Neuromuscular control

Question 1

Which muscle fibres convey sensory information about the status of the muscle.

Answer 1

Intrafusal muscle fibres

Question 2

Where are the Alpha motor neurons, and what is their function?

Answer 2

Lower motor neurons of the spinal cord
Innervate the extrafusal muscle fibres (contractile element).
Activation causes muscle contraction

Question 3

What contains all alpha motor neuron innervating a single muscle?

Answer 3

Motor neuron pool

Question 4

Define Motor neuron

Answer 4

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.

Question 5

How many muscle fibres does one motor neuron supply?

Answer 5

600

Question 6

How many neurons innervate a single muscle fibre?

Answer 6

Under normal circumstances, muscle fibres are only innervated by one neuron.

Question 7

What are the 3 types of motor neuron?

Answer 7

Type 1 - Slow
Type 2a - Fast fatigue resistant
Type 2b - Fast fatiguable

Question 8

What are the characteristics of the type 1 motor unit?

Answer 8

smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity

Question 9

What are the characteristics of the type 2a motor unit?

Answer 9

larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity

Question 10

What are the characteristics of the type 2b motor unit?

Answer 10

larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity

Question 11

What are the different types of motor neuron unit classified by?

Answer 11

Amount of tension generated
Speed of contraction
Fatiguability

Question 12

What are the 2 mechanisms by which the brain regulates the force that a single muscle can produce?

Answer 12

Recruitment

Rate coding

Question 13

What happens in Recruitment?

Answer 13

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

Question 14

What happens in Rate coding?

Answer 14

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.

Question 15

What are Neurotrophic factors?

Answer 15

Are a type of growth factor
Prevent neuronal death
Promote growth of neurons after injury

Question 16

What are motor unit & fibre characteristics dependent on?

Answer 16

The nerve that innervates them.

however motor neuron has some effect on the properties of the muscle fibres that it innervates.

Question 17

Which type of muscle fibres change after training?

Answer 17

2b to 2a

Question 18

What type of muscle change occurs in cases of severe deconditioning or spinal cord injury?

Answer 18

Type 1 to 2

Question 19

What is the role of Ageing in muscle fibre changes?

Answer 19

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).

Question 20

Define reflex action

Answer 20

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.

Question 21

What is the difference between reflexes & voluntary movements?

Answer 21

Once they are released, they can’t be stopped.

Question 22

What is the Jendrassik manoeuvre?

Answer 22

Try clenching the teeth, making a fist, or pulling against locked fingers when having patellar tendon tapped. The reflex becomes larger

Question 23

How does the Jendrassik maneouvre work?

Answer 23

Reducing the amount of inhibition that the brain and upper regions of the CNS exert under normal conditions.

Question 24

What is the role of Higher centres of the CNS in the stretch reflex?

Answer 24

Higher centres of the CNS exert inhibitory and excitatory regulation upon the stretch reflex

Question 25

What is the role of gamma motor neurons?

Answer 25

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.

Question 26

What are the mechanisms of Descending (supra spinal) control of reflexes?

Answer 26

Activation of alpha motor neurons
Activation of inhibitory interneurons
Activation of proprio spinal neurons
Activation of gamma motor neurons
Activation terminals of afferent fibres

Question 27

What is Hyper-reflexia?

Answer 27

Overactive reflexes
characteristic of upper motor neuron lesion
loss of descending inhibition

Question 28

What is Babinski’s sign?

Answer 28

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).

Question 29

What is Hyporeflexia?

Answer 29

Below normal or absent reflexes

Associated with lower motor neuron disease.

Question 30

What is the hallmark of a pure corticospinal tract lesion?

Answer 30

Abdominal reflexes (superficial reflexes) absent while the other reflexes (deep tendon reflexes) are brisk