The Sensory SystemSensory ReceptorsSensory receptors recieve input, generate receptor potentials, and with enough summation, generate action potentials in theneurons they are part of or synapse with.

There are 5 types based on the type of stimuli they detect:

1. Mechanoreceptors ­ pressure receptors, stretch receptors, and specialized mechanoreceptors involved in movement andbalance.

2. Thermoreceptors ­ skin and viscera, respond to both external and internal temperature

3. Pain receptors ­ stimulated by lack of O2, chemicals released from damaged cells and inflammatory cells

4. Chemoreceptors ­ detect changes in levels of O2, CO2, and H+ ions (pH) as well as chemicals that stimulate taste andsmell receptors

5. Photoreceptors ­ stimulated by light

General SensesA. Proprioceptors

1. Stretch receptors located in joints, ligaments, and tendons (respond to either stretch or compression)

a. Muscle spindles – modified muscle fibers with sensory nerve endings wrapped around the middle (and alsofound at the ends). Detect stretch and stimulate a reflex contraction; think about banging on your patellarligament (just an extension of a quadriceps tendon) and watching your knee jerk up – the quadricepscontracted in response to the stretch of the patellar ligament, which stretched muscle spindles. Also (just foryour information) impulses are sent to the hamstring group (the antagonists) to cause them to relax, so theydon’t oppose the contraction of the quadriceps.

1. Purpose – maintain some degree of continuous contraction (partial sustained contraction) or muscle tone

B. Cutaneous Receptors

1. Touch Receptors: fine touch

a. Meissner’s corpuscle – fine touch, discrimination; found concentrated in places where you need to have a lot ofresponsiveness to a little input.

b. Merkel disks ­ found deep at the junction of the epidermis and dermis.

c. Root hair plexus ­ at the base of hair follicles.

2. Touch Receptors: pressure sensitive

a. Ruffini’s endings and Krause's end bulbs – encapsulated pressure sensors, dermis (and elsewhere), respond tocontinuous pressure

b. Pacinian corpuscles – deep pressure sensors, onion shaped capsule (layers of Schwann cells enclosed in aconnective tissue membrane), respond to on­off pressure or vibration

3. Temperature

a. Free nerve endings, some responsive to heat and others responsive to cold

4. Pain

a. Free nerve endings, respond to chemicals released from damaged tissues.

C. Pain receptors

1. Somatic nociceptors

a. From skin and skeletal muscle

2. Visceral nociceptors

a. Receptors that help maintain internal homeostasis

1. Respond to stretch, lack of O2, chemicals released from damaged cells and inflammatory cells.

2. Referred pain – visceral pain afferents travel along the same pathways as somatic pain afferents, sosometimes the brain interprets the visceral pain as the more common somatic pain. Example – Often painfrom the heart felt during a heart attack is perceived as a pain that originates in the left arm.

Special Sense OrgansNow we will talk about the special sense organs rather than general receptors that detect things like carbon dioxide, oxygen,pH, etc.

Senses of Taste and SmellTaste buds and olfactory cells (smell receptors) detect chemicals, thus they are chemoreceptors.

A. Sense of Taste

1. Taste Buds – located in papillae on the tongue, hard palate, pharynx, and epiglottis.

2. Five types of tastes (Really there are at least six – the sixth is water. You might not recognize it, but yourhypothalamus will, and it affects fluid retention/excretion.)

a. Bitter – back of the tongue, evolutionarily important because plant alkaloids, which often are poisonous, arebitter. Keeps you from swallowing potentially toxic stuff, unless you’ve trained yourself to recognize the taste ofquinine (found in tonic water) and can knock back a gin and tonic without problems.

b. Sour – this is a good taste, the taste of citrus fruits, which contain vitamin C. Located at the sides of thetongue.

c. Salty – another good taste because craving salt provides with sodium and other minerals. Specific receptorslocated along the lateral margins of the tongue.

d. Sweet – Again, another good taste, because glucose the main fuel of the body. Your brain really really likes torun on glucose. Receptors located on the anterior superior surface of the tongue.

e. Umami ­ basically a monosodium glutamate receptor. Yeah, the stuff called flavor enhancer, the stuffknowledgeable diners insist not be used in their Chinese food. Tastes kind of like beef or chicken broth, orsometimes described as steak. The receptor detects the amino acid glutamate.

3. How the Brain Receives Taste Information

a. Taste buds open at a taste pore and consist of supporting cells and sensory epithelial cells with microvilli that bindcertain chemicals and depolarize (send a nerve impulse) in response to that chemical.

b. The brain integrates the different taste signals coming in to give an overall combination effect.

B. Sense of Smell

1. Olfactory Cells – located in the superior region of the nasal cavity. Don’t really know how many different smells wecan detect.

2. Cells are structurally alike but sensitive to different chemicals.

3. Patterns of stimulation (which combinations of cells are stimulated) determine the characteristics of an odor.

4. At least 50 different primary smells (we don’t even have the words in the English language to describe them all) butprobably somewhere between 2000 and 4000 different chemicals detected.

5. And if that isn’t enough, the sense of smell and taste interact. No sense of smell, no taste discrimination. (Everhave a cold and notice food doesn’t taste as good?)

6. Also – really closely tied in to the limbic system (the emotional brain). You really remember smells – have you everexperienced being away from the home you grew up in for some period of time and noticed that when you return fora visit the smells of that house can bring the memory of your entire childhood back instantly?

Sense of VisionPhotoreceptors – rods and cones, located in the eye, but first:

A. Accessory Organs of the Eye

1. Eyebrows, Eyelids and Eyelashes

a. Conjunctiva – mucous membrane lining the inner surface of the eyelid and anterior portion of the eye except forthe cornea, keeps tears from getting back into the orbits

b. Eyelashes act as filters to keep particulate matter out of eye

2. Lacrimal Apparatus

a. Lacrmal gland – produces tears, flow over eye to lacrimal sac

b. Lacrimal sac and ducts

c. Lacrimal canals lead into lacrimal sac

d. Nasolacrimal duct drains into the nose

3. Extrinsic Muscles – move the eyes; three pairs

a. Superior and inferior rectus – roll eye up and down

b. Lateral and medial rectus – turn eye in and out

c. Superior and inferior oblique – rotate the eye counterclockwise or clockwise

B. Anatomy and Physiology of the Eye

1. Layers (coats, or tunics)

a. Sclera – outer, white, fibrous connective tissue except for cornea, which covers the iris and is clear

b. Choroid – middle layer, pigmented to absorb stray light rays

1. Ciliary body – anterior portion of choroid, contains ciliary muscle, which rounds up the lens toaccommodate for near vision

2. The lens consists of cells that have lost their nucleus and organelles and are filled with clear proteinscalled crystallins. These proteins allow light to pass through. The lens is attached to the ciliary body byligaments, preset for distant vision, rounds up to focus light rays reflected from close objects

3. Posterior cavity is behind lens, filled with vitreous humor, thick, gelatinous

4. Anterior cavity is between cornea and lens, filled with aqueous humor

i. Produced by the ciliary body, fluid is filtered from blood plasma. Circulates to the Canal of Schlemm,located at the place where the cornea and iris meet. Blockage of this exit canal results in pressuredue to build up of aqueous humor. This pressure compresses arteries and nerve fibers of the retinadie, leading to blindness. This condition is known as glaucoma.

5. Iris – forward (anterior) most part of choroid, consist of smooth muscle, makes a ring with a hole (the pupil)in the middle through which light passes. The iris can contract in different ways to either dilate the pupil(open it further) or constrict the pupil.

c. Retina – inner layer of the eye, contains three layers of cells: inner layer of ganglionic cells, whose axons together make up the optic nerve, a middle layer of bipolar cells, which synapse with both the ganglionic cellsand the sensory cells located in the layer closest to the choriod, the rods and cones.

1. The place where the optic nerve exits the eye has no photoreceptor cells and is known as the blind spot.

2. Rods

a. Located in the periphery of the eyes

b. Sensitive to dim light but don’t detect much detail or color

i. So things may look a little fuzzy and gray at in the dark (well, in the dim I suppose. In the darkyou wouldn’t see anything)

c. Good for peripheral vision since they are located around the edges of your field of vision

d. Active molecule is rhodopsin, a combination of the pigment opsin and the pigment retinal

i. Light breaks the molecule rhodopsin to its components and this generates the nerve impulses

ii. In bright light most rhodopsin is broken down, the period of adjustment to dim light is the periodwhen rhodopsin is being re­synthesized

iii. Retinal comes from vitamin A; Vitamin A deficiency is characterized by night blindness

3. Cones

a. Function in bright light

b. Detect fine detail and color

i. Three kinds of cones based on the color they detect

a. Blue

b. Green

c. Red

ii. Lack of one type of cone is the cause of color blindness

iii. Lack of red makes green more visible and red not, etc.

iv. Red­green most common because they are sex­linked (carried on the X chromosome, and youonly have one active X chromosome in each cell, especially if you are male)

v. Complete color blindness is rare

c. Cones are most concentrated in the fovea centralis, a small area in the center of the macula lutea(yellow spot)

i. Staring straight at an object focuses light rays on the fovea centralis, which is why scanning anarea allows greater awareness of detail than fixing on one spot (a good idea when driving, etc.)

ii. This is also why staring straight at an object in the dark (dim light) is less effective than observingwith peripheral vision

2. Function of the Lens

a. Light rays reflected from objects must be bent so that they converge at a point. This is called the focal point,and should occur exactly at the retina. The distance from the lens, which bends the light rays so they willconverge, and the focal point, is the focal distance. Obviously the focal distance needs to be exactly the sameas the distance from the lens to the retina.

b. The lens is preset for distant vision; objects at a distance of about 20 feet and further are automatically focusedon the retina.

c. Light rays from closer objects diverge more, and would normally come to a focal point behind the retina (if thatwere possible). To bend light rays reflected from closer objects more so that they focus on the retina the lensmust round up. This process is called accommodation.

d. The ciliary muscles are relaxed for distant vision, which allows the ciliary body to move back and away from thelens. This pulls the suspensory ligaments taut, which holds the lens flat.

e. The ciliary muscles contract, moving the ciliary body forward and toward the lens, relaxing the suspensoryligaments and allowing the lens to become more round, to accommodate for close vision.

3. Stereoscopic Vision

a. When the eyes focus on an object each sees it from a slightly different angle

b. Optic nerves from each eye carry nerve impulses generated by light waves to the optic chiasma, where axonsfrom the right side of each eye travel to the right occipital lobe and axons from the left side of each eye travel tothe left occipital lobe.

c. The left and right hemispheres communicate with each other to construct a three dimensional interpretation ofthe object.

4. Vision Problems:

a. If the eyeball is too long the flat lens focuses distant objects in front of the retina. Since light rays reflected fromcloser objects diverge more, and the focal distance is longer, the focal point moves back to the retina withoutthe lens having to accommodate, and near vision is OK, but distant vision is blurred. You can’t flatten the lensany flatter than it already is, so you’re stuck. This is known as myopia, or nearsightedness. It can becorrected by placing a concave lens in front of the eye, which diverges the light rays a bit before they enter theeye. This increases the focal length and allows the relaxed lens to focus precisely on the retina.

b. If the eyeball is too short, the focal point from distant objects is behind the retina, but the lens can round up tomove the focal point forward, like accommodating for near vision, and distant objects appear to be in focus. The problem comes when objects close to the eye cause the focal length to be longer, and the lens, which isalready rounded up, can’t round up any more. This causes close objects to be blurred and is known ashyperopia, or farsightedness. Hyperopia can be corrected by placing a convex lens in front of the eye, whichconverges the light rays a bit before they enter the eye. This decreases the focal distance so the lens canfocus distant objects without rounding up and can round up enough to focus near objects.

c. A normal part of the aging process is loss of elasticity by the lens, which inhibits its ability to round up and focuson close objects. This age­related farsightedness in an eye with a perfectly good shape is called presbyopia(“old vision”) and usually begins to be noticed around 40 years of age. Presbyopia can also be treated withconvex lenses, but since the focal length is normal this correction will cause distant vision to be blurred, sopeople commonly wear half glasses in order to be able to look over them at distant objects and peer downthrough them at close objects. This makes negotiating stairs a challenge, especially if someone was myopic tobegin with and must then wear bifocals (Think about it).

d. Astigmatism results from the surface of the lens or cornea being uneven, which causes light to be focused onthe retina in lines rather than as a single point.

e. Cataracts are clouding of the lens due to damage from things like ultraviolet rays, cigarette smoke, and othertoxic things. The lens eventually becomes so clouded that a person with cataracts is functionally blind eventhough the photoreceptors are fine. To correct cataracts the lens can be removed and replaced with an artificiallens. Obviously the artificial lens can’t accommodate for close vision so it has to be preset for one or the otherand supplemented with contacts or glasses. Forget what the book says.

Sense of HearingA. Anatomy of the Ear

1. External Ear

Nope, no ear­specific receptors here, although I’ll bet when your mother grabs you up by the pinna (external earflap, or “Mom’s handle”) when you are misbehaving in the grocery store you have some pain receptors that starttalking to you. The parts are:

a. Pinna

b. External auditory canal, containing hairs, sweat glands, and ceruminous glands, which secrete ear wax.

2. Middle Ear

a. Begins at tympanic membrane (eardrum)

b. Contains three bones that link the tympanic membrane and the inner ear, called ossicles. These bonesconduct sound vibrations from the tympanic membrane to the fluid of the inner ear.

i. Malleus (hammer)– in contact with the tympanic membrane

ii. Incus (anvil) ­ lies between the malleus and stapes

iii. Stapes (stirrup) ­ lies between the incus and the bony wall that separates the middle ear and the inner ear. The stapes actually comes in contact with a membrane­covered opening in the wall called the ovalwindow.

c. The posterior wall of the middle ear opens into the mastoid sinuses.

d. The auditory tubes lead from the middle ear to the nasopharynx, which allows air pressure on either side of thetympanic membrane to be equalized when atmospheric pressure changes (like when you ascend to 35,000 feetin an airplane). Yawning or chewing gum helps open the auditory tubes and equalize the air pressure. Don’tyou wish babies that fly on planes could chew gum? Or take the train?

e. Otitis media is inflammation of the middle ear commonly due to infection. Fluid can build up and exert pressureon the tympanic membrane. If you get enough exudate built up (yeah, OK, pus) it can block the auditory tubeand eventually the pressure can blow the eardrum out. This is why in children with frequent ear infections

tubes are sometimes placed in the eardrum (myringotomy). This allows the pressure to equalize, and usuallythe tubes fall out by themselves as the eardrum heals from the incision to place them in.

3. Inner Ear

a. Where the action is; mechanoreceptors for both hearing and balance. Located in the temporal bone.

b. Vestibule – chamber that lies medial to the middle ear. Outer wall is the oval window. Filled with fluid(perilymph) and contains to membranous sacs, the saccule and the utricle. The saccule and the utricle houseequilibrium receptors called maculae that respond to gravity and changes of head position

i. Saccule – filled with fluid (endolymph) and continuous with ducts leading to the cochlea.ii. Utricle – filled with fluid (endolymph) and continuous with ducts leading into the semicircular canals.

c. Semicircular Canals – three channels that run through the temporal bone, posterior to the vestibule. Eachchannel is lined with a membrane and filled with endolymph. The canals are oriented at right angles to eachother in the three planes of space. Each has an enlarged area at the end that is continuous with the utriclecalled the ampulla. Each ampulla houses an equilibrium receptor called a crista ampullaris, which detectsrotational or angular movements of the head.

d. Coclea – a spiral, bony chamber anterior to the vestibule, that resembles a snail. Lined with membrane, filledwith endolymph, contains the organ of Corti, which senses sound.

4. Sound Pathway

a. Sound waves travel down the auditory canal to the tympanic membrane, where they make it vibrate.

b. Ossicles in turn vibrate and transmit the vibrations to the oval window. The vibrations are amplified about 20times by the ossicles.

c. The oval window vibrates and sends pressure waves through the endolymph in the cochlea.

i. The cochlea consists of three tubes, the vestibular canal, which originates at the oval window, thetympanic canal, which is continuous with the vestibular canal and ends at the round window, and thecochlear canal, which is enclosed and lies between the vestibular canal and the tympanic canal.

ii. The cochlear canal is separated from the vestibular canal by the vestibular membrane, and from thetympanic canal by the basilar membrane.

iii. Hair cells are supported on the basilar membrane and their cilia are embedded in the tectorial membrane. These hair cells compose the organ of Corti.

d. When sound waves pass from the oval window, through the vestibular canal, and on to the tympanic canal,they cause the basilar membrane to vibrate.

i. This bends the cilia in the hair cells and causes nerve impulses to be sent through the cochlear branch ofthe vestibulochchlear nerve, through the brain stem, and on to the temporal lobe where they areinterpreted as sound.

e. Sound waves reach the round window, where the membrane can bulge to absorb the energy and preventbackwash of the endolymph.

Sense of Equilibrium

A. Rotational Equilibrium Pathway

1. Used when the body is moving (dynamic equilibrium), detects angular or rotational equilbrium.

2. Receptors (the cristae ampularis) are found in the ampulla of the semicircular canals and contain hair cells.

3. Hair cells in the ampulla have cilia embedded in a gel­like mass, the cupula. Changes in acceleration causechanges in endolymph flow, which pushes on the gel, bends the cilia, and transduces a nerve impulse.

B. Gravitional Equilibrium Pathway

1. Detects linear acceleration, movement in a straight line.

2. Hair cells in the maculae have cilia that project into an otolithic membrane, which contains calcium carbonatecrystals called otoliths. When the head starts or stops moving in a linear direction the otolithic membrane slidesaround, bends the cilia of the hair cells, and transduces a nerve impulse.