Chapter 16 - Lower Motor Neuron Circuits and Motor Control.

Question 1

The neural circuits responsible for the control of movement can be divided into four distinct but highly interactive subsystems. Which?

Answer 1

1. Local circuity within the gray matter of the spinal cord and the tegmentum of the brainstem.
2. Upper motor neurons with cell bodies in the brainstem or cerebral cortex and whose axons descend to synapse with the local circuit neurons.
3. Cerebellum
4. Basal Ganglia

Question 2

According to the schematic simplification of the neural structures involved in the control of movement: what is in direct contact with the basal ganglia?

Answer 2

The basal ganglia is in direct contact with the descending systems (upper motor neurons).

Question 3

According to the schematic simplification of the neural structures involved in the control of movement: what is in direct contact with the cerebellum?

Answer 3

The cerebellum is in direct contact with the descending systems (upper motor neurons).

Question 4

According to the schematic simplification of the neural structures involved in the control of movement: what is in direct contact with the local circuit neurons?

Answer 4

The local circuit neurons, a part of the spinal cord and brainstem circuits, is innervated by the descending motor system and sensory inputs. It innervates the motor neuron pools.

Question 5

According to the schematic simplification of the neural structures involved in the control of movement: what is in direct contact with the local neuron pools?

Answer 5

The local neuron pools, a part of the spinal cord and brainstem circuits, is innervated by the local circuit neurons, and innervates the skeletal muscles.

Question 6

According to the schematic simplification of the neural structures involved in the control of movement: what is in direct contact with the descending system?

Answer 6

The descending system (upper motor neurons) is innervated by the basal ganglia and cerebellum, and itself innervates mainly the local circuit neurons, but also the motor neuron pools.

Question 7

Which neurons, and why, comprise the “final common path” for initiating movement, according to British neurophysiologist Charles Sherrington?

Answer 7

All commands for movement, whether reflexive or voluntary, are ultimately conveyed to the muscles by the activity of the lower motor neurons. This is the “final common path”.

Question 8

The upper motor neurons have cell bodies in..

Answer 8

The brainstem or cerebral cortex.

Question 9

The upper motor neurons have cell bodies in the brainstem or cerebral cortex, and have axons that synapse with..

Answer 9

the local circuit neurons or (more rarely) with the lower motor neurons directly.

Question 10

The upper motor neuron pathways that arise in the cortex are essential for the initiation of voluntary movements and for complex spatiotemporal sequences of skilled movement. In particular, which cortical areas?

Answer 10

In particular cortical areas in the frontal lobe:

• Brodmann’s area 4 (primary motor cortex)
• Brodmann’s area 6 (premotor cortex)

Question 11

Brodmann’s area 4, the primary motor cortex is located where?

Answer 11

It is located in the posterior portion of the frontal lobe.

Question 12

Brodmann’s area 6, the premotor cortex is located where?

Answer 12

Situated just anterior to the primary motor cortex (BA4)

Question 13

Name one particular function of the cerebellum.

Answer 13

The cerebellum acts via its efferent pathways to the upper motor neurons as a servomechanism, detecting and attenuating the difference, or “motor error”, between an intended movement and the movement actually perfomed. The cerebellum uses this information about discrepancies to mediate both real-time and long-term reductions in these inevitable motor errors.

Question 14

define attentuate.

Answer 14

to weaken or reduce in force, intensity, effect, quantity, or value:

Question 15

Name two diseases associated with the basal ganglia.

Answer 15

Parkinson’s Disease and Huntington’s disease.

Question 16

What is a motor neuron pool?

Answer 16

A motor neuron pool is the combined group of all the lower motor neurons that innervate a single muscle.

Question 17

Looking at a cross section of a part of the spinal cord, which motor neuron pool would be the most lateral?

• The neurons that innervate the muscles of the shoulder?
• The motor neurons that innervate the fingers.

Answer 17

The motor neurons that innervate the fingers would be located the most lateral out of the two. This is because the mapping of motor neuron pools follows the organization where the pools that innervate the distal parts lie farthest from the midline.

Question 18

The mapping of motor neuron pools follows the organization where the pools that innervate the distal parts lie farthest from the midline. What is a possible explanation for this?

Answer 18

Many of the neuron pools close to the midline have axonal branches that cross the midline in the commissure of the spinal cord to innervate lower motor neurons on the medial part of the contralateral hemicord. This arrangement ensure that groups of axial muscles on both sides of the body act in concert to maintain and adjust posture. In contrast, local circuit neurons in the lateral region of the intermediate zone have shorter axons that typically extend fewer than five segments and are predominantly ipsilateral. This more restricted pattern of connectivity provides the finer and more differentiated control that is exerted over the muscles of the distal extremities, such as that required for the independent movement of individual fingers during manipulative tasks.

Question 19

Two types of lower motor neurons are found in the motor neuronal pools of the ventral horn. Which?

Answer 19

Small γ (gamma) motor neurons and α (alpha) motor neurons.

Question 20

What is the function of the γ (gamma) motor neurons in the ventral horn?

Answer 20

The function is to regulate the sensory input to the brainstem motor pools by setting the intrafusal muscles fibers to an appropriate length.

Question 21

What is the function of α (alpha) motor neurons in the ventral horn?

Answer 21

They innervate the extrafusal muscle fibers, which are teh striated muscle fibers that actually generate the forces needed for posture and movement.

Question 22

What are the extrafusal muscle fibers?

Answer 22

Extrafusal muscle fibers are the skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement.

Question 23

A given extrafusal fiber is innervated by how many neurons?

Answer 23

Most extrafusal skeletal muscle fibers in mature mammals are innervated by only a single α motor neuron.

Question 24

Most extrafusal skeletal muscle fibers in mature mammals are innervated by only a single α motor neuron. But there are far more muscle fibers than motor neurons. How can this be?

Answer 24

Individual motor axons branch within muscles to synapse on many extrafusal fibers. These fibers are typically distributed over a relatively wide area within the muscle, presumably to ensure that the contractile force is spread evenly.

Question 25

British neurophysiologist Charles Sherrington, who won The Nobel Prize in Physiology or Medicine 1932together with Edgar Douglas Adrian “for their discoveries regarding the functions of neurons”, coined the term “motor unit”. What is meant by this term today?

Answer 25

The relationship between an α motor neuron and the muscle fibers it innervates is called a motor unit.

Question 26

The relationship between an α motor neuron and the muscle fibers it innervates is called a motor unit. We differentiate between three types of motor units. Which?

Answer 26

1. slow (S) motor units.
2. fast fatigable (FF) motor units.
3. fast fatigue-resistant (FR) motor units.

Question 27

What are slow (S) motor units? Also give an example of their use.

Answer 27

Slow motor units consist of small motor neurons that innervate relatively few muscle fibers. These muscle fibers contract slowly and generate relatively small forces; but, because of their rich myoglobin content, plentiful mitochondria, and rich capillary beds, these small fibers are resistant to fatigue. They are thus important for activities that require sustained muscular contraction, such as the maintenance of an upright posture.

Question 28

What are fast fatigable (FF) motor units? Also give an example of their use.

Answer 28

FF motor units consist of large α motor neurons that innervate larger, pale muscle fibers that generate more force. However, these fibers have sparse mitochondria and are therefore easily fatigued. They are especially important for brief exertions that require large forces, such as running or jumping.

Question 29

What are fast fatigue-resistant (FR) motor units?

Answer 29

FR motor units are of intermediate size and are not quite as fast as FF motor units. They generate about twice the force of a slow motor unit and, as the name implies, are resistant to fatigue.

Question 30

What can the properties of the different motor units tell us about their “reason”?

Answer 30

These distinctions among different types of motor unit explain how the nervous system produces movements appropriate for different circumstances.

Question 31

In the 1960s, Elwood Henneman and his colleagues at Harvard Medical School measured the tension changes in the muscle after stimulating sensory nerves or upper motor pathways that project to a lower motor neuron pool. What were their findings?

Answer 31

Increasing of decreasing the number of motor units active at any one time changes the amount of force produced by a muscle. Henneman found that, in experimental animals, only the smallest motor units in the pool are activated by weak synaptic stimulation. When synaptic input to the motor pool increases, progressively larger motor units are recruited.

Question 32

What is meant by the orderly recruitment of motor units?

Answer 32

It harkens back to the findings in the original experiments of Elwood Henneman and colleagues in the 1960s. They discovered that: When synaptic input to the motor pool increases, progressively larger motor units are recruited.

Question 33

The orderly recruitment of motor units is also simply called…

Answer 33

the size principle.

Question 34

What is the size principle?

Answer 34

Increasing of decreasing the number of motor units active at any one time changes the amount of force produced by a muscle. Henneman found that, in experimental animals, only the smallest motor units in the pool are activated by weak synaptic stimulation. When synaptic input to the motor pool increases, progressively larger motor units are recruited. This systematic relationship has come to be known as the size principle.

Question 35

What are the muscle spindles?

Answer 35

The sensory receptors embedded within most muscles.

Question 36

There are two structural and functional classes of intrafusal fibers. Which?

Answer 36

1. Nuclear bag fibers

2. Nuclear chain fibers

Question 37

What are intrafusal muscle fibers?

Answer 37

Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle. They constitute the muscle spindles.

Question 38

One muscle spindle, the sensory receptors embedded within most muscles, contain what amount of the two fiber classes?

Answer 38

Most muscle spindles contain two or three nuclear bag fibers and at least twice that many nuclear chain fibers.

Question 39

There are two groups of afferent sensory axons that contact the nuclear bag fibers and the nuclear chain fibers respectively. What are they called and what are their specializations?

Answer 39

Ia afferents are large-diameter axons that are coiled around the central part of each class of intrafusal fiber. These tend to respond phasically to small streches. This is because Ia afferent activity is dominated by signals transduced by the dynamic subtype of nuclear bag fiber whose biomechanical properties emphasize the VELOCITY of fiber strech.

Group II afferents are nearly as large in diameter and form endings mainly on the nuclear chain fibers. They signal the level of sustained fiber stretch by firing tonically at a frequency in proportion to the degree of strech, with little dynamic sensitivity.

Question 40

The group Ia and group II axons are large axons. What does this tell us about their function?

Answer 40

Large axons have higher signal velocity. This is probably because they mediate very rapid reflex adjustments when the muscle is stretched.

Question 41

Explain the stretch reflex circuitry.

Answer 41

Stretching a muscle spindle leads to increased activity in Ia afferents and an increase in the activity of α motor neurons that innervate the same muscle. Ia afferents also excite the motor neurons that innervate synergistic muscles, and they indirectly inhibit the motor neurons that innervate antagonists. If you can draw something similar to the circuitry on p. 363 you pass!

Question 42

The type of reflex circuitry that we see in the stretch reflex is called a monosynaptic reflex. Why?

Answer 42

When a reflex arc consists of only two neurons in an animal (one sensory neuron, and one motor neuron), it is defined as monosynaptic.

Question 43

The monosynaptic reflex arc is variously referred to as..

Answer 43

the strech reflex arc, deep tendon or the myotatic reflex arc.

Question 44

The stretch reflex is a positive or negative feedback?

Answer 44

It’s a negative feedback because it counteracts the stimuli.

Question 45

The α motor neurons innervate muscle fibers. What do the γ motor neurons do?

Answer 45

The γ motor neurons control the functional characteristics of the muscle spindles by modulating their level of excitability.

Question 46

What is meant by “the gain” of a reflex?

Answer 46

The level of γ motor neuron activity is often referred to as γ bias, or gain, and can be adjusted by upper motor neuron pathways as well as by local reflex circuitry. If the gain of a reflex is high, then a small amount of stretch applied to the intrafusal fibers will produce a large increase in their firing rates.

Question 47

Give an example of when you want the “gain” of the stretch reflex to be low.

Answer 47

During voluntary stretching, such as warming up for athletic performance, the gain of myotatic reflexes must be reduced to facilitate the lengthening of muscle fibers and other elastic elements of the musculotendinous system that is desirable under these circumstances.

Question 48

In experiments with cats, which conditions showed very high γ motor neuron activation?

Answer 48

During difficult movements, such as walking across a narrow beam, and during unpredictable conditions such as being picked up.

Question 49

What are Golgi tendon organs?

Answer 49

Goldi tendon organs are encapsulated afferent nerve endings located at the end of a muscle and a tendon.

Question 50

What is a tendon?

Answer 50

A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. (NO: sene)

Question 51

Mention a famous tendon ;)

Answer 51

The Achilles tendon!

Question 52

The Golgi tendon organ is innervated by which class(es) neurons?

Answer 52

Just one: the Ib sensory axon.

Question 53

What types of movements are the Golgi tendon organs most sensitive to?

Answer 53

When a muscle actively contracts this leads to an increase in the tension of the collagen fibrils in the tendon organ and compression of the intertwined sensory receptor. As a result, Golgi tendon organs are exquisitely sensitive to increases in muscle tension that arise from muscle contraction.

Question 54

How sensitive are the Golgi tendon organs to passive stretch?

Answer 54

When a muscle is passively stretched, most of the change in length occurs in the muscle fibers, since they are more elastic than then fibrils of the tendon. Hence, the Golgi tendon organ is relatively insensitive to stretch.

Question 55

How does the Golgi tendon circuit look?

Answer 55

The lb axons from Golgi tendon organs contact inhibitory local circuit neurons in the spinal cord that synapse, in turn, with the α motor neurons that innervate the same muscle. The Golgi tendon circuit is thus a negative feedback system that regulates muscle tension; it decreases the activation of a muscle when exceptionally large forces are generated and, in this way, it protects the muscle.

Question 56

The muscle spindle system is a feedback system that monitors and maintains muscle …..

Answer 56

length (you don’t get top score if you don’t actually understand this)

Question 57

The Golgi tendon system is a feedback system that monitors and maintains muscle ….

Answer 57

force (you don’t get top score if you don’t actually understand this)

Question 58

Figure 16.14 is named: “Spinal cord circuitry for the flexion-crossed extension reflex”. What do you think the figure shows?

Answer 58

It shows a schematic of the spinal cord neurons involved in a reflex called the flexion-crossed extension reflex. This is a withdrawal reflex caused by stimulation of the cutaneous receptors in a limb, i.e. the foot. Stepping on a tack leads to activation of spinal cord local circuits that serve to withdraw (flex) the stimulated extremity and extend the other extremity to provide compensatory support.

Question 59

Like all other reflex pathways, local circuit neurons in the flexion reflex pathway receive converging inputs from several different sources, including other spinal cord interneurons and upper motor neurons. The functional significance of this is unclear, but something interesting has been observed following damage to descending pathways. What?

Answer 59

Under normal conditions, a harmful stimulus is required to evoke the flexion reflex; following damage to descending pathways, however, other types of stimulation such as squeezing a limb, can sometimes produce the same response.

Question 60

Locomotion (walking, running, etc.) can be thought of as a cycle of two phases. Which?

Answer 60

1. A stance phase

2. A swing phase

Question 61

The stance phase of locomotion is when what is happening?

Answer 61

The limb is extended and placed in contact with the ground to propel the animal forward.

Question 62

The swing phase of locomotion is when what is happening?

Answer 62

The limb is flexed to leave the ground and then brought forward to begin the next stance phase.

Question 63

Increases in the speed of locomotion result mostly from..

Answer 63

a decrease in the amount of time it takes to complete a cycle. Most often achieved by shortening the stance phase.

Question 64

Which brain region, when stimulated, can trigger locomotion?

Answer 64

The mesencephalic locomotor region.

Question 65

Is the mesencephalic locomotor region vital to locomotion?

Answer 65

Following transections of the spinal cord at the thoracic level, a cat’s hindlimbs will still make coordinated locomotor movements if the animal is supported and placed on a moving treadmill.
It appears that each limb has its own central pattern generator responsible for the alternating flexion and extension of the limb during locomotion.
In humans these findings are true as well, but they are less impressive.

Question 66

What is the “lower motor neuron syndrome”?

Answer 66

The complex of signs and symptoms that arise from damage to lower motor neurons of the brainstem and spinal cord is referred to as the “lower motor neuron syndrome”.

Question 67

What are the symptoms of “lower motor neuron syndrome”?

Answer 67

1. paralysis (loss of movement)
2. paresis (weakness)
3. areflexia (loss of reflexes)
4. loss of muscle tone
5. atrophy (loss of muscle due to disuse)