Neuromuscular Control Flashcards
Where are alpha motor neurons?
Occupy anterior horn - ventral horn of grey matter in spinal cord
What do the lower motor neurons of the brainstem and spinal cord inntervate? What does activation cause?
- extrafusal muscle fibres of the skeletal muscles
2. activation causes muscle contraction
What does the motor neuron pool contain?
all alpha motor neurons innervating a single muscle
What is a motor unit?
a single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force
How many motor neurons and skeletal muscle fibres do humans have?
- 420,000 motor neurons
- 250 million skeletal muscle fibres.
How many muscle fibres does each motor neurone supply on average?
600
What does the stimulation of one motor unit cause?
contraction of all the muscle fibres in that unit
What are the slow (S, type I) motor units?
- smallest diameter cell bodies
- small dendritic trees
- thinnest axons
- slowest conduction velocity
- For posture muscles, as slow long time
What are the fast, fatigue resistant (FR, type IIA) motor units?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
- Calf, high
What are the fast, fatiguable (FF, type IIB) motor units?
- larger diameter cell bodies
- larger dendritic trees
- thicker axons
- faster conduction velocity
- Fast twitch muscle in calf
Is one single motor neuron innervate one muscle fibre?
An individual neuron may innervate 10 or 100 of muscle fibres, none of muscle fibres will innervated be motor neuron 1 will be innervated by motor neurons 2 - one neuron always wins (at baby more and then one neuron wins)
Why do you want more motor neurons to innervate muscle fibres?
If one motor neuron innervates thousands of muscle fibres the control over that muscle is not as refined as 10 muscle fibres by one motor neuron - innervation ratio relates to function of muscle - small ratio, fine control muscle
How are the 3 different motor unit types classfied?
- amount of tension generated
- speed of contraction
- fatiguability
- Look at graphs
How does the CNS regulate the force that a single muscle can produce?
- Recruitment
2. Rate coding
How are motor units recruited?
Not random
By what principal are motor units recruited?
- Size principal
- Smaller units recruited first - generally slow twitch units
- As more force required, more units are recruited
- Slow then fast recurited
What does recruitment allow?
-Fine control - e.g. when writing, under which low force levels are required
At what frequencies to motor units fire at?
- Motor unit can fire at range of frequencies
- Slow units fire at lower frequency
What happens as the firing rate increases?
Force produced by the unit increases
When does summation of rate coding occur?
When units fire at frequency too fast to allow the muscle to relax between arriving at action potentials
When will a muscle waste?
Even when have arterial supply, need nerve supply otherwise will waste
What do motor neurons supply?
- Neurotrophic factors
- Prevent neuronal death
- Promote growth of neurons after injury
What are motor unit and fibre characteristic dependent on?
the nerve which innervates them
What happens in cross innervation?
If a fast twitch muscle and a slow muscle are cross innervated, the soleus becomes fast and the FDL becomes slow.
Can the motor neuron effect the properties of the muscle fibres it innervates?
Yes
Can fibre types change properties under different conditions?
Yes
What are some examples of fibre types changing conditions?
- Type IIB to IIA most common following training
- Type I to II possible in cases of severe deconditioning or spinal cord injury
- Microgravity during spaceflight results in shift from slow to fast muscle fibre types
How do fibre types change with ageing?
- loss of type I and II fibres but also preferential loss of type II fibres
- This results in a larger proportion of type I fibres in aged muscle (evidence from slower contraction times).
How are reflexes different from voluntary movements?
Can’t be stopped once started
What is a reflex?
- An automatic response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outward to an effector (as a muscle or gland) without reaching the level of consciousness.
- An involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli.
What is a monosynaptic (stretch) reflex?
- Sensory receptor
- Sensory neurone
- Motor neuron
- Motor neuron (to antagonist)
- Inhibition of antagonist or other
- Excitatory to muscle that needs to be contracted
What are different types of reflexes?
automatic (knee jerk) and stereotyped behaviours (sneeze, cough) in response to stimulation of peripheral receptors
Can reflexes be influenced?
- Try clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped. The reflex becomes larger
- Jendrassik manoeuvre
Why does Jendrassik manoeuvre happen?
reducing amount of inhibition the the brain brain and upper regions on CNS exert on reflexes under normal conditions - removes descending inhibition form the brain
What do the higher centres of the CNS exert on reflexes?
inhibitory and excitatory regulation upon stretch reflex
What type of control domination in normal conditions?
Inhibitory (brain effect on reflex pathways)
What does decerebration reveal?
The excitatory control from (descending) supraspinal areas - tonic contraction and higher reflex
When can rigidity and spasticity result?
Brain damage giving over active or tonic stretch reflex
Where do alpha neurons go out to?
Extrafusal muscles
What do gamma neurons do?
Innervate intrafusal muscle (sensitise the sensory organ - don’t contribute to contraction of musucle)
What action does descending (supraspinal) control have?
- Activating alpha motor neurons: by normal upper motors neurons lower motor neurons
- Activating inhibitory interneurons
- Activating propriosponal nuerons
- Activating gamma motor neurons
- Activating terminals of afferent fibres
What are intrafusal muscle fibres?
Altering sensitively of sensory organs in muscle so when stretched cause signal to go back to spinal cord to generate reflex contraction of muscle
What is hypereflexia?
Overactive reflex
What happens in hyper-reflexia?
Loss of descending inhibition
What is hyperfelxia associated with?
upper motor neuron lesions
What is clonus?
- Involuntary and rhythmic muscle contractions
- Hypereflexia
What is babinski’s sign?
1; When sole stimulated with blunt instrument the big toe:
-Curls downwards - normal
-Curls upwards – abnormal in adults. This is a positive Babinski sign.
3. Associated with upper motor neuron lesions
Note: Toe curls upwards in infants – this is normal
What is hypo-reflexia?
- Below normal or absent reflexes
- lower motor neuron disease
What happens in upper motor neuron lesion?
- Loss of voluntary movement - pathway lost
- Inhibitory control removed so hypereflexia
