Descending pathway Flashcards
(127 cards)
1
Q
What are the two traditional classifications of descending motor pathways?
A
Pyramidal and extrapyramidal tracts
2
Q
What do pyramidal tracts pass through?
A
Pyramid in the medulla
3
Q
What tract is usually synonymous with pyramidal tracts?
A
Corticospinal tract
4
Q
What are extrapyramidal tracts?
A
Descending pathways that do not pass through the pyramid in the medulla
5
Q
What does pyramidal tract disease refer to?
A
Disease interrupting corticospinal tract fibers
6
Q
Name four extrapyramidal tracts
A
Reticulospinal, vestibulospinal, rubrospinal, tectospinal tracts
7
Q
Why is pyramidal-extrapyramidal classification inappropriate physiologically?
A
Not all corticospinal fibers pass through the pyramid
8
Q
Which corticospinal tract fibers bypass the pyramid?
A
Most fibers in the anterior corticospinal tract
9
Q
Do pyramidal tract signs in patients match isolated corticospinal tract lesion signs in animals?
A
No
10
Q
Where do extrapyramidal tracts originate from anatomically?
A
Brainstem
11
Q
What do extrapyramidal diseases clinically refer to?
A
Diseases of basal ganglia and cerebellum
12
Q
What is the major descending pathway involved in basal ganglia function?
A
Corticospinal tract
13
Q
How are descending pathways classified physiologically?
A
Lateral system and medial system pathways
14
Q
What is the basis for physiological classification of descending pathways?
A
Termination site and function
15
Q
Where do lateral system pathways descend?
A
Lateral column of spinal cord
16
Q
Name two lateral system pathways
A
Lateral corticospinal tract and rubrospinal tract
17
Q
Where are fibers of lateral system pathways located in spinal cord?
A
Lateral funiculus
18
Q
On which motor neurons do lateral system fibers terminate?
A
Lateral group of motor neurons
19
Q
Which muscles are innervated by lateral group motor neurons?
A
Distal limb muscles
20
Q
Function of lateral system pathways?
A
Regulation of skilled voluntary movements
21
Q
Where do medial system pathways descend?
A
Medial and anterior columns of spinal cord
22
Q
Name four medial system pathways
A
Reticulospinal, vestibulospinal, tectospinal, anterior corticospinal tracts
23
Q
Where are fibers of medial system pathways located?
A
Medial and anterior funiculi
24
Q
On which motor neurons do medial system pathways terminate?
A
Medial group of motor neurons
25
Which muscles are innervated by medial group motor neurons?
Proximal limb muscles and axial muscles
26
Function of medial system pathways?
Regulation of posture
27
Are corticospinal tract, pyramidal tract and upper motor neurons synonymous?
Not strictly
28
What are the two groups of corticospinal tracts?
Lateral and anterior corticospinal tracts
29
What percentage of corticospinal fibers form the lateral corticospinal tract?
0.8
30
What is the most important descending pathway for skilled voluntary activity?
Lateral corticospinal tract
31
What percentage of corticospinal fibers form the anterior corticospinal tract?
0.2
32
Function of anterior corticospinal tract?
Control of posture
33
From where do corticospinal fibers originate?
Primary motor cortex (area 4) and other cortical areas
34
Where are Betz cells located?
Fifth layer of precentral gyrus
35
How many Betz cells are present in the cortex?
About 30,000
36
How many axons are present in the corticospinal tract?
About 1 million
37
Name other cortical motor areas contributing to corticospinal tract
Premotor cortex, supplementary motor area, primary somatosensory cortex, parietal cortex
38
Motor cortex contributes how many fibers to corticospinal tract?
0.6
39
Sensory cortex contributes how many fibers to corticospinal tract?
0.4
40
Through which structure do corticospinal fibers converge?
Posterior limb of internal capsule
41
What is the radiating pattern of subcortical fibers called?
Corona radiata
42
After internal capsule, where do corticospinal fibers descend?
Cerebral peduncles of ventral brainstem
43
Where do corticospinal fibers pass through in the medulla?
Medullary pyramid
44
What percentage of corticospinal fibers decussate after the pyramid?
0.8
45
Where do decussated fibers descend?
Lateral funiculus as lateral corticospinal tract
46
What type of connections do lateral corticospinal fibers make?
Monosynaptic connections with anterior horn cells
47
Where do non-decussating corticospinal fibers descend?
Ipsilaterally in anterior funiculus as anterior corticospinal tract
48
Where do anterior corticospinal fibers cross?
At spinal cord segments during termination
49
Where do lateral corticospinal tract fibers terminate?
Lateral group of motor neurons in ventral horn
50
What percentage of lateral CST fibers terminate directly on motor neurons?
0.3
51
Where do 70% of lateral CST fibers terminate?
On interneurons
52
Function of lateral corticospinal tract?
Controls skilled voluntary movements
53
Where do anterior corticospinal fibers terminate?
On interneurons of same side
54
Where do interneurons from anterior CST terminate?
Medial group of motor neurons (opposite and same side)
55
Function of motor neurons innervated by anterior CST?
Supply proximal limb muscles and axial muscles
56
Primary function of motor cortex?
Initiation, planning and control of movement
57
Role of corticospinal tracts?
Transmit central command from motor cortex to spinal cord
58
What activities are impaired by lateral CST lesion?
Skilled voluntary activities like writing, painting
59
Isolated lateral CST lesion common in humans?
Very uncommon
60
What other tracts are usually affected with corticospinal tract disease?
Corticobulbar tracts
61
Is pure corticospinal tract disease seen in humans?
No
62
Effect of anterior CST lesion in animals?
Inability to maintain posture while walking, climbing
63
Why is postural deficit minimal in humans after anterior CST lesion?
Tract is underdeveloped and other pathways intact
64
Common site of corticospinal tract lesion?
Internal capsule (capsular lesion)
65
Why does capsular lesion cause severe deficits?
All fibers pass through narrow tunnel of internal capsule
66
Clinical result of complete interruption of corticospinal fibers?
Contralateral hemiplegia
67
Which ascending systems pass near internal capsule?
Basal ganglia and cerebellar systems
68
What other systems are affected in capsular lesions?
Extrapyramidal systems
69
What is pyramidal tract disease due to capsular lesion called?
Complete upper motor neuron paralysis
70
Most common cause of capsular lesion?
Rupture of Charcot's artery (lenticulostriate branch of MCA)
71
Alternate name for Charcot's artery?
Artery of cerebral hemorrhage
72
What percentage of intracerebral hemorrhages does Charcot's artery account for?
More than 60%
73
From where does rubrospinal tract originate?
Red nucleus in the midbrain
74
Inputs to red nucleus?
Motor cortex and cerebellum
75
When do rubrospinal fibers cross over?
Immediately after origin
76
Where do rubrospinal fibers descend?
Contralateral brainstem and lateral column of spinal cord
77
Termination site of rubrospinal tract?
Lateral group of motor neurons
78
Which muscles are innervated by rubrospinal tract?
Distal limb muscles
79
Effect of rubrospinal tract?
Excites flexors, inhibits extensors
80
Function of rubrospinal tract?
Controls skilled voluntary movements
81
Effect of rubrospinal tract lesion in animals?
Deficit in distal limb muscles, especially flexors
82
If lateral CST is intact after rubrospinal lesion, is deficit permanent?
No, deficit is temporary
83
From where do vestibulospinal tracts originate?
Vestibular nuclei
84
How many vestibular nuclei are there?
Four
85
Name the vestibular nuclei
Lateral, medial, superior, inferior
86
Sources of input to vestibular nuclei?
Otolith organs and semicircular canals
87
Function of vestibulospinal tracts?
Maintain posture in response to head position and acceleration
88
Other connections of vestibular nuclei?
Superior colliculi, cerebellum, reticular formation
89
Functions regulated via vestibular nuclei connections?
Eye position and body balance during movement
90
Origin of lateral vestibulospinal tract (LVST)?
Lateral vestibular nucleus (Deiter�s nucleus)
91
Course of LVST?
Descends ipsilaterally through brainstem and spinal cord
92
Where does LVST terminate?
Medial group of interneurons and motor neurons in ventral horn
93
Extent of LVST in spinal cord?
Entire rostrocaudal extent
94
Effect of LVST?
Excites motor neurons of proximal extensor muscles
95
Input to lateral vestibular nucleus?
Semicircular canals and otolith organs
96
Function of LVST?
Controls posture in response to angular and linear head acceleration
97
What happens to LVST in decerebrate animals?
Becomes hyperactive
98
Why does LVST become hyperactive in decerebrate animals?
Loss of cortical inhibitory control
99
Hallmark of decerebrate animals?
Extensor rigidity
100
Effect of brainstem lesion in humans on LVST?
Facilitates LVST activity, causes rigidity of neck, arms, legs
101
Origin of medial vestibulospinal tract (MVST)?
Medial vestibular nucleus
102
Course of MVST?
Descends ipsilaterally through brainstem and ventral funiculus
103
Extent of MVST?
Up to midthoracic spinal cord
104
Where does MVST terminate?
Medial group of interneurons and motor neurons
105
Function of MVST?
Adjusts posture and head position in response to angular acceleration
106
Where is the reticular formation located?
Core of brainstem
107
Sources of input to reticular formation?
Spinal cord, vestibular nuclei, cerebellum, hypothalamus, tectum, cortex
108
Main projections of reticular formation?
Cortex, thalamus, spinal cord
109
What motor neurons are influenced by reticulospinal tracts?
Gamma motor neurons
110
Two important reticular nuclei?
Nucleus reticularis pontis, nucleus gigantocellularis
111
What are the two reticulospinal tracts?
Pontine reticulospinal and medullary reticulospinal tracts
112
Function of reticulospinal tracts?
Control of posture
113
Origin of pontine reticulospinal tract?
Nucleus reticularis pontis oralis and caudalis
114
Course of pontine reticulospinal tract?
Descends ipsilaterally in medial funiculus
115
Termination of pontine reticulospinal tract?
Medial group of interneurons and motor neurons
116
Function of pontine reticulospinal tract?
Excites proximal extensor muscles, regulates posture
117
Origin of medullary reticulospinal tract?
Nucleus gigantocellularis in medulla
118
Course of medullary reticulospinal tract?
Descends ipsilaterally in ventral funiculus
119
Function of medullary reticulospinal tract?
Inhibits extensor motor neurons
120
Origin of tectospinal tract?
Superior colliculus (deep layers)
121
When do tectospinal fibers cross over?
Immediately after origin
122
Course of tectospinal tract?
Descends in ventral funiculus to midcervical spinal cord
123
Where does tectospinal tract terminate?
Medial group of interneurons and motor neurons
124
Is tectospinal tract ipsilateral or contralateral?
Contralateral
125
Smallest descending tract?
Tectospinal tract
126
Input to superior colliculus?
Visual stimuli
127
Function of tectospinal tract?
Regulates contralateral head movement in response to visual stimuli
