Correlative Neuroanatomy of the Sensory System

8/17/2019 Correlative Neuroanatomy of the Sensory System

1/8

01 of 08[SESE, SISON, STA. MARIA]

01-01

OS 202B Integration and Control Systems

Correlative Neuroanatomy of the Sensory System Date: March 1, 2016 Trans: 03-06

Lecturer: Jose Leonard Pascual, MD

OUTLINEI. Introduction

A. Sensory ModalitiesB. How We See ThingsC. Basic Layout of the Sensory System

1. Neurons2. Dermatomes

II. Pain A. Tracing the Pain PathwayB. How We Feel PainC. Pain/Temperature Pathway from BodyD. Pain/Temperature Pathway from Head

III. TouchNote: This trans is based on Dr. Pascual’s slides. Italicizednotes are lifted from 2019 trans.

Learning Objectives

FORM: Identify the important neuroanatomical structuresinvolved with the transmission of sensation (receptor,nerve, nerve root ganglion, ascending tract, thalamus,cerebral cortex)

FUNCTION: To be able to trace the pathways below fromreceptor to cortex

I. INTRODUCTION

A. Sensory Modalities1. Pain and temperature - noxious stimuli; extreme situations2. Light touch - subtle stimulus3. Vibration sense - repetitive light touch4. Proprioception - position and movement

Different sensory modalities each have their ownreceptorso Thermoreceptors – heato Meissner’s corpuscle – toucho Nociceptor – paino Pacinian corpuscle – pressureo Photoreceptors - light

B. How We Sense Things

Nerve receptor picks up the stimulus

o Free nerve endings: PAIN (meaning they are nakeddendrites)

o Pain in the fingers must ASCEND to the brain so it’s AFFERENT

The stimulus is sent to the thalamuso Stimuli received by several receptors travel together

within the peripheral nerves, reach their respectivenerve ganglia (eg. dorsal root ganglia) and enter theCNS

o Receptor peripheralnerve nerveganglia CNS(via ascending tract)

The thalamus relays the stimulus to the cerebral cortex

The cerebral cortex understands the stimulus in its context

C. Basic Layout of the Sensory System1. Neurons

a. First Order Neurons

b. Second Order Neurons Receives impulses from first order neuron Location:

o Neurons of the body: lies in the dorsal horn of thespinal cord

o Neurons of the face: lies in the spinal nucleus ofCN V

Crosses the midline and ascends into the thalamus

c. Third Order Neurons

From the thalamus to the cerebral cortex

Figure 1. Organization of Sensory Pathways.

2. Dermatomes

Dermatomes: areas on the skin supplied by a singlespecific spinal nerve root

Each dermatome segment corresponds to one spinalnerve

There is NO C1 dermatomeo C1 is a purely motor peripheral nerve

Two or three vertebral bodies separate the spinal nerve

from its origin in the spinal cordo e.g. a lesion affecting the site of origin of T10 is not

necessarily at the level of T10 vertebra; it would most probably be at the level of T8 vertebra

C1-7 exit ABOVE their corresponding vertebral bodies

C8 exits ABOVE T1

T1 downwards exit BELOW their corresponding vertebralbodies

From the spinal cord exits a dorsal root (sensory), whichfurther becomes a dorsal root ganglion and a ventral root(motor)

Table 1. Easy-to-remember dermatomal segments.

Anterior Posterior

C3 Front of neck(“lovebite”)

C2

Back of the head

C6 Thumb

Also innervates thearea at the angle of the jaw and earlobe. DoNOT include said areawhen testing for CN V

function.

C7 Middle finger

C8 Little finger

T4 Nipple

T10 Umbilicus

L1 Inguinal area(bikini line)

S2-3 Genitalia

L4 Knee & big toe C5 Shoulder

L5

Shin

S2-5

Buttocks

S1 Little Toe


2/8

2 of 3[SESE; SISON; STA. MARIA]

OS202B Correlative Neuroanatomy of the Sensory System

02 of 08

Figure 2. Diagram of dermatomal segments.

There is a difference between the length of the spine andlength of the cord

o Spine > cord

o

There is a difference between a vertebral level and acord level

o Cord will terminate at L1-L2, which is important whendoing a spinal tap

In every interval between each vertebral body, there is agap where the spinal nerves will exit

Figure 3. Spinal nerves coming out of the roots.

II. PAIN

A. Tracing the Pain Pathway

This pathway mediates nociceptive stimulio Two different modalities, one pathway = Pain and

Temperature

Each of those sensations have corresponding nociceptorso Pain (pinprick): A-delta mechanical receptorso Deep pain (tissue damage): C-polymodal (free nerve

endingso Heat/cold: Free nerve endings

B. How We Feel Pain Unmyelinated nerve endings are preferentially stimulated

(nociceptors)o If it’s myelinated, you will feel the pain very rapidly

and very painfully

Impulse travels along the nerve assigned to the areaaffected (dermatome)o Each spinal nerve has a dorsal root ganglion (DRG)

and each DRG is assigned a cord segment

Pain is carried from periphery into CNS by dorsal rootgangliao The trigeminal ganglion is structurally similar to a

dorsal root ganglion

The stimulus is passed to clusters of neurons whose

axons cross to the other side of the CNSo Dorsal horn grey matter and spinothalamic tract

(spinal cord to thalamus)

Second order neuron = where decussationoccurs

The tracts either ascend or descend to cross over to thecontralateral side o Trigeminal spinal nucleus and tract

The spinal trigeminal tract will cross at the levelof C2 segment and ascend together with thespinothalamic tract

All of the decussations must pass through thespinal cord

Figure 4. Spinal trigeminal and spinothalamic tract.

The tracts terminate in the thalamus, which thenprocesses the stimuli and relays them to the cerebralcortex

Lesion at the spinothalamic tract area ABOVE the level ofC2 will manifest as loss of pain and temperature sensationat the CONTRALATERAL half of the face and body


3/8



03 of 08

C. Pain/Temperature Pathway from the Body1. First Order Neurons: Lies within the DRG

Dendrites travel within the spinal nerve, passing throughthe intervertebral foramen to reach the DRG

Axons from the DRG ascend or descend (more often) oneor two spinal cord segments before synapsing on thedorsal horn

Axons of nociceptive receptor cells travel 1-2 spinal cordsegments up or down within the dorsolateral fasciculus(Lissauer’s tract) and synapse within the second orderneuron within the dorsal horn of the spinal cord

Figure 5. Lissauer’s tract (Red).

2. Second Order Neurons

Lies in the postmarginal nucleus within the outermostlayer (lamina 1) of the dorsal horn

The second order neuron’s axons immediately decussatewithin the anterior commissure to ascend within the lateralfasciculus as the spinothalamic tract

Going caudally to rostrally, the spinothalamic tract fibersare pushed laterally by each spinal nerves decussatingfiberso Tumor growing from outside = affects outermost

fibers = ascending loss of pain and temperaturesensation

o Tumor growing from inside of spinal cord = affectsinnermost tract fibers = descending loss of pain andtemperature sensation (i.e. cervical nerve fibers firstbefore thoracic, lumbar, and sacral)

At the level of the medulla, the spinothalamic tractascends within the reticular formation adjacent to thespinal tract and nucleus of the trigeminal nerve

o If this is harmed, the patient will manifest with crossnumbness (ipsilateral half of face and contralateralextremities)

3. Third Order Neurons

The axons of the secondary order neuron synapse withthe third order neurons within the ventroposterolateral(VPL) thalamus (“L” for limbs)

D. Pain/Temperature Pathway from the Head

The segmental innervation of the somatosensory systemof the body continues also in the head

Onion-skin pattern of innervationo Upper cervical cord and caudal medulla = more

peripheral areas of face and heado Upper medulla = more central areas (nose, cheeks,

lips)o Pons = mouth, teeth, pharynx

1. First Order Neurons

1st order neuron: the free nerve endings are the tips of thedendrites and its soma lies within the Gasserian/trigeminal/semilunar ganglion (or in ganglia of VII/IX/X)

CN V (trigeminal) ganglion

o Dendrites pass through respective foramina to reachtrigeminal ganglion

Supraorbital fissure: V1

Foramen rotundum: V2

Foramen ovale: V3

o From the trigeminal ganglion, CN V arises andpierces through the middle cerebellar peduncle of thepons

o Axons of nociceptive receptor cells within CN V enterthe pons via the sensory root

o Descends within the spinal tract of CN V to terminateon the pars caudalis of the spinal nucleus of CN V(second order neuron)

CN IX (petrosal) and CN X (jugular) ganglion

o The dendrites travel within the glossopharyngeal andvagus nerves, which enter the skull via the jugular

foramen Both penetrate the brain stem

o Axons of the nociceptive receptor cells within CN IXand X enter the medulla

o They join the spinal tract of CN V to terminate on thepars caudalis of the spinal nucleus of CN V (secondorder neuron) as well as in the dorsal horn of theupper cervical cord

2. Second Order Neurons

Spinal Nucleus of CN V o Pain and temperature o Decussates in the cervical cord (C2) to ascend as the

trigeminal lemniscus together with the spinothalamic

tract

Divided into: o Pars oralis

o Pars interporalis

o Pars caudalis

Histologically indistinguishable from the dorsalhorn of the spinal cord

Figure 6. Spinal nucleus divisions.

The second order neuron cell body lies within the parscaudalis of the spinal nucleus of CN V, whose axonsdecussate at the level of C2 and ascend as the trigeminallemniscus

3. Third Order Neurons

Axons of second order neurons synapse with tertiaryneurons within ventroposteromedial (VPM) nucleus ofthe thalamus (“M” for mukha)

4. Postcentral gyrus

Primary somatosensory cortex

Brodmann’s areas 3, 2, and 1

The more important the area, the higher the number of

neurons allocated o Face and hands have the biggest representation of

the primary somatosensory cortex


4/8



04 of 08

Figure 7. Primary somatosensory cortex and associated homunculus.

E. Pain Modulation

Within the spinal cord: substantia gelatinosa

Within the brainstem:o Periaqueductal gray of rostral braino Periventricular gray of diencephalono Reticular formation: nucleus raphe magnus

III. TOUCH

A. How We Feel TouchGeneral Pathway1. Touch receptors travel within nerves of their assigned

dermatome2. Touch impulses travel UNCROSSED to the brainstem and

terminate in nuclei assigned for touch and vibration3. Fibers from those nuclei CROSS over within the

brainstem and ascend to the thalamus4. The thalamus relays the touch stimulus to the ipsilateral

sensory cortex and the rest of the brain

Differences with the Pain Pathway

First order neuron always travels upwards uncrossed

Second order neuron is in the medulla

Only the second order neuron decussates

B. Tactile Pathway from the Body to the Head

This pathway mediates the following sensations:o Touch (tactile), including

Two-point discrimination Stereognosis (determine object by touching it) Graphesthesia (determine what’s written on skin)

Pressure (deep touch)

Vibration

Limb position

Limb motion

Each of these sensations have correspondingmechanoreceptors:

Light Toucho Tactile (hairless skin): Meissner's corpuscleso Tactile (hairy skin): hair follicle receptors

Pressureo Merkel discso Ruffini endings

Vibration: Pacinian corpuscles

Limb position & motion: muscle spindles

1. First Order Neuron

Soma lies within the dorsal root ganglion

Axon fibers from the leg enter lumbar/sacral spinal cordvia the dorsal root and are funneled medially into thefasciculus gracilis (tract of Goll)

Axon fibers from the arm enter cervical/thoracic spinalcord via the dorsal root and are funnelled medially into the

fasciculus cuneatus (Tract of Burdach) Fibers coming from the upper body (including the arms)

push the fibers from the lower body medially, leading tosomatotopic organization of the posterior columns

Together, the axons within the posterior columns ascendto the brainstem uncrossed

Figure 8. Arrangement of fasciculi gracilis and cuneatus in the spinalcord. Note that f. gracilis is pushed medially

by the f. cuneatus.

2. Second-order Neuron

Within the nucleus gracilis / nucleus cuneatus

Fasciculus gracilis and cuneatus terminate in theirrespective nuclei (nucleus gracilis and nucleuscuneatus), which lie under their respective tubercles(gracile tubercle and cuneate tubercle)

Myelinated fibers from each nuclei reach the brainstem,travel up the midline, and cross anteriorlyo The fibers are now seen as internal arcuate fibers

in the medulla, forming a structure in the midlineknown as the medial lemniscus on the contralateralside

o Lamination of the medial lemniscus (at the level of themedulla): Fibers from gracile nucleus are in the anterior

half Fibers from cuneate nucleus are in the posterior

half

Figure 9. Lamination of the medial lemniscus. Nucleus gracilis is blueand anterior; nucleus cuneatus is purple and posterior.

At the level of the pons, the medial lemniscus undergoes

dorsolateral rotationo The medial lemniscus now appears flattened and on

a medial lateral axiso Fibers from the legs are now more lateral


5/8



05 of 08

Figure 10. Dorsolateral rotation of the medial lemniscus; Laterallyplaced fibers from the leg.

3. Third-order Neuron

The medial lemniscus can now ascend withoutdecussating to the VPL nucleus of the thalamus (similarto pain pathway)

C. Tactile Pathway from the Head1. First Order Neuron

Large pseudounipolar cells within the Gasserian ganglion

Axons enter the pons via the sensory root of CN V andpass dorsomedially to tegmentum of pons

Principal target of light touch for face is the principalsensory nucleus of CN V

Counterpart of the gracilis and cuneatus

2. Second Order Neuron

Lies within the principal sensory nucleus of CN V

Main sensory nucleus / principal nucleus o Analogous to posterior column nuclei of the cordo Concerned with proprioceptive sensations of the heado Decussates and joins contralateral medial lemniscus,

which ascend to terminate in the thalamus (ventraltrigeminal tract)

o Axons of these second order neurons travel within theascending trigeminothalamic tract and terminate on

third order neurons within the VPM nucleus

Figure 11. A more complete somatotopic representation(homunculus) can be seen for the tactile pathway at the level of the

pons.

IV. LOCALIZING LESIONS IN SOMATOSENSORYPATHWAYS

Look at the pattern of the sensory phenomena (e.g., pain,numbness, etc):o Peripheral nerveo Segmental dermatomeo Spinothalamic tracto Postcentral gyrus

Case 1Your friend was hurt in a vehicular accident. On yourneurologic examination you found that he has:

Loss of sensation below the umbilicuso On the right side: cannot feel light touch/vibrationso On the left side: cannot feel pinprick

Answer: Lesion at R T10

Below umbilicus = T10

Cannot feel light touch on R side = lesion on R side (nodecussation of nerves for light touch pathway)

Cannot feel pinprick on L side = decussation of nerves

Case 2 The security guard at the mall suddenly feels dizzy. You did aneurologic examination and found that he has:

No sensation of pinprick on the left side of his body

No sensation of pinprick on the right side of his face

Difficulty swallowing (may be problem of medulla) Answer: Cross numbness (see p. 3 under “Pain/TemperaturePathway to Head”)

Lesion at C2 level; CN V fibers have not decussated whilespinothalamic tract fibers have already decussated

Case 3The fishball vendor suddenly feels his right hand is clumsy andhas right sided weakness of his face, arm and leg. You do aneurological examination and find that he has:

No problems understanding you and follows what you say

Shallow nasolabial fold on the right

Weak right arm and leg

Walks like a drunk person and teeters to the right

He has lost his vibration sense and light touch on rightside

Answer: Lesion at L and stroke at the medial lemniscus atthe medulla (affected spinothalamic tract). Slight involvementof the pyramidal (corticospinal tract) tract.

Case 4The banana-Q vendor suddenly slumps to the ground andsnores loudly. You were nearby and did a neurologicexamination and found that he has:

Very drowsy mental state

Cannot feel anything on left side of face and body (evenwith very painful stimuli)

Answer: L-sided hemianaesthesia. Stroke at the R thalamus damaged R spinothalic tract, disruption of ARAS. Snoring issign of decreased sensorium and loss of consciousness.Corona radiata and internal capsule are affected.

V. VISION

This part is heavily based on sir Pascual’s lecture slides:

https://drive.google.com/file/d/0B_x7FtVlOWK_OERzX0VvQjBf eVU/view It is recommended that you check the slides while you read thetrans because there are a number of pictures there that maybe useful and the trans might have too much pictures if theyare included as they are part of a pathway.

A. How We See

Light and color stimulate photoreceptors in the retina

The visual impulse reaches the retinal ganglion cells,whose axons converge into the optic disc

The axons continue on as optic nerve

Half of the axons from each eye cross over to thecontralateral optic tract within the optic chiasm

Visual impulses within the optic tract reach the thalamus The thalamus sends forth optic radiations within the

subcortical white matter, which terminate within the visualoccipital cortex

Summary: Light and color Photoreceptors in the retina Retinal ganglion cells axons Optic disc (papilla) Optic nerve Optic tract Thalamus

Figure 12. The ganglion cell neurons and axons that form the opticnerve.


6/8



06 of 08

B. Visual Pathway

Sighto What objects look likeo Where objects areo These would be recognition and location which are

primitive

The optic nerve exits the orbit via the optic foramen andunite to form the optic chiasm

Due to the refractive properties of the lenses of our eyes,the image that falls on our retinas will be invertedupside down and flipped horizontally:

Figure 13. Image seen by eye (left); image refracted in the retina(right).

Visual stimuli from the temporal halves of the image fall onthe nasal halves of the retina

Visual stimuli from the nasal halves of the image fall onthe temporal halves of the retina (That’s why at the levelof the retina, visual fields in each optic nerve arerepresented as inverted mirror images).

Figure 14. Representation of image stimulus as received by thetemporal and nasal halves of the eyes.

Blindness can actually root from the optic nerve to theretina itself

At the optic chiasm, the nasal fibers from each opticnerve decussate to the contralateral optic tract

At the optic chiasm, the temporal fibers continue on theipsilateral optic tract

o Snakes and crocodiles have Ipsilateral UncrossedVisual Fibers

o This is actually needed to have front facing eyesand for eye-hand coordination

o ALBINOS - Do not have front facing eyes (OpticChiasm) thus they also have poor eye-handcoordination

From the optic chiasm, optic tracts emanate from eachside and pass posterolaterally along the surface of thehypothalamus and cerebral peduncles

Axons of the retinal ganglion cells will terminate on thelateral geniculate nucleus (LGN) of the thalamus on eachside

Figure 15. The Visual Pathway.

Third order neuron: Lies within the LGN o Sends projections to the primary visual cortex or

cerebral (occipital) cortex (optic radiations a.k.a.geniculocalcarine/geniculostriate pathways)

o Representation of the field of vision in the LGN The area of the sharpest visual acuity is

subserved by the macula of the retina (haslarge amount of neurons)

This small area is greatly represented within theLGN and in the visual cortex (seeing sharpimage is very important to us and our brains)

o The optic radiations enter the retrolenticular portion ofthe posterior limb of the internal capsule

o From the internal capsule, the fibers of the opticradiation sweep to the lateral surface of the lateralventricle The more dorsal (parietal) fibers proceed directly

posteriorly, through the parietal lobe and finallythe occipital lobe

The more ventral (temporal) fibers loop anteriorlyover the inferior (temporal) horn of the lateralventricle (Also known as the Meyer’s Loop, which is the anterior most extension of the opticradiations to the temporal horn)

Figure 15. Location of Meyer’s Loop.


7/8



07 of 08

Figure 16. Location of Meyer’s Loop in imaging (left) and axial cutsection (right) of the brain.

o Superior fibers of the optic radiation pass straightthrough the parietal lobe to the occipital cortex Only the inferior quadrant of the image appears

in the superior (parietal) optic radiations The superior fibers terminate in the cuneus Sees the ground and used for locating objects

o Inferior fibers must fist loop around the temporalhorn of the lateral ventricle before going to theoccipital cortex

Only the superior quadrant of the image appearsin the inferior (temporal) optic radiations

The inferior fibers terminate on the lingual

(medial occipitotemporal) gyrus o Optic radiations as a whole will have the image

appear as just the contralateral half

Figure 17. The contralateral halves.

Visual cortex: composed of primary visual (striate) cortexand the extrastriate visual cortical areaso Primary visual cortex (V1), also known as the striate

cortex

C. Visual Acuity

As previously mentioned, the macula has a greatrepresentation of the visual cortex

While the occipital cortex receives most of its blood supplyfrom the posterior cerebral artery (PCA), the macularvision area receives additional (collateral) blood supplyfrom the middle cerebral artery (MCA)

Figure 18. Blood supply of the primary visual cortex.

D. Localizing Lesions

At the level of the optic nerve

o Damage to the retina or one of the optic nervesbefore it reaches the chiasm results in a loss ofvision that is limited to the eye of origin

o Anterior to the chiasm - loss of one eye ormonocular

At the level of the optic chiasm (lateral)

o Damage to optic chiasm AWAY from midlinestructures can affect fibers that run through inferiorfibers or Meyer’s loop

o Results to loss of superior ipsilateral visual fieldo Also known as left superior quadranopiao May result to macular loss of vision

At the level of optic chiasm (central)

o Damage to the middle portion of the optic chiasm(often the result of pituitary tumors, pharyngealtumors, rathke pouch tumors) results in damagedfibers of the nasal retina

o Intact temporal retinao Resulting loss of vision is confined to the temporal

visual field images from the temporal visual fieldfall onto nasal retina

o Called bitemporal/heteronymous hemianopia (dueto cut nasal fibers)

o Also includes loss of lateral aspect of body spaceo Brain tumors or problems with the cavernous sinus

Pituitary gland and optic nerves are near

At the level of optic tract

o Interruption of the L optic tract results in loss of sightin the R visual field (i.e., blindness in the temporalvisual field of R eye and nasal visual field of L eye)

o Called homonymous hemianopia (in this case, Rhomonymous hemianopia)

o Pathway was disrupted before the chiasm


8/8

8 f 3


08 f 08

At the superior fibers of the optic radiation crossingthe parietal lobe

o Results in loss of interior contralateral visual fieldo Called Inferior right quadranopiao “Pie on the floor” lower quadrant o “Pie on the sky” upper quadrant temporal lobe

problem

Transecting the whole optic radiations

o Similar to transecting an optic tract o Manifests also as homonymous hemianopia

Transecting the striate cortex

o Results in the contralateral homonymoushemianopia with macular sparing

o Selecting preservation of foveal vision has notbeen clearly understood, although this is a common

feature of damage in the striate cortexo Loss of blood supply, PCA was blocked, occipital lobe

lost blood supply but macular area is persistent

In the primary visual cortex (“Cortical blindness”) o Blindness may not be due to eye damage, but cortical

lesion Pupils still react to light, but cannot see anything Patient may experience hallucinations (Optic

nerve is working but the brain is blind) Results to visual field defects/scotoma

o Occipital lobe lesion: black pigmentationso Temporal lobe lesion: angel-like visionso Micropsia: objects appear smaller than actual sizeo Macropsia: objects appear larger than actual size

E. CaseYour girlfriend has a vague headache the past few years, andrecently it has gotten worse. She now tends to bump intopeople while walking into BSLR-East. Her neurologicalexamination only showed this:

Damage to optic chiasm resulting to bitemporalhemianopsia.

END OF TRANSCRIPTION

TRANSERS’ NOTE

“I got fire for a heart, I’m not scared of the dark. You’ve neverseen it look so easy. I got a river for a soul and baby you’re aboat. Baby, you’re my only reason… All my life you stood byme when no one else was ever behind me. All these lights,they can’t blind me. With your love, nobody can drag medown .” –Drag Me Down by One Direction [#PUMPUPSONG#READTHOSETRANSES]“My heart, your heart. Sit tight like book ends. Pages betweenus. Written with no end. So many words we’re not saying.Don’t wanna wait til it’s gone. You make me strong .” –Strongby One Direction [#GOTTABESTRONGFORTRANSES]“You and me got a whole lot of history. We can be the greatest

team that the world has ever seen. You and me got a whole lotof history. So don’t let it go, we can make some more, we canlive forever .”-History by One Direction [Lets make history.]

APPENDIX A: Summary of the Lesions in the Visual Pathway

Date post:	06-Jul-2018
Category:	Documents
Upload:	jonathan-gorospe
View:	221 times
Download:	0 times

Correlative Neuroanatomy of the Sensory System

Documents