Lecture 20.
Hearing & Mechanoreception (lateral line)
Both senses are used to detect movement or vibrations in H2O.
Water is 800X denser than air.
Water is incompressible, whereas air is quite compressible.
1) Sound travels faster & farther in H20 than in air
a) speed – 1,433 m/s in H2O, 355 m/s in air
sound is 4X faster in water than in air
b) tell story about snorkeling and thinking that motor boat is about to hit you when it was really quite some distance away.
Fish have two systems to detect sound & vibration
1) inner ear
2) neuromast system – lateral line
Both systems rely on hair cells. Draw a hair cell. Show inhibitory & excitatory reactions. Important this is that when the hairs bend, the cells fire.
A. Inner ear
1. 3- Semicircular canals (elasmobranches & Osteichthyes)
The Purpose of the Semi-Circular Canals is to inform the fish on acceleration/balance/orientation in space – yaw, pitch, roll & acceleration/deceleration.
draw out
a) each oriented in a different plane draw out
b) each is filled with endolymph and an ampulla (bulbous area)
c) these canals inform fish of angular acceleration in space. draw out
d) each canal has an ampulla (bulbous area): hair cells embeeded in a gelatinous matrix called cupula. draw out
e) with fish movement, the endolymph displaces the cupula which bends the sensory hairs - electrical signal from hair cells --> goes to central nervous system
draw out 3 semicircular canals with utriculus w/ otolith, sacculus w/ otolith, & lagena with otolith.
otoliths & Gravity
3 semi-circular canals terminate on a central fluid-filled chamber, utriculus – which conatins an otolith (or ear stone)
utriculus used to detect gravity – tricky if you are negatively buoyant.
1.relies on differential “weight” of fluids and otolith
otoliths & Sound
sacculus & lagena (which are off to the side) also contain an otolith. These two structures are used to detect sound
In all 3 structures, an otolith overlays hair cells. draw out; also show slide
d. purpose of otolith is to magnify sound.
1. bone is 3X density of H2O. fish flesh is ≈ H2O in density;
w/o otoliths, fish is nearly transparent to sound
2. fish & whole body vibrates w/ H2O, but due to increased density, the otoliths vibrate at a different frequency (actually lags behind the rest of the fish).
3. sensory hairs make contact w/ otolith & detect differences in amplitude & phase of otolith vs. fish vibration. output to auditory centers of brain
3. Near field vs. far-field.
a. hearing in near-field is detected directly by innear ear (and lateral line) w/o need for amplification.
b. some fish have a Weberrian apparatus draw out
c. other fish have an extended swimbladder draw out
d. exploits the fact that gas is over 100X more compressible than H2O
e. swimbladder pulsates & stimulates innear ear – similar to system in humans where middle ear bones “hammer” on eardrum to stimulate auditory system.
B. Other facts about water
1. While sound carries much farther in H2O than air, it is more difficult to displace H2O than air.
C. Mechanosensory stimuli
1. fish can detect water movements – humans can do this in an extremely gross sense. When you stand in the creek, you can tell which direction the water is coming from. Fish in a creek can not only do this, but can gain information on the other fish moving in the creek. – We can’t do this.
2. sense of “distant touch”
3. mechano-receptors called “neuromasts” that use hair cells similar to those in inner ear.
a. neuromasts consist of several hair cells bound together by a gelatinous capsule draw out.
b. H2O movement displaces cupula which bends sensory hairs - signal output.
4. neuromasts occur in lots of places show slide, around head, lateral line, canal around head, sometimes on trunk of body.
a. some have neuromasts on exterior, but most have them in a pit (also called pores) or canal. draw out lateral line & neuromasts
5. Most teleosts & Elasmobranchs have well-developed lateral line canal.
6. Directional sense – neuromasts continuously send impulses to brain, but are asymmetrical in firing.
a. activate when flexed in one direction; inactivate when flexed in another direction.
7. some neuromasts are exterior, other in pits or canals.
a. More active fish have more neuromasts in pits & canals.
1. may protect neuromasts from excessive stimulation during swimming so they can still detect H2O displacement, but not react to noise.
8. lateral line system is essential for schooling; if you cut nerve from lateral line; schooling behavior is eliminated.
9. fish place lateral lines in such a way that they are not unduly stimulated by fins.