Lecture 11: Spinal Cord, Brainstem, & Cortex Control of Motor Function II Flashcards
What is the definition of the pyramidal system?
These are tracts that pass through the medullary pyramidals. (other motor pathways are extrapyramidal)
Which two tracts make up the pyramidal system?
> corticospinal tract
> corticobulbar tract
Where do upper motor neurons originate?
motor cortices
What percentage of upper motor neurons decussate in pyramids and form the lateral corticospinal tracts?
75-85%
Where do most upper motor neurons synapse?
With association neurons in spinal cord central gray matter.
You know that 75-85% of upper motor neurons decussate in pyramids and form the lateral corticospinal tracts. Where do the remaining upper motor neurons decussate?
Remainder decussate near synapse with lower motor neurons and form anterior corticospinal tracts.
How are upper motor neurons classified?
Classified according to where they synapse in the ventral horn:
- medial activation system
- lateral activation system
- nonspecific activating system
What muscles are innervated by the medial activation system?
postural and girdle muscles
What muscle control is the lateral activation system associated with?
Associated with distally located muscles used for fine movements.
What system of upper motor neurons facilitates local reflex arcs?
nonspecific activating system
What makes up the lateral corticospinal tract?
Corticospinal fibers that have crossed in medulla.
What makes up the anterior corticospinal tract?
Uncrossed corticospinal fibers that cross near level of synapse with LMSs.
What does the lateral corticospinal tract supply?
ALL levels of spinal cord.
What does the anterior corticospinal tract supply?
Neck and upper limbs.
What are the 3 origins of the corticospinal tract (pyramidal tract)?
> primary motor cortex
pre-motor cortex
somatosensory area
True or False:
In the corticospinal tract, some fibers do not cross but continue down ipsilaterally in ventral corticospinal tract.
True
What is the pathway of the corticospinal tract?
site of origin -> internal capsule -> medullary pyramids -> X in lower medulla (most fibers) -> lateral columns of spinal cord (lateral corticospinal tract).
Where are giant pyramidal (Betz) cells located?
in motor cortex
What do giant pyramidal (Betz) cells do?
send collaterals back to cortex
What percentage of the fibers in the corticospinal tract are giant pyramidal (Betz) cells?
3%
What are the functions of the corticospinal tract?
> Adds speed and agility to conscious movements.
- especially movements of the hand
Provides a high degree of motor control.
- (i.e., movement of individual fingers)
What are some symptoms of a patient with a corticospinal tract lesion?
> reduced muscle tone > clumsiness > weakness > not complete paralysis > NOTE: complete paralysis results if both pyramidal and extrapyramidal systems are involved (as is often the case).
In the corticospinal tract, where do other fibers from the cortex, besides giant pyramidal (Betz) cells, go?
> pass into caudate nucleus and putamen
pass to red nucleus
pass to reticular substance and vestibular nuclei
large numbers of fibers pass to pontine nuclei
What does the corticobulbar tract innervate?
the head
Where do most fibers of the corticobulbar tract terminate?
In reticular formation near cranial nerve nuclei.
What do association neurons of the corticobulbar tract synapse with?
lower motor neurons
True or False:
Association neurons in the corticobulbar tract leave the reticular formation and synapse in cranial nerve nuclei and also lower motor neurons.
True
large neurons from the magnocellular region of the red nucleus give rise to what tract?
rubrospinal tract
Where does the rubrospinal tract decussate?
decussates in lower brain stem
What synapses in the magnocellular portion of the red nucleus?
- fibers from primary motor cortex (corticorubral pathway)
- branches from corticospinal tract
True or False:
The magnocellular region of the red nucleus has a somatotopic representation of all the muscles of the body.
True
What does stimulation of the red nucleus result in?
> stimulation of flexors
> inhibition of extensors (antigravity muscles)
What does the extrapyramidal system include?
> pathways that contribute to motor control but that are not part of the corticospinal system
> descending motor tracts that do not pass through medullary pyramids or corticobulbar tracts
> rubrospinal tracts
vestibulospinal tracts
reticulospinal tracts
What are the characteristics of the rubrospinal tract?
> originates in red nucleus
decussates in midbrain
descends in lateral funiculus (column)
function is closely related to cerebellar function
What do patients with a lesion in the rubrospinal tract present with?
> impairment of distal arm and hand movement
> intention tremors (similar to cerebellar lesions)
Where does the rubrospinal tract originate?
red nucleus
Where do fibers in the rubrospinal tract decussate?
midbrain
In which column does the rubrospinal tract descend?
lateral funiculus
Is the function of the rubrospinal tract closely related to cerebellar function?
Yes
Where does the vestibulospinal tract originate at?
in vestibular nuclei
Where does the vestibular nuclei receive major input from?
vestibular nerve (CN VIII)
In which column does the vestibulospinal tract descend?
anterior funiculus
What does the vestibulospinal tract fibers descending in the anterior funiculus synapse with?
LMNs to extensor muscles
You know that the vestibulospinal tract fibers synapse with LMNs to extensor muscles. What are they primarily involved in?
Maintenance of upright posture.
Where does the reticulospinal tract originate at?
various regions of reticular formation
Which column does the fibers of the reticulospinal tract descend?
anterior portion of lateral funiculus
What is the function of the reticulospinal tract?
thought to mediate larger movements of trunk and limbs that DO NOT require balance or fine movements of upper limbs
What does the pontine reticular nuclei do?
Stimulatory effect on both extensors and flexors, but especially extensors (antigravity muscles).
What do the fibers of the pontine reticular nuclei make up?
pontine reticulospinal tract (anterior column)
What do the fibers of the medullary reticular nuclei make up?
medullary reticulospinal tract (lateral column)
What does the medullary reticular nuclei do?
Inhibitory effect on both extensors and flexors, especially extensors (antigravity muscles)
Does the pontine reticular nuclei or medullary reticular nuclei have inhibitory effects on extensors and flexors?
medullary reticular nuclei
pontine reticular nuclei have stimulatory effect
Do the fibers from the pontine reticular nuclei or medullary reticular nuclei run in the anterior column?
pontine reticular nuclei fibers
medullary reticular nuclei fibers run in lateral column
What are the components of the vestibular apparatus?
> Utricle
Saccule
Semicircular canals
In the vestibular apparatus, what kind of lymph are the ducts filled with and what kind of lymph surrounds the ducts?
Ducts are filled with endolymph and surrounded by perilymph.
What does the vestibular apparatus detect?
Detects angular (semicircular canals) and linear (utricle and saccule) acceleration of the head.
What is the vestibular apparatus involved in?
Reflex adjustments of head, eyes, and postural muscles.
What does the vestibular apparatus provide for us?
A stable visual image and steady posture.
True or False:
Each semicircular canal has an enlargement at one end called the ampulla, which contains hair cells.
True
True or False:
The utricle and saccule each contain a macula, which contains hair cells.
True
What is the vestibular apparatus?
A membranous labyrinth consisting of three semicurcular canals and vestiuble (also includes cochlea).
Is the macula of the utricle or saccule located on a horizontal plane?
Macula of Utricle
What does the macula of the utricle do?
Plays role in determining orientation of head when head is upright.
What does the macula of the saccule do?
Signals head orientation when person is lying down.
In which plane is the macula of the saccule located?
vertical plane
What are the characteristics of the macula?
Each macula is covered by a gelatinous layer:
> contains large number of embedded small calcium carbonate crystals (statoconia)
> contains thousands of hair cells which project cilia into the gelatinous layer
> the weight of the statoconia bends cilia in the direction of gravitational pull
What are the characteristics of a hair cell?
> has 50-70 small cilia (stereocilia)
has 1 large cilium (kinocilium) off set to one side
tips of stereocilia are connected together and to kinocilium
What are fuctions of the hair cell in the macula?
> bending of sterocilia towards kinocilium opens hundreds of cation channels causing receptor membrane depolarization and excitation
> bending of cilia in opposite direction closes channels and hyperpolarizes receptor membrane
> hair cells are oriented such that bending the head in different directions causes different groups of hair cells to depolarize
What happens in the semicircular canals when the head begins to rotate in any direction?
> inertia of the fluid in one or more of the semicircular canals remains stationary while semicircular canal rotates with the head
> fluid flows from the duct and through the ampulla and causes the cupula to bend to one side
> hundreds of hair cells within each cupula detect this bending and send signals via the vestibular nerve
How is the semicircular canal aligned when the head is bent forward 30 degrees?
> lateral ducts are horizontal
anterior ducts are in vertical planes projecting forward and 45 degrees outward
posterior ducts are in vertical planes projecting backward and 45 degrees outward
What are the characteristics of the ampulla found at one end of each semicircular canal duct?
> filled (along with duct) with endolymph
crista ampullaris
- small crest within each ampulla
cupula
- loose mass of gelatinous tissue on top of the crista
What is the ampulla filled with?
Filled (along with duct) with endolymph (high[K]; low [Na])
What is the crita ampullaris?
small crest within each ampulla
What is the cupula?
loose mass of gelatinous tissue on top of the crista
