Human Physiology: Sensory Transduction

Sensory Transduction

introduction to sensory transduction

- steps in sensory processing

- transduction: conversion of external energy into electrical energy (as graded potentials)

- encoding: translation of the qualities of the stimulus into the language of the nervous system (action potentials)

- interpretation: organization and routing of signals to the appropriate integration areas of the CNS (mapping)

- elements of transduction

- physical stimulus: actual physical cause (light, vibrations, chemicals, heat)

- modality: perceived sensation (vision, hearing, taste, temperature)

- receptor: specialized structures that respond specifically to a particular form of energy (photoreceptors, mechanoreceptors, baroreceptors)

- doctrine of specific nerve energies: each sensory modality is subserved by a class of specialized receptor cells

adequate stimulus

- modality: particular type of sensory stimulus

- adequate stimulus: the physical stimulus that a receptor best responds to

- photoreceptors: respond to light

- nociceptors: respond to stimuli intense enough to cause tissue damage

- Pacinian corpuscles: mechanoreceptors sensitive to vibration (touch and pressure)

- receptors can respond to other types of stimuli

- input will still be perceived based on the type of receptor

- this can be demonstrated by production of a visual artifact by putting pressure on the eyeball

receptor potential

- types of receptors

- modified axon terminus e.g. Pacinian corpuscle

- separate receptor cell e.g. retinal photoreceptor

- generation of receptor potential

- graded local potential: V_m depolarization and membrane permeability increases with stimulus intensity

- generator region: specialized ending or region where receptors respond to sensation by alterations in V_m

- receptor potential: local depolarization caused by permeability change resulting from sensory input

- respondent ions: generally Na⁺, K⁺, and possibly Cl^-

- predominant role: Na⁺ ions, as V_m is being depolarized from a negative value

- Pacinian corpuscle

- ionic participants: predominantly Na⁺ (90%), but other ions involved as well

- sensory mechanism: stretch-activated mechanosensitive ion channels

- note: sensory mechanisms are unknown for most receptors, due to measurement difficulties

induction of an action potential in the adjacent membrane

- receptor potentials cause formation of potential gradients between generator region, adjacent membrane

- extracellular current: flows towards the generator region

- intracellular current: flows away from the generator region

- adjacent depolarization may then initiate an action potential that is propagated into the CNS

comparison of receptor potential to action potential

- the receptor potential is a purely local response

- amplitude of the receptor potential declines passively with distance from the generator region of the receptor

- rate of decline: depends on the length constant (λ) of the axon with which the receptor is located

- the amplitude of the generator potential is a graded potential

- action potentials: all or nothing responses (amplitude of AP does not vary significantly with stimulus intensity)

- generator potentials: amplitude varies directly with stimulus intensity

- stronger sensation (size, length) cause regions adjacent to generator regions to be more frequently depolarized

- in this way, stronger sensations lead to more frequent firing of action potentials

tonic receptors encode stimulus intensity (magnitude)

- tonic (slow adapting) receptors: evoke repetitive action potentials in response to a maintained stimulus

- frequency of action potentials varies with stimulus intensity

- correlation for most tonic receptors:

- encoding intensity

- low level stimuli: individual receptor provides information about stimulus intensity, magnitude

- high level stimuli: individual receptors are saturated, so information is encoded by number of active receptors

- thresholds are normally distributed with respect to stimulus intensity

- with increasing stimulation, more receptors of progressively higher threshold are recruited

- variability: most likely determined more by geometry and location than variability in threshold V_m

phasic receptors encode velocity and rate of change (acceleration)

- phasic (rapidly adapting) receptors: encode velocity of a stimulus or its rate of change

- adaptation: allows some receptors to encode both magnitude and rate of application of a stimulus

- stretch receptors: in joint capsules and tendons; receptor potential increases with increased stretch rate

- velocity sensors: receptors specialized to detect velocities; serve important functions in motor control

- adaptation in the Pacinian corpuscle

- Pacinian corpuscle: specialized phasic receptor where terminus is incased in a laminated capsule

- tonic vs. phasic receptors

- adaptation speed refers to speed by which V_m retuens to rest

- Pacinian corpuscle: acts phasically with capsule, tonically without capsule

- tonic: transient stimulus causes an initial burst of APs, followed by more regular APs

- phasic: transient stimulus causes an initial burst of APs, followed by no APs (rapid adaptation)

- physical properties may play a role in adaptation pattern of other mechanoreceptors, tonic or phasic

- memory device: phasic are fast

- mechanism of sensation in the Pacinian corpuscle

- compression: initial sensation that generates a receptor potential

- adaptation: laminae squeeze to the side, and stop tripping receptor potential

- recoil: fluid returns to receptor, once again generating a receptor potential