The Sensory System

Sensory Receptors

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:

  1. Mechanoreceptors - pressure receptors, stretch receptors, and specialized mechanoreceptors involved in movement and balance.

  2. Thermoreceptors - skin and viscera, respond to both external and internal temperature

  3. Pain receptors - stimulated by lack of O2, chemicals released from damaged cells and inflammatory cells

  4. Chemoreceptors - detect changes in levels of O2, CO2, and H+ ions (pH) as well as chemicals that stimulate taste and smell receptors

  5. Photoreceptors - stimulated by light

General Senses

  1. Proprioceptors

    1. Stretch receptors located in joints, ligaments, and tendons (respond to either stretch or compression)

      1. 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. 

        1. Purpose – maintain some degree of continuous contraction (partial sustained contraction) or muscle tone

  1. Cutaneous Receptors

    1. Touch Receptors: fine touch

      1. Meissner’s corpuscle – fine touch, discrimination; found concentrated in places where you need to have a lot of responsiveness to a little input. 

      2. Merkel disks - found deep at the junction of the epidermis and dermis.

      3. Root hair plexus - at the base of hair follicles.

    2. Touch Receptors: pressure sensitive

      1. Ruffini’s endings and Krause's end bulbs – encapsulated pressure sensors, dermis (and elsewhere), respond to continuous pressure

      2. 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

    3. Temperature

      1. Free nerve endings, some responsive to heat and others responsive to cold

    4. Pain

      1. Free nerve endings, respond to chemicals released from damaged tissues.

  1. Pain receptors

    1. Somatic nociceptors

      1. From skin and skeletal muscle

    2. Visceral nociceptors

      1. Receptors that help maintain internal homeostasis

        1. Respond to stretch, lack of O2, chemicals released from damaged cells and inflammatory cells.

        2. 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.

  1. Sense of Taste

    1. Taste Buds – located in papillae on the tongue, hard palate, pharynx, and epiglottis.

    2. 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 fluid retention/excretion.)

      1. 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.

      2. Sour – this is a good taste, the taste of citrus fruits, which contain vitamin C.  Located at the sides of the tongue.

      3. Salty – another good taste because craving salt provides with sodium and other minerals.  Specific receptors located along the lateral margins of the tongue.

      4. 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.

      5. 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.
  1. How the Brain Receives Taste Information

    1. Taste buds open at a taste pore and consist of supporting cells and sensory epithelial cells with microvilli that bind certain chemicals and depolarize (send a nerve impulse) in response to that chemical.

    2. The brain integrates the different taste signals coming in to give an overall combination effect.

  1. Sense of Smell

    1. Olfactory Cells – located in the superior region of the nasal cavity.  Don’t really know how many different smells we can detect.
    2. Cells are structurally alike but sensitive to different chemicals.

    3. Patterns of stimulation (which combinations of cells are stimulated) determine the characteristics of an odor.

    4. 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 detected.

    5. 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?)

    6. 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 back instantly? 

Sense of Vision

Photoreceptors – rods and cones, located in the eye, but first:

  1. Accessory Organs of the Eye

    1. Eyebrows, Eyelids and Eyelashes

      1. 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

      2. Eyelashes act as filters to keep particulate matter out of eye

    2. Lacrimal Apparatus

      1. Lacrmal gland – produces tears, flow over eye to lacrimal sac

      2. Lacrimal sac and ducts

      3. Lacrimal canals lead into lacrimal sac

      4. Nasolacrimal duct drains into the nose

    3. Extrinsic Muscles – move the eyes; three pairs

      1. Superior and inferior rectus – roll eye up and down

      2. Lateral and medial rectus – turn eye in and out

      3. Superior and inferior oblique – rotate the eye counterclockwise or clockwise

  1. Anatomy and Physiology of the Eye
    1. Layers (coats, or tunics)

      1. Sclera – outer, white, fibrous connective tissue except for cornea, which covers the iris and is clear

      2. Choroid – middle layer, pigmented to absorb stray light rays

        1. Ciliary body – anterior portion of choroid, contains ciliary muscle, which rounds up the lens to accommodate for near vision

        2. The lens consists of cells that have lost their nucleus and organelles and are filled with clear proteins called crystallins.  These 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

        3. Posterior cavity is behind lens, filled with vitreous humor, thick, gelatinous

        4. Anterior cavity is between cornea and lens, filled with aqueous humor

          1. Produced 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.

        5. 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 passes.  The iris can contract in different ways to either dilate the pupil (open it further) or constrict the pupil.

      3. 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.

        1. The place where the optic nerve exits the eye has no photoreceptor cells and is known as the blind spot.
        2. Rods

          1. Located in the periphery of the eyes

          2. Sensitive to dim light but don’t detect much detail or color

            1. So 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)

          3. Good for peripheral vision since they are located around the edges of your field of vision

          4. Active molecule is rhodopsin, a combination of the pigment opsin and the pigment retinal

            1. Light breaks the molecule rhodopsin to its components and this generates the nerve impulses

            2. In bright light most rhodopsin is broken down, the period of adjustment to dim light is the period when rhodopsin is being re-synthesized

            3. Retinal comes from vitamin A; Vitamin A deficiency is characterized by night blindness

        3. Cones

          1. Function in bright light

          2.  Detect fine detail and color

            1. Three kinds of cones based on the color they detect

              1. Blue

              2. Green

              3. Red

            2. Lack of one type of cone is the cause of color blindness

            3. Lack of red makes green more visible and red not, etc.

            4. Red-green 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 male)

            5. Complete color blindness is rare

          1. Cones are most concentrated in the fovea centralis, a small area in the center of the macula lutea (yellow spot)

            1. Staring 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.)

            2. This is also why staring straight at an object in the dark (dim light) is less effective than observing with peripheral vision


    1. Function of the Lens

      1. 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 retina.

      2. The lens is preset for distant vision; objects at a distance of about 20 feet and further are automatically focused on the retina.

      3. 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.

      4. 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.

      5. 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 close vision.

    1. Stereoscopic Vision

      1. When the eyes focus on an object each sees it from a slightly different angle

      2. 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 lobe.

      3. The left and right hemispheres communicate with each other to construct a three dimensional interpretation of the object.

    1. Vision Problems:

      1. 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.

      2. 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.

      3. 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 about it).

      4. 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.
      5. 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

  1. Anatomy of the Ear
    1. External Ear
    2. 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 parts are:

      1. Pinna

      2. External auditory canal, containing hairs, sweat glands, and ceruminous glands, which secrete ear wax.

    3. Middle Ear

      1. Begins at tympanic membrane (eardrum)

      2. 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 ear.

        1. Malleus (hammer)– in contact with the tympanic membrane

        2. Incus (anvil) - lies between the malleus and stapes

        3. 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.

      3. The posterior wall of the middle ear opens into the mastoid sinuses.

      4. 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?

      5. 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.

    4. Inner Ear

      1. Where the action is; mechanoreceptors for both hearing and balance.  Located in the temporal bone.

      2. 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

        1. Saccule – filled with fluid (endolymph) and continuous with ducts leading to the cochlea.
        2. Utricle – filled with fluid (endolymph) and continuous with ducts leading into the semicircular canals.

      3. 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.

      4. 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.

    5. Sound Pathway

      1. Sound waves travel down the auditory canal to the tympanic membrane, where they make it vibrate.

      2. Ossicles in turn vibrate and transmit the vibrations to the oval window.  The vibrations are amplified about 20 times by the ossicles.

      3. The oval window vibrates and sends pressure waves through the endolymph in the cochlea.

        1. 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.
        2. The cochlear canal is separated from the vestibular canal by the vestibular membrane, and from the tympanic canal by the basilar membrane.
        3. 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.

      4. 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.

        1. 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.

      5. Sound waves reach the round window, where the membrane can bulge to absorb the energy and prevent backwash of the endolymph.

Sense of Equilibrium

  1. Rotational Equilibrium Pathway

    1. Used when the body is moving (dynamic equilibrium), detects angular or rotational equilbrium.

    2. Receptors (the cristae ampularis) are found in the ampulla of the semicircular canals and contain hair cells.

    3. 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.

  2. Gravitional Equilibrium Pathway

    1. Detects linear acceleration, movement in a straight line.

    2. 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.