02 - Sensory Receptors

sensory modalities

each sensation has its own receptor type but uses same electrical language

• photoreceptor → vision

• chemoreceptor → taste, smell

• mechanoreceptor → touch, proprioception, hearing

• thermoreceptor → temperature

• nociceptor → pain

labeled line theory

each modality signals separately to higher brain centers

• action potentials look alike, but it allows brain to know what signal is based on where it came from

transduction vs perception

• transduction → converts stimulus to electrical signal

• perception → brain’s interpretation

  • context is important in perception

periphery sensory receptor

most touch axons are myelinated, so they are fast

• cell body in dorsal root ganglion

• one branch to skin (receptor)

• one branch to spinal cord, which goes to brain

mechanoreceptor model

PIEZO2 channel = molecule that transduce touch into electrical signal

• mechanical force stretches membrane

• stretch opens mechanosensitive ion channels

• sodium enters, potassium leaves → depolarization

  • channels open stochastically → more pressure = more channels open

stochastic channel opening

individual ion channels open all-or-nothing

• open/close probabilistically

• electric potential → sum of all currents from all channels in cell

• graded receptor potential → many channels opening together

formation of action potential

• stimulus opens mechanosensitive channels

• graded receptor potential forms

• potential spreads locally (decremental)

• voltage-gated Na⁺ channels activated

• action potentials generated

  • stronger stimulus → more action potentials

  • longer stimulus → more firing duration

• AP propagate without decrement

• NT released at terminal

slowly vs rapidly adapting receptors

• slowly adapting → responds to sustained pressure

  • potential is the largest at the beginning and persists for whole duration of stimulus

  • action potential duration is increased, lasting the duration of the stimulus

• rapidly adapting → responds to changes in pressure

  • potential is shorter, peaking when there are changes in pressure

  • no action potential when at steady-state

cutaneous mechanoreceptors

• Meissner’s corpuscle → superficial, rapidly adapting, small receptive field

• Merkel cells → superficial, slowly adapting, small receptive field

• Pacinian corpuscle → deep, rapidly adapting, large receptive field

  • most sensitive receptor to vibration

• Ruffini endings → deep, slowly adapting, large receptive field

Pacinian corpuscle

deep mechanoreceptor that is rapidly adapting and has large receptive field

• response to stimuli dominated by response at time of transition in pressure

• when capsule removed → response changes from rapid to slow

  • capsule converts sustained pressure into transient signals

receptive field

area of skin where stimulation affects a neuron’s firing

• one axon = one receptive field

• small receptive field → high spatial resolution

• large receptive field → poor localization, loss of definition

• skin is covered with overlapping receptive fields

two-point discrimination

tests minimum distance to detect two separate points

• discrimination due to density of receptors and cortical representation

  • best discrimination → fingers, lips

  • worst discrimination → back, thighs

temperature sensation

separate receptors for warm and cold sensations

• cold fibers → 5-36ºC

  • send signals to small myelinated Aδ fibers

• warm fibers → 30-45ºC

  • send signals to unmyelinated C fibers

• for temperatures outside the ranges of cold and warm fibers, they are perceived as pain

TRP receptors

transient receptor potentials (TRP) respond to different temperature ranges

• TRPV1 → activates with heat and capsaicin

• TRPM8 → activates with cold and menthol

three neuron chain

• first order neuron → signal from receptor ending travels to dorsal root ganglion cells to medulla ipsilaterally

  • synapse at cuneate nucleus

  • if from the face, first synapse is in trigeminal nucleus

• second order neuron → axon crosses midline and travels up to thalamus (midbrain)

  • synapse at ventral posterior nucleus

• third order neuron → axon travels to primary somatosensory cortex

processing at relay stations

• convergence → many signals combine into one

  • convergence along sensory pathways towards CNS

• divergence → one signal divides into many

• lateral inhibition → inhibit nearby axons

  • regulates size of receptive field to sharpen spatial contrast

  • strongly activated neurons inhibit neurons

  • weak signals suppressed

• descending modulation → cortex sends signals down

somatosensory cortex organization

amount of area devoted to each part of the body is proportional to number of axons from that body part or surface or density of sensory innervation of the region

• medial → legs

• lateral → face

• hands and lips → large representation

  • due to small receptive fields and high spatial resolution

subdivisions of somatic sensory cortex

• areas 3b & 1 → inputs form touch receptors

• areas 3a & 2 → inputs from proprioreceptors

• area 5 → combines input from both kinds of receptors (association cortex)

cortical columns

cells in cortex are arranged in columns of similar fields

• cells stacked vertically respond to same skin region

• different layers = different processing

plasticity

map in somatic sensory cortex is not fixed, and are modifiable with experience

• amputated finger → cortical map reorganizes

higher-order cortical processing

receptive fields of cortical cells are larger than those of peripheral neurons, varying in different regions of somatic sensory cortex

• areas 1 & 2 → cells respond to much larger receptive areas

• area 3b → cells respond to very small receptive fields

• area 5 → some cells respond to touch from either hands

  • shows convergence

• orientation-sensitive neuron & direction-sensitive neuron → cortex can extract patterns, not just intensity