Chapter 17 Sense Organs n General senses n

Chapter 17 Sense Organs n General senses n. Taste n. Smell n. Hearing n. Equilibrium n. Vision

The general senses Refers to the senses that are relatively simple in structure and physiology (touch, pressure, etc). n Their receptors tend to have a structure that is specialized to detect a specific stimulus. n They may so simple as to be simple nerve dendrites. n Unencapsulated receptors are sensory dendrites that lack a connective tissue wrapping (e. g. free nerve endings, Merkels discs and hair receptors). n Encapsulated receptors are dendrites with a glial cell wrap or connective tissue covering (e. g. Meissners corpuscles, Pacinian corpuscles muscle spindles). n

Receptor Classification based on origin of stimulus 1. 2. 3. Exteroceptors- receptors sensitive to changes outside of body. Are found on or near the body surface: pain, pressure, touch, temperature and special sense organs (eye, ear, nose, mouth). Interoceptors- visceroceptors receive stimuli from viscera; stretch, temperature, chemical, taste. Proprioceptors- located in skeletal muscles, joints, tendons, and ligaments; perceive stretch in these organs

Proprioceptors n Encapsulated receptors that monitor stretch in muscles and tendons. Ex. Muscle spindles Extrafusal fibers Intrafusal

Classified by Stimulus Modality 1. 2. 3. 4. 5. Mechanoreceptors- respond to mechanical forces: touch, pressure, stretch, vibration, and itch. Thermoreceptors- respond to temperature changes Chemoreceptors- respond to chemicals in solution and blood chemistry. Photoreceptors- respond to changes in light-eye Nociceptors- respond to pain and harmful stimuli leading to pain.

Classification by Structure 1. Free nerve endings- are found in all tissues of body; especially abundant in epithelial and connective tissue. - respond to pain, temperature, itch and light pressure Ex. - Merckels discs, hair follicle receptors 2. Encapsulated nerve endings- enclosed in a capsule of connective tissue; vary in shape and distribution. Ex. - Meissner’s, Pacinian and Ruffini’s corpuscles

Taste “Gustation” n Taste receptors are in taste buds- chemoreceptors Occur on tongue, palate, cheeks, pharynx and epiglottis as papillae. - - Tongue has 4 types of papillae: a). Filoform; b). Foliate; c). Fungiform and d). Circumvallate. In adults last 2 types function in taste on surface of tongue: - n Fungiform papillae widely distributed on tongue Circumvallate papillae form a “V” at back of tongue Filoform papillae do not sense taste, but the texture of foods. Taste bud = 50 -100 epithelial cells consisting of supporting, basal and gustatory cells

Taste buds

Tastes n There are five tastes that we can discriminate when presented to the tongue and mouth - Sweet - Sour - Bitter - Salty - Umami Taste is perceived by taste receptors than relayed to brain via: CN VII (Facial), IX (Glossopharyngeal) and X (Vagus) nerves. n Taste is highly dependent on smell for full perception of the odor, without smell we lose much n

Taste pathways to CNS via cranial nerves VII = ant 2/3 tongue IX = post 1/3 tongue and pharynx X = pharynx and epiglottis

Smell “Olfaction” n n n Chemoreceptors located in superior nasal concha and septum in the olfactory epithelium (nasal mucosa). ~ 5 sq. cm. Olfactory epithelium is pseudostratified columnar Odors must be in liquid state to be perceived, thus they mix with mucus prior to be sensed by receptors. ~ 10, 000 different odor molecules and ~ 1, 000 different receptor types. Average human can detect 2, 000 - 4, 000 odors. Olfactory receptor cells are bipolar and in cribriform plate of ethmoid bone. Olfactory nerve dendrites have olfactory cilia that bind the odors to them. The odor is relayed to the olfactory nerve axon which synapses with the mitral cell dendrites of the olfactory bulb → brain.

Olfactory pathway

Hearing and Equilibrium Is mediated through the structures of the inner ear. n These structures are all embedded in the temporal bones of the cranium n Hearing involves the perception of sound waves entering the external auditory canal which is received by the middle ear and relayed to the inner ear where it is then registered in the cochlea. n Equilibrium is maintained and perceived by the structures in the semi-circular canals of the inner ear. n

Outer ear anatomy Pinna – auricle covered with skin and made of elastic cartilage; helix and lobule n External auditory meatus- ear canal lined with skin and ceruminous glands passes through temporal bone n Tympanic membrane – “ear drum” separates outer ear from middle ear and is innervated with sensory fibers from trigeminal and vagus nerves. Responds to air vibrations or sound waves and transfers energy to middle ear ossicles. n

Hearing and Equilibrium

Middle ear anatomy n n n Three middle ear ossicles joined together: Malleus – looks like a hammer attaches to tympanic membrane and connected to the incus. Incus - Looks like an anvil and connected to the stapes. Stapes- looks like a stirrup – footplate attaches to oval window of inner ear where it creates ossicilations in the scala vestibuli Middle ear muscles: stapedius inserts on stapes; tensor tympani inserts on malleus. Contract during loud noises and relieve tension on ossicles to protect inner ear. Eustachian tube connects middle ear to nasal cavity

Middle ear anatomy

Inner ear anatomy Housed in a maze of temporal bone passages called the bony labyrinth. n Bony labyrinth is filled with perilymph and is lined with periosteum; perilymph is continuous with CSF n n n Bony labyrinth consists of: -a). vestibule, b). cochlea and c). semicircular canals Membranous labyrinth lines inside of bony labyrinth. It is lined with epithelium and is filled with endolymph (high in Na+, K+). n Consists of semicircular ducts, utricle and saccule and the cochlear duct.

Inner ear anatomy

Structures of hearing n Cochlea –spiraling chamber of bony labyrinth. Looks like a snails shell, and spirals around modiolus. n Lined with a membranous labyrinth with 3 separate chambers: n Scala vestibuli –filled with perilymph abuts oval window. n Cochlear duct –filled with endolymph and contains organ of Corti n Scala tympani –filled with perilymph abuts round window. n Scala vestibuli an scala tympani are joined by the apex of cochlea. heliocotrema at

Organ of Corti

Structures of hearing

Innervation from CN-VIII

Cochlear portion of VIII pathway to CNS for hearing.

Structures of equilibrium Vestibule houses the saccule and utricle which each contain a macculae that are receptor cells for head position when the head is still = static equilibrium. They also monitor head movements in a straight line = linear acceleration. n Semicircular canals are lined with a membranous semicircular duct. n Ampulla is bulge at each end of canal and inside is the membranous ampulla which contains a crista ampullaris. n These receptors detect head rotation or angular rotation. n

Macculae structure In utricle and saccule. Respond to: - Static equilibrium - Linear acceleration

Structures of equilibrium n Semicircular canals are lined with a membranous semicircular duct. Ampullae is bulge at each end of semicircular canals. n Membranous ampullae contains a crista ampullaris. n These receptors detect head rotation or angular rotation and dynamic equilibrium. n

Semicircular ducts with ampullae and Crista Ampullaris. Respond to: -Detect angular rotation -Dynamic equilibrium n

The Eye and Vision Visual receptors account for ~ 70% of all sensory input in the body. 40% of the cerebral cortex in involved in processing visual perception. n Eye balls are the sense organs for vision n Eye is a sphere ~ 1” in diameter and is located in the orbital cavity of the skull n

Accessory structures of eye Eyebrows n Eyelids with tarsal and ciliary glands. n Eyelashes n Lateral and medial canthus (angles at corner of eye n Conjunctiva n Lacrimal apparatus n Extrinsic eye muscles (6) n

Accessory structures of eye

Accessory structures of eye

Eye accessory structures n Eyelids “palpebrae” protect eyes from foreign objects and bright light. - Two lids separated by palpebral fissure - - Angle of eyes is canthus- medial and lateral canthi Lacrimal caruncle is at median canthus Levator palpebrae muscle raises and lowers eye lids Tarsal glands in eyelids secrete oily substance that keeps eyes moist. Chalazion Eyelashes on the margin of each lid help to keep substances from entering eye; ciliary glands open into lash follicle. Sty

Eye accessory structures n Conjunctiva - transparent mucous membrane covering inner surface of eyelids (palpebral conjunctiva) and anterior surface of eye (bulbar conjunctiva), but not the cornea. -made up of stratified columnar epithelium with numerous goblet cells that secrete mucous and keep eyeball and lids moist. -when eye is closed a slit-like space forms= conjunctival sac. n Lacrimal apparatus keeps surface of eye moist with tears from lacrimal glands. From upper lateral position, tears washover eyes when you blink eyes and move to medial canthus to drain into lacrimal puncta → → canaliculi → lacrimal sac and → nasolacrimal duct → nasal cavity. =- Lacrimal fluid “tears” contain mucus, antibodies and lysozymes that destroy bacteria.

Eye accessory structures Extrinsic eye muscles control all movements of eyeball in the socket. n Six muscles: n Superior rectus: CN III- elevates eye n Inferior rectus: CN III- depresses eye n Medail rectus: CN III- moves eye medially n Lateral rectus: CN VI- moves eye laterally n Superior oblique: CN IV- depresses eye and turns it laterally n Inferior oblique: CN III- elevates eye and turns it laterally III- occulomotor; IV- trochlear; VI- abducens n

Extrinsic eye muscles

Eye anatomy The eye is a sphere with a bulge (cornea) at the front and a stem at the back (optic nerve). The outside is covered by a tough outer covering called fibrous tunic. n Consists of 3 tunics (fibrous, vascular and sensory) and 2 chambers (anterior and posterior) separated by a lens and iris. n The anterior chamber is filled with aqueous humor n Posterior chamber is filled with vitreous body n n The visual receptor field (retina) occupies the major portion of the posterior wall of the eye and light

Fibrous tunic n The fibrous tunic consists of the cornea and sclera. Cornea is avascular and transparent and allows light to pass through. It is made of 100’s of sheets of collagen fibers sandwiched between two layers of epithelium. Highly innervated and sensitive to touch or particles on it. n Sclera is the white of the eye which protects, shapes and serves as the anchor site for the extrinsic eye muscles n Limbus is junction between sclera and cornea- stem cells here between cornea and conjunctiva allow for continual renewal of cornea. n Scleral venous sinus allows for drainage of aqueous humor n

Vascular tunic n Consists of three parts: choroid, ciliary body and iris. Choroid highly vascular dark brown pigmented layer that covers 5/6 of posterior chamber. -Melanocytes produce melanin that accounts for dark brown color of choroid. n Ciliary body anterior to choroid consists of ciliary muscle (sm. m. ), and the ciliary process. Radiating off of ciliary process are fine fibrils that attach to the iris and control its thickness. n Iris is the colored portion of the eye and constricts and dilates to regulate the amount of light entering into the eye. n

Sensory tunic “Retina” n Consists of two layers; thin pigmented layer and a thick neural layer n n outer layer – pigmented layer - beginning of visual pathway to brain inner layer – neural layer contains rods and cones -layer where light rays are deciphered and converted into an impulse to be relayed to brain. - ganglion cells- synapse with bipolar cells and make a 90 degree turn on retinal surface to go into optic nerve. - bipolar cells- synapse with rod and cone receptors to synapse with ganglion cells.

Sensory tunic - Rods= Blk/Whte - Cones = Color

Sensory tunic

Sensory tunic “Retina” n n optic disc (blind spot) – site where optic nerve exits eye ball. macula lutea containing the fovea centralis -small flat yellowish spot in exact center of posterior eye -contains only CONES, and no bipolar or ganglion cells to scatter light - Macula lutea is the area of highest visual acuity

n Optic disc and Macula Optic disc Fovea centralis Macula lutea

The LENS n n n Defined: composed of proteins called crystallins arranged in layers much like an onion. It is completely transparent and lacks any blood vessels. Attachments: Lens is enclosed in a clear connective tissue capsule and is held in place by encircling zonular fibers which attach it to the ciliary processes. Lens divides the eye into two cavities: n anterior chamber – lies between the cornea and iris - is filled with aqueous humor a watery fluid that nourishes the lens and cornea. n posterior chamber – lies between the iris and in front of

The Photoreceptors n RODS- receptor for black and white light n Location – (~ 250 million) in the pigmented layer of the retina n Functions – low light and peripheral vision receptors n CONES- receptors for colored light n n Location – (~6 million) in the pigmented layer of retina and macula lutea Functions – operate in bright light and are high acuity color receptors.

The Photoreceptors

Visual pathway