5.1 the motor system

Question 1

where are lower motor neurones found?

Answer 1

cell bodies are found in the ventral horn and in cranial nerve motor nuclei e.g oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus etc.

Question 2

what are lower motor neurones controlled by?

Answer 2

upper motor neurones, which will depend through the cord or brainstem and synapse onto LMN

Question 3

what reflex do LMN participate in?

Answer 3

spinal reflexes, particularly the deep tendon reflex

Question 4

how can LMN be activated?

Answer 4

usually activated by incoming impulses from sensory neurones that communicate with muscle spindles (muscle stretch reflex)

Question 5

why do babies outgrow their primitive spinal reflexes?

Answer 5

due to maturation of descending upper motor neurone neurone pathways e.g plantar reflex in babies goes from up going to down going in babies to adults

Question 6

True or false: most projections onto lower motor neurones are stimulatory.

Answer 6

FALSE. most projections onto lower motor neurones are stimulatory e.g we have the ability to sit still

Question 7

what are the signs of a lower motor neurone lesion?

Answer 7

the following signs will be seen in muscles supplied by the damaged lower motor neurones:

• weakness
• areflexia e.g no knee jerk reaction
• wasting (loss of trophic support from LMN)
• hypotonia (rigid muscles when moving joints)
• fasciculations (due to up regulation of muscle nAChRs to try to compensate for denervation)

Question 8

where are upper motor neurones found?

Answer 8

the primary motor cortex (pre central gyrus)

Question 9

where do UMN synapse onto LMN?

Answer 9

in the ventral horn or cranial nerve motor nuclei

Question 10

what is the net effect of UMN on LMN?

Answer 10

inhibition via inhibitory inrwenwueonwa

there is a net inhibitory effect = sit still. when there is a stimulatory effect, will override inhibitory effect and you’ll get a response e.g knee jerk reflex.

Question 11

what is the role of the lateral cortico-spinal tract in a limb?

Answer 11

involved with fine motor control in the limbs, particularly the distal extremities

Question 12

what do UMN that supply facial structures leave the brainstem to form?

Answer 12

to supply structures innervated by cranial nerves and not spinal nerves, leave the pathway in the brainstem and form the corticobulbar tract, which innervates LMN in the cranial nerve motor nuclei

Question 13

how is the facial nerve motor nucleus different to other motor nuclei?

Answer 13

its split into two halves

• upper half supplies superior face (mainly occipitofrontalis)
• one the inferior face

Question 14

how can you get forehead sparing in an UMN lesion?

Answer 14

the part of the facial motor nucleus that supplies the upper half of the face receives UMNs from both hemispheres, wheres the lower face only receives contralateral UMN input

hence, lesions involving the face will spare the forehead (as opposed to true facial nerve palsies which effect all muscles of facial expression)

Question 15

what are the signs of an UMN lesion?

Answer 15

weakness (due to loss of excitatory inputs onto LMN from UMN)

hypertonia (due to loss of descending inhibition)

hyperreflexia

extensor plantar reflexes

Question 16

what is spinal shock?

Answer 16

a phenomenon that occurs in the days immediately following an UMN lesion.

Initially, there is flaccid paralysis with areflexia (like in LMN lesions) but then the tone increases (hypertonia) and reflexes become exaggerated (hyperreflexia).