The Somatic Sensory System

Functional Organization of the System, which serves:

Sense of:

• Touch

• Limb position

• Pain

• Temperature

• Regulation of arousal

• Sensory regulation of movement

• Ascending SC Pathways, which serve the trunk and limbs.

• [the trigeminal system, which transmits somatic sensory info from the head, has a similar organization, but will be discussed later].

I. Functional Anatomy of two 1° Somatic Sensory Pathways

A. Dorsal-column-Medial lemniscal system – mediates touch and limb position.

B. Anterolateral system – mediates pain and temp sense and to some degree, crude touch.

Both systems decussate before reaching the brain, although at a different anatomical location.

So, 1 side is receiving info from the opposite side of the body.

Medial LemniscalSystem without theCerebellum (Fig. 5-1a)

Dorsal Column-Medial LemniscalSystem (Fig. 5-2a):

Anterolateral Systemwithout cerebellum(Fig. 5-5b)

AnterolateralSystem (Fig. 5-2b):

4 Essential Differences

1. Different types of sensory receptors:

DC: mechanical receptors

AL: receptors sensitive to noxious or thermal stimuli.

A pure lesion of the DC system only crude touch and position sense.

A pure lesion of the AL system decr sensation to pain.

4 Essential Differences2. Distinct relay nuclei in SC and brainstem.

DC: dorsal column nuclei in the medulla

AL: dorsal horn of the SC (close to point of entry).

3. Distinct levels of decussation: for both systems, the 2nd-order neuron decussates shortly after synapsing.

DC: in medulla

AC: in SC.

-important implications for lesions?

4 Essential Differences4. Distinct pathways which synapse in different

diencephalon regions:

DC: travels in 1 path in dorsal column DC n. VPL of thalamus.

AC: lateral and ventral columns of SC via 3 different paths, which synapse in different brain regions:

i. Spinothalamic tract – (a) brings info re painful stimuli to separate regions of CPL (along with DC, serves ability to discriminate and localize the stimulus). (b) also brings the info to intralaminar n. (note diffuse projections) emotional aspects of pain.

4 Essential Differencesii. Spinoreticular tract – synapsing on reticular

formation of pons and medulla, along with intralaminar n. of the thalamus.

iii. Spinomesencephalic tract – terminates on midbrain tectum and PAG

Integration of all feedback regulation of

senses for orientation pain transmission in SC.

II. Regional Anatomy: Following the course of these systems all the way from the sensory receptors

to the brain

A. Somatic sensory receptor.

B. Entry and initial pathways of DR axons in the SC.

C. Pathway through the medulla and the somatic sensory decussation.

D. Projections to the reticular formation (medulla + pons) from the AL system + paths in midbrain.

E. Descending pain supression pathways from the brainstem.

F. Thalamic n. and processing of somatic sensory info.

G. The 1° somatic sensory cortex and its organization.

H. 2° (higher-order) somatic sensory cortical areas.

II. Regional Anatomy: Somatic Sensory Neuronal Path

II. Regional Anatomy: A. Somatic Sensory Receptors

DRG neurons transmit sensory info neural signals. Located close to SC in intervertebral foramen:Neurons are pseudounipolar.Distal terminal = sensory receptor structure:

1. bare nerve endings: nociceptors, thermoreceptors sense noxiousor damaging stimuli

II. Regional Anatomy: A. Somatic Sensory Receptors

2. Encapsulated receptors: mechanoreceptors

5 morphological types shown in Fig. 5-3 (previous slide).

Which receptor types serve DC and AL systems?

Note: mechanoceptors have the largest-diameter axons and are the fastest-conducting, covered with thick myelin sheath (what kind of cell provides this sheath?)

After DRG cell, axons enter SC at the dorsal root.

Dermatome: Area of skin innervated by axons in a single dorsal root segment (Fig. 5-4, next slide).

II. Regional anatomy: B. Entry and initial pathways of DR axons in the SC.

1. 2 systems are segregated even at the level of entry.

Large-diameter axons serving touch and position enter at dorsal column.

The smaller-diameter axons serve pain and temperature sensation lateral to the DC: Lissauer’s Tract

{This overlies the dorsal horn}

Once inside the SC, DRG axons branch into:

II. Regional anatomy: B. Entry and initial pathways of DR axons in the SC.

a. Segmental branches enter gray matter and synapse immediately (i.e., reflex circuits, interneurons).

b. Ascending branches carry sensory info upward (as we have noted).

c. Descending branches synapse on interneurons below level of entry (i.e., limb withdrawal reflexes).

II. Regional anatomy: B. Entry and initial pathways of DR axons in the SC.

2. Within the SC, both gray and white matter have a topographical organization.

a. Rexed’s laminae of gray matter and separation of terminations.

DC: segmental branches of large-diameter neurons: skin L III-VI

muscle L V, VI, VII, IX

AL: segmental branches of small-diameter neurons L I, II only.

Rexed’s Laminae (Fig. 5-5c)

II. Regional anatomy: B. Entry and initial pathways of DR axons in the SC.

Somatotopy:

b. White matter topographically (somatotopically) organized according to what part of body the sensory info comes from.

Dorsal column:

lower body medial portion of DC {gracile fascicle}

upper body mostly lateral {cuneate fascicile}

This ensures that the local relationship in the periphery are preserved in the CNS.

*Clinical e.g,: lower sc injury atrophy and demyelination of gracile fascicle (medial only). (fig. 5-8)

AL System: axons decussate in sc before ascending (ventral commissure). (fig. 5-1b)

AL axons: lower body lateral

upper body medial (fig. 5-1b)

White Matter Somatotopy (Fig. 5-7)

Lower spinal cord lesion (Fig. 5-8)

II. Regional anatomy

C. Pathways through the Medulla and the Somatic Sensory Decussation.

1. DC nuclei:

fine-tuning functions (not just processing info)

gracile fascicle gracile nucleus (medial)

cuneati fascicle cuneate nucleus (lateral)

decussation (interal arcuate fibers) contralateral ML (fig. 5-11)

Note: ML is now ventral to central canal, but still dorsal to the pyramids.

2. AL fibers: still anterolateral (fig. 5-1b)

D. Projections to the Reticular Formation of the Medullas and Pons from the AL system.

1. Spinoreticular tract (AL system) reticular formation (arousal, emotional aspects of pain)

Myelin stained section (Fig. 5-5a)

Pathways through Medulla (Fig. 5-11)

II. Regional anatomy

2. In midbrain, the AL system travels dorsal to the ML (fig. 5-11)

spinothalamic (to thalamus) + spinomesencephalic tracts superior colliculus + periaquaductal grey (PAG) (fig. 5-15)

E. Descending Pain Suppression Pathway from Brainstem.

PAG (in midbrain) raphe n. (in medulla) – uses 5-HT projections to dorsal horn (in SC).

5-HT suppresses pain transmission in dorsal horn by:

i. Inhibiting ascending AL system pain projection neurons.

ii. Exciting inhibitory (enkephalin-producing) interneurons in dorsal horn.

F. Thalamic Nuclei and Somatic Sensory Information.

Thalamus is nodal point and relay n. for transmission of sensory info from SC and brainstem cerebral cortex.

DC-ML and AL systems VP n. and posterior n. (VPL for discrimination)

Pathways through Midbrain (Fig. 5-15)

Spinoreticular termination (Fig. 5-14A)

II. Regional anatomy

Spinothalamic intralaminar n. (affective aspects of pain sensation).

Lateral VP n. (VPL) is 1° input to 1° somatic sensory cortex (postcentral gyrus).

VPM sends info from the face and perioral structures (trigeminal system) (fig. 5-16 ).

Somatotopic Organization: functional units of neurons from some part of the body and the same sensory modality cluster together and project to specific location in postcentral gyrus (cortical columns).

G. The 1° Somatic Sensory Cortex and its Organization.

posterior limb

VPL --------- 1° cortex

of internal capsule

II. Regional anatomy

1. Columnar Organization:

cortical columns:

– receive inputs from the same peripheral location.

- receive inputs from the same class of sensory receptor

VPL ---------------------- layer 4 (and deep layer 3)

thalamocortical axons

*{this is why Layer 4 is the most highly developed (thickest) in sensory cortices}

2. Somatotopic Organization:

See homonculus.

Lower body: lateral portion of VPL to medial portion of 1° cortex

Upper body: medial VPL to lateral 1° cortex

Cortical Somatotopy: Homunculus (Fig. 5-16B)

Pathways through Pons (Fig. 5-14b)

Ventral Posterior nuclei of the Thalamus (Fig. 5-17b)

II. Regional anatomy

Note: greater representation for body parts with richer sensory innervation, such as the fingers.

*the representation is not static, however. Rather, it is based on use.

(if a body’s sensory paths are damaged from a particular area, its cortical representation atrophies (shrinks)).

3. Modality-specific organization.

1° somatic sensory cortex contains 4 Brodman’s areas:

1, 2, 3a, 3b. (fig. 5-19)

2, 3a: limb position, shape discrimination

1, 3b: touch perception

4. Efferent projections:

a. Corticocorticol (“ascending”) higher-order sensory cortices, motor cortex.

- process info, planning and controlling from layers 2, 3.

Brodman’s Areas (Fig. 5-19b)

II. Regional anatomy b. Callosal

Through corpus callosum to opposite side of brain (coordinate sensory input from both sides of the body) – also located in layers 2, 3

c. Descendingstriatum

VPL of thalamus

brainstem (DC nuclei) “gate-keeper” or filter function:

dorsal horn of SC

Regulating the quantity of somatic sensory info ascending through the CNS.

H. Higher-order somatic sensory cortical areas.

Much info passes from 1° cortex to other areas of parietal lobes, which process the sensory info. For perception and higher brain functions.

e.g., Posterior parietal cortex – for perception of body image.

Clinical Correlation

Sensory deficits that follow SC injury permit the localization of the trauma.

- Deficits at the level of and caudal to lesion.

- Brown-Sequard Syndrome: - SC hemisection

- As I present the “S” and “O” of this case, consider importantly,

1. The modality affected

2. The laterality

3. The level (body regions affected)

-to help localize the lesion.

Clinical Correlation

S

A 30-yr old man presents with progressive numbness and weakness in his left leg and hip and left trunk. He noted no difficulties on the right side, except for observations that he felt no pain when he accidently banged his right foot against a door.

Medical history (-); basically healthy.

O

On physical exam, patient was found to have marked diminishment in sensation of blunt pressure from ambilicus downward on left side.

L: light touch was also diminished:

2-point discrimination nearly absent (normally ~ 3 cm)

Clinical Correlation

• Graphesthesia absent (draw letter “A” on leg)

• Vibration sense diminished on left leg hip (tuning fork).

• Position sense: Patient showed a (+) Romberg sign (falling to left when made to stand erect with eyes closed).

• All the preceding was intact on the right side.

• Temp, pain sensation were intact on left side, but diminished on right side from lower hip downward

– inability to distinguish head of pins from sharp points.

- inability to discriminate test tubes filled with warm or cold H2O.

A

• Test: MRI showed a tumor encroaching on the SC from the left at the level of T10.

Clinical Correlation

• How do these symptoms relate to crossing-over of the 2 pathways?

• Why is the loss of pain, temp a couple of segments lower than the lesion?*

P

• Treatment Plan:

Diagnose tumor (myoma) – not invasive (pressing on SC).

- progressive return of functions after surgical removal (85% within 1 year)

*The axons of the AL system decussate over a distance of 1-2 spinal segments before ascending to the brainstem and diencephalon.