The nervous system responds to external signals through nerve cells or nerve fibers (neurons). Surface and external

Receptors also feed signals into the system from the environment.  Collectively, sensations from the external environment are collected and sent to the CNS from extero, vicero and propioreceptors through sensory neurons.

The special spenses (smell, taste, eye, ear and balance) play a significant role serving as exteroreceptors or antennas, that collect and transmit external sensations from the environment to the brain.  The following discussions about the special senses are presented.


I.   Sensation

·        General sense of Awareness to changes

1.           Detect changes in the external environment

2.           Respond to the changes

3.           Maintain homeostasis


Characteristics of Sensation

1.     Projection (area of origin of sensation)

2.     Intensity: debt of sensation (light, medium, heavy)

3.     Contrast (comparison between intensities)

4.     Adaptation (voluntary suppression of sensation)

5.     Memory of sensation: Memory of sensation stored even after sensation


Pathways of Sensation

·        Receptors on the skin

·        Sensory neurons carry the message

·        Sensory tracts involve the white matter in CNS

·        Sensory area (cerebral cortex feels and interprets sensation)


Classification of Receptors

Receptors (similar to antennas) fall into two categories:

1.           Generalized receptors

·        Widespread throughout the body

·        Examples: Receptors for touch, pressure, pain and temperature

2.           Specialized receptors (special senses)

·        Located in organs with pathways in the brain

·        Examples: smell, taste, sight, hearing, balance

·        Also classified as somatic and visceral senses

·        Somatic Senses:  those receptors that are located on the body wall.  Examples are cutaneous (skin) receptors and those within muscle, joints, ligaments and tendons.

·        Visceral Senses:  receptor are located within visceral organs (e.g GI, lungs, etc).

·        Senses are also classified according to the location and type of stimuli they receive. There are three kinds of receptors

1.           Exteroreceptors: receive stimuli from the

    environment; they are located near the surface of   the body. Examples:

    -  Rod and cones on retina:  photoreceptors

    -  Hair cells in the ear:  mechanoreceptors

    -  Olfactory receptors in the nose: chemoreceptors

-   Taste receptors on the tongue: chemoreceptors

-   Skin receptors: tactile, mechano, thermo, nociceptors


2.           Visceroreceptors:  sensory receptors that produce sensations from organs or internal tissues.

-   Sensations include pain, hunger, thirst, fatigue or nausea. 

-   Visceroreceptors located within the circulatory system are sensitive to blood pressure changes are called barroreceptors

3.     Proprioceptors:  sensory nerves that relay information about the body position, equilibrium and movement.

-  They are located in the ear, joints, muscles and tendons


Types of Sensations

1.        Cutaneous sensation :

·        Contains free nerve endings for pain, touch and pressure

·        Protects the skin and provide info about temperature (heat and cold), pain, touch and pressure (covered nerved endings); 

·        Sensory areas in parietal lobes

·        Receptors for touch and pressure are encapsulated (ie they are covered)

2.        Muscle sensation: Provide awareness of our muscles.

a.      Stretch receptors in muscles; sensory nerves located in the parietal lobes

b.     Cerebellum coordinate voluntary motion


3.        Taste sensation:  Chemoreceptors located in the taste buds of the tongue;

a.       they detect chemicals present in food and in


b.      Sweet, sour, bitter and salty are four basic tastes

c.      Pathways; Facial and Glossopharyngeal nerves

 in temporal and parietal lobes


4.        Smell sensation: Chemoreceptors in the upper nasal cavities detect volatile (vaporized) chemicals

a.      Pathways: Olfactory nerves to olfactory bulbs connected to olfactory areas in the temporal lobe

b.     Smell and taste sensations are synergistic


5.        Hunger and Thirst

a.      Receptors in the hypothalamus

b.     Detect changes in nutrient levels and project to the stomach.

c.     Osmoreceptors detect changes in body water concentration (water-salt ratios). Thirst projected to the mouth/pharynx.


Sensory Areas in the Brain

The lobes of the brain participate in recognizing and interpretation of sensations.  These areas are :

·        Parietal lobe:      Generalized sensations (cutaneous sensations) muscular sensations are received and interpreted in this area. It also functions in speech comprehension and verbal articulation of thought and emotions. It also interpretes texture and shape of objects.

·        Temporal lobe:  Contains auditory centers that receive sensory neurons from the cochlea of the ear. It also interpretes some sensory experiences and stores information of both auditory and visual sensations.

·        Occipital lobe:   Concerned mainly with interpreting visual sensations.  It integrates eye movements by directing and focusing the eye. It is also responsible for visual association, that is, relating visual images with past experiences.

·        Frontal lobe:  The frontal lobe initiates voluntary motor sensations for the movement of skeletal muscles. It analyzes sensory experiences and provide information pertaining to the individual. In addition it is also involved in sensation related to emotions, reasoning, memory, judgment, planning and speaking.





Smell Sensation (Olfaction)


Olfaction involves chemical receptors which detect and transmit sensations of vaporizing chemicals emitted into the nasal passages.

·        Olfaction or the sense of smell involves chemicals. The receptors for smell are the olfactory hair cells located on the roof of each nasal cavity in a path of cells called the olfactory epithelium or membrane.

·        Odors or scents chemical vapors. Chemoreceptor cells (neurons) perceive the chemicals as they vaporize.

·        Air-bone chemical molecules bind to the Olfactory hairs or cilia which extend from dendrites of these neurons on the watery membrane surface.

·        Impulses are sent to the olfactory bulb (a structure whose neurons synapse with the dendrites of the receptor cells.

·        Axons of the neurons of the olfactory bulb travel to the brain by way of the olfactory nerve (Olfactory nerve #1) to olfactory areas of the temporal lobes.

·        Humans can distinguish tens of thousands of odors however, the receptors for smell are more sensitive in animals than in humans

·        Receptors for smell are sensitive, act quickly within a short period (about a minute) and adapt very quickly.

·        Smell influences our sense of taste and vice versa.


Taste Sensation (Gustation)

Taste buds are the receptors for taste and respond to chemicals dissolved in food.

·        Taste sensation work with smell sensation or they enhance each other. 

·        The receptors for taste are located in specialized organs called taste buds which are numerous on the surface of the epithelium covering the tongue in humans and mamals. The taste buds are also located on the papillae on the upper surface of the tongue and also on the roof of the oral cavity, the pharynx and larynx.

·        There are three kinds of papillae: vallate (back of tongue), fungiform (middle) and filiform (front or apex of tongue).

        -  Vallate: largest but least abundant, and “V-shaped”.

-         Fungiform: knob-like appearance; present on tip and  sides of tongue

-         Filiform: short, threadlike; most numerous

·        Vallate and fungiform are involved in the perception of all four primary taste: sweet, sour, bitter and salty. Filiform the most abundant is not involved in perception of taste.

·        Sweet taste is perceived on the tip of the tongue; salty taste occurs especially on the sides and most of the tongue; sour taste occurs on the sides; and bitter taste on the back.

·        Sourness is related to concentration of hydrogen ions in food (acids). Organic molecules taste sweet.             

·        Taste buds (contain receptor cells) are chemoreceptors stimulated by chemicals present in foods we eat. The chemicals dissolve in the saliva and enter the taste pores of the taste buds.

·        The taste pores contain taste hairs, part of the receptors of the taste buds. Chemicals dissolved in water bind to the receptor hairs and stimulate the receptor cells. The cells in turn stimulate the dendrites of the sensory nerve (#7 and #9). Impulses are then transmitted to the taste centers (parietal-temporal lobes) in the cerebral cortex.

·        Taste buds respond to all four primary flavors but are generally preferentially responsive to one


Visual Sensation

The eye transmits visual sensations of light (photosensations), and color sensations of all images. Transmission of these sensations occur through photoreceptors in the eye.


·        Eyelids and eyelashes; keep dust out

·        Lacrimal glands produce tears, cleans the eyes

·        Eyeballs: protected by the bony socket

·        Six extrinsic muscles move the eyeballs; Nerves #s 3, 4 and 6 cranial nerves stimulate the eyes. All six muscles originate from the back of the of the eye orbit and insert on the surface of the eye.


Structure of the eye

·        Eye coverings: sclera, choroids, retina, ciliary body

1.     Sclera: outermost covering (fibrous connective tissue

2.     Choroid: middle layer—dark pigment absorbs light

3.     Ciliary body/ligaments: suspend and change shape of lense for light refraction

4.     Iris—control diameter (aperture) of the eye

5.     Retina: innermost layer—contains rods and cones

a.       Cones: detect color, abundant in the Fovea Centralis

b.      Rods: detect light ; abundant around the edges of the retina

c.           Optic nerves conduct images to the eye and brain

6.        Posterior cavity: contains vitreous humor (semi solid fluid that keeps the eye in position

7.        Anterior cavity: contains aqueous humor produced from capillaries; maintains the lens and cornea



·        Vision occurs by refraction (bending and focusing of light rays) thru cornea, aqueous humor, lens and vitreous humor.

·        Ciliary muscles adjust lens for long or short vision.

     In long vision, ciliary muscle relaxes resulting in thining of the lense. In short vision, the muscle contracts, and the lens thickens.

·        Vision occurs when light stimulates the rods and cones causing chemical reactions in the retina.

·        Chemical breakdown in rods, and light absorption (red, blue and green) in cones produces impulses carried by optic nerves.

·        The optic nerves intersect before entering the brain. The point of intersect is called optic chiasma.  Within the optic chiama, fibers from one eye crosses over to the opposite side. The crossing over of the fibers


Disorders of the eye

·        Astigmatism:  unequal focusing of light rays on the retina resulting from unequal curvature of the cornea or lens. It causes part of the image to appear blurred.

·        Blindness:  partial or lack of vision. It may be caused by a numbers of disorders such as cataract, glaucoma and detachment or deterioration of the retina. It may also result from damage to the optic nerve #II.

·        Cataract:  cloudiness or apacity of the lens which impairs or prevents vison. It is common in older people and a leading cause of blindness. May be repaired by surgical removal and replacement of the lens by plastic lens to restore good vision

·        Color blindness:  inability to perceive certain colors or all colors (in rear cases).  The most common type of the disease is Red-green color blindness characterized by difficulty distinguishing red and green colors due to the absence of red or green cones. Color blindness is sex-linked hereditary trait and occurs more often in males than in females.

·        Conjuctivitis:  an inflammation of the conjuctiva. It may be caused by allergic reaction, physical or chemical causes or infections. Inflammation caused by viral or bacterial infection is called pinkeye (highly contagious).

·        Farsightedness (hyperopia): blurred vision that result from light rays that improperly focused behind the retina. It is caused by the eye being shorter than normal.

·        Glaucoma: A condition in which more aqueous humor is produced from the eye than is removed. The condition produced a high intraocular pressure that compress blood vessels resulting in damaging or dying of receptor cells. It is also an inherited disease.

·        Nearsightedness (Myopia): is blurred vision caused by light rays being incorrectly focused in front of the retina. It is commonly caused by the eye being longer than normal.

·        Presbyopia: diminished ability of the lens to accommodate for near vision due to a decrease in its elasticity. It is caused by natural aging and prevent around age 60 and over.

·        Retinoblastoma:  cancer of immature retinal cells and constitute about 2% of cancers in children.

·        Strabismus: disorder of the extrinsic eye muscle in which the eyes are not directed toword the same object simultaneously.  Treatment may include eye excise , corrective lens or surgery.


Auditory Sensation

The ear picks up sound vibrations that travel in air or water and transmit the signals through the vestibulocochlear (Acoustic) nerve # 8. 


 The ear consist of:

1.           Consist of outer auricle, middle ear and inner ear.

The outer ear (auricle) outer ear is a canal that leads to the middle ear.  It directs sound waves (vibrations)   into the ear.

2.    The middle ear contains the eardrum connected to the auditory bones (incus, malleus and stapes) and Eustachian tube (auditory tube);

3.    Inner ear consist of bony-space lined with tissue called membranous labyrinth. The space contains a fluid called perilymph. The structures within the inner ear (choclear, utricles, saccule and semicircular canals) are filled with another fluid called endolymph.

4.           The Cochlear contains the organ of Corti (receptors or hair cells containing cranial nerve #8.

5.    Uticle and saccule are sacs that contain the hair cells embedded in otoliths (calcium carbonate crystals). Gravity causes these hair cells to bend as the position  of head changes. The bending of the receptors carry impulses to the cerebellum  and temporal lobes of the brain that handles balance

6.    Semi circular canals (3): ampula contains hair cells that bend during motion and sends impulses to the brain via cranial nerves #8. Impulses as interpreted changing speeds, stopping or starting motions.