representative ascending tracts georgia bishop phd professor and vice chair department of...
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REPRESENTATIVE ASCENDING TRACTS
Georgia Bishop PhD
Professor and Vice ChairDepartment of Neuroscience
OBJECTIVES
1. Be able to trace the ascending fiber tracts listed and discussed in this TLM. For each tract, you
should be able to:
a) Identify the location of the neurons that give rise to the tract;
b) Define the primary motor function of the ascending tracts
c) Name the location and immediate anatomical relationships of each tract in different
subdivisions of the brainstem and forebrain;
d) Describe the course of each tract from its origin to its ultimate site of termination including:
1. the location of any second and third order neurons.
2. where it relays
3. whether it is ipsilateral or contralateral.
4. If contralateral, identify the location where axons cross the midline (decussation).
2. Be able to identify select fiber tracts at all levels of the brainstem on cross sectional images
from an atlas and on MRI/CT scans.
DESCRIBE THE LOCATION AND RELATIONSHIPS OF RELEVANT ASCENDING TRACTS IN THE CNS ON GROSS BRAIN SPECIMENS, CROSS SECTIONAL MATERIAL, AND RADIOGRAPHIC IMAGES.
Information must be transmitted bidirectonally between spinal cord and cerebral cortex with relays in the brainstem
CONDUIT FUNCTION OF BRAINSTEM
Dorsal (Posterior) Column-medial LemniscusAnterolateral System: Spinothalamic Spinomesencephalic Spinoreticular SpinohypothalamicSpinocerebellar (Dorsal And Ventral)CuneocerebellarSpinocervicalSpino-olivarySpinotectal
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REPRESENTATIVE EXAMPLES OF ASCENDING TRACTS
DRG
From Tactile Receptors andproprioceptors
Modality: 1. Low Threshold Cutaneous Receptors for: Touch, Pressure, Vibration, Fine Form And Texture Discrimination, Form Recognition Of 3-dimensional Objects (Stereognosis)2. Joint And Muscle Receptors. Conscious Awareness Of Body Position (Proprioception), Limb Movement In Space (Kinesthesia)
1ST ORDER NEURON: DORSAL ROOT GANGLIA CELLS.
Primary afferent axons enter via dorsal root to enter ipsilateral posterior column. Below T5 only fasciculus gracilis. Above T5, 2 tracts – fasciculus gracilis (FG) and fasciculus cuneatus (FC).
2ND ORDER NEURONS IN CAUDAL MEDULLA IN N. GRACILIS AND N. CUNEATUS.
Axons arising from neurons in these nuclei cross the midline as the internal arcuate fibers and form the medial lemniscus (L: ribbon).
1ST ORDER NEURONS
Input From Lower Limb And Trunk (T6-S5) Forms Fasciculus Gracilis
Input From Upper Limb And Trunk (C1-T5) Forms Fasciculus Cuneatus
3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex
2ND ORDER NEURONS
3RD ORDER NEURONS
L4
C8
CAUDALMEDULLA
PONS
ROSTRALMIDBRAIN
N. GRACILISN. CUNEATUS
MEDIAL LEMINISCUS
THALAMUS(VPL)
FG FC
DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS
PERIPHERAL PROCESS:Large diameter, heavily myelinated axons carry information from muscle spindles/joint receptors (proprioception)
Slightly smaller diameter, myelinated axons, carry information from cutaneous receptors (touch, vibration)
CENTRAL PROCESS:Enters through dorsal root and courses directly to posterior column where they ascend to brainstem
MUSCLE SPINDLE/JOINT RECEPTORS
CUTANEOUS RECEPTORS
DRG CELL BODY
PERIPHERAL PROCESS
CENTRAL PROCESS
DORSAL (POSTERIOR) FUNICULUS
FIRST ORDER NEURON
INPUT FROM T6-S5 (LOWER LIMB)– FORMS FASCICULUS GRACILIS (LATIN: SLENDER) IN THE DORSAL HORN (COLUMN) OF THE SPINAL CORD
INPUT FROM C1-T6 (UPPER LIMB)– FORMS FASCICULUS CUNEATUS (LATIN: WEDGE) IN THE DORSAL HORN (COLUMN) OF THE SPINAL CORD.
S4
FG
L2
FG
T7
FG
C5 C3
FGFC FC
FGFC FC
NOTE: As axons enter from the upper limb, those present in the dorsal column from the lower limb are shifted medially.
DORSAL COLUMN – MEDIAL LEMNISCUS SPINAL CORD
1st order axons arising from neurons in the DRG ascend to TERMINATE In the ipsilateral N. Cuneatus and N. Gracilis in caudal medulla. The axons of the DRG neurons SYNAPSE on neurons in these nuclei. This is the end of the dorsal column and the beginning of the Medial Lemniscus
N. GRACILIS
LL
N. CUNEATUS(TRACT STILL PRESENT)
UL
INTERNALARCUATE FIBERS(IAF)
MEDIAL LEMNISCUS(ML)
2nd order neurons in N. Gracilis and Cuneatus give rise to axons that course ventrally in an arc = INTERNAL ARCUATE FIBERS. CROSS MIDLINE
These axons collect on either side of midline to form MEDIAL LEMNISCUS in the medullary tegmentum. This tract ascends through medulla, pons, midbrain to terminate in thalamus
UL
LL
PYRAMIDS
IOC
SpV
X
DORSAL COLUMN – MEDIAL LEMNISCUS CAUDAL MEDULLA
MLLL
UL
IOC
PY
ML REMAINS AS A VERTICAL COLUMN IN THE TEGMENTUM IMMEDIATELY ADJACENT TO THE MIDLINE AS IT ASCENDS THROUGH THE MEDULLA
MLF
MEDIAL LONGITUDINAL FASCICULUS (MLF) “CAPS” THE ML. FIBER TRACT RELATED TO VESTIBULAR SYSTEM AND CONTROL OF EYE MOVEMENTS. MORE ON THIS TRACT LATER.
DM-XSOL
VES
SpV
ML
MLF
DORSAL COLUMN – MEDIAL LEMNISCUS ROSTRAL MEDULLA
XII
BP
MAIN SENSORY V
MOTOR V
V
MES. V
As ventral brainstem expands to form basilar pons the ML is “pushed” dorsally and assumes a more horizontal orientation.
UL remains close to midline while lowerlimb “swings” laterally
UL LL
ML ULLL
ML
AB
VII
G
FA
MIDLINE
DORSAL COLUMN – MEDIAL LEMNISCUS: MID PONS
Axons from the main sensory nucleus of V carrying touch information from the face, cross the midline and join the medial leminiscus in the pons. These axons are located medially. They will terminate in the ventral posterior medial (VPM) nucleus of the thalamus, rather than the ventral posterior lateral (VPL) thalamic nucleus.
Small number of axons remain uncrossed as the dorsal trigeminal tract. These carry information from the inside of the oral cavity and end in VPM; significance is unknown.
NOTE, Axons from the spinal trigeminal nucleus (pain and temparature) do not join the medial leminiscus.
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MIDLINE
From touch receptors
TRIGEMINAL AXONS
TEG
TECTUM
Tr
BP BP
Tr
UL
LL
In the rostral pons, the ML moves laterally as the cerebral peduncle replaces the pons.
UL
LL
FAFA
SC
CP
PAG
DORSAL COLUMN – MEDIAL LEMNISCUS ROSTRAL PONS
IPF
CP
SN
SC
CA
IPF
In the midbrain, the ML moves slightly more laterally and dorsally in the tegmentum interpeduncular fossa fully develops and shifts ventral structures laterally.
UL
LL
UL
LL
FA
FA
TECTUM
ML
PAG
RN
VTA
TEG
OC
DORSAL COLUMN – MEDIAL LEMNISCUS MIDBRAIN
CC
DM
LP
VPL
VPM
CM
SN
RN
CP
IC
3rd order neurons are located in the ventral posterior lateral (UL and LL) and ventral posterior medial (face) nuclei of the thalamus.
Axons arising from 3rd order neurons course through the internal capsule as they project to the parietal lobe.
DORSAL COLUMN – MEDIAL LEMNISCUS THALAMUS
AP
M
L A
A
DM
DM
VA
VPL
VPM
VL
PUL
IL
DL
MG
LG
VL
3rd order neurons are located in the ventral posterior lateral (UL and LL) and ventral posterior medial (face) nuclei of the thalamus.
THALAMIC NUCLEI
PARIETAL LOBE(POSTCENTRAL GYRUS = PRIMARY SENSORY CORTEX)
FRONTAL LOBE
CENTRALSULCUS
SENSORY CORTEX
SPINOTHALAMIC TRACT: Conscious awareness of nature of a painful stimulus (burning, stinging, aching) and where it is located. Also conveys temperature information.
Small diameter, lightly myelinated axons.
1ST ORDER NEURON: DORSAL ROOT GANGLIA CELLS.
Primary afferent axons enter via dorsal root and synapse on neurons in the superficial portion of the dorsal horn
2ND ORDER NEURONS IN DORSAL HORN OF SPINAL CORD AT ALL LEVELS.
Axon crosses midline in anterior commissure and forms spinothalamic (anterolateral) tract in the anterior half of the lateral funiculus
3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex and insular cortex.
1ST ORDERNEURON
3RD ORDERNEURON
2ND ORDERNEURON
Other Aspects Of Pain Are Mediated by other pathways that occupy same space as spinothalamic tract. These End in:
1. Reticular Formation Throughout Brainstem (Attention)2. Limbic System (Emotion, Memory)3. Hypothalamus (Autonomic Response)4. Periaqueductal Grey (Intrinsic Pain Control Mechanisms)
Anterolateral System: Multiple Tracts That Convey Different Aspects Of Pain Including Location And Intensity of Painful Stimulus, Emotional Response to Pain, Autonomic Response to Pain, Increased Attention to Painful Input.
SPINOTHALAMIC TRACT
DRG
L4
C8
UPPER LIMB
LOWER LIMB
MEDULLA
PONS
MIDBRAIN
THALAMUS (VPL)
SYNAPSE
SYNAPSE
ANTEROLATERAL SYSTEM
L2
C3
UL
S4
T7
TRUNK
LLLL
AXONS ENTERING THE TRACT FROM THE LOWER LIMB ARE “PUSHED” LATERAL AS AXONS FROM TRUNK AND UPPER LIMB ARE ADDED.
L2
T6
C6
From Pain AndTemperatureReceptors
1ST ORDER
2ND ORDER
SPINOTHALAMIC TRACT - SPINAL CORD
TEGML
IOC
PY
XII
SOL
XNC
NG
NC
SpV
IOC
ML
PY
STT maintains a position in the lateral, ventral cord until it reaches medulla. As ventral portion of medulla (inferior olive and pyramid) become prominent, the tract is moved dorsally. In mid and rostral medulla, it is located just dorsal to the inferior olivary complex (IOC).
SPINOMEDULLARY JUNCTIONCAUDAL MEDULLA
NOTE: This tract is not as distinct as ML in myelin stained sections due to smaller diameter, lightly myelinated axons.
LL
UL
STTSTT
STTSTT
LL
UL
SPINOTHALAMIC TRACT (STT) - MEDULLA
TrigeminalGanglion
Spinal TrigeminalNucleus
Pain & TemperatureAfferents from Face
Spinal VTract
Cells of origin for afferents carrying pain and temperature information from the face are in the trigeminal ganglion.
Axons enter in the pons and DESCEND via the spinal trigeminal tract to the medulla where they synapse on 2nd order neurons in the spinal trigeminal nucleus.
Axons of 2nd order neurons, located in spinal nucleus of V, cross the midline and join the contralateral spinothalamic tract as it courses rostrally through the pons and midbrain
Axons from trigeminal nucleus terminate in ventral posterior medial nucleus of the thalamus whereas axons from upper and lower limb terminate in ventral posterior lateral nucleus of the thalamus.
Pain & TemperatureAfferents from Body
SPINAL TRIGEMINAL NUCLEUS – PAIN AND TEMPERATURE INPUT FROM FACE FROM V, VII, IX AND X
V
MAIN SENSORY V
BP
MOTOR V
V
MES. V
STT
In the pons, the STT, like the ML, is shifted dorsally by the expanding basilar pons. It lies just lateral to the ML through the rest of its course to the thalamus.
STT
TEG
Tr
BP BP
Tr
UL
LL
UL
LL
FAFA
FAULLL
FAUL
LL
STT
STT
ML
ML ML
Mid Pons
Rostral Pons
SPINOTHALAMIC TRACT (STT) - PONS
In The Midbrain, The STT Shifts a Bit More Dorsally as Cerebral Peduncle Replaces the Basilar Pons
SN
SC
CA
IPF
PAG
RN
VTA
TECTUM
TEGMENTUM
ML
ML
CP
STT
STTUL
LL
FA
INSULA
SPINOTHALAMIC TRACT (STT) - MIDBRAIN
Ascending fibers terminate on 3rd order neuron in VPL (upper and lower limb) or VPM (face) thalamus.
3rd order neuron projects through the posterior limb of the internal capsule to somatotopically appropriate area of somatosensory cortex.
LLUL
FACE
INSULA
Insula –activated by stimuli that make us feel comfortable or uncomfortable including pain, temperature, fatigue or even watching someone in pain. Also responds to vestibular changes.
INSULA
SPINOTHALAMIC TRACT (STT) - THALAMUS AND CORTEX
SUMMARY OF ASCENDING TRACTS
TRACT 1ST ORDER 2ND ORDER 3RD ORDER NEURON NEURON NEURON MODALITY
DORSAL COLUMN TOUCH, PRESSURE, VIBRATIONMEDIAL LEMNISCUS: FORM RECOGNITION, TEXTURE, PROPRIOCEPTION FASICULUS CUNEATUS DRG NEURON C1-T6) IPSI. N. CUNEATUS CONTRA. VPL
THALAMUS
FASCICULUS GRACILIS DRG NEURON (T6–S5) IPSI. N. GRACILIS CONTRA. VPL (Axons cross in caudal THALAMUS
Medulla to form Medial Lemniscus)
ANTEROLATERAL SYSTEM SPINOTHALAMIC TRACT DRG NEURON (C1-S5) IPSI. DORSAL HORN CONTRA. VPL PAIN PERCEPTION, TEMPERATURE
SPINAL CORD THALAMUS (Tract crosses in spinal cord)
TRIGEMINAL TRIGEMINAL GANGLION MAIN SENSORY NUCLEUS V CONTRA VPM (Axons cross in medulla To join Medial Lemniscus)
TRIGEMINAL TRIGEMINAL GANGLION SPINAL NUCLEUS V CONTRA VPM (Axons cross in medulla to join Spinothalamic Tract)
Ascending Tracts
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