C. SPECIAL SENSES
1.Sense of smell
2.Receptors for smell are the dendrites of the olfactory nerves (Cr. N I) located in the superior nasal cavity. They pass through those olfactory formina of the ethmoid bone and synapse at the olfactory bulb. The pathway continues on as the olfactory tract which is interpreted in the temporal lobe of the cerebral cortex.
3.Sense of taste. Receptors are at the base of pores on tongue.
4.Taste buds are the receptors and are at the base of the taste pores (that is why substances must be liquified for taste). The pathway travels along cranial nerves VII (anterior tongue), IX (middle tongue) and X (back of tongue & throat) to be interpretted in the pariteal lobe of the cerebral cortex.
5.Fibrous tunic made primarily of connective tissue is the outer layer and includes sclera & cornea. Vascular tunic = Uvea contains vascular & pigmented structures and is the middle layer. It includes the ciliary body, iris, pupil and choroid. The nervous tunic is the inner layer and includes the retina, rods and cones and optic nerve.
6. The iris is pigmented smooth muscle and regulates the light coming through the pupil. A large pupil lets in a lot of light.
7. The retina is the inner layer of the wall of the eyeball and is the location of the receptors for vision, the rods and cones.
8. The optic disc is the location where the optic nerve exits the eyeball. It is the blind spot because there are no rods or cones there.
9. The lens is just posterior to the iris. Its function is focusing through changing its shape. The aqueous humor is the fluid anterior to the lens in the anterior cavity and the vitreous humor is the fluid in the posterior cavity, posterior to the lens. The function of the fluids is to maintain shape of the eyeball and nourishment.
10. The palpebral fissure is the space between upper and lower eyelids. The medial & lateral corners of your eyes are the commissures, the caruncle is the red lump that is a gland at the medial commissure, the tarsal plate is in the eyelid making it firm, the conjunctiva is the mucous membrane lining the eyelids and then reflecting onto the eyeball. The meibomian gland are eyelid gland making an oily substance so that your eyelids don’t stick, palpebrae are eyelids, sebaceous ciliary gland are the oil gland for your eyelashes (cilia).
11. The lacrimal gland makes the tears and is located superior/lateral to eyeball. The tears coat the eyes and drain through the lacrimal puncta on the medial edge of the eyelids, passing through the lacrimal canal and finally through the nasolacrimal duct into the nose. Tears function to clean eye, moisturize it, keep microorganisms at a minimum.
12. Rods & cones on the retina. Rods tend to be more peripheral and cones more centrally located.
13. The bending of light rays so that they all converge on one spot called the focal point. If they all converge on one point and that point is on your retina, you have clear vision.
14. The light rays are refracted by the cornea and again by the lens. Hanging from the ciliary body are suspensory ligaments attached to the retina so that the lens can be changed in shape. A straighter lens does not bend the light rays very much, a more curved lens bends the light rays a lot.
15. Emmetropia is perfect 20/20 vision., accomodation is the ability to change the shape of your lens to change from far vision where the lens is relatively straight to close vision where the lens is very curved. You loose this ability with age. The near point of vision is the closest point that you can still focus. It is very close to the eyes in the young and farther away in older people.
16. Your frontal lobe controls voluntary fixation., wanting to look at something. Then, as the image becomes out of the field of view, your occipital lobe controls recentering your eyes during involuntary fixation so that the object is again viewable.
17. Each eye sees a slightly different field of view. This difference allows for judging distance/3D/depth.
18. If your eyes are not fixating in a coordinated fashion, such as after being drunk, the corresponding image does not land on the eyes’ fovea centralis. You have double vision, or diplopia. Strabismus is when your eyes point different directions & with time you fix images with one eye and the other becomes functionally blind. Rhodopsin is the photopigment in rods requiring vitamin A for its synthesis.
19. As with any receptor, it turns light into depolarization response. Rods respond more to grays/blacks/whites and cones to bright colors.
20. You have many more rods. They are peripheral, very sensitive to light – meaning they could work even if there is just a slight glimmer of moonlight, show convergence in their pathway to the optic nerve – meaning that the image is not too sharp since info. is pooling, needs vitamin A for their synthesis and respond to black & white. Cones are less numerous and are very dense in fovea centralis. They are centrally located. There are red, green and blue responding cones but we see more colors due to the blending of these three cones. They are not very sensitive to light and work well in very bright light. They do not converge, so that each cone has its own “line” to the brain. This results in very precise and clear images. That is why your fovea centralis is your spot with the most accurate vision because it is filled with numerous cones. Cones allow you to see colors. They do not work well in the dark.
21. Rods and cones synapse with bipolar neurons which synapse with ganglion cells. The ganglion cells actually comprise the optic nerve. Horizontal and amacrine cells reach between pathways and can inhibit weakly stimulated pathways (lateral inhibition).
22. From the optic nerve, passing through the optic foramen, half of the pathway crosses at the optic chiasma. The pathway continues on as the optic tract, sends messages to the lateral geniculate body in the thalamus, (as well as the superior colliculi for visual reflex responses) then continues to the occipital lobe of the cerebral cortex for interpretation of the image.
23. Visual acuity is your accuracy to distinguish detail and is measured with the Snellen eye chart. Remember when interpreting numbers on the chart, that the first number is you. If your vision is 20/60, you must stand 20 feet from the chart to see clearly what the “normal” person can stand 60 feet from the chart. Visual resolving power is similar to acuity in that it is the ability to tell that 2 closely spaced points are indeed 2. It would be best if the image lands on fovea centralis where your receptors are the densest.
24. The quickest reponse is pupillary constriction in bright light and then pupillary dilation in dark areas. Then you can make/break down rhodopsin, but this does take a few minutes. If you make more rhodopsin you are more sensitive to the light and can see better in the dark. In bright light, you break down rhodposin.
25. Direct is ipsilateral. If you shine a light in your right eye, the right pupil constricts. However, the left also constricts and this contralateral response is the indirect reflex.
26. Outer ear contains the pinna (elastic cartilage), the ear canal = external auditory meatus, and the eardrum = tympanic membrane.
27. Ear canal, as these are earwax glands
28. The auditory tube connects the middle ear with the throat. This allows pressure release in the middle ear, so that you don’t explode your eardrum with too much pressure.
29. Middle ear contains the auditory tube described above, and three little ossicles that vibrate & as they do so amplify the sound waves. Outer to inner, the ossicles are the malleus, incus and stapes. The malleus touches the tympanic membrane and the stapes touches the oval window. The round window is also a connection between the middle ear and the inner ear. It can bulge in either direction to help with pressure stabilization in the inner ear.
30. The inner ear has the pattern of having a osseous labyrinth lined by a membranous labyrinth. The osseous one contains perilymph fluid and the membranous contains endolymph for fluid. The vestibule is bony and lined by the membranous utricle and saccule that function in equilibrium, discussed below. The 3 semicircular canals/ear are placed at 90 degrees from each other and are bony. They are lined with the semi circular ducts that have a widened portion called the ampulla. They function in equilibrium too and are described below. The cochlea is the last bony portion that is shaped like a snail shell. It is lined with the cochlear duct which houses the receptors for hearing. As you recall from skeletal system, all of these parts are housed within the temporal bone.
31. Described above.
32. Sound waves are collected from the pinna, travel down the ear canal and vibrate the tympanic membrane. This in turn, vibrates the ossicles which magnify the sound wave. The stapes pushes against the oval window making the perilymph in the scala vestibuli move, which in turn makes the perilymph in the lower channel, the scala tympani, move. This causes the endolymph in the cochlear duct to move at the same frequency, thus stimulating the Organ of Corti that sits on the floor of the cochlear duct. This is the location of tiny hairs that will be stimulated according to the frequency of the sound wave. The hairs nearest the apex of the Organ are long and stimulated by low frequency (low tone) sound waves and those nearest the base of the Organ are short tiny hairs that respond to higher frequence pulses (high tones).
33. A reflex to protect from loss of hearing stops the wild vibrating of the ossicles.
34. Location is based on the disparity between the two ears. If the sound were on the right, the sound waves reach the right ear first and is louder in the right ear. The loudness is the amplitude of the sound wave – the strength of the sound wave. The pitch is based on the frequency of the sound wave with high frequency being high pitches.
35. The otoliths in the utricle and saccule move as you accelearate/decelerate thus bending the hair cells leading to a nerve impulse.
36. Again, the otoliths in the utricle and saccule move as your head moves.
37. As you spin and accelerate/decelerate, the hair cells (=crista) in the ampulla of the semicircular ducts are stimulated leading to a nerve impulse.
38. This is a characteristic eyeball movement to try to fix on an object while you spin. If you are spinning to the right, your eyeballs move slowly to the left and then quickly catch up to you by flicking to the right. It is a normal response.
39. The semicirular canals are all located 90 degrees from each other providing information about your equilibrium. This information, coupled with the vestibule and proprioceptors all over the body, as well as visual information goes to the cerebellum your balance headquarters.