Physiology - LMN Lesions

Question 1

Name examples of disorders affecting the LMN.

Answer 1

Trauma

Motor Neuron Disease (amytrophic lateral sclerosis - ALS), Guillian Barre Syndrome, polio

Question 2

List the characteristics of a LMN lesion.

Answer 2

Decrease in power 
Decrease in muscle tone
Decrease in tendon reflexes 
Muscle atrophy - denervation 
Normal plantar response (flexion or none)

Question 3

Explain why loss in power occurs?

Answer 3

Can no longer innervate the muscle leading to power loss

Question 4

Explain why there is a decrease in tendon reflexes.

Answer 4

Monosynaptic loop is broken. Peripheral nerve (sensory axon or LMN) is damaged

Question 5

Explain why muscle atrophy occurs.

Answer 5

Denervation atrophy occurs because there is an increase in proteolysis due to loss of LMN input
Decreased activation of muscles = decreased contractile proteins = decreased muscle bulk

Question 6

What determines resting tone.

Answer 6

Muscle structure and tissue properties
Tonic activity of stretch reflex arc
Incomplete relaxation

Question 7

What is muscle tone?

Answer 7

Resistance of muscles to stretch

Question 8

What is the tendon reflex?

Answer 8

Response to stretch of muscles evoked by striking tendon

Normally - defend the muscle length against brief rapid stretch

Question 9

What other things cause hypotonia?

Answer 9

Cerebellar disease

Muscle disease

Question 10

What happens in disuse atrophy?

Answer 10

Decrease in protein synthesis - muscles are no longer used

Question 11

List the functions of the spinal cord showing how it acts as a mini brain.

Answer 11

Withdrawal reflex
Rhythmic motor patterns
Process descending commands (UMN) for voluntary movement