The nervous system responds
to external signals through nerve cells or nerve fibers (neurons). Surface and
external
Receptors also feed signals into the system from the
environment. Collectively, sensations
from the external environment are collected and sent to the CNS from extero,
vicero and propioreceptors through sensory neurons.
The special spenses (smell,
taste, eye, ear and balance) play a significant role serving as exteroreceptors
or antennas, that collect and transmit external sensations from the environment
to the brain. The following discussions
about the special senses are presented.
1.
Detect
changes in the external environment
2.
Respond
to the changes
3.
Maintain
homeostasis
1.
Projection
(area of origin of sensation)
2.
Intensity:
debt of sensation (light, medium, heavy)
3.
Contrast
(comparison between intensities)
4.
Adaptation
(voluntary suppression of sensation)
5.
Memory
of sensation: Memory of sensation stored even after sensation
·
Receptors
on the skin
·
Sensory
neurons carry the message
·
Sensory
tracts involve the white matter in CNS
·
Sensory
area (cerebral cortex feels and interprets sensation)
Receptors (similar to
antennas) fall into two categories:
1.
Generalized
receptors
·
Widespread
throughout the body
·
Examples:
Receptors for touch, pressure, pain and temperature
2.
Specialized
receptors (special senses)
·
Located
in organs with pathways in the brain
·
Examples:
smell, taste, sight, hearing, balance
·
Also
classified as somatic and visceral senses
·
Somatic
Senses: those receptors that are
located on the body wall. Examples are
cutaneous (skin) receptors and those within muscle, joints, ligaments and
tendons.
·
Visceral
Senses: receptor are located within
visceral organs (e.g GI, lungs, etc).
·
Senses
are also classified according to the location and type of stimuli they receive.
There are three kinds of receptors
1.
Exteroreceptors:
receive stimuli from the
environment; they are located near the surface of the body. Examples:
-
Rod and cones on retina:
photoreceptors
-
Hair cells in the ear:
mechanoreceptors
-
Taste
receptors on the tongue: chemoreceptors
-
Skin
receptors: tactile, mechano, thermo, nociceptors
2.
Visceroreceptors: sensory receptors that produce sensations
from organs or internal tissues.
-
Sensations
include pain, hunger, thirst, fatigue or nausea.
-
Visceroreceptors
located within the circulatory system are sensitive to blood pressure changes
are called barroreceptors
3.
Proprioceptors: sensory nerves that relay information about
the body position, equilibrium and movement.
- They are
located in the ear, joints, muscles and tendons
1.
Cutaneous sensation :
·
Contains
free nerve endings for pain, touch and pressure
·
Protects
the skin and provide info about temperature (heat and cold), pain, touch and
pressure (covered nerved endings);
·
Sensory
areas in parietal lobes
·
Receptors
for touch and pressure are encapsulated (ie they are covered)
2.
Muscle sensation: Provide awareness of our muscles.
a.
Stretch
receptors in muscles; sensory nerves located in the parietal lobes
b.
Cerebellum
coordinate voluntary motion
3.
Taste sensation: Chemoreceptors located in the
taste buds of the tongue;
a.
they detect chemicals present in food and in
saliva
b.
Sweet, sour, bitter and salty are four basic
tastes
c.
Pathways; Facial and Glossopharyngeal nerves
in temporal and parietal lobes
4.
Smell sensation: Chemoreceptors in the upper nasal cavities detect volatile
(vaporized) chemicals
a.
Pathways:
Olfactory nerves to olfactory bulbs connected to olfactory areas in the
temporal lobe
b.
Smell
and taste sensations are synergistic
5.
Hunger and Thirst
a.
Receptors
in the hypothalamus
b.
Detect
changes in nutrient levels and project to the stomach.
c.
Osmoreceptors
detect changes in body water concentration (water-salt ratios). Thirst
projected to the mouth/pharynx.
The
lobes of the brain participate in recognizing and interpretation of
sensations. These areas are :
·
Parietal lobe: Generalized sensations
(cutaneous sensations) muscular sensations are received and interpreted in this
area. It also functions in speech comprehension and verbal articulation of
thought and emotions. It also interpretes texture and shape of objects.
·
Temporal lobe: Contains auditory centers
that receive sensory neurons from the cochlea of the ear. It also interpretes
some sensory experiences and stores information of both auditory and visual
sensations.
·
Occipital lobe: Concerned mainly with
interpreting visual sensations. It
integrates eye movements by directing and focusing the eye. It is also
responsible for visual association, that is, relating visual images with past
experiences.
·
Frontal lobe: The frontal lobe initiates
voluntary motor sensations for the movement of skeletal muscles. It analyzes
sensory experiences and provide information pertaining to the individual. In
addition it is also involved in sensation related to emotions, reasoning,
memory, judgment, planning and speaking.
Smell Sensation (Olfaction)
Olfaction involves chemical receptors which detect
and transmit sensations of vaporizing chemicals emitted into the nasal
passages.
·
Olfaction
or the sense of smell involves chemicals. The receptors for smell are the
olfactory hair cells located on the roof of each nasal cavity in a path of
cells called the olfactory epithelium or membrane.
·
Odors
or scents chemical vapors. Chemoreceptor cells (neurons) perceive the
chemicals as they vaporize.
·
Air-bone
chemical molecules bind to the Olfactory hairs or cilia which extend from
dendrites of these neurons on the watery membrane surface.
·
Impulses
are sent to the olfactory bulb (a structure whose neurons synapse with
the dendrites of the receptor cells.
·
Axons
of the neurons of the olfactory bulb travel to the brain by way of the
olfactory nerve (Olfactory nerve #1) to olfactory areas of the temporal
lobes.
·
Humans
can distinguish tens of thousands of odors however, the receptors for smell are
more sensitive in animals than in humans
·
Receptors
for smell are sensitive, act quickly within a short period (about a minute) and
adapt very quickly.
·
Smell
influences our sense of taste and vice versa.
Taste buds are the receptors for taste and respond
to chemicals dissolved in food.
·
Taste
sensation work with smell sensation or they enhance each other.
·
The
receptors for taste are located in specialized organs called taste buds
which are numerous on the surface of the epithelium covering the tongue in
humans and mamals. The taste buds are also located on the papillae on
the upper surface of the tongue and also on the roof of the oral
cavity, the pharynx and larynx.
·
There
are three kinds of papillae: vallate (back of tongue), fungiform
(middle) and filiform (front or apex of tongue).
- Vallate: largest
but least abundant, and “V-shaped”.
-
Fungiform:
knob-like appearance; present on tip and
sides of tongue
-
Filiform:
short, threadlike; most numerous
·
Vallate
and fungiform are involved in the perception of all four primary taste: sweet,
sour, bitter and salty. Filiform the most abundant is not involved in
perception of taste.
·
Sweet
taste is perceived on the tip of the tongue; salty taste occurs
especially on the sides and most of the tongue; sour taste occurs on the
sides; and bitter taste on the back.
·
Sourness is related to concentration of hydrogen ions in food (acids). Organic
molecules taste sweet.
·
Taste
buds (contain receptor cells) are chemoreceptors stimulated by chemicals
present in foods we eat. The chemicals dissolve in the saliva and enter the taste
pores of the taste buds.
·
The
taste pores contain taste hairs, part of the receptors of the
taste buds. Chemicals dissolved in water bind to the receptor hairs and
stimulate the receptor cells. The cells in turn stimulate the dendrites of the
sensory nerve (#7 and #9). Impulses are then transmitted to the taste
centers (parietal-temporal lobes) in the cerebral cortex.
·
Taste
buds respond to all four primary flavors but are generally preferentially
responsive to one
The eye transmits visual sensations of light (photosensations), and color sensations of all images. Transmission of these sensations occur through photoreceptors in the eye.
·
Eyelids
and eyelashes; keep dust out
·
Lacrimal
glands produce tears, cleans the eyes
·
Eyeballs:
protected by the bony socket
·
Six
extrinsic muscles move the eyeballs; Nerves #s 3, 4 and 6 cranial nerves
stimulate the eyes. All six muscles originate from the back of the of the eye
orbit and insert on the surface of the eye.
·
Eye
coverings: sclera, choroids, retina, ciliary body
1.
Sclera:
outermost covering (fibrous connective tissue
2.
Choroid:
middle layer—dark pigment absorbs light
3. Ciliary body/ligaments:
suspend and change shape of lense for light refraction
4.
Iris—control
diameter (aperture) of the eye
5.
Retina:
innermost layer—contains rods and cones
a.
Cones: detect color, abundant in the Fovea
Centralis
b.
Rods: detect light ; abundant around the
edges of the retina
c.
Optic
nerves conduct images to the eye and brain
6.
Posterior
cavity: contains vitreous humor (semi solid fluid that keeps the eye in
position
7.
Anterior
cavity: contains aqueous humor produced from capillaries; maintains the lens
and cornea
Physiology
·
Vision
occurs by refraction (bending and focusing of light rays) thru cornea, aqueous
humor, lens and vitreous humor.
·
Ciliary
muscles adjust lens for long or short vision.
In long vision, ciliary muscle relaxes resulting in thining of
the lense. In short vision, the muscle contracts, and the lens thickens.
·
Vision
occurs when light stimulates the rods and cones causing chemical reactions in
the retina.
·
Chemical
breakdown in rods, and light absorption (red, blue and green) in cones produces
impulses carried by optic nerves.
·
The
optic nerves intersect before entering the brain. The point of intersect is
called optic chiasma. Within the
optic chiama, fibers from one eye crosses over to the opposite side. The
crossing over of the fibers
·
Astigmatism: unequal focusing of light
rays on the retina resulting from unequal curvature of the cornea or lens. It
causes part of the image to appear blurred.
·
Blindness: partial or lack of vision. It
may be caused by a numbers of disorders such as cataract, glaucoma and
detachment or deterioration of the retina. It may also result from damage to
the optic nerve #II.
·
Cataract: cloudiness or apacity of the
lens which impairs or prevents vison. It is common in older people and a
leading cause of blindness. May be repaired by surgical removal and replacement
of the lens by plastic lens to restore good vision
·
Color blindness: inability to perceive certain
colors or all colors (in rear cases).
The most common type of the disease is Red-green color blindness
characterized by difficulty distinguishing red and green colors due to the
absence of red or green cones. Color blindness is sex-linked hereditary trait
and occurs more often in males than in females.
·
Conjuctivitis: an inflammation of the
conjuctiva. It may be caused by allergic reaction, physical or chemical causes
or infections. Inflammation caused by viral or bacterial infection is called pinkeye
(highly contagious).
·
Farsightedness (hyperopia): blurred vision that result from light rays that
improperly focused behind the retina. It is caused by the eye being shorter
than normal.
·
Glaucoma: A condition in which more aqueous humor is produced from the eye than
is removed. The condition produced a high intraocular pressure that compress
blood vessels resulting in damaging or dying of receptor cells. It is also an
inherited disease.
·
Nearsightedness (Myopia): is blurred vision caused by light rays being incorrectly
focused in front of the retina. It is commonly caused by the eye being longer
than normal.
·
Presbyopia: diminished ability of the lens to accommodate for near vision due to a
decrease in its elasticity. It is caused by natural aging and prevent around
age 60 and over.
·
Retinoblastoma: cancer of immature retinal
cells and constitute about 2% of cancers in children.
·
Strabismus: disorder of the extrinsic eye muscle in which the eyes are not
directed toword the same object simultaneously. Treatment may include eye excise , corrective lens or surgery.
The
ear picks up sound vibrations that travel in air or water and transmit the
signals through the vestibulocochlear (Acoustic) nerve # 8.
The ear consist of:
1.
Consist
of outer auricle, middle ear and inner ear.
The outer ear (auricle)
outer ear is a canal that leads to the middle ear. It directs sound waves (vibrations) into the ear.
2. The middle ear contains the eardrum
connected to the auditory bones (incus, malleus and stapes) and Eustachian
tube (auditory tube);
3. Inner ear consist of bony-space lined with tissue
called membranous labyrinth. The space contains a fluid called perilymph.
The structures within the inner ear (choclear, utricles, saccule and semicircular
canals) are filled with another fluid called endolymph.
4.
The
Cochlear contains the organ of Corti (receptors or hair cells
containing cranial nerve #8.
5. Uticle and saccule are sacs that contain the
hair cells embedded in otoliths (calcium carbonate crystals).
Gravity causes these hair cells to bend as the position of head changes. The bending of the
receptors carry impulses to the cerebellum and temporal lobes of the brain that handles balance
6. Semi circular canals (3): ampula contains hair cells that
bend during motion and sends impulses to the brain via cranial nerves #8.
Impulses as interpreted changing speeds, stopping or starting motions.