Motor Systems- Spinal Cord Flashcards
What are the main functions of the motor cortex? (3)
- To receive sensory input and translate that into a plan for movement.
- It controls fine motor control
- Motor learning
What are the five main components of the motor system and what is the role of each?
- Spinal cord- final output of signal to the muscles
- Brainstem nuclei- postural and motor control of trunk, head and neck
- Motor cortex- translates sensory information into motor plan and execution. Fine motor control and motor learning site
- Cerebellum- coordination and motor learning. Planned movement is “quality checked” with executed movement
- Basal ganglia- overall level of motor activity is set. “go/don’t go”
What are the main functions of the brainstem nuclei for motion?
- Postural
2. Movement of head, neck and trunk
What are the two main functions of the cerebellum?
- Coordination and motor learning
2. Quality check to make sure the plan for movement matches the execution of movement
What is the main function of the basal ganglia in terms of motor activity?
Control center where overall level of motor activity is set (“go or don’t go”)
What is the “final common pathway” of the motor system?
Motorneurons in the brainstem and spinal cord
What is a motoneuron pool?
Where is the motoneuron pool arranged?
All the motoneurons innervating a single muscle.
The pool is arranged in a column within the Vental horn of the spinal cord over several spinal levels.
What rexed lamina contain motoneuron pools?
9
How are proximal, distal, flexor and extensor muscles arranged in the spinal cord?
Proximal muscles are medial to distal muscles
Flexors are above (more dorsal to) extensors in the ventral gray
How were lower motor neurons localized to the ventral horn of the spinal cord?
Motor neurons were labeled with retrograde transport tracer
What is a motor unit?
A motoneuron and the muscle unit (matched extrafusal fibers) that it innervates
How many muscles fibers does one motoneuron innervate?
It depends on the properties of the motor unit.
Large limb muscles can have 1000 muscle fibers to one motoneuron. Whereas fine control oculomotor muscles may have a motor unit with less than 10 fibers.
How are motor unit types characterized?
By speed of contraction and fatigability which are functions of the myosin isotope and metabolic enzyme profile
What is the contraction speed, fatigability and force for:
- S fibers
- FR fibers
- FF fibers
- Slow, fatigue resistant, low force
- Fast, fatigue resistant, medium force
- Fast, fatigable, most force
Can a motor unit consist of multiple muscle fiber types?
No- the muscle fiber type is uniform within a motor unit
How can muscle fiber type be modified?
By the pattern of motor neuron firing activity
Which muscle fiber type has the lowest threshold for activation?
S then FR then FF
What is motor unit plasticity?
Changes in motor neuron firing activity will induce changes to the muscle fiber type
What are the two ways to increase levels of total muscle force?
- Recruiting more motor units
2. Increasing the firing rate of the motor neurons of the active units
What would you expect to see if the firing rate of the motor neuron was :
- 5Hz
- 20Hz
- 80Hz
- 100Hz
- Single muscle twitches
- Temporal summation
- Unfused tetanus
- Fused tetanus
As the firing neuron approaches its highest rate, the mechanical response of the fiber is doing what?
Approaching fused tetanus
What are the size of the motoneurons innervating: S-fibers FR-fiber FF-fiber How does this affect recruitment order? What is this principle called?
Smallest to largest.
Because S motoneurons are the smallest, they will have the most resistance.
Because v =ir, for the same stimulus, it will have a larger change in voltage and will depolarize first.
This is called the size principle for motoneuron recruitment
At what current will the motoneuron begin to fire repetitively?
1.5x the synaptic current necessary to cause recruitment
As the synaptic current increases, what happens to the individual motor neuron and the neurons around it?
The frequency of firing of the neuron that was already recruited increases.
Other motor neurons are recruited.
What is a lower motor neuron?
What is an upper motor neuron?
A motoneuron of the spinal cord or brainstem that directly innervates a muscle unit
A nerun that projects to the LMN pool in the ventral horn
What inherited disorder strictly effects LMNs?
What infections?
What inherited disorder affects BOTH UMN and LMN?
- Spinal muscular atrophy
- Polio, west Nile
- Amyotrophic lateral sclerosis (ALS-Lou Gherigs)
What are the four major hallmarks of a LMN deficit?
- Focal atrophy and wasting of the muscle
- Fasciculation (involuntary small twitching)
- Weakness with decreased tendon reflex
- EMG abnormalities: fibrillation potentials , altered motor unit recruitment pattern with huge motor units (loss of fine manipulation)
What is capable of modulating spinal reflex?
What is the tendon reflex typically used for?
Why is the tendon reflex a good test for the neurological exam?
- Descending supraspinal control
The tendon reflex usually is a reliable input-output test of function that does not require volitional control
It is a good test because it does not rely on subjective reporting of sense or level of effort for motion. You can perform these reflexes on a patient in a coma
If you record AP after initiating the bicep tendon reflex, what would you see for:
- Sensory neuron
- Interneurons
- Motor flexor
- Motor extensor
- AP
- Activation to fire an inhibitory signal on the antagonist muscle
- AP
- Hyperpolarization because the tricep will be inhibited by the interneuron
What are the muscle fibers within the muscle spindle called?
What type of neurons innervate these fibers?
Intrafusal fibers and are innervated by gamma motor neurons
What are the force generating muscle fibers called?
What neurons innervate them?
Extrafusal fibers and they are innervated by alpha motor neurons
What is that arrangement of intrafusal and extafusal muscle fibers?
What is the arrangement of GTO to extafusal fibers?
They are in parallel so the spindles can optimally sense stretch of the muscle
GTO are in series with the extafusal fibers to detect the total force (tension) acting on the muscle
What are the afferents from muscle spindles?
What are the afferents from Golgi tendon organs?
Ia proprioception fibers that detect stretch of the muscle
Ib proprioception fibers that detect tension on the tendon
What is the muscle spindle sensitive to? Mechanical orientation? Afferents? Efferents? Action on the agonist? Action on the antagonist?
Length Parallel Ia and II fibers gamma motor neurons Excitation Inhibition
What is the GTO sensitive to? Mechanical orientation? Afferents? Efferents? Action on agonist? Action on antagonist?
Tension/force Series Ib fibers None Inhibition Excitation
How does the GTO inhibit the agonist muscle?
Activation of the Ib inhibitory interneuron that protects the muscle from injury
What mediates polysynaptic spinal reflexes?
Cutaneous afferents
What is the flexor reflex/crossed extensor reflex?
What does it allow coordination of?
What would the afferent stimuli be?
Interneurons in the spinal cord not only have agonist/antagonist pairings for specific mono synaptic reflexes, but they can also cross the body of the spinal cord to coordinate with motor neuron pools on both sides of the cord.
When painful stimuli is applied to the right foot, flexors in the right leg are activated and extensors are inhibited so you can pull up your leg away from the pain. At the same time the left Flexors will be inhibited and left extensors will be activated so you don’t fall
At what CNS level is the polysynaptic spinal reflex mediated?
Spinal cord level (no brain input necessary)
Where is the circuitry for locomotion located?
How is it connected?
In the lumbar enlargement of the spinal cord and it connects to a similar cervical enlargement by the fasciculus proprius
What tasks need to be accomplished by neural circuitry in the spinal cord to coordinate locomotion?
What carries out these functions?
- Flexors and extensors (tendon stretch reflex)
- Movement between right and left limbs (crossed extensor Reflex)
- Movement between upper and lower limbs
- Regulation of rhythm and speed for stepping
- Changing pattern of limb movement (forward to backward)
These tasks are accomplished by central pattern generators made of neural circuits (either rhythm generator interneurons or pattern network)
What are the two main types of central pattern generators?
What can influence these pathways?
- Rhythm generators
- Pattern network
These pathways can be influenced by inputs from spinal cord, supraspinal level, periphery, or by drugs and neurotransmitters
