• Composed of sensory receptors throughout the body.
– General senses
• Touch, pressure, pain, heat, cold, stretch, vibration.
– Special senses
• Vision, hearing, equilibrium, olfaction, taste.
• Relative to the source of the stimuli.
– Exteroceptors
– Interoceptors
– Propioceptors (sketetal muscle). Conscious.
• General structure
– Modified dendrites.
• Free nerve endings
• Merkel discs in epidermis
• Hair follicle receptors
• Meissner corpuscles: light touch
• Ruffini corpuscles: deep pressure and stretching.
• Pacinian corpuscles: pressure receptor.
• Muscle spindles
• Golgi tendon organs
• Receptor physiology
– Act as signal transducers. Direct relationship between importance and number of clustering of the receptors; punctuate distribution.
Special Senses
- Vision
- Olfaction
- Taste
- Hearing and equilibrium
1. Vision
· 70 percent of all sensory receptors are in the eyes
· Each eye has over a million nerve fibers
· Protection for the eye
1. Most of the eye is enclosed in a bony orbit made up of the lacrimal (medial), ethmoid (posterior), sphenoid (lateral), frontal (superior), and zygomatic and maxilla (inferior)
2. A cushion of fat surrounds most of the eye
Organ: Eye ball
Accessory structures
· Lacrimal apparatus.
§ Lacrimal gland: secretes tears; solution of salts and lyzozyme.
§ Lacrimal canals: drains lacrimal fluid from eyes
§ Nasolacrimal canal: empties lacrimal fluid into the nasal cavity
· Eyelids (papebrae) - brush particles out of eye or cover eye
· Medial and lateral canthus
· Caruncle
· Conjunctiva - Mucus membrane that lines inner surface of eyelids. Stratified squamous and stratified cilindrical epithelium.
· Eyelashes - trap particles and keep them out of the eye
· Ciliary glands - lubricate eyeball, modified sweat glands between the eyelashes- secrete acidic sweat to kill bacteria, lubricate eyelashes
· Tarsal glands: lubricate eyeball
· 6 extrinsic eye muscles
Internal structure of the eye
Composed of 3 layers or tunics
External
• Cornea (transparent) and Sclera (white).
Middle
• Anterior
– Iris: radial smooth muscle (III ocoulomotor)
– Uvea: vasculariazed.
– Cilliar body(muscles) : control the curvature of the lens (cristaline).
– Cilliary processes: secrete aqueous humor.
• Posterior
– Choroid: dark pigment.
Inner
• Retina: sensory region
Additional internal structures
· Humor vitreous
· Macula lutea (yellow spot): high cone density
· Fovea centralis: area of maximum (very high amount of cones) sight acuity.
· Sclera venous sins( Schlemm canal)
· Ciliary zonule
Histology of the retina
• Sensory tunic
– Outer pigmented epithelial layer
– Inner neural layer.
• Photoreceptors: cones or rods.
• Bipolar neurons
• Ganglion
• Axons of ganglions
2. Olfaction
· Olfactory receptors are in the roof of the nasal cavity
§ Neurons with long cilia
§ Chemicals must be dissolved in mucus for detection
· Impulses are transmitted via the olfactory nerve
· Interpretation of smells is made in the cortex (olfactory area of temporal lobe)
Sensory structure:
Olfactory epithelium. Chemoreceptor.
§ Olfactory receptor cell: bipolar neuron (I)
§ Support cells: pseudo stratified ciliated epithelium.
§ Basal cells.
3. Taste
· Taste buds house the receptor organs
· Location of taste buds
§ Most are on the tongue
§ Soft palate
§ Cheeks
The Tongue and Taste
· The tongue is covered with projections called papillae
§ Filiform papillae – sharp with no taste buds
§ Fungifiorm papillae – rounded with taste buds
§ 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
§ Have gustatory hairs (long microvilli)
§ Hairs are stimulated by chemicals dissolved in saliva
· Impulses are carried to the gustatory complex (pareital lobe) by several cranial nerves because taste buds are found in different areas
§ Facial nerve
§ Glossopharyngeal nerve
§ Vagus nerve
Taste Sensations
- Sweet receptors
- Sugars
- Saccharine
- Some amino acids
- Sour receptors
- Acids
- Bitter receptors
- Alkaloids
- Salty receptors
- Metal ions
- Umami
- Glutamate, aspartate (MSG, meats)
Sensory structure
a. Taste bud. Chemoreceptor. Mechanoreceptor. Thermo receptor.
i. Basic tastes: sweet, sour, salt, bitter.
ii. Types (location in figure)
1. Fungiform papillae (mushrooms)
2. Circunvallate papilla (globes)
3. Filiform papilla
iii. Cell types
1. Gustatory (taste) cells: bipolar neurons.
2. Support cells
4. Hearing and balance
Eye Reflexes
· Internal muscles are controlled by the autonomic nervous system
· Bright light causes pupils to constrict through action of radial (iris) and ciliary muscles
· Viewing close objects causes accommodation
· External muscles control eye movement to follow objects- voluntary, controlled at the frontal eye field
· Viewing close objects causes convergence (eyes moving medially)
The Ear
· Houses two senses
· Hearing (interpreted in the auditory cortex of the temporal lobe)
· Equilibrium (balance) (interpreted in the cerebellum)
· Receptors are mechanoreceptors
· Different organs house receptors for each sense
Organ: Ear (sensory receptor for hearing and equilibrium).
· The ear is divided into three areas
§ Outer (external) ear
§ Middle ear
§ Inner ear
· Involved in hearing only
· Structures of the external ear
§ Auricle
§ External acoustic meatus
§ Tympanic membrane
§ Ceruminous glands
· Structures of the Middle ear
• Tympanic cavity
– Osicles
» Malleus (hammer)
» Incus (anvil)
» Staples (stirrup). Connects with the oval window.
• Pharingotimpanic tube. Pressure equalization of middle ear and environment.
· Structures of the Inner ear
• Bony and membranous labyrinth
– Cochlea: hearing
– Vestible: equilibrium
– 3 perpendicular semicircular canals: equilibrium
Hearing physiology
• Sound waves travel from the outer ear (timpanic membrane)à middle ear (Malleus,Incus, staples, oval window)à inner ear (scala tympani)/scala vestibulià stimulates the stereocillia of the organ of cortià VIII
• The round window serves as a pressure relief valve.
Equilibrium physiology
• Located in the vestibular apparatus
– Vestible
• Utricle and saccule (sacs) within vesitble.
– Receptor: Macculae (static equilibrium)
» Hair cells (stereocillia) are embedded in the otholitic membrane which contain CaCO3(otoliths). Respond to vertical acceleration.
– Membranous semicircular ducts
• Ampulla (located at the base of each perpendicular duct). Mechism of dynamic equilibrium (angular acceleration).
– Receptor: Crista ampullaris. Stereocilia covered by a gelatinous cupula. Endolymph stimulates the recptor.
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