Functional Hierarchy of the motor System

Question 1

What type of neuron supplys skeletal muscles?

Answer 1

Alpha-motor neurons (A type of LMN)

Question 2

What structures are involved in controlling movement?

Answer 2

• Descending control pathways from the cerebral cortex (e.g. corticospinal tract) (planning movements)
• Basal Ganglia (Initiation/termination)
• Cerebellum
• Local Spinal Cord & brain stem circuits

Question 3

Where do UMNs sit and what would be manifestions of a lesion?

Answer 3

They start in the primary motor cortex or brainstem.

Spasticity (Increased Tone)
Weakness
Wasting in chronic lesions due to lack of use
Hyperreflexia

Question 4

Where do LMNs sit and what would be the manifestations of a lesion?

Answer 4

In the cranial nerve nuclei or grey matter of the spinal cord

• Flaccid Paralysis
• Hyporeflexia
• Fasciculation
• Muscle Atrophy

Question 5

There is a spatial map of the bodies muscles in the spinal cord how is it arranged?

Answer 5

Medial -> Lateral

So the more medial the grey matter the more medial muscles it supplies

Question 6

How does the stretch reflex work?

Answer 6

• Stretch the tendon
• Pulls on the Muscle Spindle Fibres
• Trigger 1a afferent fibres which enter spinal cord through the Dorsal Root Ganglion

The 1a afferent fibres make two connections:

• One is a monosynaptic link to an alpha-motor neuron which returns through the ventral root to the agonist triggering contraction
• Second is a polysynaptic link through an inhibitory interneuron to an alpha-motor neuron and to the antagonistic muscle causing relaxation

Question 7

What stretch reflexes can we test and what vertebral level do they originate?

Answer 7

Biceps Jerk - C6
Triceps Jerk - C7
Patellar Tendon - L4
Achilles Tendon - S1

Question 8

Which is faster the flexor or stretch reflex?

Answer 8

The stretch reflex because the nociceptive sensory fibres involved in the flexor reflex have a smaller diameter and therefore a slower conduction than 1a afferent fibres

Question 9

How does the Flexor (withdrawal) reflex occur?

Answer 9

• Nociceptor triggered
• Increased APs in cutaneous afferent fibres
• Afferent fibres pass up and down the spinal cord to reach all the flexors of the effected limb
• Through a polysynaptic excitatory link they trigger contraction in all the limb’s flexors

Question 10

How do we stop ourselves from falling over when our lower limb uses the flexor reflex?

Answer 10

Contralateral limb extension

Some interneurons cross the spinal cord to excite the contralateral extensors and inhibit contralateral flexors

Question 11

How does pain facilitation work?

Answer 11

When multiple impulses come along one after another they increase in strength.

This occurs between the same inputs e.g. nociceptors
And between different inputs e.g. pain coupled with a stretch reflex can cause it to be very exaggerated

Question 12

What is the Golgi tendon organ reflex?

Answer 12

GTO reflex

• When there’s an excessive load on a muscle
• GTO is stretched (indicating risk of damaging tendon/muscle)
• Afferent fibres –> Inhibitory motorneuron –> Alpha-motor neuron –> Muscle relaxes

Question 13

How is it possible to override a reflex?

Answer 13

• Every alpha-motorneuron receives >10000 synapses
• Some of these come from descending cortical inputs
• Descending impulses acting on the alpha-motorneurons will outdo those of the reflex

Question 14

What do gamma-motorneurons do?

Answer 14

They come on descending pathways from the brain and innervate muscle spindle fibres

Question 15

How do gamma-motorneurons affect muscle tone?

Answer 15

Excess stimulation of muscle spindles

• -> Excess stimulation of 1a afferent fibres
• -> Excess muscle activity
• -> Spasticity

And vice versa

Question 16

Define Spinal Shock?

Answer 16

A result of spinal injury:

• Hyporeflexia
• Clonus
• Autonomic Dysfunction

The reflexes gradually return over a few weeks and autonomic dysfunction takes longer to resolve, possibly it never properly does