Chapter 12: The Somatic Sensory System

Somatic sensation includes:

Touch

pain

temperature

proprioception

Skin receptors on glabrous skin include

free nerve endings

meissner corpuscle

merkel’s disk

Ruffini endings

Pacinian

Skin receptors on hairy skin include:

merkel’s disk

ruffini endings

hair follicle receptor

free nerve endings

pacinian

2 layers of skin

epidermis, dermis

Merkel’s disk

light touch

align with top of fingerprint

highest spatial resolution → form and texture

Meissner’s corpuscle

on valleys of fingerprints

fine touch

transducing low frequency vibrations (textured objects moved across skin) 3-40 Hz

grip and sensing slippage

Ruffini endings

sensitive to stretching

determine position of fingers

Pacinian

deep pressure, high frequency vibrations 250-350 Hz (through objects), filter out low frequency stimuli

Receptive field

area of skin innervated by somatosensory nerve ending

small receptive field

merkel’s disk

meissner corpuscles

• more sensitive

large receptive field

Ruffini endings

Pacinian corpuscles

• less sensitivity

slow adapting

merkel’s disk

ruffini ending

rapidly adapting

meissner corpuscles

Pacinian corpuscles

how are receptors distributed on skin?

The concentration of different receptors differ for each area of the skin depending on the function

ex) high concentration of meissner’s corpuscle on fingertips for fine touch

hair follicle receptor

deflection of hair → EPSP or IPSP

can be fast or slow adapting

guard hairs: more sensitive to deflection

Function of corpuscle

corpuscle: 20-70 concentric layers of connective tissue

continued pressure → layers slip and pressure diffuses → potential dissipate

removing stimulus cause deflexion of axon

→ sensitive to vibrations

Tip of sensory axons

unmyelinated tip with mechanically-gated ion channels

opening of ion channels at tip of sensory axons

1. stretch axon membrane

2. force to extracellular protein

3. force to intracellular protein

two-point discrimination

smallest d between 2 points touching skin at same time where you feel 2 points instead of 1

Primary afferent axons

somatosensory receptors to spine → brain

Pathway

primary afferent axons enter spinal cord through dorsal root ganglia → A beta neurons to:

1) dorsal column → dorsal column nuclei (medulla) - decussation → VP thalamus → S1 (Brodmann’s 3)

2) dorsal horn → second-order sensory neurons (reflexes)

A alpha neurons

proprioceptors of skeletal muscle → reflexive contraction of muscle

• muscle spindle

A beta neuron

mechanoreceptors of skin

primary afferent axons

A gamma

pain, temperature

C

temperature, pain, itch

dermatomes

C1 has none because it only as muscle innervation

1:1 correspondence between dermatome and spinal segments

Spinal cord ends at

L3, cauda equina below L3

Cortical areas

Brodmann’s 1, 2, 3

Brodmann’s 3a

recieve projections from VP

proprioception

Brodmann’s 3b

recieve projections from VP

to area 1, 2 (texture and size)

Brodmann’s 1

Texture

Brodmann’s 2

Size, shape

somatotopy of cortex

cortical columns for slow vs rapid adapting neurons

features with higher sensation have more representation on cortex

somatosensory cortex has plasticity depending on

changes in function

Phantom pain

outgoing motor commands but no feedback from muscle spindles → muscle continues to contract at dangerous rates

Pain

nociceptors

Nociceptors are

free nerve endings

K+/Bradykinin/Histamine/Prostaglandins

Temperature

A gamma fiber creates

first pain

sharper, shorter

C fiber creates

second pain

dull, more sustained

temperature

thermoreceptors

thermoreceptor receptor fields

not evenly distributed: cold vs hot spots 1mm wide on skin

each neuron contains one thermoreceptor channel type

what can bind to thermoreceptors

unrelated chemicals

at very high or low temperatures

nociceptors get activated, thermoreceptor activation does not decrease or increase infinitely

Pathway of pain

C fibers → dorsal root ganglion → synapse at zone of lissauer → decussate at spine → (spinothalamic tract) → VP thalamus

Pain afferents contain:

• glutamatergic NT (synaptic vesicles)

• substance P (storage granules)

  • only in high frequency stimulation

  • cause swelling, release of histamines, inflammation, hyperalgesia

Pain sensitization

caused by nociceptors being stimulated by chemicals during inflammation

• prostaglandins

• bradykinin

• histamine (from mast cells)

Hyperalgesia

painful stimuli perceived as more painful

allodynia

pain from normally painless stimulus

pain downregulation pathway

PAG (midbrain) → Raphe nuclei (medulla) → dorsal horn: suppress incoming nociceptive signals

Painkiller placebo

activates the descending control pathway to downregulate pain