7. The Motor System

Question 1

Where is the motor cortex?

Answer 1

In the pre-central gyrus.

Question 2

Where are the UMN and LMN found considering the CNS and PNS?

Answer 2

UMN is entirely in CNS. The LMN is mostly in the PNS but the body and start of axon is in the CNS.

Question 3

What signs would a CNS lesion mostly present with?

Answer 3

UMN signs although it could present with LMN if the cell body or proximal axon is affected.

Question 4

Where are the nuclei for the cranial nerves found?

Answer 4

In the brainstem.

Question 5

Describe the knee reflex arc.

Answer 5

Tendon hammer hits tendon, quadriceps fibres are pulled and stretched, primary neurone goes through dorsal root ganglia and synapses on secondary neurone which goes to quads to cause contraction - knee reflex.

Question 6

How does the knee reflex arc also include control over the hamstrings?

Answer 6

A collateral branch is sent off from the primary neurone at L3 level to the L5 level vertebrae, here is send an inhibitory interneurone to act on secondary neurone to cause relaxation of the hamstrings.

Question 7

What makes complex reflex arcs complex?

Answer 7

Descending inhibition after the UMN has matured.

Question 8

What is the adult reflex in Babinski’s reflex?

Answer 8

Toe goes down and foot moves up and away from ‘noxious’ stimulus.

Question 9

What is the baby reflex in Babinski’s reflext?

Answer 9

Whole limb draws away and toes move backwards.

Question 10

Why is the baby’s response to Babinski’s reflex different to the adult response?

Answer 10

The baby doesn’t have developed pathways involved in the reflex - it’s a primitive sign.

Question 11

What are the signs of LMN damage?

Answer 11

Weakness, wasting, areflexia or hyporeflexia, hypotonia, fasciculation, fibrillation.

Question 12

What is meant by fasciculation and fibrillation?

Answer 12

Fasciculation - uncoordinated muscle contraction, fibrillation - uncoordinated contraction but picked up by electrodes.

Question 13

Where are upper motor neurones?

Answer 13

In the motor cortex.

Question 14

What is the general effect of UMN on LMN?

Answer 14

Inhibitory, small amount of excitatory but predominantly inhibitory.

Question 15

What is the internal capsule sandwiched between?

Answer 15

Thalamus and lentiform nucleus.

Question 16

What is the shape of the internal capsule in transverse section?

Answer 16

V-shaped.

Question 17

What is the topographical representation of the internal capsule?

Answer 17

Genu (bend) - fibres to the face, posterior limb - upper limb, trunk, lower limb (centre to posterior part).

Question 18

What is the pathway of the lateral corticospinal tract?

Answer 18

UMN from cortex to corona radiata, through internal capsule, to cerebral peduncle, then decussation of the pyramids in the pons, descend to meet the LMN at the level of the muscle and synapse with muscle.

Question 19

What is the pathway of the ventral corticospinal tract?

Answer 19

UMN from cortex to corona radiata, through internal capsule, descends ipsilaterally to level of muscle, decussates and synapses with LMN which synapses with muscle.

Question 20

How are the corticospinal tracts divided into lateral and ventral?

Answer 20

80% lateral, 20% ventral.

Question 21

What is the ventral corticospinal tract important for?

Answer 21

Precise control of distal muscles.

Question 22

What is the lateral corticospinal tract important for?

Answer 22

Coarse control particularly of trunk for posture.

Question 23

What supplies the cranial nerve nuclei?

Answer 23

Corticobulbar corticonuclear supply.

Question 24

What is the supply to the upper facial nucleus?

Answer 24

Bilateral innervation from UMN.

Question 25

What is the supply to the lower facial nucleus?

Answer 25

Innervation from contralateral UMN.

Question 26

What clinical signs would a lesion of the UMN supplying facial nuclei on one side cause?

Answer 26

Loss of lower half on contralateral side but forehead sparing as UMN on other side still supplies forehead.

Question 27

What clinical signs would a lesion of the facial nerve on one side cause?

Answer 27

Loss of motor function of face on ipsilateral side.

Question 28

What is the role of the tectospinal tract?

Answer 28

Unconscious reflex actions of the head in response to visual and auditory stimuli.

Question 29

Which structures does the tectospinal tract connect?

Answer 29

Midbrain tectum to cervical region.

Question 30

What is the path of the rubrospinal tract?

Answer 30

From magnocellular red nucleus of the midbrain, decussates, descends in lateral part of brainstem tegmentum, then travels to lateral funiculus of spinal cord and courses adjacent to lateral corticospinal tract.

Question 31

What is the role of the rubrospinal tract?

Answer 31

Small contribution to motor control.

Question 32

What is the role of the vestibulospinal tract?

Answer 32

Vestibular nuclei receive information from CN VIII about orientation of the head, commands go through tract to alter tone, extend and change position of limbs and head to support posture and maintain balance. It allows gaze fixing with posture compensation to stop head having to move.

Question 33

Where does the vestibulospinal tract run?

Answer 33

In lateral vestibulospinal tract and medial longitudinal fasciculus.

Question 34

What is the path of the reticulospinal tract?

Answer 34

From reticular formation to the lower motor neurones to act on trunk and proximal limb flexors and extensors. In ventral region of spinal column.

Question 35

What is the function of the reticulospinal tract?

Answer 35

Muscle tone, keeps body paralysed whilst sleeping.

Question 36

What are the UMN signs immediately after lesion?

Answer 36

Initially flaccid paralysis - 'spinal shock'.

Question 37

What are the UMN signs about a month after the lesion?

Answer 37

Weakness, hypertonia -> spasticity and clasp knife reflex, hyperreflexia, extensor plantars in Babinski sign (primitive reflexes).

Question 38

Why is there UL flexion and LL extension in UMN damage?

Answer 38

All muscles have hypertonia and in the UL the flexors are stronger than the extensors so the UL stays flexed and vice versa in the LL.

Question 39

Explain what is meant by clasp knife reflex.

Answer 39

Keep pulling on arm and it won't move but will suddenly jolt and move with high force.

Question 40

Explain how in UMN lesions, there is a clasp knife reflex.

Answer 40

Normally, the UMNs inhibit the action of LMN. So in UMN signs, the inhibition of LMN is lost meaning the muscle stay flexed (in UL). However, when enough force is applied when pulling the joint, it begins to act on high pressure golgi tendon organs which inhibit neurones to the muscle and provide sudden relaxation under high force.