Somatic Nervous System & Special Senses

Chapter 12

Special Senses

Part B

Sciences of Special Senses

• Ophthalmology – is the science that deals with the eye and its disorders

• Otorhinolaryngology – deals with all the other senses.

Olfaction: Smell

• Sense of smell • Requires 10 million to 100 million

receptors

Structures

• Olfactory epithelium • Located in the upper portion of the nasal

cavity • Consists of three types of cells

• Olfactory receptors • Supporting cells • Basal stem cells

Olfactory Receptors

• Are the stimulated by olfactory hairs, which project from knob-shaped tip of the olfactory receptor• Odorants (chemicals in the air) stimulate

the olfactory hairs • Short life span – only about 1 month

• See nose hairs are good

Supporting Cells

• Columnar epithelial cells of the mucous membrane lining of the nose

• Provide physical support, nutrients, and electrical insulation for the olfactory receptors

•

Basal Cells

• Cells that are responsible for producing new olfactory receptors

Olfactory glands

• Responsible for the production of mucus that moistens the surface the surface of the olfactory epithelium and serves as a solvent for inhaled odorants.

Stimulation of Olfactory Receptors

• Unique because adaptation to a chemical is very rapid• You may smell something bad but over

time if you remain in that environment you won’t smell it anymore.

Hyposmia

• Reduced ability to smell, affects over half of those over the age of 65 and 75% over 80.

• Caused by head injury, Alzheimer’s disease or Parkinson’s disease

• Also by certain drugs, such as antihistamines, or steriods

Gustation: Taste

• Five primary tastes• Sour• Sweet• Bitter• Salty• Umani

Umami

• Describe as a “savory” taste

Taste Buds• Taste buds – are the location of receptors for

taste• Taste buds are found on the papillae, which are

the bumps on the tough

• Number of tastes bud decrease with age• Location

• Mostly on the tongue • Some on the roof of mouth, throat and epiglottis

Tastants

• Chemicals that activate the gustatory receptors

Stimulation

• Tastants are dissolved in saliva which then allows them to enter the pores and come into contact with the gustatory hairs.

• Stimulation of hairs causes an electrical impulse to be sent to the brain

Vision

• More than half the sensory receptors in the human body are found in the eye

• The largest part of the cerebral cortex is devoted to vision

Careers Associated with vision

• Ophthalmologist – physician who specializes in the diagnosis and treatment of eye disorders with drugs, surgery and corrective lenses

• Optometrist – has a doctorate of optometry and is licensed to test the eyes and treat visual defects b y prescribing corrective lenses

• Optician – technician who fits, adjusts and dispenses corrective lenses using the prescription supplied by an ophthalmologist or optometrist

Accessory structures of the eye

• Accessory structures – are eyebrows, eyelashes, eyelids, muscles the move eyeballs and lacrimal (tear) apparatus

Eyelashes and Eyebrows

• Protect the eyeballs from foreign objects, perspiration, and direct rays from sun

Eyelids

• Upper and lower • Shade the eye during rest • Protect eyes from excessive light • Protect eyes from foreign objects • Spread lubricating secretions over the

eyeballs by blinking

Eye Muscles

• Six muscles control the movement of the eyeball • Right, left, up, down and diagonally

Lacrimal apparatus

• Group of glands, ducts, sacs that produce and drain lacrimal fluid as tears

• Lacrimal glands (one for each eye)• About the size and shape of an almond

and are responsible for secreting lacrimal through the lacrimal ducts to the eyeballs

• Tears can also pass through the nasolacrimal duct, which allows lacrimal to drain into your nasal cavity

Lacrimal

• Is a watery solution containing salts, some mucus, and a bacteria killing enzyme called lysozyme

Layers of the eyeball

• Adult eyeball measures about 2.5 cm across

• Three layers • Fibrous Tunic• Vascular Tunic• Retina

Fibrous Tunic

• Outer coat of the eyeball • Consists of

• Cornea – transparent fibrous coating that colors the iris (colored portion of your eye)

• Also helps focus light • Sclera – is the “white” of the eye, dense

connective cove • Gives the shape of the eye, makes it more rigid and

protects inside of eyeball • Conjunctiva – epithelial layer, covers the anterior

surface of eyeball and lines inner surface of eyelid

Corneal Transplant

• If your cornea is damaged it can be replaced with a donor cornea

Vascular Tunic• Middle Layer of the eye• Composed of:

• Choroid – thin membrane that lines most of the internal surfaces of the sclera

• Contains many blood vessels that nourish the retina• Ciliary body – consists of

• ciliary processes, which are folds on the inner surface of the ciliary body whose capillaries secrete a fluid called aqueous humor and the

• ciliary muscles, a smooth muscle that alters the shape of the lens for viewing objects up close or at a distance

• Lens – transparent structure that focuses light rays onto the retina

• Constructed of many layers of elastic protein fibers • Attached to ciliary muscles by zonular fibers

Iris and pupil

• Part of the vascular tunic • Iris

• Colored portion of the eye• Muscle that regulates the amount of light

let into the eye • Pupil

• Hole in the center of the eye through which light enters

Retina

• Portion that lines posterior of the eye about ¾ of the eyeball

• Consists of two layers • Neural layer • Pigmented layer

Neural Layer

• Consists of three distinct layers • Photoreceptor layer • Bipolar cell layer • Ganglion cell layer

Photoreceptor layer

• Two types of cells • Rods – allow us to see shades of gray in

dim light (6 million) • Cones – allow us to see color (120 million)

• Found in central fovea in the center of the macula lutea

Pigmented Layer

• A sheet of melanin is located between the choroid and the neural part of the retina• Melanin absorbs stray light, helping to

keep the image sharp and clear

Pathway

• Photoreceptor layer to • Bipolar cell layer to • Ganglion cell layer to• Optic nerve

Interior of Eyeball

• Two cavities divided by the lens • Anterior cavity • Vitreous cavity

Anterior cavity

• Contains aqueous humor that helps maintain the shape of the eye and supplies oxygen and nutrients to the lens and cornea.

Vitreous Chamber

• Contains the vitreous body, which helps maintain the shape of the eyeball and keeps the retina attached to the choroid• Vitreous body – develops in embryonic life

and is not replaced • Choroid – provides blood supply and

absorbs scattered light

Intraocular pressure

• Pressure in the eye • Maintains the shape of the eye • Keeps the retina pressed against choroid

• Normal pressure is 16mm of Hg