Lecture_13__Special_Senses

Mechanical Signals in Special Senses

  • Hearing: Involves cochlea and hair cells (do NOT regenerate).

  • Equilibrium and Balance: Utricle and saccule; Semicircular canals also play a role in balance.

Chemical Signals in Special Senses

  • Smell:

    • Nasal Epithelium: Contains sensory cells (DO regenerate).

  • Taste:

    • Taste Buds: Sensory cells here can regenerate.

Electromagnetic Signals in Special Senses

  • Vision:

    • Photoreceptors found in the retina (do NOT regenerate).

The Ear and Hearing

Structure of the Ear

  • External Ear: Collects sound.

  • Middle Ear: Contains auditory ossicles (malleus, incus, stapes) and Eustachian tube.

  • Internal Ear (Labyrinth): Involves cochlea and vestibular apparatus for hearing and balance.

Mechanisms of Hearing and Balance

  • Hearing and balance rely on mechanical bending of stereocilia.

  • Semicircular Canals: Each contains a crista ampullaris to detect rotational movements.

  • Cochlea houses the Spiral Organ (Organ of Corti) for sound perception.

Properties of Sound

Characteristics of Sound Waves

  • Sound: Pressure disturbances created by vibrating objects.

    • Pitch: Related to frequency; normal human hearing range is 20-20,000 Hz.

    • Higher frequency = higher pitch.

  • Amplitude: Affects loudness; greater amplitude corresponds to louder sounds.

Transmission of Sound to the Internal Ear

Steps of Sound Transmission

  1. Sound enters the external ear.

  2. Transmission occurs in the middle ear.

  3. Stapes transmit sound through the oval window into the cochlear duct.

  4. Sound waves move through the cochlear duct, bending stereocilia to activate hair cells.

Membranous Labyrinth of the Cochlea

Components of Cochlea

  • Scala Vestibuli and Scala Tympani: Contain perilymph.

  • Cochlear Duct: Contains endolymph (~160 mM K+).

  • Spiral Organ: Hair cells within the organ transduce sound into electrical signals.

Organ of Corti

Anatomy and Function

  • Contains Tectorial Membrane and Hair Cells.

  • Stereocilia: Protrude into endolymph; crucial for sound transduction.

Excitation of Hair Cells

Mechanism of Signal Transduction

  • Bending of stereocilia opens mechanically gated ion channels, resulting in depolarization and release of neurotransmitter glutamate.

  • Bending towards tallest stereocilia: Causes depolarization.

  • Bending away: Results in hyperpolarization and reduced neurotransmitter release.

Frequency Detection in Organ of Corti

Frequency Encoding

  • Basilar Membrane:

    • High-frequency sounds affect basal areas (base).

    • Medium frequencies affect middle areas.

    • Low frequencies resonate at the apex.

  • Pitch and loudness are encoded based on hair cell activation and action potential frequency.

Hearing Problems

Types of Deafness

  • Sensorineural Deafness: Damage to the neural structures from hair cells to the auditory cortex.

    • Often due to hair cell loss from exposure to loud noises.

  • Conduction Deafness: Blocked sound conduction from middle to internal ear.

    • Common causes: impacted earwax, perforated eardrum, otosclerosis, otitis media.

Equilibrium and Balance

Vestibular System Components

  • Utricle and Saccule: Respond to linear head movements and gravity; house macula for equilibrium sensing.

  • Semicircular Canals: Monitor rotational acceleration; detects angular movements through crista ampullaris.

Activation of Smell Receptors

Process of Smelling

  1. Odorant binds to receptor proteins in olfactory epithelium.

  2. Activates G-protein signaling to generate cAMP.

  3. Influx of Na+ and Ca2+ leads to membrane depolarization.

  4. Increased neurotransmitter release occurs upon depolarization.

  5. Adaptation occurs with prolonged stimulus exposure.

Taste and Taste Buds

Structure and Function

  • Taste buds consist of 50-100 taste receptor cells.

    • Replace every 7-10 days with stem cells.

  • Basic Taste Sensations: Salty, Sour, Sweet, Bitter, and Umami.

Activation of Taste Receptors

Mechanism of Taste Perception

  • Binding of tastant leads to depolarization of taste cells, leading to neurotransmitter release.

  • Saltiness: Na+ influx.

  • Sourness: H+ effects.

  • Sweet, Bitter, and Umami: Signal via G-protein gustducin.

  • Rapid adaptation occurs in response to stimuli.

Light and Vision

Light Characteristics

  • Eyes perceive visible light from 400-700 nm.

  • White objects reflect all light; black objects absorb all.

Refraction and Lenses

  • Light bends when passing through different media.

  • Bi-convex lenses can refract light, influencing image formation.

The Retina

Structure and Function

  • Composed of an outer pigment layer and an inner neural layer.

  • Three neuron types: photoreceptors, bipolar cells, and ganglion cells.

  • Pathway of signal propagation from photoreceptors to the brain via ganglion cell axons.

Functional Anatomy of Photoreceptors

Types

  • Rods: Responsible for night vision; contain rhodopsin.

  • Cones: Function in daylight and color vision; contain three types for red, green, and blue wavelengths.

Sensing Light (Phototransduction)

Light Activation Process

  1. Light activates retinal, leading to a signaling cascade via transducin and phosphodiesterase.

  2. Reduction of cGMP causes closure of cGMP-gated cation channels, leading to hyperpolarization.

Comparison of Rods and Cones

Feature

Rods

Cones

Color

Single pigment (grayscale)

1 of 3 pigments (RGB)

Sensitivity

High (night vision)

Low (need bright light)

Acuity

Low (converge to ganglion)

High (one cone per ganglion)

Location

Peripheral retina

Central retina

Light Dark Adaptation

Processes During Transition

  • Light Adaptation: Rapid adjustment in bright light; rods burn out, cones adapt.

  • Dark Adaptation: Takes longer; rods recover function and increase sensitivity.