Chapter 10

• Sensory receptors are categorized based on the type of stimuli they respond to.

Sensory Receptor Categories

• Chemoreceptors: Sensitive to changes in chemical concentration
• Pain receptors: Detect tissue damage
• Thermoreceptors: Respond to temperature differences
• Mechanoreceptors: Respond to changes in pressure or movement
• Photoreceptors: Respond to light, are found in the eye, with rods and cones being photoreceptors

Sensory Terminology

• Sensory adaptation: The nervous system's ability to become less responsive to a maintained stimulus
• Perception: Conscious awareness of stimuli
• Sensation: Occurs when receptors are stimulated and send impulses to the brain
• Interpretation: Assigning meaning to sensory information
• Projection: The brain sends a sensation back to its point of origin, allowing a person to pinpoint the area of stimulation
• Referred pain: Visceral pain that feels as if it is coming from another body area; results from common nerve pathways leading from skin and internal organs

Sensory Receptors in the Skin

• Specialized nerve endings detect touch, pressure, vibration, temperature, and pain

Primary Taste Sensations

• Sweet, such as table sugar
• Sour, such as a lemon
• Salty, such as table salt
• Bitter, such as caffeine or quinine
• Umami (savory): A response to certain amino acids associated with meats and their chemical relatives (e.g., monosodium glutamate/MSG)

Anosmia

• Anosmia is the partial or full loss of smell

Lysozyme

• Lysozyme is an enzyme in tears that kills bacteria, reducing the risk of eye infections

Ear Structures

• Outer ear structures include:
  • Auricle (pinna): Collects sound
  • External acoustic meatus (external auditory canal): An S-shaped tube transporting sound toward the eardrum
  • Tympanic membrane (eardrum): Lies at the end of the external acoustic meatus and vibrates with sound waves
• Middle ear structures include:
  • Tympanic membrane: Vibrates the malleus, which vibrates then the incus and then finally the stapes
  • Stapes: Vibrates fluid inside the oval window of the inner ear
  • Vibrations in the fluid: Stimulate hearing receptors in the inner ear
  • Auditory ossicles: Transmit and amplify sound waves
• Inner ear structures include:
  • Membranous labyrinth: Inside a bony (osseous) labyrinth within the temporal bone
  • Perilymph: Fluid between the two labyrinths
  • Endolymph: Fluid inside the membranous labyrinth
  • Cochlea: Within the labyrinths, functions in hearing
  • Semicircular canals: Within the labyrinths, function in equilibrium

Pathway of Sound to the Organ of Corti

• Sound energy passes through: tympanic membrane (eardrum), auditory ossicles (malleus, incus, stapes), oval window, cochlear fluid, and finally the basilar membrane within the cochlea (where the organ of Corti is located)

Auditory Ossicles (Tympanic Membrane to Oval Window)

• Malleus (hammer): Attached to the tympanic membrane
• Incus (anvil): Situated between the malleus and stapes
• Stapes (stirrup): Directly connects to the oval window

Function of Auditory Ossicles

• Auditory ossicles transfer and amplify air vibrations into the inner ear to be processed as sound

Eustachian Tube

• The auditory (eustachian) tube connects the middle ear to the nasopharynx
• It helps maintain equal air pressure on both sides of the eardrum (required for normal hearing)
• Mucous membrane infections of the throat can travel up the auditory tube to the middle ear and cause middle ear infections

Conductive Deafness vs. Sensorineural Deafness

• Conductive deafness: Occurs due to interference with the transmission of sound vibrations to the inner ear; the eardrum may harden or tear due to disease or injury.
• Sensorineural deafness: Results from damage to the cochlea, auditory nerve, or auditory pathways, loud sounds, tumors, brain damage, or certain drugs can cause sensorineural hearing loss.

Tunics of the Eye

• Fibrous tunic:
  • Outermost layer of the eye
  • Made up of the cornea and sclera
  • The cornea is transparent (covers the iris)
  • The sclera is white (covers the rest of the eye)
  • Protects the eye and maintains its shape
• Vascular tunic (uvea):
  • Middle layer of the eye
  • Made up of the iris, ciliary body, and choroid
  • The iris controls the amount of light that enters the eye
  • The choroid contains blood vessels that nourish the retina
• Retina:
  • Contains photoreceptor cells, nerve cells, and a pigmented layer

Light Passage to the Photoreceptors

• Light passes through these structures: cornea, pupil, anterior chamber, lens, posterior chamber, retina

Conjunctiva

• The conjunctiva is a membranous covering lining the eyelids and on the anterior surface of the sclera of the eye covered by the eyelids

Iris Muscle Contraction

• Contraction of the radial muscles of the iris increases the diameter of the pupil; pupil dilates allowing more light to enter
• Contraction of the circular muscles of the iris (sphincter pupillae) decreases the diameter of the pupil, constricting the pupil and reducing light entry

Rhodopsin and Iodopsin

• Rhodopsin: Primarily found in rod photoreceptor cells and responsible for vision in low light conditions
• Iodopsin: Found in cone photoreceptor cells, responsible for color vision
• Rod cells: Contain rhodopsin for detecting light intensity (black and white vision)
• Cone cells: Contain iodopsin for detecting different wavelengths of light and clear images

Fovea Centralis

• The fovea centralis is a depressed region of the retina at the center of the macula lutea, consisting of densely packed cones which provides the sharpest color vision

Accommodation

• Accommodation is the ability of the lens to adjust shape to facilitate focusing on objects at different distances
• The ciliary muscle contracts to change the shape of the lens, allowing the eye to focus on objects at different distances; it is a smooth muscle