Motor Control- Descending Control (UMNs)

Question 1

Corticospinal tract

Question 2

Rudiments of motor control. You can see cross sections of the brain, brain stem, and spinal cord at various levels to demonstrate the path of the corticospinal tract. Note once again the origins of the upper motor neuron cell body in the precentral gyrus, its path through the internal capsule, its crossover in the pyramidal decussation, and its path through the spinal cord as the lateral corticospinal tract.

Question 3

What are the main pathways that regulate LMNs?

Answer 3

Corticospinal (precentral gyrus)

Vestibulospinal (vestibular nuclei)

Reticulospinal (reticular formation)

Rubrospinal (red nucleus)

Colliculospinal (superior colliculus)

Local circuit neurons

Dorsal root sensory neurons

Question 4

This slide presents the cortical brain areas that control, or otherwise, influence movement.

In the early 1900s, a neuroanatomist named Brodmann gave numerical identities to all major cortical gyri. A few of these numerical identities may come up on board exams and thus you should be familiar with the numerical names of a few that we discuss in this course.

Question 5

The central sulcus marks the caudal boundary of what Brodmann area?

Answer 5

4 (shown in light green) which contains the primary motor cortex where the large motor neurons, called Betz cells, reside. These neurons give rise to the axons that make up the corticospinal or pyramidal tract.

Question 6

Other areas that influence the outflow of activity from the primary motor cortex come from the premotor area (shown in blue) and the supplementary motor area (shown in dark green). Both areas reside in Brodmann area __

Answer 6

6.

Question 7

The primary somatosensory cortex, important in planning movement, is found in Brodmann areas _,_, and _

Answer 7

3,1 and 2, (shown in light orange).

Question 8

The parietal association cortex (shown in dark orange) is found in Brodmann areas __ and __

Answer 8

5 and 7, and is indirectly associated with motor control.

Question 9

Brain lesions have taught us that the primary motor and sensory gyri, shown as purple and blue respectively, contain a somatotopic map of the head, face, body, and limbs. This slide presents these somatotopic maps called homunculi with the sensory homunculus shown on the left and the motor homunculus shown on the right.

Answer 9

Within the motor cortex underlying each of the specific body regions are motor neurons that send axons to control movements in that specific body part. Note the large areas of cortex dedicated to functions that require very precise control, such as the lower face and tongue for articulation and the hand area for dexterity.

Knowledge of these maps becomes useful when the clinician relates neurologic symptoms and signs to the location of a lesion in the brain. For example, occlusion of the right anterior cerebral artery causes an ischemic stroke in the right paramedian brain where the leg is represented. Such a stroke would produce contralateral leg weakness but would not be expected to compromise control and movement of the hand or face.

Question 10

This slide presents the somatotopic organization of the corticospinal tract. Note that the fibers from the upper extremity, body, and lower extremity are color coded. The right middle figure shows the internal capsule.

. The posterior portion of this structure, i.e. the portion caudal to the angle, is the posterior limb and the portion anterior to the angle is the anterior limb.

Question 11

. The corticospinal and corticobulbar fibers, destined for the spinal cord and brainstem respectively, travel in the posterior limb of the internal capsule and maintain a somatotopic organization.

Question 12

Describe the route of corticobulbar fibers

Answer 12

The corticobulbar fibers pass nearest to the angle of the internal capsule, called the genu, with the arm, trunk, and leg fibers arranged in that order as one moves posterior-laterally. These fibers maintain their somatotopic arrangement in the brainstem.

In the cerebral peduncle, the face or bulbar fibers are most ventral-medial and the legs are dorsal-lateral. After the pyramidal fibers have crossed in the lower medulla and upper cervical spinal cord, they maintain a somatotopic arrangement such that the leg fibers are most lateral and arm fibers are most medial; the few trunk fibers that travel in the lateral corticospinal tract lie between the leg and arm fibers. Note also that the majority of the trunk fibers have not crossed over but travel on the ipsilateral side of the spinal cord as the anterior or ventral corticospinal tract.

Question 13

LMNs are somatotopically arranged such that the neurons serving the extremities are located laterally in the ventral horn while the motor neurons serving the trunk muscles lie medially.

Answer 13

Note that the descending corticospinal fibers innervating the arm are positioned medially rather than peripherally in the spinal cord. The arm fibers peel off first to synapse on their target motor neurons in the ventral horns and in the process get out of the way of corticospinal fibers descending to innervate the legs.

The ventral corticospinal tract synapse on trunk and axial muscle motor neurons lying most medially and near the tract.

Question 14

This slide presents the vestibulospinal tract. Where does its fibers originate?

Answer 14

from the vestibular nuclei.

Question 15

(It is not important that you learn the anatomy of the vestibulospinal tract pathway but know that this tract along with the reticulospinal tract terminate where?

Answer 15

at the cervical and thoracic spinal cord levels to innervate neck and trunk muscles and that such innervation provides information coming from the vestibular formation to control coordinated movements of neck and trunk muscles.

Question 16

The reticulospinal tract. These fibers originate from neurons where?

Answer 16

in the reticular system located diffusely in the brainstem.

Question 17

It is not important that you learn the anatomy of the the reticulospinal tract pathway but you should know that this tract along with the vestibulospinal tract terminates largely where?

Answer 17

at the cervical and thoracic spinal cord level to innervate neck and trunk muscles and that such innervation provides information coming from the reticular formation to control coordinated movements of the head and trunk muscles.

Question 18

This slide presents the rubrospinal tract with fibers that originate in the red nucleus. This tract travels closely with what?

Answer 18

the lateral corticospinal tract and participates in the control of arm muscles.

Question 19

This slide presents the colliculospinal tract also known as the tectospinal tract. These fibers originate where?

Answer 19

from the superior colliculus.

Question 20

Where does the colliculospinal tract terminate largely?

Answer 20

at the cervical spinal cord level to innervate neck muscles and that such innervation provides information coming from the superior colliculus to coordinate head movements with eye movements.

Collicular vision and the colliculospinal pathway is what turns your head and eyes toward a bug crawling on the wall before you think about it. It’s a reflex reaction to pay immediate attention to sudden disturbances in your visual periphery, a very useful reflex from an evolutionary standpoint if you didn’t want to get eaten or if you were scouring the landscape for something to eat.

Question 21

A brain lesion, such as a stroke, at the points in the corticospinal tract indicated by the black arrows, would cause patients to develop what?

Answer 21

weakness of the contralateral face and limbs. (Note that such lesions occur above or rostral to the pyramidal decussation)

Question 22

If the lesion to the corticospinal tract occurred below the decussation, shown by the blue arrow, what would happen?

Answer 22

the patient would develop ipsilateral weakness of the arm and leg when the lesion was high in the cervical cord or only in the ipsilateral leg if the lesion lay below the cervical cord.

Question 23

If the lesion of the corticospinal tract involved only the anterior or ventral horn cells, or the peripheral nerves, as indicated by the red arrows, how would it present?

Answer 23

the patient would develop weakness of the ipsilateral limb served by that lower motor neuron population or peripheral nerve. Thus, muscle weakness is a symptom and sign of both upper and lower motor neuron lesions.

Question 24

Importantly, additional differences in signs and symptoms distinguish upper from lower motor neuron weakness. In general, upper motor neurons influence lower motor neurons by inhibiting the local reflex arcs. Lesions that interrupt that inhibition cause the local spinal reflex systems to go unchecked and to become hyperactive. Thus, upper motor neuron or corticospinal tract lesions increase muscle tone and muscle stretch reflexes.

Answer 24

In contrast, lower motor neuron or peripheral nerve lesions decrease muscle tone and muscle stretch reflexes.

Question 25

Since the motor neuron maintains a neurotrophic relationship with the muscle that it innervates, lesions of either the motor neuron or peripheral nerve block trophic substances from reaching the muscle which then atrophies. Lesions to upper motor neurons do not interrupt this trophic relationship so minimal or no muscle atrophy occurs.

Answer 25

With the loss of lower motor neuron input to muscles, the muscle fiber membrane becomes hyperexcitable and may spontaneously discharge creating contractions of small muscle groups called fasciculations.

Question 26

This slide shows the pathologic reflex called the Babinski Sign.

As one strokes the lateral sole of the foot with a blunt object, enough to cause discomfort, the response in normal adults is plantar flexion, that is the big toe goes down. This is shown on the left.

Answer 26

Following injury to the upper motor neurons or corticospinal tract, the same maneuver elicits an extensor plantar response, that is the big toe goes up with or without fanning of the other toes as shown on the right.

Question 27

T or F. The Babinski Sign is a primitive reflex that exists in newborns

Answer 27

T. BUT it and disappears with maturation of the upper motor neuron system at 3 to 6 months of age.

Question 28

Once again both conditions are characterized by motor weakness. Upper motor neuron lesions produce so called spastic motor responses that feature increased muscle tone, hyperactive muscle stretch reflexes, and clonus. What is clonus?

Answer 28

Clonus is characterized by a repetitive plantar extension of the foot when the foot is forcefully flexed upward. It reflects the muscle stretch reflex that is unchecked and continues to operate through a feedback loop.

In addition, upper motor neurons lesions produce an extensor plantar response, the Babinski sign. Lower motor neuron lesions cause a flaccid muscle response with decreased muscle tone and decreased muscle stretch reflexes. In addition, fasciculations of the muscle may be seen and the muscle will become atrophic in a short period of time.

Question 29

Answer 29

4.

Question 30

Answer 30

UMN lesio likely- 1 and 4

Question 31

Answer 31

B.

D- cutamen

Question 32

Answer 32

F.

Flexing the neck stretches the dorsal columns (lermet sign) and will cause tingling in the lower extremities

Question 33

Answer 33

2.

Question 34

Answer 34

3.

Question 35

Answer 35

4 and 5

Question 36

Answer 36

3 and 4. Get an MRI of the neck

Question 37

Answer 37

1.