Lecture 7 Descending pathways (Corticospinal) & Brain areas

Question 1

What is the principal function of the premotor cortical areas?

Answer 1

The programing of complex motor activity, which is then relayed to the primary motor cortex, where the execution of motor activity is initiated.

Question 2

What are the four regions that make up the premotor cortical areas?

Answer 2

The premotor cortex (PMC), supplementary motor cortex (SMA), frontal eye field (FEF), and the cingulate motor cortex.

Question 3

How do the premotor cortical areas differ from the primary motor cortex in terms of the movements they control?

Answer 3

The premotor cortical areas control complex movements involving multiple muscle groups, while the primary motor cortex primarily controls individual muscle contractions.

Question 4

What is the function of the frontal eye field (FEF)?

Answer 4

The frontal eye field (FEF) coordinates eye movements, particularly those involved in voluntary visual tracking of moving objects.

Question 5

How does the posterior parietal area contribute to motor tasks?

Answer 5

The posterior parietal area helps with tactile discrimination, recognizing object shapes by touch, and guiding movements that require both visual and tactile input.

Question 6

What might be associated with the cingulate motor cortex due to its location in the limbic system?

Answer 6

The cingulate motor cortex, located in the limbic system, may be associated with the motivational or emotional aspects of movement.

Question 7

What are the key regions involved in motor control that control the contralateral side of the body?

Answer 7

The primary motor cortex (MI), premotor cortex (PMC), supplementary motor area (SMA), and frontal eye fields (FEF) are among the key regions that control the contralateral side of the body

Question 8

Which motor area projects to synapses on individual muscle extensors and flexors?

Answer 8

The primary motor cortex (MI) projects to synapses on individual muscle extensors and flexors, ultimately controlling fine motor movements.

Question 9

What is the primary characteristic of the primary motor cortex (MI) in terms of somatotopy?

Answer 9

The primary motor cortex (MI) is highly somatotopic, meaning it has a precise mapping of the body’s motor functions.

Question 10

What distinguishes the somatotopic organization of premotor areas compared to the primary motor cortex (MI)?

Answer 10

Premotor areas have a less somatotopic organization compared to the highly somatotopic primary motor cortex (MI). This means they are less precisely mapped with specific body parts.

Question 11

How do basal nuclei influence motor control through their projections?

Answer 11

Basal nuclei project to the thalamus, indirectly affecting ascending projections to the motor cortex, thus playing a role in motor control and coordination

Question 12

Which three regions of the brain control motor activity?

Answer 12

Motor activity is controlled by the cerebral cortex, basal nuclei, and cerebellum.

Question 13

How does the cerebellum contribute to motor activity?

Answer 13

The cerebellum fine-tunes movements by receiving input from the motor cortex and sensory pathways, ensuring timing, speed, and precision in motor control.

Question 14

What is the role of the cerebral cortex in motor activity?

Answer 14

The cerebral cortex plans and executes complex voluntary motor activities.

Question 15

Where is the Primary Motor Cortex (MI) located?

Answer 15

The Primary Motor Cortex (MI) is located in the precentral gyrus of the frontal lobe.

Question 16

What is the primary projection of the Primary Motor Cortex (MI)?

Answer 16

MI directly projects to the spinal cord.

Question 17

What is the main function of the Primary Motor Cortex (MI)?

Answer 17

MI is responsible for controlling voluntary muscle movements and enables precise and fine motor control.

Question 18

Where is the Premotor Cortex (PMC) situated?

Answer 18

PMC is situated in the frontal lobe, anterior to the primary motor cortex.

Question 19

What is the primary function of the Premotor Cortex (PMC)?

Answer 19

Control the muscles involved in posture and help direct the body and limbs toward a desired direction during movement.

Question 20

Where is the Supplementary Motor Area (SMA) located?

Answer 20

SMA is located in the superior frontal gyrus.

Question 21

What are the primary functions of the Supplementary Motor Area (SMA)?

Answer 21

The SMA is primarily involved in coordinating more elaborate movements, coordinating movements across both sides of the body, and controlling trunk and proximal limb muscles.

Question 22

What are the primary projections of the Supplementary Motor Area (SMA)?

Answer 22

SMA projects to the primary motor cortex, spinal cord, and other motor regions.

Question 23

Posterior Parietal Area Projections:

Answer 23

Primary Motor Cortex (MI), SMA, Premotor Cortex (PMC)

Question 24

Where does PMC send projections to?

Answer 24

Projecting to the reticular formation spinal cord and M1,

Question 25

What is the organization of the cortex in terms of layers, and how many distinct layers are there?

Answer 25

The cortex is organized into 6 distinct layers, but not all of these layers are present to the same extent in different regions.

Question 26

Which cortical layer is primarily responsible for housing the cell bodies of long-range projection neurons?

Answer 26

Layer 5 of the cortex is primarily responsible for housing the cell bodies of long-range projection neurons.

Question 27

What are the two primary descending motor pathways originating from the motor cortex, terminating in the spinal cord and brainstem?

Answer 27

The corticospinal (terminating in the spinal cord) and corticonuclear (terminating in the brainstem) tracts are the two main descending motor pathways originating from the motor cortex.

Question 28

What are the seven descending motor pathways that ultimately
exert their influence on muscle activity?

Answer 28

Three of
these pathways, the lateral corticospinal, the anterior corticospinal, and the corticonuclear tracts derive their fibers from the
sensorimotor cortex, whereas the other four tracts, the tectospinal, rubrospinal, reticulospinal, and vestibulospinal tracts,
derive their fibers from the brainstem

Question 29

All of the descending
tracts terminate in the spinal cord with the exception of the _____, which terminates in the brainstem.

Answer 29

corticonuclear tract

Question 30

Where do upper motoneurons from the general corticospinal pathway terminate in the spinal cord?

Answer 30

Upper motoneurons terminate on interneurons/motoneuron in the spinal cord or spinal nerves.

Question 31

Where do upper motoneurons from the SMA and PMC in the general corticospinal pathway terminate in the spinal cord?

Answer 31

Upper motoneurons terminate on interneurons/motoneuron in the spinal cord or spinal nerves.

Question 32

Where do upper motoneurons from the PPC and SI in the general corticospinal pathway terminate in the spinal cord?

Answer 32

They terminate in the dorsal (sensory) grey matter or on second order ascending neurons in the nucleus gracilis or nucleus cuneatus.

Question 33

1/3 proportion of fibers in the general corticospinal pathway come from which areas? (3)

Answer 33

1/3 of fibers from MI
1/3 from SMA + a little PMC
1/3 from PPC and SI

Question 34

What are the primary roles of pyramidal cells in the corticospinal tract, and where do their axons synapse in the spinal cord gray matter?

Answer 34

Pyramidal cells, also known as upper motoneurons (UMNs), play a central role in the corticospinal tract. Their axons synapse with interneurons in laminae V-VII, alpha motoneurons (innervating skeletal muscle fibers), or gamma motoneurons (innervating muscle spindles) in the spinal cord gray matter.

Question 35

What is the main function of the corticospinal tract in the nervous system?

Answer 35

Pathway for voluntary motor movements, and it plays a crucial role in controlling precise and skilled movements, including fine motor control.

Question 36

What are upper motor neurons, and what is their role in the corticospinal tract?

Answer 36

Located in cerebral cortex or brainstem that send signals down the corticospinal tract. They initiate and plan voluntary muscle movements, conveying motor commands from the brain to the spinal cord.

Question 37

What are lower motor neurons, and what is their role in the corticospinal tract?

Answer 37

Located neurons in the spinal cord’s ventral horn and directly innervate skeletal muscles. They receive signals from upper motor neurons and execute the final motor commands, activating the muscles and causing movement.

Question 38

What types of muscles do the lateral and anterior corticospinal tracts preferentially stimulate?

Answer 38

The lateral corticospinal tract primarily controls the movement of muscles in the distal limbs(hands and fingers), while the anterior corticospinal tract is involved with movement of the proximal muscles of the trunk, neck, and shoulders.

Question 39

How do most axons in the anterior corticospinal tract decussate, and what does this mean for motor control?

Answer 39

Only about 10-15% of the axons in the anterior corticospinal tract decussate in the spinal cord just before they synapse with lower motor neurons. Most of them continue on the same side (ipsilaterally) of the spinal cord, influencing motor neurons on the same side of the body

Question 40

How do most axons in the lateral corticospinal tract decussate, and what does this mean for motor control?

Answer 40

Approximately 90% of the axons in the lateral corticospinal tract decussate in the caudal medulla (at the base of the pyramids) or cross over to the other side of the brainstem, in a bundle of axons called the pyramidal, meaning that axons from one side of the cerebral cortex control muscles on the opposite side of the body, allowing for contralateral motor control.

Question 41

What is the consequence of damage to the corticospinal tract, leading to upper motor neuron syndrome?

Answer 41

Damage to the corticospinal tract results in upper motor neuron syndrome, characterized by symptoms like weakness or paralysis, hyperactive reflexes, decreased motor control, and variable muscle tone.

Question 42

Steps of Corticospinal Tract

Answer 42

Upper Motor Neurons Leave Cortex: Upper motor neurons from the motor cortex begin their journey.

Brainstem Entry: They descend to the brainstem, entering the midbrain through large fiber bundles called cerebral peduncles.

Medullary Formation: Continuing down, in the medulla, these fibers form two bundles known as pyramids, which create visible ridges on the exterior of the brainstem.

Decussation at Pyramidal Decussation: At the base of the pyramids, approximately 90% of the corticospinal tract fibers cross over to the other side of the brainstem in a bundle of axons called the pyramidal decussation.

Entry into Spinal Cord: These fibers synapse with lower motor neurons in the contralateral ventral grey horn, which, in turn, innervate skeletal muscles on the opposite side of the body.

Non-Decussated Fibers: The remaining 10% of the fibers continue into the spinal cord on the same side where they started, forming the ventral or anterior corticospinal tract. These fibers stay on the same side of the spinal cord and synapse with lower motor neurons in the anterior grey horn. These lower motor neurons then exit the spinal cord and reach skeletal muscles on the same side of the body, causing them to contract

Question 43

How does a stroke on the right side of the brain impact postural muscles, and why?

Answer 43

A stroke on the right side of the brain would not affect postural muscles as much since they have bilateral control, meaning both sides of the brain can influence these muscles, helping to maintain posture and stability.

Question 44

What would be the effect of a stroke on the right side of the brain on distal muscles?

Answer 44

A stroke on the right side of the brain would affect distal muscles on the left side of the body.

Question 45

What type of control does the anterior corticospinal tract provide for muscle groups like those in the back, abdomen, hip, and shoulder?

Answer 45

The anterior corticospinal tract provides postural and proximal control

Question 46

What type of control does the lateral corticospinal tract enable for skeletal muscles?

Answer 46

The lateral corticospinal tract enables upper motor neurons (UMNs) to have contralateral control of skeletal muscles, with cell bodies on the right half of the brain controlling skeletal muscles on the left half of the body.

Question 47

Where do axons from the motor cortex (primary) terminate in the spinal cord?

Answer 47

Axons from the motor cortex terminate in the ventral horn of the spinal cord, where they synapse with interneurons and lower motor neurons that innervate skeletal muscle.

Question 48

Where do axons from the sensory cortex terminate in the spinal cord, and what is their function there?

Answer 48

Axons from the sensory cortex terminate in the dorsal horn of the spinal cord. In this region, they function in reflexes and modulate the transmission of sensory information to higher brain centers.

Question 49

What is the role of the internal capsule in the corticospinal pathway?

Answer 49

Serves as a pathway for motor commands from the cerebral cortex to the spinal cord, controlling voluntary movements of the limbs and trunk.