lecture 36 - body sensation (sensory receptors) 1 + 2 - PoNF

function of sensory receptors

provide sensations of touch, pressure, temperature, pain

why are sensory receptors known as transducers

they convert stimuli into frequency of action potentials

What are sensory receptors? (cutaneous receptors)

sensory neurons or cells that are able to respond to environmental stimuli

name the 3 main types of sensory receptors

mechanoreceptors

proprioceptors

nociceptors

function of mechanoreceptors

stimulated by mechanical stimuli

pressure

stretch

give skin sensations of touch and pressure

function of proprioceptors

mechanoreceptors in joints/muscles

signal information about body or limb position

Nociceptors function

respond to painful stimuli

Thermoreceptors

respond to changes in temperature

Chemoreceptors

respond to chemicals (e.g., smell, taste, changes in blood chemistry)

sensory modality

the stimulus type that activates a particular receptor

e.g. touch, pressure, joint angle, pain

what does sensory receptor transduction involve

opening or closing of ion channels

sensory transduction

adequate stimulus causes graded membrane potential change (only a few mV)

activates stretch sensitive ion channels so ions flow across membrane and change membrane potential locally

receptor/generator potential

change in membrane potential in response to stimulus

adequate stimulus in cutaneous mechanoreceptors and proprioceptors

membrane deformation

What is a receptor potential?

the graded potential in sensory receptor caused by a stimulus

how is a receptor potential generated

stimulus triggers ion flow through membrane locally

if depolarisation reaches area with voltage gated ion channels (1st node of Ranvier) then APs fire

lowest stimulus intensity - no APs

highest stimulus intensity - most APs

function of 1st node of ranvier

send AP when depolarised

function of general nodes of ranvier

saltatory conduction

frequency coded stimulus intensity

larger stimulus - larger receptor potential - higher frequency of APs

if stimulus persists, what can some mechanoreceptors do?

adapt - continuous mechanical stimulation causes a drop off in action potentials so brain can process new or changing events

action potentials persist for some - nociceptors

Adaptation of mechanoreceptors

adapt to maintained stimulus and only signal change or novel event

rapid : cease to fire

slow : still firing but slower

Rapid Adapting Mechanoreceptors

Pancinian corpuscle

Meisseners corpuscles

- action potentials when stimulus is increased and decreased only

Slowly adapting mechanoreceptors

Merkel and Ruffini

- action potentials only throughout high intensity stimulus

nociceceptors

DO NOT ADAPT

Pancinian Corpuscle

an encapsulated ending of a sensory nerve that acts as a receptor for pressure and vibration

Pancinian Corpuscle structure

myelinated nerve - naked nerve ending

enclosed by a connective tissue capsule of layered membrane lamellae and each layer is separated by fluid

Pancinian corpuscle response to stimuli

mechanical stimulus deforms the capsule and nerve ending

this stretches the nerve ending and opens ion channels

Na+ influx - local depolarisation - receptor potential

action potentials generated and fires when myelination begins

does the Pancinian corpuscle show rapid adaptation

yes

rapid adaptation of Pancinian corpuscle

mechanical stimulus causes deformation of capsule

nerve ending stretches

opening of ion channels

open ion channels causes local depolarisation causing a generator potential

action potentials fired - brain detects

fluid rapidly redistributes within capsule lamellae which spreads the stimulus impact out laterally minimising downward deformation

downward force causing mechanical stretch to nerve endings stops so action potentials stop firing

stimulus withdrawn

Pancinian corpuscle response normally

rapidly adapting, ON/OFF response

what happens if capsule is removed from Pancinian capsule

bare nerve endings, loss of adaptation, continues to produce receptor potential

what is the receptive field

area or range of sensory input that can trigger a response in a particular neuron

What is 2 point discrimination?

Ability to discriminate between two closely spaced points touching the skin

What does the 2 point discrimination test depend on?

receptive field size

neuronal convergence

what causes low acuity

high convergence and a large secondary receptive field

lateral inhibition

allows us to locate stimulus precisely