(10) Motor Pathways of the CNS

Question 1

which laminae of gray matter contains LMN

Answer 1

IX

Question 2

which laminae of gray matter contains intermediate sensory neurons

Answer 2

I-IV

Question 3

differences in organization of LMN synapses with UMN that control distal extremiteis vs trunk muscualture

Answer 3

Distal:
UMN are from lateral corticospinal tract and synapse on ipsilateral side

Trunk:
UM: are from anterior corticospinal trant and synapse bilaterally

Question 4

size of LMN cell body is proportional to…

Answer 4

the number of fibers it innervates

Question 5

type of muscle fibers that tend to be small in size and thus the ration of muscle fibers per neuron is small

Answer 5

slow fatigue resistant fibers

Question 6

type of muscle fiber that las more mitochondria, myoglobin, and denser capillary beds

Answer 6

slow fatigue resistant fibers

Question 7

type of metabolism utilized by slow vs fast muscle fibers

Answer 7

slow = aerobic 
fast = anaerobic/glycolysis

Question 8

describe muscle stretch reflex

Answer 8

stretched sensed by muscle spindles which send signal to 1a sensory neurons. 1a neurons synapse with LMN and transduce signal to them. LMN sends signal to muscle fibers which contract. 1a sensory neurons also synapse with inhibitor interneurons to inhibit contraction of antagonistic muscle

Question 9

what type of afferent fibers do muscle spindles give rise to?

Answer 9

Ia and II

Question 10

what type of afferent fibers do golgi tendon organs give rise to?

Answer 10

IIb

Question 11

___ motor neuron innervates muscle spindles

Answer 11

gamma

Question 12

sensory organ that generates the sensory info that triggers a muscle reflex arc

Answer 12

muscle spindle

Question 13

significance of gamma motor neurons

Answer 13

innervate muscle spindle intrafusal muscle fibers to keep the stretch receptors from becoming relaxed and non-functional. (intrafusal muscle fibers contract with the extrafusal muscle fibers)

Question 14

type of info carried on type II afferents from muscle spindles

Answer 14

muscle length: inc # APs = inc length and vice versa

Question 15

type of info carried on type Ia afferents from muscle spindles

Answer 15

muscle length and velocity: inc # APs = inc length and velocity and vice versa

Question 16

when muscle fibers are static the frequency of APs from type II and Ia afferents reflects …

Answer 16

fiber length

Question 17

function of golgi tendon organs

Answer 17

prevent generation of excessive (and potentially harmful) force
*generates APs that inhibit contraction and promote relaxation

Question 18

describe the negative feedback control provided by the GTO

Answer 18

stretch of the GTO causes it to generate an AP which it sends to the 1b inhibitory interneuron which inhibits LMN/contraction of the muscle.
**1b inhibitory interneuron also activate another LMN of the antagonistic muscle to cause it to contract

***they also send signals to descending pathways

Question 19

What is the flexor reflex

Answer 19

uncomfortable stimulus will produce a reflex withdrawal

Question 20

Describe the organization/process of reflex withdrawal

Answer 20

stimulus → excitatory interneuron → activate agonist muscle to contract and inhibit contraction of antagonist

Question 21

Where do the cell bodies of UMN originate

Answer 21

precentral gyrus

Question 22

Where do UMN decussate?

Answer 22

medulla (aka pyramidal decussation)

Question 23

Describe the pathway taken by UMN to from the cortex to the spinal cord

Answer 23

precentral gyrus → posterior limb of internal capsule →midbrain and pos → medulla where they decussate → lateral corticospinal tract of spinal cord

Question 24

large areas of the cortex are dedicated to…

Answer 24

functions that require precise control (i.e. face, tongue, and hands)

Question 25

location of leg fibers in posterior limb of internal capsule

Answer 25

posterior

arms are anterior and trunk is between them

Question 26

location of leg fibers in spinal cord

Answer 26

leg is lateral

Question 27

in the corticospinal tract, not all the fibers decussitate. fibers controlling ___ are do not decussitate

Answer 27

trunk

Question 28

somatotrophic organization of LMN in spinal cord

Answer 28

in ventral horn:
extremities are lateral and trunk is medial

(flexors posterior and extensors anterior??)

Question 29

tract that relays fibers from motor cortex to cranial nerve motor nuclei

Answer 29

corticobulbar tract

Question 30

tract that terminates in cervical and thoracic spinal cord levels to control coordinated movements of trunk and neck muscles

Answer 30

vestibulospinal and recticulospinal tracts

Question 31

travels closely with lateral corticospinal tract and participates in control of arm mucles

Answer 31

rubrospinal tract

Question 32

terminates in cervical spinal cord to control movements of the head and eye

Answer 32

colliculospinal (tecto-) tract

Question 33

lesions in the corticospinal tract would cause pts to develop weakness in (ipsilateral or contralateral) face and limbs

Answer 33

above pt of decussation = contralateral
below pt of decussation = ipsilateral

pt of decussation = caudal medulla, pyramidal decussation

Question 34

effect of corticospinal tract lesions on local spinal reflexes

Answer 34

they go unchecked and are therefore hyperactive

→ increased muscle tone and reflexes

Question 35

causes fasciculations (why?)

Answer 35

LMN lesions bc the muscle cell membrane becomes hyperexcitable and may spontaneously discharge → fasciculations

Question 36

lesion that blocks the release of trophic proteins to the muscle, leading to atrophy

Answer 36

UMN and LMN lesions

Question 37

Positive Babinski sign looks like… and indicates….

Answer 37

stroking lateral sole of foot will cause extension of big toe and fanning of other toes in a person with a lesion of the UMN in the corticospinal tract

Question 38

babies will have a positive babinski sign until age…

Answer 38

3 to 6 months

Question 39

+ Clonus looks like … and indicates…

Answer 39

when the foot is forcefully flexed, the foot will engage in repetitive plantar extension. this indicates and UMN lesion bc it is leaving the muscle stretch reflex unchecked and hyper active