1. REPRESENTATIVE ASCENDING TRACTS
Georgia Bishop PhD
Professor and Vice Chair Department of Neuroscience
In this module, we will describe how sensory information is transmitted from the periphery to the cerebral cortex. 2 representative tracts will be described in detail as these will be related to their function later in this week. You should be aware that there are several ascending tracts some of which were discussed by Dr. McTigue in her lecture on the spinal cord. The purpose of this module is to introduce you to the concept of synaptic relays in transmitting information from the spinal cord, where DRG axons enter the CNS to the cerebral cortex. We will follow these tracts through the various levels of the brainstem, identifying their specific location at different levels. This is relevant when considering lesions that may only involve small portions of the brainstem and understanding the resulting neurological symptoms.

2. 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.
These are the objectives for this module.

3. CONDUIT FUNCTION OF BRAINSTEM
Information must be transmitted bidirectonally between spinal cord and cerebral cortex with relays in the brainstem
For the nervous system to function properly, communication must be established between different parts of the central nervous system. For example, sensory information has to be relayed to the cerebral cortex for proper interpretation. Likewise, control of movement arising in the cerebral cortex must be transmitted to neurons in the spinal cord. The pathways used to transmit this information can be identified at each level of the brainstem.

4. 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 **
There are numerous tracts that carry information from the periphery to different parts of the central nervous system. For the most part, the name of the tract tells you where it originates and where it ends. For example, the spinothalamic tract begins in the spinal cord and ends in the thalamus. We will only focus on 2** of the many different ascending tracts including the dorsal column-medial lemniscus pathway which transmits general sensory information and the spinothalamic tract which conveys information on pain and temperature.

5. DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS
DRG
From Tactile Receptors and
proprioceptors
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) FG FC
Input From Lower Limb And Trunk (T6-S5) Forms Fasciculus Gracilis
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).
PONS
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).
2ND ORDER
NEURONS
N. GRACILIS
N. CUNEATUS
This appears to be a very complex slide. However, let me just say that everything you need to know about the dorsal column-medial lemniscus pathway is shown on this slide including cells of origin, relays, location at various levels, and site where axons cross the midline to reach the contralateral side. Subsequent slides will allow us to go into a bit more detail. If you learn and understand the information on this slide you will have mastered this pathway. The information on this slide is essential to your understanding of how general sensory information is transmitted from the periphery to the cerebral cortex. . The first critically important concept is that sensory information related to touch is not relayed directly from the spinal cord to higher brain centers. Rather there are 2 tracts with a relay or synaptic junction in between. The first tract is called the dorsal column pathway and as its name implies, it is located in the dorsal column of the spinal cord. There is then a relay of information to neurons in the brainstem which in turn create a new pathway that extends from the medulla to the thalamus. A third pathway extends from the thalamus to the cerebral cortex. Think about a relay race where the baton is passed off by the runners at different stages of the race. Let's begin the summary. The modalities *** carried by this pathway include low threshold cutaneous receptors which detect touch, pressure and vibration. In addition information on the fine form and texture discrmination are carried by these receptors. Putting several pieces of information together, form recognition, that is the 3 dimensional shape of an object can be detected (stereognosis). For example, putting your hand in your pocket and feeling an object that you identify as a key, even without seeing it, based on shape, texture, etc. In addition, sensory information from muscle and joint receptors are carried in this pathway. These axons relay information on where the body is in space (proprioception) or how the limbs are moving in space (kinesthesia). As you heard in the lecture on the spinal cord, sensory information from the*** lower limb is carried in a tract called the fasciculus gracilis. This tract is formed by the central processes of dorsal root ganglion neurons located between T6 and S5. Sensory information from the upper limb ***is carried in a tract called the fasciculus cuneatus which is formed from the central processes of dorsal root ganglion neurons located between C1 and T5. Let's summarize the pathway before looking at it in detail. The first order neuron,*** that is the cell of origin, are dorsal root ganglion neurons. The central processes or axons of these neurons enter the spinal cord through the dorsal root and gather in the dorsal or posterior column where they ascend to the medulla. Below T5 only the fasciculus gracilis is present. Above T5, both the fasciculus gracilis and the fasciculus cuneatus are present. Gracilis is medial to cuneatus. These axons synapse on neurons in the nucleus gracilis or nucleus cuneatus ***which we described in a previous presentation. These nuclei are located in the caudal medulla and are defined as the second order neuron. The axons of neurons located in these two nuclei leave and form another pathway that arcs ventrally and medially and are called internal arcuate axons. They all gather together on the midline, just medial to the inferior olivary nuclei (described previously) to form a tract called the ***Medial lemniscus. These axons ascend on the contralateral side of the medulla, pons and midbrain until they reach the thalamus in the diencephalon *** where another synapse occurs on neurons in a subdivision of the thalamus called the ventral posterior lateral nucleus (VPL). Axons arising from VPL project to areas of the forebrain that are specialized for processing sensory information.
CAUDAL
MEDULLA
MEDIAL
LEMINISCUS
3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex C8
1ST ORDER
NEURONS L4

6. 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
That was a lot of information on the previous slide. Let’s break it down into smaller bits on the rest of the slides in this module. Dorsal root ganglion cells give rise to a two processes. One process projects to the periphery and other projects into the spinal cord. The peripheral processes *** have different sizes and amount of myelin. The larger the process or the heavier the myelin, the faster action potentials are conducted to the CNS. For the dorsal column-medial lemniscus system, the axons are large and heavily myelinated. Those carrying proprioceptive information from muscles and joints are the largest diameter and thus will have the fastest conducation rate. Those carrying sensory information from cutaneous receptors are just slightly smaller and thus just a bit slower. The central processes *** enters the spinal cord via the dorsal root. In the dorsal column-medial lemniscus pathway ***these axons collect in the dorsal or posterior funiculus and ascend to the brainstem.
PERIPHERAL PROCESS
MUSCLE SPINDLE/
JOINT RECEPTORS
CUTANEOUS
RECEPTORS

7. 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 FC FG FC
This is a series of cross sections from S4 to C3. Sensory input arising from the lower limb and lower trunk form the fasciculus gracilis***. Between S5 and T6 only the fasciculus gracilis is present and it occupies the entire dorsal funiculus. Above T6 the fasciculus gracilis is still present, but it is pushed medially as ***Sensory input from the upper limb and upper trunk is added on laterally forming the fasiculus cuneatus. At C5 and C3, the fasiculus gracilis occupies the medial part of the dorsal funiculus. Axons added on *** as the fasciculus cuneatus are located laterally. C5 C3
NOTE: As axons enter from the upper limb, those present in the dorsal column from the lower limb are shifted medially.

8. 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 LL
2nd order neurons in N. Gracilis and Cuneatus give rise to axons that course ventrally in an arc = INTERNAL ARCUATE FIBERS. CROSS MIDLINE
N. CUNEATUS
(TRACT STILL
PRESENT) UL
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
INTERNAL
ARCUATE
FIBERS
(IAF)
This section is at the junction of the medulla and spinal cord. As we saw in a previous presentation, 3 nuclei are evident on the dorsal surface. These are the nucleus gracilis, the nucleus cuneatus and the spinal nucleus of V. Axons ascending in the fasciculus gracilis *** leave their respective fiber tracts and synapse on neurons in the nucleus gracilis. Likewise, axons ascending in the fasciculus cuneatus *** leave the tract and synapse on neurons in the nucleus cuneatus. These neurons are the first relay of sensory information. These are the second order neurons. Axons arising from these neurons*** leave their respective nucleus and course ventrally and medially forming an arc. This fiber bundle is called the Internal Arcuate Fibers. These axons cross the midline just dorsal to the pyramids and the inferior olivary nucleus (IOC). After crossing the midline *** these axons coalesce to form a large fiber tract called the medial lemniscus. Within the medial lemniscus, axons from the nucleus gracilis are ventral to those arising from the nucleus cuneatus. One way to remember this is to think about a person standing on top of a pyramid (for the pyramidal tract). ****However, our little man doesn't have a head because axons from the spinal trigeminal nucleus do not join this bundle. This is important to remember. Although they are anatomically related to each other, the nucleus gracilis and nucleus cuneatus carry general sensory information. The neurons in the spinal trigeminal nucleus relay information on pain and temperature from the face and will be discussed when we follow a different pathway from the spinal cord. The head will be added shortly so our figure won't be headless for long. LL
MEDIAL
LEMNISCUS
(ML)
PYRAMIDS
IOC X

9. 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 ML LL UL
IOC PY
This is a section through the rostral or open medulla. A key landmark here is the presence of the large inferior olivary nucleus as well as several cranial nerve nuclei on the dorsal surface of the brainstem including the hypoglossal nucleus, the dorsal motor nucleus of the vagus, the nucleus solitarius and the caudal end of the vestibular nuclei. This should be a review of these nuclei as they were discussed in a previous presentation. Nothing really changes with respect to the medial lemniscus.*** It remains on the midline, just above the pyramids with the same somatotopic organization. The one minor change is the addition of a new fiber tract that is located just dorsal to the medial lemniscus. This is called the medial longitudinal fasciculus or MLF. This tract arises from neurons located in the vestibular nuclei. We will discuss this tract in more detail in a presentation on the vestibular system. The axons of the MLF and ML are difficult to separate. Just know that once the vestibular nuclei appear, the most dorsal part of the large midline fiber tract are part of the medial longitudinal fasciculus. The portion of the fiber tract located more ventrally is the medial lemniscus. You will not be asked to define the border between these two tracts. ML

10. 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 UL LL BP
MAIN
SENSORY V
MOTOR V V
MES. V AB G
VII FA ML UL LL
This section is at the level of the mid pons. For review, several previously described structures are visible in this section including the facial motor nucleus and facial nerve, the motor, main sensory, and mesencephalic nuclei of the trigrminal nerve, and the abducens nucleus. The basilar pons has expanded ventrally and basically pushes the medial lemniscus dorsally into the tegmentum ***. As the basilar pons pushes the medial lemniscus into the tegmentum, it also “knocks it over”. Instead of a vertical orientation, the tract assumes a horizontal orientation at this level. The somatotopic organization follows this shift***. Here is our figure of the man standing on the pyramid. As the basilar pons expands, think of the body being knocked over medially. The lower limb gets kicked out sideways and laterally and knocks our figure over in such a way that it grasps the midline for stability. Thus, the upper limb remains near the midline, holding on for dear life and the lower limb is moved laterally.
MIDLINE

11. 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.
From touch receptors
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.
At last, we can add the head to our person. We are now at the level of the main sensory nucleus of V. Axons arising from neurons in the trigeminal ganglion carrying general sensory information from the face enter *** the brainstem at this level and synapse *** on neurons in the main sensory nucleus of V. The axons of these neurons cross the midline and join the medial lemniscus on it's medial side. Now all of the sensory information from the body and the head are joined in one common pathway that continues to ascend through the brainstem. As we'll see shortly, the axons from the face will terminate in a different portion of the thalamus; they will end in the ventral posterior medial nucleus whereas general sensory information from the body terminates in the ventral posterior lateral nucleus of the thalamus. For unknown reasons***, a few axons relaying information from the inside of the mouth do not cross the midline; they remain on the ipsilateral side. Just a reminder *** axons carrying pain information from the face do not join the medial lemniscus. We'll see where these axons are very soon.
NOTE, Axons from the spinal trigeminal nucleus (pain and temparature) do not join the medial leminiscus.

12. 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
This is a cross section at the junction of the pons and midbrain. The large cerebral peduncle is evident and we now see the narrow cerebral aqueduct rather than the expanded 4th ventricle. Another landmark, is the presence of the tectum indicating we are entering the midbrain. You can also see that the deep interpeduncular fossa is beginning to develop on the ventral surface of the brainstem. ***In the rostral pons, the medial lemniscus begins to shift a bit laterally as the ventral cleft deepens. The relationship of the lower limb, upper limb and face are retained. CP
IPF BP

13. 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 RN
VTA
TEG OC UL LL FA ML CP SN
In the midbrain proper, the interpeduncular fossa is well developed. We can review several structures we identified in a previous presentation including the cerebral aqueduct, periaqueductal grey, superior colliculus, oculomotor nucleus, red nucleus, substantia nigra and cerebral peduncle. The medial lemniscus *** shifts even more laterally. As before *** the relative position of different regions of the body is retained.
IPF

14. 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. DM LP
VPL
VPM CM SN RN CP IC
This is a transverse section through the thalamus. The area enclosed by the red box *** is shown at higher magnification in this image. Midline is to the right. Due to the plane of section, portions of the midbrain are also evident on this section including the substantia nigra, cerebral peduncle and red nucleus. Axons in the medial lemniscus *** enter the thalamus terminating as described on the previous slide. ***Axons arising from neurons in the thalamus will project to portions of the cerebral cortex that are involved in processing sensory information. This is in the parietal lobe. Axons arising from VPL *** carry sensory information to regions of the cortex that process information from the body. ***Axons arising from neurons in VPM relay sensory information from the face. CC
Axons arising from 3rd order neurons course through the internal capsule as they project to the parietal lobe.

15. THALAMIC NUCLEI
3rd order neurons are located in the ventral posterior lateral (UL and LL) and ventral posterior medial (face) nuclei of the thalamus. A DM VA
VPL
VPM VL
PUL IL DL MG LG A P M L
This is a diagram of the thalamus. Rostral is to the right and caudal to the left. Toward the caudal end of the thalamus, we see 2 subdivisions called the ventral posterior lateral (VPL) and ventral posterior medial (VPM) nuclei. This is location of the third order neuron in the relay of sensory information to the cerebral cortex. Axons that arose in nucleus gracilis in the medulla carrying general sensory information from the lower part of the body as well as axons that arose in the nucleus cuneatus carrying general sensory information from the upper part of the body synapses on neurons in VPL. Axons that arose from neurons in the main sensory nucleus of the trigeminal nerve synapse on thalamic neurons in VPM.

16. SENSORY CORTEX
CENTRAL
SULCUS
PARIETAL LOBE
(POSTCENTRAL GYRUS = PRIMARY SENSORY CORTEX)
FRONTAL
LOBE
Just a reminder, the area of the cortex that processes sensory information is located just posterior to the central sulcus. The primary sensory cortex is the postcentral gyrus colored in blue. Areas posterior to the postcentral gyrus are called Association Areas and they are involved in higher order processing and integration of sensory information. More about this later.

17. 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.
Small diameter, lightly myelinated axons.
THALAMUS
(VPL)
3RD ORDER
NEURON
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)
MIDBRAIN
SPINOTHALAMIC TRACT
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
PONS
That concludes our discussion on the ascending pathways that carry general sensory information to the cortex. In the last part of this module, we'll focus on pathways that carry pain and temperature information. As for the dorsal column-medial lemniscus pathway, this slide summarizes the entire system. You should know the information on this slide. We'll look at it in more detail on subsequent slides. The anterolateral system is made up of multiple tracts that convey different aspects of pain such as the location of the painful input and the intensity (is it a pinprick or a deep knife cut). Other parts of the system are involved in other aspects of pain. The primary tract *** that conveys information on location and nature of a painful stimulus is called the spinothalamic tract. Unlike the pathway that conveys general sensory information, the axons in this tract are small diameter and lightly myelinated meaning a higher threshold is needed to activate these axons. *** Other aspects of pain are mediated by axons that are spatially contiguous. Rather than reaching the cortex for conscious awareness, these axons leave at different levels of the brainstem. Some synapse in the reticular formation and are involved in getting your attention to painful input. Others terminate in the limbic system for an emotional overlay, or a memory of a previous painful event. Some terminate in the hypothalamus to elicit an autonomic response to pain such as changing blood pressure. Finally, some terminate in the periaqueductal grey. These are very important as they activate a pathway that modulates pain. More on this later in the block. Let's define the spinothalamic ***pathway. ***The first order neuron is in the dorsal root ganglion. The central process enters the spinal cord via the dorsal root and synapses on neurons in the ipsilateral dorsal horn. This is different from the pathway we just described. ***The axons of the second order neurons located in the dorsal horn cross the midline in the spinal cord and form a tract in the ventral, lateral funiculus of the spinal cord. These axons ascend through the medulla, pons and midbrain all the way to the thalamus where the third order neuron is located. *** The ascending axons synapse on third order neuron, located in the ventral posterior lateral nucleus of the thalamus. The axons of these neurons project to the parietal lobe of the cerebral cortex for conscious awareness of the painful stimulus and where it is located.
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
MEDULLA
SYNAPSE
UPPER LIMB
DRG
3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex and insular cortex. C8
SYNAPSE
LOWER LIMB
1ST ORDER
NEURON
2ND ORDER
NEURON L4

18. SPINOTHALAMIC TRACT - SPINAL CORD
From Pain And
Temperature
Receptors
1ST ORDER
2ND ORDER T7
TRUNK L2 S4 LL LL
Let’s first look at the input at the level of the spinal cord. At sacral levels *** painful input from the lower limb enters via the central process*** of the DRG neuron an synapses on the second order neuron in the dorsal horn of the spinal cord. The axon of the second order neuron crosses the midline and begins to form an ascending tract in the ventral part of the contralateral lateral funiculus. As one ascends through the spinal cord ***the same pattern is followed as information from other parts of the lower limb are added on. Around T7 *** information from the trunk is added on. Finally, in cervical levels *** input from the upper limb is added. *** Note as progressively higher levels of the body come in, the input from the lower limb is pushed a bit more dorsally and laterally. C3 UL
AXONS ENTERING THE TRACT FROM THE LOWER LIMB ARE “PUSHED” LATERAL AS AXONS FROM TRUNK AND UPPER LIMB ARE ADDED.

19. 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.
TEG ML
IOC PY
XII
SOL X NC NG NC
SpV
IOC ML PY
Let’s follow the tract into the medulla. *** This is a section at the level of the spinomedullary junction. We see several nuclei we already discussed as well as the medial lemniscus. The spinothalamic tract retains the same**** relative position it had in the spinal cord on the lateral aspect of the medulla. ***This section is a bit more rostral. You should review the location of nuclei and tracts discussed previously. The spinothalamic tract *** retains it’s lateral position. *** One thing to note is that this tract is not as distinct as the medial lemniscus since the axons are smaller and only lightly myelinated. Thus, they do not stand out in a section stained to detect myelin.
STT LL UL LL UL
STT
SPINOMEDULLARY JUNCTION
CAUDAL MEDULLA

20. SPINAL TRIGEMINAL NUCLEUS – PAIN AND TEMPERATURE INPUT FROM FACE FROM V, VII, IX AND X
Ganglion
Cells of origin for afferents carrying pain and temperature information from the face are in the trigeminal ganglion. V
Pain & Temperature
Afferents from Face
Spinal V
Tract
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.
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
Pain & Temperature
Afferents from Body
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.
Remember, the spinal nucleus of V which conveys pain sensations from the face is located in the medulla. ***The cells of origin for this information are located in the trigeminal ganglion. ***Axons enter at the level of the pons and descend in the spinal trigeminal tract to the medulla where they synapse on neurons in the spinal nucleus of V. This would be analagous to the axons arising from DRG neurons that synapse on second order neurons in thespinal cord. Axons from the second order neurons in the spinal nucleus of V *** cross the midline and join the spinothalamic tract on the contralateral side. As for the ***dorsal column medial lemniscus pathway, the axons from the body and the face run together to the thalamus. Those from the body will terminate in the VPL and those from the face in VPM of the thalamus.

21. 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
TEG Tr BP UL LL FA
STT
MAIN
SENSORY V BP
MOTOR V V
MES. V ML ML
In the pons, the spinothalamic tract, like the medial lemniscus is shifted dorsally as the basilar pons expands. The two tracts are essentially adjacent to each other at this level. In the rostral pons, the two tract are contiguous with the medial lemniscus located medially the spinothalamic tract dorsal and lateral. They almost seem to form an L shape (with the L lying on its back).
STT
STT ML
Rostral Pons
Mid Pons

22. SPINOTHALAMIC TRACT (STT) - MIDBRAIN
In The Midbrain, The STT Shifts a Bit More Dorsally as Cerebral Peduncle Replaces the Basilar Pons
INSULA SN SC CA
IPF
PAG RN
VTA
TECTUM
TEGMENTUM ML CP
STT LL UL FA
The relationship between the medial lemniscus and spinothalamic tract is maintained in the midbrain. Both tracts shift a bit more dorsally and laterally as the cerebral peduncles form.

23. SPINOTHALAMIC TRACT (STT) - THALAMUS AND CORTEX
INSULA LL UL
FACE
Ascending fibers terminate on 3rd order neuron in VPL (upper and lower limb) or VPM (face) thalamus.
INSULA
At the level of the diencephalon, the axons synapse on third order ** neurons in either the VPL thalamus for the body or the VPM for information arising in the face. The axons of these third order neurons project to the parietal lobe and to a part of the cortex called the insula *** which is buried deep under the parietal and temporal lobes.
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.

24. SUMMARY OF ASCENDING TRACTS
TRACT ST ORDER ND ORDER RD ORDER
NEURON NEURON NEURON MODALITY
DORSAL COLUMN TOUCH, PRESSURE, VIBRATION
MEDIAL 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
to join Spinothalamic Tract)
This table summarizes the information you need to know about each tract.

25. Ascending Tracts

26. Thank you for completing this module
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