Sense Organs´╗┐

Unit Overview: In this section you will learn about how you have six different special senses. They are: taste, smell, balance, hearing, and sight. You also have fast and slow-twitch fibers in your body. You will learn that vibrations in your ear go through three different bones to protect you. Also later on you will read and learn about how the structures for hearing and equilibrium are located in your inner ear.

1. Characteristics of sensory receptors
Lets say for instance that you touch a blanket. First off sensory receptors change the energy from our outside world into nerve impulses that are sent to the CNS, then impulses are sent to electrochemical nerves which then send impulses to the brain. This all helps you figure out what kind of fabric the blanket is and everything else that your sensory receptors can pick up. "The brain also interprets impulses arriving from the auditory nerve as sound and from the optic nerve as sight," Fox, Stuart.
Sensory receptors can be grouped by what functions they perform in relationship to what kind of energy they transduce. These include: chemoreceptors (sense chemical stimuli in the blood, for example-the taste buds on your tongue), photoreceptors (found in the rods and cones in the retina located in the eyes), thermoreceptors (your hot and cold sensors on your skin), and mechanoreceptors (mechanical deformation of a recptor cell, for example-your touch, pressure, or hair receptors found in the ear and on the outer surface of you skin). When you bump into the edge of a countertop for example, do you feel pain? If you do the recptor that is working is called the nociceptors (pain receptor) which respond with making you feel pain when there is tissue damage. Pain can be reduced or increased depending on what the person was expecing, their emotions, and concepts.
Sensory receptors can also be grouped by the type of information that they bring to the brain. They include: proprioceptors, cutaneous receptors, special senses, exeroceptors, and interoreceptors. Proprioceptors, which include your muscle spindles, golgi tendon organs, and joint receptors. These help move and control your skeleton. Muscle spindles and golgi tendon organs are located within the muscles in the body. Cutaneous receptors include your hot, cold, touch, pressure, and pain receptors. A persons special senses include sight, hearing, taste, balance, and smell. Exteroceptors are receptors that respond from things outside of the persons body. For example, seeing, touching, and hearing. Interoreceptors are receptors that responds to things happening inside of the body. For example, regualtion on breathing and what your organs are doing.
Tonic receptors are slow-twich fibers, meaning that they contract slowly, that allow a person to do something for a long period of time. For example a cross country runner. Tonic receptors require less tension and they get their energy by using oxygen. On the other hand you have phasic receptors which are fast-twich fibers. They conract the muscle quickly and they don't use energy, so they get tired faster than tonic receptors. Since they are fast-twich receptors, they are stronger than your tonic receptors. You could also look at phasic as your quick burst of energy outlet. They are also known as the on, off stimulus. Phisic receptors also come in handy for quick reflexes or example if you touch a hot burner on the stove top and quickly pull your hand back.

Your sense of taste is called gustation. The receptors are found on the dorsal surface of your tongue and are called taste buds. In each of these taste buds there are 50-100 specialized epithelial cells (taste buds) that are like little hairs which help with the sensors with the exteroreceptores, where they are covered and soaked in saliva. The sopharyngeal nerve carries the taste information to the medulla oblongata. Then the information in carried to the thalamus to be sent over to the cervral cortex. Information is also sent to the perfrontal cortex, which comes to play an important role in finguring out the flavor. Taste buds contain microvilli which make the different kinds of flavors depending on what kind of chemical come in contact with them.


There are four main catagories of taste. These include: salty, sour, sweet, and bitter. There is now a new, fifth kind of taste that can be add for the amino acid glutamate called umami. In each microvilli, there are receptors for each type of sense. For example if you eat something sweet, only the receptor for sweet is going to send a signal to the brain by releasing transmitters, saying that what you are tasting is sweet. Taste can also be subjective to whether the food or drink that you put in your mouth is hot or cold. Sodium ions are the certain chemical that gives you the taste of salty. Sour taste however is produced by the chemial of hydrogen ions, which means that any acid is therefore going to give you the taste of sour. G-protein-coupled receptors are for sweet, bitter, and umami. Any toxin that you take in is going to give you the taste of bitter. Umami is going to respond to proteins, giving the sense for the "meaty" taste. G-protein-coupled receptors detect when the flavor is sweet.
Olfactory epithelium contains olfactory receptors that give you the sense of smell. The stem cells every one to two months stimulates the production of new neurons to replace the ones that were damaged due to the environment. Each receptor is bipolar and each one contains one dendrite that goes into the nasal cavity where it hits the cilia. Cilia is little hairs that consists of receptors proteins that attract odorant molecules. Every axon from each olfactory sensory neuron then sends the information containing what type of odor it is by binding together to stimulate the certain receptor protein. It is then produced into the action potential, were it than b e sent to the brain into the olfactory bulb in the cerebral cortex where it then makes a second-order neuron. From there it goes back to the cerebral cortex where the sense of smell is detected. Olfactory receptors are stimulated by the person breathing out of their nose. If you breath through your nose while chewing food you will get the sense of taste rather than the sense of smell.

2. Ears & Hearing

Sound waves can travel in all different directions from a source. Sound makes movements in the tympanic membrane and middle-ear ossicles. These are transmitted into the fluid-filled cohclea. This produces vibrations of the basilar membrane. Sound waves are characterized by their frequency and intensity. Frequency is measured in heartz. Pitch is directly related to frequency; so the greater the frequency, the higher its pitch. Intensity of a sound is directly related to the amplitude of the sound waves. It is measured in units of decibels.

The outer ear has two structures: the pinna, or auricle, and the external auditory meatus. Sound waves in the external auditory meatus produce very small vibrations of the tympanic membrane.

The middle ear is between the tympanic membrane on the outer side and the cochlea on the inner side. The middle ear ossicles (malleus, incus, stapes) are in this cavity. For protection, the vibrations of the tympanic membrane are transferred through three bones instead of just one.


Eyes & Vision


The cornea and lens on the photoreceptive retina focuses on light from an observed object. The focus is maintained on the retina at different distances. The eyes transduce energy in the electromagnetic spectrum into nerve impulses. Light of longer wavelengths in the infarared regions of the spectrum is felt as heat but does not have enough energy to excite the photoreceptors.

Light enters the eye from the right side of this figure and is focused on the retina.

3.Vestibular Apparatus and Equilibrium, The Retina, Neural Processing of Visual Information

Vestibular Apparatus and Equilibrium

The structures for equilibrium and hearing are located in the inner ear within the membranous labyrinth.

  • The structure involved in equilibrium, known as the vestibular apparatus, consists of the otolith organs (utricle and saccule) and the semicircular canals.

  • The utricle and saccule provide information about linear acceleration, whereas the semicircular canals provide information about angular acceleration.

  • The sensory receptors for equilibrium are hair cells that support numerous stereocilia and one kinocilium.


  • When the stereocilia are bent in the direction of the kinocilium, the cell membrane becomes depolarized.

  • When the stereocilia are bent in the opposite direction, the membrane becomes hyperpolarized.

The stereocilia of the hair cells in the utricle and saccule project into the endolymph of the membranous labyrinth and are embedded in a gelatinous otolithic membrane.
  • When a person is upright, the stereocilia of the utricle are oriented vertically; those of the saccule are oriented horizontally.
  • Linear acceleration produces a shearing force between the hairs and the otolithic membrane, thus bending the stereocilia and electrically stimulating the sensory endings.


The three semicircular canals are oriented at nearly right angles to each other, like the faces of a cube.
  • The hair cells are embedded within a gelatinous membrane called the cupula, which projects into the endolymph.
  • Movement along one of the planes of a semicircular canal causes the endolymph to bend the cupula and stimulate the hair cells.
  • Stimulation of the hair cells in the vestibular apparatus activates sensory neurons of the vestibulocochlear nerve (VIII), which projects to the cerebellum and to the vestibular nuclei of the medulla oblongata.
    • The vestibular nuclei in turn send fibers to the oculomotor center, which controls eye movements.
    • Spinning and then stopping can thus cause oscillatory movements of the eyes called nystagmus.


The Retina


The retina contains rods and cones, photoreceptor neurons that synapse with bipolar cells.

  • When light strikes the rods, it causes the photodissociation of rhodopsin into retinene and opsin.
    • This bleaching occurs maximally with a light wavelength of 500 nm.
    • Photodissociation is caused by the conversion of the 11-cis to the all-trans form of retinene, that cannot bond to opsin.

  • In the dark, more rhodopsin can be produced, and increased rhodopsin in the rods, makes the eyes more sensitive to light. This increased concentration of rhodopsin is partly responsible for dark adaptation.

  • The rods provide black-and-white vision under conditions of low light intensity. At higher light intensity, the rods are bleached out and the cones provide color vision.

In the dark, a constant movement of Na+ into the rods produces what is known as a "dark current."
  • When light causes the dissociation of rhodopsin, the Na+ channels become blocked and the rods become hyperpolarized in comparison to their membrane potential in the dark.
  • When the rods are hyperpolarized, they release less neurotransmitter at their synapses with bipolar cells.
  • Neurotransmitters from rods cause depolarization of bipolar cells in some cases, and hyperpolarization of bipolar cells in other cases; thus, when the rods are in light and release less neurotransmitter these effects are inverted.

According to the trichromatic theory of color vision, there are three systems of cones, each of which responds to one of three colors: red, blue, or green.
  • Each type of cone contains retinene attached to a different type of protein.
  • The names for the cones signify the region of the spectrum in which the cones absorb light maximally.
The fovea centralis contains only cones; more peripheral parts of the retina contain both cones and rods.
  • Each cone in the fovea synapses with one bipolar cell, which in turn synapses with one ganglion cell.
    • The ganglion cell that receives input from the fovea thus has a visual field equal to only that part of the retina which activated its cone.
    • As a result of this 1:1 ratio of cones to bipolar cells, visual acuity is high in the fovea but sensitivity to low light levels is less than in other regions of the retina.
  • In regions of the retina where rods predominate, large number of rods provide input to each ganglion cell (there is great convergence). As a result, visual acuity is impaired, but sensitivity to low light levels is improved.
The right half of the visual field is projected to the left half of the retina of each eye.
  • The left half of the retina sends fibers to the left lateral geniculate body of the thalamus.
  • The left half of the right retina also sends fibers to the left lateral geniculate body. This is because these fibers decussate in the optic chiasma.
  • The left lateral geniculate body thus receives input from the left half of the retina of both eyes, corresponding to the right half of the visual field; the right lateral geniculate receives information about the left half of the visual field.
    • Neurons in the lateral geniculate bodies send fibers to the striate cortex of the occipital lobes.
    • The geniculostriate system is involved in providing meaning to the images that form on the retina.
  • Instead of synapsing in the geniculate bodies, some fibers from the ganglion cells of the retina synapse in the superior colliculus of the midbrain, which controls eye movements.
    • Since this brain region is also called the optic tectum, this pathway is called the tectal system.
    • The tectal system enables the eyes to move and track an object; it is also responsible for the pupillary reflex and the changes in lens shape that are needed for accommodation.

Neural Processing of Visual Information

The area of the retina that provides input to a ganglion cells is called the receptive field of the ganglion cell.
  • The receptive field of a ganglion cell is roughly circular, with an "on" or "off" center and an antagonistic surround.
    • A spot of light in the center of an "on" receptive field stimulates the ganglion cell; whereas a spot of light in its surround inhibits the ganglion cell.
    • The opposite is true for ganglion cells with "off" receptive fields.
    • Wide illumination that stimulates both the center and the surround of a receptive field affects a ganglion cell to a lesser degree than a pinpoint of light that illuminates only the center or the surround.
  • The antagonistic center and surround of the receptive field of ganglion cells provide lateral inhibition, which enhances contours and provides better visual acuity.
Each lateral geniculate body receives input from both eyes relating to the same part of the visual field.
  • The neurons receiving input from each eye are arranged in layers within the lateral geniculate.
  • The receptive fields of neurons in the lateral geniculate are circular, with an antagonistic center and surround, much like the receptive field of ganglion cells.

Cortical neurons involved in vision may be either simple, complex, or hypercomplex.
  • Simple neurons receive input from neurons in the lateral geniculate; complex neurons receive input from simple cells; and hypercomplex neurons receive input from complex cells.
  • Simple neurons are best stimulated by a slit or bar of light that is located in a precise part of the visual field and has a precise orientation.
  • Complex cells respond best to a straight line that has a particular orientation and that moves in a particular direction. The position of the line in the visual field is not important.
  • Hypercomplex cells respond best to lines that have a particular length or have a particular bend or corner.

Essential Questions:
-Use a flow chart to describe how sound waves in the air within the external auditory meatus are transduced into the movements of the basilar membrane (hair cells).
1. Sound waves funneled into External Auditory Meatus.
2. External Auditory Meatus channels sound waves to tympanic membrane.
3. Mechanical force by the bones of the ossicles traveling to the oval window to produce a fluid wave in the cochlear duct.
4. Fluid wave causes differential movement of the basilar membrane.

-Describe how light is transmitted through the structures of the eye, refracted, and photoreceptors are stimulated to send the CNS to be interpreted. In other words, trace the path of light and the neural impulses sent to the brain.

Stimulated by light, the rods and cones trigger electrical signals in bipolar cells. Bipolar cells transmit both excitatory and inhibitory signals to ganglion cells. The ganglion cells become depolarized and generate nerve impulses. The axons of the ganglion cells exit the eyeball as cranial nerve II, the optic nerve and extend posteriorly to the optic chiasm. In the optic chiasm, about half of the axons from each eye cross to the opposite side of the brain. After passing the optic chiasm, the axons, now part of the optic tract, terminate in the thalamus. Here the neurons of the optic tract synapse with thalamic neurons whose axons pass to the primary visual areas in the occipital lobes. Because of the crossing at the optic chiasm, the right side of the brain receives signals from both eyes for interpretation of visual sensations from the left side of an object, and the left side of the brain receives signals from both eyes for interpretation of visual sensations from the right side of an object.

-How this applies to PTA:
Our sense organs are one of the most important systems in our body, it is what helps us stay safe and sense any dangers around us. As PTA's this system is very important that we really understand how it works, because of the types of treatments we use to help our patients. Depending on how well our patient senses or feels certain things will determine what we as PTA's can and can't use to treat them. Example if a Patient can't feel the difference between sharp and dull pressure or light and deep pressure or if something is hot or cold then we can't do Traction, Hot/Ice packs, Whirlpool, Ultrasound, some forms of ambulation, Deep Massage, Estem/ TENs units. So it can really limit what types of treatments we can give a patient. If a Patient has an equilibrium issue then we can put then on a tilt table to help their body to adjust to being in different positions. We can also teach them how to do Otolith repositioning if they suffer from Vertigo or severe dizziness. Or a PT can do a CV test to check if there are any limitations we need to abide by for doing any kind of traction or head and neck exercises when weight is being used in that area. If a Patient has trouble seeing then we as PTA's can do a house assessment test and put up bright colored tape or rubber on the edges of for example tables, counters, cabinets, lips of stairs, wall corners, etc... so that they don't hit the edges and get hurt or cause bruising or skin tears if they are elderly, or have the live space rearranged by removing anything they might trip over or sharp edges be modified some how or removed.

1. Fox, Stuart I. "Human Physiology." New York: McGraw-Hill, 2011. Print.