Photoreceptors & Thermoreceptors

Photoreceptor function

transduce light from EM radiation to APs

Photoreceptor workflow (simple)

absorb photons → change membrane potential → action potential → fibers/nerves/tracts → vision

Rods

good in dim conditions, sensitive to light, low acuity, more abundant in retina

Cones

good for bright condition, rapid response to variations in color, good acuity, less abundant in retina

Cones location

concentrated in macula and fovea

Intrinsically photosensitive retinal ganglion cells

impacted by light to influence circadian rhythm and pupillary reflex, DOES NOT CONTRIBUTE TO SIGHT

Outer Segment

captures light and converts to electrical signals

phototransduction

light waves to APs

Connecting Cilium

connects outer and inner segments, traffics proteins to outer segment

Inner segment

metabolic and biosynthetic machinery/cell organelles

Nuclear Region

continuous with inner segment, houses nucleus

Synaptic Region

contains synaptic vesicles to transmit glutamate to secondary neurons

Outer segment of rods

disks are structurally isolated, contains rhodopsin

Outer segment in cones

disks not structurally isolated, contain photopsins

Outer segment recycling

destroyed distally by phagocytosis, and created proximally

Rod photoreceptor proteins

Retinal + opsin (general) = rhodopsin

Cone photoreceptor proteins

retinal + specialized opsins

Intrinsically photosensitive ganglion cell proteins

melanopsin

Melanopsin function

reflexive responses to circadian rhythms and pupillary reflexes

S cone

detects blue light

M cone

detects green light

L cones

detect red light

Color of rod detection

black

Photoreceptor output strength

more incoming photons, brighter colors perceived

Photoreceptor detection limitations

do not measure wavelength or detect color, will absorb based on sensitivity

Phototransduction cascade start

photon absorbed in opsin disk, 3d configuration change, will activate transducin

Amplification of phototransduction

each opsin activates 100 transducins

Phototransduction cascade molecules

Opsin shape changed → activates transducin → activates PDE → coverts cGMP to GMP

Impact of decreased cGMP

close Na+ channels leading to hyperpolarization and Ca2+ channels closing

Impact of closed Ca2+ channels

Ca2+ influx, decreased glutamate, AP fires

Thermoreceptor specificity

non-specialized, understands relative changes in temperature

Thermoreceptor types

Hot and cold, relative to body temperature

Warm receptors

increase warm, increase AP firing rate/decrease warm, decrease AP firing rate

Cold receptors

increase cold, increase AP firing rate/decrease cold, decrease AP firing rate

High temperature response

will feel cold before feeling the burn

Kraus End Bulb Location

throughout body

Kraus End Bulb Detection

cold temperatures

Ruffini Ending Detection

extended heat, stretch, pressure

Ruffini Endings

enlarged dendritic endings, destroyed in 3rd degree burns

Ruffini Ending Speed

Slow adapting