Corticospinal and Corticobulbar pathways Flashcards
Where do corticospinal and corticobulbar pathways begin and end?
Begin from neurons in the cerebral cortex and end on lower motor neurons in the spinal cord and brainstem
What do the corticospinal and corticobulbar pathway neurons serve as and what do their axons form?
Neurons serve as upper motor neurons and their axons form major descending motor control pathways that modify motor activity
As the corticospinal and some of the corticobulbar fibers descend through the CNS what structure do they traverse?
Medullary pyramids
What are the corticospinal and corticobulbar pathways that traverse the medullary pyramids known as?
Pyramidal system
What are the descending motor pathways that do not traverse the pyramids known as?
Extrapyramidal system
What are some examples of extrapyramidal system?
Vestibulospinal tract
Medial longitudinal fasciculous
Tectospinal and tectobulbar tracts
Where do corticospinal and corticobulbar tracts originate?
From neurons in frontal and parietal lobes
Which part of the frontal lobe do corticospinal and corticobulbar originate from?
Precentral gyrus and anterior paracentral lobule
What percent of pyramidal fibers are contributed by the frontal lobe?
30%
What neurons do pyramidal fibers of corticospinal and corticobulbar in the frontal lobe originate from?
From giant pyramidal neurons known as Betz cells
What brodmann map area is associated with the frontal lobe where pyramidal fibers originate from?
4
What is the frontal lobe where corticospinal and corticobulbar originate from known as?
Primary motor cortex (M1)
What does the primary motor cortex do?
Muscle contractions on the contralateral body and muscle contractions are single muscles or muscles that move a single joint or digit
Is the Primary motor cortex somatopically organized?
Motor homonculus
- similar to sensory homonculous of primary somatosensory cortex
What is the premotor and supplementary motor areas (of the frontal lobe) correspond anatomically with?
Premotor = superior and middle frontal gyrus
Supplemental motor area = medial frontal gyrus
What are the Brodmann map area for the premotor and supplementary motor areas?
6 and some of 8
What percent of the pyramidal fibers originate in the premotor and supplementary motor areas?
30%
What do premotor and supplementary motor areas do?
1) contraction on contralateral side of body (synergistic or patterned movements
2) programming of skilled movements (without thought required)
3) supplementory area - plays a role in planning for execution of movement
What areas of the parietal lobe are included in the pyramidal system?
Postcentral gyrus, posterior paracentral lobule and superior parietal lobule
What percent of the pyramidal fibers originate in the parietal lobe that are included in the pyramidal system?
about 40%
True or False: all descending fibers that originate from the parietal lobe that includes the pyramidal system are upper motor neurons?
False, some of the descending fibers that originate here are not upper motor neurons; instead they terminate on sensory neurons in the brainstem and spinal cord
What are corticospinal and corticobulbar and how do they exit the cerebral hemisphere?
Projection fibers that leave the cerebral hemisphere through the posterior limb of the internal capsule
How is the internal capsule organized?
Anterior to posterior: Pyramidal system Somatosensory system Visual data Auditory data
Where do fibers from the posterior limb of the internal capsule continue to?
Fibers continue into the cerebral crus of the midbrain
What is the somatotopical organization in the cerebral crus of the midbrain?
Pyramidal fibers are in middle 1/3 of cerebral crus
Where in the pons do corticospinal and corticobulbar descend through?
Through the basilar pons in (fascicle-like) bundles
What do the corticospinal and corticobulbar form as they enter the medulla?
Form the pyramis and maintain the somatopical organization
As pyramidal fibers descend through the brainstem, where do corticobulbar fibers terminate?
Terminate on motor nuclei of cranial nerves on lower motor neurons
What happens to a majority (85%) of the corticospinal fibers that descend through the pyramids?
85% of the fibers decussate in the pyramidal decussations
Where do the decussated corticospinal fibers descend through in the spinal cord and what do they form?
Descend through the lateral funiculus and form the lateral corticospinal tract
How are the fibers in the lateral corticospinal tract somatopically organized?
Medial = fibers that terminate in cervical cord intermediate = fibers that terminate in upper extremity Lateral = fibers that terminate in lumbosacral
Where do most fibers of the lateral corticospinal tract terminate?
In the cervical cord segments
Where and how do most corticospinal fibers ultimately influence lower motor neurons in?
The ventral horn
1) some directly on LMNs
2) many end on interneurons that then influence LMNs
What percentage of corticospinal fibers in the pyramids do not decussate?
15%
Where do the uncrossed corticospinal fibers that descend and what do they form?
Descend into the anterior funiculus form the anterior corticospinal tract
Where do most of the anterior corticospinal tract fibers ultimately decussate?
At the segmental level although some do terminate on the ipsilateral side
Where do most of the anterior corticospinal tract fibers terminate?
Most terminate in the cervical cord
How do most fibers of the of the anterior corticospinal tract influence LMN activity?
Both directly and indirectly influence lower motor neurons
Where do some corticospinal fibers that originate in the pariental lobe terminate, instead of onto sensory neurons?
On dorsal horn neurons and modify the transmission of sensory data through ascending pathways
What do corticospinal and corticobulbar tracts help to provide for?
voluntary control of motor activities that involve the distal parts of the extremities and face
What skilled movements do corticospinal and corticobulbar tracts help control?
Face muscles: 1) eye 2) tongue 3) facial expression Distal Extremities: 1) Fingers 2) Toes
What factors do corticospinal and corticobulbar tracts allow the skilled movement areas to be performed with?
Speed
Agility
Precision
What does a pure lesion of the corticospinal tracts do?
Hypotonia of contralateral muscles - especially of distal extremities and face
Decreases muscle tone -> flaccid (limpness)
What happens initially with a pure lesion of the corticospinal tracts
Initial period of flaccid paralysis contralateral to lesion - especially of distal extremities and face
What happens after several months with a pure lesion of the corticospinal tracts?
Motor control returns and any chronic loss of voluntary motor control is minimal (therefore no permanent paralysis)
What persists after several months with a pure lesion of the corticospinal tracts?
Permanent difficulty performing highly skilled activities with the distal extremities (ie: picking up and manipulating small objects) and execution of movements that require much attention. Loss of speed, agility and precision of movements
Where do corticobulbar projections originate from?
Upper motor neurons that are located in the inferior 1/3 of the precentral gyrus, the premotor area and areas of the parietal lobe
Where do corticobulbar projections descend through to reach the brainstem?
Descend through the posterior limb of the internal capsule and into the brainstem where most terinate on interneurons that influence LMNs of cranial nerves.
What motor nuclei of the cranial nerves receive corticobulbar innervation?
Oculomotor nuclear complex trochlear nucleus trigeminal motor nucleus abducens nucleus facial motor nucles nucleus ambiguus hypoglossal nucleus
What does the oculomotor nuclear complex innervate?
includes LMNs, innervates skeletal muscle of III nerve
What does the trochlear nucleus innervate?
includes LMNs, innervates skeletal muscle of IV nerve
What does the trigeminal motor nucleus innervate?
includes LMNs, innervates skeletal muscle of V nerve
What does the abducens nucleus innervate?
includes LMNs, innervates skeletal muscle of VI nerve
What does the facial motor nucleus innervate?
includes LMNs, innervates skeletal muscle of VII nerve
What does the nucleus ambiguus innervate?
includes LMNs of IX, X and cranial root of XI nerve
What does the hypoglossal nucleus innervate?
includes LMNs, innervates skeletal muscle of IV nerve
Which nuclei do not receive direct corticobulbar innervation?
CN III, IV, VI
What cranial nerves follow direct corticobulbar innervation?
CN V, VII, IX, X, cranial root XI and XII
Which motor nuclei are innervated by corticobulbar fibers on the contralateral side?
V, VII nucleus ambiguus and XII
What is significant about corticobulbar fibers that end ipsilaterally?
Most LMNs of cranial nerves receive bilateral corticobulbar innervation and there is no significant paralysis of most muscles (since innervated by cranial nerves following unilateral lesions of corticobulbar fibers)
What are some exceptions to the pattern of bilateral corticobulbar fiber innervations?
The neurons of facial motor nucleus that innervate:
1) facial expression of lower 1/2 of face
2) neurons of the hypoglossal nucleus (sometimes)
These only receive from controlateral corticobulbar innervation
Why is an understanding of the pattern of corticobulbar innervation to LMN’s of cranial nerves important clinically?
Unilateral corticobulbar (upper motor neuron) lesions will only affect muscles of facial expression only lower 1/2 of face and muscles of tongue (all other muscles innervated by cranial nerves are unaffected)
Why are the innervation of motor nuclei II, IV and VI different than other motor nuclei?
The activity of the LMNs of these nerves are coordinated to ensure that both eyes maintain visual fixation on the same object in the visual field
What are vergence movements?
When eyes move in opposite directions
What are the two types of vergence movements?
Divergence
Convergence
What is divergence
both eyes abduct (focus on distant object)
What is convergence?
both eyes adduct
What are conjugate movements?
when eyes move in parallel
What are involuntary conjugate movements?
include visual tracking maneuvers when watching an object move across the visual field (ie: smooth pursuit movement = automatic tracking movements)
What is the area of the cortex that controls the automatic tracking of movements?
Occipital lobe, also vestibular driven movements
What are the voluntary conjugate movements that occur when scanning a landscape or a page known as and how do they appear?
Known as saccadic eye movements
- series of brisk, jerky, fast , excursional movements
What area of the cerebral cortex controls saccadic eye movements?
The area of the cerebral cortex that controls this activity is located in the posterior part of the middle frontal gyrus and is called the frontal eye field
What brodmann’s map area is the frontal eye field?
8
What does unilateral stimulation of the frontal eye field result in?
conjugate, saccadic movements of eyes to the opposite side
Do the visual cortex or FEF neurons directly project axons to the motor nuvlei of III, IV or VI
no
Where do the visual cortex or FEF neurons project axons to?
Superior colliculus in the midbrain
Where do visual cortex or FEF neurons that have reached the superior colliculus project to?
Send fibers to other neurons of the brainstem that coordinate conjugate movements of the eyes
What does conjugate movement of the eyes in the horizontal plane require and what does it form?
The participation of special neurons in the brainstem which form the paramedian pontine reticular formation
Where is the paramedian pontine reticular formation found?
located in the lower pons (near VI nucleus)
Where does the paramedian pontine reticular formation receive input from?
Superior colliculus
Vestibular info
Vestibular nuclear complex
Where do paramedian pontine reticular formation send axons through and to?
Through the ascending medial longitudinal fasciculus to the Nuclei of III, IV, VI
What do the neurons of the paramedian pontine reticular formation do when activated?
Neurons of the paramedian pontine reticular formation ensures that LMNs of III, IV, and VI are activated such that the eyes move ina conjugate fashion in the horizontal plane
What occurs with lesions in the frontal eye field?
- Paralysis of voluntary gaze to side opposite of lesion
- preservation of smooth pursuit movements
What occurs with lesions in the paramedian pontine reticular formation?
1) paralysis/loss of lateral gaze towards the lesioned side
2) includes loss of voluntary gaze and smooth pursuit
