Touch (discriminativesensation)

Proprioception (joint position andmovement senses)

Pain

generic receptor neuron somatosensory receptor neuron

axon

Touch

(Axon Ending Types)

touch receptor endings in skin

Ruffini corpuscle, proprioception (senses skin stretch)

at rest

activated

Deep touch, vibration Skin stretch sensation

Meissner corpuscle

Merkel disk receptors

Light touch

Touch

Nerve axon types

fine touch receptor

cutaneous nerve

dull pain

sharp pain

axon diameter (microns)

Num

ber

of a

xons touch receptors proprioceptors

dull pain“fullness”?

sharp paincrude touchheat, cold(itch, tickle?)

Touch

tracts

dorsal columns

dorsal

ventral

touch receptor neuron

dorsal roots

cuneate fasciculus

gracile fasciculus

Cuneate fasiculus: upper body

Gracile fasiculus: lower body

Not facial sensation (done by CNV)

dorsal column nuclei(cuneate n. and gracile n.)

medial lemniscus

VPL (ventral posterior lateral n.)

thalamus

main sensory trigeminal n.

VPM (ventral posterior medial n.)

medial lemniscus

thalamus

trigeminal ganglion

Facial Sensation

Lesions of SI cause... Loss of ability to localizeobjects

Loss of ability to recognizeobjects by feel

Loss of ability to localizepain

Preservation of ability to distinguish modalities (touch, vibration, heat,cold, pain)…but less sensitive to all

The map in somatosensory cortex is plastic

Lesions of PNS vs CNS

• Lesion of PNS axon = regeneration and regrowth

• Lesion of CNS axon = complete cell death

Proprioception

axon diameter (microns)

Num

ber

of a

xons proprioceptors

Golgi tendon

Golgi tendon organSignals muscle tensionResponds when the muscle

actively contracts, but not

when an external force pulls

on the muscle.

Spindle afferents: sense muscle length

… it reports the difference between desired and actual length

Active relaxation of muscle: no intrafusal lengthening. No spindle afferent response

Passive stretch: intrafusal muscle fiber lengthens. Spindle fires a response

Proprioceptive information follows 3 pathways...

Local (in spinal cord)

(results in the stretch reflex)

motor neuron

inhibitoryinterneuron

muscle spindle afferent

Golgi tendonorgan

+

Proprioceptive information follows 3 pathways...

To cerebellum

(Keeps the cerebellum informed about the actual movements and allows it to coordinate, smooth and refine movements)

dorsal nucleusof Clarke

external cuneate nucleus

dorsal spino-cerebellar tractTract continues to

cerebellum

Proprioceptive information follows 3 pathways...

To cerebral cortex

(We can consciously perceive proprioception.)

Pain

pain receptor neuron(nociceptor)

dorsal horn

nociceptor Lissauer’s tract

pain & temperature neurons

dorsal horn

pain receptor neuron(nociceptor)

dorsal horn

dorsal horn

anterior white commissure

spinothalamic tract

nameless tract

spinothalamic tract

VMpo (ventral medial nucleus, posterior part)

CM (central medial nucleus)

spinal trigeminal tract

trigeminal ganlion

Pain Sensation from Face

spinothalamic tract

VMpo (ventral medial nucleus, posterior part)

spinal trigeminal tract

spinal trigeminal nucleus

Receptor types in pain-temperature pathway:

sharp pain

dull pain (aching, burning)

heat

cold

crude touch

“fullness” (bladder, stomach, etc.)

itchtickle

free nerve endings

touch receptor

cutaneous nerve

dull pain (unmyelinated)

sharp pain

axon diameter (microns)

Num

ber

of a

xons touch receptors

proprioceptors

dull pain“fullness”?

sharp paincrude touchheat, cold(itch, tickle?)

sharp pain nociceptor = A delta fiber

activated by intense mechanical stimulationor high heat (> 45 deg C)

polymodal nociceptor = dull pain nociceptor = C fiber

activated by substances released by tissue damage and noxious stimuli:

• Bradykinin• Prostaglandins• Histamine• K+

• by acid (protons)• by heat > 42o C• by intense mechanical stimulation• by noxious substances (for example, capsaicin)

INNERVATES ALL TISSUE EXCEPT THE BRAIN AND LENS OF EYE

or thermal

• Hyperalgesia: intense pain in response to mildly painful stimulus (pinprick)

• Allodynia: pain in response to completely innocuous stimulus (touch)

Referred Pain: heart and left arm pain travel in the same track

Descending pain modulation

Neurons of the descending pain modulation system are activated by opium and its derivatives (morphine, etc.)

Endogenous opioid transmitters

• endorphins• enkephalins• dynorphins

• What activates descending pain modulation system?

• STRESS!

• fear

• hunger

• thirst

• fatigue

• prolonged motor activity

• hypnosis

Excitation of neurons in the rostral medulla causes inhibition of nociceptor neurons in the spinal cord.

opiates activate pathway here

… and here

opiates inhibit nociceptors here

Neuropathic pain syndromes:

tic douloureux (trigeminal neuralgia)

Chronic facial pain from vessels pinching on the trigeminal nerve. The C fibers are the smallest and easiest to stimulate to fire an AP.

Treatment: Surgery to reroute the offending vessel

Neuropathic pain syndromes:

tabes dorsalis

• The largest axons in the dorsal root ganglia (a beta and proprioceptors) are systematically destroyed.

• The dorsal columns degenerate. • Patient looses discriminative touch and

proprioception. • Locomotion becomes awkward and stumbling.• Also suffers from “lightening-like” stabbing pain

Neuropathic pain syndromes:

thalamic pain

• Spontaneous burning or crushing pain on one side of the body

• May be from lesion in VMpo or MD???• Narcotic meds not effective• electrical stimulation of precentral gyrus can

improve symptoms

Neuropathic pain syndromes:

phantom limb pain

• Narcotics not very effective• From reorganization of the somatoscopic

maps?