Sensation and Sensory Receptors

Sensation

• General senses use tiny receptors.
• Special senses use large, complex receptors involving bipolar neurons.
• Sensory receptors respond to stimuli.
• General senses include touch, pain, pressure, cold, heat, stretch, and vibration.

General Sensory Receptors

• Exteroreceptors react to the external environment and are close to the body surface
• Interoreceptors respond to stimuli within internal visceral organs.
• Proprioreceptors are internal and limited to skeletal muscles, tendons, joints, ligaments, and CT.
• Chemical changes influence interoreceptors.
• Simple cutaneous receptors in skin are receptor structures of special senses and are located in the epidermis/dermis.

Nonecapsulated Receptors

• Free nerve endings (epithelium, CT) detect pain, heat, and cold.
• Tactile (Merkel) discs in the stratum basale that detect light touch are in the epidermis and dermis
• Hair follicle receptors detect light touch and hair bending.

Encapsulated Receptors

• Tactile corpuscles (dermal papillae) detect light pressure and stretch.
• Bulbous corpuscles (dermis) detect deep pressure and stretch.
• Lamellar corpuscles (dermis, subcutaneous, periosteum, tendons) detect deep pressure, stretch, and vibration.
• Muscle spindles (skeletal muscle) help coordinate the amount of stretching.

Distribution of Receptors

• Receptors are not uniformly distributed throughout the body
• Punctate distribution involves clustering at certain points.
• Touch sensitivity is most desired in touch clusters.
• Tactile localization determines the body portion being touched.
• The touch field corresponds to a field in the somatosensory cortex.
• Free nerve endings are at the fingertips.

Adaptation and Referred Pain

• Adaptation occurs when a stimulus is applied for a while, slowing receptor discharge and decreasing awareness.
• Referred pain is perceived to be coming from a different area.

Ear (External)

• Auricle (pinna) collects and directs sound waves, featuring elastic coverage with skin.
• The lobule is the earlobe.
• The external auditory meatus is a canal carved into the temporal bone that transmits waves into the tympanic membrane.
• The tympanic membrane separates the external and middle ear; it transmits sound waves to auditory ossicles.

Middle Ear

• The middle ear is filled with air.
• Malleus (hammer) is attached to the eardrum.
• Incus (anvil) receives vibrations from the malleus.
• Stapes (stirrup) receives vibrations from the incus, then transmits/amplifies vibrations to the oval window.
• Oval window is an oval-shaped membrane deep to stapes; will transmit vibrations to perilymph of scala vestibuli
• The pharyngotympanic tube connects the middle ear to pharynx, equalizing air pressure.

Internal Ear

• Sound waves travel as pressure waves through liquid.
• Cochlea (spiral organ) is responsible for hearing and has receptors.
• Vestibule (utricle and saccule) contains maculae with receptors for static equilibrium and linear acceleration of the head.
• Semicircular canals' ampullae have receptors for rotational acceleration of the head.

Cochlea

• Scala Vestibuli: upper chamber containing perilymph (sodium-rich)
• Scala Media: middle chamber (potassium-rich) containing the spiral organ with hair cells (nerve endings for cochlear nerves and on the basilar membrane)
• Scala Tympani: lower chamber containing perilymph (sodium-rich)

Weber and Rinne Tests

• Weber Test: a tuning fork is struck and placed centrally on the patient's head.
• Air and bone conduction are compared in the Rinne Test.
• No hearing loss or equal hearing loss in both ears means that the sound should be equal in both ears during testing

Semicircular Canals

• The semicircular canals are filled with endolymph, and hair cells are activated by the bending of hairs for rotational movement
• They measure rotational acceleration of the head.
• The base of each duct is an ampulla, containing a receptor region called the crista ampullaris (tuft of hair cells).
• Head movement depolarizes hair cells, sending impulses to the vestibular division of the 8th cranial nerve.

Utricle and Saccule (Vestibule)

• These structures contain maculae.
• Hair cells monitor head position and movement in a straight line.
• Each contains kinocilium and stereocilia embedded in the otilith membrane (gel).
• Bending towards kinocilium leads to depolarization.

Taste and Smell

• Taste and smell use chemoreceptors.
• Olfactory epithelium (pseudostratified) is the organ of smell.
• It contains olfactory sensory neurons (bipolar), supporting columnar cells, and olfactory stem cells in the basal surface, which will become new sensory neurons.

Taste Buds

• Taste buds are specific receptors on papillae, which are peg-like projections on the tongue's dorsal surface.
• Vallate papillae have a V-formation in the posterior surface.
• Foliate papillae are on the side walls.
• Fungiform papillae are on the surface, and they are the most numerous.
• Gustatory epithelial cells includes microvili called gustatory hairs that project towards taste pores, along with precursor basal epithelial cells.