THE FIVE SENSES

The 5 Senses• Sight (Eye)• Hearing (Ear)• Taste (Tongue)• Smell (Nose)• Touch (Skin)

The Sixth Sense

Sight – the Eye

The Eye• About 25mm (1 inch) in diameter• Consists of:• 3 tunics (layers)• A lens• 2 principal cavities

Fibrous Tunic (Outer Layer)•2 parts:•Sclera•Made of dense connective tissue•White part of the eye•Cornea• Forms the anterior surface of the eye•Transparent

Vascular Tunic (Middle Layer)•3 Parts:•Choroid• Thin, vascular layer• Supplies nutrients and oxygen to the eye• Absorbs light•Ciliary Body• Thickened, anterior portion of the vascular tunic• Contains muscle fibers that regulate the shape of the lens

• Iris• Forms the most anterior part of the vascular tunic• Consists of pigment (gives the eye color) and regulates the diameter of the pupil

Internal Tunic (Inner Layer/Retina)•2 Types of Photoreceptors• Rods• Approximately 100 million per eye• Responsible for night (black and white) vision• Cones• Approximately 7 million per eye• Responsible for daytime (color) vision

•Optic Disk• “Blind spot” of the eye – no rods or cones• Forms the start of the optic nerve•Optic Nerve (cranial nerve II)•Transmits information from the retina to the brain

Cow Eye•Tapetum•Extra layer between retina and choroid•Night Vision

Lens•Focuses light rays for near and far vision•Biconvex structure•Composed of tightly arranged proteins•Held in place by the suspensory ligament that attaches to the ciliary body

Cavities of the Eye• Interior of the Eye is separated by the lens into:•Anterior cavity• Further divided into an anterior and posterior chamber by the iris• Contains a watery substance called aqueous humor

•Posterior cavity• Contains a jellylike substance called vitreous humor

Vision•The lens must change shape to focus light on the retina•The farther away an object, the flatter the lens must become•Adjustments in shape are controlled by the ciliary muscles in the ciliary body

•Light rays hit the photoreceptors in the retina•Nerve impulses are carried along the optic nerve to the optic chiasma and to the occipital lobe of the cerebrum

Hearing – The Ear

The Ear•Organ of hearing and equilibrium (balance)•Three regions:•Outer Ear•Middle Ear• Inner Ear

The Outer Ear•Open to the external environment•Directs sound waves to the middle ear•Consists of:•Auricle (pinna)•Directs sound waves to the external auditory canal•External auditory canal• A 2.5cm fleshy tube that fits into the bony external acoustic meatus

•Tympanic membrane (eardrum)• Conducts sound waves to the middle ear

The Middle Ear•The air-filled space medial to the tympanic membrane•Structures include:• Auditory ossicles• Three small bones that extend from the tympanic membrane to the vestibular window• Hammer, anvil, and stirrup (amplify sound waves)

• Auditory muscles• Two tiny muscles that function to reduce the pressure of loud sounds

• Auditory (eustachian) tube• Connects the middle ear cavity to the pharynx• Drains moisture from the middle ear cavity to equalize pressure on both sides of the eardrum

The Inner Ear•Contains the organs of hearing, balance, and equilibrium•Bony labrynth•Semicircular canals (3) & Ampulla: contain receptors that are sensitive to the rotation of the head•Vestibule: contains receptors that are sensitive to gravity and linear motions of the head• Contains fluids called endolymph and perilymph•Vestibular window: membrane covered opening from the middle ear to the inner ear; transfers sound waves from the auditory ossicles to the cochlea•Cochlea: contains hair cells that vibrate and send nerve impulses to the cochlear nerve (cranial nerve VIII)

Hearing1. Sound waves are funneled by the auricle into the

external auditory meatus2. The sound waves strike the tympanic membrane,

causing it to vibrate3. Vibrations of the tympanic membrane are amplified as

they pass through the hammer, anvil, and stirrup4. The vestibular window is pushed back and forth by

the stirrup setting up pressure waves in the perilymph of the cochlea

5. The pressure waves are propagated to the endolymph contained within the cochlear duct

6. Stimulation of hair cells within the spiral organ of the cochlea causes the generation of nerve impulses in the cochlear nerve, which pass to the pons of the brain

Taste – The Tongue

Tongue/Taste•Taste buds•Peglike projections of the tongue called lingual papillae•One taste bud contains a cluster of 40-60 gustatory cells, each innervated by a sensory neuron

•Taste sensations•Sweet (front)•Sour (sides)•Bitter (back)•Salty (front sides)•Taste•Sensations are transmitted to the brain stem, then to the thalamus, and finally to the cerebral cortex

Smell – The Nose

Smell•Receptors for smell are located in the nasal mucosa of the superior nasal concha•Airborne chemicals become dissolved in the mucous layer lining the superolateral part of the nasal cavity•The olfactory nerve (cranial nerve I) transmits impulses related to smell•Sensations are conveyed along each olfactory tract to the olfactory portions of the cerebral cortex

Touch – The Skin