10/7 - ASCENDING AND DESCENDING PATHWAYS Flashcards Preview

Neuro 3050: Structure & Function of the Nervous System > 10/7 - ASCENDING AND DESCENDING PATHWAYS > Flashcards

Flashcards in 10/7 - ASCENDING AND DESCENDING PATHWAYS Deck (66):
1

CONDUIT FUNCTION OF BRAINSTEM

Information is transmitted bidirectonally between spinal cord and cerebral cortex, often with relays in the brainstem.

For the nervous system to function properly, communication must be established between different parts of the CNS. 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.

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General Rule for Ascending Pathways

These carry sensory information to the CNS.
Thus, the first order neurons are located within a peripheral ganglion – Dorsal Root Ganglion (DRG) for body, cranial nerve ganglion for head.

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Primary Neuron

first order neuron

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REPRESENTATIVE EXAMPLES OF ASCENDING TRACTS

Dorsal (Posterior) Column-medial Lemniscus

Spinothalamic


There are numerous tracts that carry information from the periphery to the CNS. 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: the dorsal column-medial lemniscus pathway which transmits general sensory information and the spinothalamic tract which conveys information on pain and temperature.
 

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spinothalamic tract

conveys information on pain and temperature

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dorsal column-medial lemniscus

transmits general sensory information

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DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS

CONSCIOUS PERCEPTION OF SENSORY INFORMATION

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)

Large heavily myelinated axons

Input From Lower Limb And Trunk (T6-S5) Forms a Bundle of Axons Called the Fasciculus Gracilis

Input From Upper Limb And Trunk (C1-T5) Forms a Bundle of Axons Called the Fasciculus Cuneatus

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Fasciculus Gracilis

Input From Lower Limb And Trunk (T6-S5) Forms a Bundle of Axons Called the Fasciculus Gracilis

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Fasciculus Cuneatus

Input From Upper Limb And Trunk (C1-T5) Forms a Bundle of Axons Called the Fasciculus Cuneatus

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Stereognosis

Form Recognition Of 3-dimensional Objects

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Kinesthesia

Limb Movement In Space

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Proprioception

Conscious Awareness Of Body Position

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DORSAL (POSTERIOR) COLUMN – MEDIAL LEMNISCUS

NEURON

1st ORDER NEURON: DORSAL ROOT GANGLIA CELLS.
Primary afferent axons enter via dorsal root to enter ipsilateral dorsal 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).

3rd ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex

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FIRST ORDER NEURON IS IN A _____

FIRST ORDER NEURON IS IN A DRG

Dorsal root ganglion cells give rise to a 2 processes. One process projects to the periphery and other projects into the spinal cord.

The peripheral processes have different sizes and amounts 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.

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PERIPHERAL PROCESS of a DRG:

Carry information from cutaneous receptors (touch, vibration)as well as muscle spindles/joint receptors (proprioception).

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CENTRAL PROCESS of a DRG:

Enters through dorsal root and courses directly to posterior column where they ascend to brainstem

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Everything you need to know about the dorsal column-medial lemniscus pathway

How general sensory information is transmitted from the periphery to the cerebral cortex.

The modalities carried by this pathway include low threshold cutaneous receptors which detect touch, pressure and vibration. Information on the fine form and texture discrmination are also 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).

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 DRG neurons located between C1 and T5. The first order neuron, aka 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. 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 the Medial lemniscus tract. 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.

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DORSAL COLUMN – MEDIAL LEMNISCUS
SPINAL CORD

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.

As Axons Enter From The Upper Limb, Those Present In The Dorsal Funiculus From The Lower Limb Are Shifted Medially.

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INPUT FROM T6-S5 (LOWER LIMB)– FORMS FASCICULUS ____ (LATIN for ____)

INPUT FROM T6-S5 (LOWER LIMB)– FORMS FASCICULUS GRACILIS (LATIN for SLENDER)

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INPUT FROM ___ (_____ limb)– FORMS FASCICULUS CUNEATUS (LATIN for ____)

INPUT FROM C1-T6 (UPPER LIMB)– FORMS FASCICULUS CUNEATUS (LATIN for WEDGE)

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CAUDAL MEDULLA- DORSAL

Dorsal surface of the caudal medulla. We will focus on the dorsal or tegmental region of the brainstem. We still see the Nucleus gracilis medially, the nucleus cuneatus lateral to it, and the spinal nucleus and tract of V, most lateral.

The major change is that the nuclei are getting larger and the fiber tracts are getting smaller as they terminate in their respective nuclei. The fasciculus gracilis is almost completely gone, whereas there are still some axons remaining in the fasciculus cuneatus.

The Spinal Tract of V is actually getting larger as we go rostrally due to the fact that it enters the brainstem at the level of the pons and descends to the medulla. Thus as we move, rostrally, we will encounter more and more of these axons.

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DORSAL COLUMN – MEDIAL LEMNISCUS

2nd ORDER NEURONS

1st order fibers arising from DRG ascend to TERMINATE in the ipsilateral N. Cuneatus and N. Gracilis in caudal medulla.

2ND ORDER NEURONS in N. Gracilis and Cuneatus give rise to axons that course ventrally in an arc = INTERNAL ARCUATE FIBERS. CROSS MIDLINE

2nd order axons collect on either side of midline to form MEDIAL LEMNISCUS (L. Ribbon or band) in the medullary tegmentum. This tract ascends through medulla, pons, midbrain to terminate in thalamus

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DORSAL COLUMN – MEDIAL LEMNISCUS

2nd ORDER NEURONS
(in other words)

The junction of the medulla and spinal cord. 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 2nd 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!! 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 follow a different pathway from the spinal cord.

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DORSAL COLUMN – MEDIAL LEMNISCUS

Rostral Medulla

Medial Lemniscus REMAINS AS A VERTICAL COLUMN IN THE TEGMENTUM IMMEDIATELY ADJACENT TO THE MIDLINE AS IT ASCENDS THROUGH THE MEDULLA

Cross section through the rostral/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.

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. This tract is involved in 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.

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DORSAL COLUMN –
MEDIAL LEMNISCUS

MID PONS

As ventral brainstem expands to form basilar pons, the Medial Lemniscus is “pushed” dorsally and assumes a more horizontal orientation.

UL remains close to midline while lower
limb “swings” laterally.

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DORSAL COLUMN –
MEDIAL LEMNISCUS

MID PONS
summary

This section is at the level of the mid pons. 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.

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TRIGEMINAL AXONS

Axons from the main sensory carrying touch information from the face, cross the midline and join the medial leminiscus in the pons. These axons are located medially and 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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TRIGEMINAL AXONS
(in other words)

At last, we can add the head to our person. We are 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. 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. Remember: axons carrying pain information from the face do not join the medial lemniscus.

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DORSAL COLUMN – MEDIAL LEMNISCUS

MIDBRAIN

In the midbrain, the ML moves slightly more laterally and dorsally in the tegmentum as the ventral motor area of the midbrain replaces the basilar pons.

In the midbrain proper, the interpeduncular fossa is well developed. We can review structures we identified previously, 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.

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THALAMIC NUCLEI :
3rd ORDER NEURON

3RD ORDER NEURONS are located in the ventral posterior lateral (UL and LL) and ventral posterior medial (face) nuclei of the thalamus.

Diagram of the thalamus. 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 3rd 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.

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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.

Axons arising from 3rd order neurons course through the internal capsule as they project to the parietal lobe.

Transverse cross section through the thalamus. 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 terminate in the thalamus. 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. Just for reference, nuclei labeled DM (dorsomedial), CM (centromedian) and LP (lateral posterior) are other thalamic nuclei.

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SENSORY CORTEX

Remeber: 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. Areas posterior to the postcentral gyrus are called Association Areas and they are involved in higher order processing and integration of sensory information.

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DORSAL COLUMN MEDIAL LEMNISCUS:

FASICULUS CUNEATUS Summary

1ST ORDER NEURON:
DRG NEURON (C1-T6)

2ND ORDER NEURON:
IPSI. N. CUNEATUS

3RD ORDER NEURON: CONTRA. VPL of THALAMUS

MODALITY:
TOUCH, PRESSURE, VIBRATION, FORM RECOGNITION, TEXTURE, PROPRIOCEPTION

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DORSAL COLUMN MEDIAL LEMNISCUS:

FASCICULUS GRACILIS Summary

1ST ORDER NEURON:
DRG NEURON (T6–S5)

2ND ORDER NEURON:
IPSI. N. GRACILIS
(Tracts cross in caudal Medulla)

3RD ORDER NEURON: CONTRA. VPL of THALAMUS

MODALITY:
TOUCH, PRESSURE, VIBRATION, FORM RECOGNITION, TEXTURE, PROPRIOCEPTION

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DORSAL COLUMN MEDIAL LEMNISCUS:

TRIGEMINAL Summary

1ST ORDER NEURON: TRIGEMINAL GANGLION

2ND ORDER NEURON:
MAIN SENSORY NUCLEUS V

3RD ORDER NEURON: CONTRA. VPM

MODALITY:
TOUCH, PRESSURE, VIBRATION, FORM RECOGNITION, TEXTURE, PROPRIOCEPTION

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ANTEROLATERAL SYSTEM

Think PAIN

Anterolateral System:
Multiple Tracts That Convey Different Aspects of Pain Including: Location & Intensity of Painful Stimulus, Emotional Response to Pain, Autonomic Response to Pain, and 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.

Other Aspects Of Pain Are Mediated by other pathways that occupy same space as spinothalamic tract. These End in:

1. Reticular Formation throughout the brainstem (Attention)
2. Limbic System (Emotion, Memory)
3. Hypothalamus (Autonomic Response)
4. Periaqueductal Grey (Intrinsic Pain Control Mechanisms)

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anterolateral system &
spinothalamic tract

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 the spinothalamic tract. Unlike the pathway that conveys general sensory information, the axons in this tract are in 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.

The spinothalamic pathway. The 1st 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 previously described. The axons of the 2nd 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 to the thalamus where the 3rd order neuron is located. The ascending axons synapse on 3rd order neuron, located in the ventral posterior lateral (VPL) 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.

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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

2ND ORDER NEURONS IN DORSAL HORN OF SPINAL CORD AT ALL LEVELS.
Axon CROSSES MIDLINE and forms spinothalamic (anterolateral) tract in the anterior half white matter of the spinal cord.

3RD ORDER NEURON IN THALAMUS (VENTRAL POSTERIOR LATERAL NUCLEUS).
Axons arising from these neurons project to primary sensory (parietal) cortex and insular cortex.

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SPINOTHALAMIC TRACT - SPINAL CORD

AXONS ENTERING THE TRACT FROM THE LOWER LIMB ARE “PUSHED” LATERAL AS AXONS FROM TRUNK AND UPPER LIMB ARE ADDED.

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 and synapses on the 2nd order neuron in the dorsal horn of the spinal cord. The axon of the 2nd 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.

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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 Medial Lemniscus in myelin stained sections due to smaller diameter, lightly myelinated axons.

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SPINOTHALAMIC TRACT (STT) - MEDULLA

Review

Cross Section at the level of the spinomedullary junction.

We see several nuclei including 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 rostral. The spinothalamic tract retains it’s lateral position.

Note: the spinothalamic 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.

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SPINAL TRIGEMINAL NUCLEUS:

PAIN & TEMPERATURE INPUT FROM FACE FROM V, VII, IX AND X

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 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.

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SPINAL TRIGEMINAL NUCLEUS:

PAIN & TEMPERATURE INPUT FROM FACE FROM V, VII, IX AND X

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.

In addition, the few sensory fibers found in cranial nerves VII, IX, and X also synapse in the spinal trigeminal nucleus. These 2nd order neurons 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.

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SPINOTHALAMIC TRACT (STT) Pons

In the pons, the STT, like the ML, is shifted dorsally by the expanding basilar pons. The STT lies just lateral to the ML through the rest of its course to the thalamus.

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).

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SPINOTHALAMIC TRACT (STT)
Midbrain

In The Midbrain, The STT Shifts a Bit More Dorsally as Cerebral Peduncle Replaces Basilar Pons

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.

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SPINOTHALAMIC TRACT (STT)
THALAMUS AND CORTEX

Ascending fibers terminate ON 3RD ORDER NEURON in VPL (upper & lower limb) or VPM (face) thalamus.

3rd order neuron projects 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.

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SPINOTHALAMIC TRACT (STT)
THALAMUS AND CORTEX

Summary

At the level of the diencephalon, the axons synapse on 3rd order neurons in either the VPL thalamus for the body or the VPM for information arising in the face.

The axons of these 3rd 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.

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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.

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ANTEROLATERAL SYSTEM:

SPINOTHALAMIC TRACT

1ST ORDER NEURON:
DRG NEURON (C1-S5)

2ND ORDER NEURON:
IPSI. DORSAL HORN SPINAL CORD
(Tract crosses in spinal cord)

3RD ORDER NEURON: CONTRA. VPL of THALAMUS

MODALITY:
PAIN PERCEPTION, TEMPERATURE

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ANTEROLATERAL SYSTEM:

TRIGEMINAL

1ST ORDER NEURON: TRIGEMINAL GANGLION

2ND ORDER NEURON:
SPINAL NUCLEUS V

3RD ORDER NEURON: CONTRA. VPM of THALAMUS

MODALITY:
PAIN PERCEPTION, TEMPERATURE

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ALPHA OR LOWER MOTOR NEURONS

Motor neurons in spinal cord and cranial nerve nuclei that project to skeletal muscles.

Final common pathway to innervate muscles.
When lost ---> flaccid paralysis.

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UPPER MOTOR NEURONS

Neurons in cortex and other areas of brainstem that project to motor neurons in spinal cord or cranial nerve nuclei with somatic motor component.

Damage results in spastic paralysis as reflexes go “unchecked”.

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MOTOR CONTROL COMPONENTS

Motor neurons in spinal cord and cranial nerve nuclei that project to skeletal muscles.

Final common pathway to innervate muscles.
When lost ---> flaccid paralysis.

Neurons in cortex and other areas of brainstem that project to motor neurons in spinal cord or cranial nerve nuclei with somatic motor component.

Damage results in spastic paralysis as reflexes go “unchecked”.

Basal ganglia, cerebellum, association areas of motor cortex play vital role in the choice, design and monitoring of movement but have no direct effect on lower motor neurons.

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Motor neurons in the spinal cord are capable of activating muscles through ______ .

Motor neurons in the spinal cord are capable of activating muscles through various reflex loops (e.g., extensor reflex, flexor withdrawal reflex).

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The function of ___ is to regulate the activity of reflex loops and ultimately the firing rate of motor neurons in the ____.

The function of descending tracts is to regulate the activity of these reflex loops and ultimately the firing rate of motor neurons in the spinal cord.

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DESCENDING CONTROL OF MOVEMENT

Cells of origin are located in primary motor cortex

The portion of the frontal lobe between the central and pre-central sulcus = PRIMARY MOTOR CORTEX

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CORTICOSPINAL (PYRAMIDAL) TRACT

Function – classically defined as mediating voluntary movement, especially of the digits

Cells of origin: pyramidal cells in motor regions of frontal lobe.

Axons course through internal capsule (large fiber tract located lateral to thalamus).

Axons descend ipsilaterally through cerebral peduncle in the midbrain, basilar pons and rostral pyramidal tract in the medulla.

On reaching caudal medulla 85% cross to the contralateral side in the pyramidal decussation to form lateral corticospinal tract.

15% remain ipsilateral to form anterior corticospinal tract.

Terminate either directly on motor neurons or on interneurons that contact motor neurons.

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CORTICOBULBAR TRACT

Neurons in the cerebral cortex also project to motor nuclei related to cranial nerves. Arise from face region of precentral gyrus.

Motor Nucleus of V
Facial Motor Nucleus
Hypoglossal Nucleus (XII)

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CORTICOSPINAL/CORTICOBULBAR TRACT

COURSE

Pyramids >>> Pons >>> Cerebral Peduncle (Midbrain) >>> Internal Capsule


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CORTICOSPINAL TRACT

In the medulla, the fibers regroup to form the pyramidal tract (pyramid). However, there is a significant decrease in size of this fiber tract compared to that of the cerebral peduncle.

Comparison of size of cerebral peduncle (before pons) and pyramid (after pons) to show how many axons leave the tract in the pons and rostral medulla.

In the pons, the CST is broken up into fascicles that course between collections of neurons forming pontine nuclei.

Many cortical axons, not contributing to the corticospinal tract, project to brainstem nuclei in the midbrain, pons and medulla, especially cranial nerve nuclei with a motor component.

At the junction of the spinal cord and medulla, 85% of the axons in the corticospinal tract cross to the contralateral side. Note: they also move dorsally and laterally. This is called the lateral corticospinal tract.
15% of the fibers remain on the ipsilatral side in the same ventral position. This is called the anterior corticospinal tract.

PYRAMIDAL DECUSSATION

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PYRAMIDAL DECUSSATION

At the most caudal pole of the pyramids, the corticospinal axons cross over the midline and continue their descent on the contralateral (to the cell of origin) side. This crossover point is called the PYRAMIDAL DECUSSATION.

The crossing fibers enter the lateral funiculus of the spinal cord where they are called the LATERAL CORTICOSPINAL TRACT. LCST axons exit the tract to terminate upon neurons in the spinal cord gray matter along its entire length.

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LATERAL CST AXONS TERMINATE ON:

Motor Neurons Located Laterally (Upper Limb & Digits)

Interneurons That Project To Motor Neurons

Neurons At Base Dorsal Horn

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ANTERIOR CST AXONS TERMINATE ON:

Motor Neurons Located Medially (Axial Muscles).

Some Cross To Contralateral Side

End In Cervical And Thoracic Segments

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RUBROSPINAL TRACT

Function: alternate route for mediation/modulation of movements. primary effect is on flexor muscles, but also can have an effect on extensors. May excite flexor and inhibit extensor.
Also an important source of afferents to the inferior olivary nucleus.

Cells of origin: red nucleus in the midbrain.
Receives input from cerebral cortex and cerebellum and modulates activity of motor neurons in the spinal cord.

Axons cross midline to form the rubrospinal tract (RST) in midbrain.
Most terminate in cervical spinal cord but others will traverse the entire length of the spinal cord.

In spinal cord, axons are located just ventral to lateral corticospinal tract.

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Rubrospinal Tract
again....

Originate in contralateral red nucleus. Axons cross at level of origin in midbrain.

Rubrospinal tract crosses in midbrain.

In the spinal cord, individual axons terminate at specific levels (minimal collateralization).

In humans, few extend below cervical cord.

Terminate medially and more dorsally where they are excitatory to proximal upper limb flexors.

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AFFERENTS TO RED NUCLEUS

Primarily from cerebellum and a few from cerebral cortex