Chapter 18 - Special Senses

Olfaction (Smell)
     Chemicals that we perceive as smell are detected by olfactory organs that consist of:
  Olfactory epithelium - This epithelium consists of olfactory receptor cells, supporting cells, and basal cells (stem cells).
  Lamina propria - The lamina propria of olfactory epithelium contains olfactory glands which produce mucus.
  Olfactory Pathway
     The olfactory receptor cells are highly modified bipolar neurons. The axons of these cells collect into 20 or more bundles that pass through the cribriform plate and synapse with second-order neurons in the olfactory bulb.
     The axons of the second-order neurons travel in the olfactory tract to reach the olfactory cortex, the hypothalamus, and portions of the limbic system.
Gustation (Taste)
     Gustatory receptors are distributed over the dorsal surface of the tongue in taste buds. Taste buds are recessed into the surrounding epithelium and contain gustatory cells, transitional cells, and basal cells. The gustatory cells have microvilli (taste hairs) that extend through a narrow opening of the taste bud called the taste pore.
     Taste buds are found on the sides of three of four types of papillae found on the surface of the tongue:
  1. Filiform papillae - are "thread-shaped" papillae that do not have taste buds but are responsible for the tongue's roughness.
       The remaining three papillae have taste buds along their side walls.
  2. Fungiform papillae - are "mushroom-shaped" papillae are scattered over the anterior two-thirds of the tongue.
  3. Foliate papillae - are creases and folds along the posterior sides of the tongue.
  4. Circumvallate papillae - (circum - around; vallate - walled) are papillae in which a central bump is surrounded by a "wall" of tissue. The circumvallate papillae are arranged in an inverted V formation at the boundary between the root and body of the tongue.
Equilibrium and Hearing
     The ear is divided into three anatomical regions:
  External Ear
     The external ear collects and directs sound waves to the eardrum.
     The structures of the external ear include:
  Auricle (pinna) - This is what is commonly known as the ear. It is a flexible fold of skin supported internally by elastic cartilage that surrounds the external acoustic meatus.
  External acoustic meatus (canal) - This canal permits sound waves to strike the tympanic membrane.
  Ceruminous glands - These glands are found in the skin lining the external acoustic meatus. The waxy secretion of this gland slows the growth of microorganisms.
  Tympanic membrane (tympanum) - This is a delicate, semi-transparent membrane that separates the external and middle ears.
  Middle Ear
     The middle ear consists of an air-filled space called the tympanic cavity. The tympanic cavity is connected with the nasopharynx by the auditory tube (Eustachian tube). The tympanic cavity also communicates with the mastoid sinuses in the mastoid process. It is through this connection that infections can spread from the middle ear to the meninges of the brain to cause meningitis.
     Auditory Ossicles
     Three tiny bones called ear ossicles are found in the middle ear and transfer the vibrations of the tympanic membrane to the fluid-filled chambers of the inner ear. The ear ossicles act as levers that amplify the force of vibration delivered to the inner ear.
     The ear ossicles are called:
  1. Malleus (Hammer) - attaches to the tympanum
  2. Incus (Anvil) - middle ossicle
  3. Stapes (Stirrup) - The base or, foot plate, of this bone covers the oval window on the inner ear.
     The magnitude of the force that reaches the inner ear can be reduced by two muscles that dampen the vibrations produced by loud sounds.
  1. Tensor Tympani M. - attaches to the malleus and by contracting stiffens the tympanic membrane reducing vibration.
  2. Stapedius M. - attaches to the stapes and by contracting reduces the vibrations of the stapes.
  Inner Ear
     The receptors for equilibrium and hearing are located within fluid-filled chambers and tubes that form the membranous labyrinth. The fluid of the membranous labyrinth is called endolymph and differs from extracellular fluid in having a high potassium and low sodium concentration.
     The membranous labyrinth is surrounded and protected by a shell of bone called the bony labyrinth. The contours of the bony labyrinth closely resemble the membranous labyrinth. The space between the membranous and bony labyrinth is filled with a fluid called perilymph that closely resembles CSF.
     The bony labyrinth can be divided into three regions the vestibule, semicircular canals and cochlea. The bony wall of the bony labyrinth is solid except at two locations at the base of the cochlea:
  1. Round window - is a round opening in the bone that is spanned by a thin, flexible membrane.
  2. Oval window - is an oval opening that is sealed by the foot plate of the stapes which is held in position by an annular ligament that attaches the outer edge of the footplate to the edge of the oval window.
  Vestibular Complex and Equilibrium
     The vestibular complex is the part of the inner ear that preserves physical equilibrium by detecting rotation, gravity and acceleration.
  Semicircular Canals
     The anterior, posterior and lateral semicircular canals are designed to detect rotation. Each semicircular canal contains a semicircular duct. Each semicircular canal has an expanded portion which contains within it an expanded portion of the semicircular duct called the ampulla which contains the hair cells.
     The hair cells are the receptor cells. They are associated with supporting cells in a raised portion of the lining of the ampulla called a crista. The apical surface of the hair cell has long microvilli called stereocilia (hence, "hair" cell) and one cilium called a kinocilium. The kinocilium and stereocilia are embedded in a gelatinous substance called the cupula which nearly fills the space within the ampulla.
     Rotational movements in different planes are detected by the mechanical distortion of the stereocilia as fluid moves within the semicircular ducts.
  Utricle and Saccule
      The utricle and saccule are interconnected membranous sacs of the membranous labyrinth found in the vestibule. The endolymph of the utricle is confluent with that of the semicircular ducts and the endolymph of the saccule is confluent with that of the cochlear duct. The utricle and saccule are interconnected by a narrow endolymphatic duct that ends in a blind pouch called the endolymphatic sac.
     Both the utricle and saccule contain hair cells similar to those found in the semicircular canals. In each sac the hair cells are concentrated on a oval spot in the wall called a macula ("spot"). The kinocilia and stereocilia of the hair cells are embedded in a gelatinous mass that has crystals of calcium carbonate embedded on its surface. This gelatinous mass with its crystals is called an otolith and the crystals are called statoconia.
     The difference in density between the crystals and the gelatinous matrix causes a mechanical distortion of the stereocilia of the hair when the head is tilted or the body experiences acceleration. Hence, the hair cells detect the position of the head in space and linear acceleration.   
  Cochlea and Hearing
     The cochlea coils about 2.5 turns around a central hub called the modiolus. The sensory neurons that form the cochlear nerve have their cell bodies in the modiolus in a ganglion called the spiral ganglion.
     The cochlear duct is present within the cochlea flanked by the vestibular duct and the tympanic duct which contains perilymph. At the base of the cochlea the oval window is in contact with the vestibular duct and the round window is in contact with the tympanic duct. The vestibular and tympanic ducts are confluent at the tip of the cochlea at a place called the helicotrema.
  Organ of Corti
     The hair cells that are responsible for hearing are within the organ of Corti, or spiral organ. The organ of Corti rests on a basilar membrane which separates the cochlear duct from the tympanic duct. The hair cells are arranged in an inner row and outer rows that follow the turns of the cochlear duct.
     The stereocilia (a kinocilium is lacking) of the the hair cells are in contact with an overhanging tectorial ("roof") membrane that is attached to the inner wall of the cochlear duct.
  Sound Detection
     Sound is detected when a pressure wave of a frequency between 20 - 20,000 Hz strikes the tympanic membrane causing it to vibrate. The vibration is transferred to the oval window by the ear ossicles and creates a pressure wave within the fluid of the cochlea because of the presence of the round window. The pressure wave moves the basilar membrane relative to the more rigid tectorial membrane causing the stereocilia to bend.
     The basilar membrane is relatively narrow and stiff at the base and wide and loose at the tip of the cochlea. As a consequence, the maximum vibration any location along the length of the basilar membrane depends upon the frequency of the sound waves. By comparing the relative movement of stereocilia along the length of the basilar membrane the brain perceives frequency, or pitch. The loudness of a sound is perceived when the greater force of a louder sound causes an increase in the range of movement of the stereocilia of the hair cells.
  Accessory Structure of the Eye
Eyelids - (Palpebrae)
     The free margins of the eyelids form the palpebral fissure. The edges where the upper and lower eyelids meet are called the lateral and medial canthus. The palpebral margins are lined by eyelashes.
     Glands associated with the eyelids include:
Glands of Zeis - These are sebaceous glands associated with the eyelashes.
Tarsal glands - (Meibomian glands) - These glands line the inner margin of the lid and produce a lipid-rich product that prevents the lids from sticking together.
Lacrimal caruncle - This is a mound of tissue in the medial canthus that produces thick secretions. 
     The eyelids are supported internally by sheets of connective tissue that form the tarsal plate.     
     The conjunctiva is the epithelium that lines the inner surface of the eyelids and continues onto the outer surface of the eye. The palpebral conjunctiva lines the inner surface of the eyelid and the bulbar conjunctiva lines the anterior surface of the eye. The conjunctiva  consists of a specialized stratified squamous epithelium that is thick until it lines the transparent cornea where it is relatively thin. 
Lacrimal Apparatus
     The lacrimal apparatus produces, distributes and removes tears. Tears are continually produced and reduce friction, remove debris, prevent bacterial infection and provide nutrients and oxygen for the conjunctiva. 
     The lacrimal apparatus consists of:
1. Lacrimal Gland
  The lacrimal, or tear gland produces a secretion that is watery, slightly alkaline and contains the enzyme lysozyme which has an antimicrobial function. The gland is located in a depression in the frontal bone in the superior lateral orbit. Ten to 12 ducts release the tears into the superior, lateral region of the fornix which is the pocket formed where the palpebral and bulbar conjunctiva meet. 
2. Superior and Inferior Lacrimal Canaliculi
  Blinking sweeps the tears over the surface of the eye toward the medial canthus. The tears drain into canals called the superior and inferior lacrimal canaliculi through pores called the superior and inferior lacrimal puncta, respectively. 
3. Lacrimal Sac
  The tears drain next into the lacrimal sac which is found on a groove of the lacrimal bone. 
4. Nasolacrimal Duct
  The nasolacrimal duct is found within a passage created by the lacrimal bone and maxilla and opens into the nasal cavity near the inferior meatus. 
     The eyeball is hollow and is divided into two cavities:
1. Posterior cavity
  This cavity takes up most of the internal space and extends anteriorly to the lens. It contains a gelatinous vitreous body and is also called the vitreous chamber. 
2. Anterior cavity
  This cavity is in the front of the lens and is filled with a clear liquid called aqueous humor. It is further subdivided into anterior and posterior chambers by the iris. 
     The wall of the eye has three layers called tunics:
1. Fibrous Tunic
      The outermost layer of the eye is the fibrous tunic. Most of the fibrous tunic is the sclera. The sclera consists of dense fibrous connective tissue and provides mechanical support and physical protection. The fibrous connective tissue of the sclera also serves as attachment sites for the extraocular muscles. 
     The anterior portion of the fibrous tunic is transparent because of the precise alignment of collagen fibers and the absence of blood vessels. This portion is called the cornea. The cornea is primarily responsible for focusing light rays onto the retina. 
2. Vascular Tunic
     As its name implies, this tunic contains numerous blood vessels. It is also the location of the intrinsic eye muscles.
     The vascular tunic includes:
  The iris contains blood vessels, pigment cells and smooth muscles. The smooth muscles control the diameter of the central opening of the iris called the pupil.
Ciliary body
  The iris is attached to the ciliary body which begins at the junction between the cornea and sclera and extends to the ora serrata. The ciliary body is primarily the ciliary muscle the mass of which projects into the interior of the eye. The epithelium that covers the ciliary body is thrown into folds called ciliary processes. Suspensory ligaments are attached to the anterior rim of the ciliary body and holds the lens in position.
  The remaining vascular tunic contains extensive capillaries and is called the choroid. The choroid supplies oxygen and nutrients to the outer portion of the retina.
3. Neural Tunic
     The neural tunic is the retina. The retina consists of two layers:
1. Pigmented layer
  The pigmented layer is a single-cell layer of pigmented cells that absorb light after it passes through the retina. The pigmented layer provides vital metabolic support for the photoreceptors of the retina. It extends anteriorly to cover the ciliary body and the iris.
2. Neural layer
  The neural layer consists of three layers of cells that include the photoreceptors, cells that initiate the processing of visual information and blood vessels that supply the neural layer. The neural layer extends anteriorly to the boundary called the ora serrata.
     The layers of the neural layer from the outermost layer to the innermost are:
Visual receptors
  There are two types of photoreceptors:
  Rods are more sensitive to light but there is only one type of rod and color discrimination is not possible with rods.
  Cones are less sensitive to light but there are three types of cones with sensitivities in different regions of the light spectrum. Cones provide color discrimination and greater detail.
  The cornea and lens focuses light on a spot of the retina that contains only cones and appears as a yellow spot called the macula lutea. In the center of the macula lutea there is a depression called the fovea which provides the greatest visual acuity.
Bipolar cells
  The photoreceptors synapse with bipolar cells. The transmission of visual information through the bipolar cells is modulated by horizontal cells found in this layer.
Ganglion cells
  The bipolar cells synapse with the ganglion cells whose axons carry impulses to the brain in the optic nerves and tracts. The transmission at this layer is modulated by amacrine cells. The axons of the ganglion cells leave the retina by converging on the optic disc and penetrating through the wall of the eye. The central retinal artery and vein emerge onto the surface of the retina at the optic disc.
  Chambers of the Eye
     The anterior and posterior chambers of the anterior cavity of the eye are filled with aqueous humor. Aqueous humor is continually produced at the ciliary processes of the ciliary body. The aqueous humor passes through the pupil into the anterior chamber and drains into the canal of Schlemm (scleral venous sinus) at the limbus of the cornea.
     The posterior cavity of the eye is behind the lens and is filled with a gelatinous body called the vitreous body. The vitreous body maintains the shape of the eye, supports the lens, and presses the neural layer against the pigmented layer.
  Visual Pathways
     The visual information received and initially processed in the retina travels to the lateral geniculate nucleus of the thalamus. The fibers carrying information from the medial half of the retina crosses over (decussates) to the other side (contralateral) of the brain at the optic chiasm. The information is then relayed from the thalamus to the visual cortex.
     The partial decussation of the fibers from the retina result in the information from the right visual field going to the left occipital cortex and the information from the left visual field going to the right occipital cortex. A comparison of the information coming from each eye in the overlapping visual fields gives us depth perception.