Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 50.2 In Hearing and Equilibrium, Mechanoreceptors Detect Moving Fluid or Settling Particles
Hearing and balance are mechanosensory processes that rely on the deflection of hair cell projections by movement of fluid or particles. In both invertebrates and vertebrates, specialized structures detect sound, gravity, and motion using variations of the same basic mechanism.
1. Gravity and Sound Detection in Invertebrates
- Many invertebrates use statocysts, fluid-filled chambers with mechanoreceptors and dense granules called statoliths
- Statoliths settle during movement, stimulating hair cells to signal orientation
- Example: crayfish statoliths manipulated with magnets to study gravity sensing
- Insect hairs detect frequencies, like mosquito wingbeats
- Some insects use tympanic membranes to detect air vibrations
2. Structure of the Vertebrate Ear
- Outer ear: pinna and auditory canal funnel sound to the tympanic membrane
- Middle ear: three bones (malleus, incus, stapes) transmit vibrations to the oval window
- Eustachian tube: connects middle ear to pharynx for pressure equalization
- Inner ear: includes semicircular canals (balance) and cochlea (hearing)
3. Cochlea and Hearing
- Cochlea: spiral structure with vestibular canal, cochlear duct, and tympanic canal
- Contains the organ of Corti with hair cells on the basilar membrane topped by the tectorial membrane
- Vibrations create fluid waves that deflect the basilar membrane
- Bent hairs open ion channels → receptor potential → neurotransmitter release → auditory nerve signals
4. Sound Volume and Pitch
- Volume: stronger pressure waves bend more hairs → higher action potential rate
- Pitch: detected based on vibration location on the basilar membrane
- High pitch = base (narrow, stiff)
- Low pitch = apex (wide, flexible)
- Each region connects to a specific auditory cortex area
5. Equilibrium: Linear and Rotational
- Utricle and saccule: detect gravity and linear acceleration with otoliths in gel
- Tilt shifts otoliths → hair cells bend → neurotransmitter release changes
- Semicircular canals: detect rotational movement
- Hair cells in cupula respond to fluid movement during head rotation
- Three canals in different planes detect motion in all directions
6. Hearing and Balance in Other Vertebrates
- Frogs, reptiles, birds: surface tympanic membranes and one middle ear bone
- Fish: use skeletal conduction or swim bladders, with otoliths in inner ears
- Lateral line system (fish, amphibians):
- Hair cells in body canals detect water flow and vibrations
- Cupula bends with water movement, generating signals
In a Nutshell
Both hearing and equilibrium rely on mechanoreceptors—hair cells that detect fluid motion or particle displacement. Structures like the cochlea and semicircular canals transform mechanical signals into electrical ones, enabling animals to detect sound, maintain balance, and navigate their environments. Variations on this mechanism are found across invertebrates, vertebrates, and even aquatic species.