Bio 210 Notes for Chapter 16

The Special Senses

The Chemical Senses: Taste and Smell

Taste and smell receptors are classified as chemoreceptors because they respond to chemicals dissolved in an aqueous solution.  Taste receptors (gustatory receptors) respond to chemicals dissolved in saliva and smell receptors (olfactory receptors) respond to chemicals dissolved in fluids lining the nasal cavity.

    Taste Buds and the Sense of Taste

Most taste buds are located on papillae which are peglike projections on the surface of the tongue.  There are three types of papillae as follows:

A single taste bud is made of 40 to 100 epithelial cells of three major types as follows:

There are four basic taste sensations (sweet, sour, salty, and bitter).  However, most of what we taste is a combination of the basic taste sensations.  

Taste pathway

Other factors influence the taste of food.  For example, spicy foods activate pain receptors in the mouth and smell of the food enhances the taste of it.

    The Olfactory Epithelium and the Sense of Smell

The organ of smell is called the olfactory epithelium and it is located in the roof of the nasal cavity.   appreciate certain smells.

Olfactory Pathway

Homeostatic Imbalances of the Chemical Senses

anosmias

The Eye and Vision

The adult eye is spherical in shape with a diameter of about one inch.  Only the anterior one-sixth of the eye is visible and the posterior five-sixths is enclosed by a fat pad and the bones of the orbit.

    Accessory Structures of the Eye (Eyebrows, Eyelids, Conjunctiva, Lacrimal Apparatus, and Extrinsic Eye Muscles)

Eyebrows

Eyelids

  • palpebrae

  • palpebral fissures.  

  • canthi

  • lacrimal caruncle 

  • tarsal plates

  • orbicularis oculi and levator palpebrae superioris

  • tarsal glands (Memobian glands)- The oil produced by these glands help to keep eyelashes separated.  

  • ciliary glands -modified sweat glands called .  is called a 

  • chalazion - inflammation of a tarsal gland  is called

  • sty-inflammation of any of the smaller glands 

Conjunctiva

  • palpebral conjunctiva 

  • ocular conjunctiva

  • conjunctivitis (pinkeye) 

Lacrimal Apparatus

  • lacrimal gland 

  • lacrimal puncta 

  • lacrimal canals 

  • lacrimal sac

  • nasolacrimal ducts 


Extrinsic Eye Muscles 

The extrinsic eye muscles are six small skeletal muscles attached to the surface of each eye that control the movements of the eye.  They are as follows:

  • superior rectus muscle - moves eye superiorly

  • inferior rectus muscle - moves eye inferiorly

  • lateral rectus muscle - moves eye laterally

  • medial rectus muscle - moves eye medially

  • superior oblique muscle - rotates the eye downward and slightly laterally

  • inferior oblique muscle - rotates the eye upward and laterally.

Recall that these muscles are controlled by three pairs of cranial nerves (III,IV, and VI).

The extrinsic eye muscles basically produce the following types of eye movements:

  • saccades - small, jerky movements

  • scanning movements - slow, tracking movements

Diplopia (double vision) 

Strabismus 

    Structure of the Eyeball

The eyeball consists of three layers or tunics (fibrous layer, vascular layer, and sensory layer).  It internal cavities are filled with fluids called humors that help to maintain its shape.  The eyeball is divided into two segments (an anterior segment and a posterior segment).  The lens is the structure that divides the eye into these two segments.

        Fibrous Tunic

        Vascular Tunic

The vascular tunic is also called the uvea and is the middle layer of the eyeball.  It has three distinct regions ( choroid, ciliary body, and iris).  

        Sensory Tunic

The sensory tunic is the innermost layer of the eyeball and is also called the retina.  There are two layers to the retina:

The photoreceptors of the neural layer are sensitive to light energy.  When they are activated, they spread currents to the bipolar cells which then activate ganglion cells.  The axons of ganglion cells exit the eyeball through the optic nerve.

The optic disc is the area of the eyeball where the optic nerve exits.  It is also called the blind spot of the eye since no photoreceptors are in this area.  

Thee are two types of photoreceptors as follows:

macula lutea

fovea centralis

The retina receives its blood supply two sources as follows:

Head trauma or jerking movements of the head can result in retinal detachment.  In this condition, the pigmented layer and the nervous layer separate vitreous humor seeps between them.  The can cause permanent blindness if the two layers are not reattached.  

        Internal Chambers and Fluids

The lens internally divides the eye into two segments as follows:

       The iris divides the anterior segment into two chambers as follows:

Typically aqueous humor is produced at the same rate it is drained.  If aqueous humor drainage is blocked, a condition known as glaucoma develops.  

        Lens

The lens is a biconvex, flexible, and transparent structure that functions to focus light onto the retina.  It is held in place by suspensory ligaments of the ciliary body.  Like the cornea, the lens is avascular.

The lens has two regions as follows:

Cataracts result when crystallin proteins clump together.  

Physiology of Vision

    Overview: Light and Optics

Light is a form of energy.  Visible light is actually not one wavelength but a spectrum or range of wavelengths.  Objects have color because they reflect a certain wavelength in the visible spectrum. 

Another thing that is important to know about light is that it bends or is refracted easily.  

Lenses are structures that are designed to bend light.  The lens in the human eye is convex (thicker in the middle than on the sides).  Convex lenses bend light so that it converges or intersects into a single focal point.  Images formed by convex lens are also upside down and reversed from left to right.     

    Focusing of Light on the Retina

As light enters the eye, it is bent three time.  

The far point of vision is defined as the distance beyond which no change in lens shape is needed for focusing.  In a normal (emmetropic) eye, this distance is 20 feet.  

During distant vision, the ciliary muscles are completely relaxed and the suspensory ligaments are pulled taught.  This causes the lens to be stretched and it gets thin.  Therefore, it is at its lowest refractory power ( i.e. it bends light the least amount possible).  Because light from distant images reach the eyes as nearly parallel ray, it does need to be bent that much to be focused onto the retina.

During near vision the following events occur:

Myopia is more commonly called nearsightedness.  In this condition, a person can focus properly on images close up but not in the distance.  Distant images are blurred because they are focused in front of the retina and not on the retina. Concave lenses that diverge the light  before it enters the eye correct this condition.

Hyperopia is more commonly called farsightedness.  In this condition, a person can focus properly on images in the distance but not close up.  Close images are blurred because they are focused behind the retina.  Convex lenses that converge the light before it enters the eye correct this condition.

Astigmatism is a condition in which unequal curvatures in different parts of the lens or cornea exist.  This also leads to blurred vision. Special cylindrical lenses are used to correct this condition. 

 

    Photoreception

Photoreception is the process by which the eye detects light energy.  

Two types of photoreceptors are rods and cones.  

Color blindness is due to a lack of one or more cone type.  This condition is more common in males than females and is inherited as a sex-linked condition.

In the dark, a molecule called cGMP is bound to sodium channels on the membranes of the outer segments of photoreceptor cells.  As long as cGMP is bound to sodium channels, they will remain open and sodium flows freely into the outer segment which produces a depolarizing current that ultimately holds the calcium gates at the synaptic endings open.  As you know, when calcium gates are open on synaptic endings, calcium flows in and neurotransmitters are released.  The neurotransmitter released here is glutamate.

When light triggers retinal to detach from its opsin, cGMP is destroyed and therefore the sodium channels close.  This causes a hyperpolarizing current to be generated which inhibits the release of neurotransmitters from the synaptic endings of the photoreceptor cells.  We will see how inhibiting neurotransmitter release by the photoreceptor cells plays a role in visual processing later.  

The retina must adjust in light and dark situations.  In light adaptation (going from dark to light like when you turn on the bathroom light in the middle of the night) rods have to be "turned off" and cones "turned on".  Initially the retina is set for dim light  and is very sensitive to light.  Therefore, the retina must decrease its sensitivity to light which takes about 60 seconds.  As cones are activated visual acuity and color vision continues to improve for about 5-10 minutes.

In dark adaptation (going from light to dark like when you walk into a dark movie theater) cones have to be "turned off" and rods "turned on".  Initially the retina is set for bright light and is less sensitive to light.  Therefore, the retina must increase its sensitivity to light which takes anywhere from 20-30 minutes.  The reason why dark adaptation takes longer is that rhodopsin of rods has been broken down by the bright light so it must be regenerated for the rods to properly function again.

Night blindness (nyctalopia) is a condition is which rods do not function properly.  It is commonly caused by a lack of vitamin A.  Recall that vitamin A is needed for the synthesis of retinal.       

    The Visual Pathway to the Brain

    Stereoscopic Vision and Depth Perception

Stereoscopic vision is possible because visual fields of each eye overlap and the primary visual cortices receive inputs from both visual fields.  This type of vision allows for depth perception and depends on both eyes working together.  Therefore, if one eye or optic nerve is lost depth perception is seriously impaired.  Why is depth perception not impaired if a person has a stroke in the right or left primary visual cortex?  

    Visual Processing

A very simple explanation of visual processing (how information received by rods and cones is turned into vision) is summarized as follows:

The Ear: Hearing and Balance

    Structure of the Ear

The ear is divided into three regions - outer ear, middle ear, and inner ear.  The outer and middle ear are only involved in hearing while the inner ear functions in both hearing and equilibrium.

        The Outer Ear

The external or outer ear consists of two structures as follows:

        The Middle Ear

The middle ear (also called the tympanic cavity) is a small, air-filled space.  It is lined by a mucous membrane and contains or is associated with the following structures:

        The Inner Ear

The inner ear has a very complicated twisting, and winding structure.  For this reason is is also called a labyrinth which means "maze".  The inner ear can be divided into the following major regions:

The inner ear can also be divided into the following regions based on the shapes that the bony and membranous labyrinth form:

  1. scala vestibuli - superior to the cochlear duct; filled with perilymph; 

  2. scala tympani - inferior to the cochlear duct; filled with perilymph; 

  3. scala media - the cochlear duct; filled with endolymph; the roof of the scala media next to the scala vestibule is called the vestibular membrane; floor of scala media next to scala tympani is called basilar membrane (each basilar membrane holds an organ of Corti); external wall of scala media is called stria vascularis.

The vestibulocochlear nerve exits the inner ear and carries balance and sound information to the brain for interpretation.

     

    Mechanisms of Hearing

Sound waves travel into the external auditory meatus and cause the tympanic membrane to vibrate.  The vibrating tympanic membrane causes the ear ossicles to vibrate.  The last ear ossicle (stapes) hits the inner ear at a membrane called the oval window and fluids of the inner ear are set into motion.  The greater the intensity of the sound, the greater the vibration of the tympanic membrane, the greater motion of the stapes, and hence the greater the impact on the oval window and the greater the movement of inner ear fluids.

As the stapes vibrates back and forth, it sets the perilymph of the scala vestibuli into back and forth motion.  This causes the basilar membrane to swing up and down which causes the basal part of the cochlear duct to oscillate much like a turning jump rope.  Sound waves of low frequency (below 20Hz) create pressure waves that simply travel through the entire cochlea (through the scala vestibuli, scala tympani, and out through a membrane covered opening called the round window of the cochlea).  These sound waves do not stimulate organs of Corti and are therefore, not "heard".  Sound waves of higher frequency (above 20Hz) create  pressure waves that travel from the scala vestibuli through the cochlear duct to the perilymph of the scala tympani.  This sets the basilar membrane into motion. Fibers of varying lengths span the width of the basilar membrane. Different frequencies of sounds waves will cause different fibers to resonate (reverberate).   The fibers that resonate determine how the basilar membrane will vibrate. 

The vibrating basilar membrane can cause tiny hairs in the organ of Corti to bend.  Recall that the organ of Corti sits on top of the basilar membrane.  It is made of supporting cells and receptor cells called cochlear hair cells.  The hair cells are arranged into rows (one row of inner hair cells and three rows of outer hair cells).  On top of the hair cells is a gel-like structure called the tectorial membrane.  The hair cells contact afferent fibers of the cochlear nerve (part of the vestibulocochlear nerve that carried sound information to the brain).  Depending on the vibrating pattern of the basilar membrane, the tectorial membrane will move to stimulate certain hair cells.  When hair cells are stimulated, they stimulate sensory neurons which have axons in the cochlear nerve.  The brain interprets the sound based on which hair cells and sensory neurons were activated.     

 

    The Auditory Pathway to the Brain

  Homeostatic Imbalances of Hearing

In general there are two types of deafness as follows:

Tinnitus is defined as an abnormal ringing in the ear.  It can result from cochlear nerve degeneration, an ear infection or as a side effect of certain drugs.

Meniere's Syndrome is a disorder of the semicircular canals and cochlea.  Therefore a person with this condition often loses their hearing and has attacks of nausea and vertigo.

Mechanisms of Equilibrium and Orientation

Although the ear is not the only organ that sends information to the brain to help us maintain balance, it is an important one.  Receptors in the vestibule of the inner ear monitor static equilibrium while receptors in the semicircular canals monitor dynamic equilibrium.

    The Maculae and Static Equilibrium

The maculae are sensory receptors for static equilibrium.  Recall that the vestibule contains two endolymph containing sacs (the saccule and the utricle) that are made from the membranous labyrinth.  The saccule wall contains one macula and the utricle wall contains another macula.  These maculae respond to straight line acceleration forces of the head but not rotational movements.  

Each macula is a flat structure composed of supporting cells and hair cells.  The hair cells possess cilia which are embedded in a gelatinous structure called the otolithic membrane.  

The macula in the utricle is horizontal so that the cilia of the hair cells extend vertically into the otolithic membrane when the head is in a upright position.  When the head moves in the horizontal plane (like tilting of the head laterally) the otolithic membrane is displaced and the hairs are bent.  This activates sensory neurons which ultimately deliver information to the brain to advise it of the position of the head.

The macula in the saccule is vertical so that the cilia of the hair cells extend horizontally into the otolithic membrane when the head is in the upright position.  When the head moves in the vertical place (like when you ride an elevator) the otolithic membrane is displaced and the hairs are bent.  This activates sensory neurons which ultimately deliver information to the brain about the position of the head. 

    The Crista Ampullaris and Dynamic Equilibrium

The crista ampullaris is the receptor for dynamic equilibrium.  Each semicircular canal contains one in its ampulla.  Each cristae is made of supporting cells and hair cells.  The cilia of the hair cells project into a gel-like structure called a cupula.  When the head rotates in one direction, the endolymph in the semicircular ducts moves in the opposite direction which ultimately displaces the cupula and thus the cilia of the hair cells.  Movement of the cilia activates the hair cells which in turn activate sensory neurons that deliver information to the brain via the vestibular nerve.

    The Equilibrium Pathway to the Brain

Balance information from the inner ear, visual receptors, and somatic receptors (proprioceptors) are delivered to the cerebellum and the brain stem.  The brain stem uses balance information to control the reflexes needed to help us maintain balance.  The cerebellum uses balance information to coordinate skeletal muscle activity so that head position, posture, and balance are maintained during body movements.

 

 

Regina Hoffman, Ph.D.
Copyright © 1999. All rights reserved.