Tactile Mechanoreceptors
Non-encapsulated
- Free nerve endings (pain and temp)
- Merkle cells (fine touch)
- hair receptors
Encapsulated
- Meisner (touch)
- Pacinian (vibration)
- Ruffini (stretch)
Proprioception Mechanoreceptors
Detect
Muscle stretch
Joint position
Muscle tension
Major Somatosensory Pathways
DC-ML and Trigeminal
-Fine tactile and proprioceptive
ALS and Spinal Trigeminal
-Pain, Temp, Crude touch
Dorsal Spinocerebellar and Cuneocerebellar
-Unconscious proprioceptive
Anterolateral System (ALS)
Pain, Temp, Crude Touch
also called SPINOTHALAMIC TRACT
unencapsulated nerves
-nocioreceptors, mechanoreceptor, thermo
Thermo receptors
Heat 29-45
Cold 5-40
Ion TRP channels
Nociceptors
Mechanical, thermal, chemical stimuli
Adelta = fast sharp pain
C = slow dull/burning pain
Sensitization of Pain
DEC threshold for Nociceptor
Occurs in damaged tissue:
Prostaglandins, bradykinin, etc
ALLODYNIA: normal stimuli perceived as painful
First order ALS
Axon start at receptor
Cell bodies in Dorsals Root Ganglion
Enter spinal chord and branch
Collateral branches go up and down DORSOLATERAL TRACT OF LISSAUER
Synapse with 2nd Order in dorsal column
Second Order ALS
Receive info from 1st order in dorsal horn
axons DECUSSATE in the ANTERIOR/VENTRAL WHITE COMMISSURE
Go to anterolateral portion of white matter (on contralateral side)
Join SPINOTHALAMIC TRACT all way up
Higher parts of body add to medial side (lower parts are lateral)
ALS in brainstem
Caudal medulla = anterolateral
Rostral Medulla = Lateral
Pons= lateral tegmentum (lateral dorsal pons)
Midbrain = lateral again
Third Order ALS
2nd order go to VPL nucleus of thalamus
3rd order branch off to different parts of brain
Sensory Homunculus
Third order sensory fibers from thalamus mainly go to PRIMARY SOMATOSENSORY CORTEX
Located in post central gyrus
Amount of cortex allotted depends on density of receptors, not size of region
Organized by location (starting medial) Genitals Legs Hip Trunk Neck Arm Elbow hand Fingers Thumb Eye Nose Face Luis Teeth Tongue
Pain modulation
- feed forward from receptor
2. Descending pathway from CNS
Descending Pain Modulation
- CNS and Periaqueductal gray matter in midbrain
- activated by opiates
- goes to substantia gelatinous a
- release serotonin, norepi, and opiates
Gate Control Theory
Mechanosensory fibers act as gate to modulate pain
Uses inhibitory neurons located in dorsal column
Spinothalamic Tract Lesions
Contralateral loss of pain starting a few segments below lesion
Pain eventually comes back through alternate pathways
Fine Touch, Vibration, Pressure, Proprioception
Go through DC-ML system
Fast, myelinated fibers
First Order DC-ML
Cell bodies in dorsal root
Some collaterals axons synapse in dorsal horn
Most axons go straight up DORSAL COLUMN to brain stem
As go up body, axons add laterally to dorsal column (legs most medial)
Somatotopy
Point for point correspondence of an area of the body to a specific point in CNS
ALS: lower more medial
DC at T6
- Fasiculus GRACILIS = Lower body = medial
- Fasiculus CUNEATUS = Upper body = lateral
DC Junction
Caudal Medulla
First order fibers synapse in DORSAL COLUMN NUCLEI
- nucleus GRACILIS
- Nucleus CUNEATUS
Second Order DC-ML
Begin at DC Nuclei in caudal medulla
Make up INTERNAL ARCUATE FIBERS
-decussate
Become MEDIAL LEMNISCUS
- on cotralateral side
- immediately dorsal to pyramids
Go up to VPL of thalamus
Third order DC-ML
Second order synapse in VPL of thalamus
Go through internal capsule
End at primary sensory cortex in
post central gyrus
DC-ML Lesions
Deficits of fine, discriminative touch
Can’t identify objects by hand
Loss of position sense
Clumsy movement
ROMBERG SIGN
-unsteadiness when eyes closed
Trigeminal Nerve Fibers
Cell bodies in Trigeminal Ganglion (Semilunar)
-lateral wall of cavernous sinus
Proprioceptive (primary) nerve fibers in BRAIN STEM
- woooaahhhh
- mesencephalic nucleus of Trigeminal
CN V enters brainstem through middle cerebellar peduncle in mid pons
Trigeminal Nuclei
Mid pons
- Principal Nucleus -
- Spinal Nucleus
Principal Nucleus
- Fine Sensory nucleus immediately dorsal to nerve entry
- most SECOND ORDER neurons decussate and go up anterior/ventral trigeminothalamic tract
- a few go up ipsilateral posterior/dorsal trigem tract - Motor nucleus immediately medial to nerve entry
ALL GO TO VPM IN THALAMUS
Third order Trigem
Start in VPM
Go to face area of primary somatosensory cortex
post central gyrus
Spinal Tract of V
Small diameter fibers
Pain and temperature
Go caudal immediately after nerve entry to upper cervical spinal cord
IPSILATERAL
Terminate in SPINAL NUCLEUS of trigem
Trigeminothalamic Tract
Secondary nerve fibers from BOTH the principal nucleus and spinal nucleus of trigem
DECUSSATE
Go up next to medial lemnicus to VPM
Unconscious Proprioception
Muscle stretch and tension
Goes to cerebellum (Spinocerebellum)
Pathways are IPSILATERAL
Dorsal Spinocerebellar Pathway
First Order nerve fibers from muscles, joints, etc
terminate in nucleus dorsalis/Clarks nucleus in spinal cord
Second Order nerve fiber rise IPSILATERALLY in dorsal part of lateral funiculus
Go through inferior cerebellum
Cuneocerebellar Tract
Upper limb equivalent
First Order from receptors go up spinal cord with Fasiculus Cuneatus to the EXTERNAL CUNEATE NUCLEUS
Second Order fibers begin at ECN and go through inferior cerebellar peduncle