The Sensory System
Sensory receptors recieve input, generate receptor potentials,
and with enough summation, generate action potentials in the neurons
they are part of or synapse with.
There are 5 types based on the type
of stimuli they detect:
- Mechanoreceptors - pressure receptors, stretch receptors, and specialized
mechanoreceptors involved in movement and balance.
- Thermoreceptors - skin and viscera, respond to both external and
- Pain receptors - stimulated by lack of O2, chemicals released
from damaged cells and inflammatory cells
- Chemoreceptors - detect changes in levels of O2, CO2,
and H+ ions (pH) as well as chemicals that stimulate taste
and smell receptors
- Photoreceptors - stimulated by light
- Stretch receptors located in joints, ligaments, and tendons (respond
to either stretch or compression)
- Muscle spindles – modified muscle fibers with sensory nerve endings
wrapped around the middle (and also found at the ends). Detect stretch
and stimulate a reflex contraction; think about banging on your patellar
ligament (just an extension of a quadriceps tendon) and watching
your knee jerk up – the quadriceps contracted in response to the
stretch of the patellar ligament, which stretched muscle spindles. Also
(just for your information) impulses are sent to the hamstring group
(the antagonists) to cause them to relax, so they don’t oppose the
contraction of the quadriceps.
Purpose – maintain some degree of continuous contraction (partial
sustained contraction) or muscle tone
- Cutaneous Receptors
- Touch Receptors: fine touch
- Meissner’s corpuscle – fine touch, discrimination; found concentrated
in places where you need to have a lot of responsiveness to a little
- Merkel disks - found deep at the junction of the epidermis and dermis.
- Root hair plexus - at the base of hair follicles.
- Touch Receptors: pressure sensitive
- Ruffini’s endings and Krause's end bulbs – encapsulated pressure
sensors, dermis (and elsewhere), respond to continuous pressure
- Pacinian corpuscles – deep pressure sensors, onion shaped
capsule (layers of Schwann cells enclosed in a connective tissue
membrane), respond to on-off pressure or vibration
- Free nerve endings, some responsive to heat and others
responsive to cold
- Free nerve endings, respond to chemicals released from
- Pain receptors
- Somatic nociceptors
- From skin and skeletal muscle
- Visceral nociceptors
- Receptors that help maintain internal homeostasis
- Respond to stretch, lack of O2, chemicals released
from damaged cells and inflammatory cells.
- Referred pain – visceral pain afferents travel
along the same pathways as somatic pain afferents, so sometimes
the brain interprets the visceral pain as the more common somatic
pain. Example – Often pain from the heart felt during a heart
attack is perceived as a pain that originates in the left arm.
Special Sense Organs
Now we will talk about the special sense organs
rather than general receptors that detect things like carbon dioxide,
oxygen, pH, etc.
Senses of Taste and Smell
Taste buds and olfactory cells (smell receptors) detect chemicals,
thus they are chemoreceptors.
- Sense of Taste
- Taste Buds – located in papillae on the tongue,
hard palate, pharynx, and epiglottis.
- Five types of tastes (Really there are at least
six – the sixth is water. You
might not recognize it, but your hypothalamus will, and it affects
- Bitter – back of the tongue, evolutionarily
important because plant alkaloids, which often are poisonous, are
bitter. Keeps you from swallowing potentially toxic stuff, unless
you’ve trained yourself to recognize the taste of quinine (found
in tonic water) and can knock back a gin and tonic without problems.
- Sour – this is a good taste, the taste of citrus
fruits, which contain vitamin C. Located at the sides of the tongue.
- Salty – another good taste because craving
salt provides with sodium and other minerals. Specific receptors
located along the lateral margins of the tongue.
- Sweet – Again, another good taste, because
glucose the main fuel of the body. Your brain really really likes
to run on glucose. Receptors located on the anterior superior
surface of the tongue.
- Umami - basically a monosodium glutamate receptor. Yeah,
the stuff called flavor enhancer, the stuff knowledgeable diners
insist not be used in their Chinese food. Tastes kind of like beef
or chicken broth, or sometimes described as steak. The receptor
detects the amino acid glutamate.
- How the Brain Receives Taste Information
- Taste buds open at a taste pore and consist of supporting
sensory epithelial cells with microvilli that bind certain chemicals
and depolarize (send a nerve impulse) in response to that chemical.
- The brain integrates the different taste signals coming in to give an overall
- Sense of Smell
- Olfactory Cells – located in the superior region
of the nasal cavity. Don’t really know how many different smells we
- Cells are structurally alike but sensitive to
- Patterns of stimulation (which combinations
of cells are stimulated) determine the characteristics of an odor.
- At least 50 different primary smells (we don’t
even have the words in the English language to describe them all)
but probably somewhere between 2000 and 4000 different chemicals
- And if that isn’t enough, the sense of smell
and taste interact. No sense of smell, no taste discrimination. (Ever
have a cold and notice food doesn’t taste as good?)
- Also – really closely tied in to the limbic
system (the emotional brain). You really remember smells – have
you ever experienced being away from the home you grew up in for
some period of time and noticed that when you return for a visit
the smells of that house can bring the memory of your entire childhood
Sense of Vision
Photoreceptors – rods and cones, located in the eye, but first:
- Accessory Organs of the Eye
- Eyebrows, Eyelids and Eyelashes
- Conjunctiva – mucous membrane lining the inner
surface of the eyelid and anterior portion of the eye except for
the cornea, keeps tears from getting back into the orbits
- Eyelashes act as filters to keep particulate
matter out of eye
- Lacrimal Apparatus
- Lacrmal gland – produces tears, flow over eye
to lacrimal sac
- Lacrimal sac and ducts
- Lacrimal canals lead into lacrimal sac
- Nasolacrimal duct drains into the nose
- Extrinsic Muscles – move the eyes; three pairs
- Superior and inferior rectus – roll eye up and down
- Lateral and medial rectus – turn eye in and out
- Superior and inferior oblique – rotate the eye counterclockwise
- Anatomy and Physiology of the Eye
- Layers (coats, or tunics)
- Sclera – outer, white, fibrous connective tissue
except for cornea, which covers the iris and is clear
- Choroid – middle layer, pigmented to absorb
stray light rays
body – anterior portion of choroid, contains ciliary muscle, which
rounds up the lens to accommodate for near vision
- The lens consists of cells that have lost their
nucleus and organelles and are filled with clear proteins called
proteins allow light to pass through. The lens
is attached to the ciliary body by ligaments, preset for distant
vision, rounds up to focus light rays reflected from close objects
cavity is behind lens, filled with vitreous humor, thick, gelatinous
cavity is between cornea and lens, filled with aqueous humor
by the ciliary body, fluid is filtered from blood plasma. Circulates to
the Canal of Schlemm, located at the place where the cornea and iris meet. Blockage
of this exit canal results in pressure due to build up of aqueous humor. This
pressure compresses arteries and nerve fibers of the retina die, leading
to blindness. This condition is known as glaucoma.
- Iris – forward
(anterior) most part of choroid, consist of smooth muscle, makes
a ring with a hole (the pupil) in the middle through which light
iris can contract in different ways to either dilate the pupil
(open it further) or constrict the pupil.
- Retina – inner layer of the eye, contains three
layers of cells: inner layer of ganglionic cells, whose axons together
make up the optic nerve, a middle layer of bipolar cells, which
synapse with both the ganglionic cells and the sensory cells located
in the layer closest to the choriod, the rods and cones.
place where the optic nerve exits the eye has no photoreceptor
cells and is known as the blind spot.
- Located in the periphery of the eyes
- Sensitive to dim light but don’t detect much
detail or color
things may look a little fuzzy and gray at in the dark (well, in
the dim I
suppose. In the dark you wouldn’t see anything)
- Good for peripheral vision since they are located
around the edges of your field of vision
- Active molecule is rhodopsin, a combination
of the pigment opsin and the pigment retinal
breaks the molecule rhodopsin to its components and this generates the
bright light most rhodopsin is broken down, the period of adjustment to
dim light is the period when rhodopsin is being re-synthesized
comes from vitamin A; Vitamin A deficiency is characterized by night blindness
- Function in bright light
- Detect fine detail and color
kinds of cones based on the color they detect
of one type of cone is the cause of color blindness
of red makes green more visible and red not, etc.
most common because they are sex-linked (carried on the X chromosome, and
you only have one active X chromosome in each cell, especially if you are
color blindness is rare
- Cones are most concentrated in the fovea centralis,
a small area in the center of the macula lutea (yellow spot)
straight at an object focuses light rays on the fovea centralis, which
is why scanning an area allows greater awareness of detail than fixing
on one spot (a good idea when driving, etc.)
is also why staring straight at an object in the dark (dim light) is less
effective than observing with peripheral vision
- Function of the Lens
- Light rays reflected from objects must be bent
so that they converge at a point. This is called the focal point,
and should occur exactly at the retina. The distance from the
lens, which bends the light rays so they will converge, and the
focal point, is the focal distance. Obviously the focal distance
needs to be exactly the same as the distance from the lens to the
- The lens is preset for distant vision; objects
at a distance of about 20 feet and further are automatically focused
on the retina.
- Light rays from closer objects diverge more,
and would normally come to a focal point behind the retina (if
that were possible). To bend light rays reflected from closer
objects more so that they focus on the retina the lens must round
up. This process is called accommodation.
- The ciliary muscles are relaxed for distant
vision, which allows the ciliary body to move back and away from
the lens. This pulls the suspensory ligaments taut, which holds
the lens flat.
- The ciliary muscles contract, moving the ciliary
body forward and toward the lens, relaxing the suspensory ligaments
and allowing the lens to become more round, to accommodate for
- Stereoscopic Vision
- When the eyes focus on an object each sees
it from a slightly different angle
- Optic nerves from each eye carry nerve impulses
generated by light waves to the optic chiasma, where axons from
the right side of each eye travel to the right occipital lobe and
axons from the left side of each eye travel to the left occipital
- The left and right hemispheres communicate
with each other to construct a three dimensional interpretation
of the object.
- Vision Problems:
- If the eyeball is too long the flat lens focuses
distant objects in front of the retina. Since light rays reflected
from closer objects diverge more, and the focal distance is longer,
the focal point moves back to the retina without the lens having
to accommodate, and near vision is OK, but distant vision is blurred. You
can’t flatten the lens any flatter than it already is, so you’re
stuck. This is known as myopia, or nearsightedness. It
can be corrected by placing a concave lens in front of the eye,
which diverges the light rays a bit before they enter the eye. This
increases the focal length and allows the relaxed lens to focus
precisely on the retina.
- If the eyeball is too short, the focal point
from distant objects is behind the retina, but the lens
can round up to move the focal point forward, like accommodating
for near vision, and distant objects appear to be in focus. The
problem comes when objects close to the eye cause the focal length
to be longer, and the lens, which is already rounded up, can’t
round up any more. This causes close objects to be blurred and
is known as hyperopia, or farsightedness. Hyperopia can
be corrected by placing a convex lens in front of the eye, which
converges the light rays a bit before they enter the eye. This
decreases the focal distance so the lens can focus distant objects
without rounding up and can round up enough to focus near objects.
- A normal part of the aging process is loss
of elasticity by the lens, which inhibits its ability to round
up and focus on close objects. This age-related farsightedness
in an eye with a perfectly good shape is called presbyopia (“old
vision”) and usually begins to be noticed around 40 years of age. Presbyopia
can also be treated with convex lenses, but since the focal length
is normal this correction will cause distant vision to be blurred,
so people commonly wear half glasses in order to be able to look
over them at distant objects and peer down through them at close
objects. This makes negotiating stairs a challenge, especially
if someone was myopic to begin with and must then wear bifocals (Think
- Astigmatism results from the surface of the lens or cornea being uneven,
which causes light to be focused on the retina in lines rather
than as a single point.
- Cataracts are clouding of the lens due to damage from things like ultraviolet
rays, cigarette smoke, and other toxic things. The lens eventually
becomes so clouded that a person with cataracts is functionally
blind even though the photoreceptors are fine. To correct cataracts
the lens can be removed and replaced with an artificial lens. Obviously
the artificial lens can’t accommodate for close vision so it has
to be preset for one or the other and supplemented with contacts
or glasses. Forget what the book says.
Sense of Hearing
- Anatomy of the Ear
- External Ear
Nope, no ear-specific receptors here, although
I’ll bet when your mother grabs you up by the pinna (external ear
flap, or “Mom’s handle”) when you are misbehaving in the grocery
store you have some pain receptors that start talking to you. The
- External auditory canal, containing hairs,
sweat glands, and ceruminous glands, which secrete ear wax.
- Middle Ear
- Begins at tympanic membrane (eardrum)
- Contains three bones that link the tympanic
membrane and the inner ear, called ossicles. These bones conduct
sound vibrations from the tympanic membrane to the fluid of the inner
- Malleus (hammer)– in contact with the tympanic
- Incus (anvil) - lies between the malleus and
- Stapes (stirrup) - lies between the incus and
the bony wall that separates the middle ear and the inner ear. The
stapes actually comes in contact with a membrane-covered opening
in the wall called the oval window.
- The posterior wall of the middle ear opens into
the mastoid sinuses.
- The auditory tubes lead from the middle ear
to the nasopharynx, which allows air pressure on either side of the
tympanic membrane to be equalized when atmospheric pressure changes
(like when you ascend to 35,000 feet in an airplane). Yawning or
chewing gum helps open the auditory tubes and equalize the air pressure. Don’t
you wish babies that fly on planes could chew gum? Or take the train?
- Otitis media is inflammation of the middle ear
commonly due to infection. Fluid can build up and exert pressure
on the tympanic membrane. If you get enough exudate built up (yeah,
OK, pus) it can block the auditory tube and eventually the pressure
can blow the eardrum out. This is why in children with frequent
ear infections tubes are sometimes placed in the eardrum (myringotomy). This
allows the pressure to equalize, and usually the tubes fall out by
themselves as the eardrum heals from the incision to place them in.
- Inner Ear
- Where the action is; mechanoreceptors for both
hearing and balance. Located in the temporal bone.
- Vestibule – chamber that lies medial to the
middle ear. Outer wall is the oval window. Filled with fluid (perilymph)
and contains to membranous sacs, the saccule and the utricle. The
saccule and the utricle house equilibrium receptors called maculae
that respond to gravity and changes of head position
- Saccule – filled with fluid (endolymph) and
continuous with ducts leading to the cochlea.
- Utricle – filled with fluid (endolymph) and
continuous with ducts leading into the semicircular canals.
- Semicircular Canals – three channels that run
through the temporal bone, posterior to the vestibule. Each channel
is lined with a membrane and filled with endolymph. The canals are
oriented at right angles to each other in the three planes of space. Each
has an enlarged area at the end that is continuous with the utricle
called the ampulla. Each ampulla houses an equilibrium receptor
called a crista ampullaris, which detects rotational or angular movements
of the head.
- Coclea – a spiral, bony chamber anterior to
the vestibule, that resembles a snail. Lined with membrane, filled
with endolymph, contains the organ of Corti, which senses sound.
- Sound Pathway
- Sound waves travel down the auditory canal to
the tympanic membrane, where they make it vibrate.
- Ossicles in turn vibrate and transmit the vibrations
to the oval window. The vibrations are amplified about 20 times
by the ossicles.
- The oval window vibrates and sends pressure
waves through the endolymph in the cochlea.
- The cochlea consists of three tubes, the vestibular
canal, which originates at the oval window, the tympanic canal,
which is continuous with the vestibular canal and ends at the round
window, and the cochlear canal, which is enclosed and lies between
the vestibular canal and the tympanic canal.
- The cochlear canal is separated from the vestibular
canal by the vestibular membrane, and from the tympanic canal by
the basilar membrane.
- Hair cells are supported on the basilar membrane
and their cilia are embedded in the tectorial membrane. These
hair cells compose the organ of Corti.
- When sound waves pass from the oval window,
through the vestibular canal, and on to the tympanic canal, they
cause the basilar membrane to vibrate.
- This bends the cilia in the hair cells and causes
nerve impulses to be sent through the cochlear branch of the vestibulochchlear
nerve, through the brain stem, and on to the temporal lobe where
they are interpreted as sound.
- Sound waves reach the round window, where the
membrane can bulge to absorb the energy and prevent backwash of the
Sense of Equilibrium
- Rotational Equilibrium Pathway
- Used when the body is moving (dynamic equilibrium), detects
angular or rotational equilbrium.
- Receptors (the cristae ampularis) are found in the ampulla of the semicircular canals and contain hair cells.
- Hair cells in the ampulla have cilia embedded
in a gel-like mass, the cupula. Changes in acceleration cause changes in endolymph
flow, which pushes on the gel, bends the cilia, and transduces
a nerve impulse.
- Gravitional Equilibrium Pathway
- Detects linear acceleration,
movement in a straight line.
- Hair cells in the maculae have cilia that project
into an otolithic membrane, which contains calcium carbonate crystals
called otoliths. When the head starts or stops moving in a linear
direction the otolithic membrane slides around, bends the cilia of
the hair cells, and transduces a nerve impulse.