Neuromuscular Control

Question 1

How does the grey and white matter look on a cross section of the spinal cord?

Answer 1

White matter on the outside

Butterfly shape of grey matter on the inside

Question 2

What is the alpha motor neuron? Where is it located?

Answer 2

Occupy the anterior / ventral horn of the grey matter of the spinal cord
They are the lower motor neurons of the brainstem and spinal cord
Innervate the extrafusal muscle fibres (muscle fibres with contractal element in them) - activation = muscle contraction

Question 3

What are intrafusal muscle fibres?

Answer 3

Contain sensory organs - responds to stretch and tension within the muscle
Communicates sensory info about the status of the muscle - brings about reflex activity

Question 4

What is a motor neuron pool?

Answer 4

Contains all the alpha motor neurons innervating a single muscle

Question 5

What is a motor unit?

How does this develop before birth?

Answer 5

A single motor neuron and all the muscle fibres it innervates
Smallest functional unit required to produce force

Before birth, there may be an overlap in the muscle fibres that receive innervation from multiple neurons, but one neuron wins eventually
So a muscle fibre is only innervated by one motor neuron
But one motor neuron may innervate many muscle fibres (avg. 600)
Stimulation of one motor unit causes contraction of all the muscle fibres in that unit

Question 6

What are the 3 types of motor unit?

How are they distributed in muscles?

Answer 6

Slow (S, type 1) - smallest diameter cell bodies, small dendritic trees, thinnest axons, slowest conduction velocity

Fast, fatigue resistant (FR, type IIA) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity

Fast, fatiguable (FF, type IIB) - larger diameter cell bodies, larger dendritic trees, thicker axons, faster conduction velocity

Randomly distributed, not congregated in one area

Question 7

For the 3 different types of motor units, what is the: force generated, speed of contraction, and fatiguability?

Answer 7

Type I (S) = least force, continues to generate it’s own max force for over an hour

Type IIA (FR) = more force, continues to generate max force but dies after 10 minutes

Type IIB (FF) = greatest force, loses ability to generate it’s max force quickly

Question 8

So how does the function of the muscle relate to motor unit?

Answer 8

Function = dependent on the type of motor unit that makes up most of the muscle e.g. for standing and posture = mainly S

Question 9

What are the 2 main mechanisms by the CNS that can regulate muscle force? And how do they work?

Answer 9

Recruitment = turn on number of neurons required for muscle fibre contraction, so more force required = more neurons turned on. Governed by the ‘size principle’ = slow units recruited first, fast units recruited afterwards . Allows fine control

Rate coding = motor neurons fire at range of frequencies, increased firing rate = increased force production by unit

Question 10

What is meant by muscle wastage? How does it occur?

Answer 10

i.e. if a muscle still has it’s blood supply, but not it’s nerve supply, the muscles can no longer be contracted

Question 11

What are neurotrophic factors?

Answer 11

Neurotrophic factors - growth factor, prevents neuronal death, promotes growth of neurons after injury

Question 12

What are motor unit and fibre characteristics dependent on?

Answer 12

The nerve that innervates them
e.g. if the innervation to slow and fast muscles are swapped, the properties of the muscle also swap due to the swapped innervation

Question 13

Is there plasticity of the different motor units? Which are common / uncommon?

Answer 13

Fibre types can change properties under many different conditions 
Type IIB (FF) to IIA (FR) most common following training 
Type I to II - only in cases of injury / severe deconditioning (e.g. in spaceflights)

Question 14

Ageing is associated with which type of motor unit loss?

Answer 14

Loss of types I and II fibres, but preferential loss of II as larger propertion of type I fibres = slower contraction time

Question 15

What is a reflex?

Answer 15

An involuntary, automatic response to a stimulus without reaching the level of consciousness
Magnitude and timing depends on intensity and onset of stimulus
Reflexes differ from voluntary movements in that once they are released, they can’t be stopped.

Question 16

How and why can reflex amplitude be altered?

Use the Jendrassik manoeuvre as an example:

Answer 16

Descending (supraspinal) control of reflexes:

CNS exerts inhibitory and excitatory regulation on reflexes. CNS has resting level of inhibition. Decerebration (separation of the cerebral cortex from the lower spinal cord) results in stronger, overactive and tonic reflex due to lack of inhibition

Knee-jerk reflex made more intense if tested whilst patient is clenching their teeth, making a fist and/or pulling against looked fingers
Removes descending inhibitor from the brain- Jendrassik manoeuvre

Question 17

What are the 5 different pathways that produce descending (supraspinal) control of the reflexes?

Answer 17

1. Activating alpha motor neurons - lower motor neurons = voluntary contraction
2. Activating inhibitory interneurons
3. Activating propriospinal neurons - activates nearby muscles of the spinal cord
4. Activating gamma motor neurons (responsible for altering sensitivity of sensory receptors within muscles)
5. Activating terminals of afferent fibres

Question 18

What is hyper-reflexia? A lesion in which neurons is it caused by?

How can the heightened reflex activity be seen clinically?

Answer 18

Overactive reflexes
Loss of descending inhibition
Associated with upper motor neuron lesions
(Loss of voluntary movement)

For example:

Clonus: See involuntary and rhythmic muscle contractions when the muscle is stretched out (until there is inhibition of the reflexive contractions)

Question 19

What is Babinski’s sign in hyper-reflexia?

Answer 19

Sole stimulated with blunt instrument normally leads to toes curling downwards (particularly big toe) - normal plantar response

In an adult with an upper motor lesion - toe curls upwards instead of downwards
Indicative of corticospinal tract lesion

Upward curl = normal in healthy infants (as their corticospinal tract has not developed)

Question 20

What is hypo-reflexia? A lesion in which neurons is it caused by?

Answer 20

Below normal or absent reflexes

Associated with lower motor neuron lesions / diseases

Question 21

What are the 3 ways motor units are classified?

Answer 21

force generated, speed of contraction, and fatigability

Question 22

What is summation?

Answer 22

Summation occurs when units fire at frequency too fast to allow the muscle to relax between arriving action potentials