Transcript Somatic and Special Senses

Somatic and
Special Senses
Receptors and Sensations

Types of Receptors
 Chemoreceptors – Stimulated by
changes in the chemical concentration
of substances
 Pain Receptors – stimulated by tissue
damage
 Thermoreceptors – stimulated by
changes in temperature
 Mechanoreceptors – stimulated by
changes in pressure or movement
 Photoreceptors – stimulated by light
energy
Sensations (perceptions)
A sensation is a feeling that occurs when the
brain interprets sensory impulses.
Sensations depend on which region of the brain
receives an impulse.
At the same time a sensation forms, the cerebral
cortex causes the feeling to seem to come
from the stimulated receptors: this process is
called projection because the brain projects
the sensation back to its apparent source.
Projection allows a person to pinpoint the region
of stimulation, thus, the eyes seem to see
and the ears seem to hear.
Sensory Adaptations
Sensory adaptation is an adjustment made when
sensory receptors are continuously
stimulated.
As receptors adapt, impulses leave them at
decreasing rates, until finally, these receptors
may stop sending signals.
Impulses can be triggered only if the stimulus
strength changes.
A person entering a room with a strong odor
experiences sensory adaptation.
At first the sense seems intense, but it becomes
less and less noticeable as the smell
(olfactory) receptors adapt.
Somatic Senses
Touch and Pressure
Senses
Sensory nerve fibers
– common in
epithelial tissues
where their free
ends are between
epithelial cells;
associated with the
sensation of touch
and pressure
Touch and Pressure
Senses
Meissner’s corpuscles – small, oval masses of flattened
connective tissue cells within connective tissue
sheaths
Two or more sensory nerve fibers branch into each
corpuscle and end within it as tiny knobs.
Abundant in the hairless portions of the skin, such as the
lips, fingertips, palms, soles, nipples and external
genital organs.
Respond to the motion of objects that barely contact the
skin, interpreting impulses from them as sensation of
light touch.
Touch and Pressure
Senses
Pacinian corpuscles – relatively large
structures composed of connective tissue
fibers and cells
Common in the deeper subcutaneous
tissues and in muscle tendons and joint
ligaments.
Respond to heavy pressure and are
associated with the sensation of deep
pressure.
Touch and Pressure
Senses
Ruffini’s corpuscle – deeply located in the dermis
and are variant’s of Meissner’s corpuscles with a
more flattened capsule.
Mediate sensations of crude and persistent touch.
Slow adapting and permit the fingers to remain
sensitive to deep pressure for long periods. (ex.
Grasping a steering wheel for a long time.
Temperature Senses
Heat receptors – most sensitive to
temperatures above 77oF and become
unresponsive at temperatures above
113oF (temperatures near or above this
stimulate pain receptors, producing a
burning sensation)
Temperature Senses
Cold receptors – most sensitive to
temperatures between 50oF and 68oF (
temperatures below 50oF stimulate pain
recpetors, producing a freezing
sensation)
Temperature Senses
Both heat and cold receptors rapidly adapt.
Within about a minute of continuous
stimulation, the sensation of heat or cold
begins to fade.
Sense of Pain
Sensing pain consists of free nerve endings that are
widely distributed throughout the skin and internal
tissues, except in the nervous tissue of the brain,
which lacks pain receptors.
Functions as protection.
Adapts poorly, if at all.
Once a pain receptor is activated, it may send impulses
into the central nervous system for some time.
It is believed that injuries promote release of certain
chemicals that build up and stimulate pain receptors.
Deficiency of oxygen-rich blood (ischemia) in a tissue or
stimulation of certain mechanoreceptors also trigger
pain sensation
Visceral pain
Pain receptors are the only receptors in
viscera whose stimulation produces
sensations.
Pain may feel as if it is coming from some
part of the body other than the part being
stimulated (referred pain).
Referred pain arises from common nerve
pathways that carry sensory impulses
from skin areas as well as viscera.
Pain nerve fibers
Acute pain fibers
1. relatively thin, myelinated nerve fibers
2. conduct impulses rapidly and are associated
with the sensation of sharp pain, which
typically originates from a restricted area of
the skin and seldom continues after the painproducing stimulus stops
3. usually sensed as coming only from the skin
Pain nerve fibers
Chronic pain fibers
1. thin, unmyelinated nerve fibers
2. conduct impulses more slowly and produce a
dull, aching sensation that may be diffuse
and difficult to pinpoint
3. pain may continue for some time after the
original stimulus ceases
4. felt from the skin as well as in deeper tissues
Pain nerve fibers
An event that stimulates pain receptors
usually triggers impulses on both acute and
chronic fibers – causes a dull sensation.
Pain impulses that originate from the head
reach the brain on sensory fibers of cranial
nerves.
All other pain impulses travel on the sensory
fibers of spinal nerves, and they pass into
the spinal cord by way of the dorsal roots of
these spinal nerves.
Within the spinal cord, neurons process pain
impulses in the gray matter of the dorsal
horn, and the impulses are transmitted to
the brain.
Regulation of Pain
Impulses
Awareness of pain arises when impulses
reach the thalamus. The cerebral cortex
determines pain intensity, locates the
pain source, and mediates emotional
and motor responses to the pain.
Regulation of Pain
Impulses
1. Areas of gray matter in the midbrain, pons,
and medulla regulate movement of pain
impulses from the spinal cord.
2. Impulses from special neurons in these brain
areas descend in the lateral funiculus to
various levels of the spinal cord.
3. These impulses stimulate ends of certain
nerve fibers to release biochemicals that can
block pain signals by inhibiting presynaptic
nerve fibers in the posterior horn of the spinal
cord.
Regulation of Pain
Impulses
Inhibiting substances released in the posterior
horn include neuropeptides called enkephalins
(have morphine-like actions) and the
monamine serotonin (stimulates other neurons
to release enkephalins).
Endorphins are another group of neuropeptides
with pain-supressing, morphine-like actions.
Endorphins are in the pituitary gland and the
hypothalamus.
Both provide natural control.
Special Senses
Sense of Smell
Sense of smell is associated with complex
sensory structures in the upper region
of the nasal cavity.
Olfactory receptors
Olfactory receptors are chemoreceptors,
which means that chemicals dissolved
in liquids stimulate them.
Receptors function closely with taste to aid
in food selection
Olfactory Organs
yellowish brown
masses that cover
the upper parts of the
nasal cavity, the
superior nasal
conchae, and a
portion of the nasal
septum
Olfactory Organs
contains olfactory receptors
that are neurons
surrounded by columnar
epithelial cells; hair-like cilia
cover tiny knobs at the
distal ends of these
neuron’s dendrites; the cilia
project into the nasal cavity
and are the sensitive parts
of the receptors; chemicals
enter the nasal cavity as
gases, but they must
dissolve at least partially in
the watery fluids that
surround the cilia before
receptors can detect them
Olfactory nerve pathways
Receptors send nerve impulses along axons of
the receptor cells to neurons located in
enlargements called olfactory bulbs, which lie
on either side of the crista galli of the ethmoid
bone, where they are analyzed.
As a result, impulses travel along olfactory
tracts to the limbic system.
The major interpreting areas (olfactory cortex) for
these impulses are located within the temporal
lobes and the base of the frontal lobes, anterior
to the hypothalamus
Olfactory nerve pathways
Sense of Taste
Taste Buds – special organs of taste;
occur primarily on the surface of the
tongue and are associated with tiny
elevations called papillae; also found in
smaller numbers in the roof of the
mouth and walls of the pharynx
Sense of Taste
Taste receptors (gustatory cells) – modified
epithelial cells that function as receptors;
structure is somewhat spherical with an
opening the taste pore, on its free surface;
tiny projections called taste hairs protrude
from the outer ends of the taste cells and
extend from the taste pore, and are the
sensitive parts of the receptor cells; taste
cells are wrapped in a network of nerve
fibers; before a chemical can be tasted, it
must dissolve in the watery fluid (provided by
salivary glands) surrounding the taste buds
Taste receptors (gustatory
cells)
Taste sensations:
1. Sweet – front
of tongue
2. Sour – sides
of tongue
3. Salty – all the
way around
the rim of the
tongue
4. Bitter – back
of tongue
Taste sensations:
Flavor results from one of the primary
sensations or from combination of
two or more. Experiencing flavors
involves taste (concentration of
stimulating chemicals), as well as the
sensations of odor, texture (touch),
and temperature. Chemicals in some
foods may stimulate pain receptors.
Taste receptors undergo sensory
adaptation relatively rapidly.
Taste nerve pathways
Sensory impulses from taste receptors in
the tongue travel on fibers of the facial,
glossopharyngeal, and vagus nerves
into the medulla oblongata. From there,
the impulses ascend to the thalamus
and are directed to the gustatory cortex,
which is located in the parietal lobe,
along a deep portion of the lateral
sulcus.
Sense of Hearing
The ear is the organ of
hearing, as well as
functioning in the
sense of
equilibrium.
External Ear
Aurical – outer,
funnel-like structure
that collects sound
waves traveling
through the air
External auditory
meatus –
S-shaped tube; leads
inward through the
temporal bone for
about 2.5 cm
Middle Ear
tympanic cavity –
air filled space in
the temporal
bone
Middle Ear
eardrum (tympanic
membrane) –
semitransparent membrane
covered by a thin layer of
skin on its outer surface and
by mucous membrane on
the inside; has an oval
margin and is cone-shaped,
with the apex of the cone
directed inward; pressure of
sound waves moves it back
and forth in response and
thus produces the vibrations
of the sound wave source
Auditory ossicles
3 bones attached by tiny ligaments to the
wall of the tympanic cavity, and they are
covered by mucous membranes; bridge
the eardrum and the inner ear,
transmitting vibrations between these
parts
Auditory ossicles
malleus (hammer)–
attaches to the
eardrum; when
the eardrum
vibrates, the
malleus vibrates
in unison and
causes the incus
to vibrate
Auditory ossicles
incus (anvil)–
receives
vibrations from
the malleus and
transmits them to
the stapes
Auditory ossicles
stapes ( stirrup)– receives
vibrations from the
incus; held by
ligaments to an
opening in the wall of
the tympanic cavity
called the oval
window, which leads
to the inner ear;
vibrations of the
stapes at the oval
window moves fluid
within the inner ear,
which stimulates the
hearing receptors
Auditory ossicles
The auditory ossicles also help to
increase (amplify) the force of
vibrations as they pass from the
eardrum to the oval window.
Auditory Tube (Eustachian
tube)
connects each middle ear to the throat;
conducts air between the tympanic
cavity and the outside of the body by
way of the throat (nasopharynx) and
mouth; helps maintain equal air
pressure on both sides of the eardrum,
which is necessary for normal hearing;
this function is noticeable during rapid
changes in altitude
Auditory Tube (Eustachian
tube)
Inner Ear
The Inner ear is a complex system of communicating
chambers and tubes called a labyrinth.
Each ear has two such structures: the osseous
labyrinth, a bony canal in the temporal bone, and a
membranous labyrinth, a tube that lies within the
osseous labyrinth and has a similar shape.
Between the osseous and membranous labyrinths is a
fluid called perilymph that cells in the wall of the
bony canal secretes.
The membranous labyrinth contains another fluid called
endolymph.
Inner Ear
Cochlea – functions in hearing
* contains a bony core and a thin, bony shelf that
winds around the core like the thread of a screw
* the shelf divides the osseous labyrinth of the
cochlea into two compartments, the upper
(scala vestibule) leads from the oval window to
the apex of the spiral and the lower (scala
tympani) extends from the apex of the cochlea
to a membrane-covered opening in the wall of
the inner ear called the round window
Cochlea
The portion of the membranous labyrinth within
the cochlea is called the cochlear duct and
lies between the two bony compartments and
ends as a closed sac at the apex of the
cochlea
It is separated from the scala vestibule by a
vestibular membrane (Reissner’s membrane)
and from the scala tympani by a basilar
membrane which contains many thousands of
stiff, elastic fibers, whose lengths progressively
increase from the base of the cochlea to its
apex
Cochlea
organ of Corti
contains hearing receptors; located on the upper
surface of the basilar membrane and
stretches from the apex to the base of the
cochlea; its receptor cells (hair cells) are
organized in rows and have many hair-like
processes that project into the endolymph of
the cochlear duct; above these hair cells is a
tectorial emebrane attached to the bony
shelf of the cochlea, passing over the
receptor cells and contracting the tips of the
hairs
Cochlea
organ of Corti
Steps in the Generation of
Sensory Impulses from
the Ear
1. Sound waves enter external auditory meatus.
2. Waves of changing pressure cause eardrum to
reproduce vibrations coming from sound wave
source.
3. Auditory ossicles amplify and transmit vibrations
to end of stapes.
4. Movement of stapes at oval window transmits
vibrations to perilymph in scala vestibule.
5. Vibrations pass through vestibular membrane
and enter endolymph of cochlear duct.
Steps in the Generation of
Sensory Impulses from the
Ear 6. Different frequencies of vibration in endolymph
stimulate different sets of receptor cells.
7. As a receptor cell depolarizes, its membrane
becomes more permeable to calcium ions.
8. Inward diffusion of calcium ions causes vesicles at
the base of the receptor cell to release
neurotransmitter.
9. Neurotransmitter stimulates ends of nearby
sensory neurons.
10. Sensory impulses are triggered on fibers of
cochlear branch of vestibulocochlear nerve.
11. Auditory cortex of temporal lobe interprets
sensory impulses.
Auditory Nerve Pathways
Nerve fibers associated with hearing enter the
auditory nerve pathways, which pass into
auditory cortices of the temporal lobes, where
they are interpreted.
Some fibers cross over, so that impulses arising
from each ear are interpreted on both sides of
the brain.
Consequently, damage to a temporal lobe on one
side of the brain does not necessarily cause
complete hearing loss in the ear on that side.
Sense of Equilibrium
Inner Ear
semicircular canals
- provide a sense
of equilibrium
Static Equlibrium
 Senses the position of the head, maintaining
stability and posture when the head and body are
still.
 The organs for static equilibrium are located within
the vestibule, a bony chamber between the
semicircular canals and cochlea.
 The membranous labyrinth inside the vestibule
consists of 2 expanded chambers: a utricle and a
saccule. Each of these chambers has a tiny
structure called a macula on its anterior wall.
 Macula contain numerous hair cells, which serve
as sensory receptors.
Static Equlibrium
 The head bending forward, backward, or to the
side stimulates hair cells. The movement tilts the
gelatinous masses of the macula bending the hair
cells, which signal the nerve fibers associated with
them.
 The resulting impulses travel into the CNS on the
vestibular branch of the vestibulocochlear nerve.
 These impulses inform the brain of the head’s
position.
 The brain responds by sensing motor impulses to
skeletal muscles, which contract or relax to
maintain balance.
Dynamic Equilibrium
 The three semicircular canals detect motion of the
head and aid in balancing the head and body during
sudden movement.
 The canals lie at right angles to each other and each
corresponds to a different anatomical plane.
 Suspended in the perilymph of the bony portion of
each canal is a membranous canal that ends in a
swelling called an ampulla.
 The ampulla contains the sensory organs of the
semicircular canals, crista ampullaris.
 Each crista ampullaris contains a number of sensory
hair cells and supporting cells. The hair cells extend
upward into a dome-shaped gelatinous mass called
the capula.
Dynamic Equilibrium
 Rapid turns of the head or body stimulate the hair cells
of the crista ampularis.
 The semicircular canals move with the head or body, but
the fluid inside the membranous canals remains
stationary. This action bends the capula in one or more
of the canals in a direction opposite that of the head or
body movement. The hairs in the capula also bend
stimulating the hair cells to signal their associated nerve
fibers, sensing impulses to the brain.
 Parts of the cerebellum are important in interpreting
impulses from the semicircular canals and helping to
maintain balance.
Sense of Equilibrium
Other sensory structures aid in maintaining
equilibrium.
Certain mechanoreceptors, particularly those
associated with the joints of the neck, inform
the brain about the movements.
The eyes detect changes in posture that result
from body movements.
Such visual information is so important that even
if the organs of equilibrium are damaged, a
person may be able to maintain normal
balance by keeping the eyes open and
moving slowly.
Vertigo
Vertigo is the feeling that you or your
environment is moving or spinning. It differs
from dizziness in that vertigo describes an
illusion of movement.
When you feel as if you yourself are moving, it's
called subjective vertigo, and the perception
that your surroundings are moving is called
objective vertigo.
Unlike nonspecific lightheadedness or
dizziness, vertigo has relatively few causes.
Causes of Vertigo
http://www.emedicinehealth.com/vertigo/page2_em.htm
Sense of Sight
Visual Accessory Organs
Visual accessory organs are housed within
the pear-shaped orbital cavity of the
skull.
This orbit contains fat, blood vessels,
nerves, and connective tissue.
Eyelid - has four layers
1. skin – the thinnest skin of the body; covers the
lid’s outer surface and fuses with its inner lining
near the margin of the lid
2. obicularis oculi muscle – acts as a sphincter
and closes the lids when it contracts
3. levator palpebrae superioris muscle – raises
the upper lids and thus helps open the eye
4. conjunctiva – mucous membrane that lines the
inner surfaces of the eyelids and folds back to
cover the anterior surface of the eyeball (except
for its central portion [cornea])
Lacrimal apparatus
1. lacrimal gland – located in the orbit and secretes
tears continuously; moistens and lubricates the
surface of the eye and the lining of the lids; tears exit
through tiny tubules and flow downward and medially
across the eye; tears contain an enzyme (lysozyme)
that is an antibacterial agent, reducing the risk of eye
infections
2. superior and inferior canaliculi – two small ducts
which collect tears that flow into the lacrimal sac,
which lies deep in a groove of the lacrimal bone, and
then into the nasolacrimal duct, which empties into
the nasal cavity
Extrinsic Muscles
Extrinsic Muscles arise from the bones of
the orbit and insert by broad tendons on
the eye’s outer surface.
6 extrinsic muscles move the eye in
various directions:
Extrinsic Muscles
1. superior rectus – rotates eye upward
and toward midline; oculomotor nerve III
2. inferior rectus – rotates eye downward
and toward midline; oculomotor nerve III
3. medial rectus – rotates eye toward
midline; oculomotor nerve III
Extrinsic Muscles
1. lateral rectus – rotates eye away from
midline; abducens nerve VI
2. superior oblique – rotates eye
downward and away from midline;
trochlear nerve IV
3. inferior oblique – rotates eye upward
and away from midline; oculomotor
nerve III
Structure of the Eye
Outer Tunic (fibrous tunic)
Cornea




comprises the anterior sixth of the outer tunic
and bulges forward
window of the eye and helps focus entering
light rays
composed largely of connective tissue with a
thin layer of epithelium on its surface
transparent because it contains few cells and
no blood vessels and its cells and
collagenous fibers form regular patterns
Outer Tunic (fibrous tunic)
Sclera
a. continuous with the cornea
b. white portion of the eye
c. makes up the posterior 5/6th of the outer tunic
d. opaque due to many large, disorganized,
collagenous, and elastic fibers
f. protects the eye and is an attachment for the
extrinsic muscles
g. in the back of the eye, the optic nerve and
certain blood vessels pierce the sclera
Middle Tunic (vascular tunic)
Choroid coat
 posterior 5/6th of the globe of the eye
 is loosely joined to the sclera and is honey
combed with blood vessels, which nourish
surrounding tissue
 contains many pigment-producing
melanocytes, which produce melanin to absorb
excess light and thus keep the inside of the eye
dark
Middle Tunic (vascular tunic)
Ciliary body
a. thickest part of the middle tunic
b. extends forward from the choroid coat and
forms an internal ring around the front of the
eye
c. contains many radiating folds called ciliary
processes and groups of muscle fibers that
constitute the ciliary muscles
Ciliary body
d. many strong, delicate fibers, called
suspensory ligaments, extend inward
from the ciliary processes and hold the
transparent lens in position
e. the body of the lens lies directly behind
the iris and pupil and is composed of
differentiated epithelial cells called lens
fibers
f. the cytoplasm of the lens fibers is the
transparent substance of the lens
Ciliary body
g. the lens capsule is a clear, membrane-
like structure composed largely of
intercellular material whose elastic
nature keeps it under constant tension
h. the suspensory ligaments attached to
the margin of the capsule are also under
tension and pull outward, flattening the
capsule and the lens inside
i. when the fibers contract, the choroid coat
is pulled forward and the ciliary body
shortens relaxing the suspensory
ligaments
Ciliary body
j. the lens thickens in response and is now
focused for viewing closer objects than before
k. the lens thickens in response and is now
focused for viewing closer objects than before
l. to allow focus on more distant objects, the ciliary
muscles relax, tension on suspensory
ligaments increases, and the lens becomes
thinner and less convex again
m. this ability of the lens to adjust shape to
facilitate focusing is called accommodation
Iris
a. thin diaphragm composed mostly of
connective tissue and smooth muscle fibers
b. from the outside, the iris is the colored
portion of the eye
c. extends forward from the periphery of the
ciliary body and lies between the cornea and
lens dividing the space (anterior cavity)
d. separated into an anterior chamber
(between the cornea and iris) and a posterior
chamber (between the iris and vitreous body,
and containing the lens)
Iris
e. the epithelium on the inner surface secretes a
watery fluid called aqueous humor into the
posterior chamber
f. the aqueous humor circulates from this chamber
through the pupil, a circular opening in the center of
the iris, and into the anterior chamber
g. aqueous humor fills the space between the cornea
and lens to nourish and aid in maintaining shape of
the front of the eye
h. aqueous humor leaves the anterior chamber
through veins and a special drainage canal, the
scleral venous sinus (canal of Schlemm) located in
its walls
Iris
i.
the smooth muscle fibers of the iris are
organized into two groups:
1. circular set – acts as a sphincter; when it
contracts, the pupil gets smaller, and the
amount of light entering decreases
2. radial set – when these muscles
contract, the pupil’s diameter increases, and
the amount of light entering increases
Inner Tunic
Retina
a. contains the visual receptor cells
(photoreceptors)
b. nearly transparent sheet of tissue that
is continuous with the optic nerve in the
back of the eye and extends forward as
the inner lining of the eyeball and ends
just behind the margin of the ciliary
body
Retina
c. has a number of distinct layers:
1. macula lutea – yellowish spot in the central region
which has a depression in its center called the fovea
centralis (region of the retina that produces the
sharpest vision
2. Optic disk – medial to the fovea centralis; where
nerve fibers from the retina leave the eye and join the
optic nerve; a central artery and vein also pass through
the optic disk; these vessels are continous with the
capillary network of the retina, and with vessels in the
underlying choroid coat which supply blood to the inner
tunic; since the optic disk region has no receptor cells, it
is commonly known as the blind spot
Retina
d. the space bounded by the lens, ciliary
body, and retina is the largest
compartment of the eye and is called
the posterior cavity and is filled with a
transparent, jelly-like fluid called
vitreous humor
e. vitreous humor with collagenous fibers
comprise the vitreous body, which
supports the internal parts of the eye
and helps maintain its shape
Light Refraction




Light refraction is the bending of light rays due to
focusing.
When light waves pass at an oblique angle from a
medium of one optical density into a medium of a
different optical density light refraction occurs.
This process occurs at the curved surface
between the air and the cornea and the curved
surface of the lens itself.
A lens with a convex surface (such as in the eye)
causes light waves to converge.
Light Refraction
 The convex surface of the cornea refracts light
waves from outside objects.
 The convex surface of the lens, and to a lesser
extent, the surfaces of the fluids within chambers of
the eye then refract the light again.
 If eye shape is normal, light waves focus sharply on
the retina.
 The image that forms on the retina is upside down
and reversed from left to right.
 The visual cortex interprets the image in its proper
position.
Visual Receptors
Visual receptors are modified neurons that are
located in a deep portion of the retina and are
closely associated with a layer of pigmented
epithelium.
The epithelial pigment absorbs light waves not
absorbed by the receptor cells, and together with
the pigment of the choroid coat, keeps light from
reflecting off surfaces inside the eye.
Visual receptors are stimulated only when light
reaches them.
A light image focused on an area of the retina
stimulates some receptors, and impulses travel
from them to the brain.
Visual Receptors
The impulse leaving each activated receptor provides
only a fragment of the information required for the
brain to interpret a total scene. There are two distinct
kinds of receptors:
1. Rods – long, thin projections at their ends; hundreds of
times more sensitive to light, therefore can provide
vision in dim light; produce colorless vision; provide
general outlines of objects give less precise images
because nerve fibers from many rods converge, their
impulses are transmitted to the brain on the same nerve
fiber
2. Cones – have short, blunt projections; detect color;
provide sharp images
Visual Receptors
The fovea centralis, the area of sharpest vision,
lacks rods but contains densely packed cones
with few or no converging fibers. Also in the
fovea centralis, the overlying layers of the
retina and the retinal blood vessels are
displaced to the sides, more fully exposing
receptors to incoming light. Consequently, to
view something in detail, a person moves the
eyes so that the important part of the image
falls on the fovea centralis.
Visual Pigments
Both rods and cones contain light-sensitive
pigments that decompose when they
absorb light energy.
Visual Pigments
Rods – contain the light-sensitive biochemical called
rhodopsin (visual purple).
In the presence of light rhodopsin breaks down into molecules
of a colorless protein called opsin and a yellowish
substance called retinal (retinene) that is synthesized from
vitamin A.
Decomposition of rhodopsin alters the permeability of the rod
cell membrane, as a result, a complex pattern of nerve
impulses originate in the retina and then travel along the
optic nerve into the brain where they are interpreted as
vision.
In bright light there is more decomposition.
In dim light there is less decomposition and more regeneration
of rhodopsin
Visual Pigments
Cones – the light-sensitive pigments are similar to
rhodopsin in that they are composed of retinal
combined with protein.The protein, however differs.
Three different sets of cones each contain an
abundance of one of the three different visual
pigments.
The wavelength of light determines the color that the
brain perceives from it.
For example, the shortest wavelengths are perceived
as violet, and the longest as red.
Visual Pigments
The three cone pigments are:
1. erythrolabe – sensitive to red light
waves
2. chlorolabe – sensitive to green light
waves
3. cyanolabe – sensitive to blue light
waves
Visual Pigments
The color a person perceives depends on
which set of cones or combination of sets the
light in a given image stimulates.
If all three sets of cones are stimulated, the
person senses the light as white, and if none
are stimulated, the person senses black.
Different forms of colorblindness result from
lack of different types of cone pigments.
Visual Nerve Pathways
The axons of the retinal neurons leave the eyes to form the optic nerves.
Just anterior to the pituitary gland, these nerves give rise to the X-shaped
optic chiasma, and within the chiasma, some of the fibers cross over.
The fibers from the nasal (medial) half of each retina cross over, but those
from the temporal (lateral) sides do not.
Fibers from the nasal half of the left eye and the temporal half of the right
eye form the right optic tract, and fibers from the nasal half of the
right eye and the temporal half of the left eye form the left optic tract.
Just before the nerve fibers reach the thalamus, a few of them enter nuclei
that function in various visual reflexes.
Most of the fibers, however, enter the thalamus and synapse in its posterior
portion.
From this region, the visual impulses enter nerve pathways called optic
radiations, which lead to the visual cortex of the occipital lobes.
Visual Nerve Pathways
Just before the nerve fibers reach the thalamus, a
few of them enter nuclei that function in various
visual reflexes.
Most of the fibers, however, enter the thalamus and
synapse in its posterior portion.
From this region, the visual impulses enter nerve
pathways called optic radiations, which lead to
the visual cortex of the occipital lobes.