Fri March 28
MECHANORECEPTORS
• All animals can sense physical contact with their
bodies.
Such
stimuli
are
detected
by
mechanoreceptors.
• The simplest mechanoreceptors are the free nerve
endings found in the skin.
• Complex
mechanoreceptors
have
accessory
structures that have the ability to transfer mechanical
energy to the cell membrane that is specialized for
converting it into electrical potential energy, i.e.
transduction.
• Mechanoreceptors respond to being mechanically
pushed and pulled through touch, pressure, gravity,
stretch, and movement.
• As their contour changes, mechanoreceptors supply
information to the animal about shape, texture,
weight, and the landscape of objects in the external
environment.
• Through the use of mechanoreceptors we can feel,
maintain balance, and even hear.
• Feeling occurs when mechanoreceptors detect touch,
pressure, and pain as objects come in contact with the
skin.
• Another kind of mechanoreceptor is responsible for
proprioception, or balance, which enables an animal
to know the position of its body.
• Proprioceptors are located within muscles, tendons,
and joints.
• Mechanoreceptors also facilitate hearing. For
terrestrial vertebrates, hearing occurs when sound
waves in the air become pressure waves in the
cochlear fluid, resulting in depolarization of sensory
neurons.
• The cochlear nerve then transmits these impulses to
the brain.
Various Accessory Structures Associated with
Receptor Cells of Acoustico-Lateralis System (lateral
line system) in Vertebrates
Various Accessory Structures for Mechanoreception
• Muscle stretch receptors of various kinds found in
arthropods and vertebrates.
• Hair-like sensory bristles extends from exoskeleton
of arthropods.
• Auditory structure of vertebrate ear.
INSECT HAIR CELL
• In insects the displacement of bristles causes action
potential to generate.
• Here axon is bathed in endolymph which has high
concentration of K+ ions.
• When displacement of bristle occurs then cation
channels open and cause K+ influx to increase
positivity.
• It has been found that there is a channel protein with
six transmembrane helices that are voltage-gated ion
channels.
ACTIVATION OF A MECHANORECEPTOR
• The stimulus that activates a mechanoreceptor cell is
a stretch or distortion of its plasma membrane.
• Ion channels that are activated by stretch are
probably found in all organisms.
• These ion channels activate or inactivate by change
in tension of the plasma membrane.
HAIR CELL RECPTOR MECHANISM
• The hair cells of vertebrates are extraordinary
sensitive
mechanoreceptors
that
transduce
mechanical stimuli into electrical signals.
• Hair cells are named for the many cilia that project
from the apical end of these cells.
MECHANISM OF TRANSDUCTION IN HAIR
CELL
 It has been found that when stimulus arise,
displacement of stereocilia towards kinocilium cause
depolarization, and when move in opposite direction.
 Fishes and amphibians have a set of these receptors
called “lateral line system” which is based on hair
cells and detects motions in the surrounding water.
 The vertebrate organs of hearing and the organs that
reports the position and movement of the body with
respect to gravity (called vestibular organs OR organs
of equilibrium) are based on hair cell receptors.
 In these organs, hair cells consist of kinocilium and
sterocilia. But in some kinocilium is not present. It is
proved by removal of kinocilium that kinocilium is
not necessary for transduction.
TRANSDUCTION IN HAIR CELL
• At rest 15 % of ion channels are open in hair cells,
which cause resting membrane potential of -60 mV.
• Hair cells do not produce action potentials, instead,
these release neurotransmitter onto an afferent
neuron.
• The amount of neurotransmitter released depends on
membrane potential in the hair cell and determines
the frequency of APs in afferent neuron.
MECHANISM OF THE HAIR CELL AT THE
LEVELS OF KINOCILIUM AND STEREOCILIA
• Tip Links
• Ion channels in attached with the tip links
• Muscle motor Myosin beta
Hearing receptors in the semicircular canals