Exam 2 week 9 ppt 5 Descending Tracts Flashcards
Name the Descending Tracts (6)
- •Pyramidal Tracts
- –Lateral & Anterior corticospinal tracts
- •Extrapyramidal Tracts
- –Vestibulospinal
- –Reticulospinal
- –Tectospinal
- –Rubrospinal
What are the two main categories that descending tracts are listed under?
Pyramidial Tracts
Extrapyramidal Tracts
(however nonsensical this may seem)
Corticospinal Tract: pathway of UMN fibers
Originate where?
Descend how and where?
Terminates where?
- –Originating in primary motor, motor association & primary somatosensory cerebral cortex & anterior cingulate gyrus
- –Descending longitudinally through the:
- Internal capsule
- Cerebral peduncle
- Pyramids of the pons & medulla
- –Terminating on LMNs in the spinal cord or on interneurons which relay to LMN
Corticospinal Tract: pathway of UMN fibers
where do they originate?
–Originatie in primary motor, motor association & primary somatosensory cerebral cortex & anterior cingulate gyrus
Corticospinal Tract: pathway of UMN fibers
Descend how and where?
Descends longitudinally through the:
- Internal capsule
- Cerebral peduncle
- Pyramids of the pons & medulla
Corticospinal Tract: pathway of UMN fibers
Terminate where?
–Terminate on LMNs in the spinal cord or on interneurons which relay to LMN
LCST
Lateal corticospinal tract
what are the names of the two corticospinal tracts?
anterior corticospinal tract
lateral corticospinal tract
Details on the pathway of the Lateral corticospinal tract
Decussation?
position in spinal cord?
levels it ends in spinal cord?
- –85–90% of the corticospinal tract fibers
- –Cross midline in the pyramidal decussation
- –Descends in the contralateral lateral funiculus
- –55% of axons end in cervical cord, 25% in lumbosacral cord & 20% in thoracic cord
Lateral corticospinal tract (LCST) comprises 85–90% of the corticospinal tract fibers. They Cross midline in the pyramidal decussation of the caudal medulla. They then Descends in the contralateral lateral funiculus. Of these fibers 55% of axons end in cervical cord, 25% in lumbosacral cord & 20% in thoracic cord
Details on the pathway of the Lateral corticospinal tract
Decussation?
–85–90% of the corticospinal tract fibers
–Cross midline in the pyramidal decussation
Details on the pathway of the Lateral corticospinal tract
position in spinal cord?
–Descends in the contralateral lateral funiculus
Details on the pathway of the Lateral corticospinal tract
levels it ends in spinal cord?
–55% of axons end in cervical cord, 25% in lumbosacral cord & 20% in thoracic cord
ACST
Anterior Corticospinal Tract
Details on the pathway of the Anterior corticospinal tract
Decussation?
position in spinal cord?
levels it ends in spinal cord?
–Smaller contigent of uncrossed fibers
–Descends in the ventral funiculus
–Majority ends in cervical spinal cord
Details on the pathway of the Anterior corticospinal tract
Decussation?
–Smaller contigent of uncrossed fibers
Details on the pathway of the Anterior corticospinal tract
position in spinal cord?
–Descends in the ventral funiculus
Details on the pathway of the Anterior corticospinal tract
levels it ends in spinal cord?
–Majority ends in cervical spinal cord
Corticospinal Tracts: two things both have in common about where they end and function
- •Both pathways tend to end on spinal interneurons
- •Alpha-gamma coactivation to maintain force in shortening contractions (concentric loading mode)
Both pathways tend to end on spinal interneurons or directly on LMN
These descending axons from both parts end on both alpha and ganma MN to produce Alpha-gamma coactivation to maintain force in shortening contactions (concentric loading mode)
Explain the Somatotopical organization of the Corticospinal Tracts
All of it
- –Primary motor cortex
- UE & face ventro-laterally
- LE & trunk dorso-medially
- –Rotate in corona radiata
- –Internal capusule – posterior limb
- Face & arms rostral
- Trunk & leg more caudal
- –Cerebral Peduncle
- UE & face ventro-medial
- LE & trunk dorso-lateral
- –Brainstem – probably retains this relationship
- –Spinal cord – lateral corticospinal tract
- UE medial
- LE & trunk lateral
The axons in the corticospinal tracts are Somatotopically organized.
Neurons In the Primary motor cortex are arranged so that neurons which innervate muscle of the the UE & face ventro-laterally and those that innervate muscles of the LE & trunk dorso-medially positioned in the motor homunculus pattern. Their axons then Rotate as they pass through the corona radiata. In the Internal capsule these axons descend through the posterior limb with the axons which eventually will innervate the Face & arms are more rostral (closer to the genu) and those that innervate eventually the Trunk & leg are more caudal. In the cerebral peduncle those axon which carry information destined for the UE & face descend more ventro-medial while those with information destined fo the LE & trunk descend more dorso-lateral. This relationship is maintain as the pyramids pass through the brainstem.In the lateral corticospinal tract of the spinal cord those axons ending in the cervical spinal cord on LMN which innervate the upper extremity descend medially and those descending further are progressively more lateral as illustrated here. The anterior corticospinal tract mostly ends in the cervical spinal cord.
Explain the Somatotopical organization of the Corticospinal Tracts
Primary Motor Cortex
–Primary motor cortex
- UE & face ventro-laterally
- LE & trunk dorso-medially
The axons in the corticospinal tracts are Somatotopically organized.
Neurons In the Primary motor cortex are arranged so that neurons which innervate muscle of the the UE & face ventro-laterally and those that innervate muscles of the LE & trunk dorso-medially positioned in the motor homunculus pattern. Their axons then Rotate as they pass through the corona radiata
Explain the Somatotopical organization of the Corticospinal Tracts
where does the relationship rotate?
Rotate in corona radiata
Neurons In the Primary motor cortex are arranged so that neurons which innervate muscle of the the UE & face ventro-laterally and those that innervate muscles of the LE & trunk dorso-medially positioned in the motor homunculus pattern. Their axons then Rotate as they pass through the corona radiata
Explain the Somatotopical organization of the Corticospinal Tracts
Internal Capsule
–Internal capusule – posterior limb
- Face & arms rostral
- Trunk & leg more caudal
In the Internal capusule these axons descend through the posterior limb with the axons which eventually will innervate the Face & arms are more rostral (closer to the genu) and those that innervate eventually the Trunk & leg are more caudal
Explain the Somatotopical organization of the Corticospinal Tracts
Cerebral Peduncle
–Cerebral Peduncle
- UE & face ventro-medial
- LE & trunk dorso-lateral
In the cerebral peduncle those axon which carry information destined for the UE & face descend more ventro-medial while those with information destined fo the LE & trunk descend more dorso-lateral. This relationship is maintain as the pyramids pass through the brainstem
Explain the Somatotopical organization of the Corticospinal Tracts
brainstem
Cerebral Peduncle
- UE & face ventromedial
- LE & trunk dorsolateral
Brainstem – probably retains this relationship
In the cerebral peduncle those axon which carry information destined for the UE & face descend more ventro-medial while those with information destined fo the LE & trunk descend more dorso-lateral. This relationship is maintain as the pyramids pass through the brainstem
Explain the Somatotopical organization of the Corticospinal Tracts
Spinal cord (LCST)
–Spinal cord – lateral corticospinal tract
- UE medial
- LE & trunk lateral
In the lateral corticospinal tract of the spinal cord those axons ending in the cervical spinal cord on LMN which innervate the upper extremity descend medially and those descending further are progressively more lateral as illustrated here. The anterior corticospinal tract mostly ends in the cervical spinal cord
Name the two Vestibulospinal Tracts
Lateral Vestibulospinal Tract
Medial Vestibulospinal Tract
LVST
Lateral Vestibulospinal Tract
Lateral Vestibulospinal Tract: describe the pathway
begining?
descends in what part of spinal cord?
ends where?
- –Arises from lateral vestibular nucleus
- –Descends ipsilaterally thru lateral brainstem & lateral aspect of ventral funciculus
- –Ends on neurons in medial motor nucleus that innervate paraspinal & proximal limb extensors - antigravity muscles
Lateral Vestibulospinal Tract: describe the pathway
begining?
–Arises from lateral vestibular nucleus
Lateral Vestibulospinal Tract: describe the pathway
descends in what part of spinal cord?
–Descends ipsilaterally thru lateral brainstem & lateral aspect of ventral funciculus
Lateral Vestibulospinal Tract: describe the pathway
ends where?
–Ends on neurons in medial motor nucleus that innervate paraspinal & proximal limb extensors - antigravity muscles
Lateral Vestibulospinal Tract: function with example
- Major role is to regulate posture and balance
- Example of function: if body leans forward, the action of the LVST is to contract the gastrocnemius (stabilize the ankle) and hamstrings (stabilize the knee) and pull the person back upright
MVST
Medial Vestibulospinal Tract
Medial Vestibulospinal Tract: describe the pathway
begining?
descends in what part of spinal cord?
ends where?
- –Arises from medial vestibular nucleus
- –Descends bilaterally thru medial brainstem & ventromedially in spinal cord
- –Ends on medial motor nucleus of cervical spinal cord motor neurons innervate the neck muscles
Medial Vestibulospinal Tract: describe the pathway
begining?
–Arises from medial vestibular nucleus
Medial Vestibulospinal Tract: describe the pathway
descends in what part of spinal cord?
Descends bilaterally thru medial brainstem & ventromedially in spinal cord
Medial Vestibulospinal Tract: describe the pathway
ends where?
–Ends on medial motor nucleus of cervical spinal cord motor neurons innervate the neck muscles
what are the names of the two Reticulospinal Tracts?
- Pontine (medial) Reticulospinal Tract
- Medullary (Lateral) REticulospinal Tract
Reticulospinal Tracts: What is distictive about the size and conduction of their axons?
•Composed of large fiber axons with fast conduction
Medial Vestibulospinal Tract: function (3)
- –Functions to stabilize the head in space
- –Functions in maintaining gaze when body is moving as in walking
- –May play a dynamic role in head tracking movements required to stabilize voluntary gaze on moving object
where (in general) do the Reticulospinal Tracts arise?
•Arise from several nuclei in brainstem reticular formation
Medial Reticulospinal tract: pathway
Alternate name?
origin?
Decussation?
position in spinal cord?
where does it end?
Alternate name: Pontine Reticulospinal Tract
Origin: Pontine Reticular Formation Nuclei
- RF nuclei recieves inputs from vestibular nuclei and cerebellum
Decussation: bilateral but mostly ipsilateral
Descends length of the spinal cord in anterior funiculus
Mono & polysynaptic excitatory inputs on motor neurons innervating postural muscles
Medial Reticulospinal tract: pathway
Alternate name?
Alternate name: Pontine Reticulospinal Tract
Medial Reticulospinal tract: pathway
origin?
Origin: Pontine Reticular Formation Nuclei
- RF nuclei recieves inputs from vestibular nuclei and cerebellum
Medial Reticulospinal tract: pathway
Decussation?
Decussation: bilateral but mostly ipsilateral
Medial Reticulospinal tract: pathway
position in spinal cord?
Descends length of the spinal cord in anterior funiculus
Medial Reticulospinal tract: pathway
where does it end?
Mono & polysynaptic excitatory inputs on motor neurons innervating postural muscles
Medial Reticulospinal Tract: function
–Establish postural stability – make postural sets
The principle function of the Pontine (or Medial) Reticulospinal Tract is to establish postural stability the is to make postural sets
Lateral Reticulospinal tract: pathway
Alternate name?
origin?
Decussation?
position in spinal cord?
where does it end?
Alternate Name: Medullary Reticulospinal Tract
Origin: Medullary reticular formation nuclei
- RF nuclei receive inputs from cerebral cortex & red nucleus
Decussation; Bilateral but mostly ipsilateral
Descends length of the spinal cord in lateral funiculus
Polysynaptic inhibitory inputs on motor neurons innervating postural muscles
Lateral Reticulospinal tract: pathway
Alternate name?
Alternate Name: Medullary Reticulospinal Tract
Lateral Reticulospinal tract: pathway
origin?
Origin: Medullary reticular formation nuclei
Lateral Reticulospinal tract: pathway
Decussation?
Decussation; Bilateral but mostly ipsilateral
Lateral Reticulospinal tract: pathway
position in spinal cord?
Descends length of the spinal cord in lateral funiculus
Lateral Reticulospinal tract: pathway
where does it end?
Polysynaptic inhibitory inputs on motor neurons innervating postural muscles
Lateral Reticulospinal tract: function
–Readies body for movement by breaking postural sets
The Medullary (or Lateral) Reticulospinal Tract readies body for movement by breaking postural sets
Explain the importance of the coordination of the Reticulospinal tracts
- •Balance of tracts allows for proper regulation of posture & movement
- •Trunk & lower extremity postural activation before lifting movement
- –Stability precedes mobility
- –Proximal stability via the fast acting excitatory action of the pontine reticulospinal tract
- –The distal break of posture at the knees is medullary reticulospinal followed by corticospinal tract action
It is a Balance of the two reticulospinal tracts that allows for proper regulation of posture & movement. For example, there needs to be Trunk & lower extremity postural activation before lifting movement to stablize the trunk and proximal extremity to allow motion of the distal extremities. Remember proximal Stability is required for and precedes distal mobility
Fast acting excitatory action of the pontine reticulospinal tract produces the proximal stability upon which the medullary reticulospinal produces the distal break of posture at the knees which is followed by corticospinal tract action to produce the movement
Tectospinall tract: pathway
origin?
Decussation?
position in spinal cord?
where does it end?
- •Originates is the superior colliculus
- •Also projections from inferior colliculus for orienting (turning head) toward sounds with similar descending connections as portion from superior colliculus
- Decussates emmediately
- •Descends dorsomedially thru brainstem & ventro-medially thru spinal cord
- •Ends in cervical spinal cord on ventral horn motor neurons to link head & neck with movement in the visual surround
: The tectospinal tract originates is the superior colliculus & decussates immediately upon arising from the superior colliculus. It Descends dorsomedially thru brainstem and ventromedially thru spinal cord and ends in cervical spinal cord on ventral horn motor neurons to link head & neck with movement in the visual surround
Tectospinall tract: pathway
origin?
- •Originates is the superior colliculus
- •Also projections from inferior colliculus for orienting (turning head) toward sounds with similar descending connections as portion from superior colliculus
Tectospinall tract: pathway
Decussation?
Decussates emmediately
Tectospinall tract: pathway
position in spinal cord?
•Descends dorsomedially thru brainstem & ventro-medially thru spinal cord
Tectospinall tract: pathway
where does it end?
•Ends in cervical spinal cord on ventral horn motor neurons to link head & neck with movement in the visual surround
Tectospinall tract:
function
- •Ends in cervical spinal cord on ventral horn motor neurons to link head & neck with movement in the visual surround
- •Example of function is turning of head with rapidly approaching object
- •Also projections from inferior colliculus for orienting (turning head) toward sounds with similar descending connections as portion from superior colliculus
: An example of tectospinal tract function is turning of head and putting up the hands when there is a rapidly approaching object. Baseball players are trained to overcome this reflex when they are told to keep their eye on the ball. Also projections from inferior colliculus for orienting (turning head) toward sounds with similar descending connections as portion from superior colliculus
Rubrospinal tract: pathway
origin?
Decussation?
position in spinal cord?
where does it end?
- •Arises from very small magnocellular part of red nucleus - about 200 neurons
- •Decussates near its origin in the ventral tegmentum
- •Descends in dorsolateral brainstem & lateral funiculus of spinal cord
- •Ends on cervical motor neurons to wrist and hand
The Red nucleus of the midbrain has a large parvocellular portion and a small magnocellular portion. The rubrospinal tract arises from the smaller magnocellular part of red nucleus which consists of about 200 neurons. Axons from these neurons immediately decussates near their origin in the ventral tegmentum. The rubrospinal tract fibers Descend in dorsolateral brainstem & lateral funiculus of spinal cord to end on cervical motor neurons to wrist and hand
Rubrospinal tract: pathway
origin?
•Arises from very small magnocellular part of red nucleus - about 200 neurons
Rubrospinal tract: pathway
Decussation?
•Decussates near its origin in the ventral tegmentum
Rubrospinal tract: pathway
position in spinal cord?
•Descends in dorsolateral brainstem & lateral funiculus of spinal cord
Rubrospinal tract: pathway
where does it end?
•Ends on cervical motor neurons to wrist and hand
Rubrospinal tract:
Function
- •Some evidence to suggest that it plays an anticipatory muscle action when learning a new motor skill or modifying a previously learned task
- •Cerebellar input to spinal cord? Damage in other primates leads to cerebellar symptoms of intention tremor & ataxia
Some is evidence to suggest that The rubrospinal tract plays a role in the anticipatory muscle action when learning a new motor skill or modifying a previously learned task. There is evidence to suggest that the rubrospinal tract represents an indirect means for the cerebellum to influence spinal motor neurons. This feeling results form studies in non-human primates where damage leads to cerebellar symptoms of intention tremor & ataxia
TDIP
Tonic Descending Inhibitory Pathway
What is the Tonic Decending Inhibitory Pathway?
- –Brain structures may control the amount of inhibition, excitation, or the balance of inhibition and excitation existing in the interneuronal pool of the spinal gray
Brain structures may control the amount of inhibition, excitation, or the balance of inhibition and excitation existing in the interneuronal pool of the spinal gray. This influence is sometimes referred to as the Tonic descending inhibitory pathway (TDIP)
Explain the Suprasegmental Control of Reflexes
- •Suprasegmental structures that exert control over reflexes include:
- –Reticular formation, cerebellum, and cerebral cortex
- •This tonic inhibitory control reduces the excitability of lower motor neurons & magnitude of spinal reflexes
Suprasegmental structures that exert control over reflexes include: the Reticular formation, cerebellum, and cerebral cortex. This tonic inhibitory control reduces the excitability of lower motor neurons & magnitude of spinal reflexes
