General & Special Senses
Lecture 7
Chapter 9
X. The Sensory System
A. Introduction
1. Receptors detect environmental changes and stimuli.
2. Different receptors are sensitive to and receive different
sensory stimuli.
a. Chemoreceptors – medulla in the lower brain stem, aortic
and carotid bodies detect O2, CO2, and H+ levels in the blood.
Receptors of the small intestine detect H+ levels in the
lumen.
b. Pain receptors – located in the skin and internal organs.
c. Thermoreceptors – sense heat (temperature) in the skin and
control body temperature in the hypothalmus of the brain.
d. Mechanoreceptors – sense stretch and pressure within the
skin, muscles, and joints.
e. Photoreceptors – light sensitive neurons in the retina of the
eye.
3. Sensation
a. Sensations are feelings resulting from sensory stimulation.
b. A particular part of the sensory cortex always interprets
impulses reaching it in the same way.
c. The brain projects a sensation back to the region of
stimulation.
4. Sensory adaptations are adjustments made by sensory
receptors to continuous stimulation in which impulses are
triggered at slower and slower rates.
B. Pain receptors
1. Pain receptors are free nerve endings stimulated by tissue
damage.
2. The only receptors in visceral organs that provide sensation are
pain receptors, (visceral pain).
3. The sensations produced from visceral receptors are likely to
feel as if they are coming from some other part or location.
This is called “referred” pain.
4. Somatic pain originates in the skin, skeletal muscles, joints and
tendons.
5. Stimulated nerve fibers from amputated limbs can tend to result
in “phantom pain”.
6. Pain experience has two components: pain stimulus and reaction
to pain stimulus which determines the amount of suffering.
C. Sense of Smell (Olfaction)
1. Olfactory organs
a. The olfactory organs consist of receptors and supporting cells
in the nasal cavity.
b. Olfactory receptors are neurons with cilia that are sensitive to
gaseous/dissolved chemicals.
c. Nerve impulses travel from the olfactory receptors through the
olfactory nerves, olfactory bulbs, and olfactory tracts to
interpreting centers in the olfactory portion of the cerebral
cortex.
You perceive odors (smells) in your brain!
2. Olfactory stimulation
a. Olfactory impulses may result when various gaseous molecules
combine with specific binding sites on the cilia of the receptor
cells.
b. Olfactory receptors adapt rapidly.
D. Sense of Taste
1. Taste receptors
a. Taste buds consist of receptor cells and supporting cells located
in papillae on the tongue.
b. Taste cells have taste hairs that are sensitive to particular
chemicals dissolved in water.
c. Taste hair surfaces seem to have receptor sites to which
chemicals combine.
2. Taste sensation (gustation)
a. The five primary taste sensations are: sweet, sour, salty, bitter,
and umami.
b. Various taste sensations result from the stimulation of one or
more sets of taste receptors.
c. Much of taste also involves the sense of smell.
E. Sense of Hearing
1. The external ear composed of three parts collects sound waves
created by vibrating objects.
a. The visible “ear” that collects and directs sound is the auricle or
pinna.
b. The external auditory meatus directs sound into the skull.
c. The tympanic membrane vibrates to begin the conversion of
sound energy to mechanical energy.
2. Middle ear (air filled space)
a. The three auditory ossicles, (malleus, incus, & stapes) of the
middle ear conduct and multiply the energy of vibration to the
oval window of the inner ear.
b. Eustachian tubes connect the middle ear to the throat and
function to help maintain equal air pressure on both sides of
the eardrums so they are free to vibrate in response to sound.
3. Inner ear
a. The inner ear consists of a complex system of interconnected
tubes and chambers, the osseous (bony) filled with perilymph,
and membranous (membrane) labyrinths filled with endolymph.
b. The organ of Corti contains the hearing receptors (hair cells) that
are stimulated by vibrations in the fluids of the inner ear.
c. Different frequencies of vibrations are thought to stimulate
different receptor hair cells.
4. Steps in hearing:
a. Sound waves collected by auricle (pinna)
b. Waves pass through External Auditory Meatus
c. Tympanic Membrane vibrates
d. Malleus connected on the medial side of Tympanic Membrane
moves.
e. Malleus moves the incus
f. Incus moves the stapes
g. Stapes pushes on the oval window. Movement of stapes is 20
times greater than that of the Tympanic Membrane
h. Perilymph in Scala Vestibuli moves
i. Perilymph wave energy causes the Basilar Membrane in the
cochlear duct to move up and down.
j. Organ of Corti hair cells within the cochlear duct (containing
endolymph) move up and down
k. Hair cells touch the Tectorial Membrane
l. Hairs bend and send nerve impulses down the cochlear branch
of the Vestibulocochlear Nerve to the temporal lobe of the
cerebrum.
m. Different frequencies cause different regions of the Basilar
Membrane to vibrate causing different regions of Organ of
Corti hair cells to be stimulated = perception of different
frequencies.
o. Movement of perilymph in the Scala Vestibuli goes to the end
of the cochlea and moves the perilymph in the Scala Tympani
which pushes on the Round Window so the wave energy is lost,
hearing stops.
F. Sense of Equilibrium
1. Static equilibrium is concerned with maintaining the stability of the
head and body when these parts are motionless.
2. Dynamic equilibrium is concerned with balancing the head and
body when they are moved or rotated suddenly. Sensory organs of
dynamic equilibrium are located in the Semicircular canals.
3. Other parts that help with the maintenance of equilibrium include
the eyes and mechanoreceptors associated with certain joints
called proprioceptors.
G. Sense of Sight
1. Visual accessory organs include the eyelids, lacrimal apparatus
(lacrimal gland produces tears, Nasolacrimal duct drains the eye to
the nasal cavity), and extrinsic muscles to move the eye.
2. Structure of the Eye
a. The wall of the eye has an outer, middle, and inner layer that
functions as follows:
1.) The outer white layer (sclera) is fibrous, protective, and
shapes the eye. Its transparent anterior portion (cornea)
protects and refracts (bends) light entering the eye.
Astigmatism is blurry vision in parts of the visual field caused
by unequal curvatures “waviness” in the cornea.
2.) The middle layer (choroid) is the vascular layer and contains a
brown pigment that helps to absorb light to avoid visual
confusion.
3.) the inner layer (retina) contains the visual receptor cells
b. The lens is a transparent, elastic structure whose shape is
controlled by the action of the ciliary muscles that are part of
the ciliary body. The lens changes shape to refract light.
c. The iris is a muscular diaphragm that controls the amount of light
entering the eye. The pupil is the hole in the middle of the iris.
d. Spaces within the eye are filled with fluids that help to maintain
the shape of the eye.
1.) Anterior cavity (in front of the lens) is filled with aqueous
humor
2.) Posterior cavity (larger and behind the lens) is filled by
vitreous humor.
3. Refraction of light
a. Light waves are refracted primarily by the cornea and lens.
b. The lens must be thickened (accommodation) to focus on
objects closer than 20 feet away by use of the ciliary muscles.
4. Visual receptors
a. The visual receptors are called rods and cones
1.) Rods (more sensitive to low light) provide black & white vision
that is poor in detail
2.) Cones require higher intensity light to provide highly detailed
color vision.
b. Visual pigments
1.) A light-sensitive pigment in rods decomposes in the presence
of light to trigger nerve impulses that our visual cortex
perceives as vision
2.) Color vision seems to be related to the presence of three sets
of cones (blue, green, red) containing different light-sensitive
pigments. Cones are most highly concentrated at the Macula
Lutea of the retina. This is the area of most acute color
vision and light hits it when you look directly at objects.
5. Stereoscopic vision
a. Stereoscopic vision involves the perception of distance and
depth (depth perception)
b. Stereoscopic vision occurs because of the formation of two
slightly different retinal images that the brain superimposes and
interprets as one image in three dimensions
6. Visual nerve pathways
a. Nerve fibers from the retina form the optic nerves. Nerve fibers
exit the back of the eye at the optic disc causing the “blind spot”.
Because nerve fibers fill this area, no cones or rods are present.
b. Some nerve fibers from each eye cross over in the optic chiasm.
This ensures that all areas of the visual field perceived by both
eyes are processed by one side of the brain into a coherent
image.
7. Shape of the eyeball
a. If elongate (too long):
1.) The focus point of the image is in front of the retina, the
image that hits the retina is out of focus.
2.) This is Myopia “short sighted vision” you can’t see far away
so you are described as being “nearsighted”.
b. If eyeball is (too short):
1.) The focus point of the image occurs past (behind) the retina
so the image that hits the retina is out of focus.
2.) This is hyperopia “farsightedness” You can’t see close
objects!
8. Aging
a. As you get older the lens gets harder and will not thicken as or
accommodate as much for close vision. This is usually corrected
by glasses or contact lenses.