Neuromuscular Control Flashcards
What is an alpha motor neuron?
The lower motor neurons of the brainstem and the spinal cord
Occupy ventral (anterior horn) of grey matter of the spinal cord
What is the function of an alpha motor neuron?
Innervate the extrafusal muscle fibres of the skeletal muscles
Activation causes muscle fibre contraction
What is a motor neuron pool?
Motor neuron pool contains all alpha motor neurons innervating a single muscle
Define 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.
On average how many muscle fibres does each motor neuron supply?
600 muscle fibres
A muscle fibres is only innervated by one motor neuron
What does the stimulation of one motor unit cause?
contraction of all the muscle fibres in that unit
What happens during development to motor neuron?
Initially many neurons innervate a muscle fibre
Eventually one will win
1 neuron will innervate the muscle fibre
What increases when there are fewer muscle fibres that a neuron innervates?
More dexterity
Increased fine motor control
More refined
Small innervation ratio
What are the three types of motor unit?
Slow (S, type I)
Fast, fatigue resistant (FR, type IIA)
Fast, fatiguable (FF, type IIB)
What are the main features of type I motor units?
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
What are the main features of type IIA motor units?
larger diameter cell bodies larger dendritic trees thicker axons faster conduction velocity fatigue resistant
What are the main features of type IIB motor units?
larger diameter cell bodies larger dendritic trees thicker axons faster conduction velocity fatiguable
What are the three types of motor unit defined by (defining factors)?
amount of tension generated
speed of contraction
fatiguability
What are the two mechanism by which the brain regulates the force that a single muscle can produce?
Recruitment
Rate coding
What are the main features of recruitment?
Governed by the “size principle”.
Smaller units are recruited first (these are generally the slow twitch units).
As more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.
What are the main features of rate coding?
A motor unit can fire at a range of frequencies.
Slow units fire at a lower frequency.
As the firing rate increases, the force produced by the unit increases.
Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials
(S–>FFR–>FF)
What is an example of an S motor unit?
Those involved in posture (low force over an extended period of time)
What is the order of derecruitment?
Opposite to recruitment
What are neurotrophic factors?
Prevent neuronal death
Promote growth of neurons after injury
What are motor unit and fibre characteristics dependent on?
nerve which innervates them
If a fast twitch muscle and a slow muscle are cross innervated, the slow becomes fast and the FDL becomes slow.
Describe the main features of motor unit plasticity
Fibre types can change properties under many different 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
What is ageing associated with (motor unit changes)?
with 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)
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 the magnitude and timing of a reflex dependent on?
intensity and onset of the stimulus
Can a reflex be stopped once released?
No
Can reflexed 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.
Reduces the amount of inhibition the brain provides over reflexes
What do higher centres of the CNS provide over reflexes?
exert inhibitory and excitatory regulation upon the stretch reflex
Inhibitory control dominated in normal conditions
Decerebration reveals the excitatory control from supraspinal areas
What is the descending control of reflexes?
- Activating alpha motor neurons
- Activating inhibitory interneurons
- Activating propriospinal neurons
- Activating gamma motor neurons
- Activating terminals of afferent fibres
What are gamma motor neurons?
responsible for altering the sensory activity of the sensory organs that are housed within muscles
sensitise the organ
lower motor neurons
What are intrafusal fibres?
House the sensory organs
Tell the CNS the status of contraction of the muscle
What is hyper-reflexia?
Overactive reflex
Loss of descending inhibition
Associated with upper motor neuron lesion
What is clonus?
Involuntary and rhythmic muscle contractions
Loss of descending inhibition
Associated with upper motor neuron lesions
What is Babinski sign?
When sole stimulated with blunt instrument the big toe:
Curls downwards - normal
Curls upwards – abnormal in adults. This is a positive Babinski sign.
Associated with upper motor neuron lesions
Note: Toe curls upwards in infants – this is normal.
What is hyporeflexia?
Below normal or absent reflexes
Associated with lower motor neuron diseases
