Lecture 9: Spinothalamic, Posterior Column, Thalamo-cortical Pathways

Question 1

What are the two pathways for somethesis?

Answer 1

Protopathic (anterolateral)

Epicritic (lemniscal)

Question 2

What does the protopathic pathway detect?

Answer 2

pain, crude touch and temperature sensation

Question 3

What does the epicritic pathway detect?

Answer 3

fine touch including information about form, texture, touch, pressure, slippage, vibration and position of muscles and joints; proprioception

Question 4

What type of spatial and temporal resolution is found with the protopahtic pathway?

Answer 4

low

Question 5

What type of spatial and temporal resolution is found with the epicritic pathway?

Answer 5

high

Question 6

What fiber type is associated with the protopathic pathway?

Answer 6

small, slowly conducting, lightly myelinated and unmyelinated

Question 7

What fiber type is associated with the epicritic pathway?

Answer 7

large, rapidly conducting, myelinated

Question 8

What is the ascending tract associated with the protopathic pathway?

Answer 8

Lateral Spinothalamic Tract

Question 9

What is the ascending pathway associated with the epicritic pathway?

Answer 9

Posterior and Posterolateral Columns

Question 10

Pacinian Corpsucle

Answer 10

• Large, lamellar, rapidly adapting mechanoreceptor
• Detects gross pressure and vibratory skin stimuli (`250Hz)
• Found throughout sub skin, joints, muscle, and mesentery.

Question 11

Meissner’s Corpuscle

Answer 11

• Rapidly adapting skin mechanoreceptor
• Sensitive to light touch and vibration (<50Hz).
• Located in glaborous (hairless) skin right below the epidermis.

Question 12

Merkel’s Disks

Answer 12

• Slowly adapting mechanoreceptors
• Located in skin and mucosa
• In glaborous skin, found clustered beneath the ridges of fingerprints.
• In hairy skin, they cluster in specialized epithelial structures called “touch domes” or “hair disks”

Question 13

Ruffini Organs

Answer 13

• Slowly adapting mechanoreceptors
• Found only in deep layers of glaborous skin.
• Respond to sustained pressure and skin stretch
• Thought to be responsible for deleting objects slipping along the skin and also contribute to position sense.

Question 14

Free Nerve Endings

Answer 14

• Nerve endings that are not associated with any accessory organs.
• Typically responsive to temperature and nociceptive stimuli.

Question 15

What is typical of pain receptors?

Answer 15

They are usually relatively unspecialized free nerve endings.

Question 16

What type of axons are usually attached to pain receptors?

Answer 16

Adelta or C fibers (Type III or IV Afferents)

Question 17

Where do touch and afferents send their collaterals?

Answer 17

Primarily to the dorsal horn with a few in the ventral horn.

Question 18

Where do Aalpha afferents come from and project to?

Answer 18

They come from muscles receptors (spindles and golgi tendon organs) and project to the ventral horn, as well as deep layers of the dorsal horn.

Question 19

Where do Abeta fibers come from and project to?

Answer 19

They include many cutaneous and joint receptors (and secondary spindle organs) and project mostly to the dorsal horn.

Question 20

Lissauer’s Tract

Answer 20

Some axons traveling in the protopathic (spinothalamic) pathway may travel a short way (2-3 spinal cord segments) before synapsing in the dorsal horn. Therefore, it takes 2-3 spinal cord segments for all afferents entering at a given level to cross to the contralateral cord. Thus, lesions at a given spinal cord segment result in loss of input form 2-3 levels below the injury.

Question 21

Clarke’s Nucleus

Answer 21

• Is located in lamina VII of the intermediate zone of the spinal cord and is found approximately at the level of T2-L2 and is involved in unconscious proprioception.
• Proprioceptive afferents from the lower body terminate here in the spinal intermediate zone.
• Is the origin of the dorsal spinocerebellar tract and thus is the primary input to the cerebellum from spinal cord proprioceptive afferents.

Question 22

Substantia Gelatinosa

Answer 22

• Lamina I and II of the spinal cord gray matter.

* ***This is the site of first modualtion of pain and temperature information.

Question 23

Lissauer’s Tract (Posterolateral Fasiculus)

Answer 23

• Composed of sensory fibers carrying pain and temperature that ascend or descend several spinal cord levels before synapsing in the dorsal horn.
• Also contains short axons of projections of neurons from lamina I and II.

Question 24

What are the three divisions of the spinothalamic pathway?

Answer 24

1. Spinothalamic
2. Spinoreticular
3. Spinomesencephalic

Question 25

What is the spinothalamic pathway important for?

Answer 25

Discriminative aspects of pain.

i. e. reports to the brain that “a sharp object is sticking in my right foot”
* *Axon collaterals of these fibers synapse in the brainstem.

Question 26

What is the spinoreticular pathway important for?

Answer 26

General arousal and emotional aspects of pain.

i. e. signals “Ouch! That hurts!”
* *Axon collaterals of these fibers synapse in the reticular formation in the pons and medulla.
* *Come from all layers of the dorsal horn.

Question 27

What is the spinomesenphalic pathway important for?

Answer 27

Activating descending, efferent pathways that modulate pain.

i. e. initiates pathways that modulate pain–> “Ah, that’s better.”
* *Axon collaterals in the periaqueductal gray matter of the mesencephalon.
* *Fibers come from layers I and V of the dorsal horn.

Question 28

How are afferents from the spinothalamic pathway added to the cord as it ascends?

Answer 28

Axons from the cervical dorsal horn are added more medially that those from the lumbar which is opposite form how they are added in the epicritic pathway.

Question 29

Adelta Fibers

Answer 29

• Mediate first pain, which is a sharp sensation that can be readily localized on the body.
• These fibers synapse in Rexxed’s Lamina I and II (substantia gelatinosa), especially layer I which is the marginal zone.
• This is the initial site of integration of pain information and short axons run up and down the cord form these layers (these short fibers that run between spinal cord segments are know as the propriospinal tract).
• Interneurons in layers I and II also project to layer V and are the ones that send their axons across the midline to ascend the cord.

Question 30

C Fibers

Answer 30

• Smallest axons
• Associated with second pain (duller less localized aspect of pain)
• Synapse in the substantia gelatinous (esp. layer I marginal zone).
• The layer I neurons send their axons across the midline and up the cord.

Question 31

Explain the phenomena of referred pain.

Answer 31

• For fine touch, Aalpha and Abeta fibers enter the dorsal horn and their main axon ascends the cord in their ipsilateral posterior columns.
• These afferents send collaterals into the dorsal and ventral horn.
• *Some collaterals of the epicritic system synapse on the same layer V cells that protopathic pain fibers do.

Question 32

What are the components of the trigeminal nuclei?

Answer 32

1. Motor Nucleus of V (motoneurons of masticatory muscles and a few others)
2. Chief Sensory Nucleus of V (cell bodies of second order neurons for the epicritic system for the head/face)
3. Mesencephalic Nucleus of V (cell bodies of primary afferents for muscle spindles and golgi tendon organs for masticatory muscles)
4. Spinal Nucleus of V (cell bodies of second order cells for the protopathic system for the head/ face)

Question 33

What is the chief sensory nucleus of the trigeminal nuclei analogous to?

Answer 33

Posterior Column Nuclei

Question 34

What is the mesenphalic nucleus of the trigeminal nuclei analogous to?

Answer 34

Analogous to a displaced DRG, found within the brainstem.

Question 35

What is the spinal nucleus analogous to?

Answer 35

Dorsal Horn (especially the substantia gelatinosa)

Question 36

Trigeminal of Gasserian Ganglion

Answer 36

• Analogous to the DRG of the spinal cord.

- Location of the cell bodies of the somatosensory afferents of the head /face.

Question 37

Where do the trigeminal projections synapse at?

Answer 37

VPM Nucleus

Question 38

Describe the arrangement of muscle receptors for the muscles of mastication.

Answer 38

The cell bodies for the afferents are located in MESENCEPHALIC NUCLEUS OF CNV, within the brainstem, rather than the trigeminal ganglion. The central processes of these cells project bilaterally to the MOTOR NUCLEUS OF CNV (basis for monosynaptic stretch reflex).

**All other afferents have their cells bodies in the trigeminal ganglion and their central processes project to the brainstem.

Question 39

Describe the pathway for fine touch in the trigeminal lemniscus.

Answer 39

The afferents project to the CHIEF SENSORY NUCLEUS OF CNV and first synapse here. These second order axons cross to the CONTRALATERAL VPM which then project to the PRIMARY SOMATOSENSORY CORTEX.

Question 40

Describe the trigeminothalamic tract.

Answer 40

Protopathic afferents have their cells bodies in the TRIGEMINAL GANGLION and their central processes project to the IPSILATERAL TRACT OF V where they synapse. The second order axons project to the CONTRALATERAL VPM which in turn projects to the PRIMARY SOMATOSENSORY CORTEX via the POSTERIOR LIMB OF THE INTERNAL CAPSULE.

Question 41

Primary Somatosensory Cortex

Answer 41

• Known as S1.

- Includes Brodmann’s Arease 3a, 3b, 2 and 1.

Question 42

S2 (Parietal Operculum)

Answer 42

-A unimodal (only somatosensory) association cortex where further processing of information concerning somesthesis occurs.

Question 43

Borden’s Areas 5 and 7

Answer 43

-In the superior parietal lobe and are another unimodal (mostly) area for somesthesis.

Question 44

Area 3a

Answer 44

receives primarily muscle afferents and nociceptive info from the thalamus (proprioception and pain)

Question 45

Area 3b

Answer 45

receives primarily cutaneous inputs (slowly and rapidly adapting: tactile perception)

Question 46

Area 1

Answer 46

gets primary input form 3b- cutaneous; more detailed info about texture is processed here.

Question 47

Area 2

Answer 47

gets primary projection from both 3a and 3b, thus this area integrates pain, tactile, and proprioceptive information to allow position and edge detection, size perception, etc.