special senses

Hearing: The reception of an air sound wave that is converted to a fluid wave that ultimately stimulates mechanosensitive cochlear hair cells that send impulses to the brain for interpretation.

Sound Definition: A pressure disturbance (alternating areas of high and low pressure) produced by a vibrating object and propagated by molecules of the medium (air).

Sound waves are generated when an object vibrates, leading to:
- Air Molecule Displacement: Air molecules are pushed forward into adjacent areas, resulting in an area of high pressure due to compression.
- Rarefaction: As the object returns to its original position, fewer air molecules are present, creating an area of low pressure.
- Transmission of Wave Energy: The kinetic energy from the vibrating object is transferred to air molecules, which continues to transfer to nearby molecules, although the wave energy declines over time and distance.

Waves: Combinations of compressions and rarefactions moving outward from the sound source.
Two Physical Properties:
1. Frequency:
- Definition: The number of waves that pass a given point in a period of time.
- Measurement Unit: Hertz (Hz, waves per second).
- Human Hearing Range: 20–20,000 Hz, most sensitive frequencies at 1500-4000 Hz.
- Pitch: Perception of different frequencies; higher frequency = higher pitch.
- Wavelength: Distance between two consecutive crests; shorter wavelength means higher frequency.
1. Amplitude:
- Definition: Height of sound wave crests.
- Perceived Loudness: Subjective interpretation of sound intensity, measured in decibels (dB).
- Normal Range: 0-120 dB; normal conversation ~50 dB; pain threshold ~120 dB; risk of hearing loss from prolonged exposure above 90 dB (e.g., amplified rock music at 120 dB).

Pathway of Sound:
1. Tympanic Membrane (Eardrum): Sound waves enter the external acoustic meatus, striking the tympanic membrane and causing it to vibrate. Higher intensities lead to increased vibration.
2. Auditory Ossicles: Transfer vibration from the tympanic membrane to the oval window; tympanic membrane size (~20x larger than oval window) amplifies this vibration by approximately 20x.
3. Scala Vestibuli: Stapes rocks back and forth on the oval window with each vibration, producing wave motions in perilymph, which ends at the round window causing it to bulge.
- Helicotrema Path: Waves with frequencies below hearing threshold travel through helicotrema and scala tympani to round window.
- Basilar Membrane Path: Sounds in hearing range move through cochlear duct, vibrating the basilar membrane at frequencies specific to the sound.

Resonance: Refers to the movement of different areas of basilar membrane in response to specific frequencies.
- Structure Along Length:
  - Fibers near the oval window: Short and stiff, resonate with high frequencies.
  - Fibers near cochlear apex: Longer and floppier, resonate with lower frequencies.

Role of Outer Hair Cells:
- Efferent neurons convey messages from the brain to the ear, controlling outer hair cell contraction and altering basilar membrane stiffness.
- Functions:
1. Fine-tunes responsiveness of inner hair cells by amplifying the motion of the basilar membrane.
2. Protects inner hair cells from loud noises by reducing motion of the basilar membrane.

Neural Pathway: Impulses from cochlear bipolar cells reach auditory cortex via the following pathway:
1. Spiral ganglion ➔ 2. Cochlear nuclei (medulla) ➔ 3. Superior olivary nucleus (pons-medulla) ➔ 4. Lateral lemniscus (tract) ➔ 5. Inferior colliculus (midbrain auditory reflex center) ➔ 6. Medial geniculate nucleus (thalamus) ➔ 7. Primary auditory cortex.
- Cross-Input: Some fibers cross over while others do not, allowing both auditory cortices to receive input from both ears.

Perception of Pitch: Determined by impulses from hair cells at different locations along the basilar membrane.
Detection of Loudness: Relative loudness is indicated by increased action potential frequency experienced by hair cells.
Localization of Sound: Based on relative intensity and timing of sound waves reaching both ears; timing increases on one side lead to the perception of sound originating from that side.

Definition: Equilibrium refers to responses to head movement and relies on input from the inner ear, eyes, and stretch receptors.
Vestibular Apparatus: Contains equilibrium receptors in semicircular canals and vestibule.
- Vestibular Receptors: Monitor static equilibrium.
- Semicircular Canal Receptors: Monitor dynamic equilibrium.

Maculae: Sensory receptor organs monitoring static equilibrium, located in each saccule and utricle wall, assessing head position and controlling posture.
Activation of Maculae Receptors:
- Hair cells continuously release neurotransmitter; changes in acceleration/deceleration alter neurotransmitter levels, leading to action potential frequency changes sent to the brain.

Definition: Receptors for rotational acceleration found in the ampulla of each semicircular canal; respond primarily to angular movements of the head.
Vestibular Nystagmus:
- Linked to eye movements during rotation, causing eyes to drift opposite to head rotation and then rapidly jump in the direction of rotation after cerebellar compensation.

Information Processing: Equilibrium information travels to reflex centers in the brain stem for swift reflexive responses, preventing falls.
Input Modes for Balance:
- Vestibular receptors, visual receptors, and somatic receptors (skin, muscles, joints).

Deafness Types:
1. Conduction Deafness: Blocked sound conduction to inner ear fluids (causes: earwax, perforated eardrum, otitis media, otosclerosis).
2. Sensorineural Deafness: Damage to neural structures from cochlear hair cells to auditory cortex (often from gradual hair cell loss).

Cochlear Implants: Effective for congenital or noise-induced cochlear damage by converting sound energy to electrical signals, allowing deaf children to learn to speak.
Tinnitus: A ringing or buzzing sound in the absence of external stimuli, often due to cochlear nerve degeneration or inner ear inflammation.
Ménière’s Syndrome: A labyrinth disorder affecting cochlea and semicircular canals, causing vertigo and nausea, treated with motion sickness meds or surgery.
Developmental Aspects: Newborns can hear, reflexively responding; by four months, infants can turn toward familiar voices. Hearing issues increase after age 60, particularly high-frequency loss (presbycusis).
Congenital Abnormalities: Include issues like missing pinnae or closed meatuses, with maternal rubella linked to sensorineural deafness.