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Anatomy of the
Sensory Organs
Sensory Function and Vision…
The General Senses…
• Sensory receptors
– specialized cells that monitor the environment
and relay information to the CNS.
– Free nerve endings are the simplest type:
they are the dendrites of sensory neurons
– Complex receptors (eyes) are housed in
organs
– Some receptors respond to only one kind of
stimulus
All sensory receptors send info to
the CNS via an action potential…
• At the CNS, info is routed according to the
stimulus and its location
• The stronger the stimulus, the higher the
frequency of action potentials
• Some receptors adapt, that is their
sensitivity to a stimulus is reduced if the
stimulus is continually applied (smell)
– The RAS can heighten or reduce awareness
of sensory information
General versus special senses…
• General sense receptors
included those for
temperature, pain,
pressure, touch, vibration
& proprioception (body
position)
• These receptors are very
simple in nature
“Special” senses
• Special senses
monitor vision,
hearing, olfaction,
gustation, and
equilibrium through
specialized sense
organs
• These sense organs
are highly specialized
“General sense” receptors…
• Are simple, found everywhere and are classified
by their stimulus:
• Nociceptors – sensitive to pain
• Thermoreceptors – respond to heat
• Mechanoreceptors–respond to touch/pressure
• Chemoreceptors – response to chemicals
• Only 1% of the info they provide ever reaches
the cerebral cortex (our consciousness)
Nociceptors/Pain receptors:
• found in joints, periosteum of bone and skin
• do not adapt!
• 2 types of axons carry the painful sensations:
– Fast pain sensations (localized, shooting pain) are
carried by myelinated axons
– Slow pain sensations (generalized, aching pain) are
carried by unmyelinated axons: it is difficult to pinpoint
the stimulus location
• Referred pain is the perception of pain coming
from a body area that is NOT stimulated:
– Pain originating in viscera is felt on body surfaces
Referred pain:
• For example, pain
produced by a heart
attack may feel as if it
is coming from the
arm because sensory
information from the
heart and the arm
converge on the
same nerve cells in
the spinal cord.
Thermoreceptors…
• Free nerve endings in skin, muscle, liver,
and hypothalamus
• they adapt quickly
• Cold receptors respond to temps <50
– 4X’s as numerous as warm receptors
• Warm receptors respond to temps >113
• Both are structurally identical
Mechanoreceptors…
• Membrane distortion opens mechanically
regulated ion channels to create impulses.
There are 3 classes:
– Tactile – respond to touch
– Baroreceptors – respond to pressure
– Proprioceptors – respond to changes in body
position
Tactile receptors…
• May be simple or complex,
superficial or deep, fine
(provide detailed information)
or crude (provide little
information)
• Merkel’s – fine touch and
pressure
• Pacinian – deep pressure
• Meissner’s – fine touch
and pressure in select
areas
• Ruffini – pressure or
distortion in deep dermal
layers
Baroreceptors are….
• stretch receptors that
monitor changes in
organ pressure in
distensible organs
• rapidly adapting
• Generate an AP from
their dendrites when
organs are stretched
or change position
• They monitor BP,
respiration, digestion,
and urinary control
Chemoreceptors…
Only respond to dissolved chemicals
• Rapidly adapting: found in olfaction, taste
& the CNS at:
– Medulla – receptors are sensitive to pH/CO2
changes in CSF– trigger respiratory
adjustments
– Aortic/Carotid bodies – sensitive to changes
in pH/CO2/O2 blood levels – trigger
adjustments in respiration and cardiovascular
activity
Proprioceptors….
• Monitor joint position & muscle contraction
• DO NOT ADAPT
• Structurally complex – are 2 types
– Tendon organs – monitor tendon strain
– Muscle spindles – monitor muscle length
• Most information from these receptors is
monitored subconsciously
• Are vital for normal skeletal motor function
Olfaction -The Sense of Smell
• Consists of paired olfactory organs made of
epithelium containing olfactory receptors,
supporting cells & stem cells
• the receptor cells are highly modified
neurons (chemoreceptors)
• Contains olfactory glands that produce a
multifunctional mucus
• Sensory Pathway:
olf.epithelium>olf.bulbs>olf.tracts>cortex
• http://www.bbc.co.uk/science/humanbody/bo
dy/factfiles/smell/smell_ani_f5.swf
The olfactory organs…
Gustation….taste
• http://www.bbc.co.uk/science/humanbody/body/f
actfiles/taste/taste_ani_f5.swf
• Taste buds are organs containing gustatory &
supporting cells that lie within papillae
• Chemicals contact taste hairs which change the
MP of taste cells & leads to an AP in the sensory
neuron
• 4 primary taste sensations – sweet, salt, sour,
bitter
• Sensory Pathway: sensory receptors>medulla>
thalamus>primary sensory cortex
A complex sensory organ: the eye.
• is surrounded by accessory structures that
act to protect, lubricate, and support it
• is a light, compact, durable, and highly
specialized hollow organ that weighs
about 8 oz and measures 1 inch in
diameter.
• is divided into anterior (aqueous) &
posterior (vitreous) cavities.
• its walls are made of 3 “tunics”
Accessory structures of the eye…
•
•
•
•
•
•
eyelids (palpebrae)
eyelashes & brows
exocrine glands
lacrimal apparatus
Conjunctiva
6 extrinsic occulomotor
muscles:
– the inferior, superior, lateral
and medial rectus muscles
– the superior and inferior
oblique muscles
Eye anatomy…..
• http://www.macula.org
/anatomy/eyeframe.ht
ml
• The hollow eye is divided
into 2 cavities:
• An anterior cavity which
contains aqueous humor
• A posterior cavity which
holds vitreous humor
• Humors act to stabilize
eye shape and provide
nutrients
The Tunics of the eye…
• Fibrous - the sclera &
anterior cornea
• Vascular – contains
blood vessels,
lymphatics, choroid &
intrinsic muscles of the
iris &ciliary bodies (they
support the lens)
• Neural – the retina, it
contains the rods and
cones (photoreceptor
cells), bipolar &ganglion
cells
Retinal organization …
• The retina is made of several cell layers:
– Photoreceptor cells – rods lie along the
periphery & cones lie at the back of the retina
– Bipolar cells synapse with the rods and cones
– Ganglion cells synapse with the bipolar cells
– The axons of the ganglion cells form the optic
nerve
– http://www.macula.org/anatomy/retinaframe.html
http://www.macula.org/anatomy/anatomy.html
• Macula lutea – area on
the retina where the
visual image forms, it
contains only cones with
the greatest numbers at
the fovea centralis
• Optic Disc or “blind spot”
is the area where the
ganglion cell axons exit
the eye to form the optic
nerve
Other structures of the eye:
• Lens – held in place by suspensory ligaments, it
functions to focus the visual image onto the
retina
• Cornea – clear portion of the fibrous tunic it is
contiguous with the sclera
• Iris – part of the vascular tunic, it contains blood
vessels, pigment, and 2 smooth muscle layers to
control the width of the pupil.
• Ciliary body – a thick region of the choroid that
encircles the lens and supports the suspensory
ligments of the lens
Accommodation- focusing an image on the retina by
changing lens shape
• light bends/refracts as it passes from 1 medium
to another. In the eye it goes through the cornea,
a. humor, lens, & v. humor. The refraction of light
is constant through all but the lens
• The lens changes shape to keep the image
focused on the retina for greatest visual acquity
• Accommodation occurs with response to light
and to the distance of the object being viewed
• http://www.kscience.co.uk/animations/eye.
swf
The Physiology of Vision…
How is it that we see?
• Photoreceptors respond to visible light:
• Rods – sensitive to photons (energy) but
not their wavelength (color) allow for vision
at night/dim light
• The 3 types of Cones (green, red, blue)
need bright light & are responsive to
wavelength: they allow us to “see” color
• Lack of functional cones = colorblindness
The visual pathway….
•
•
•
•
Photoreceptor stimulation
Bipolar cell activation
Stimulation of ganglion cell’s whose axons form the…
Optic nerve that cross at the diencephalon and goes to
the thalamus that routes info to the visual cortex of
occipital lobe and the reflex centers of brain stem
• At the optic chiasm, a partial crossover of nerve fibers
occurs
• http://www.sumanasinc.com/webcontent/anisam
ples/neurobiology/visualpathways.html