Lectures 9 & 10 - Somatic Motor System Flashcards
(105 cards)
1
Q
What does the somatic motor system control?
A
Skeletal muscle
2
Q
Where do our lower motor neurons emanate from?
A
- Ventral horns of the spinal cord
2. Brainstem
3
Q
What are the 3 sources of input for LMNs? What is this called?
A
- Spinal interneurons
- Sensory neurons from muscle spindles
- Upper motor neurons in the brain
Final Common Pathway
4
Q
What are the 2 types of lower motor neurons? What does each innervate?
A
- Alpha: extrafusal muscle fibers
2. Gamma : intrafusal muscle fibers
5
Q
What is the function of gamma motor neurons?
A
Gain function in motor reflexes
6
Q
What is another name for a myotatic reflex?
A
Stretch reflex
7
Q
What motor neurons do myotatic reflexes involve?
A
Both alpha and gamma
8
Q
Describe the 4 steps of a myotatic stretch.
A
- Perception of stretch of intrafusal muscle fibers by muscle spindle
- Afferent signal sent through Ia sensory axons to spinal chord
- Ia sensory neurons synapse with homonymous alpha motor neurons and inhibitory interneurons for the antagonist muscle at the ventral horn
- Alpha motor neurons secrete ACh to cause contraction of the extrafusal muscle fiber
9
Q
What is the role of the gamma motor neurons in myotatic reflexes?
A
TBD
10
Q
What kind of neurons are LMNs? Purpose?
A
Multipolar
It receives input from several different sources
11
Q
What does a motor unit consist of? What kind of property is this?
A
Alpha motor neuron + all of the muscle fibers it innervates
Divergent property
12
Q
What does a motor neuron pool consist of?
What kind of property is this?
A
All of the alpha motor neuron innervating a particular muscle
Convergent property
13
Q
According to which 2 characteristics do we classify motor units?
A
- Size = power (contractile force)
2. Type of muscle fiber: red or white
14
Q
What are the 2 types of motor units classified by size? Describe each
A
- Small = small alpha motor neuron + small diameter muscle fibers = low contractile force
- Large = large alpha motor neuron + large diameter muscle fibers = high contractile force
15
Q
What are the 2 types of motor units classified by type? Describe each
A
- Red: slow twitch due to slow myosin ATPase and low fatigability due to oxidative metabolism
- White: fast twitch due to fast myosin ATPase and high fatigability due to glycolytic metabolism
16
Q
How does the size of a motor unit affect its EPSP threshold? What does this mean?
A
Larger motor units have a higher EPSP threshold, meaning they contract with more APs than smaller motor units
17
Q
What 5 characteristics of alpha motor neurons INCREASE with increased motor unit size?
A
- Cell body size
- Dendritic complexity
- Short-term EPSP potentiation with repeated activation
- Axonal diameter
- Number of axonal branches
18
Q
What 5 characteristics of alpha motor units DECREASE with increased motor unit size?
A
- Input resistance
- Excitability
- Ia EPSP amplitude
- PSP decay constant
- Duration of after-hyperpolarization
19
Q
Do motor units exhibit plasticity?
A
YUP
20
Q
What particular experience shows that motor units have plasticity?
A
- Soleus muscle involved in postural control: longer time for motor unit to peak (red fiber) and long duration
- Gastrocnemius muscle involved in fast movement: shorter time for motor unit to peak (white fiber) and short duration
However, when chronically stimulating a gastroscnemius muscle, it learns to take longer to peak and last longer: can use oxidative phosphorylation
21
Q
What does the after-hyperpolarization of a neuron refer to?
A
The undershoot phase
22
Q
How does training affect motor units?
A
- Increase firing rate
2. Increase torque
23
Q
What are the 3 types of motor units based on both size and muscle fiber type?
Characteristics for each:
- Size/diameter?
- Strength?
- Type?
- Mito?
- Capillaries?
- Mb?
- ATP synthesis main pathway?
- Main function?
A
- Slow motor units: small, weak, red fibers, many mito, many capillaries, high Mb, oxidative phosphorylation, and are used for sustained contractions (eg: posture)
- Fast fatigable motor units: large, strong, white, few mito, few capillaries, low Mb, glycolysis, and used for sprints, jumping and other fast movements
- Fast contracting fatigue-resistant motor units: medium size and force, red/pink so fatigue resistant (intermediate rate of fatigue) AND fast contracting, many mito, many capillaries, high Mb, oxidative phosphorylation
24
Q
What are the 3 factors regulating contractile force of a motor unit?
A
- LMN firing rate
- % of active motor units
- Size of active motor units
25
What type of motor units for walking?
Fast contracting fatigue-resistant motor units
26
What is the size principle?
Recruitment of motor units increases the force available
27
How are motor neuron pools organized in the spinal cord?
- Axial, proximal, and flexor muscle control is closer to central canal
- Distal and extensor muscle control is closer to the ventral horns
28
Where are LMN that control proximal and distal muscles mainly found in the spinal cord?
Cervical and lumbosacral spinal cords
29
Where are LMN that control axial muscles mainly found in the spinal cord?
Thoracic spinal cord
30
What does the organization of motor pools allows us to predict?
Weakness from location of ventral horn injury and vice versa
31
How have we been able to figure out motor neuron pools organized in the spinal cord?
Dye injected in muscle will travel back to the spinal cord and dye the area of the grey matter that controls it
32
What are the 2 types of spinal reflex pathways? Explain each
1. Monosynaptic pathway: no interneuron
| 2. Polysynaptic pathway: interneuron involved
33
What is an example of a polysynaptic reflex pathway?
Pain withdrawal reflex
34
What is an example of a monosynaptic reflex pathway?
Patellar tendon reflex
35
What is the definition of a reflex?
Communication between afferent and efferent pathways that do not require conscious thought
36
What is the basic explanation of a myotatic reflex?
Pull on a muscle, it will contract to pull back
37
Is the myotatic reflex considered monosynaptic or polysynaptic? What to note?
Monosynaptic
However the inhibition of the antagonist muscle does involve an inhibitory interneuron
38
What are muscle spindles?
Sensory receptors on the intrafusal fibers within the belly of a muscle that primarily detect changes in the length of this muscle
39
How does the muscle spindle affect the rate of firing of the Ia afferent neuron?
Relaxed: low firing frequency
Stretched: high frequency of firing
40
How does the firing of the Ia afferent neuron due to muscle spindle stretching evolve over time?
It dissipates over time
41
How does the reverse myotatic reflex function? Explain how it works.
Opposes contraction
Golgi tendon organ (nerve endings connecting muscle and tendons) sends information via the Ib afferent neuron which synapses with an inhibitory interneuron in the spinal cord to oppose contraction when you lift something that is too heavy for you
42
Activity of muscle spindle and Golgi tendon when the muscle is passively stretched?
1. Muscle spindle: high firing of Ia afferent neuron
| 2. Golgi tendon: low firing of Ib afferent neuron
43
Activity of muscle spindle and Golgi tendon when the muscle is actively contracted?
1. Muscle spindle: low firing of Ia afferent neuron
| 2. Golgi tendon: high firing of Ib afferent neuron
44
What is the cross extension reflex?
Reflex that provides structural support during a flexion reflex of the opposite leg
45
What function of somatic control does the basal gaglia have?
Initiation
46
What function of somatic control does the association cortex have?
Planning
47
What function of somatic control does the cerebellum have?
Coordination and refinement
48
Compare damage to UMNs and LMNs.
- UMNs: increased muscle tone, decreased strength, increased stretch reflexes (hypereflexia), and moderate atrophy
- LMNs: decreased muscle tone, decreased stretch reflexes (hyporeflexia), pronounced weakness, and severe atrophy
49
What are the 3 brain structures involved in somatic control that send information to the UMNs? Which ones first send information to the thalamus?
1. Basal ganglia**
2. Association cortex
3. Ceberellum**
50
What are the 6 signs/symptoms of LMN damage?
1. Paresis
2. Flaccid paralysis
3. Hyporeflexia
4. Visible fasciculations
5. Fibrillations (non-visible)
6. Muscle atrophy
51
What is paresis?
Weakness
52
What are fasciculations?
Pathological spontaneous depolarizations of single LMNs in a motor unit due to sick but not yet dead LMNs
53
How are muscle fibrillations detected?
EMG = Electromyography
54
What are muscle fibrillations?
Pathological spontaneous depolarizations of individual muscle fibers with dead or absent LMNs
55
What does the lateral white matter of the spinal cord contain?
Axons from the motor cortex
56
What does the medial white matter of the spinal cord contain?
Axons from the brainstem
57
Do UMN of the cerebral cortex cross the midline?
YUP
58
What do UMNs from the cerebral cortex control? Describe the pathway.
Lateral white matter UMNs => LMNs in the lateral ventral horn => distal muscles
59
What are distal limb muscles used for?
Skilled movements
60
What do UMNs from the brainstem control? Describe the pathway.
Medial white matter UMNs => LMNs in the medial ventral horn => axial, proximal, and flexor muscles
61
What are axial and proximal limb muscles used for?
Posture and balance
62
What is a Brodmann's area?
Area of the cerebral cortex
63
What does the central sulcus of the brain divide?
Pre and post central gyrus
64
How do cortical UMNs communicate with the rest of the body?
Descending axons that form pyramidal tracts:
1. Corticospinal tract
2. Corticobulbar tract
65
What does the Brodmann area 6 include? Where is it located?
Secondary motor cortex:
1. Premotor area
2. Supplementary motor area
Rostral to Area 4
66
What does the Brodmann area 4 include? Where is it located?
The primary motor cortex
Between area 6 and the central sulcus
67
What are Betz cells? Where are they found?
Largest UMNs in M1
Layer 5
68
How many layers does the primary motor cortex have?
6
69
What is the association motor cortex?
Cerebral cortex outside the primary areas
70
What is the function of M1?
Execute movement
71
What is the function of the secondary motor cortex?
To plan movements and the organization of complex multi-joint movements
by priming LMNs
72
Role of corticobulbar tract?
Connects UMNs to LMNs in the brainstem and controls motor output of non-ocular motor cranial nerves (aka not CN III)
73
Role of corticospinal tract?
Conducts impulses from the brain to the spinal cord LMNs to control lateral pathways
74
What determines which areas of the cerebral cortex take up more space on the spatial map relating the brain to the body's musculature?
Muscles necessary for fine motor control take up more space
75
What are the 3 main cranial nerve nuclei targets of the corticobulbar tract? Ipsilateral, contralateral, or bilateral?
5, 7, and 12:
1. Trigeminal nucleus
2. Facial nucleus
3. Hypoglossal nucleus
ALL BILATERAL
76
What is pyramidal decussation? What does this mean?
Crossing over of the corticospinal tracts that occurs where the medulla meats the spinal cord
Superior to decussation: contralateral control
Inferior to decussation: ipsilateral control
77
From where does the premotor area receive input? Main function?
From visual cortex and posterior parietal cortex
Coordinates movements to respond to external cues
78
Main function of supplementary motor area?
Coordinates movements (especially sequenced ones) to respond to internal cues
79
Does corticospinal activity correlate with muscle force?
YUP
80
Which activity happens first: M1 or secondary motor cortex?
Secondary motor cortex
81
What is apraxia?
Inability to execute skilled movement on cue (without paralysis)
Due to SMA or PMA damage
82
When are AP firing and where when you observe an activity and try to reproduce it?
In the PMA: both during observance and execution
83
What are extrapyramidal tracts?
UMN descending axons from the midbrain to the spinal cord that represent an indirect pathway to communicate with LMNs
84
What is the Babinski sign?
This test is run on infants to test for normal development. If you run a probe from bottom to top on plant of foot, you should witness toe flexion.
Toe extension (fanning up) is a positive result and indicates UMN damage or poor development.
85
What are the 4 extrapyramidal tracts? Does each connect to lateral pathways or ventromedial pathways?
1. Colliculospinal tract: ventromedial pathways
2. Rubrospinal tract: lateral pathways
3. Reticulospinal tract: ventromedial pathways
4. Vestibulospinal tract: ventromedial pathways
86
What is spastic paralysis?
Increase in muscle tone due to UMN damage
87
Role of colliculospinal tract? Location?
Connect superior colliculus (in tectum) to LMNs in cervical cord to control the head movement in response to visual stimuli
Right next to anterior corticospinal tract
88
Role of rubrospinal tract? Location?
Connect red nucleus in pons to LMNs in cervical cord to control upper limb movement (arms)
Right next to lateral corticospinal tract
89
Role of reticulospinal tract? Location?
Connect reticular formation to LMNs at all levels of the spinal cord for control of posture and locomotion
Right next to anterior corticospinal tract
90
Role of vestibulospinal tract? Location?
Connect vestibular nuclei to LMNs at all levels of the spinal cord for control of posture and locomotion
Right next to anterior corticospinal tract
91
Are large or small lesions of M1 more common?
Large lesions
92
What part of the body will lesions to M1 affect?
Contralateral portions
93
Primary Lateral Sclerosis (PLS): UMN or LMN damage?
UMN
94
Amyotrophic Lateral Sclerosis (ALS): UMN or LMN damage?
BOTH!
95
What is clonus?
Spasticity in feet of patients with UMN damage where the feet will uncontrollably flex the foot after the doctor dorsoflexes it
96
What are primary afferent sensory nerves?
Sensory nerves from the intrafusal fibers to the spinal cord
97
Why do alpha neurons of larger motor units have a decreased PSP decay constant?
Because they last longer
98
Does the Ia afferent neuron fire signals when the muscle is contracted?
NOPE
99
Which will fatigue faster: slow motor units or fast fatigue-resistant motor units?
Fast fatigue-resistant motor units
100
Where is the medial white matter of the spinal cord?
In between the ventral horns
101
Are cranial nerves LMNs or UMNs?
LMNs
102
What are the 2 types of corticospinal tracts? What does their names indicate?
1. Lateral
2. Anterior
Indicate where they are located in spinal cord
103
Where are UMNs located?
1. Motor cortex
| 2. Brainstem
104
What are the 3 parts of the motor cortex?
1. Primary Motor Cortex
2. Premotor Cortex or Area
3. Supplementary Motor Area
105
Where do the UMNs that innervate the cranial nerves come from?
Motor cortex
