One of the characteristics of a living organism is its ability to respond to stimuli. The human sensory system is highly evolved and processes thousands of incoming messages all at once. This amazing involvedness allows us to be aware of our surroundings and take the appropriate actions. This unit taught us about sensory receptors and understanding the different types of sensory stimuli that the body detects. We learned to name the components of the eye and its accessory organs that aid in vision. The major regions of the ear and the process of hearing was also explained. We also learned to identify the modalities of taste and to explain how they are produced. How the structures of the vestibular apparatus functions to produce a sense of equilibrium was another big part of this unit, and is explained below on this WIKI page.


Sensory Receptors
Sensory receptors are dendrites of sensory neurons specialized for receiving specific kinds of stimuli. Sensory receptors are classified by three methods:

Classification by receptor complexity.
Free nerve endings are dendrites whose terminal ends have little or no physical specialization. Encapsulated nerve endings are dendrites whose terminal ends are enclosed in a capsule of connective tissue. Sense organs (such as eyes and ears) consist of sensory neurons with receptors for the special senses (vision, hearing, smell, taste, and equilibrium) together with connective, epithelial, or other tissues.

Classification by location. Exteroceptors occur at or near the surface of the skin and are sensitive to stimuli occurring outside or on the surface of the body. These receptors include those for tactile sensations, such as touch, pain, and temperature, as well as those for vision, hearing, smell, and taste. Interoceptors respond to stimuli occurring in the body from visceral organs and blood vessels. These receptors are the sensory neurons associated with the autonomic nervous system. Proprioceptors respond to stimuli occurring in skeletal muscles, tendons, ligaments, and joints. Theses receptors collect information concerning body position and the physical conditions of theses locations.

Classification by type of stimulus detected. Mechanoreceptors respond to physical force such as pressure (touch or blood pressure) and stretch. Photoreceptors respond to light. Thermoreceptors respond to temperature changes. Chemoreceptors respond to dissolved chemicals during sensations of taste and smell and to changes in internal body chemistry such as variations of O2, CO2, or H+ in the blood.

The video above is a great in-depth explaination of sensory receptors!

Equilibrium is maintained in response to two kinds of motion:
Linear acceleration is a change in velocity when traveling horizontally or vertically, such as starting to walk/stopping or jumping up and down.
Rotational acceleration is a change in velocity when traveling in any direction other than horizontally or vertically, such as turning the head, spinning tumbling, or rocking.

The perception of equilibrium occurs in the vestibular apparatus, which consists of the vestibule and the semicircular canals. Motion in these two structures is detected as follows:
The vestibule is the primary detector of changes in linear acceleration. A sensory receptor called a macula is located in the walls of the saccule and utricle, the two bulblike sacs of the vestibule. A macula contains several receptor cells called hair cells, from which several stereocilia and a single kinocilium extend into a glycoprotein gel, the otolithic membrane. Calcium carbonate crystals called otoliths pass through the otolithic membrane, increasing its density and therefore responsiveness to changes in motion. Changes in linear motion cause the otolithic membrane to move forward and backward in the utricle or up and down in the saccule. The movement of the otolithic membrane causes similar movements in the embedded stereocilia of the hair cells, which in turn initiate graded potentials.
The semicircular canals are the primary detector changes in rotational acceleration. The three canals, individually called the anterior, posterior, and lateral canals, are arranged at right angles to one another. The expanded base of each canal, called an ampullae, contains a sensory receptor, or crista ampullaris. Like the maculae of the vestibule, each crista contains numerous hair cells whose stereocilia and kinocilium protrude into a gelatinous matrix, the cupula (which is similar to the otolithic membranes of the maculae). Changes in rotational motion cause the cupula and the embedded stereocilia to move, which stimulates the hair cell to generate a graded potential.

Graded potentials in the hair cells of the maculae and cristae result in changes in the amounts of neurotransmitter secreted. In response to these changes, action potentials are generated in the fibers of the vestibular nerve which then joins the vestibulocochlear nerve. From here, the nerve impulses travel to the cerebellum and to the vestibular nuclei of the medulla oblongata. The vestibular nuclei then send fibers to the oculomotor center of the brain stem and to the spinal cord. Neurons in the oculomotor center control eye movements, and neurons in the spinal cord stimulate movement of the head, neck and limbs. Movement of the eyes and body produced by these pathways help to maintain balance.

Smell and Taste

The sense of smell, or olfactory sense, occurs in olfactory epithelium that occupies a small area in the roof of the nasal cavity. The olfactory receptor cells are bipolar neurons whose dendrites have terminal knobs with hairlike cilia protruding beyond the epithelial surface. The cilia, or olfactory hairs, initiate an action potential when they react with a molecule from an inhaled vapor. However, molecules of the vapor must first dissolve in the mucus that covers the cilia before they can be detected. The action potential is transmitted along the axons of the olfactory receptor cells (which form the olfactory nerves) to the olfactory bulbs, where they synapse with sensory neurons of the olfactory tract.
Other cells of the olfactory epithelium include columnar supporting cells and basal cells. The basal cells continually divide to produce new olfactory receptor cells that, because of their short life, need regular replacement. The replacement of olfactory receptor cells is unusual because most other nerves cells cannot be replaced.
The mucus that lines the olfactory epithelium is produced by olfactory glands that occupy the connective tissue above the olfactory epithelium.
The sense of taste, or gustatory sense, occurs in the taste buds. Located primarily on the tongue, taste buds reside in papillae, the bumps on the tongue that give it a rough texture. The taste bud consists of supporting cells, basal cells, and gustatory receptor cells, and taste pores, located at the top. A long microvilli, or gustatory hair, from each gustatory receptor cell within the taste bud projects through the taste pore. Gustatory hairs generate action potentials when stimulated by chemicals that are dissolved in the saliva.

Basal cells are actively dividing epithelial cells. The daughter cells of basal cells develop into supporting cells, which subsequently mature into gustatory receptor cells. Because they are easily damaged by the activities that occur in the mouth, gustatory receptor cells are short-lived and replaced about every ten days.

An individual gustatory receptor cell responds to only five taste sensations: sweet, bitter, salty, sour, and umami. All other tastes arise from a mixture of these five tastes in combination with olfactory sensations associated with the substance tasted. Each taste bud has the ability to detect each of the five sensations. Taste can also be influenced by the temperature and texture of food, which stimulate receptors around the taste buds in the tongue.


In my job as a CNA in an elder care facility, I work very closely with many residents that have hearing loss. I also have personal experience because my husband also suffers from moderate sensorineural hearing loss (caused by his job in the military) and tinnitus. Hearing impairment is very common among elderly people and can seriously affect their quality of life, personal safety, and ability to function independently. It limits their ability to interact socially with family and friends and to receive and interpret information.
There are two major categories of deafness: conduction deafness and sensorineural deafness. Conductive hearing loss happens when something blocks the sounds that are carried from the tympanic membrane to the inner ear. Ear wax buildup, fluid in the middle ear, abnormal bone growth, a punctured eardrum, or a middle ear infection can cause this type of hearing loss. Sensorineural hearing loss can result from a wide variety of pathological processes and from exposure to extremely loud noises such as gunshots and loud concerts. This type of hearing loss results from damage to parts of the inner ear, the auditory nerve, or hearing pathways in the brain.

There are many different types of hearing loss, one of them is presbycusis. Presbycusis is age-related hearing loss. It becomes more common in people as they get older. People with this kind of hearing loss may have a hard time hearing what others are saying or may be unable to stand loud sounds. The decline is slow. It can be caused by sensorineural hearing loss. Presbycusis may be caused by aging, loud noise, heredity, head injury, infection, illness, certain prescription drugs, and circulation problems such as high blood pressure. The degree of hearing loss varies from person to person. Also, a person can have a different amount of hearing loss in each ear.

Tinnitus accompanies many forms of hearing loss, including those that sometimes come with aging. People with tinnitus may hear a ringing, roaring, or some other noise inside their ears. Tinnitus may be caused by loud noise, hearing loss, certain medicines, and other health problems, such as allergies and problems in the heart and blood vessels. Often it is unclear why the ringing happens. Tinnitus can come and go, it can stop completely, or it can stay. For my husband, it is a constant ringing sound that never goes away. He wears hearing aids to make it easier for him to hear the sounds he needs to hear, but the ringing never stops. Maskers, small devices that use sound to make tinnitus less noticeable, help other people. It also helps to avoid things that might make tinnitus worse, like smoking, alcohol, and loud noises.

There are many devices that can help a person with hearing loss to regain their independence and help them to hear a bit better. The effectiveness of each will depend on the type of hearing loss they suffer from. They include hearing aids, telephone amplifying devices, TV or radio listening devices, assistive listening devices (for places like church, movie theaters, and other public places), doorbell, smoke detector, and alarm clock alerts, just to name a few. Cochlear implants can also be helpful for some who suffer from sensorineural deafness.
Some things to keep in mind when talking with anyone who suffers from hearing loss are:
Face the person and talk clearly.
Speak at a reasonable speed; do not hide your mouth, eat, or chew gum.
Use facial expressions or gestures to give useful clues.
Repeat yourself if necessary, using different words.
Include the hearing-impaired person when talking. Talk with the person, not about the person, when you are with others. This helps keep the hearing-impaired person from feeling alone and excluded.
Be patient & kind!


1. 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).

Sound waves are collected by the outer ear and channeled to the tympanic membrane. ↓↓↓↓↓↓↓↓↓↓↓ When sound waves are transmitted from the tympanic membrane along the middle ear ossicles, it causes the stapes to vibrate the oval window. ↓↓↓↓↓↓↓↓↓↓↓ Vibrations on the oval window produce pressure waves in the per lymph of the scala vestibuli and the scala tympani. These vibrations are transferred to the basilar membrane↓↓↓↓↓↓↓↓↓↓↓ Vibrations of the basilar membrane move the hair cells of the organ of Corti. The stereocilia of the hair cells bend when they move against the tectorial membrane. The bending generates a graded potential in the hair cell, which causes the release of a neurotransmitter at its base. The neurotransmitter, in turn, generates an action potential in dendrites of the cochlear nerve. Cell bodies of the cochlear nerve assemble in the spiral ganglia, and its axons merge with the vestibulocochlear nerve.↓↓↓↓↓↓↓↓↓↓↓ Pressure waves in the perilymph of the scala tympani cause the round window to bulge into the middle ear. This allows vibrational movements of the perilymph (and indirectly the endolymph) that, as an incompressible fluid, would not otherwise be able to vibrate within the surrounding rigid temporal bone.

2. 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.

Light rays bounce off all objects. If a person is looking at a particular object, such as a tree, light is reflected off the tree to the person's eye and enters the eye through the cornea. next, light rays pass through an opening in the iris, called the pupil. The iris controls the amount of light entering the eye by dilating or constricting the pupil. In bright light, the pupils shrink to prevent too much light from entering. In dim light, the pupil enlarges to allow more light to enter the eye. Light then reaches the lens. The lens focuses light rays onto the retina by bending or refracting them. The cornea does most of the refraction and the lens fine-tunes the focus. The lens can change its shape to provide clear vision at various distances. If an object is close, the ciliary muscles of the eye eyeDiagramLG.jpgcontract and the lens becomes rounder. To see a distant object the same muscles relax and the lens flattens. Behind the lens and in front of the retina is a chamber called the vitreous body, which contains a clear, gelatinous fluid called vitreous humor. Light rays passes through the vitreous before reaching the retina. The retina lines the back two-thirds of the eye and is responsible for the wide field of vision that most people experience. For clear vision, light rays must focus directly on the retina. When light focuses in front of or behind the retina, the result is blurry vision. The retina contains millions of specialized photoreceptor cells called rods and cones that convert light rays into electrical signals that transmit to the brain through the optic nerve. Rods and cones provide the ability to see in dim light and to see in color, respectively. The macula, located in the center of the retina, is where most of the cone cells are located. The fovea, a small depression in the center of the macula, has the highest concentration of cone cells. The macula is responsible for central vision, seeing color, and distinguishing fine detail. The outer portion (peripheral retina) is the primary location of rod cells and allows for night vision and seeing movement and objects to the side (peripheral vision). The optic nerve, located behind the retina, transmits signals from the photoreceptor cells to the brain. Each eye transmits signals of a slightly different image, and the images are inverted. Once they reach the brain, they are corrected and combined into one image.


All information was obtained from Human Physiology; Stuart Ira Fox; 12th Edition, Burton’s Microbiology For The Health Sciences; Paul G. Engelkirk and Janet Duben-Engelkirk; 9th Edition, http://www.wikipedia.org/_, http://www.howstuffworks.com/,_ http://www.physioweb.org/, and http://www.webmd.com/.
All sources for photos can be accessed directly by clicking on the photos.