REPRESENTATIVE
ASCENDING TRACTS
Georgia Bishop PhD
Professor and Vice Chair
Department of Neuroscience
OBJECTIVES
DESCRIBE THE LOCATION AND RELATIONSHIPS OF RELEVANT ASCENDING
TRACTS IN THE CNS ON GROSS BRAIN SPECIMENS, CROSS SECTIONAL
MATERIAL, AND RADIOGRAPHIC IMAGES.
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.
CONDUIT FUNCTION OF BRAINSTEM
Information must be transmitted bidirectonally between spinal cord and cerebral cortex with
relays in the brainstem
REPRESENTATIVE EXAMPLES OF ASCENDING TRACTS
**
**
Dorsal (Posterior) Column-medial Lemniscus
Anterolateral System:
Spinothalamic
Spinomesencephalic
Spinoreticular
Spinohypothalamic
Spinocerebellar (Dorsal And Ventral)
Cuneocerebellar
Spinocervical
Spino-olivary
Spinotectal
DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS
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)
3RD ORDER
NEURONS
THALAMUS
(VPL)
Input From Lower Limb And Trunk (T6-S5) Forms Fasciculus
Gracilis
FG
FC
ROSTRAL
MIDBRAIN
Input From Upper Limb And Trunk (C1-T5) Forms Fasciculus
Cuneatus
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).
PONS
2ND ORDER
NEURONS
N. GRACILIS
N. CUNEATUS
CAUDAL
MEDULLA
MEDIAL
LEMINISCUS
DRG
3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR
LATERAL NUCLEUS).
C8
Axons arising from these neurons project to primary sensory
(parietal) cortex
L4
From Tactile
Receptors and
proprioceptors
1ST ORDER
NEURONS
FIRST ORDER NEURON
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
DORSAL (POSTERIOR) FUNICULUS
CENTRAL PROCESS
DRG CELL BODY
PERIPHERAL PROCESS
MUSCLE SPINDLE/
JOINT RECEPTORS
CUTANEOUS
RECEPTORS
DORSAL COLUMN – MEDIAL LEMNISCUS SPINAL CORD
INPUT FROM T6-S5 (LOWER LIMB)– FORMS FASCICULUS GRACILIS (LATIN: SLENDER) IN THE DORSAL HORN
(COLUMN) OF THE SPINAL CORD
FG
FG
FG
S4
T7
L2
INPUT FROM C1-T6 (UPPER LIMB)– FORMS FASCICULUS CUNEATUS (LATIN: WEDGE) IN THE DORSAL HORN
(COLUMN) OF THE SPINAL CORD.
FG
FG
FC
FC
FC
FC
C5
C3
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 CAUDAL MEDULLA
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
N. CUNEATUS
(TRACT STILL
PRESENT)
2nd order neurons in N. Gracilis and Cuneatus give
rise to axons that course ventrally in an arc =
INTERNAL ARCUATE FIBERS. CROSS MIDLINE
LL
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
SpV
UL
LL
IOC
X
INTERNAL
ARCUATE
FIBERS
(IAF)
MEDIAL
LEMNISCUS
(ML)
PYRAMIDS
DORSAL COLUMN – MEDIAL LEMNISCUS ROSTRAL MEDULLA
ML REMAINS AS A VERTICAL COLUMN IN THE TEGMENTUM IMMEDIATELY ADJACENT TO THE MIDLINE AS IT ASCENDS THROUGH
THE MEDULLA
MEDIAL LONGITUDINAL FASCICULUS (MLF) “CAPS” THE ML. FIBER TRACT RELATED TO VESTIBULAR SYSTEM AND CONTROL
OF EYE MOVEMENTS. MORE ON THIS TRACT LATER.
VES
SOL
DM-X
XII
MLF
MLF
SpV
UL
ML
ML
IOC
PY
LL
DORSAL COLUMN – MEDIAL LEMNISCUS: MID PONS
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 lower
limb “swings” laterally
ML
LL UL
MES. V
MAIN
SENSORY V
AB
G
MOTOR V
VII
FA
V
ML
UL
BP
MIDLINE
LL
TRIGEMINAL AXONS
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.
☺
MIDLINE
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.
From touch
receptors
DORSAL COLUMN – MEDIAL LEMNISCUS ROSTRAL PONS
TECTUM
In the rostral pons, the ML moves laterally
as the cerebral peduncle replaces the
pons.
SC
PAG
Tr
Tr
TEG
LL
LL
UL
UL
FA
FA
CP
IPF
BP
BP
DORSAL COLUMN – MEDIAL LEMNISCUS MIDBRAIN
In the midbrain, the ML moves slightly more laterally and dorsally in the tegmentum interpeduncular fossa
fully develops and shifts ventral structures laterally.
TECTUM
SC
CA
PAG
OC
LL
LL
UL
UL
FA
ML
TEG
SN
RN
CP
VTA
IPF
FA
DORSAL COLUMN – MEDIAL LEMNISCUS THALAMUS
3rd order neurons are located in the ventral posterior lateral (UL and LL) and ventral posterior medial (face) nuclei of
the thalamus.
LP
DM
VPL
IC
CM
VPM
RN
SN
CP
CC
Axons arising from 3rd order neurons course through the internal capsule as they project to the parietal lobe.
THALAMIC NUCLEI
3rd order neurons are located in the ventral posterior lateral (UL and LL) and ventral posterior medial
(face) nuclei of the thalamus.
PUL
M
P
DL
A
DM
L
A
DM
VL
A
IL
VL
VA
MG
VPM
VPL
LG
SENSORY CORTEX
CENTRAL
SULCUS
FRONTAL
LOBE
PARIETAL LOBE
(POSTCENTRAL GYRUS = PRIMARY SENSORY CORTEX)
ANTEROLATERAL SYSTEM
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: Conscious awareness of nature of a
painful stimulus (burning, stinging, aching) and where it is
located. Also conveys temperature information.
3RD ORDER
NEURON
THALAMUS
(VPL)
Small diameter, lightly myelinated axons.
Other Aspects Of Pain Are Mediated by other pathways that occupy same
space as spinothalamic tract. These End in:
1.
2.
3.
4.
Reticular Formation Throughout Brainstem (Attention)
Limbic System (Emotion, Memory)
Hypothalamus (Autonomic Response)
Periaqueductal Grey (Intrinsic Pain Control Mechanisms)
MIDBRAIN
SPINOTHALAMIC TRACT
1ST ORDER NEURON: DORSAL ROOT GANGLIA CELLS.
PONS
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.
MEDULLA
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).
SYNAPSE
UPPER LIMB
DRG
C8
SYNAPSE
LOWER LIMB
1ST ORDER
NEURON
Axons arising from these neurons project to primary sensory
(parietal) cortex and insular cortex.
L4
2ND ORDER
NEURON
SPINOTHALAMIC TRACT - SPINAL CORD
From Pain And
Temperature
Receptors
C6
1ST ORDER
T6
2ND ORDER
L2
TRUNK
LL
LL
S4
T7
L2
UL
AXONS ENTERING THE TRACT FROM
THE LOWER LIMB ARE “PUSHED”
LATERAL AS AXONS FROM TRUNK
AND UPPER LIMB ARE ADDED.
C3
SPINOTHALAMIC TRACT (STT) - MEDULLA
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).
NOTE: This tract is not as distinct as ML in myelin stained
sections due to smaller diameter, lightly myelinated
axons.
SOL
NG
NC
X
XII
NC
TEG
ML
SpV
STT
LL
STT
UL
STT
ML
STT
IOC
LL
UL
IOC
PY
PY
SPINOMEDULLARY JUNCTION
CAUDAL MEDULLA
SPINAL TRIGEMINAL NUCLEUS – PAIN AND TEMPERATURE INPUT FROM FACE FROM V, VII,
IX AND X
Trigeminal
Ganglion
Pain & Temperature
Afferents from Face
Spinal V
Tract
V
Spinal
Trigeminal
Nucleus
Pain & Temperature
Afferents from Body
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.
SPINOTHALAMIC TRACT (STT) - PONS
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
Tr
Tr
STT
MES. V
MAIN
SENSORY V
LL
LL
UL
FA
UL
FA
TEG
LL
LL
UL
UL
FA
MOTOR V
FA
V
BP
ML
ML
BP
STT
STT
ML
Rostral Pons
BP
SPINOTHALAMIC TRACT (STT) - MIDBRAIN
In The Midbrain, The STT Shifts a Bit More Dorsally as Cerebral Peduncle Replaces the Basilar Pons
TECTUM
SC
STT
CA
LL
STT
PAG
UL
INSULA
FA
TEGMENTUM
ML
ML
RN
SN
CP
VTA
IPF
SPINOTHALAMIC TRACT (STT) - THALAMUS AND CORTEX
INSULA
LL
UL
FACE
INSULA
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.
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.
SUMMARY OF ASCENDING TRACTS
TRACT
1ST ORDER
NEURON
2ND ORDER
NEURON
3RD ORDER
NEURON
DORSAL COLUMN
MEDIAL LEMNISCUS:
TOUCH, PRESSURE, VIBRATION
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
(Axons cross in caudal
Medulla to form Medial
Lemniscus)
CONTRA. VPL
THALAMUS
TRIGEMINAL
ANTEROLATERAL
SYSTEM
SPINOTHALAMIC TRACT
TRIGEMINAL
MODALITY
TRIGEMINAL GANGLION MAIN SENSORY NUCLEUS V
(Axons cross in medulla
To join Medial Lemniscus)
DRG NEURON (C1-S5)
IPSI. DORSAL HORN
SPINAL CORD
(Tract crosses in spinal
cord)
TRIGEMINAL GANGLION SPINAL NUCLEUS V
(Axons cross in medulla
to join Spinothalamic Tract)
CONTRA VPM
CONTRA. VPL
THALAMUS
CONTRA VPM
PAIN PERCEPTION, TEMPERATURE
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