Upper Motor Neurones and Control of Movement

Question 1

describe the system which allows smooth walking action without input from upper motor neurones or motor cortex

Answer 1

2 adjacent excitatory pacemaker interneurons
- 1 involved in extension, 1 flexion
pacemaker interneurone excites efferent nerve to flexor muscle but also excited another inhibitory neurone which inhibits other pacemaker interneurone involved with extension, therefore flexion is excited and extension is inhibited
after a brief time delay, the inhibited pacemaker interneurone involved with extension then fires an AP which excites an inhibitory interneurone which inhibits the pacemaker interneurone involved with flexion, therefore extension is excited and flexion is inhibited
this cycle continues to allow smooth walking

Question 2

there are 3 levels of hierarchy of motor control, what is the highest level?

Answer 2

highest level is involved with strategy of movement

structures involved = neocrotical association areas and basal ganglia

Question 3

middle level of motor control?

Answer 3

tactical (how you will perform the plan made by higher level - what sequence of muscle contractions etc)
structures = motor cortex and cerebellum
(tactics must be stored in memory for a time)

Question 4

low level of motor control?

Answer 4

involved in execution (activation of motor pools and interneuron pools that command the movement and make essential postural adjustments - to prevent falling while performing movement etc)
structures = brain stem and spinal cord

Question 5

descending spinal tracts arise from where?

Answer 5

cerebral cortex (area concerned with motor comman - precentral gyrus - brodmann areas 6 and 4)

Question 6

descending spinal tracts are concerned with what?

Answer 6

control of movement
muscle tone
spinal reflexes
spinal autonomic functions
modulation of sensory transmission to higher centres

Question 7

2 divisions of descending pathways?

Answer 7

lateral pathways
- lateral corticospinal tract
- rubrospinal tract
ventromedial pathways
- pontine reticulospinal tract
- meduallry reticulospinal tract
- lateral vestibulospinal tract
- tectospinal tract
- ventral corticospinal tract

Question 8

features and functions of lateral descending pathways?

Answer 8

under control from the cerebral cortex
important for voluntary movement of distal muscles (particularly discrete, skilled movements - e.g fractioned movement of arms - hands - fingers)

Question 9

damage to what pathways can cause the loss of fractionated movement?

Answer 9

lateral pathways

- couldn’t move fingers and other joints independently, everything would move as one when trying to perform a task

Question 10

functions of ventromedial pathways?

Answer 10

not under direct control of motor cortex
under control from nuclei in the brainstem
important for control of posture and locomotion (not so much involved in in fine, skilled or fractioned movement)

Question 11

what is the major lateral pathway and describe its path?

Answer 11

corticospinal tract
cell bodies in motor cortex (BAs 4 and 6) and somatosensory areas of parietal cortex
axons course to the base of the medulla forming a tract (medullary pyramid)
85% of fibres cross here at the decussation of pyramids to form the lateral corticospinal tract (most important)
other 15% stay ipsilateral and form the ventral corticospinal tract and decussate more caudally

Question 12

where do axons of the corticospinal tract terminate?

Answer 12

dorsolateral region of the ventral horn of the spinal cord

- area of LMNs and interneurons controlling distal muscles (particularly flexors)

Question 13

describe the more minor lateral pathway

Answer 13

rubrospinal tract
cell bodies located in red nucleus (receives input from motor cortex and cerebellum)
axons decussate at the ventral tegmental decussation and descend the spinal cord ventrolateral to the lateral corticospinal tract, terminating in the ventral horn

Question 14

functions of rubrospinal tract?

Answer 14

exerts control over limb flexors by exciting LMNs of these muscles

Question 15

lesions of the lateral pathways is associated with what symptoms?

Answer 15

loss of fractionated movement
slowing and impairment of accuracy of voluntary movements
little effect on normal posture (sitting, standing etc)

Question 16

lesion of the corticospinal tract specifically causes what symptoms?

Answer 16

initially causes deficits as profound as a lesion of the whole lateral pathways but over time, major recovery can occur as rubrospinal tract is preserved and can compensate for functions
only permanent deficit = weakness of distal flexors and inability of move finger independently)

Question 17

describe the path of vestibulomedial pathway (vestibulospinal tract)?

Answer 17

cell bodies in vestibular nuclei (lateral and medial) that receive input via CN VIII, cerebellar input also important
axons from lateral vestibular nucleus (Deiter’s) descend ipsilaterally as the latral vestibulospinal tract as far as the lumbar spinal cord
- holds body upright and balanced postire by fascilitating extensor MNs of antigravity muscles
axons from medial vestibular nucleus descend as the medial vestibulospinal tract as far as the cervical spinal cord
- these activate cervical spinal circuits that control neck and back muscles guiding head movements

Question 18

optic tectum is another name for what?

Answer 18

superior colliculus

Question 19

main inputs to superior colliculus?

Answer 19

axons of retina
axons of neurones which are in visual analysing areas of visual cortex
also has input about sound and from somatosensory systems

Question 20

describe the path of the tectospinal tract?

Answer 20

cell bodies in the superior colliculus
axons decussate in the dorsal tegmental decussation and descend close to the midline as the tectospinal tract to the cervical spinal cord influencing muscles of neck, upper trunk and shoulders

Question 21

superior colliculus acts as what?

Answer 21

map of external world guiding

e.g - orientation of the head and eyes to an important new visual stimulus

Question 22

both the pontine (medial) and medullary (lateral) reticulospinal tracts arise from where and are controlled by what?

Answer 22

arise from reticular formation (diffuse mesh of neurones located along length and at the core of the brainstem)
controlled by descending signals from the cortex

Question 23

describe the pontine reticulospinal tract?

Answer 23

descends ipsilaterally
enhances antigravity reflexes of the spinal cord
helps to maintain a standing posture by facilitating contraction of the extensors of the lower limbs

Question 24

describe the medullary reticulospinal tract?

Answer 24

descends bilaterally
opposes the action of the medial tract
releases antigravity muscles from the reflex control