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GENERAL SOMATIC

AFFERENTS

Luis Emmanuel O. Esguerra, MD

Department of Anatomy

UERM Memorial Medical Center

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Review of Terms

Functional Components of PeripheralNerves

General and Special

Somatic and Visceral Afferent and Efferent

“Neuron” = nerve cell body Nucleus vs. Ganglion

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Objectives

Describe the components of the pathways for

general somatic sensation: Sensory receptors.

Peripheral fiber tracts.

Ascending fiber tracts within the CNS. Thalamus and Primary Sensory Cortex.

Describe the pathways for general somaticsensation from the body. Describe the general pattern.

Trace the pathways:

Anterolateral (Spinothalamic) Tract System

Dorsal Column (Medial Lemniscal) System

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Objectives

Describe the pathways for unconscious

proprioception. Describe the pathways for general somatic

sensation from the face region.

Describe the general pattern. Trace the pathways for different general somatic

sensations from the face region.

Dorsal Trigeminothalamic Tract Ventral Trigeminothalamic Tract

Trace the pathway involved in the direct andconsensual corneal blink reflexes.

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SOMATIC SENSORY

(ASCENDING) PATHWAYS

Sensory Receptors

Peripheral somatic afferent fibers

(cranial and spinal nerves)

Fiber tracts and their nuclei of origin

Thalamus (relay center)

Primary sensory cortex (post-centralgyrus)

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MODALITIES OF

SOMATIC SENSATION

PAIN: First – fast, sharp

Second – slow, dull/burning

THERMAL: Warmth

Cold

POSITION SENSE (PROPRIOCEPTION): Unconscious or conscious

Static limb position and kinesthesia

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MODALITIES OF

SOMATIC SENSATION

TOUCH-PRESSURE:

Simple or crude or light touch

Tactile discrimination

2-point discrimination

Stereognosis

Graphesthesia

VIBRATION SENSE

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CLASSIFICATION OF

SENSORY RECEPTORS

Based on source of stimuli

Exteroceptive receptors (exteroceptors)

Interoceptive receptors (interoceptors)

Based on adequate stimuli Mechanoreceptors

Thermoreceptors Photoreceptors [rods and cones (retina)]

Chemoreceptors

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MECHANORECEPTORS Tactile [touch-pressure, vibration]

Non-encapsulated Nerve endings around hair follicles

Merkel’s discs (expanded tip)

Encapsulated

Meissner’s corpuscles

Pacinian corpuscles Ruffini’s corpuscles

Krause’s end bulbs

Golgi-Manzoni

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Sensory nerves

Paciniancorpuscle

Nerve endingsof hair follicle

Meissner’s corpuscle

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MECHANORECEPTORS

Proprioception

Golgi tendon organs

Neuromuscular spindles

Encapsulated receptors

Auditory

Organ of Corti (cochlea)

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THERMORECEPTORS

Respond to temperature differences ~2°C

Warmth

Not yet identified, probably naked endings

of small unmyelinated nerve fibers Cold

Naked endings of myelinated nerve fibersbranching into skin

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Sensory nerves

Free nerve endings

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CHEMORECEPTORS

Pain

Naked/free nerve endings

Taste

Taste buds (tongue)

Olfaction

Olfactory epithelium (nasal cavity)

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SOMATIC AFFERENT FIBERS

Fiber Specific Theories Different receptors and their fibers each

detect a particular quality of sensation

[Doctrine of Specific Nerve Energies] Separate neural channels carry this

information from the specific receptors to

the place in the brain specific to thesensation [Labeled Line Principle]

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CLASSIFICATION OF PERIPHERALSOMATIC AFFERENT FIBERS

Slow (second,burning) pain

Warmth

Fast (first,sharp) pain

Cold

Crude touch

2-pointdiscrimination

Vibration

Consciousproprioception

C (IV)Aδδδδ (III)Aαααα-ββββ

(Ib & II)

noneNON-MUSCLE[skin/deep]

Flower spray endings

of NM spindles

Golgi

tendon

Annulospiralendings of

NM spindles

II (Aββββ)IbIa

smallest; slowestlargest; fastest

MUSCLE

UNMYELINATEDMYELINATED

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ASCENDING FIBER TRACTS

Spinothalamic (Anterolateral) System Pain

Thermal sense

Crude touch

Dorsal Column (Medial Lemniscal) System Conscious proprioception

Vibration sense

Pressure – deep touch Tactile discrimination

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HypothalamusHypothalamus

Brain stemBrain stem

Cerebral cortexCerebral cortex

ThalamusThalamus

(medial)(medial)

Basal nucleiBasal nuclei

(lateral to thalamus)(lateral to thalamus)

CerebellumCerebellum

Spinal cord

MidbrainMidbrain

PonsPons

MedullaMedulla

Brain component Brain component

Cerebral cortexCerebral cortex

Basal nucleiBasal nuclei

ThalamusThalamus

HypothalamusHypothalamus

CerebellumCerebellum

Brain stemBrain stem(midbrain,(midbrain, ponspons,,

and medulla)and medulla)

Sherwood Human Physiology 5 2004

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THE THALAMUS

Dorsal portion of

the diencephalon(largest)

Functions: Relay center

Modifies &integrates afferentimpulses

Adds emotional

tone Regulates cortical

activity

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DIVISIONS OF

THE THALAMUS

Intralaminar group

Centromedian (CM)

Intralaminar nuclei

Anterior nuclear group

Medial nuclear group Median

Medial

Medial dorsal (MD) ordorsomedial (DM)

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DIVISIONS OF

THE THALAMUS

Lateral nuclear group

Lateral (dorsal) nuclear Lateral dorsal (LD)

Lateral posterior (LP)

Pulvinar

Ventral nuclear Ventral anterior (VA)

Ventral lateral (VL)

Ventral posterior Ventral posterolateral

(VPL)

Ventral posteromedial(VPM)

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PRIMARY SENSORY CORTEX

Located in the post-central gyrus

Somesthetic Center or Somatosensory Area I

Brodmann areas 3, 1 & 2

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PRIMARY SENSORY CORTEX

Snell Clin Neuroana 6 2006

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Sherwood Human Physiology 5 2004

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GENERAL PATTERN

(EXCLUDING FACE REGION):

1. Receptors

2. Neuron 1 [N1]: DRG (no synapse)

3. Neuron 2 [N2]: (decussation)

• Dorsal horn of spinal cord (anterolateral system)

• Nucleus gracilis or nucleus cuneatus of medulla (dorsalcolumn system)

4. Neuron 3 [N3]: VPL of the thalamus

• thru posterior limb of internal capsule• forming corona radiata

5. Sensory cortex: post-central gyrus

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GENERAL PATTERN:

IMPORTANT POINTS

The System is crossed. Sensory info from

one side of the body projects to the oppositethalamus & cortex

N1 in DRG

N2 in dorsal horn or homologous nuclei

N3 in thalamus [VPL]

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SPINOTHALAMIC

(ANTEROLATERAL) SYSTEM

Lateral Spinothalamic Tract

For pain and thermal sensation

Anterior (Ventral) Spinothalamic Tract

For light touch, itch, tickle, sexual sensation

Spinotectal Tract

Spinoreticular Tract

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Manter & Gatz 10 2003

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Manter & Gatz 10 2003

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Lateral

Spinothalamic Tract

Subserves:

Pain Thermal sensation

Pathway for “fast”

sharp pain


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Lateral

Spinothalamic Tract

1. Receptors

• Free nerve endings


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Lateral

Spinothalamic Tract

2. N1: Dorsal Root

Ganglion• Axons enter spinal

cord via lateral divisionof dorsal root

• Enter at zone of

Lissauer with fibersrunning 1 to 2segments up or down


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Lateral

Spinothalamic Tract

3. N2: Dorsal horn cells

• Substantia gelatinosa(internuncial neurons)

• Rexed laminae I-V(tract cells)

• Axons cross tocontralateral side inventral commissure &ascend at lateralfuniculus

• Fibers from lower cordlocated dorsolaterally,higher levelsventromedially


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Lateral

Spinothalamic Tract

4. N3: VPL of the

thalamus• Thalamic radiations in

the posterior limb ofthe internal capsule

• Axons form corona

radiata to sensorycortex

5. Postcentral gyrus(Brodmann areas 3,1, & 2)


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Spinoreticular Tract• Fibers to reticular

formation

• For general alertness andarousal in response topainful stimuli

to intralaminar thalamicnuclei (for dull aching

pain)

Spinotectal Tract• Fibers to superior

(rostral) colliculus [for eye

and head orientation]

• For orientation towardpotentially harmful painfulstimuli


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PAIN THEORIES

Intrinsic Pain Suppression Mechanism

Neurons in substantia gelatinosa secretesubstance P for pain sensation

Brain secretes morphine-like substances

(enkephalins, endorphins) to act on opiatereceptors in Lamina II

Gate Control Theory of Pain

Stimulation of myelinated fibers with rubbingof pain-stimulated area suppress fine painfibers

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Anterior

Spinothalamic Tract

Subserves:

Light (crude) touch Itch

Tickle

Sexual sensation Some authors do not

differentiate from

lateral spinothalamictract and group themall together asAnterolateral System


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Anterior

Spinothalamic Tract

1. Receptors

• Hair follicle nerveendings

• Merkel’s discs

• Meissner’s corpuscles

• Pacinian corpuscles


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Anterior

Spinothalamic Tract

2. N1: Dorsal Root

Ganglion• Axons enter spinal

cord via lateral divisionof dorsal root

• Enter at zone ofLissauer with fibersrunning 1 to 2segments up or down


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Anterior

Spinothalamic Tract

3. N2: Dorsal horn cells

• Substantia gelatinosa• Axons cross to

contralateral side inventral commissure(some do not) &ascend at anteriorfuniculus

• Fibers from lower cordlocated dorsolaterally,

higher levelsventromedially

• Join LST at medulla asspinal lemniscus


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Anterior

Spinothalamic Tract

4. N3: VPL of the



• Axons form coronaradiata to sensorycortex



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IN CASE OF UNILATERAL SPINALIN CASE OF UNILATERAL SPINALCORD TRANSECTION: CORD TRANSECTION:

Light touch sensation may never be

completely lost because it has bilateralrepresentation by the Anterior

Spinothalamic Tract.

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Dorsal Column

Pathway

Also known as Medial

Lemniscal System For conscious

proprioception, vibration,deep pressure, deep

touch and tactilediscrimination

Proprioception includes joint position sense,

kinesthetic sense andvibratory sense


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Dorsal Column

Pathway

1. Receptors

• Meissner’s corpuscles• Pacinian corpuscles

• Ruffini’s corpuscles(pressure)

• Golgi tendon organs


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Dorsal Column

Pathway

2. N1: Dorsal Root

Ganglion• Axons enter ipsilateral

dorsal funiculus viamedial division ofdorsal root

• fasciculus gracilis :fibers from sacral,lumbar and lower 6thoracic

• fasciculus cuneatus :fibers from upper 6thoracic and allcervical


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Dorsal Column

Pathway

3. N2: Nucleus gracilis

and n. cuneatus inmedulla

• Axons cross toopposite side as

internal arcuate fibers

• Form the mediallemniscus in thecontralateral side


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Dorsal Column

Pathway

4. N3: VPL of the



• Axons form coronaradiata to sensorycortex



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IN CASE OF UNILATERAL SPINALIN CASE OF UNILATERAL SPINALCORD TRANSECTION: CORD TRANSECTION:

Light touch sensation may never be

completely lost because it may bemediated also by the Dorsal Column

System.

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Ataxia

Unsteady, awkward and poorly coordinated

movement “Sensory” Ataxia - unsteady gait due to a

lesion in the Dorsal Column Pathway

Romberg sign –

With eyes closed, patient sways or falls to oneside when asked to stand with feet placed closetogether

Used to detect dorsal column ataxia

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PATHWAYS FOR UNCONSCIOUS

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PATHWAYS FOR UNCONSCIOUS

PROPRIOCEPTION

Cuneocerebellar

[CCT]

Posterior/dorsalspinocerebellar

[PSCT]

Finecoordination of

individualmuscles

Rostralspinocerebellar

[RSCT]

Anterior/ventralspinocerebellar

[ASCT]

General aspectsof movement

UPPER LIMBSLOWER LIMBS

GENERAL PATTERN OF PATHWAYS

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GENERAL PATTERN OF PATHWAYS

FOR UNCONSCIOUS PROPRIOCEPTION

1. Receptor: NM spindle, Golgi tendon, touch endings

CCTRSCTPSCT

* with crossed fibers (caudal midbrain – superior cerebellar peduncle)

Note: 2-neuron linkage

4. Cerebellum and Vermis

Ext. arcuate fibers Lateral funiculus

L-VI (dorsalhorn at cervical

enlargement)

L-VII (medialintermediate

horn) [T1-L2]

Accesory cuneatenucleus

Nucleuscentrobasalis

Nucleusdorsalis of Clark

Dorsal horn

MEDULLASPINAL CORD

3. N2:

(via fasciculuscuneatus) [C1-T5]

2. N1: DRG

ASCT*

TRIGEMINOTHALAMIC TRACT

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TRIGEMINOTHALAMIC TRACT

GENERAL PATTERN (FACE REGION):

1. Receptors

2. Neuron 1 [N1]: equivalent to DRG (no synapse)

• Gasserian ganglion [CN V]

• Geniculate ganglion [CN VII]

• Superior ganglia [CN IX & X]

• Mesencephalic nucleus of CN V (proprioception only)

3. Neuron 2 [N2]: sensory nuclei of CN V at brainstem

• Fibers cross to form Ventral Trigeminothalamic Tract

• Some fibers do not cross and form Dorsal TrigeminothalamicTract

4. Neuron 3 [N3]: VPM of the thalamus• Also known as arcuate nucleus or semilunar nucleus

5. Sensory cortex: post-central gyrus

Trigeminal

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Trigeminal

NuclearComplex

Sensory nuclei Main sensoryMain sensory

SpinalSpinal

MesensephalicMesensephalic Motor nucleus ofMotor nucleus of

CN VCN V

Trigeminothalamic

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Trigeminothalamic

Tract [TTT]for pain, temperaturefor pain, temperatureand light touch and light touch

Pattern:1. Receptors

2. N1: Gasserian,geniculate & superior

ganglia fibers enter brainstem

at appropriate levelfor each nerve

descend and enteripsilateral spinal tractof V

Trigeminothalamic

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g

Tract [TTT]

Pattern (cont’d):3. N2: spinal nucleus of V

mainly

fibers cross tocontralateral side ofbrainstem & enterVentral TTT

4. N3: VPM of thalamus

Fibers enter posterior

limb of internalcapsule

5. Postcentral gyrus (“facearea” of Brodmannareas 3, 1, & 2)

for pain, temperaturefor pain, temperatureand light touch and light touch

Trigeminothalamic

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g

Tract [TTT]


2. N1: Gasserian,geniculate & superior

ganglia3. N2: main sensory n. of

V & rostral part of spinaln. of V

most fibers cross some do not cross

and form Dorsal TTT

for tactile sensationfor tactile sensationand vibratory sense and vibratory sense

Trigeminothalamic

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g

Tract [TTT]

Pattern (cont’d):3. N3: VPM of thalamus

Fibers enter posteriorlimb of internal

capsule4. Postcentral gyrus (“face

area” of Brodmannareas 3, 1, & 2)

for tactile sensationfor tactile sensationand vibratory sense and vibratory sense

Trigeminothalamic

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g

Tract [TTT]


2. N1: mesencephalicnucleus of V

3. N2: main sensory n. ofV or reticular formation

fibers ascendipsilateral brainstemin Dorsal TTT

4. N3: VPM of thalamus5. Postcentral gyrus

* for unconscious proprioception,N2 fibers pass into

cerebellum

for consciousfor consciousproprioception proprioception

DIRECT CORNEAL BLINK

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REFLEXLEFT

R orbicularis L

oculi

RIGHT

Touch Cornea

CN V1 (ophthalmicdiv)

Gasserian ganglion

Spinal tract of V

Spinal nucleus of V

Motor nucleus of VII

CONSENSUAL CORNEAL

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BLINK REFLEXLEFTRIGHT

Touch Cornea

CN V1 (ophthalmicdiv)

Gasserian ganglion

Spinal tract of V

Spinal nucleus of V


medial

longitudinal

fasciculus


R orbicularis L

oculi

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REFERENCES Snell, R.S. Clinical Neuroanatomy , 7th ed.,

Lippincott Williams & Wilkins, 2010.

Carague-Lansang, E.N., et al. Lansang Notes:Modules in Neuroanatomy , 4th ed., UERMMMC:Quezon City, 2006.

Moore, K.L. and Dalley, A. F. Clinically OrientedAnatomy , 6th ed., Lippincott Williams & Wilkins,2010.

Gilman, S. and Newman, S.W. Manter and

Gatz’s Essentials of Clinical Neuroanatomy andNeurophysiology , 10th ed., F.A. Davis Co., 2003.

Netter, F.H. Atlas of Neuroanatomy andNeurophysiology , Icon Custom Comm., 2002.

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