Chapter 17 - Upper Motor Neuron Control of the Brainstem and Spinal Cord

Question 1

What are the targets of upper motor neurons?

Answer 1

They target the local circuit neurons within the spinal cord.

Question 2

A midsection of the spinal cord has a butterfly-looking area. What is it comprised of?

Answer 2

The area is the gray matter area.

Question 3

What is meant by the horns of the internal spinal cord?

Answer 3

The gray matter area that looks like a butterfly has four wings. The wings are called horns. They are either dorsal or ventral horns.

Question 4

The axons of the upper motor neurons descend across two (four) areas of the spinal cord. Which, and where are they positioned - both in the “butterfly” and dorsal/ventral.

Answer 4

The lateral white matter, a circular column between butterfly wing tips; and the medial white matter, an area beneath the butterfly. The lateral white matter is dorsal to the medial white matter.

Question 5

Axons in the lateral white matter descend from upper motor neurons in …

Answer 5

the cerebral cortex.

Question 6

Axons in the medial white matter descend from upper motor neurons in …

Answer 6

the brainstem

Question 7

The upper motor neurons in the cerebral cortex reside in several adjacent and highly interconnected areas in …

Answer 7

the posterior frontal lobe.

Question 8

The upper motor neurons in the cerebral cortex all receive regulatory input from the … via …

Answer 8

basal ganglia and cerebellum via relays in the ventrolateral thalamus.

Question 9

Why, mainly, is the precentral gyrus named the primary motor cortex?

Answer 9

This area is special because you can elicit movements by electrical stimulation with a low intensity current. The low threshold for eliciting movements is an indicator of a relatively large and direct pathway from the primary area to the lower motor neurons of the brainstem and spinal cord.

Question 10

In which cortical layer will you find the upper motor neurons of the primary motor cortex?

Answer 10

In cortical layer 5.

Question 11

The largest neurons (by soma) in the human central nervous system is found in the primary motor cortex. Which layer and what are their names?

Answer 11

The Betz cells are located in cortical layer 5.

Question 12

Why are the Betz cells so important?

Answer 12

They’re important because they are a part of the brain, but they’re mostly known for being huge. In fact, they account for no more than 5% of the projection to the spinal cord. The big bulk of neurons in the primary motor cortex are non-Betz pyramidal neurons.

Question 13

The axons of the upper motor neurons descend in the …. tracts

Answer 13

The axons of the upper motor neurons descend in the cortiobulbar and corticospinal tracts.

Question 14

The axons of the upper motor neurons descend in the cortiobulbar and corticospinal tracts. Two names, why?

Answer 14

The names are used to distinguish between axons that terminate in the brainstem (“bulbar” refer to brainstem nuclei) or spinal cord.

Question 15

The axons of the upper motor neurons descend in the cortiobulbar and corticospinal tracts. Along their course, these axons pass through the posterior limb of the internal capsule in the forebrain to enter the cerebral peduncle at the base of the midbrain. Then they pass through the base of the pons, where they are scattered among the transverse pontine fibers and nuclei of the basal pontine gray matter. They coalesce again on the ventral surface of the ———, where they form the ——-.

Answer 15

They coalesce again on the ventral surface of the medulla, where they form the medullary pyramids.

Question 16

Where do the corticobulbar tract axons terminate?

Answer 16

They do, as their name implies, terminate in the brainstem (brainstem nuclei looks like bulbs). They leave the corticobulbar tract pathway at the appropriate levels of the brainstem.

Question 17

The crossing over of the corticospinal tract happens where?

Answer 17

At the caudal end of the medulla, almost 90% of the axons cross the midline.

Question 18

How does the corticospinal tract synapse with lower motor neurons?

Answer 18

Some axons synapse directly with certain α neurons, whilst the majority terminate among pools of local circuit neurons. This mean that the α neurons that are directly innervated by upper motor neurons are more reliant on the signals from the cortex and brainstem.

Question 19

What are “motor maps”?

Answer 19

Motor maps refer to the physical space occupied by the faculty of moving certain body parts. Musculature used in tasks that require fine motor control is represented by a greater area of motor cortex than is the musculature requiring less precise motor control.

Question 20

Intracortical stimulation has given us new information about motor maps, mention one important finding.

Answer 20

When microstimulation was combined with recordings of muscle electrical activity, even the smallest currents capable of eliciting a response initiated the excitation of several muscles (and the simultaneous suppression of others), suggesting that organized movements rather than individual muscles are represented in the map.

Question 21

Why is the homonuculus drawing slightly incorrect?

Answer 21

Because the representation of muscle movement is not organized at the level of individual muscles or body parts, and the distribution of muscle fields among neighboring cortical neurons is neither spatially continuous or temportally fixed.

Question 22

Michael S. A. Graziano and his colleagues at Princeton University conducted a study in 2005 that further supported the theory that the motor cortex is organized by movement rather than individual muscles. What did he do?

Answer 22

He extended the technique of cortical microstimulation that previous studies had done. He made the stimulation last a bit longer than previously, to a timescale that more closely corresponds to the duration of volitional movements. When such stimuli are applied to the precentral gyrus of the macaque monkey, the resulting movements are sequentially distributed across multiple joints and are strikingly purposeful. Examples of motor patterns frequently elicited were movements of the hand to the mouth, as if to feed, movements that bring the hand to central space as if to inspect an object, and defensive postures as if to protect the body from an impending collision.

Question 23

Some of the axons in the corticospinal tract arise from neurons in the premotor cortex. How many percent is “some”?

Answer 23

More than 30%!

Question 24

What is the principal difference between the premotor and primary motor cortex?

Answer 24

It lies in the strength of their connections to lower motor neurons, with more upper motor neurons in the primary motor cortex making monosynaptic connections to α motor neurons.

Question 25

One particular divison in the ventrolateral portion of the premotor cortex has received considerable attention in the recent years. Why?

Answer 25

A discovery in 1996 by Rizzolatti et al. of neurons in this area that did not just respond to preparations for the execution of particular movements, but also when the same actions was observed being performed by another (monkey or human).

Question 26

What appears to be the function of the premotor cortex?

Answer 26

We lack a complete understanding of the neurons in this part of the cortex, but it seems that neurons in the premotor cortex rather than directly commanding the initiation of a movement, these neurons appear to encode the monkey’s intention to perform a particular movement.

Question 27

Mention findings from the original mirror neuron experiments.

Answer 27

1. The neurons responded much less when the same actions were pantomimed without the presence of a behavioral goal.
2. They respond during the observation of goal-directed behavior even when the final stage of the action is hidden from view.

Question 28

Mention the type of difficulties that have been observed in patients with lesions in their premotor cortex.

Answer 28

1. Monkeys with a lesion in this are are severely impaired when trying to do visually cued conditional tasks, even though they can respond to the visual stimulus and perform the movement in a different setting.
2. Patients have difficulty learning to select a particular movement to be performed in response to a visual cue, even though they understand the instructions and can perform the movements.
3. Patients have difficulties in performing movements in response to verbal commands.

Question 29

A rostral division of the lateral premotor cortex in the human brain is famously known for something… hmmm :)

Answer 29

A rostral divison of the lateral premotor cortex in the human brain, especially in the left hemisphere, has evolved to play a special role in organizing vocal tract articulators that are involved in the production of speech sounds. This region is called Broca’s area.

Question 30

How does the medial premotor cortex differ from the lateral?

Answer 30

Like the lateral, the medial premotor cortex mediates the selection of movements. However, this region appears to be specialized for initiating movements specified by internal rather than external cues.

Question 31

What is the main function of the upper motor neurons in the brainstem?

Answer 31

1. Maintenance of balance
2. Regulation of posture
3. Orientation of visual gaze.

Question 32

Where in the brainstem are the upper motor neurons that are responsible for functions such as the maintenance of balance, regulation of posture, and the orientation of visual gaze?

Answer 32

1. Nuclei of the vestibular complex
2. the reticular formation
3. superior colliculus.

Question 33

Axons from the vestibular division of the eight cranial nerve transmit information from the semicircular canals and otolith organs. The major target of these axons is…

Answer 33

The vestibular nuclei.

Question 34

The information from the semicircular canals and otolith organs eventually reaches the vestibular nuclei. Neurons in the vestibular nuclei transmit this information to ..

Answer 34

1. Most of the cells have descending axons that terminate in the medial region of the spinal cord gray matter.
2. Some of them extend more laterally to contact the neurons that control the proximal muscles of the limbs.

Question 35

Neurons in the medial vestibular nucleus have a specific function. Which?

Answer 35

Neurons in the medial vestibular nucleus give rise to a medial vestibulospinal tract that terminates bilaterally in the medial ventral horn of the cervical cord.
There, the medial vestibulospinal tract regulates head position by reflex activation of neck muscles in response to the stimulation of the semicircular canals resulting from rotational acceleration of the head.

Question 36

There is a medial vestibulospinal tract. There is also a.. that roughly does….

Answer 36

lateral vestibulospinal tract! It transmits information about limb extensor (antigravity) muscles when the otolith organs signal deviations from stable balance and upright posture.

Question 37

The vestibular nuclei has many different functions. The medial vestibular nucleus helps regulate head position, and the lateral vestibular nucleus helps us keep our balance. There’s also a famous reflex concerning the eyes that has its seat in this area. Which?

Answer 37

The vestibulo-ocular reflex, which helps us maintain eye fixation when the head is moving, is also in this area.

Question 38

What is the reticular formation?

Answer 38

The reticular formation is a complicated network of circuits in the core of the brainstem that extends from the rostral midbrain to the caudal medulla. The neurons here have a lot of functions!

Question 39

The neurons in the reticular formation have at least 6 functions. Which?

Answer 39

1. Cardiovascular control
2. Respiratory control
3. Sensory motor reflexes
4. Coordination of eye movements
5. Regulation of sleep and wakefulness.
6. Temporal and spatial coordination of limb and trunk movements.

Question 40

Both the vestibular nuclei and the reticular formation is important for the maintenance of posture and balance. They’re only slightly different in their function. How?

Answer 40

Direct projections from the vestibular nuclei to the spinal cord ensure rapid compensatory FEEDBACK response to any postural instability detected by the vestibular labyrinth. In contrast, the motor centers in the reticular formation are controlled largely by other motor centers in the cerebral cortex, hypothalamus, or brainstem. The relevant neurons in the reticular formation initiate FEEDFORWARD adjustments that stabilize posture during ongoing movements.

Question 41

Mention one experiment where the feedforward adjustements in posture is clearly shown.

Answer 41

In one experiment (couldn’t find the source, is in the book), the subject’s muscle contractions in the bicep and grastocnemius were measured. The subject used his arm to pull on a handle in response to an auditory tone. Activity in the biceps muscle begins about 200 milliseconds after the tone. However, the contractions of the biceps is accompanied by a significant increase in the activity of a proximal leg muscle, the gastrocnemius. In fact, contraction of the gastrocnemius muscle begins well before the contraction of the biceps.|

Question 42

Injury to upper motor neurons is common. Why?

Answer 42

1. Because of the large amount of cortex occupied by the motor areas.
2. Because their pathways extend all the way from the cerebral cortex to the lower end of the spinal cord.

Question 43

What is spinal shock?

Answer 43

Damage to the motor cortex or the descending upper motor axons in the internal capsule causes an immediate flaccidity (lacking force; weak) of the muscles on the contralateral side of the body and lower face. This condition is called spinal shock and reflects the decreased activity of spinal circuits suddenly deprived of input from the motor cortex and brainstem.

Question 44

If spinal shock is due to damage to the motor cortex or descending upper motor axons, why is it called a shock and not a condition?

Answer 44

After several days, the spinal cord circuits regain much of their function for reasons that are not fully understood, but may include the strengthening of remaining connections and the sprouting of new connections.

Question 45

At least three types of permanent symptoms emerges in a patient recovering from spinal shock. List them.

Answer 45

1. The Babinski sign
2. Spasticity
3. Loss of the ability to perform fine movements.

Question 46

What is the Babinski sign?

Answer 46

One of the permanent symptoms that emerge in a patient recovering from spinal shock. The normal response in an adult to stroking of the sole of the foot is flexion of the big toe, and often the other toes. Following damage to descending upper motor neuron pathways, however, this stimulus may elicit extension of the big toe and a fanning of the other toes. A similar response occurs in human infants before the maturation of the corticospinal pathway.

Question 47

What is spasticity after spinal shock?

Answer 47

One of the permanent symptoms that emerge in a patient recovering from spinal shock. Spasticity is increased muscle tone, hyperactive stretch reflexes, and clonus.

Question 48

What is clonus?

Answer 48

Oscillatory contractions and relaxations of muscles in response to muscle stretching.