Motor Neurons and Muscles Flashcards
(53 cards)
1
Q
What areas of the central nervous system are involved in control of movement?
A
- basal ganglia
- cerebellum
- descending systems (motor cortex and brainstem centers)
2
Q
What areas of the peripheral nervous system are involved in control of movement?
A
- local circuit neurons
- motor neuron pools
- sensory inputs and skeletal musccles
3
Q
What is the general motor function of the basal ganglia?
A
- gating proper initiation of movement
- suppresses unwanted movements, prepares upper MN circuits for movement initiation
4
Q
What is the general motor function of the motor cortex?
A
- planning, initiating and directing voluntary movements
5
Q
What is the general motor function of the brainstem centers
A
- basic movements and postural control
6
Q
What is the general motor function of local circuit neurons?
A
- lower motor neuron integration
7
Q
What is the general motor function of the cerebellum?
A
- sensory motor coordination of ongoing movement
- detects and attentuates “error” signals
8
Q
What is the spinal cord organized into?
A
- dorsal sensory and ventral motor regions
9
Q
Where do upper motor neurons from the brainstem innervate and why?
A
- both sides of midline
- lower motor neurons in medial ventral horn
- for axial and proximal limb muscles
- for posture, balance and coordination
10
Q
Where do upper motor neurons from the cerebral cortex innervate and why?
A
- lower motor neurons in lateral ventral horn
- distal limb muscles
- skilled movements
11
Q
What is the general motor function of upper motor neurons in the superior colliculus?
A
- to initiate orienting movements of head and eyes
12
Q
Where do lower motor neurons receive from and innervate on?
A
- send axons out of brainstem and spinal cord to innervate the skeletal muscles of head and body
13
Q
Where do local circuit neurons receive from and innervate on?
A
- receive input from upper motor neurons and sensory inputs
- innervate lower motor neurons
14
Q
Where do upper motor neurons synapse?
A
- mainly with local circuit neurons and sometimes lower motor neurons
15
Q
What are upper motor neurons involved in?
A
- the initiation of voluntary movements and spatio-temporal sequences of skilled movements
16
Q
Where does planning initiation and directing sequences of movements occur?
A
- several motor cortical areas including the primary motor cortex and pre-motor cortex
17
Q
Which parts of the spinal cord are for motor and for sensory?
A
- ventral spinal cord: motor
- dorsal spinal cord: sensory
18
Q
What is a motor unit?
A
- motor neuron and the fibers it innervates
19
Q
What is a motor pool?
A
- group of motor neurons that innervate a group of muscle fiber
20
Q
What experiment displays evidence for motor pools?
A
- retrograde labeling muscle to cord
21
Q
What is the relationship of number of fibers/motor neuron?
A
- less fibers/unit equals more control and fine movements
22
Q
Polyinnervated to mononeural?
A
?
23
Q
What influences contraction strength of a muscle?
A
- increased firing rate of motor neurons
- recruitment of additional motor units
24
Q
What are the different “stages” of contraction when you increase the firing rate?
A
- single muscle twitches
- temporal summation
- unfused tetanus
- fused tetanus
25
Do all motor units within the same muscle have the same properties?
- no, different properties adapted for fast ballistic movements or slower sustained contractions
26
What are the three types of motor units?
1. fast fatigable
2. fast fatigue-resistant
3. slow
27
What are the features of slow motor neurons?
- have small motor neurons that innervate a few fibers
- fibers contract slowly and generate small forces
- lots of mitochondria, myoglobin, and capillaries
- resistant to fatigue
- ex. eye muscles
28
What are the features of fast fatigue motor units?
- larger motor neurons and muscle fibers
- fewer mitochondria
- ex. gastroc (leg)
29
What are the features of fast fatigue-resistant?
- intermediate sized motor neurons and muscle fibers
- greater force than slow motor units
- less prone to fatigue than fast fatigue motor units
- ex. soleus muscle (back)
30
What is the somatotopic organization of the lower motor neuron pools in the spinal cord?
- proximal muscles medially in ventral horn
| - distal muscles laterally in ventral horn
31
Lower motor neurons receive inputs from?
- local circuit neurons
32
Where are short-distance local circuit neurons located in the ventral horn and why?
- located more laterally
- do not have collaterals to contralateral side
- fine control of distal extremities
33
Where are medial, long distance local circuit neurons located in the ventral horn and why?
- long axons with contralateral collaterals
| - help to coordinate left and right sides (posture control)
34
What is the pathway of reflexes (where does signal travel when knee tapped and leg kicks)?
1. muscle sensory receptor activated with tap innervates on interneuron in ventral horn and on motor neuron for extensor muscle
2. a) motor neuron for extensor muscle innervates extensor muscle
2. b) interneuron innervates motor neuron for flexor muscle, which inhibits the flexor muscle
35
Which motor neurons are used in the reflex pathway?
- alpha motor neurons
36
Which sensory receptor and axon are used for the reflex pathway?
- muscle spindle - group I and II
| - recall that they are fast
37
What two kinds of motor neurons do sensory afferents form synapses with?
1. excitatory synapses with alpha - motor neurons for same and complementary muscles
2. excitatory synapses with inhibitory interneurons that inhibit motor neurons of antagonistic muscles
38
What do gamma motor neurons do?
- adjust length of intralfusal fibers to maintain tension on sensory afferents
39
What do sensory afferents do?
- Ia stretch receptors setect "length" of intrafusal fibers
40
What are the steps in the pathway of stretch reflex circuitry?
1. descending facilitation and inhibition to alpha - motor neuron
2. alpha motor neuron to muscle to produce force required to hold glass
3. disturbance (adding liquid to glass) causes length change in muscle fiber
4. spindle receptor activates increase spindle afferent discharge to alpha motor neuron
41
How does motor neuron activity regulate muscle spindle responses?
- extrafusal muscle contraction results in intrafusal fibres becoming slack which takes away stretch in afferents
- now small lengthening or further shortening cannot be signaled
- gamma motor neurons contract intrafusal fibres to restore length so afferents can again be sensitive to further stretch or relaxation
42
What is the sensory afferent of the golgi tendon organ?
- Ib afferent
43
What is the afferent of the muscle spindle?
- Ia afferent
44
What is the difference in 'set-up' of the golgi tendon and spindle afferent?
- golgi tendons are in series with extrafusal muscle fibers
| - spindle afferents are in parallel with extrafusal muscle fibers
45
What happens to the rate of firing of spindle afferents and of golgi tendon afferents when the muscle contracts?
- spindle afferents stop firing because they slacken
| - golgi tendon afferents increase firing because they are in series
46
What do the 1b afferents do to avoid injury?
- inhibit the same muscle that creates tension in them via inhibitory interneurons
- excite anatagonistic muscle
47
What does damage to the lower motor neurons (spinal cord) result in?
- paralysis (loss of movement)
- paresis (weakness)
- areflexia (loss of spinal reflexes)
- hypotonia (loss of muscle tone)
48
What is muscle "tone" due to?
- the resting level of firing of alpha motor neurons
49
What is the major contributor to tonic level of firing?
- activity in muscle spindle afferents
50
What is hypotonia assessed by?
- judging the resistance of a limb to passive stretch
51
What does damage to descending pathways result in?
- hypertonia and spasticity
- chronic contraction and resistance to passive movement
- increased sensitivity of alpha motor neurons to spindle afferents
52
What is a spinalized cat?
- cat with severed spinal cord at mid back
53
How does a spinalized cat still manage to walk on a treadmill?
- central pattern generator in spinal cord can be activated by proproiceptive inputs (muscle stretch)
- legs dragged back and then response by lifting foot and swinging forward
