Spinal Cord and Movement Control Flashcards
What are the 4 main components that contribute to the control of movement?
1- Spinal cord and brainstem ( LMN)
2- Descending systems (UMN)
3- Cerebellum
4- Basal Ganglia
Where are the 2 spinal cord enlargements?
cervical (for the arms) and lumbar (for the legs)
What is the cauda equina?
It contains all nerves to/ from the legs and pelvic floor
Where does the cauda equina begin?
At L1 (because that’s where the cord ends)
What type of neuron does this describe:
Large cell body, extensive dendritic tree, large axons and myelinated with schwann cells, rapid conduction velocity
Lower motor neurons
Where are lower motor neurons distributed?
They are distributed along the length of the spinal cord
What is found in the medial ventral horn?
Lower motor neurons to proximal muslces
What is found in the lateral ventral horn?
Lower motor neurons to distal muslces
What is the lower motor neuron pool?
Total of all lower motor neurons innervating a given muscle (it is usually distributed over 2-3 neurologic segments)
How is the spinal cord organized?
It is arranged from dorsal to ventral into various laminae (the organization is based on histological composition)
What composes a motor unit?
Cell body, axon, all the muscle fibers that the axon innervates
How many muscles does each axon innervate?
Axons only innervate 1 muscle
Where do Lower Motor Neurons synapse?
They synapse on muslce fibers (neuromuscular junctions)
What is the neurotransmitter at the site of lower motor neuron synapse?
Acetylcholine
Why are the neuromuscular junction connections described as “extremely reliable connections”?
Because every time the axon fires the muscle fiber will contract
What is the functional unit that generates force?
Sarcomere
What are the necessary components of force generation?
Ca2+ binding, rotation of myosin cross- bridge, consumption of energy (ATP)
What type of motor unit does this describe:
Slow rate of force increase during twitch and little or no force loss with repeated twitches
“S” or Slow motor units
What is the firing rate of S motor units?
60-120 ms
What type of motor unit does this describe:
Relatively fast rate of force increase during twitch and moderate force loss with repeated twitches
“FR” or Fast, fatigue resistant
What is the firing rate of FR motor units?
30-80 ms
What type of motor unit does this describe:
Fastest rate of force increase during twitch and rapid force loss with repeated twitches
“FF” or Fast, fatiguable
What is the firing rate of FF motor units?
20-40 ms
What is a “twitch”?
One single stimulus to the motor units axon
What is tetanus?
Repeated stimuli to the axon before the mechanical increase in force recovers to zero
Describe the force during a partially- fused contraction:
Total force varies a lot
What is the fatigue test?
A standardized pattern of electrical stimulation of 2 seconds of stimulation then 2 seconds of rest
During a fatigue test what happens to S, FR and FF motor units?
S- little to no force drop
FR- Eventual force drop
FF- Very rapid force drop
What are two ways to moderate force generation?
Recruitment and rate coding
What is recruitment
Recruitment is based on size principle. Smaller motor units are recruited first, then larger (previously silent) motor units are recruited.
Stronger inputs lead to recruitment of more powerful motor units
What is rate-coding?
This is an increase in the firing rate/ discharge of an already recruited motor unit. Can lead to a partially-fused output and even a fully fused force at highest firing rates
Which motor units have the higher excitability….smaller or larger?
Smaller because they are more easily recruited
Why is it important that recruitment is not random?
Because it would lead to a loss of movement precision and loss of function
What motor units are recruited for standing?
S
What are FR motor units recruited for?
Walking and running
What are FF motor units recruited for?
galloping and jumping
How are motor unit types determined?
1- muscle fibers are poly-innervated at birth
2- pruning of innervation
3- Coincident with motorneuron loss within spinal cord
4- availability of tropic factors
What is EMG?
Electromyogram
What is EMG typically used for?
Commonly used for diagnosis or neuromusclar disorders
Why are EMG signals easy to record?
Because the muscle unit of a single motor unit serves as a natural biogenic amplifier of a nerve action potential
What does electrode design influence?
1- volume of muscle recorded from
2- signal characteristics
3- frequency response
4- sensitivity to noise
What are the different types of electrodes that can be used for EMG?
Surface, needle, non-selective, highly-selective
