lecture 14 - motor system 1

Question 1

What is the alternative term for the lower motor neuron?

Answer 1

Alpha-motor neuron

Question 2

What are some diseases that affect lower motor neurons?

Answer 2

Guillain-Barre Syndrome, Motor Neuron Disease/ALS, polio

Question 3

What is the tendon stretch reflex?

Answer 3

When a tendon is struck, the normal response is to defend muscle length against rapid stretch by flexing the muscle

Question 4

How do lower motor neuron lesions affect tendon reflexes?

Answer 4

Reduce tendon reflexes, because the tendon reflex arc is disrupted

Question 5

How do lower motor neuron lesions affect power?

Answer 5

Power is reduced, due to reduced innervation

Question 6

How do lower motor neuron lesions affect muscle tone?

Answer 6

They reduce resting muscle tone, because there is reduced tonic activity in the stretch reflex arc and a loss of the continuous low level excitation of the muscle at rest.

Question 7

How do lower motor neuron lesions affect muscle bulk?

Answer 7

There will be muscle atrophy due to reduced activation of muscles and therefore changes in muscle metabolism. There will also be disuse atrophy.

Question 8

How do lower motor neuron lesions affect the plantar response?

Answer 8

Plantar response is normal, because descending inhibition via the UMNs is intact

Question 9

What is the difference in the functions of the corticospinal tract and corticobulbar tracts?

Answer 9

Corticospinal provides motor supply mostly to distal muscles (trunk/limbs) via the lateral CST and a little to the proximal muscles via the medial/ventral CST. However, the corticobulbar tracts supply only the head, face and neck, particularly the muscles supplied by the cranial nerves

Question 10

What is the medial/vental corticospinal tract?

Answer 10

15% of the CST fibres that are uncrossed and mainly supply proximal muscles (head, face)

Question 11

What is the lateral corticospinal tract?

Answer 11

85% of fibres of the CST that decussate at the medulla and supply mostly the distal but some proximal muscles

Question 12

How will a lesion of the lateral CST affect proximal and distal muscles?

Answer 12

Distal function on the same side will be lost (as after decussation), but proximal function will mostly be preserved due to the function of the medial CST

Question 13

What is the decussation of the corticobulbar tract?

Answer 13

No decussation. - most cranial nerve nuclei are bilaterally innervated

Question 14

What is the role of the superior colliculus in motor control?

Answer 14

Receives input from the visual pathway and directs rapid orientation towards moving objects

Question 15

What are some examples of upper motor neuron disorders?

Answer 15

Stroke, trauma, multiple sclerosis

Question 16

How do upper motor neuron lesions affect muscle power?

Answer 16

Reduce power, by reducing nerve excitation to the LMN and therefore muscle

Question 17

How do upper motor neuron lesions affect tendon reflexes?

Answer 17

There is a reduction in inhibition of gamma fibres, increasing muscle spindle sensitivity and causing hyperreflexia

Question 18

How do upper motor neuron lesions affect muscle tone?

Answer 18

There is a loss of upper motor neuron inhibitory input to the muscles, causing excessive muscle contraction resulting in hypertonia and spasticity

Question 19

What is the difference between spasticity and hypertonia?

Answer 19

Spasticity is increased stiffness of muscles due to reduced descending inihibition, while hypertonia is increased basal muscle tone at rest

Question 20

How do upper motor neuron lesions affect muscle bulk?

Answer 20

There will be mild disuse atroophy

Question 21

How do upper motor neuron lesions affect the plantar response?

Answer 21

There is an extensor response (Babinski’s sign) as there is no UMN inhibiton of the primitive extensor response.