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NEUR531 > topic 10: spinal control of movement > Flashcards

topic 10: spinal control of movement Flashcards

1
Q

what 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 program in the spinal cord

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2
Q

based on their appearance under a microscope, the muscles in the body can be described according what categories?

A

striated and smooth

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3
Q

describe smooth muscles

A

-lines digestive tract, arteries and related structures and is innervated by nerve fibres from the autonomic nervous system
-plays a role in peristalsis (movement of material through intestines) and control of blood pressure and flow

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4
Q

what are the types of striated muscles?

A

-2 types, cardiac and skeletal

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5
Q

describe the cardiac muscle

A

-heart muscle, which contracts rhythmically even in the absence of any innervation
-innervation of the heart from the ANS functions to accelerate or slow down the heart rate

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6
Q

describe the skeletal muscle

A

-constitutes the bulk of muscle mass of the body and functions to move bones around joints, to move eyes within the head, to inhale and exhale, facial movements and produce speech
-each skeletal muscle enclosed in connective sheath that, at the ends of the muscle, forms the tendons
-within each muscle are hundred of muscle fibres, the cells of skeletal muscle, and each fibre is innervated by a single axon branch from the CNS
-somatic motor system because derived embryologically from 33 paired somites

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7
Q

what are synergists?

A

muscles that work together to cause flexion or extension –> as opposed to antagonists which work opposing one another

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8
Q

what are axial, proximal (or girdle), and distal muscles?

A

-axial - muscles responsible for movement of the trunk
-proximal- move shoulder, elbow and pelvis and knee
- distal - hands, feet and digits (fingers and toes)

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9
Q

where are the swollen regions of the dorsal and ventral horns?

A

spinal segments C3-T1 (for muscles of the arm) and L1-S3 (for leg musculature)

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10
Q

What are the categories of lower motor neurons?

A

-alpha and gamma motor neurons

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11
Q

What do alpha motor neurons trigger?

A

directly trigger generation of force by muscles

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12
Q

What is a motor unit?

A

one alpha neuron and all the muscles fibres it innervates collectively make up the elementary component of motor control –> motor unit

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13
Q

how does muscle contraction occur in terms of motor units?

A

results from the individual and combined actions of motor units

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14
Q

what is a motor neuron pool?

A

collection of alpha motor neurons that innervates a single muscle

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15
Q

what are the ways the CNS grades strength of muscles contraction?

A

-varying the firing rate of motor neurons
-or recruiting additional synergistic motor units

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16
Q

how does an alpha motor neuron communicate with a muscle fibre?

A

releasing ACh at the neuromuscular junction (the specialised synapse between a nerve and a skeletal muscle

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17
Q

what is the innervation ratio of large muscles group vs small muscles group?

A

-large-1000 muscle fibres per alpha motor neuron (e.g., leg)
-small-3 muscle fibres per alpha motor neuron

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18
Q

what are the inputs to an alpha motor neuron?

A

-dorsal root ganglion with axons that innervate a specialised sensory apparatus embedded within the muscle known as muscle spindle
-upper motor neurons in the motor cortex and brainstem
-interneurons in the spinal cord

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19
Q

What are red muscle fibres characterized by?

A

-large number of mitochondria and enzymes specialised for oxidative energy metabolism
- sometimes called slow (S) fibres, relatively slow to contract but can sustain contraction for long time without fatigue
-typically found in antigravity muscles of legs and torso
-10-20 impulses per second

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20
Q

describe the characteristics of pale muscle fibres

A

-fewer mitochondria (than red), rely mainly on anaerobic (without oxygen) metabolism
-sometimes called fast (F) fibres, contract quickly and powerfully, but fatigue more quick than slow fibres
-typically involved in escape reflexes

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21
Q

what can fast fibres be broken down into?

A

-fatigue-resistant (FR) fibres –> generate moderately strong and fast contractions and are relatively resistant to fatigue
-fast fatigue (FF) fibres –> generate the strongest, fastest contractions but are quickly exhausted when stimulated at high frequency for long period (30-60 impulses per second)

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22
Q

what are the types of muscles fibres and corresponding motor units?

A

S, FR, FF –> 1 slow and 2 fast motor units

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23
Q

what is hypertrophy?

A

a consequence of increased activity, exaggerated growth of muscles fibres

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24
Q

What is atrophy?

A

prolonged inactivity, or degeneration of muscle fibres

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25
Q

what does ACh produce in postsynaptic neuron of muscle cell?

A

-large EPSP due to activation of nicotinic ACh receptors
-sufficient to evoke AP in muscle fibre

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26
Q

What is excitation-contraction coupling?

A

-AP triggers release of Ca2+ from organelle inside the muscle fibre. which leads to contraction of fibre
-relaxation occurs when Ca2+ levels are lowered by reuptake into organelle

27
Q

how are muscle fibres formed? and what happens as a result

A

early in foetal development by fusion of muscle precursor cells, or myoblasts, which are derived from the mesoderm –> this fusion leaves each cell with more than one cell nucleus, so individual muscle cells are said to be multinucleated –> fusion elongates the cell (hence the name fibre), and fibres range from 1 to 500 mm in length

28
Q

What are muscle fibres enclosed in?

A

By an excitable cell membrane called the sarcolemma

29
Q

what are myofibrils?

A

cylindrical structure within the muscle fibre, contract in response to AP sweeping down the sarcolemma

30
Q

What are myofibrils surrounded by?

A

sarcoplasmic reticulum (SR), an extensive intracellular sac that stores Ca2+ (similar in appearance to smooth endoplasmic reticulum of neurons)

31
Q

how do AP sweeping along the sacrolemma gain access to the sarcoplasmic reticulum deep inside the fibre?

A

by way of a network of tunnels called T tubules, interior of T tubule is continuous with extracellular fluid

32
Q

describe the places where T tubule comes in close apposition to SR

A

-There is a specialized coupling of the proteins in the two membranes. A voltage-sensitive cluster of four calcium channels, called a tetrad, in the T tubule membrane is linked to a calcium release channel in the SR.
-The arrival of an action potential in the T tubule membrane causes a conformational change in the voltage sensitive tetrad of channels, which opens the calcium release channel in the SR membrane. Some Ca 2+ flows through the tetrad channels, and even more Ca 2+ flows through the calcium-release channel, and the resulting increase in free Ca within the cytosol causes the myofibril to contract.

33
Q

what is the myofibril divided into?

A

into segments by disks called Z lines

34
Q

what is a sarcomere?

A

a segment composed of 2 Z lines and the myofibril in between

35
Q

what is anchored to each side of the Z lines?

A

a series of bristles called thin filaments

36
Q

what is between and among the 2 sets of thin filaments?

A

a series of fibres called thick filaments

37
Q

when does muscle contraction occur in terms of filaments?

A

when a thin filaments slide along the thick filaments, bringing the adjacent Z lines towards one another –> i.e., sarcomere becomes shorted in length –>sliding-filament model

38
Q

what is the major thick and thin filament protein?

A

thick - myosin
thin - actin

39
Q

how do actin and myosin interact to cause contraction?

A

the exposed “heads” of the myosin molecules bind actin molecules and then undergo conformational change that causes them to pivot –> this pivoting causes the thick filament to move with respect to the thin filament, ATP then binds to the myosin heads and the heads disengage and “uncock” so that the process can repeat itself –> repetition of this cycle enable myosin heads to “walk” along the actin filament

40
Q

when the muscles is at rest, why cant myosin interact with actin? and enables binding?

A

because myosin attachment sites on actin molecule covered by complex of 2 proteins: tropomyosin and troponin
-Ca2+ initiated muscle contraction by binding to the troponin and causing tropomyosin to shift its position, thereby exposing the site where myosin binds to actin
-contraction continues as long as Ca2+ and ATP are available –relaxation occurs when the Ca2+ is sequestered by the Sr

41
Q

summarise the excitation phase of the excitation-contraction coupling

A
  1. an AP occurs in alpha motor neuron axon
  2. ACh release by axon terminal of the alpha motor neuron at the neuromuscular junction
  3. nicotinic receptor channels in the sarcolemma open, and postsynaptic sarcolemma depolarises (EPSP)
  4. voltage-gated sodium channels in the sarcolemma open and AP is generated in muscle fibre, which sweeps down sarcolemma and into the T tubules
  5. depolarisation of the T tubules causes Ca2+ release from the SR
42
Q

summarise the contraction phase of the excitation-contraction couple

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
43
Q

summarise the relaxation phase of the excitation-contraction coupling

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
44
Q

what is a muscle spindle?

A

also called stretch receptor, consists of several types of specialised skeletal muscle fibres contained in a fibrous capsule –> the middle third of the capsule is swollen, giving the structure the shape. in middle (equatorial) region, group ia sensory axons wrap around the muscle fibre of the spindle –> spindles and associated ia axons, specialised for detection of changes in muscles length (stretch), are examples of proprioceptors

45
Q

what is proprioception?

A

informs us about how our body is positioned and moving in space

46
Q

describe the group ia axons

A

-thickest myelinated axons –> fast
-enter spinal cord via dorsal roots, branch repeatedly, for excitatory synapses upon both interneurons and alpha motor neurons of ventral horns

47
Q

what is the stretch reflex (myotatic reflex)?

A

-when muscles is pulled on, it tends to pull back (contract)
-involves sensory feedback from the muscle
-concluding that motor neuron must receive continual syaptic input from the muscles

48
Q

what is the monosynaptic stretch reflex arc?

A

the ia axon and alpha motor neurons on which it synapses

49
Q

what are intrafusal fibres?

A

-inside muscle spindle, modified skeletal muscle fibres within its fibrous capsule

50
Q

what are extrafusal fibres?

A

-more numerous than intrafusal fibres, they lie outside the spindle and form the bulk of the muscle

51
Q

what is an important difference between intrafusal and extrafusal fibres?

A

-only extrafusal fibres are innervated by alpha motor neurons
-intrafusal fibres receive their motor innervation by another type of lower motor neuron called gamma motor neuron

52
Q

describe the gamma motor neuron

A

-They innervate intrafusal muscle fibres at the two ends of the muscle spindle
-activation of these fibres causes a contraction of the two pole of the muscle spindle, thereby pulling on the noncontractile equatorial region and keeping ia axons active
-alpha activation alone decrease ia activity, while gamma activation alone increases ia activity

53
Q

what is the gamma loop?

A

gamma motor neuron –> intrafusal muscle fibre –> iA afferent axon –> alpha motor neuron –> extrafusal muscle fibres
-by regulating the set point of the stretch feedback loop, the gamma loop provides additional control of alpha motor neurons, and muscle contraction

54
Q

what do Golgi tendon organs monitor? and there size

A

-monitors muscle tension, or force of contraction
-about 1mm long and 0.1mm wide

55
Q

where are golgi tendon organs located and innervated by

A

-located at the junction of muscle and tendon –>situated in series with one another
-innervated by group Ib sensory axons –> slightly smaller than Ia axons

56
Q

What is within the Golgi tendon organ? and what happens when muscle is contracted?

A

-thin branches of the Ib axon entwine among the coils of collagen fibrils –> when muscle contracts, the tension on the collagen fibrils increases –> as fibrils straighten and squeeze the ib axons, mechanosensitive ion channels activated and AP trigged

57
Q

What does the axons ia and ib encode information for?

A

-ia from spindle encodes muscle length
-ib from Golgi tendon organ encodes muscle tension

58
Q

what happens as the ib axons enter the spinal cord

A

-branch repeatedly, synapse on special interneurons called inhibitory interneurons in the ventral horn

59
Q

where are proprioceptive axons in the joints?

A

connective tissues of joints, especially within the fibrous tissue surround the joints (joint capsule) and ligaments –>response to changes in angle, direction and velocity of movement in joint

60
Q

what is reciprocal inhibition?

A

the contraction of one set of muscles accompanied by the relaxation of their antagonist muscles

61
Q

what is the flexor reflex (flexor withdrawal reflex)?

A

-excitatory, polysynaptic reflex arc used to withdraw a limb from an aversive stimulus –> slower than stretch reflex
-activated by small, myelinated A-delta nociceptive axons
-enter spinal cord, branch and activate interneurons –> then excite alpha motor neurons that control all the flexor muscles of affected limb

62
Q

what is the crossed-extensor reflex?

A

inhibition of flexors on the opposite side, used to compensate for extra load impose by limb withdrawal on antigravity extensor muscles of opposite leg

63
Q

what is the central pattern generators?

A

-circuits that give rise to the rhythmic motor activity (i.e., walking)

64
Q

how is walking conducted?

A

-Walking is initiated when a steady input excites two interneurons that connect to the motor neurons controlling the flexors and extensors, respectively.
-The interneurons respond to a continuous input by generating bursts of outputs. The activity of these two interneurons alternates because they inhibit each other via another set of interneurons, which are inhibitory.
-Thus, a burst of activity in one interneuron strongly inhibits the other, and vice versa. Then, using the spinal cord circuitry of the crossed-extensor reflex (or a similar circuit), the movements of the opposite limb could be coordinated so that flexion on one side is accompanied by extension on the other.

NEUR531 (9 decks)

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