Corticobulbospinal tracts - SRS Flashcards
In the spinal cord and brainstem, lower motor neurons directly innervate what?
Muscle
What are lower motor neurons controlled by?
- Local circuits within the spinal cord and brainstem
- Upper motor neurons
What regulates upper motor neurons?
Basal ganglia and cerebellum
Where are lower motor neuron cell bodies located?
Ventral horn of the spinal cord
Brainstem motor nuclei
What are the two types of lower motor neurons that innervate skeletal muscle?
Alpha and Gamma
Where are upper motor neuron cell bodies located?
- -Cortex (Corticospinal)
- Cortex (Corticobulbar)
- –Vestibular nuclei (Vestibulospinal)
- –Superior colliculus (Tectospinal)
- –Reticular formation (Reticulospinal)
- –Red nucleus (Rubrospinal)
Where do interneurons for axial structures located?
Do they have many collaterals?
Unilateral or bilateral?
Medial, and bilateral.
Possess many collaterals
Where do interneurons for Limb structures located?
Unilateral or bilateral?
Do they have many collaterals?
More lateral, are unilateral and have fewer collaterals.
A single alpha motor neuron and all the muscle fibers it innervates = ?
Motor unit
How do we change the amount of force produced by a muscle?
Increase or decrease the amount of motor units recruited.
Identify the blanked out structures
The neuromuscular junction/motor end plate is analagous to a?
Synapse
Alpha Motor neurons from the spinal cord innervate muscles involved in excitation-contraction coupling. Characterize these fibers by speed and structure.
Fast
Myelinated
In excitation-contraction coupling, what NT is released by the alpha motor neuron?
What receptors does it bind?
What type of receptors are these?
- releases acetylcholine
- Binds to nicotinic receptors
- these are ligand-gated Na+ channels
What are the junctional folds of the neuromuscular junction often continuous?
T-Tubules
Motor units differ in the types of muscle fibers they innervate. What are the three types we covered?
Slow motor units
Fast fatigable motor units
Fast Fatigue-resistant motor units
Descrime the muscle fibers of slow motor units.
–Smaller, red muscle fibers (myoglobin, mitochrondria, capillary beds)
What size are the slow motor unit alpha-motor unit neurons?
What level of force do these generate?
How fatigable are they?
–Smaller Alpha-motor neurons
–Small force
–Resistant to fatigue
What are fatigue resistant slow motor units imporant for?
Describe their threshold for activation.
- Important for sustained muscle contraction (eg. Regulating posture)
- Lowest thresholds for activation = recruited first
Describe the size and color of Fast fatigable motor unit fibers.
What size alpha motor neurons are we dealing with here?
What is the force like?
Fatiguability?
–Larger, pale muscle fibers (few mitochondria)
–Larger Alpha motor neurons
–Large force
–Easily fatigued
In what manner of activity are fast fatigable motor units most useful?
•Brief exertions that require large forces (eg. jumping)
Where do fast fatigue resistance motor units fall on the spectrum of muscle fiber types?
What else are they known as?
–Between slow motor units and fast fatigable motor units (aka intermediate motor units)
As the synaptic activity driving motor neurons increases, in what order do we recruit the motor neuron types?
- slow motor neurons
- fast, fatigue-resistant motor neurons
- fast fatigable motor neurons
What is muscle tone?
The resting level of tension in a muscle
What does muscle tone enable us to do?
•Allows muscle to make an optimal response to voluntary or reflexive movement by keeping them in a state of readiness to resist stretch
What does denervation do to muscle?
Causes rapid atrophy
When muscle fiber degenerate, they are replaced by fatty and fibrous tissue, leading to loss of contractile proteins in muscle cells. What can this fibrous tissue cause?
Contracture
Why do muscles atrophy in the elderly?
_Loss of motor neurons _ due to vascular disease and poor nutrition.
When motor neurons are lost, motor end plates are innervated by collaterals of adjacent motor neurons, leading to an abnormal EMG signature know as?
Giant motor units
What are typical signs of lower motor neuron damage?
Paralysis or paresis
areflexia
atrophy
fibrillations
fasciculations
What are fibrillations? What are they due to?
–Rapid twitching due to change in muscle fiber excitability *denotes pathology*
What are fasciculations?
–Local, small twitch due to alpha motor neuron damage
What pathways for upper motor control of lower motor neurons are extrapyramidal? (4)
–Vestibular (vestibulospinal)
–Reticular (reticulospinal)
–Red nucleus (rubrospinal)
–Superior colliculus (tectospinal)
What pathways for upper motor neuron control of LMN’s is pyramidal?
–Lateral corticospinal
–Ventral corticospinal
–Corticobulbar
From what nuclei does the vestibulo spinal tract arise?
LAteral or medial vestibular nuclei
What does the lateral vestibular nuclei command?
**Ipsilateral: **posture, including extensors
What does the medial vestibular nuclei command?
**Bilateral: **head position stability
Identify blanked structures
Where does the tectospinal tract arise?
Superior colliculus
What does the tectospinal tract influence?
Musculaturs of neck and eyes
Postural reflexes to alarming visual stimuli and some auditory stimuli
Identify
The reticulospinal tract arises from different brainstem levels. Does it decussate?
What does it control?
Does not decussate
Affects axial and postural muscles, and provides secondary control when CST is dysfunctional
What is the reticulospinal tract largely controlled by?
Cortical motor centers
Identify
What does the rubrospinal tract innervate? What does it do?
Innervates spinal cord through thoracic levels.
It is a laterally based tract possibly involved in flexor muscles of the arm. Not entirely sure what it does in humans.
Ident
What organizes the movement of individual muscles?
Motor homunculus in the primary motor cortex
What organizes the movement of groups of muscles?
Motor homunculi in premotor and supplementary motor cortices
Where are Betz cells found? In what layer are they found?
Primary motor cortex
Layer V
What do Betz cells project through?
- Project through posterior limb of internal capsule
- Through medullary pyramids
What does the pyramidal tract provide collaterals to?
- Reticular formation
- Trigeminal
- Facial
- Nucleus ambiguus
Describe the decussation of the pyramidal tract.
80% at the caudal medulla
20% continue ipsilateral
What tract do the decussating fibers of the pyramidal tract take after decussation?
What does this innervate?
- “Lateral corticospinal tract”
- Limbs, digits (stuff needing fine motor control)
What path do the 20% of pyramidal tract fibers take?
What do they go to?
They have a second chance at decussating, where do they get this opportunity?
- “anterior or ventral corticospinal tract”
- Axial muscles
- Some of these will decussate in the anterior white commissure, some won’t at all
Ident
Ident
For bronchiomeric muscles, specifically facial expression above the eyes, what do lesions of the cortex produce?
Bilateral deficiencies (but minor)
For bronchiomeric muscles, specifically facial expression above the eyes, what do lesions of the nucleus produce?
Ipsilateral Deficiency
For muscles of facial expression below the eyes, muscles of mastication and the tongue, what does a lesion of the cortex cause?
Contralateral deficiency
For muscles of facial expression below the eyes, muscles of mastication and the tongue, what does a lesion of the nucleus cause?
Ipsilateral deficiency
Identify these locations
Identify this location
What doe a + Babinski’s sign in adults indicate?
If toes point upward instead of downward due to loss of upper motor neuron control
What is spasticity a sign of?
–Corticospinal influences probably are somewhat inhibitory to the brainstem control of postural muscles—relaxes them a bit.
What can cause loss of fine motor control?
Corticospinal damage
ALS arises due to degeneration of what?
Corticospinal and LMN
What is the average time to fatality post ALS diagnosis?
5 yrs
