Touch (discriminative sensation) Proprioception (joint position and movement senses) Pain

generic receptor neuronsomatosensory receptor neuron axon

Touch (Axon Ending Types)

touch receptor endings in skin

Ruffini corpuscle, proprioception (senses skin stretch) at rest activated

Deep touch, vibrationSkin 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) Number of axons touch receptors proprioceptors dull pain “fullness”? sharp pain crude touch heat, 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 localize objects Loss of ability to recognize objects by feel Loss of ability to localize pain 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) Number of axons proprioceptors

Golgi tendon

Golgi tendon organ Signals muscle tension Responds 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 inhibitory interneuron muscle spindle afferent Golgi tendon organ +

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 nucleus of Clarke

external cuneate nucleus dorsal spino- cerebellar tract Tract 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 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.) itch tickle

free nerve endings

touch receptor cutaneous nerve dull pain (unmyelinated) sharp pain

axon diameter (microns) Number of axons touch receptors proprioceptors dull pain “fullness”? sharp pain crude touch heat, cold (itch, tickle?)

sharp pain nociceptor = A delta fiber activated by intense mechanical stimulation or 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?