Neuromuscular control Flashcards
Which muscle fibres convey sensory information about the status of the muscle.
Intrafusal muscle fibres
Where are the Alpha motor neurons, and what is their function?
Lower motor neurons of the spinal cord
Innervate the extrafusal muscle fibres (contractile element).
Activation causes muscle contraction
What contains all alpha motor neuron innervating a single muscle?
Motor neuron pool
Define Motor neuron
A single motor neuron together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force.
Stimulation of one motor neuron unit causes contraction of all the muscle fibres in that unit.
How many muscle fibres does one motor neuron supply?
600
How many neurons innervate a single muscle fibre?
Under normal circumstances, muscle fibres are only innervated by one neuron.
What are the 3 types of motor neuron?
Type 1 - Slow
Type 2a - Fast fatigue resistant
Type 2b - Fast fatiguable
What are the characteristics of the type 1 motor unit?
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
What are the characteristics of the type 2a motor unit?
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
What are the characteristics of the type 2b motor unit?
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
What are the different types of motor neuron unit classified by?
Amount of tension generated
Speed of contraction
Fatiguability
What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?
Recruitment
Rate coding
What happens in Recruitment?
Motor units are not randomly recruited. There is an order to this.
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 happens in 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.
What are Neurotrophic factors?
Are a type of growth factor
Prevent neuronal death
Promote growth of neurons after injury
What are motor unit & fibre characteristics dependent on?
The nerve that innervates them.
however motor neuron has some effect on the properties of the muscle fibres that it innervates.
Which type of muscle fibres change after training?
2b to 2a
What type of muscle change occurs in cases of severe deconditioning or spinal cord injury?
Type 1 to 2
What is the role of Ageing in muscle fibre changes?
Ageing associated 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).
Define reflex action
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.
What is the difference between reflexes & voluntary movements?
Once they are released, they can’t be stopped.
What is the Jendrassik manoeuvre?
Try clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped. The reflex becomes larger
How does the Jendrassik maneouvre work?
Reducing the amount of inhibition that the brain and upper regions of the CNS exert under normal conditions.
What is the role of Higher centres of the CNS in the stretch reflex?
Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex
What is the role of gamma motor neurons?
responsible for altering the sensitivity of sensory organs housed in muscles. organs that when stretched cause a signal to go back to the spinal chord.
What are the mechanisms of Descending (supra spinal) control of reflexes?
Activation of alpha motor neurons Activation of inhibitory interneurons Activation of proprio spinal neurons Activation of gamma motor neurons Activation terminals of afferent fibres
What is Hyper-reflexia?
Overactive reflexes
characteristic of upper motor neuron lesion
loss of descending inhibition
What is Babinski’s sign?
When the sole is stimulated with a blunt instrument, the big toe curls upwards. Associated with upper motor neuron lesions (toe curls upwards in infants normally).
What is Hyporeflexia?
Below normal or absent reflexes
Associated with lower motor neuron disease.
What is the hallmark of a pure corticospinal tract lesion?
Abdominal reflexes (superficial reflexes) absent while the other reflexes (deep tendon reflexes) are brisk
