BIOL 3370: Lecture 12

Seismic communication in elephants

• the transmission of information through mechanical vibrations or waves along a substrate, such as the ground, plant leaves, tree branches, or spider webs

• seen in African elephants

  • communicate with high frequency sounds that travel through the air, and with low frequency sounds that travel through the ground

  • when threatened, these low frequency sounds can alert other elephants up to 10 km away.

• is this hearing, or feeling?

Mechanoreceptive Pacinian corpuscles

• touch receptors which life deep within the fatty mass of elephant feet which allow them to sense vibrations and transmit APs to the brain

  • means of allowing for seismic communication

• type of phasic receptor

Hair cells

• hair cells dont have axons (unlike touch receptors) so they can only produce graded potentials, not action potentials

  • displacement of stereocilia opens or closes ion stretch receptor channels either towards (which signals for depolarization) or away from (which signals for hyperpolarization) the kinocilium

• synapse onto afferent sensory neurons, which can then produce action potentials that are transduced to the CNS

• in the inner ear, hair cells are positioned in the cochlea

  • a K⁺ rich, endolymph filled chamber

Stereocilia displaced towards the kinocilium

• when stereocillia are displaced towards the kinocilium, the tension produced stretches K⁺ cation channels, causing a rush of K⁺ into the cell.

  • this causes the activation of voltage-gated Ca⁺⁺ channels to open in other parts of the cell membrane

• graded depolarizations (only, not APs, as mentioned above) then cause a proportional release of neurotransmitters. if enough neurotransmitters are released to cause the afferent neuron to depolarize to its voltage threshold, then an AP is generated and transduced to the CNS

Stereocilia displaced away from the kinocilium

• when stereocilia are displaced away from the kinocilium, then hyperpolarization occurs, which onsets a return to resting potential

Hair cells in African elephants

• in African elephants, sound vibrations from the ground can travel through the skeleton all the way to the ear bones, which activates auditory receptors in the cochlea of the inner ear

  • the auditory receptors are hair cells

Ear anatomy

• 3 main parts:

  • external ear

  • middle ear

  • inner ear

• outer ear canal

• ossicles

• cochlea

External ear

• funnels incoming sound-pressure waves to the tympanic membrane (ear drum)

Middle ear

• transfers the sound energy from the external ear to the fluid-filled inner ear

• ossicles

  • stapes

  • incus

  • malleus

Inner ear

• sound waves become pressure waves in the fluid-filled (endolymph) cochlea, and activate hair cells embedded within the basilar membrane

Vertebrate ear anatomy variation

• in cetaceans and sirenians there are some differences in ear anatomy, but overall have similar mechanisms for hearing

  • as with elephants, the distinction between hearing and feeling can be blurry

• outer ear

  • lacks pinnae (fleshy outer ear)

  • ear canal is filled with wax which acts as a water block and does not connect to the tympanic membrane

• sound is transmitted to the middle ear

  • sound waves/vibrations are captured by jaw bones and specialized fat layers around the jaws

  • this allows sound waves to bypass the external ear to directly vibrate the ossicles

Basilar membrane

• each region of the basilar membrane has a different width and stiffness, endowing it a specific ‘resonant frequency’

• hair cells reside in the membrane

Sound travelling through the cochlea

• sound waves travel through the cochlea’s scala vestibuli and scala tympani

• the ossicles work together to transmit sound waves from air into pressure waves in the chochlea. the stapes is the ossicle that makes this transfer of energy

• the frequency of the pressure waves are sensed at specific areas of the basilar membrane, which lifts/osscilates in response to the pressure wave

  • resonate, and causes hair cells to interact with the tectorial membrane above

• basilar membrane gets displaced by vibrations from pressure waves moving through the Scala tympani

• as the wave moves through, it lifts the membrane upwards as it goes, which forces the hair cells up into the Tectorial membrane, which is fixed in place

• this causes displacement of stereocilia towards kinocilium (which causes depolarization)

  • the stronger the pressure wave, the more displacement there is, which causes a larger graded release of neurotransmitters

• downwards movement of the basilar membrane occurs when the wave passes, which returns stereocilia to their original position away from the kinocilium (which causes hyperpolarization)

Hair cells depolarize when…

• Hair cells depolarize when the tectorial membrane displaces stereocilia TOWARDS the kinocilium

Hair cells hyperpolarize when

• Hair cells hyperpolarize when the tectorial membrane displaces stereocilia AWAY FROM the kinocilium

Localization of sound

• humans are reasonably good at this, but owls, bat and cetaceans are exceptional

• requires two ears. with only one ear, it would be difficult to localize a sound’s source

  • a single sound reaches our two ears at different times and different intensities

• differences in timing and intensity of sounds arriving at each ear allow the brain to triangulate a sound’s source

Bat echolocation

• emit high frequency sonar pulses with bounces off objects in the environment and allow them to track prey

• bats emit high-frequency cries, the shape of which varies amongst species

• in some species like the Brazilian free-tailed bats, they listen for conspecific echolocation cries, and then emit a higher frequency call to jam the signal and prevent the other bat from hearing its own echo

  • instance of intra-specific competition

Moth echolocation

• most moths can’t hear echolocation cries, and so rely mostly on visual cues when attempting to avoid predation, but some can hear echolocation

• some moths have defences, in Luna moths a unique sonar confusing defensive system has evolved

  • luna moths can’t hear the cries of bats, but natural selection has provided them with a morphological adaptation against echolocation; in flight, long tail extension creates a turbulent wake that deflects witth the echolocation signal. a bat can’t resolve its own echo! passive defence

  • tiger moths can hear a bat’s cry, and defend themselves using their own high frequency calls that jam a bat’s echo field. they then use evasive flight manuevers to escape. active defence

Tympanal organ (insect hearing)

• insect hearing is mediated by the Tympanal organ, a stretched surface cuticle which acts like an eardrum

  • sound waves striking the cuticle transmit pressure waves into an air-filled chamber

  • those pressure changes are then transmitted into a lymph-filled chondrotonal organ (analogous to the Cochlea)

  • the pressure changes mechanically bend the scolophores, opening cation channels and causing K⁺ influx