Descending Motor Pathways

Question 1

What are the requirements for movement ?

Answer 1

1. Goal or purpose,
2. Activation of relevant movement
3. Feedback of movement
4. Refinement/correction of movement 5. Cessation on completion

Question 2

Where in the brain is the motor cortex located ?

Answer 2

Anterior to central cortical sulcus, in the frontal lobe

Question 3

How much of the frontal lobe does the motor cortex occupy ?

Answer 3

1/3 of it

Question 4

What are the sub areas of the motor cortex ?

Answer 4

– Primary motor cortex
– Premotor area
– Supplementary motor area


Question 5

What does most of the primary motor cortex control ?

Answer 5

> 1/2 1° motor cortex controls hands and muscles of speech

Question 6

How many muscles does the excitation of a single motor neuron control ?

Answer 6

Excitation of a single mn ususally excites a specific movement – not one specific muscle.
I.e. A pattern of separate muscles are excited.

Question 7

Which of the primary motor cortex or premotor area gives more complex patterns of movement ?

Answer 7

Nerve signals from premotor areas give more complex “patterns” of movement.

Question 8

What is a special feature of contractions elicited by the Supplemental Motor Area ?

Answer 8

Contractions elicited are often bilateral eg grasping movements of both hands.

Question 9

Does the Supplemental motor area work by itself ?

Answer 9

Usually functions together with premotor area.

Question 10

What is the function of the supplemental motor area ?

Answer 10

Gives background movement onto which premotor and 1° motor cortex add finer control.

Question 11

How do motor signals get transmitted from cortex to the spinal cord ?

Answer 11

Through the Corticospinal tract

Question 12

What proportion of each motor area in the cortex does the Corticospinal tract originate from ?

Answer 12

30% 1°
30% premotor/ supplementary motor
40% somatosensory.

Question 13

Describe the path of motor signals from the cortex to

Answer 13

Cortex –> pyramids of medulla –> (majority)cross in lower medullae –> lateral corticospinal tractse –> mainly terminate on interneurons

Those that do not cross pass ipsilaterally in ventral corticospinal tracts.

Question 14

What pathways exist to transmit motor signals ? Are they for conscious or unconscious movements ?

Answer 14

Lateral Pathways = Conscious Movements

Ventromedial pathways = Unconscious Movements

Question 15

How are the lateral, and ventromedial pathways controlled respectively ?

Answer 15

LATERAL PATHWAY
Principally controlled by the cerebral cortex via 2 corticospinal tracts.

VENTROMEDIAL PATHWAY
Principally controlled in the brainstem.

Question 16

What do the lateral, and ventromedial pathways control respectively ?

Answer 16

LATERAL PATHWAY
General control of voluntary movement.
Mainly associated with control of distal muscles.

VENTROMEDIAL PATHWAY
Control of posture and rhythmic movements associated with locomotion and posture
Control axial and proximal muscles.

Question 17

What is posture ?

Answer 17

Position of a body and its parts relative to each other. It is a compromise between balance and movement.

Question 18

Which structure is involved in controlling/adjusting posture ?

Answer 18

-Controlled and driven by the brainstem (where the postural set is governed)

Question 19

Describe the entire posture control mechanism, especially for both unexpected and expected movement.

Answer 19

Central command –> Limb Movement –> Postural Instability –> Postural Adjustment (FEEDBACK for unanticipated postural instability)

Central command –> Postural Adjustment (FEEDFORWARD for anticipated postural instability)

Question 20

Explain how the terms postural set and compensation relate to posture.

Answer 20

Posture is adjusted predominantly by involuntary movement driven both predictively (postural set) and reactively (compensation).

Question 21

Where is sensory information for posture control integrated from ?

Answer 21

1. Muscle proprioceptors (detect changes in muscle length and or tension)
2. Vestibular apparatus (Detect movements of the head relative to the Earths gravitational field, and give sense of balance)
3. Visual inputs (detecting movements in visual field representing movement of the body)

Question 22

Where is the sensory information for posture control integrated in ?

Answer 22

Brainstem

Question 23

What are the important parts of the brainstem in the processing of information for posture control ?

Answer 23

• Reticular nuclei

* Vestibular nuclei (cranial nuclei for the vestibular nerve)

Question 24

What are the principal effects of the Lateral vestibulospinal tract ?

Answer 24

• Facilitate extensor motor neurons and inhibit flexor motor neurones innervating the ipsilateral medial and axial muscles.
• Activates alpha and Gamma MNs which causes enhanced muscle spindle mediated stretch reflexes
• Overall effect is to increase the tone to the antigravity muscles

Question 25

What is the principal effect of the medial vestibulospinal tract ?

Answer 25

Controls head and eye movements, usually in response to a moving target

Question 26

What would happen to the medial vestibulospinal tract in the case of an unplanned head movement ?

Answer 26

Head movements activate the vestibular apparatus which projects to the medial vestibular nucleus –> Medial vestibular nucleus links this information with visual and tectal sensory information, and transmits signals to the cranial nerves controlling head, neck and eye movements –> Head and eye movement are yoked together to maintain a steady gaze regardless of body movement.

Question 27

What is the principal effect of the Reticulospinal tract ?

Answer 27

Co-ordinates movement and posture. Specifically,

1. Pontine and medullary output coordinates truncal and proximal muscle activity to orientate the body either towards or away from a stimulus – primitive response.
2. Affects muscle tone in response to planned motor activity and coordinated overall distal and proximal muscle activity

Question 28

Where does the reticulospinal tract obtain sensory information from ? Where is this information processed ?

Answer 28

1. Vestibular nuclei
2. Cortical areas for voluntary movement
   3 .Proprioception, vision etc

This sensory info is processed in the brainstems reticular formation

Question 29

Which pathway/tract are responsible for maintaining posture ?

Answer 29

The ventromedial pathway, through the vestibulospinal and reticulospinal tracts

Question 30

Which pathway does each of the following belong to:

• Vestibulospinal tract
• Reticulospinal tract
• Corticospinal tract

Answer 30

ALL THREE BELONG TO THE VENTROMEDIAL PATHWAY

Question 31

Where are each of Upper Motor Neurons and Lower Motor Neurons found ?

Answer 31

UPPER MOTOR NEURONS (UMNs)
-Found in the cortex and brainstem nuclei

LOWER MOTOR NEURONS (LMNs)
-Found in the brainstem and spinal cord

Question 32

Are UMNs/LMNs restricted to the CNS ?

Answer 32

UMNs
-Restricted to the CNS

LMNs
-Not restricted to the CNS (their axons leave the CNS and innervate muscle)

Question 33

Do UMNs/LMNs contact muscles ?

Answer 33

UMNs
-Do NOT contact muscles

LMNs
-DO stimulate muscles to contract

Question 34

What is the function of each of UMNs and LMNs ?

Answer 34

UMNs:
-Transmit nerve impulse to LMNs

LMNs:
-Deliver motor functions to muscles

Question 35

Are lesions of UMNs common ? Why or why not ?

Answer 35

Relatively common as these axons are very long (therefore vulnerable).

Question 36

What do positive and negative signs mean in the context of UMN lesions ?

Answer 36

NEGATIVE is a loss of function,

POSITIVE is the appearance of an abnormal response.

Question 37

Give two specific examples of results of an UMN lesion.

Answer 37

• Extensor Planter reflex (Babinski sign) is a positive sign seen following corticospinal lesions. Corticospinal lesions lead to UMN syndrome.
• Hypertonia leading to painful spasticity due to hyperexcitable spindle neurons which induce over reactive stretch responses

NO ATROPHY OR FASCICULATION

Question 38

Define the Babinski reflex. What is a normal response ?

Answer 38

BABINSKI REFLEX
“Occurs after the sole of the foot has been firmly stroked. The big toe then moves upward or toward the top surface of the foot. The other toes fan out.”

NORMAL
All toes point down

Question 39

What do lesions of the axons of the LMNs cause ?

Answer 39

• Muscle paralysis
• Reduced motor tone
• Reduced stretch reflex
• Fasciculation
• Atrophy