Spinothalamic, Posterior Column, Thalamo-cortical Pathways

Question 1

There are two systems for somesthesis (somatosensory signaling), namely:

Answer 1

protopathic (anterolateral) pathways

epicritic (lemniscal) pathways

Question 2

Again, there are two separate systems and pathways for conveying somatosensory information from the periphery of the body to the brain. What does the protopathic system mediate?

Answer 2

The first system (protopathic) concerns pain, crude touch, and temperature sensation.

Question 3

What does the epicritic pathway mediate?

Answer 3

A second system (epicritic) concerns fine touch, including information about the form and texture of objects, pressure on the skin or body, position of muscles and joints, and vibration sense.

Question 4

How is the spatial and temporal resolution of the protopathic pathway? epicritic?

Answer 4

protopathic- low

epicritic- high

Question 5

What is the fiber type for the protopathic pathway?

Answer 5

small, slowly conducting light myelinated and unmyelinated fibers

Question 6

What is the fiber type for the epicritic pathway?

Answer 6

large, rapidly conducting, myelinated

Question 7

What is the ascending tract of the protopathic pathway?

Answer 7

lateral spinothalamic tract

Question 8

What is the ascending tract of the epicritic pathway?

Answer 8

posterior and posterolateral columns

Question 9

Where are motoneuron cell bodies located?

Answer 9

In the spinal cord ventral horn.

Question 10

How do motoneurons act?

Answer 10

The axons of motoneurons exit the ventral horn in the ventral roots and travel within spinal nerves to their target muscles. This is the “final common path” for motor activation and the efferent side of spinal reflexes.

Question 11

Sensory receptors for somesthesis are found where?

Answer 11

In the skin (cutaneous), in muscles and joints, and visceral organs.

Question 12

How do sensory receptors communicate with the CNS?

Answer 12

The receptors communicate with the CNS via the peripheral processes of pseudo unipolar sensory neurons. The cell bodies of these neurons are located outside the spinal cord in the dorsal root ganglia (DRGs).

Action potentials in the DRG cell peripheral process continue (within the dorsal roots) along the central process into the dorsal horn of the spinal cord.

Depending on the type of receptor and which somesthetic system is involved, the peripheral process may synapse in the dorsal horn or project up the spinal cord to the brainstem.

Question 13

Major landmarks:

C4: low collar shirt

C6: thumb

C8: ulnar to thumb

T4: nipples

T10: umbilicus

Answer 13

T12: Inguinal

L4: Medial side of great toe and knee

S1: lateral side of foot and shin

Question 14

What are some myelinated peripheral fiber types?

Answer 14

Aa, AB, Ay, and Adelta

B

Question 15

What do Aa fibers mediate?

Answer 15

motor to skeletal muscle (fastest fibers)

Question 16

What do Aa 1a subtype fibers mediate?

Answer 16

sensory from muscle spindle

Question 17

What do AB, 1b subtype fibers mediate?

Answer 17

sensory from Golgi tendon organ and Ruffini endings

Question 18

What do AB, II subtype fibers mediate?

Answer 18

sensory from skin receptors

Question 19

What do Ay fibers mediate?

Answer 19

motor to intrafusal fibers

Question 20

What do Adelta, III subtype fibers mediate?

Answer 20

sensory from free nerve endings for pain and temperature and hair follicles

Question 21

What do B fibers mediate?

Answer 21

preganglionic autonomic fibers

Question 22

What do C, IV subtype fibers (unmyelinated) mediate?

Answer 22

postganglionic autonomic fibers; sensory from free nerve endings for pain and temperature; smell

Question 23

There are many types of sensory receptors for somesthesis. How are they classified?

Answer 23

They can be classified by whether they concern external events (exteroreceptors), the position of joints and muscles in space (proprioceptors), or the state of internal organs (enteroreceptors).

Receptors can also be classified according to the physics of what they detect: chemicals (chemoreceptors), light (photoreceptors), temperature (thermoreceptors), mechanical force (mechanoreceptors), or pain / noxious stimuli (nociceptors).

Question 24

The Figure on the back provides another setof different types of energy that can bedetected by sensory receptors in general (not just somesthesis).

Answer 24

Note also that these forms of energy that exist in nature are transduced (transformed from one form of energy to another) by the sensory receptors into a graded electrical signal. Graded responses are integrated by the receptor and in many cases converted into action potentials for long distance transmission.

Thus, even at the earliest stages, the nervous system encodes an abstraction of the “real world”.

Question 25

What is a Pacinian corpuscle?

Answer 25

A large, lamellar, rapidly adapting mechanoreceptor that efficiently detects gross pressure and vibratory skin stimuli (optimal frequency 250 Hz). Besides subcutaneous skin, they are also found in joints, muscle, and mesentery

Question 26

What is a Meissner’s corpuscle?

Answer 26

Another rapidly adapting skin mechanoreceptor that is sensitive to light touch and vibration of < 50 Hz. They are located in glabrous skin, right below the epidermis.

Question 27

What are Merkel’s disks?

Answer 27

Slowly adapting mechanoreceptors located in skin and mucosa. In glabrous skin, these disks are found clustered beneath the ridges of the fingertips that make up fingerprints. In hairy skin, they cluster into specialized epithelial structures called “touch domes” or “hair disks”.

Question 28

What are Ruffini organs?

Answer 28

Slowly adapting mechanoreceptors found only in deep layers of glabrous skin. They respond to sustained pressure and skin stretch. They are thought responsible for detecting objects slipping along the skin and contribute to position sense.

Question 29

Free nerve endings are just what it sounds like. There are no accessory structures associated with them to form a sense organ. Free nerve endings are typically responsive to what?

Answer 29

temperature and nociceptive stimuli.

Question 30

Sensory receptors transduce the energy contained in their preferred stimuli into electrical potentials and in most cases, action potentials

What is adaptation?

Answer 30

Adaptation is a reduced response in the face of a continued, constant stimulus.

In the uppermost case, action potential firing is maintained at a constant rate as long as the stimulus is applied - there is no adaptation. In the middle case, firing is maintained for some time but then the frequency of action potentials slows, despite the maintained stimulus. This receptor shows adaptation and in this case the adaptation is slow (slowly adapting receptor). In the lower example, adaptation is rapid – the response is very brief or transient. The slowly adapting example is also transient, but persists for a longer time before adapting. Thus we can see that slow and fast are relative terms here.

Question 31

What is the composition of a Pacinian corpuscle, a rapidly adapting receptor?

Answer 31

These receptors consist of a free nerve ending encapsulated by a series of modified Schwann cells known as lamellae.

Question 32

How do Pacinian corpuscles respond to pressure?

Answer 32

When pressure is applied to the lamellae, they move relative to one another and the Pacinian corpuscle becomes distorted. This mechanical stimulus is transduced into an action potential at the onset of the stimulus and then complete adaptation for as long as pressure is maintained.

Question 33

What happens when the pressure on the lamellae of a Pacinian Corpuscle is released?

Answer 33

a second action potential occurs. **Thus this organ signals onset and offset of the stimulus but no signals are produced during a maintained stimulus**.

Question 34

How is transduction thought to occur in Pacinian corpuscles?

Answer 34

The actual transduction event is thought to be gating of stretch-sensitive ion channels in the nerve ending.

The resulting depolarizations are transduced once more into action potentials at the first Node of Ranvier of the afferent nerve. The lamellae are an accessory structure that determines adaptation in the Pacinian corpuscle. Without the lamellae, the corpuscle responds to different stimuli and in a different way.

Question 35

Protopathic somesthesis concerns pain and temperature sensation, as well as a crude form of touch (e.g., detection of pressure on the skin in a general part of the body). Within the spinal cord, protopathic information travels where?

Answer 35

in the anterolateral funiculi in the spinothalamic tract.

Question 36

What is a funiculi = fasciculi?

Answer 36

A surface feature indicating an underlying axon tract.

Question 37

Epicritic somesthesis concerns fine, discriminative touch, proprioception (information about the internal body: muscles, joints, and internal organs), as well as vibration sense. Within the spinal cord, epicritic information travels where?

Answer 37

in the posterior columns that becomes a tract known as the medial lemniscus in the brainstem.

Question 38

Describe the intitial parts of the epicritic pathway?

Answer 38

1) a subQ Pacinian corpuscle (a rapidly adapting receptor particularly sensitive to vibration) receptor is connected to a large afferent axon (Ab = type II)
2) This axonprojects (via the dorsal root) into the dorsal horn of the spinal cord.

Question 39

What happens to the axon once in the DRG in the epicritic pathway?

Answer 39

1) This axon continues into the ipsilateral (same side of body) posterior (dorsal) column of the spinal cord and then
2) ascends through the entire spinal cord to the ipsilateral brainstem, specifically the medulla.

Question 40

What is the first synapse of the epicritic pathway?

Answer 40

In one of the posterior (dorsal) column nuclei in the medulla.

For afferents that enter in the lumbar and sacral spinal cord, this synapse is in the Gracile Nucleus (Nucleus Gracilis).

The Nucleus Cuneatus serves a similar function for afferents from Thoracic and Cervical levels (including the arms).

Question 41

Within the spinal cord, axons from the lumbar spinal cord destined for the nucleus gracilis travel where?

Answer 41

in the fasciculus gracilis (most medial posterior column).

Question 42

The path for the thoracic and cervical cord to the nucleus gracilis (as part of the epiciritc pathway) travels through where?

Answer 42

1. the more lateral fasciculus cuneatus.

Question 43

_____is the cut-off for gracilis (below T6) and cuneatus (above T6)

Answer 43

~T6

Question 44

Epicritic pathway: “Gracile” means “thin”. “Cuneate” means “wedge shaped”. In tetrapods, the pathway runs in the part of the spinal cord closest to the back, thus they are called the dorsal columns. In an upright human, the back (dorsal) is also posterior.

Question 45

What happens from the initial synapse in the epicritic pathway? (at the gracile and cuneatus nuclei)

Answer 45

1) From the posterior column nuclei (gracilus and cunteatus), the axon of a secondary neuron (with cell body in posterior column nucleus) crosses the mid-line of the brainstem and runs in the contralateral (from the side of the body with the Pacinian corpuscle) medial lemniscus to the part of the thalamus involved in somesthesis, the ventral posterolateral nucleus (VPL).(synapse # 2).

Question 46

What happens from the VPL in the epicritic pathway?

Answer 46

A third order neuron sends its axon to the ipsilateral (to the VPL – still contralateral to receptor in periphery) primary somatosensory cortex.

So it takes three neurons to make it from the periphery to the cortex (and the third synapse is in the cortex).

Question 47

Where is the Primary somatosensory cortex located?

Answer 47

In the postcentral gyrus.

Question 48

Crossing the midline is also called ________

Answer 48

decussation. The brain region where axons cross the midline is also called a decussation. Axons decussate at the decussation.

Question 49

What are the initial steps of the protopathic pathway?

Answer 49

1) A free nerve ending in the skin (cutaneous) of the leg senses pain or temp (The axons attached to these free nerve endings are typically small and slowly conducting (Adelta or C fibers = type III or IV afferents)).
2) Afferents from the skin of the leg enter the spinal cord (via dorsal root) in the lumbar or sacral spinal cord.

Question 50

Pain receptors and temperature receptors are typically what?

Answer 50

relatively unspecialized free nerve endings

Question 51

Protopathic pathway;In contrast to the lemniscal pathway, protopathic peripheral axons synapse where?

Answer 51

in the ipsilateral dorsal horn (first synapse).

Question 52

Where is the second synpase in the protopathic pathway?

Answer 52

A second order neuron (soma in dorsal horn) sends its axon across the mid-line at that same level of the spinal cord to the contralateral anterolateral funiculus. The axon then turns and ascends in the anterolateral spinal cord through the brainstem to the VPL thalamus (synapse #2, contralateral to receptor).
The pathway is called the spinothalamic tract.

Question 53

Where does the protopathic pathway go from the VPL?

Answer 53

From the VPL, a third order neuron projects ipsilaterally (to the VPL) to somatosensory cortex (post central gyrus, synapse #3).

So the first synapse is in the ipsilateral dorsal horn. At the level of entry, the pathway decussates in the anterior commissure of the spinal cord and the rest of the pathway is contralateral to the peripheral receptor.

Question 54

The epicritic pathway (lemniscal – via medial lemniscus) enters the ipsilateral dorsal horn and ascends in the ipsilateral posterior column to the ipsilateral posterior column nuclei, where the first synapse occurs. The second order neuron decussates in the medulla and ascends as the contralateral medial lemniscus to the contralateral (to the receptor) VPL nucleus of the thalamus (synapse # 2). The third order neuron sends its axon (thalamic radiations) to the primary somatosensory cortex (postcentral gyrus; synapse #3).

Answer 54

The protopathic pathway enters the ipsilateral dorsal horn and synapses there (synapse #1). The second order neuron decussates at that level of the spinal cord and travels in the contralateral anterolateral spinal cord in the spinothalamic tract. This axon synapses (#2) in the VPL thalamus (contralateral to receptor). The third order neuron projects to the primary somatosensory cortex (postcentral gyrus; synapse #3).

In both pathways, the VPL and somatosensory cortex involved is contralateral to the peripheral receptor.

Question 55

T or F. The VPL always projects to its ipsilateral cortex.

Answer 55

T.

Question 56

A Pacinian corpuscle from the arm or neck (Cervical cord) relaying info via the medial meniscal apthway (aka the epicritic pathway) would first synapse where?

Answer 56

first synapse is in Nucleus Cuneatus, rather than Nucleus Gracilis

Question 57

The posterior column nuclei (Gracilis and Cuneatus) are located where?

Answer 57

in the caudal (literally towards the tail) medulla.

Question 58

The decussation of axons from the posterior column nuclei in the epicritic pathway occurs where?

Answer 58

in the caudal medulla. These crossing fibers are also called internal arcuate fibers (they arc across the midline).

Question 59

In the rostral medulla, the medial lemniscus (axons of contralateral posterior column nuclei in the epicritic pathway) runs where?

Answer 59

near the midline, towards the anterior medulla. As the medial lemniscus travels rostrally (literally towards the head – in this case towards the cortex) its path takes it gradually away from the midline, through the pons (metencephalon) and midbrain (mesencephalon) on its way to VPL.

Question 60

Projections from VPL to cortex travel where?

Answer 60

through the posterior limb of the internal capsule

Question 61

Epiciritc pathway

Answer 61

Axons enter the dorsal horn and ascend to their respective posterior column nuclei (Gracilis for lumbar; Cuneatus for Cervical). This means that means that as we ascend the spinal cord, afferents add to the pathway lateral to the lower regions (Cuneatus is lateral to Gracilis).

Question 62

Just caudal to the brainstem, the posterior column of the spinal cord can be readily seen to be separated into two fasciculi (fiber tract; a ridge indicates the underlying axons):

Answer 62

the fasciculus gracilis is medial and the fasciculus cuneatus is lateral.

Question 63

Describe the collaterals (branches) of the axons from the peripheral receptors in the epicritic pathway.

Answer 63

We have thus far concentrated on the main lemniscal pathway ascending in the spinal cord but collaterals also synapse within the ipsilateral dorsal horn and ventral horn.

Thus, in addition to conveying the peripheral information to be processed in the brainstem, thalamus, and cortex, additional processing occurs in the dorsal horn and ventral horn.

You already know about the functional significance of these projections and synapses. Proprioceptive inputs from muscle spindle organs synapse directly on alpha motoneurons in the ventral horn. This is the anatomical basis for the monosynaptic stretch reflex (also called myotatic reflex). Similarly, polysynaptic pathways link muscle and cutaneous receptors to motoneurons via interneurons in the spinal cord (basis for other reflexes).

Question 64

This diagram (Netter) summarizes the pattern of inputs to the spinal cord. The Roman numerals refer to named layers or laminae of the spinal cord. They are known as Rexed’s laminae. (I-V = dorsal horn, VI, VII = intermediate zone, VIII and IX = ventral horn).

Answer 64

Epicritic inputs are blue (touch and pressure) or purple (proprioception – includes muscle afferents and joint afferents). Note that both send axons up the ipsilateral posterior column.

The proprioceptive afferents send collaterals to deep layers of the dorsal horn (V-VII) and also to the ventral horn (some of which – type Ia afferents from spindles – are monosynaptic on motoneurons).

Question 65

Touch and pressure afferents send collaterals primarily to the dorsal horn, with a few in ventral horn. Aa afferents are from the muscle receptors (spindles and Golgi tendon organs). They project to ventral horn as well as deep layers of dorsal horn. Ab fibers include many cutaneous and joint receptors (and secondary spindle organs). These project mostly to dorsal horn.

Answer 65

Protopathic inputs are indicated in red. They synapse immediately in the ipsilateral dorsal horn (mostly layers I and V) and the second order projection is to the contralateral spinothalamic tract. Some axons travel a short way (2-3 segments) in Lissauer’s Tract before synapsing in the dorsal horn. It thus takes 2-3 spinal cord segments for all afferents entering at a given level to cross to the contralateral cord. Lesions at a given spinal cord level therefore result in loss of inputs from 2-3 segments below the lesion.

Question 66

What is the spinocerebellar tract?

Answer 66

A collection of axons that originate in the spinal cord and terminate in the cerebellum (ipsilateral side).

Question 67

What does the spinocerebellar tract do?

Answer 67

This tract conveys information about limb and joint position to the cerebellum. The input is primarily from muscle spindle organs and Golgi tendon organs (GTOs).

Question 68

The spinocerebellar tract can be divided into four components:

Answer 68

the dorsal spinocerebellar tract, the ventral spinocerebellar tract, the cuneocerebellar tract, and the rostral spinocerebellar tract.

Question 69

What does the dorsal spinocerebellar tract convey?

Answer 69

information from spindles and GTOs from the ipsilateral leg and body.

Question 70

What does the ventral spinocerebellar tract convey?

Answer 70

information from GTOs from the ipsilateral leg and body.

Question 71

What does the cuneocerebellar tract convey?

Answer 71

coveys information from spindles and GTOs from the ipsilateral arm

Question 72

What does the rostral spinocerebellar tract convey?

Answer 72

information from GTOs from the ipsilateral arm

Question 73

Describe the initial route of the dorsal spinocerebellar tract

Answer 73

1) The afferents pass via fasiculus gracilis and synapse in Clarke’s Nucleus (or Clarke’s Column: a column of relay neuron cell bodies within the medial gray matter within the spinal cord between T2-L2).

Question 74

Where do neurons from Clarke’s nucleus in the dorsal spinocerebellar tract project?

Answer 74

2) These neurons in turn project to the cerebellum via the inferior cerebellar peduncle.

Question 75

Describe the route of the cuneocerebellar tract

Answer 75

Afferents in the cuneocerebellar tract synapse in the accessory cuneate nucleus. Second order neurons in this pathway also project to the ipsilateral cerebellum via the inferior cerebellar peduncle (like the dorsal spinocerebellar tract)

Question 76

Describe the initial routes of the ventral spinocerebellar tract

Answer 76

1) afferents synapse in layer VII of the spinal cord (L3-S3 for ventral)
2) these cells project over to the contralateral lateral funiculus to synapse with neurons in the region of the superior cerebellar peduncle.
3) Some projections from the superior cerebellar peduncle cross the midline again, ending on the ipsilateral cerebellum (relative to the original afferents). Other afferents remain on the side contralateral to the original afferent.

Question 77

Describe the initial routes of the rostral spinocerebellar tract

Answer 77

1) afferents synapse in layer VII of the spinal cord (above L3 for Rostral).
2) The rostral spinocerebellar tract synapses at the dorsal horn of the spinal cord and ascends ipsilaterally to the cerebellum through the inferior cerebellar peduncle.

Question 78

Where is Clarke’s Nucleus located?

Answer 78

in lamina VII of the intermediate zone of the spinal cord. It is found at the level of T2-L2 and involved in unconscious proprioception.

Proprioceptive afferents from the lower body terminate within Clarke’s nucleus in spinal intermediate zone.

Question 79

Clarke’s nucleus is the origin of what?

Answer 79

the dorsal spinocerebellar tract (axons from cell bodies in the dorsal nucleus of Clarke travel through the ipsilateral lateral funiculus via the restiform body on their way to the cerebellum). This is the primary input to cerebellum from spinal cord proprioceptive afferents.

Question 80

This shows the tracts in fixed and stained (real) human spinal cord. The tissue was stained for myelin so the fiber tracts are dark (axons are myelinated – the funiculi) and the cell bodies (in dorsal and ventral horn – gray matter) are light.

For now, note the location of the posterior columns and how the representation of the cervical cord is lateral to that of lumbar cord. Also note neck and occiput vs. arm; upper and lower trunk vs. leg.

Question 81

This is one more view of the whole pathway, indicating relative positions of representations of various parts of the body at each level.

Answer 81

The leg afferents enter in lumbar cord. At thoracic cord the lower trunk is added next. At cervical levels, the upper limb representation is added. The upper limb (cervical representation) is added lateral to the lumbar representation. ****At the decussation, what was medial in the spinal cord (lumbar) arcs to become lateral in the brainstem (e.g., legs lateral)****

As the medial lemniscus ascends in the brain stem, the lateral representation becomes more posterior (dorsal) as well. So the legs are lateral but also posterior.

In the cortex, the representation is basically “standing on its head” - the arms are more lateral (and inferior) and the legs are represented medially (and superior).

Question 82

What is the Substantia gelatinosa?

Answer 82

Lamina I and II of the spinal cord gray matter. This is a site of first modulation of pain and temperature information.

Question 83

Lissauer’s Tract is also called what?

Answer 83

the Posterolateral fasiculus. It is composed of sensory fibers carrying pain and temperature that ascend or descend several spinal cord levels before synapsing in the dorsal horn. It also contains short axons of projections of neurons from laminae I and II.

Question 84

Describe the protopathic pathway up to the first synapse

Answer 84

Our example begins with a free nerve ending (sensing either pain, temperature, or crude touch) in the cervical spinal cord. The central process of the associated DRG neuron enters the dorsal horn via the dorsal root.

The first synapse is in the ipsilateral dorsal horn (layers I and V).

Question 85

The second order neuron of the protopathic pathway has its cell body in the ipsilateral dorsal horn. Then what happens?

Answer 85

The second order axon crosses the mid-line at the same cord level as the afferent entered the dorsal horn, turns and ascends in the anterolateral part of the contralateral cord. The tract is called the spinothalamic tract.

Question 86

Where does the spinothalamic tract run?

Answer 86

in the anterolateral spinal cord.

Question 87

How does the spinothalamic tract behave in the brainstem?

Answer 87

In the brainstem, the spinothalamic tract remains lateral. Going from the medulla into the pons and midbrain, the tract moves more dorsal while remaining lateral.

The second synapse is in the VPL thalamus (contralateral to afferent). The third order neuron (thalamic relay neuron) projects to primary somatosensory cortex (postcentral gyrus) via the posterior limb of the internal capsule.

Question 88

Lissauer’s tract is comprised of what?

Answer 88

axons of interneurons that travel 2-3 segments up / down the spinal cord from the level of afferent entry.

Question 89

NOTE: In addition to the major 3 synapse pathway from periphery to somatosensory cortex that we have described for the protopathic pathway, there are additional synapses made in brainstem, thalamus, and cortex by this system.

Besides the major termination in the VPL, collaterals of the spinothalamic tract axons also synapse where?

Answer 89

in the intralaminar nucleus and dorsomedial nucleus of the thalamus. These nuclei are important for cortical arousal.

Question 90

There are three divisions of the protopathic pathway:

Answer 90

spinothalamic, spinoreticular and spinomesencephalic. The names indicate where the axons terminate (synapse).

Question 91

The spinothalamic tract is important for what?

Answer 91

discriminative aspects of pain.

Question 92

Spinothalamic axon collaterals synapse where?

Answer 92

• In the brainstem.
• There are also synapses with neurons in the reticular formation in the pons and medulla. This is known as the spinoreticular pathway (spinoreticular tract fibers come from neurons in all layers of the dorsal horn).

Question 93

In the mesencephalon, there are connnections of the spinothalamic collaterals to what?

Answer 93

the periaqueductal gray (spinomesencephalic tract).
These spinomesencephalic fibers come form cells in layer I and V of the dorsal horn.

Question 94

The spinoreticular pathway is important for what?

Answer 94

the general arousal and emotional aspects of pain and the spinomesencephalic pathway is important for activating descending, efferent pathways that modulate pain.

So if you stepped on a nail, the spinothalamic system would report that” a sharp object is sticking in your right foot”, the spinoreticular pathway would signal “Ouch, that hurts” and the spinomesencephalic would initiate pathways to modulate the pain (ah, that’s better!). It is interesting that the highest concentration of opiate receptors in the brainstem is in the periaqueductal gray.

Question 95

Afferents of the protopathic pathway are broken into what?

Answer 95

a group that is exclusively responding to painful stimuli (in blue) and a second group of fibers that respond to both pain and temperature (red). All afferents synapse in the ipsilateral dorsal horn.

Question 96

The second order axons of the protopathic pathways cross to the contralateral spinothalamic tract. Note that as cervical level afferents are added to the axons ascending from the lumbar cord, the second order axons from cervical dorsal horn are added medially. ***This is in contrast to the epicritic system, where cervical-derived axons add lateral to the lumbar-derived axons***

Answer 96

Note that the pain afferents contribute collaterals to the spinoreticular pathway (synapse in reticular formation) and the spinomesencephalic tract.

This illustration indicates the projection to intralaminar thalamic nuclei (dorsomedial collateral is not shown but also present) in addition to the main pathway to VPL. The intralaminar nuclei project diffusely throughout the cortex, as befits a role in general arousal. The dorsomedial nucleus projects to prefrontal cortex.

Question 97

For pain, temperature, and crude touch, the afferent fibers synapse in the ipsilateral dorsal horn. What fibers mediate first pain– a sharp sensation that can be readily localized on the body?

Answer 97

The larger Adelta fibers.

Question 98

Where do Adelta fibers synapse?

Answer 98

These fibers synapse in the first couple of laminae in the dorsal horn (Rexed’s laminae I, II; especially layer I = marginal zone).

Layers I and II are also known as the substantia gelatinosa (This is an initial site of integration of pain information)

Question 99

What happens from the substantia gelatinosa?

Answer 99

Short axons run up and down the cord from these layers. Such short fibers between spinal cord segments are known as propriospinal fibers or a propriospinal tract (this can be confusing but this is a separate meaning of propriospinal – not position sense).

Interneurons in layers I and II also project to layer V (deepest part of dorsal horn). The layer V cells are the ones that send their axons across the midline and ascend the cord.

Question 100

__ fibers are the smallest axons and associated with second pain – a duller, less localizable aspect of pain.

Answer 100

C

Question 101

Where do C fibers synapse?

Answer 101

C fibers synapse in the substantia gelatinosa (especially layer I = marginal zone) and these layer I neurons send their axons across the midline and up the cord.

Question 102

Lissauer’s tract carries what?

Answer 102

non-discriminative pain and temperature information and consists of short association fibers that interconnect neighboring segments of the posterior horn. Pain afferents can travel 2-3 segments with Lissauer’s tract before terminating in the ipsilateral dorsal horn (Layers I and V). Because of this pattern pain sensation is lost for 2-3 segments below a lateral spinal cord lesion.

Question 103

For fine touch etc., Aa and Ab fibers do what?

Answer 103

enter the dorsal horn and their main axon ascends the cord in the ipsilateral posterior columns. These afferents also send collaterals into the dorsal horn and ventral horn.

Note especially that some collaterals of the epicritic system synapse on the same layer V cells that protopathic pain fibers do. It is thought that this is important for the phenomena of referred pain.

Question 104

Human spinal cord with myelin stain (myelin = dark).

The spinothalamic tract is in the anterolateral cord (contralateral to afferent entry). Note the topography, with leg lateral and arm and other cervical elements added to the medial side.

Question 105

Spinothalamic tract overview:

Answer 105

1) Axons from periphery enter the spinal cord and travel up or down 1-2 segments in the Lissauer tract and then synapse in the posterior horn
2) Axons of the secondary neuron cross the midline in the anterior white commissure and ascend as the anterolateral tract in the SC
3) In the rostal medulla, the ALT lies between the inferior olivary nuclear and the nucleus of the spinal tract of the trigeminal nerve
4) In the pons and midbrain, the ALT lies lateral to the medial lemniscus
5) The ALT terminates in the VPL of the thalamus
6) From the VPL, fibers project through the internal capsule and corona raiate to the primary somatosensory cortex in the postcentral gyrus

Question 106

For the head and face, the _________ is responsible for somesthesis.

Answer 106

trigeminal system (related to cranial nerve V)

Question 107

What are the main nuclei of CN V?

Answer 107

The motor nucleus of V (motoneurons of masticatory muscles and a few others),

The chief sensory nucleus of V (cell bodies of second order neurons for epicritic system for head/face),

the mesencephalic nucleus of V (cell bodies of primary afferents for muscle spindles and Golgi tendon organs for masticatory muscles),

and the spinal nucleus of V (cell bodies of second order cells for protopathic system for head / face).

Question 108

The chief sensory nucleus is thus analogous to the posterior column nuclei.

The mesencephalic nucleus is analogous to a displaced DRG, found within the brainstem

The spinal nucleus is analogous to the dorsal horn (especially substantia gelatinosa).

Question 109

Somatosensory afferents for the head / face have their cell bodies where?

Answer 109

in a ganglion outside of the brain known as the Gasserian or Trigeminal ganglion. Thus the trigeminal ganglion is analogous to dorsal root ganglia for the spinal cord.

Question 110

Note: While the VPL is the thalamus for somesthesis for the body, the trigeminal projections are to the _____ _____ ______

Answer 110

ventral posteromedial nucleus (VPM).

Question 111

Muscle receptors for the muscles of mastication have a special arrangement. The cell bodies for the afferents are located where?

Answer 111

in the mesencephalic nucleus of V, within the brainstem, rather than in the trigeminal ganglion. The central processes of these cells project bilaterally to the motor nucleus of V (basis for monosynaptic stretch reflex).

All of the other afferents have their cell bodies in the trigeminal ganglion. Their central processes project into the brainstem.

Question 112

For fine touch (epicritic), the afferents of CN V project where?

Answer 112

to the chief sensory nucleus of V. The first synapse is here. The second order axons cross to the contralateral VPM (thalamus). This pathway is the trigeminal lemniscus. VPM projects in turn to primary somatosensory cortex (postcentral gyrus: contralateral to afferents).

Question 113

Protopathic afferents for the face also have their cell bodies in the trigeminal ganglion. Describe this pathway

Answer 113

Their central processes project to the ipsilateral spinal tract of V. The first synapse is here. The second order axons project to the contralateral VPM via the trigeminothalamic tract. VPM in turn projects to somatosensory cortex via the posterior limb of the internal capsule.

Question 114

Question 115

What does the VPL do?

Answer 115

Somesthetic information from the body is relayed to primary somatosensory cortex via the ventroposterolateral nucleus of the thalamus (VPL).

Question 116

What are the main inputs into the VPL?

Answer 116

medial lemniscus and the spinothalamic tract

Question 117

What are the main outputs from the VPL?

Answer 117

post central gyrus (cortex 3,1,2)

Question 118

What are the main inputs into the VPM?

Answer 118

trigeminal lemniscus, trigeminothalamic (also taste)

Question 119

What are the main outputs from the VPM?

Answer 119

post central gyrus (cortex: 3,1,2)

Question 120

The primary somatosensory cortex is also known as __

Answer 120

S1.

S1 was originally described in cat as that part of the cortex most sensitive to fine touch. The original assignment of S1 in humans was done with relatively crude surface electrodes and included what is actually four different areas in the cat. Thus the post central gyrus includes Brodmann’s areas #3a, 3b, 2 and 1.

Question 121

What is S2?

Answer 121

S2 is a unimodal (only somatosensory) association cortex where further processing of information concerning somesthesis occurs.

Question 122

Where is S2 located?

Answer 122

S2 is located on the parietal operculum.

Areas #5 and #7 are in the superior parietal lobule and are another unimodal (mostly) association area for somesthesis.

Question 123

The cortex is required for perception of fine touch, vibration, pain, temperature etc. Several Brodmann’s areas are represented in the postcentral gyrus. These include areas 3, 1, and 2 (named anterior to posterior in the gyrus). Area 3 is subdivided into areas 3a and 3b.

In general, area _ receives the bulk of projections from VPL and VPM.

Answer 123

3

Question 124

What does area 3a of Brodmann’s area in the postcentral gyrus receive?

Answer 124

receives primarily muscle afferents and nociceptive (pain) information from the thalamus (proprioceptive and pain).

Question 125

What does area 3b of Brodmann’s area in the postcentral gyrus receive?

Answer 125

Area 3b receives primarily cutaneous inputs (slowly and rapidly adapting: tactile perception).

Question 126

What does area 1 of Brodmann’s area in the postcentral gyrus receive?

Answer 126

Area 1 gets its primary input from area 3b – cutaneous. More detailed information about texture is processed here.

Question 127

What does area 2 of Brodmann’s area in the postcentral gyrus receive?

Answer 127

Area 2 gets its primary projection from both areas 3a and 3b, thus this area integrates pain, tactile, and proprioceptive information to allow position and edge detection, size perception etc.

The general idea is that there is parallel processing of the different sub modalities and there is also integration between modalities [higher order (serial) processing].

Question 128

The somatosensory homunculus is a rather deformed map of the body surface that indicates which parts of the somatosensory cortex responds to touch, pain etc. on a given part of the body. Note the upside down representation (head is inferior and lateral). All sub modalities are mapped this way in somatosensory cortex

Question 129

Answer 129

4.