Bio 1611, Senses Study Guide Chapter 15, Mareib Human A and P, 8th Ed.

SKIN SENSES - See p. 161.

Touch

There are two types of touch - crude and discriminative. Meissner's corpuscles
are located in the dermis; they detect the precise location of a stimulus. They are most
concentrated in the anterior skin of the index finger and much more widely spaced on
the back. Merkel's discs also detect discriminative touch. Crude touch is sensed by
Ruffini end organ mechanoreceptors. Important touch receptors are located at the
base of hair follicles in hair root plexes. Don't rub a cat's fur the wrong way!

Pressure (deep touch over a large area) is detected by Pacinian corpuscles. Vibration
is sensed by all the touch receptors. Impulses are relayed to the ventral posterior nuclei
in the thalamus. Free nerve endings in the epidermis detect light touch and pain.
 

 Sections of Skin

Pain receptors are classified as acute (fast, A) or chronic (slow, C). Acute pain
is essentially a quick, short-term, surface phenomenon. Chronic pain starts slowly and
builds; it is felt at the surface or in deeper tissues. The pain gates in the substantia gelatinosa
of the spinal cord are influenced by endorphin-secreting neurons (see presynaptic inhibition
in the Nervous Tissue Chapter). If A receptors are stimulated by rubbing a 'banged' leg,
the pain gates are closed for the C fibers. That is why rubbing the hurt helps. See p. 490-491.

Referred pain occurs because the brain does not get accurate information about
the precise source of pain in many organs in the thoracic and abdominal cavities.
See p. 535. Therefore bladder pain can be felt in the superior-medial-posterior
portions of the thigh. Heart pain can be felt along the tract of the ulnar nerve on the
inside of the arm, the left jaw, between the shoulders, and the left thorax. Spinal nerves
serving all those regions run together back to the brain. Can you now explain the phantom
\pain emanating from a non-existent organ?

Temperature - thermoreceptors are free-nerve endings which sense hot and cold.

Proprioception or "muscle sense" includes the muscle spindles and tendon organs
discussed in the section on reflexes. This the motor cortex knows where our limbs
are at any time (even if we cant see them). See p. 486.
 

SMELL AND TASTE

Functionally, these two senses are closely related and usually combine in order to
make food "tasty." An apple is hard to distinguish in taste from an onion if you can't
smell the onion.  Both taste buds and olfactory sensory cells have ciliary processes
which bear protein receptors embedded in their membranes.  Substances that are
detected by smell or taste, lock and key to specific receptors.  You will be given
PTC taste papers to show that those who have a dominant gene will have these receptors.
Those without that gene cannot taste the PTC. Many animals have a sense of smell
much better than us because they have much more olfactory sensory cells, more varied
receptors and more nasal cavity membranes, relatively speaking. Generally, the sense
of smell is more sensitive than taste. See p. 579.

Explain why can some people taste the PTC chemical and others do not?

The four basic tastes are salt, sweet, sour and bitter.  The anterior 2/3 area of the tongue
specializes in receptors mostly for sweet, sour and salty, with acute bitter tastes mainly l
ocated at the back of the tongue.  See p. 471-473.  The cranial Facial Nerve VII innervates
the sensory functions of the front 2/3rds of the tongue, while the rear 1/3 is innervated
by the Glossopharyngeal Nerve IX. Taste sensations are relayed to the ventral posterior
nuclei  of the thalamus then on to the primary gustatory area of the inferior post-central gyri
of the parietal lobes.

VISION

PATH OF VISION - See p. 547.

Human eyes are best described by a path of vision approach.  An image consisting
of light rays first passes through a transparent cornea with a moist conjunctival covering.
Cloudy corneal or lens cataracts are apparently caused by ultra-violet radiation damage.

The light rays then pass through an aqueous humor.

If the canals of Schlemm (scleral venous sinus) are blocked,
glaucoma will result. The increasing formation of lymph-like fluid will then press against
the lens, vitreous humor, and finally the retina, which will degenerate, accompanied by
a progressive loss of vision. See p. 555.

Describe the physician's glaucoma test.

The lens can change shape through the actions of ciliary muscles and suspensory
ligaments.  In order to do accommodation for near vision, the lens is rounded by
contractions of the ciliary muscles.  For far vision the lens is thinned as the ciliary
muscles relax. As age increases, the lens hardens (presbyopia). Therefore,
bi-and trifocals may be necessary. See p. 558-560.

The radial smooth muscle of the iris regulate the amount of light coming into the interior
of the eye.  The nerves serving these muscle cells are multi-branched with axon knobs
(multi-unit smooth muscle), so a very precise opening can be maintained by innervated
individual smooth muscle cells.

If the distance from lens to retina is shorter than the focal length, far-sightedness or
hypermetropia results.  The eye ball is too compressed!  Conversely,
if the lens to retina distance is too long, the focal point is in front of the retina and
near-sightedness or myopia results.

The gelatinous vitreous humor is present in the interior of the eye.  The retina of the eye
is surrounded by a dark black light absorptive pigmented layer and a brown-red layer
called the choroid.  This dark layer absorbs scattered light rays so they are not sensed
by the retina.  This improves our day color vision.  Light rays coming perpendicular to
the macula lutea and its central fovea are better for acuity (the ability to distinguish
between two points). The sclera is a tough connective tissue wrapping of the eye outside
of the choroid. In chronic sugar diabetes poor blood flow or stokes may damage the retina.
Senile macular degeneration may occur because blood vessels have abnormally
proliferated.
 

Label the fovea, the macula and the blind spot.

Why are colors dimmed by the bright/hazy sunlight of August?

Why do some animal eyes glow in the dark?  They have a shiny reflective layer that
gathers scattered light rays.  This makes night vision brighter than ours but what
does it do for their day vision?

In the retina, there are three sensory cells which specialize in reception of the
three primary colors of light, red, green (also called yellow), and blue cone cells.
Your TV set, by the way, has three same color cathode ray guns aimed at the inside
of your TV screen.  The cones develop colored, high resolution images in bright light.
Cones contain pigment molecules called photopsins. See p. 561-565.

The rods specialize in black/gray/white images developed from weak light such as
occurs at night.  Rods contain a light sensitive pigment called rhodopsin (visual purple).
 Rhodopsin is made from Vitamin A retinal and a protein called opsin.  Do you recall
seeing a flash of purple when you come out of a very dark room into a very bright one?
 That purple flash is your rhodopsin bleaching out!  See p. 561-565.

Light changes shape of the rhodopsin molecule.  Its shape change is analogous to
stepping on a lever to open one of those flip-lid kitchen waste cans.  The retinal lid
of the rhodopsin flips up when light hits it.  This deactivates cyclic GMPs that normally
keep sodium ion gates open. Thus, the sodium ion channels close so that the dark ion
current from one end of the cell to the other is interrupted.  This results in the rod cell
hyperpolarizing in order to send a message to the bipolar cells where a neuroinhibitor
(glutamate) is inhibited, depolarizations are then sent to ganglion cells and then to the
optic nerve.  Horizontal cells inhibit the signals of cells lateral to the areas illuminated,
thus enhancing contrast. Amacrine cells  detect changes in the intensity of light.
See the Nervous Tissues chapter. Don't neurons usually send out messages by
depolarization?

Why does Vitamin A deficiency result in night blindness?
 

Cone and rod cell impulses are relayed through the retina to the cranial nerve
# II, the Optic Nerve, to the Optic Chiasma, on to the lateral geniculate nuclei
of the thalamus, then to the occipital lobes' primary vision areas, and finally,
forward to the association and general interpretation areas.
 

HEARING

The path of hearing begins with the tympanic membrane.  The ear drum
(tympanum) magnifies the vibrations of compacted air molecules (sound waves)
mechanically.  Mechanical amplification takes place because of the 22/1 ratio
of surface areas of the tympanum and to the ossicles (ear bones).  The ossicles,
in order, are the hammer, anvil and stirrup (malleus, incus, stapes).  The
stirrup pounds against the oval window membrane of the inner ear or cochlea
(the snail).  See p. 574-577.
 

Bats can hear sounds much higher than our range of 20 to 20,000 cycles per second.
In fact, they give sonar screams at 40,000 cps to locate moths to eat!  Whales can hear
around 10 cps.

Inner ear sounds are hydraulically amplified (like car brakes).  The lymph-like liquid
of the vestibular canal is like all liquids, it is not compressible!  Therefore pressure waves
travel equally in all directions.  For a simplified example, in your car, a pressure of
100 lbs/square inch can be transmitted by your brake pedal and master cylinder to the
brake pads of your four wheels which have 100 times more surface area.  Calculate
100 lbs/in2 (x) 100 in2  = 10,000 lbs pressure, plenty enough to stop your 2400 lb car!
See p. 578-579.

The pressure waves then move through the cochlear duct to the organ of corti
where sensory hair cells with ciliary processes are stimulated.  The
hair cells lie on top of a basilar membrane.  High frequency waves cause the
basilar membrane nearest to the oval window to vibrate.  Low frequency waves
travel further and cause the basilar membrane furthest away from the oval window
to vibrate. Meniere's syndrome occurs when fluid pressures rises in the
endolymph of the cochlea. Vertigo, roaring tinnitus, and eventually loss of hearing
will result.

When the basilar membrane vibrates, it causes the hair cell ciliary hair cell processes
to deflect against a tectorial membrane, Resulting hair cell depolarizations are sent to
the cochlear branch of the vestibulocochlear nerve (VIII) then to the medial
geniculate nuclei of the thalamus and then to the temporal lobes for sound development
and association. High frequency sounds can destroy the hair cell processes. Watch
out during those big rock concerts!

Deafness is classified and nerve or sensorineural if the problem lies in the
cochlear branch or the larger vestibulocochlear nerve VIII. Otherwise
if the defect is the ear drum, ossicles or cochlea, it is called conduction deafness.

EQUILIBRIUM

The sense of balance in terms of head up/head down movements is determined by
the utricle and saccule in the vestibule of the inner ear.  Hair cell ciliary
processes extend into a gelatinous mass which includes calcium carbonate
otoliths or ear stones.  As your head drops, the gel mass weighted by the otoliths
deflect the hair cell processes.  Messages are sent both to the cerebellum and higher
brain for balancing muscle movements. See p. 586.

The semicircular canals detect turning movements.  When your head/body turns,
lymph-like fluids turn also, these deflect hair cell processes located at the base of
the canals in the ampullae.  Sometimes these fluids continue to move even when
your turn has stopped.  This convinces you that you are first continuing the turn,
hen turning in the opposite direction if you are a pilot flying in a cloud.  This is why
pilots rely on instruments and not the "seat of their pants" for flying "blind.”
 
 

Study Questions

1. Give the C.N.S. path and events involved in eyesight.
2. Give the path and events involved in hearing.
3. How is the sense of equilibrium developed?
4. Compare the physiology of rods and cones.
5. Explain the cause and symptoms of glaucoma.
6. Explain macular degeneration.

e-mail:john.aliff @ gpc.edu