CBNS 120 Sensory Transduction I: Overview & Mechanoreceptors

The adequate stimulus of a sense organ is:

The specific sensory modality that most commonly activates it.

The five major classes of sensory receptors based on adequate stimulus are:

Mechanoreceptors, photoreceptors, chemoreceptors, thermoreceptors, and nociceptors.

Mechanoreceptors respond to:

Mechanical displacement or stretch (via stretch-gated cation channels).

Photoreceptors respond to:

Light (via G-protein-coupled receptors like rhodopsin).

Chemoreceptors (taste and olfactory) respond to:

Chemical molecules (most are GPCRs; some taste receptors are ion channels).

Thermoreceptors respond to:

Temperature (warm and cold receptors in skin and hypothalamus).

Nociceptors respond to:

Painful (tissue-damaging) stimuli (mechanical, thermal, chemical).

Electroreceptors (e.g., ampullae of Lorenzini) are found in:

Fish, and in the bills of platypus and spiny anteater.

Magnetoreceptors likely contain:

Magnetite (ferrous ferrite) and are found in migratory birds, bees, and some bacteria.

Receptor potential is defined as:

A graded change in membrane potential of a sensory receptor cell in response to stimulation, proportional to stimulus intensity.

Transduction in sensory systems is:

The transformation of energy first into a graded receptor potential, then into a spike-frequency code.

"Short receptors" are:

Specialized non-neuronal cells that produce graded potentials and release neurotransmitter onto primary sensory neurons (e.g., hair cells, photoreceptors).

"Long receptors" are:

Neurons with axons that conduct action potentials over long distances (e.g., skin mechanoreceptors, olfactory receptors).

Adaptation in sensory receptors is:

A decreasing sensory response to a maintained stimulus.

A slowly adapting (tonic) receptor:

Continues to fire action potentials throughout a sustained stimulus (e.g., Merkel's disks, Ruffini's endings).

A rapidly adapting (phasic) receptor:

Fires only at the onset and offset of a sustained stimulus (e.g., Meissner's corpuscles, Pacinian corpuscles).

The five types of cutaneous mechanoreceptors are:

Merkel's disks (SA), Ruffini's endings (SA), Meissner's corpuscles (RA), Pacinian corpuscles (vRA), and hair follicle receptors (RA).

Pacinian corpuscles are specialized to detect:

Vibration (very rapidly adapting, acceleration detectors).

Piezo1 and Piezo2 are:

Mechanosensitive ion channels that are trimers shaped like a propeller (triskelion) with a central pore.

The Piezo channel opens when:

The membrane is stretched, causing the dome-like structure to flatten.

Hair cells (vestibular and cochlear) are mechanoreceptors that respond to:

Sound waves and head movement/acceleration.

Moving hair bundle toward the kinocilium causes:

Depolarization (increased K+/Ca2+ influx through stretch-gated channels, increased transmitter release).

Moving hair bundle away from the kinocilium causes:

Hyperpolarization (decreased K+/Ca2+ influx, decreased transmitter release).

Tip links connect:

Stereocilia of hair cells; they are composed of cadherin 23 (CDH23) and protocadherin 15 (PCDH15).

The mechanoelectrical transduction (MET) channel in hair cells is:

A heteromeric dimer of TMC1 and TMC2 (transmembrane channel-like proteins).

Usher's syndrome (deafness) can be caused by:

A mutation that disrupts the tip-link protein protocadherin 15.

Endolymph (bathing hair bundles) has:

High K+ and low Na+ concentration, and a positive extracellular potential of +80 mV.

The resting membrane potential of hair cells relative to endolymph is approximately:

-125 mV (due to -45 mV resting potential relative to +80 mV endolymph).

Sound intensity is encoded by:

Larger amplitude sound waves → larger hair bundle movement → larger receptor potential → increased firing in auditory neurons.

Sound frequency is encoded in mammals by:

The position of hair cells along the cochlea (base = high frequency; apex = low frequency; discovered by Georg von Békésy).

Outer hair cells function as:

Cochlear amplifiers that boost basilar membrane vibrations via electromotility (change length with depolarization).

Inner hair cells function as:

True sensory receptors that transduce sound and pass afferent information to the brain.

Otoacoustic emissions (OAEs) are:

Sounds generated by outer hair cell electromotility (discovered by David Kemp, 1978).

In birds, reptiles, and amphibians, sound frequency is encoded by:

Electrical resonance properties of hair cells (endogenous membrane potential oscillations via Ca2+ and Ca2+-activated K+ channels).

During electrical resonance in non-mammalian hair cells, depolarization is produced by:

Ca2+ influx.

During electrical resonance in non-mammalian hair cells, repolarization is produced by:

Calcium-activated K+ efflux (IK(Ca)).