portions of the nasal cavity are lined by a tall, pseudostratified epithelium. Three types of cells constitute the olfactory epithelium:

Bowman's glands can also be found in the olfactory epithelium. These glands secrete watery solutions in which odiferous substances can be dissolved.