Motor control: Role of the motor cortex and spinal reflexes

Question 1

What are the components of the motor system?

Answer 1

Cortex, descending motor pathways, spinal cord, motor unit (LMN + effector organ

Question 2

What are alpha motor neurones?

Answer 2

They are the lower motoneurons that directly control muscle contraction.

Question 3

Where are the alpha motor neurones cell bodies found?

Answer 3

In the ventral horn of the spinal cord.

Question 4

How do the alpha motor neurones communicate with the muscles/effector organs?

Answer 4

These neurones make physical contact with muscles at the NMJ to allow for contraction.

Question 5

What are the 2 descending motor pathways?

Answer 5

Lateral pathway (voluntary control of movement) and Ventromedial pathway (unconscious control of movement).

E.g. ability to stand up without thinking about it.

Question 6

Describe the relationship between the sensory and motor cortex.

Answer 6

Sensory input comes into the cerebral cortex, and the sensory-motor cortex has sensory input and motor output close together.

Question 7

What other structures also input into the motor cortex?

Answer 7

Subcortical nuclei (basal ganglia), cerebellum, and interactions between brainstem and cerebellum.

Question 8

Describe the hierarchical organisation of the motor system.

Answer 8

The spinal cord is at the bottom, feeding information to the brainstem and cortex. Sensory information is sent via the thalamus to the cortex.

Question 9

What are the 3 basic types of movement?

Answer 9

Reflex (protective, closed loop), rhythmic motor patterns (repetitive movements), and voluntary movement (purposeful, open loop).

Question 10

What can govern alpha motoneurons activity?

Answer 10

Sensory input into MN (via dorsal root), spinal interneurones, and upper motor neurones (from cerebral cortex).

Question 11

Can movement occur if descending influences from the brain are severed? Why?

Answer 11

Yes, spinal cord circuits can generate movement in isolation due to central pattern generators, even without brain input.

Question 12

What does descending input from Upper Motoneurons allow?

Answer 12

Allows sophisticated, adaptable patterns of movement and is superimposed upon the intrinsic circuitry of the spinal cord.

Question 13

Define distal musculature and which pathway innervates them.

Answer 13

E.g. hands, feet, and digits, important for fine motor movements, innervated by lateral motor neurones.

Question 14

Define proximal and axial muscles and which pathway innervates them.

Answer 14

E.g. trunk muscles, important for posture, innervated by medial motor neurones.

Question 15

What is the origin and termination of descending tracts?

Answer 15

Descending tracts go from the brain to alpha MN (LMN).

Question 16

What are the 2 types of descending tracts?

Answer 16

Lateral pathways and Ventromedial pathways.

Question 17

What are the lateral descending motor pathways?

Answer 17

Corticospinal tract (direct link from motor cortex to alpha motor neurones) and Rubrospinal tract (originates in red nucleus).

Question 18

What is the origin of corticospinal fibres?

Answer 18

Contralateral projection from areas 4 and 6 of the motor cortex to lateral spinal motor neurons.

Question 19

What is the origin of rubrospinal fibres?

Answer 19

Contralateral projections from the red nucleus in midbrain.

Question 20

What are the ventromedial motor pathways?

Answer 20

Reticular output from Reticular Nuclei (Pontine and Medullary Reticulo-spinal pathways) and output from Superior colliculus and vestibular nuclei (Vestibulo-spinal & Tecto-spinal pathways).

Question 21

Where do ventromedial motor pathway fibres originate?

Answer 21

All originate from brain stem nuclei.

Question 22

Are ventromedial pathways contral- or ipsilateral?

Answer 22

They are both contra- and ipsilateral descending projections.

Question 23

What is the function of the ventromedial motor fibres?

Answer 23

Important for balance, body position, and visual input, modulating spinal reflexes and maintaining posture.

Question 24

What is the function of the pontine Reticulo-spinal tract?

Answer 24

Enhances anti-gravity reflexes of spinal cord and facilitates leg extensor muscles to maintain standing posture.

Question 25

What is the function of the Medullary Reticulo-spinal tract?

Answer 25

Allows voluntary override to enable voluntary movements.

Question 26

Where do Vestibulo-spinal and Tecto-spinal fibres originate?

Answer 26

Vestibular nucleus (Vestibulospinal tract) and superior colliculus (Tecto-spinal pathway).

Question 27

What is the function of the Vestibulospinal tract?

Answer 27

Relays gravitational sensory information to maintain head and neck position.

Question 28

What is the function of the Tectospinal tract?

Answer 28

Relays visual sensory information to orientate head and eyes to stimuli.

Question 29

Summary of descending pathways: Lateral pathways.

Answer 29

Inputs from motor cortex to SC, with direct and indirect connections, controlling voluntary movement.

Question 30

Summary of descending pathways: Ventromedial pathways.

Answer 30

Inputs from motor cortex to SC via brainstem nuclei, controlling unconscious movement.

Question 31

What may be an advantage of severing the lateral ascending spinal sensory pathways?

Answer 31

Used for relief of intractable pain, but may damage descending lateral motor pathways.

Question 32

Would severing the lateral sensory pathway affect posture? Why?

Answer 32

Posture unaffected as there is no damage to ventromedial tracts.

Question 33

Which part of the cortex does voluntary movement involve?

Answer 33

Almost all of the neocortex, involving sensory input, planning, and execution.

Question 34

What are the 2 areas of the motor cortex?

Answer 34

Area 4: primary motor cortex and Area 6: premotor and supplementary motor areas.

Question 35

Where is the primary motor cortex found?

Answer 35

Primary motor cortex (M1) is located in the precentral gyrus.

Question 36

What is Area 4 of the motor cortex?

Answer 36

Area 4 is the primary motor cortex (M1). It has the lowest stimulus threshold to cause movement and has strong synaptic links to alpha motor neurons in the ventral horn.

Question 37

What is Area 6 of the motor cortex?

Answer 37

Area 6 consists of the premotor area and supplementary motor area. Stimulation here evokes more complex movements, indicating its role in planning, imitating, and storing certain movements.

Question 38

What are the two areas of the motor cortex?

Answer 38

The motor cortex consists of Area 4 and Area 6.

Question 39

What is the function of Area 4?

Answer 39

The primary motor cortex (M1; Area 4) controls distal musculature, facilitating fine motor control.

Question 40

What is the function of Area 6?

Answer 40

Area 6 has two components: the premotor cortex (lateral) controls proximal musculature, movement sequencing, and preparation for movement, while the supplementary motor area (fronto/medial) is involved in planning and initiation, as well as bi-manual coordination.

Question 41

Where is the primary motor cortex found?

Answer 41

The primary motor cortex (M1/Area 4) is located right beside the sensory cortex.

Question 42

What is found in front of the primary motor cortex?

Answer 42

In front of the primary motor cortex are the premotor area and the supplementary motor area, collectively known as Area 6.

Question 43

What does a motor homunculus allow us to identify?

Answer 43

A motor homunculus helps identify specific parts of the cortex connected to specific body parts, showing that important body parts for movement have larger cortical areas dedicated to them.

Question 44

How much of the corticospinal tract do upper motor neurons (UMN) contribute?

Answer 44

Upper motor neurons contribute approximately 50% of corticospinal tract axons.

Question 45

Where are UMN cell bodies found?

Answer 45

UMN cell bodies are pyramidal neurons located in cortical layer V, known as Betz cells.

Question 46

How are UMN organized?

Answer 46

UMN are somatotopically organized, with specific neurons in the cortex connected to specific body parts, activating small groups of muscles.

Question 47

What does each UMN encode?

Answer 47

Each individual UMN encodes either the force or direction of movement.

Question 48

What can damage UMN?

Answer 48

Damage to UMN can occur due to conditions such as stroke or tumor.

Question 49

What are UMN lesion signs?

Answer 49

Signs of UMN lesions include initial muscle weakness, followed by spasticity, increased muscle tone (hypertonia), and increased reflex responses (hyper-reflexia).

Question 50

Which side of the body is affected if UMN is damaged?

Answer 50

Damage to UMN affects the side of the body contralateral to the damage in the brain, as corticospinal fibers decussate in the medulla.

Question 51

Is recovery possible from UMN damage?

Answer 51

Recovery is possible as the primary motor cortex circuitry shows adaptive alterations.