1. Anatomy and Physiology
Sense Organs
Anatomy and Physiology

2. Senses
Sense organs fall into two major categories; sense organs and special sense organs
Sense organs refer to receptors that function to produce general senses such as touch, temperature, and pain to initiate reflexes
Special sense organs function to produce vision, hearing, balance, taste, and smell

3. Sense Organs Sense organs are called sensory receptors
The general function of receptors are to respond to stimuli by converting them to nerve impulses
Receptor potential occurs when an adequate stimulus acts on a receptor; when it reaches a threshold, it results in an action potential (impulse) in the sensory neuron

4. Sense Organs
Receptors can adapt to the stimuli, meaning that the magnitude of the receptor potential decreases over a period in response to a continuous stimulus
Receptors are classified according to their location in the body, the specialized stimulus that causes them to respond, and their structure

5. Receptors Classified By Location
Exteroceptors are located near the body surface and respond to external stimuli; these are sometimes referred to as cutaneous receptors
Visceroceptors or interoceptors are located internally; they function in response to internal organ stimuli
Proprioceptors are specialized and are found in skeletal muscle, joints, and tendons. They provide information about body movement, orientation, and muscle stretch
Tonic receptors allow us to locate parts of our body without having to look at it
Phasic receptors allow us to feel change in body positions

6. Receptors Classified By Stimulus Detected
Mechanoreceptors respond to mechanical stimuli that change position of the receptor itself
Pressure applied to skin, stretching muscles, etc
Chemoreceptors are activated by the amount or the changing concentration of certain chemicals
Taste, smell, pH, blood glucose levels,etc

7. Receptors Classified By Stimulus Detected
Thermoreceptors are activated by changes in temperature
Nociceptors are activated by intense stimuli that results in tissue damage; pain is produced
May be caused by toxic chemicals, light, sound, pressure, heat
Photoreceptors are only found in the eyes; they respond to light

8. Receptors Classified By Structure
Free nerve endings are the simplest, most common and most widely distributed sensory receptors; these include exteroceptors, visceroceptors, and nocioceptors
Brain tissue lacks free nerve endings and is incapable of sensing painful stimuli
Slightly modified free nerve endings are responsible for itching, tickling, touch, movement, and mechanical stretching

9. Receptors Classified By Structure
Stimulated free nerve endings almost always result in the feeling of pain and is a first indication of injury or disease
Acute pain fibers mediate a sharp, intense and localized pain sensation
Chronic pain fibers mediate a less intense, but more persistent pain (dull/ aching)

10. Receptors Classified By Structure
Root hair plexuses are free nerve endings associated with the hair follicles and detect hair movement
Merkel discs are responsible for sensing light touch
There are six types of encapsulated nerve endings; they all have some type of connective tissue capsule that surrounds the dendritic ends

11. Receptors Classified By Structure
Meissner’s corpuscles are responsible for sensations of light touch and low-frequency vibration
These are concentrated in the hairless skin areas; lips, fingertips, etc
Krause’s end bulbs respond to light touch and low-frequency vibrations; it is also suggested that they are stimulated in temperatures below 65°F

12. Receptors Classified By Structure
Ruffini corpuscles are located deep in the dermis and mediate the sense of crude and persistent touch; these receptors are stimulated by temperatures ranging from 85° to 120° F
Pacinian corpuscles are found deep in the dermis and respond to deep pressure, high-frequency vibrations and stretching

13. Receptors Classified By Strucure
Muscle spindles consists of grouping muscle fibers that are encapsulated and contain sensory nerve fibers; these are anchored to the connective tissue of skeletal muscle
these provide monitoring of the strength of muscle contractions and stretching of muscles
Golgi tendon organs are located between muscle tissue and tendons; they react to excessive muscle contraction and cause the muscle to relax
This protects muscles from tearing and pulling away from the tendinous points of attachment

14. Olfactory
The olfactory neurons are bipolar and have cilia that touch the surface of the epithelium lining the nasal cavity
Olfactory receptor neurons are chemoreceptors
Olfactory receptors are replaced on a regular basis by germinative basal cells in the olfactory epithelium
Receptor potentials are generated when receptor neurons are exposed to gas molecules or chemicals dissolved in the mucus covering the nasal epithelium

15. Olfactory
Olfactory neurons rapidly adapt to continuous stimulation due to inhibition of action potentials by specialized granule cells in the olfactory bulb

16. Olfactory Pathway
Chemicals dissolved in mucus surrounds the olfactory cilia and a threshold is reached
Receptor potential activated
Action potential of neuron generated and passed to the olfactory nerves in the olfactory bulb
Impulses pass through the olfactory tract to the thalamic and olfactory centers of the brain for interpretation, integration and memory storage

17. Gustatory Gustatory receptors respond to taste stimuli
Most taste buds are associated with the elevated projections of the tongue called papillae
Filiform papillae do not contain taste buds but allow you to experience food texture

18. Gustatory
Taste buds are stimulated by chemicals dissolved in saliva; taste buds house chemoreceptors
Each taste bud contains gustatory cells that have cilia; cilia project into a taste pore that is bathed in saliva
All tastes can be detected in all areas of the tongue; It is believed, however, that each type of taste bud responds most effectively to one of the four primary taste sensations (bitter, sweet, sour, and salty)

19. Gustatory Pathway
Taste sensation begins with the creation of the receptor potential in the gustatory cells
The generation of an action potential then transmits sensory input to the brain
Nerve impulses generated in the anterior 2/3 of the tongue travel over the facial nerve
Nerve impulses from the posterior 1/3 of the tongue are conducted by the glossopharyngeal nerve
The vagus nerve also plays a minor role in taste; these sensations would begin in the walls of the pharynx and epliglottis

20. Gustatory Pathway
All three cranial nerves carry the impulses to the medulla oblongata
Relays carry impulses to the thalamus and then into the taste areas of the cerebral cortex in the parietal lobe of the brain

21. Auditory and Balance
The ear has a dual function; it plays a role in hearing, but it also functions as the sense organ for balance and equilibrium
Specialized mechanoreceptors are responsible for both functions; the mechanoreceptors are called hair cells
The ear is divided into three anatomical parts; external ear, middle ear, and the inner ear

22. Auditory and Balance
The external ear has a flap called the auricle or pinna
The tube leading from the auricle to the temporal bone is named the external auditory meatus (ear canal)
Modified sweat glands are located in the auditory canal; they secrete cerumen
When the glands become impacted it results in pain and temporary deafness
At the end of the auditory canal is the tympanic membrane (eardrum) it stretches across the end of the ear canal separating it from the middle ear

23. Auditory and Balance
The middle ear (tympanic cavity) contains three small bones called the auditory ossicles; malleus (hammer), incus (anvil), and stapes (stirrup)
The malleus attaches to the inner surface of the tympanic membrane, the other end attaches to the incus, which then attaches to the stapes; the stapes then connects to the oval window

24. Auditory and Balance The middle ear has 4 openings
The opening from the ear canal that is covered by the tympanic membrane
The oval window which is a opening into the inner ear, this is also where the stapes fits
The round window which is an opening into the inner ear, it is covered by a membrane
The eustachian tube (auditory tube) connects the ear to the back of the throat
The openings are routes for infection and are a concern of physicians

25. Auditory and Balance
The inner ear is called the labyrinth because of its shape; it consists of two main parts the bony labyrinth and the membranous labyrinth
The bony labyrinth consists of three parts; the vestibule, cochlea, and semicircular canals
The membranous labyrinth consists of the utricle, saccule, cochlear duct, and the semicircular canals
The utricle, saccule, and semicircular canals are involved in balance
The cochlea is involved in hearing

26. Auditory and Balance
Endolymph refers to the fluid in the membranous labyrinth
Perilymph refers to the fluid that surrounds the membranous labyrinth and fills the space between it and the bony walls around it
Cochlea means “snail”; it describes the shape of this part; the cochlea consists of cell bodies for sensory neurons
The cochlear duct is the only part of the internal ear concerned with hearing
The roof of the cochlear duct is known as the vestibular membrane or Reissner’s membrane
The floor of the cochlear duct is know as the basilar membrane

27. Auditory and Balance
The hearing sense organ, The Organs of Corti, rests on the basilar membrane along the entire length of the cochlear duct
Dendrites of sensory neurons are associated with the Organs of Corti; the axons of the sensory neurons extend to form the cochlear nerve
Cochlear implants are used to assist deaf persons if the Organs of Corti are damaged

28. Auditory Pathway
Sound is created by vibrations in air, fluid, or solid material
The amplitude of a sound wave determines its perceived loudness
The number of sound waves during a specific period of time (frequency) determines pitch

29. Auditory Pathway
Sound waves enter the external auditory canal and strike the tympanic membrane
Vibrations from the tympanic membrane move the malleus, that then moves the incus, then the stapes
The stapes fits against the oval window, so when it moves it exerts pressure on it ; this causes a ripple effect in the perilymph of the scala vestibuli
The ripple is transmitted through the vestibular membrane to the endolymph inside the cochlear duct and then to the Organ of Corti and the basilar membrane
The ripple then moves through the perilymph (scala tympani side) to the round window (the end point)

30. Auditory Pathway
The movement of the hair cells and thus the stimulation of the organ of corti results in an impulse being conducted from the cochlear nerve to the brainstem
Auditory impulses go through relay stations in the medulla, pons, midbrain, and thalamus before it reaches the auditory area of the temporal lobe of the brain

31. Balance
Sense organs involving balance, or equilibrium, are found in the vestibule and the semicircular canals
The utricle and the saccule function in static equilibrium, a sense that explains the position of the head relative to gravity and the sense of acceleration and deceleration of the body
The sense organs associated with dynamic equilibrium are the semicircular canals; they function to maintain balance

32. Static Equilibrium
The macula is a specialized epithelium containing receptor hair cells and supporting cells that are covered with a gelatinous matrix
Movement of the macula provides information relating to head position or acceleration
Otoliths are composed of calcium carbonate and are located in the maculas; turning the head causes the otoliths to change position and thus stimulates hair cells and neurons

33. Dynamic Equilibrium
The crista ampullaris is located in the ampulla of each semicircular canal. It is a specialized sensory epithelium
The crista is composed of hair cells that are embedded in a gelatinous cap, the cupula
The cupula serves as a float that moves with the flow of the endolymph
As the cupula moves it creates receptor potential, then an action potential. The impulse passes through the vestibular portion of the 8th cranial nerve. It reaches the medulla oblongata and then to other areas of the brain and spinal cord for interpretation, integration, and response

34. Optic The eyeball has three layers the sclera, choroid, and retina
The sclera is made up of tough white fibrous tissue
The anterior portion of the sclera is the cornea, it lies over the iris.
The iris is the colored part of the eye
The cornea is transparent, whereas the rest of the sclera is white and opaque

35. Optic The cornea and the lens lack blood vessels
The Canal of Schlemm is a venous sinus that drains the aqueous humor in the eye
If the aqueous humor is produced faster than it can be drained, glaucoma results
The choroid coat of the eye contains blood vessels and pigment. It is modified into three separate structures; the ciliary body, the suspensory ligament, and the iris

36. Optic
The ciliary body contains small ciliary muscles and ligaments that hold the lens suspended in the correct place
The iris is the colored part of the eye; the hole in the middle of the iris is called the pupil
The iris is attached to the ciliary body
The retina is made up of three layers of neurons; photoreceptor neurons, bipolar neurons, and ganglion neurons

37. Optic The photoreceptor neurons are found in the rods and cones
Cones are more dense in the fovea centralis; and become less dense outward from the fovea centralis
Rods are absent from the fovea centralis and increase density toward the periphery of the retina
Axons of the ganglion neurons extend back to the posterior eyeball into an area called the optic disc; this is where the optic nerve emerges from the eyeball
The optic disc is also called the blind spot. It does not contain rods or cones

38. Optic The eye is divided into two cavities;
The anterior cavity is filled with aqueous humor; a substance that is clear and watery
The posterior portion is larger and it is filled with vitreous humor; a substance with the consistency of a soft gelatin
Both the aqueous and the vitreous humors play a role in maintaining intraoccular pressure to prevent the eyeball from collapsing

39. Optic The aqueous humor is formed from capillaries in the ciliary body
Eyes have extrinsic and intrinsic muscles
The extrinsic eye muscles are skeletal muscles that attach the eye to the bones of the orbit
The intrinsic eye muscles are smooth muscles located within the eye; these are the iris and the ciliary muscles
The iris regulates the size of the pupil
The ciliary muscles control the shape of the lens

40. Optic
The eyebrows, eyelashes, eyelids, and lacrimal apparatus are accessory structures of the eye
Eyebrows and eyelashes protect the eye from entrance of foreign objects and they shade the eyes from direct light
The eyelash contains small glands that secrete lubricating fluid, if they become infected a sty develops
Eyelids are lined with conjuctiva, which are mucous membranes, it continues over the surface of the eyeball.
Inflammation of the conjuctiva is called pinkeye

41. Optic
The lacrimal apparatus consists of structures that secrete tears and drain them from the surface of the eyeball

42. The Process Of Seeing
Four process focus light rays so that they form a clear image
Refraction means bending light rays. Rays are refracted by the cornea, aqueous humor, lens, and vitreous humor
If there are errors in refraction, nearsightedness (myopia), farsightedness (hyperopia), and astigmatism occurs

43. Process of Seeing
Accommodation for near vision requires changes in the curvature of the lens, the constriction of the pupils and the convergence of the two eyes
Curvature of the lens takes place to achieve greater refraction (bending light) for near vision because light rays from nearer objects are divergent and not parallel (as in further vision)

44. Process Of Seeing
Contraction of the ciliary muscle pulls the choroid layer closer to the lens and loosens tension of the suspensory ligaments allowing the lens to bulge
Near vision the ciliary muscle is contracted so the lens will bulge
Far vision the ciliary muscle is relaxed so that the lens is flatter
Presbyopia is a condition where people become farsighted as the lenses lose their elasticity as the person ages

45. Process Of Seeing
The iris constricts the pupil to prevent divergent rays from entering the eye through the periphery of the cornea and lens; if divergent light entered the pupil it could not be bent enough to provide a clear image
Convergence of eyes refers process where images must fall on the same location within each retina in order to see a clear image. This is accomplished by moving the eyeball so that the visual axes are parallel

46. Process Of Seeing
Diplopia occurs when objects fall on noncorresponding points of the two retina; double vision
Strabismus (cross-eye or squint) is a condition that cannot be corrected by neuromuscular effort. The person learns to suppress one of the images

47. Photopigments
Photopigments can be broken down into a glycoprotein called opsin and a vitamin A derivative called retinal
Rods are named rhodopsin; they are highly sensitive to dim light and cause a rapid breakdown of the photopigment into opsin and retinal components
Cones are responsible for the ability to see colors; there are three different types of cones in the eye
Neural input from a varying number of cones creates color
Cones produce vision in bright light

48. Neural Pathway For Vision
Nerve fibers that conduct impulses from rods and cones reach the visual cortex in the occipital lobes of the brain via the optic nerves, the optic chiasma, optic tracts, and optic radiations

49. Sense Disorders
Otosclerosis is an inherited bone disorder that impairs conduction of sound waves by causing structural irregularities in the shape of the stapes
Tinnitus is ringing of the ears and could be a sign of otosclerosis
Excess cerumen can block sound wave conduction

50. Sense Disorders Otitis refers to a temporary ear infection
Presbycusis is the progressive loss of hearing associated with aging and degeneration of nerve tissue in the ear and vestibulocochlear nerve
Meniere’s disease is a chronic inner ear disease with an unknown cause; it results in tinnitus, progressive nerve deafness and vertigo (spinning sensations)

51. Sense Disorders
Myopia is nearsightedness; it can be corrected using concave contact lenses, glasses or refractive surgery
The image focuses in front of the retina
Hyperopia is farsightedness; it can be corrected by convex lenses and refractive eye surgery
The image focuses behind the retina

52. Sense Disorders
Astigmatism refers to irregularity in the curvature of the cornea or lens
Cataracts are cloudy spots in the eye’s lens that develop in the lens as we age, it can interfere with focusing
Conjuctivitis is pink eye and is usually caused by bacteria
Chlamydial conjuctivitis, or trachoma, is caused by the same bacteria that cause the reproductive infection
Conjuctivitis can be caused by other bacteria such as Staphylococcus and Haemophilius

53. Sense Disorders
Retinal detachment occurs when part of the retina falls away from the tissue supporting it
Usually results from aging, tumors, or blows to the head
Diabetic retinopathy is a disorder that causes small hemorrhages in the retinal blood vessels that disrupt the flow of oxygen to the photoreceptors
Glaucoma results from excessive intraoccular pressure
Nyctalopia is night blindness that can be caused by a vitamin A deficiency or degeneration of the retina

54. Sense Disorders
Scotoma refers to the loss of only the center of the visual field often associated with multiple sclerosis
Colored blindness is an inherited condition where one of the photopigments in the cones are missing