Lecture 4b-descending pathways

Question 1

what is the pyramidal motor system used for

Answer 1

how we make voluntary movements
-sometimes we want voluntary contract head muscles (facial expressoin) and sometimes we want voluntary contract body muscles (biceps)

Question 2

each voluntary motor pathway (pyramidal) requires how many neurons? what are they

Answer 2

2-upper motor neuron and lower motor neuron

Question 3

the LMN projects directly to what?

Answer 3

to muscle to cause muscle contraction via spinal nerves, etc (body) and motor CN (for head)

Question 4

what kind of neurons are the alpha and gamma motor neurons at the ventral horn gray

Answer 4

LMN
-they leave the cord as the ventral rootlets/roots which join the spinal nerve and go out to the periphery to innervate striated skeletal muscle

Question 5

what is are the 2 main UMN to consider

Answer 5

corticospinal tract and corticobulbar tract

Question 6

what does the corticospinal tract do

Answer 6

-conveys impulses from motor areas of cortex down to the LMN in the spinal cord

1. large bundle of fibers (axons) that start in cell bodies in motor areas of the cerebral cortex,
2. descends through the brainstem and lateral white columns of the spinal cord, and
3. terminates in the spinal cord’s ventral horn gray.

Question 7

what does the corticobulbar tract do

Answer 7

conveys impulses from motor areas of cortex; but instead of projecting to the cord, it sends impulses down to LMNs in the brainstem

-brainstem=bulbar

Question 8

what are extrapyramidal tracts

Answer 8

other UMNs since they also synapse on LMNs

Question 9

why can single neurons w/ very long axons project all the way from motor cortex to the contralateral half of the spinal cord?

Answer 9

bc a relay in thalamus is not required for outputs from the cortex

Question 10

where does the corticospinal tract begin?

where is that located?

Answer 10

arises/starts in the motor cortex, primarily in the primary motor cortex which is in the precentral gyrus of the frontal lobe

Question 11

what is the motor homunculus

Answer 11

the distorted map of the body
“FUTL”
face, upper ext, thorax, lower extrem

Question 12

when the fibers of the CST descend, where do they coalesce

Answer 12

post limb of the internal capsule

-V shaped white matter

Question 13

what happens when there is a lesion at the PLIC

Answer 13

contralateral spastic paralysis of the body

Question 14

after the PLIC, the CST descends through the brainstem, specifically where in the brainstem?
wehre in midbrain
wehre is pons
where in medulla

Answer 14

ventral brainstem

• crus cerebri (of the cerebral peduncles)
• base of pons aka basis pontis
• medullary pyramids

Question 15

how much of the CST axons will cross to the other side of the body? where will they do that?

Answer 15

90% -but for this course consider this a crossed tract.

-medullary pyramids of the low medulla

Question 16

where is the CST while it is in the spinal cord?

when does the CST reach the cord?

Answer 16

the lateral funiculus

1-2mm caudal to the motor decussation

Question 17

what is the goal of the UMN?

Answer 17

to synapse on the LMN

the LMN goes out to innervate striated skeletal muscles of the body

Question 18

if your brain wnats to impact upper extremities (ex. flex biceps), it leaves the lateral funiculus where?

where will it synapse at

Answer 18

at the level of cervical enlargement and will synapse on the corr. LMNs in the ventral horn gray

Question 19

where is the lateral corticospinal tract in the cord?

Answer 19

in the dorsal aspect of the lateral funiculus

diff. from brainstem-CST is in ventral brainstem but in lateral cord

Question 20

what are the steps to the corticospinal tract

Answer 20

1. begins in the primary motor cortex in the precentral gyrus of the frontal lobe
2. fibers of the CST descend and coalesce in the PLIC
3. after PLIC, the CST descends through the ventral brainstem (crus cerebri (midbrain), basis pontis (pons), medullary pyramids (medulla))
   - CN 3 and CN6
4. vast majority of CST axons cross to the other side in the medullary pyramids
5. about 1-2mm caudal to the motor decussation, we are now in the cord -called the lateral corticospinal tract now in the lateral funiculus
6. “exit the highway” and synpase ventral horn cells.
7. LMN to directly innervate striated skeletal muscle. ex. if you want to impact upper extremities it leaves the lateral funiculus at the level of the cervical enlargement and synapse on corres. LMNs in the ventral horn gray.

Question 21

what is the function of the corticospinal tract?

what does it do to alpha motor neurons and gamma motor neurons?

Answer 21

control of VOLUNTARY skilled movements of the distal muscles, particularly the distal flexor musculature

• activates alpha motor neurons (excitatory drive)
• also has inhibitory influence on certain spinal reflexes; influences muscle tone and deep tendon reflex
• inhibitory to gamma motor neurons

Question 22

waht do alpha motor neurons go out to innervate? via what?

what do gamma motor neurons innervate

Answer 22

extrafusal striated skeletal muscle
via ventral roots, etc
intrafusal muscle fibers

Question 23

what happens when the UMN is lesioned

Answer 23

increased firing of the gamma motor neurons resulting in more contraction of the poles of the muscle spindle making them hypersensitive to further changes in muscle length
-hyperreflexia and hypertonicity/spasticity

Question 24

where is the lateral CST located

Answer 24

in the dorsal-lateral funiculus of white just medial to the dorsal spinocerebellar tract

Question 25

does the CST directly innervate muscles of the arms or legs?

Answer 25

no. the CST is an UMN which synapses on the LMN. it is the LMN itself which will go out to directly innervate muscle

Question 26

when a UMN lesion (visualize the entire course of the CST in the brain and cord) occurs (ex. stroke) what happens to: 
weakness 
muscle tone
stretch reflexes (DTRs)
fascifuations
atrophy
babinski sign

Answer 26

weakness: yes, decreased strenght
muscle tone: increased (hypertonic, spastic)
stretch reflexes (DTRs): increased (hyperreflexia)
fascifuations: no
atrophy: mild (atrophy of disuse)
babinski sign: yes

Question 27

when a LMN lesion (ventral horn gray, ventral roots, etc) occurs (ex. polio) what happens to: 
weakness 
muscle tone
stretch reflexes (DTRs)
fascifuations
atrophy
babinski sign

Answer 27

weakness: yes, decreased strenght
muscle tone: decreased (hypotonic, flaccid)
stretch reflexes (DTRs): decreased or absent (hyporeflexia or areflexia)
fascifuations: yes, possibly
atrophy: severe
babinski sign: no

Question 28

UMN damage =

LMN damage =

Answer 28

spastic paresis

flaccid paralysis

Question 29

what is spastic paresis

waht is flaccid paralysis

Answer 29

type of weakness characterized by hypertonicity and hyperreflexia, partial weakness

type of weakness accopanied by hyporeflexia and hypotonicity, total weakness

Question 30

UMN lesions =

LMN lesions =

Answer 30

atrophy of disuse

wasting atrophy

Question 31

in UMN lesion, what do you lose inhibitory control over?

why do you not get as much muscle atrophy (except of disuse)?

Answer 31

gamma motor neurons

-still have extrapyramidal inputs

Question 32

whay do you have flaccid paralysis in an LMN lesion

Answer 32

bc the LMNs are dead, you have marked muscle atrophy, no deep tendon reflexes, no muscle tone

• connection btwn nervous system and striated skeletal muscle is cut off => atrophy and arefelxia
• inputs into the ventral horn cells don’t matter now cause the LMNs of the final common pathway (LMNs) are dead

Question 33

what is the CST (pyramidal system) importnat for

what are extrapyrmaidals used for

Answer 33

• fine movements of distal flexor muscles

- locomotion (ex walking), posture and balane, orienting towards a sensory stimuli