Name Period SPECIAL SENSES The Senses General senses of touch o Temperature o Pressure o Pain Special senses o Smell o Taste o Sight o Hearing o Equilibrium The Eye and Vision 70 percent of all sensory receptors are in the eyes Each eye has over a million nerve fibers Protection for the eye o Most of the eye is enclosed in a bony orbit o A cushion of fat surrounds most of the eye Accessory Structures of the Eye
Eyelids Eyelashes Meibomian or tarsal glands modified sebacious glands of the eyelid produce an oily secretion to lubricate the eye Ciliary glands modified sweat glands between the eyelashes Conjunctiva o Membrane that lines the eyelids o Connects to the surface of the eye o Secretes mucus to lubricate the eye Lacrimal apparatus o Lacrimal gland produces lacrimal fluid located above the lateral end of each eye o Lacrimal canals drains lacrimal fluid from eyes into the lacrimal sac medial location Lacrimal sac provides passage of lacrimal fluid towards nasal cavity Nasolacrimal duct empties lacrimal fluid into the nasal cavity Function of the Lacrimal Apparatus Properties of lacrimal fluid o Dilute salt solution (tears) o Contains antibodies and lysozyme, an enzyme that destroys bacteria Protects, moistens, and lubricates the eye Empties into the nasal cavity (sniffles!) Six Extrinsic Eye Muscles Muscles attach to the outer surface of the eye Produce eye movements
Structure of the Eye The wall is composed of three tunics o Fibrous tunic or sclera outside layer o Vascular tunic or choroid middle layer o Sensory tunic or retina inside layer
The Fibrous Tunic Sclera Cornea o White connective tissue layer o Seen anteriorly as the white of the eye o Transparent, central anterior portion o Allows for light to pass through o Repairs itself easily o The only human tissue that can be transplanted without fear of rejection (no blood vessels) Vascular Tunic or Choroid Layer Blood-rich nutritive tunic w/ dark pigment Pigment prevents light from scattering Modified interiorly into two structures o Cilliary body smooth muscle Attaches to lens by ciliary zonule, a ligament o Iris smooth muscle Sensory Tunic (Retina) Pigmented layer that gives eye color Pupil rounded opening in the iris Contains receptor cells (photoreceptors) o Rods (B/W) o Cones (Color) Signals pass from photoreceptors via a two-neuron chain o Bipolar neurons o Ganglion cells Signals leave the retina toward the brain through the optic nerve Neurons of the Retina and Vision Rods o Most are found towards the edges of the retina o Allow dim light vision and peripheral vision o Perception is all in gray tones
Cones o Allow for detailed color vision o Densest in the center of the retina o Fovea centralis area of the retina with only cones lateral to each blind spot No photoreceptor cells are at the optic disk, or blind spot where the optic nerve leaves the eyeball Cone Sensitivity There are three types of cones Different cones are sensitive to different wavelengths o One blue, another green and the 3 rd a range including green and red ( red cones ) Color blindness is the result of lack of one cone type
Lens Biconvex crystal-like structure Held in place by a suspensory ligament attached to the ciliary body Internal Eye Chamber Fluids Aqueous humor o Watery fluid found in chamber between the lens and cornea o Similar to blood plasma; secreted by a special area of choroid o Helps maintain intraocular pressure o Provides nutrients for the lens and cornea o Reabsorbed into venous blood through the scleral venous sinus or canal of Schlemm Vitreous humor o Gel-like substance behind the lens o Keeps the eye from collapsing o Lasts a lifetime and is not replaced Lens Accommodation Light must be focused to a point on the retina for optimal vision The eye is set for distance vision (over 20 ft away) The lens must change shape to focus for closer objects
Images Formed on the Retina Visual Pathway Photoreceptors of the retina Optic nerve bundle of axons carrying retinal impulses Optic nerve crosses at the optic chiasma Optic tracts contain fibers from the lateral side of the eye on the same side and the medial side of the opposite eye Optic tract fibers synapse w/ neurons in the thalamus Thalamus (axons form optic radiation) Visual cortex of the occipital lobe Eye Reflexes Internal muscles are controlled by the autonomic nervous system o Bright light causes pupils to constrict through action of radial and ciliary muscles o Viewing close objects causes accommodation External muscles control eye movement to follow objects Viewing close objects causes convergence (eyes moving medially)
The Ear Houses two senses o Hearing o Equilibrium (balance) Receptors are mechanoreceptors Different organs house receptors for each sense Anatomy of the Ear The ear is divided into three areas o Outer (external) ear o Middle ear o Inner ear The External Ear Involved in hearing only Structures of the external ear o Pinna (auricle) o External auditory canal The External Auditory Canal Narrow chamber in the temporal bone
Lined with skin Ceruminous (wax) glands are present Ends at the tympanic membrane The Middle Ear or Tympanic Cavity Air-filled cavity within the temporal bone o Flanked laterally by eardrum and medially by a bony wall with 2 openings, the oval window & the inferior, membrane-covered round window Only involved in the sense of hearing Two tubes are associated with the inner ear o The opening from the auditory canal is covered by the tympanic membrane o The auditory tube connecting the middle ear with the throat Allows for equalizing pressure during yawning or swallowing This tube is otherwise collapsed Bones of the Tympanic Cavity Three bones (ossicles) span the cavity o Malleus (hammer) o Incus (anvil) o Stapes (stirrip) Vibrations from eardrum move the malleus,which moves the anvil, which moves the stirrup and presses on the oval window of the inner ear These bones transfer sound to the inner ear Inner Ear or Bony Labyrinth Includes sense organs for hearing and balance Filled with perilymph A maze of bony chambers within the temporal bone o Cochlea o Vestibule o Semicircular canals Chemical Senses Taste and Smell Both senses use chemoreceptors o Stimulated by chemicals in solution o Taste has four types of receptors o Smell can differentiate a large range of chemicals
Both senses complement each other and respond to many of the same stimuli Olfaction The Sense of Smell Olfactory receptors (neurons with long cilia) are in the roof of the nasal cavity o Chemicals must be dissolved in mucus for detection and impulse transmission by the hairs (cilia) Impulses are transmitted via the olfactory nerve Interpretation of smells is made in the cortex Cortex stores snapshots of smells & is tied to the limbic system (emotions) Olfactory Epithelium The Sense of Taste Taste buds (10,000+) house the receptor organs Location of taste buds o Most are on the tongue o Soft palate o Cheeks The Tongue and Taste The tongue is covered with projections called papillae o Filiform papillae sharp with no taste buds o Fungifiorm papillae rounded with taste buds o Circumvallate papillae large papillae with taste buds Taste buds are found on the sides of papillae Structure of Taste Buds Gustatory cells are the receptors o Have gustatory hairs (long microvilli)
o Hairs are stimulated by chemicals dissolved in saliva Depolarizes the nerve and the impulse is carried to the brain Impulses are carried to the gustatory complex by several cranial nerves because taste buds are found in different areas o Facial nerve o Glossopharyngeal nerve o Vagus nerve Anatomy of Taste Buds Taste Sensations Sweet receptors o Sugars o Saccharine o Some amino acids Sour receptors o Acids Bitter receptors o Alkaloids Salty receptors o Metal ions
Developmental Aspects of the Special Senses Formed early in embryonic development Eyes are outgrowths of the brain o Developing by the 4 th week o Eyeballs enlarge until age 8 or 9 (lenses throughout life) o Babies can t see near at birth (hyperopic) All special senses are functional at birth