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Neuroscience - Structure and Function of the Human Brain > Chapter 13 - Spinal Control of Movement > Flashcards

Chapter 13 - Spinal Control of Movement Flashcards

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1
Q

What does the motor system consist of?

A

All of our muscles and the neurons that control them.

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2
Q

How many muscles are involved in behavior?

A

Almost 700 muscles need to be coordinated in various combinations in a changing and often unpredictable environment.

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3
Q

How can chickens run with their heads cut off?

A

Some complex patterns of behavior (such as running around) can be generated without the participation of the brain.

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4
Q

What are motor programs?

A

Programs in the spinal cord. They are used for the generation of coordinated movements, and they are accessed, executed, and modified by descending commands from the brain.

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5
Q

What two parts can the motor control be divided into?

A

1) The spinal cord’s command and control of coordinated muscle contraction.
2) The brain’s command and control of the motor programs in the spinal cord.

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6
Q

What is smooth muscle?

A

One of the two muscle categories. The other one is striated muscle.

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7
Q

Where is smooth muscle?

A

It lines the digestive tract, arteries, and related structures.

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8
Q

What innervates smooth muscle?

A

The nerve fibers from the autonomic nervous system.

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9
Q

How can striated muscle be categorized?

A

1) Cardiac muscle

2) Skeletal muscle

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10
Q

What is cardiac muscle? How does it work?

A

It is heart muscle. It contracts rhythmically even in the absence of any innervation.

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11
Q

How does the heart rate vary?

A

The autonomic nervous system (ANS) functions innervate the heart. This can accelerate or slow down the heart rate.

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12
Q

What is skeletal muscle?

A

It forms most of the muscle mass of the body.

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13
Q

What functions does skeletal muscle have?

A

1) Move bones around joins
2) Move the eyes within the head
3) Inhale and exhale
4) Control facial expressions
5) Produce speech.

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14
Q

How is skeletal muscle enclosed?

A

Skeletal muscle is enclosed in connective tissue sheath that, at the ends of the muscle, forms the tendons.

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15
Q

What are muscle fibers?

A

The fibers in the muscles. In each muscle, there are hundreds of muscle fibers. They are the cells of the skeletal muscle, and each fiber is innervated by a single axon branch from the central nervous system (CNS).

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16
Q

What is the somatic motor system? Why is it called that?

A

The skeletal muscle system, which is under voluntary control and generates behavior.

It is called somatic because skeletal muscle is derived embryologically from 33 paired somites.

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17
Q

What is a flexion? What causes it?

A

Movement in the direction that closes the “knife” (the join that functions like a hinge on a pocket knife).

It is caused by the brachialis major muscle. Two minor muscles are also involved: biceps brachii and coracobrachialis.

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18
Q

What is an extension?

A

Movement in the direction that opens the “knife”.

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19
Q

What are the flexors?

A

1) Brachialis
2) Biceps brachii
3) Coracobrachialis

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20
Q

What are synergists and antagonists?

A

Synergists: Muscles that work together.
Antagonists: Muscles that pull on the joint in opposite directions.

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21
Q

What are the extensors?

A

1) Tricepts brachii

2) Anconeus

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22
Q

What kind of coordinated action is required for flexions?

A

1) Relaxing antagonist extensor muscles

2) Contraction of the synergistic flexor muscles

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23
Q

What are the muscles responsible for movements of the trunk called?

A

Axial muscles.

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24
Q

What are the muscles that move the shoulder, elbow, pelvis and knee called?

A

Proximal (or girdle) muscles.

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25
Q

What are the muscles that move the hands, feet, and digits (fingers and toes) called?

A

Distal muscles.

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26
Q

What are lower motor neurons?

A

Somatic motor neurons in the ventral horn of the spinal cord that control muscle contraction.

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27
Q

What do lower motor neurons do?

A

Innervate the somatic musculature, ergo command muscle contraction.

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28
Q

What are upper motor neurons?

A

Neurons that supply input to the spinal cord.

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29
Q

What is the final common pathway for the control of behavior?

A

The lower motor neurons. They are the neurons that control muscle contraction and behavior.

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30
Q

How are the neurons organized in the lower motor neuron system?

A

1) The axons of lower motor neurons bundle together to form ventral roots.
2) Each ventral root joins with a dorsal roots to form a spinal nerve.
3) The spinal nerve exits the cord through the notches between vertebrae.

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31
Q

What are mixed spinal nerves?

A

Spinal nerves that contain both sensory and motor fibers.

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32
Q

What is a spinal segment?

A 
The segment of the spinal cord that the motor neurons connect to.
Cervical 8
Thoracic 12
Lumbar 5
Sacral 5
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33
Q

How are lower motor neurons distributed within the spinal cord?

A

Some regions of the spinal cord have swollen dorsal and ventral horns because they control a larger portion of the muscles.

E.g., C3-T1 control the innervation of the more than 50 muscles of the arm.
L1-S3 control the leg musculature.

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34
Q

Where are the motor neurons that innervate distal and proximal musculature located?

A

Mainly in the cervical and lumbar-sacral segments of the spinal cord.

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35
Q

Where are the motor neurons that innervate axial musculature?

A

At all levels of the spinal cord.

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36
Q

How are the axial muscle neurons and distal muscle neurons located to each other?

A

The axial muscle neurons are medial to the distal muscle innervating neurons.

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37
Q

How are flexor and dorsal innervating neurons located?

A

Neurons innervating flexors are dorsal to those innervating extensors.

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38
Q

How are lower motor neurons categorized?

A

1) Alpha motor neurons

2) Gamma motor neurons

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39
Q

What are alpha motor neurons?

A

Motor neurons that directly trigger the generation of force by muscles.

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40
Q

What is a motor unit?

A

A combination of an alpha motor neurons and all the muscle fibers it innervates. It is an elementary component of motor control.

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41
Q

What is a motor neuron pool?

A

A combination of alpha motor neurons that innervate a single muscle (e.g., the biceps brachii).

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42
Q

How does the CNS control muscle contraction?

A

1) Varying the firing rate of motor neurons.

2) Recruiting additional synergistic motor units.

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43
Q

How does the varying of firing rate of motor neurons work by the CNS?

A

1) Alpha motor neurons communicate with muscle fibers by releasing ACh at the neuromuscular junction.
2) ACh released in response to one presynaptic action potential causes an excitatory postsynaptic potential (EPSP) in the muscle fiber. Also called end-plate potential.
3) This EPSP is large enough to trigger one postsynaptic action potential.
4) Postsynaptic action potential causes a twitch (a rapid sequence of contraction and relaxation) in the muscle fiber.
5) A sustained contraction requires a continual barrage of action potentials.
6) High-frequency presynaptic activity causes temporal summation of postsynaptic responses.
7) Twitch summation increases the tension in the muscle fibers and smoothes the contraction.

44
Q

How does the recruiting of additional synergic motor units work by the CNS?

A

1) Extra tension provided by the recruitment of the active motor unit depends on how many muscle fibers are in that unit.
2) Innervation ratios for small muscles in e.g., eyes and digits allow more fine control.
3) Most muscles have a range of motor unit sizes, and smallest are recruited first, largest last.
4) Small neurons, as a consequence of the geometry and physiology of their soma and dendrites, are more easily excited by signals descending from the brain.

45
Q

What is the size principle?

A

The idea that the orderly recruitment of motor neurons is due to variations in alpha motor neuron size.

46
Q

What are the 3 major sources of input to alpha motor neurons?

A

1) Input from spinal interneurons
2) Sensory input from muscle spindles
3) Input from upper motor neurons in the brain

47
Q

What does the input from muscle spindles provide feedback about?

A

It provides feedback about muscle length.

48
Q

What is the input from upper motor neurons in the motor cortex and brain stem used for?

A

It provides input for the initiatuion and control of voluntary movement.

49
Q

What does the input from the interneurons in the spinal cord tell the alpha motor neuron?

A

This input may be excitatory or inhibitory and is part of the circuitry that generates the spinal motor programs.

50
Q

What are red (dark) muscle fibers characterized by?

A

A large number of mitochondria and enzymes specialized for oxidative energy metabolism.

They are also called slow (S) fibers, and are relatively slow to contract but can sustain contraction for a long time without fatigue.

51
Q

Where are the red (dark) muscle fibers / S / slow fibers located, for example?

A

E.g., in the antigravity muscles of the leg and torso, and in the flight muscles of birds that fly (as opposed to domesticated chickens).

52
Q

What are the pale (white) muscle fibers characterized by?

A

Fewer mitocontria, and relying mainly on anaerobic metabolism.

These fast (F) fibers contract rapidly and powerfully, but fatigue more quickly.

53
Q

Where are white muscle fibers / F/ fast fibers located, for example?

A

Muscles involved in escape reflexes; jumping muscles of frogs and rabbits. In humans, arm muscles.

54
Q

How can F fibers be divided?

A

Into FR (fatigue-resistant) and FF (fast-fatigable) fibers.

55
Q

What do FR fibers do?

A

They generate moderately strong and fast contractions and are relatively resistant to fatigue.

56
Q

What do FF fibers do?

A

They generate the strongest, fastest contractions but are quickly exhausted.

57
Q

How do muscle fibers relate to motor units?

A

Each motor unit can only contain muscle fibers of a single type.

Thus, there are slow motor units and fast motor units.

58
Q

How does the axon size in muscle fibers work?

A

FF units are generally biggest and have largest diameter, fastest conducting axons. (30-60 Hz impulses)

FR units have motor neurons and axons of intermediate size.

Slow units have small-diameter, slow-conducting axons. (10-20 Hz impulses)

59
Q

Say a few words about neuromuscular matchmaking.

A

In an experiment where innervation of fast muscles were replaced with innervation of slow muscles, the muscle acquired slow properties in both operation and biochemistry. This is called switch of muscle phenotype.

Neurons may also switch phenotype as consequence of synaptic activity (experience) and this may be a basis for learning and memory.

60
Q

What are proprioceptors? Give an example?

A

Muscle spindles and their associated Ia axons, specialized for the detection of changes in muscle length (stretch).

They are the receptors responsible for body sense, proprioception.

61
Q

What is a stretch reflex?

A

Also called myotatic reflex.

When a muscle is pulled on, it tends to pull back (contract). It involves sensory feedback from the muscle.

62
Q

What causes the knee-jerk reflex?

A

Tapping the tendon of quaadriceps very briefly stretches the quadriceps muscle of the thigh. Then it reflexively contracts and extends the leg.

63
Q

What are the muscle fibers within a muscle spindle called?

A

Intrafusal fibers.

64
Q

What are the muscle fibers that lie outside the muscle spindle and form the bulk of the muscle?

A

Extrafusal fibers.

65
Q

What are gamma motor neurons?

A

Motor neurons that innervate the intrafusal muscle fiber at the two ends of the muscle spindle.

They provide information about muscle length to the spinal cord when the muscles contract and the spindles would otherwise become slack.

66
Q

How do the activations of alpha and gamma motor neurons affect Ia output?

A
  • Alpha activation alone decreases Ia activity.

- Gamma activation alone increases Ia activity.

67
Q

What is the gamma loop?

A

1) Gamma motor neuron
2) Intrafusal muscle fiber
3) Ia afferent axon
4) Alpha motor neuron (Synapse with)
5) Extrafusal muscle fibers

68
Q

What is the Golgi tendon organ?

A

Sensor of skeletal muscle, that monitors muscle tension, or the force of contraction.

69
Q

Where are Golgi tendon organs located and what innervates them?

A

They are at the junction of the muscle and the tendon and innervated by group Ib sensory axons.

70
Q

How do Golgi tendon organs work?

A

Branches of Ib axons entwine among collagen fibrils. Along with contraction, tension increases, and mechanosensitive ion channels are activated and action potentials can be triggered.

71
Q

How do Ib axons from the Golgi tendon organ enter the CNS?

A

1) Enter spinal cord
2) Branch repeatedly
3) Synapse on special interneurons (Ib inhibitory interneurons) in the ventral horn
( 4) Ib inhibitory interneurons also receive inputs from other sensory receptors and from descending pathways)

72
Q

What does the Ib reflex arc do?

A

It may protect the muscle from being overloaded in extreme situations.

In normal situations, it regulates muscle tension within an optimal range. Muscle tension increases -> inhibiton of the alpha motor neuron slows muscle contraction.

This is particularly important for execution of fine motor acts.

73
Q

How does proprioception from the joints work?

A

There are a variety of proprioceptive axons in the connective tissues of joints, especially within the fibrous tissues surrounding the joints (joint capsules) and ligaments.

They respond to changes in the angle, direction, and velocity of movement in a joint.

They send a lot of information about a moving joint, but few about the resting position.

74
Q

How do the actions of Ib inputs from Golgi tendon organs on alpha motor neurons work?

A

They are polysnyaptic; they are all mediated by intervening spinal interneurons. Most of the input comes from interneurons of the spinal cord (as opposed from the Golgi tendon organs).

The interneurons are networked together in a way that allows coordinated motor programs to be generated in response to their many inputs.

75
Q

What is reciprocal inhibition?

A

The process in e.g., a simple stretch reflex where one set of muscles is relaxed and one is flexed. Interneurons play a critical role in the proper execution of these reflexes.

76
Q

How is reciprocal inhibition visible in the stretch reflex?

A

The descending pathways that activate the alpha motor neurons when flexors are commanded to contract also activate interneurons, which inhibit the alpha motor neurons that supply the antagonist muscles.

77
Q

What is the flexor (withdrawal) reflex?

A

A complex, polysynaptic reflex used to withdraw a limb from an aversive stimulus (such as foot from thumbtack).

The direction of withdrawal depends on the location of the stimulus (hot stimuli applied to palm and back of the hand trigger withdrawals in opposite directions).

78
Q

How is the flexor reflex activated?

A

By small, myelinated Aδ nociceptive axons that trigger pain.

These nociceptive axons entering the spinal cord branch profusely and activate interneurons in several different spinal segments.

These cells eventually excite the alpha motor neurons that control all the flexor muscles of the affected limb.

Inhibitory interneurons are also recruited to inhibit the alpha motor neurons that control the extensors.

79
Q

What is the crossed-extensor reflex?

A

Activation of extensor muscles and the inhibition of flexors on the opposite side of the body.

It is used to compensate for the extra load imposed by limb withdrawal on the antigravity extensor muscles of the opposite legs.

It also provides a building block for locomotion, such as walking.

80
Q

What are central pattern generators?

A

Circuits that give rise to rhythmic motor activity.

81
Q

How do neural circuits generate rhythmic patterns of activity?

A

Simplest pattern generators are individual neurons whose membrane properties endow them with pacemaker properties.

It can be that the activiation of NMDA receptors on spinal interneurons is sufficient to generate the locomotor activity in fish.

82
Q

Explain the cycle of NMDA receptor activation by glutamate. This is a part of generating rhythms in vertebrates.

A

1) Membrane depolarizes
2) Na+ and Ca2+ flow into the cell through the NMDA receptors
3) CA2+ activates potassium channels
4) K+ flows out of the cell
5) Membrane hyperpolarizes
6) Ca2+ stops flowing into the cell
7) Potassium channels close
8) Membrane depolarizes

83
Q

What is excitation-contraction coupling?

A

1) ACh release of alpha neurons causes muscle contraction
2) ACh produces a large EPSP in the postsynaptic membrane due to activation of nicotinic ACh receptors
3) Because the membrane of the muscle cell contains voltage-gated sodium channels, the EPSP is sufficient to evoke an action potential in the muscle fiber
4) This action potential (the excitation) triggers the release of Ca2+ from an organelle inside the muscle fiber
5) This leads to contraction of the fiber
6) Relaxation occurs when the Ca2+ levels are lowered by reuptake into the organelle

84
Q

How are muscle fibers formed?

A

Early in fetal development, by the fusion of muscle precursor cells, or myoblasts, which are derived from the mesoderm.

85
Q

Why are individual muscle cells multinucleated?

A

Because they are formed by fusion of muscle precursor cells and have more than one cell nucleus.

The fusion elongates the cells, and fibers can range from 1mm to 500mm in length.

86
Q

Where are muscle fibers enclosed?

A

Muscle fibers are enclosed by an excitable cell membrane called the sarcolemma.

87
Q

What are myofibrils?

A

Cylindrical structures within the muscle fiber. They contract in response to an action potential sweeping down the sarcolemma.

88
Q

What is sarcoplasmic reticulum (SR)?

A

An extensive intracellular sac that stores Ca2+ that surrounds myofibrils.

It is similar in appearance to the smooth endoplasmic reticulum of neurons.

89
Q

What are T tubules?

A

They are a network of tunnels (T for transverse) that enable access of action potentials to the sarcoplasmic reticulum deep inside the fiber.

90
Q

What is a tetrad?

A

A voltage-sensitive cluster of four calcium channels in the T tubule membrane. It is connected to a calcium release channel in the SR.

91
Q

What is a calcium release channel in the SR?

A

A channel where calcium can pass through the sarcoplasmic reticulum SR. It is connected to a tetrad (4 calcium channels) on the T tubule membrane.

92
Q

How does calcium transfer through the SR membrane work?

A

1) Action potential arrives in the T tubule membrane
2) Action potential causes a conformational change in the voltage-sensitive tetrad of channels
3) The opening of the tetrad opens the calcium release channel in the SR membrane
4) Some Ca2+ flows through the tetrad channels, and even more Ca2+ flows through the calcium release channel
5) The resulting increase in free Ca2+ within the cytosol causes the myofibril to contract.

93
Q

What are Z lines?

A

Disks that divide the myofibril into segments.

94
Q

What is a sarcomere?

A

A segment composed of two Z lines and the myofibril in between.

95
Q

What are thin filaments?

A

A series of bristles that are anchored to each side of the Z lines. The thin filaments from adjacent Z lines face each other but do not come in contact.

96
Q

What are thick filaments?

A

Between and among the two sets of filaments occurring series of fibers.

97
Q

When does muscle contraction occur?

A

When the thin filaments slide along the thick filaments, bringing adjacent Z lines toward one another.

98
Q

What is the sliding-filament model?

A

The shortening of sarcomere by movement of the thin and thick filaments.

99
Q

Why does the sliding of filaments with respect to one another occur?

A

Because of the interaction between the major thick filament protein, myosin, and the major thin filament protein, actin.

The exposed “heads” of the myosin molecules bind actin molecules and then undergo a conformational change that causes them to pivot.

This pivot causes the thick filament to move with respect ot the thin filament.

ATP then binds to the myosin heads and the heads disengage and “unlock” so that the process can repeat itself.

Repeating this cycle enables the myosin heads to “walk” along the actin filament.

100
Q

What is tropomyosin and troponin?

A

A complex that prevents myosin from interacting with actin.

101
Q

What are the 5 steps of excitation?

A

1) An action potential occurs in an alpha motor neuron axon
2) ACh is released by the axon terminal of the alpha motor neuron at the neuromuscular junction
3) Nicotinic receptor channels in the sarcolemma open, and the postsynaptic sarcolemma depolarizes (EPSP)
4) Voltage-gated sodium channels in the sarcolemma open and an action potential is generated in the muscle fiber, which sweeps down the sarcolemma and into the T tubules
5) Depolarization of the T tubules causes Ca2+ release from the SR.

102
Q

Describe the 6 steps of contraction.

A

1) Ca2+ binds to troponin.
2) Tropomyosin shifts position and myosin binding sites on actin are exposed.
3) Myosin heads bind actin.
4) Myosin heads pivot.
5) An ATP binds to each myosin head and it disengages from actin.
6) The cycle continues as long as Ca2+ and ATP are present.

103
Q

Describe the 3 steps of relaxation.

A
  1. As EPSPs end, the sarcolemma and T tubules return to their resting potentials.
  2. Ca2+ is sequestered by the SR by an ATP-driven pump.
  3. Myosin binding sites on actin are covered by tropomyosin.
104
Q

What causes rigor mortis?

A

Starving the muscle cells of ATP prevents the detachments of the myosin heads and leaves the myosin attachment sites on the actin filaments exposed for binding. The end result is the formation of permanent attachments between the thick and thin filaments.

105
Q

Define in one sentence “motor unit”. How does it differ from motor neuron pool?

A

An elementary component of motor control, composed of a single alpha motor neuron and all the muscle fibers it innervates.

It differens from a motor neuron pool in the sense that a motor neuron pool is a collection of alpha motor neurons that innervates a single muscle.

106
Q

Which is recruited first, a fast motor unit or a slow motor unit? Why?

A

Slow, because slow motor units resistant to fatigue are generally smaller.

107
Q

What is the type of proprioceptive input that can cause the death of rabbits when trying to hug them?

A

The Golgi tendon reflex / inverse myotactic reflex.

Neuroscience - Structure and Function of the Human Brain (20 decks)

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