BIOMED 10/23b SC Anatomy II

Question 1

receptor that senses change in muscle length is

Answer 1

intrafusal muscle fibers in muscle spindle

Question 2

Examining a patient who shows marks of burn on the left foot. You test his sensations and find that he can not feel pinprick and hot/cold on the plantar as well as dorsal aspect of the Left foot. Proprioception is in tact. Lesion may involve:

Answer 2

• If dorsal root is cut, lose all sensations (proprioception as well), so not dorsal root
• Right anterolateral pathway in the spinal cord
• Utilize the spinal cord location to determine the side of the injury

Question 3

If left proprioception is off, but pain and temperature are normal

Answer 3

Left DCML tract issue

Question 4

Cut dorsal root yields

Answer 4

no sensation at all from the same side

Question 5

cut DCML

Answer 5

• all proprioception and vibration gone on the same side

- Temperature and pain gone on the other side

Question 6

descending tracts include

Answer 6

lateral
medial
direct
indirect

Question 7

medial system of descending tracts entails

Answer 7

anti-gravity
Descend in the anteromedial column of the spinal cord

□ Anterior corticospinal tract
□ Vestibulospinal tracts (vestibular nuclei to spinal cord)
□ Reticulospinal (reticulum to spinal cord)tracts
□Tectospinal tract(midbrain/tectum to spinal cord)

Question 8

lateral systems of descending tracts include

Answer 8

goal directed, fine motor control
Descends in the lateral column (funiculus) of the spinal cord

□ Lateral corticospinal tract - cortex to spinal cord
□Rubrospinal tract - nuclei from which these start (red nucleus to spinal cord)

Question 9

direct system of descending spinal tract

Answer 9

project from the cerebral cortex to the spinal cord directly

Cerebral cortex -> descend at level of spinal cord

Two main:

1. lateral corticospinal tract (paramatro)
2. anterior corticospinal tract

Question 10

what does the lateral corticospinal tract do?

Answer 10

1. Arises from the cerebral
cortex and project to the
spinal cord
2. crosses at the medulla (80%)
3. descends at the lateral funiculus and at spinal cord synapses either directly on anterior horn cells or interneurons

Question 11

what do the indirect systems of the descending spinal tracts do?

Answer 11

project to the brainstem nuclei and then axons of the brainstem nuclei project to
the spinal cord

Question 12

what is the homunculus?

Answer 12

region of the cerebral cortex that is like a map of the muscles of the body

• the maps are not static

Question 13

can the homonculus be reorganized?

Answer 13

Yes! if you practice and train different body parts, the homunculus changes

Question 14

what are the main regions of the cerebral cortex?

Answer 14

1. primary motor (homunculus lives there)
2. premotor
3. supplementary motor
4. somatosensory motor

Question 15

what instance would be monosynaptic for the lateral corticospinal tract?

Answer 15

for fine motor muscles (fingers/hands/feet/NOT ELBOW); corticospinal axon moves directly from white matter to the anterior horn cells that are supplying the muscle (ONLY HERE)

Question 16

how does the anterior corticospinal tract work?

Answer 16

1. Arises from the cerebral
   cortex and project to the
   spinal cord
2. does NOT cross at the medulla (20%)
3. descends at the anterolateral funiculus and at spinal cord synapses either directly on anterior horn cells or interneurons
4. axons leave funiculus and bifurcate
   -trunk muscles on same and opposite sides
   -proximal muscles on the opposite side (hand muscles)

Question 17

what are the upper and lower motor neurons that are in the lateral and anterior CST?

Answer 17

§ Anterior horn cells - lower motor neuron sitting within spinal cord, axons project down to the muscle
§ Upper motor neuron - neurons from corticospinal cortex and descend down to spinal cord
§ Lesions have very different effets

Question 18

What is a motor unit?

Answer 18

motor neuron
muscle fiber
1. small
2. medium
3. large

Question 19

what are the small muscle fibers?

Answer 19

slow (S) - type I, slow oxidative muscle fibers

• low threshold
• quick activation

Question 20

what are the medium muscle fibers?

Answer 20

Fast Fatigue Resistant (FFR) - type IIa, fast oxidative glycolytic (FOG)

• fast fatigue resistant
• higher threshold than small

Question 21

What are the large muscle fibers/motor units?

Answer 21

fast fatigue (FF) - type IIx, Fast glycolytic
-high threshold

Question 22

what is the size principle of motor units?

Answer 22

first recruit smaller motor units, then as you need more and more motor force, there are two things that occur:

1. small motor units increase firing frequency
2. larger motor units begin to slowly get recruited and increase firing frequency

• -all motor units get activated when high forces are required
• -both occur in parallel

Question 23

if you were to have a lesion on the left corticospinal region, what would happen?

Answer 23

weakness is predominant in the right hand, but not as much in the trunk

Question 24

what does the lateral CST predominantly innervate?

Answer 24

• contralateral anterior horn
cells
• AHCs projecting to the
distal UE and LE
• For hand muscles,
innervation is
monosynaptic

Question 25

what does the anterior CST predominantly innervate?

Answer 25

• contralateral and
ipsilateral anterior horn
cells via interneurons
• AHCs projecting to the
proximal muslces
• Synapses on interneurons
that project to the AHCs.

Question 26

what are the types of AHCs or lower motor neurons?

Answer 26

– Alpha MN- go to the extrafusal muscle fibers
(skeletal muscle fibers)
– Gamma MN- go to the intrafusal muscle fibers of
the muscle spindle

Question 27

indirect descending pathways

Answer 27

project to the brainstem (midbrain, pons, medulla) nuclei and then axons of the brainstem nuclei project to the spinal cord

Cerebral cortex -> synapse at brainstem -> synapse at level of spinal cord

Question 28

Rubrospinal Descending Motor Tract

Answer 28

– Crosses immediately and descends in the
lateral funiculus
– Most rubrospinal fibers terminate at cervical
and thoracic levels, but some extend to all
cord levels,
– have an inhibitory effect on extensor
muscles and an excitatory effect on the
flexor muscles
– Alternate pathway to the spinal cord

Question 29

reticulospinal descending motor tract

Answer 29

Important in the sense that it plays a significant role in maintenance of muscle tone/spasticity after stroke/SCI

1. Medial (pontine) - facilitates spinal circuits has intrinsic excitability
2. Lateral (medullary) - inhibits extensor tone relies on cortical input for its function

Question 30

vestibulospinal tracts

Answer 30

Indirect descending

1. lateral vestibulospinal tract
2. medial vestibulospinal tract

Question 31

lateral vestibulospinal tract

Answer 31

– Arises from the lateral
vestibular nucleus
– Descends ipsilaterally in
the anterior funiculus
– Increases extensor
motor neurons
controlling antigravity
muscles

Question 32

medial vestibulospinal tract

Answer 32

– Arises from the medial
and inferior vestibular
nuclei
– Descends bilaterally to
synapse on cervical and
upper thoracic spine
– Maintains to stabilize the
head in relation to the
body

Question 33

spinal cord arrangement of white matter

Answer 33

funiculus -> lateral -> Rubrospinal -> anterolateral

Question 34

spinal reflexes

Answer 34

• Spinal cord reflexes are central nervous
system (CNS) pathways that lie entirely within
the spinal cord.
• The sensory afferent fibers that evoke these reflexes enter the spinal cord and activate spinal motor neurons directly or indirectly through a chain of one or more spinal interneurons.

Question 35

stretch reflex

Answer 35

• Input from periphery to center -> alpha motor neurons and then activating the muscle
• Sensory organ = muscle spindle (stretch on intrafusal muscle fibers go to central ns and activate alpha motor neurons and get a contraction while causing inhibition of antagonistic muscle)