Sensory Systems Lab

Learning Objectives

  • Explain the structure of the retina and the drainage system for the aqueous humor in the eye.
  • Describe the organization of the cochlea and the organ of Corti in the ear.
  • Explain the structural components of the taste buds and olfactory epithelium.
  • Explain the role of the canal of Schlemm in glaucoma.

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Pre-Lab Reading

Introduction

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.

Overview of the Eye

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 outer corneo-scleral layer
  • the intermediate uveal layer
  • the inner retinal layer

Corneo-scleral Layer

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

  • The cornea is located in the anterior portion of this layer. It is transparent and functions to roughly focus images onto the retina.
  • The sclera is located in the posterior portion of this layer. The sclera is opaque and it is the site of insertion for the extraocular muscles.

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

Uveal Layer

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

  • The choroid is located in the posterior portion. It lies between the sclera and the retina and is heavily pigmented. It serves two main functions: it supports the retina and absorbs light that has passed through the retina. This prevents it from reflecting and decreasing the integrity of the image formed on the retina.
  • The ciliary body is located in the anterior portion of the uveal layer, right behind the limbus. It is attached to the lens via the suspensory ligament (zonule). The ciliary body contains smooth muscle that controls the shape of the lens .
  • The iris extends from the ciliary body to cover the front of the lens. It regulates the amount of light reaching the retina. The aperture of the iris is known as the pupil.

Retinal Layer

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.

Chambers of the Eye

There are three chambers within the eyeball:

  • The anterior chamber lies between the iris and the cornea.
  • The posterior chamber lies between iris and lens.
  • The vitreal cavity occupies the bulk of the eye between the lens and posterior wall.

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.

Transmission of Visual Information

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.

Sub-layers of Retina

Layers of the Retina

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

  • Photoreceptor cells: rod cells and cone cells ("the 1st neuron" in the visual information transmission path)
  • Intermediate neurons: bipolar cells, horizontal cells, and amacrine cells . ("the 2nd neuron")
  • Retinal ganglion cells : these are cells of the optic tract neurons ("the 3rd neuron")
  • Pigmented epithelial cells
  • Neuron support cells

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 pigmented epithelium is in contact with Bruch's membrane, which separates retina from the choroid. The pigmented epithelium is a light-absorbing layer that reduces random reflections of unabsorbed light.
  • The photoreceptor layer contains photosensitive outer segments of rods and cones.
  • The outer limiting membrane is not a true membrane, but simply represents the junction between Muller cells, which provide structural and nutritional support for the retinal neuron, and the photoreceptor cells.
  • The outer nuclear layer contains cell bodies of the rods and cones.
  • The outer plexiform layer contains synapses between axons of photoreceptors and dendrites of intermediate neurons.
  • The inner nuclear layer contains cell bodies of intermediate neurons and Muller cells.
  • The inner plexiform layer contains synapses between intermediate neurons and ganglion cells of the optic tract.
  • The ganglion cell layer contains cell bodies of ganglion cells.
  • The optic nerve fiber layer contains axons of ganglion cells.
  • The inner limiting membrane separates the retina from the vitreous body.

Overview of the Ear

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.

External Ear

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.

Middle Ear

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.

Internal Ear

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 vestibule lies medial to the tympanic cavity. It houses the sensory epithelia, or maculae, of the saccule and utricle, two components of the membranous labyrinth important for maintenance of balance. The vestibule also contains the oval window, through which vibrations are transmitted from the middle ear.
  • The semicircular canals are another component of the vestibular system involved in regulating static and dynamic balance. The three canals are organized as loops located perpendicular to one another.
  • The cochlea consists of a bony canal that makes 2.75 spiral turns around an axis of bone known as the modiolus . A shelf of bone from the modiolus forms a ridge known as the spiral lamina . Nerve bundles of the cochlear division of the auditory nerve (VIII) enter the base of the modiolus. The cell bodies of the spiral (cochlear) ganglion are located within the modiolus along the inner wall of the cochlear canal.

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 vestibuli, located between the roof of the labyrinth and the vestibular membrane, is lined by mesenchymal cells and contains perilymph.
  • The scala media, located between the vestibular membrane and basilar membrane, contains endolymph.
  • The scala tympani, located between the basilar membrane and the floor of the labyrinth, is lined by mesenchymal cells and contains perilymph.

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:

  • Sustentacular cells, which serve a support function.
  • Hair cells, which are embedded within the supporting cells and have distal sensory sterocilia. Inner hair cells primarily transduce vibrations into neural signals. Outer hair cells undergo electromotile vibrations that assist with tuning that broadens the frequency range and improves frequency selectivity, thus allowing humans to hear sounds of both low and high frequency and to tune in to those that are most valuable, such as the frequencies of speech and music.

Transmission of Auditory Information

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.

Overview of Taste Buds

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

  • Filiform papillae, which are the most numerous and appear short and bristled. Their surface is keratinized.
  • Fungiform papillae, which are scattered among the filiform papillae and are larger, redder, and more globular. They are not keratinized.
  • Circumvallate papillae, which contain the most taste buds. There are between 6 and 14 circumvallate 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:

  • Gustatory (taste) cells, which appear light and contain microvilli.
  • Sustentatcular (support) cells, which appear dark, contain microvilli, and secrete a glycoprotein substance into the taste pore.

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.

Overview of Olfactory Epithelium

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

  • Olfactory receptor cells are bipolar neurons with a single dendritic process that extends toward the surface of the epithelium and forms an olfactory knob. The olfactory knob gives off long cilia that interact with odiferous substances. On the other end, these cells give off non-myelinated axons. The axons group together and pass through the cribiform plate to reach the olfactory bulb of the forebrain.
  • Sustentacular (support) cells are supporting epithelial cells. They have microvilli on their luminal surfaces that are mixed together with the cilia of the olfactory receptor cells. These cells are thought to provide mechanical and physiological support for the receptor cells.
  • Basal epithelial cells serve a regenerative function.

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

Pre-Lab Quiz

  1. Describe how the three chambers of the eye are defined.
  2. Answer:
  3. Describe the flow of the aqueous humor from the site of secretion to the site of drainage.
  4. Answer:
  5. What is the blind spot?
  6. Answer:
  7. Explain the series of events that allow for the transduction of a sound wave into a neural impulse.
  8. Answer:
  9. What are the five taste senses and what is the structural difference between the taste buds that sense each?
  10. Answer:
  11. What is the function of Bowman's gland in the olfactory epithelium?
  12. Answer:

Slides

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  1. Eye
  2. Ciliary Body
  3. Ciliary Body Epithelium
  4. Cornea
  5. Lens
  6. Flow of Aqueous Humor
  7. Retina
  8. Fovea
  9. Optic Nerve
  10. Inner Ear
  11. Organ of Corti
  12. Hair Cells
  13. Circumvallate Papilla
  14. Taste Buds
  15. Olfactory Epithelium

Virtual Microscope Slides

  1. Eye
  2. At low power, identify some of the key structures of the eye: cornea, iris, ciliary body, lens, optic nerve, anterior chamber, posterior chamber, and vitreal cavity.
  3. Chochlea
  4. After identifying the major components of the cochlea, trace the path of a sound wave as it proceeds through the different chambers.

Pathology

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  1. Glaucoma

Quiz

  1. What occupies the space behind the lens?
  2. Answer: Vitreous body
  3. What is produced by the finger-like processes?
  4. Answer: Aqueous humor
  5. Identify the bottom layer of this structure.
  6. Answer: Optic nerve fiber layer
  7. Name this structure.
  8. Answer: Cochlea
  9. Identify this structure.
  10. Answer: Organ of Corti
  11. What occupies the middle compartment?
  12. Answer: Endolymph
  13. Cataracts develop for a variety of reasons, including exposure to ultraviolet light, diabetes, and high blood pressure. The result is an expanding opacity in the lens that impedes normal vision. Explain the normal structure of the lens and why damage to this structure may lead to cataracts.
  14. Answer: The lens normally has cells and collagen fibers oriented in a particular way to allow for the transmission of light. Damage to these proteins can cause them to become denatured or to change shape, and damage to cells can cause them to produce proteins in the incorrect configuration. Either way, the proteins of the lens become improperly organized and cataracts occur.