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Neuroscience 101 > 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 a 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 never 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 joints
2) Move the eyes within the head
3) inhale and exhale
4) control facial expressions
5) produce speeech

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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 flexion? What causes it?

A

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

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

Movemet 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 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 behaviour?

A

Neurons that supply input to the spinal cord.

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30
Q

How are 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.

31
Q

What are mixed spinal nerves?

A

Spinal nerves that contain both sensory and motor fibers.

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

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.

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.

35
Q

Where are the motor neurons that innervate axial musculature?

A

At all levels of the spinal cord.

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.

37
Q

How are flexor and dorsal innervating neurons located?

A

Neurons innervating flexors are dorsal to those innervating extensords.

38
Q

How are lower motor neurons categorized?

A

1) Alpha motor neurons

2) Gamma motor neurons

39
Q

What are the alpha motor neurons?

A

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

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.

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)

42
Q

How does the CNS control muscle contraction?

A

1) Varying the firing rate of motor neurons.

2) Recruiting additional synergistic motor units.

43
Q

How does the varying of firing rate the 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

What are striated muscles?

A

Cardiac and skeletal muscles.

45
Q

Proximal muscles

A

Muscles that move the shoulder, pelvis, and knee

46
Q

What are the two categories of lower motor neurons?

A

alpha and gamma motor neurons.

Alpha motor neurons trigger the generation of forces by muscles. Gamma motor neurons are neurons that innervate intrafusal fibers. Intrafusal fibers are skeletal muscles fibers within its fibrous capsule.

47
Q

What happens in ALS?

A

The large alpha motor neurons degenerate causing motoric issues.

48
Q

Sarcolemma

A

An excitable cell membrane enclosing the muscle fiber.

49
Q

Sarcoplasmic reticulum

A

An extensive intracellular sac that stores ca2+.

50
Q

What are the steps of excitation in excitation-contraction coupling?

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 posynaptic sarcolemma depolarizes.
  4. Voltage-gated sodium channels 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 tubuules causes ca2+ release from the sarcoplasmic reticulum.
51
Q

What are the steps of contraction in excitation-contraction coupling?

A
  1. Ca2+ binds to troponin.
  2. Myosin binding sites on actin are exposed.
  3. Myosin heads bind action.
  4. Myosin heads pivot.
  5. Myosin heads disengage at the expense of ATP.
  6. The cycle continues as long as Ca2+ and ATP are present.
52
Q

How do (at least some) pacemaker neurons work?

A
  1. Na+ and Ca2+ flows into the cell through NMDA receptors.
  2. Ca2+ activates potassium channels.
  3. K+ flows out of the cell.
  4. the membrane hyperpolarized which causes ca2+ flow to stop.
  5. The potassium channels close.
  6. the membrane depolarizes and we start over.
53
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.

54
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).

55
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.

56
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.

57
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)

58
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.

59
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.

60
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.

61
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.

62
Q

What are the muscle fibers within a muscle spindle called?

A

Intrafusal fibers.

63
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.

64
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.

65
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

66
Q

What is the Golgi tendon organ?

A

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

67
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.

68
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.

69
Q

What are central pattern generators?

A

Circuits that give rise to rhythmic motor activity.

70
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.

71
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

72
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.

73
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.

Neuroscience 101 (12 decks)

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