Special Senses Dr. Michael P. Gillespie
Special Senses • Receptors for the special senses – smell, taste, vision, hearing, and equilibrium – are anatomically distinct from one another. • These receptors are concentrated in very specific locations in the head. • They are usually embedded in the epithelial tissue within complex sensory organs such as the eyes and ears. • The neural pathways for the special senses are more complex than those of the general senses.
Smell & Taste • Smell and taste are chemical senses. They involve the interaction of molecules with the receptors. • Impulses from these sense propagate to the limbic system and higher cortical areas. Consequently, they evoke emotional responses and memories.
Chemosensor • A chemosensor, also known as a chemoreceptor, is a sensory receptor that transduces a chemical signal into an action potential. • A chemosensor detects certain chemicals in the environment.
Olfactory Receptors • There are between 10 – 100 million receptors for olfaction (sense of smell). • They are contained in the olfactory epithelium (5cm2)
Olfactory Receptors • 3 Kinds of cells • Olfactory receptors • 1st order neurons – bipolar • Olfactory hairs – cilia that project from the dendrite – respond to chemicals called odarants • Supporting cells • Columnar epithelium of mucous membrane • Support, nourish, detoxify chemicals • Basal cells • Stem cells – produce new olfactory cells (live approx. 1 month)
Olfactory Glands (Bowman’s) • Bowman’s glands are in the connective tissue that supports the epithelium. • They produce mucous which moistens the epithelial surface and dissolves the odorants.
Innervation • Branches of the facial nerve (CN VII) innervate the supporting cells and olfactory glands. • They stimulate the olfactory glands and the lacrimal glands. • The lacrimal glands produce tears from pepper and ammonia.
Physiology of Olfaction • There are hundreds of “primary” odors. • Humans can recognize about 10,000 different odors. • Different combinations of olfactory receptors stimulate different patterns of activity in the brain.
Odor Thresholds and Adaptation • All special senses have a low threshold including olfaction. • Methyl mercaptan can be detected with as little as 1/25 billionth of a milligram/ mL air. • Adaptation is a decrease in sensitivity. It occurs rapidly. 50% of the decrease occurs within the first second or so and then very slowly after that.
Olfactory Pathway • There are approximately 20 olfactory foramina on either side of the nose in the cribiform plate of the ethmoid bone. • 40 or so bundles of axons form right and left olfactory nerves (CN I). • They terminate in the olfactory bulbs – below the frontal lobes of the cerebrum.
Olfactory Pathway • Axons of the olfactory bulbs form the olfactory tract which projects to the primary olfactory area of the cerebral cortex. • Some project into the limbic system and hypothalamus (emotional and memory evoked responses. • Olfactory sensations are the only sensations that reach the cerebral cortex without first synapsing in the thalamus.
Olfactory Pathway • The primary olfactory area has axons that extend to the orbitofrontal area (frontal lobe) – region for odor identification.
Hyposmia • Hyposmia is a reduced ability to smell. • Women have a keener sense of smell than men, especially at ovulation. • Smoking impairs the sense of smell. • Age deteriorates the olfactory receptors. • Affects 50% over 65 and 75% over 80 years of age.
Hyposmia • Neurological changes impair the receptors. • Head injury, Alzheimer’s, Parkinson’s. • Medications impair receptors. • Antihistamines, analgesics, steroids.
Aromatherapy • Effects of smells on our psychology have been claimed. • Lavender, Orange Blossom, Rose, and Sage are said to be calming. • Sandlewood, Patachouli and Jasmine are said to alleviate mild depression. • Association areas of the brain.
Survival Function • Our sense of smell serves a survival function to help us select non-poisonous foods. • There are very few naturally occurring toxic vapors that are odorless. • Synthetic vapors often give false impressions to our senses. Our natural preferences can no longer be relied upon.
Gustation (Taste) • Chemical sense. • There are only 5 primary tastes that can be distinguished. • Sour, sweet, bitter, salty, umani (receptors stimulated by MSG) • All other flavors are combinations of the 5 primary tastes and smell.
Taste Receptor Cells • Contrary to popular belief, there is no tongue ‘map’. • Responsiveness to the five basic modalities – bitter, sour, sweet, salty, and umami – is present in all areas of the tongue. • The taste receptor cells are tuned to detect each of the five basic tastes. • A given gustatory receptor may respond more strongly to some tastants than others.
Taste Buds and Papillae • There are approximately 10,000 taste buds. • Most are on the tongue. • There are some on the soft palate, pharynx, and epiglottis. • Each taste bud is an oval body with 3 kinds of epithelial cells. • Supporting cells. • Gustatory receptor cells (life span of approx. 10 days). • Basal cells.
Epithelial Cells on Taste Buds • The Supporting Cells surround approximately 50 gustatory receptor cells in a taste bud. • The Gustatory Receptor Cells synapse with 1st order neurons. ! 1st order neurons contacts many gustatory receptor cells. • The Gustatory Hair (microvillus) projects through the taste pore. • The Basal Cells are stem cells at the periphery of the taste bud. They produce supporting cells which will develop into gustatory cells.
Taste Buds • The taste buds are found in elevations on the tongue. • Vallate (circumvallate) papillae • Fungiform papillae • Foliate papillae • Filiform papillae
Vallate (Circumvallate) Papillae • About 12 very large circular vallate papillae form an inverted V-shaped row at the back of the tongue. • Each of these papillae contains approximately 100-300 taste buds.
Fungiform Papillae • The Fungiform (mushroom like) papillae are mushroom shaped elevations scattered over the entire surface of the tongue. • They contain about 5 tastebuds each.
Foliate Papillae • The foliate (leaflike) papillae are located in small trenches on the lateral margins of the tongue, but most of their taste buds degenerate in early childhood.
Filiform Papillae • Filiform papillae cover the entire surface of the tongue. • They are pointed, threadlike structures that contain tactile receptors but no taste buds. • They increase friction between the tongue and the food, making it easier for the tongue to move food into the oral cavity.
Tastants • Tastants are chemicals that stimulate gustatory receptors. • Tastants dissolve in saliva. They can then make contact with the plasma membrane of the gustatory hairs, which are the sites of taste transduction. • This generates a receptor potential, which in turn triggers nerve impulses with first-order sensory neurons.
Tastant Stimulation of Gustatory Receptors • Different tastants stimulate the gustatory receptors in different ways to generate the receptor potential. • The sodium ions in salty foods enter the gustatory receptor cells via Na+ channels in the membrane. • The hydrogen ions in sour tastants flow in through H+ channels.
Tastant Stimulation of Gustatory Receptors • Other tastants (sweet, bitter, and umami) do not enter the gustatory receptor cells. They bind to receptors on the plasma membrane. They trigger second messengers in the cell. • All tastants ultimately result in the release of neurotransmitters from the gustatory receptor cell. • Different foods taste different because of the patterns of nerve impulses in groups of first-order neurons that synapse with the receptors.
Taste Thresholds • The threshold for taste varies for each of the primary tastes. • The threshold for bitter substances (i.e. quinine) is the lowest. • Poisonous substances are often bitter. • The threshold for sour substances (i.e. lemon) is somewhat higher. • The thresholds for salty substances and sweet substances are similar and higher than the others.
Taste Adaptation • Complete adaptation of a taste can occur in 1-5 minutes if continuous stimulation.
Gustatory Pathway • Three cranial nerves contain axons for the gustatory pathways. • Facial Nerve (CN VII) – serves taste buds in anterior 2/3 of the tongue. • Glossopharyngeal Nerve (CN IX) – serves taste buds in the posterior 1/3 of the tongue. • Vagus Nerve (CN X) – serves taste buds in the throat and epiglottis. • Impulses propagate to the gustatory nucleus in the medulla oblongata.
Gustatory Pathway • Some axons carrying taste signals project into the limbic system and the hypothalamus. • Others project to the thalamus and from there to the primary gustatory area in the parietal lobe of the cerebral cortex. • This allows us to perceive taste.
Taste Aversion • The taste projections to the hypothalamus and limbic system account for the strong association between taste and emotions. • Sweet foods evoke reactions of pleasure, while bitter foods can evoke reactions of disgust. This is true even in newborn babies. • Animals learn to avoid foods that upset the digestive system. This is known as taste aversion. • Certain medications cause upset stomach and can cause taste aversion to all foods.
Vision • Sight is extremely important for human survival. • More than half of the sensory receptors in the human body are located in the eyes.
Electromagnetic Radiation • Electromagnetic radiation is energy in the form of waves that radiate from the sun. • Many types: • Gamma rays, X-rays, UV rays • Visible light • Infrared radiation, Microwaves, Radio waves • The range of electromagnetic radiation is known as the electromagnetic spectrum.
Wavelength • The distance between two consecutive peaks of an electromagnetic wave is the wavelength. • Wavelengths range from short to long. • Gamma rays – short than a nanometer. • Radio waves – greater than a meter • The eyes are responsible for the detection of visible light.
Wavelength • The color of visible light depends upon its wavelength. • Wavelength of 400 nm is violet • Wavelength of 700 nm is red • An object will absorb certain wavelengths of light and reflect others. It will appear the color of the wavelengths it reflects. • White – reflects all wavelengths of visible light. • Black – absorbs all wavelengths of visible light.
Accessory Structures of the Eye • Eyelids • Eyelashes • Eyebrows • Lacrimal apparatus • Extrinsic eye muscles
Eyelids • The palpebrae or eyelids (palprbra – singlular) shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and spread lubricating secretions over the eyeballs. • The upper eyelid contains the levator palpebrae superioris muscle and is more moveable than the lower. • The lacrimal caruncle is a small, reddish elevation on the medial border and contains sebaceous (oil) and sudoriferous (sweat) glands.
Eyelids • The Meibomian glands are embedded in the eyelids and secrete fluid that prevents the eyelids from adhering to each other. Infection of the glands produces a cyst known as a chalazion. • The bulbar conjunctiva passes from the eyelids to the surface of the eyeball and covers the sclera (“white” of the eye). The conjunctiva is vascular. Irritation or infection cause bloodshot eyes.
Eyelashes and Eyebrows • The eyelashes and eyebrows help protect the eyeballs from foreign objects, perspiration, and the direct rays of the sun. • Sebaceous ciliary glands are located at the base of the hair follicles of the eyelashes. The release a lubricating fluid into the follicles. • Infection of these glands results in a sty.
Lacrimal Apparatus • The lacrimal apparatus is a group of structures that produces and drains lacrimal fluid or tears. • The lacrimal ducts empty tears onto the surface of the conjunctiva of the upper lid.
Lacrimal Apparatus • The fluid passes into through the lacrimal puncta, into lacrimal canals, to the lacrimal sac, and then into the nasolacrimal duct. • The lacrimal glands are supplied by parasympathetic fibers of the facial nerves (VII). • Tears are cleared away by either evaporation or by passing into the lacrimal ducts.