Chapter 16 - Lower Motor Neuron Circuits and Motor Control. Flashcards Preview

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Flashcards in Chapter 16 - Lower Motor Neuron Circuits and Motor Control. Deck (67):
1

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

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

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?

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

3

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

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

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?

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.

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?

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.

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?

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.

7

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

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".

8

The upper motor neurons have cell bodies in..

The brainstem or cerebral cortex.

9

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

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

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?

In particular cortical areas in the frontal lobe:
- Brodmann's area 4 (primary motor cortex)
- Brodmann's area 6 (premotor cortex)

11

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

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

12

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

Situated just anterior to the primary motor cortex (BA4)

13

Name one particular function of the cerebellum.

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.

14

define attentuate.

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

15

Name two diseases associated with the basal ganglia.

Parkinson's Disease and Huntington's disease.

16

What is a motor neuron pool?

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

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.

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.

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?

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.

19

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

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

20

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

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

21

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

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

22

What are the extrafusal muscle fibers?

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.

23

A given extrafusal fiber is innervated by how many neurons?

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

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?

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.

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?

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

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?

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

27

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

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.

28

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

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.

29

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

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.

30

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

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

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?

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.

32

What is meant by the orderly recruitment of motor units?

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.

33

The orderly recruitment of motor units is also simply called...

the size principle.

34

What is the size principle?

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.

35

What are the muscle spindles?

The sensory receptors embedded within most muscles.

36

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

1. Nuclear bag fibers
2. Nuclear chain fibers

37

What are intrafusal muscle fibers?

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.

38

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

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

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?

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.

40

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

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

41

Explain the stretch reflex circuitry.

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!

42

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

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

43

The monosynaptic reflex arc is variously referred to as..

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

44

The stretch reflex is a positive or negative feedback?

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

45

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

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

46

What is meant by "the gain" of a reflex?

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.

47

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

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.

48

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

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

49

What are Golgi tendon organs?

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

50

What is a tendon?

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)

51

Mention a famous tendon ;)

The Achilles tendon!

52

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

Just one: the Ib sensory axon.

53

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

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.

54

How sensitive are the Golgi tendon organs to passive stretch?

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.

55

How does the Golgi tendon circuit look?

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.

56

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

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

57

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

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

58

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

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.

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?

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.

60

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

1. A stance phase
2. A swing phase

61

The stance phase of locomotion is when what is happening?

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

62

The swing phase of locomotion is when what is happening?

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

63

Increases in the speed of locomotion result mostly from..

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

64

Which brain region, when stimulated, can trigger locomotion?

The mesencephalic locomotor region.

65

Is the mesencephalic locomotor region vital to locomotion?

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.

66

What is the "lower motor neuron syndrome"?

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".

67

What are the symptoms of "lower motor neuron syndrome"?

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)