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MSS & Skin > Somatic Nervous System & Muscle Contraction > Flashcards

Somatic Nervous System & Muscle Contraction Flashcards

1
Q

What are the 3 types of muscle?

A

Cardiac
Skeletal
Smooth

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2
Q

Describe features of Cardiac muscle

A

Cardiac muscle is striated & involuntarily controlled

Found in the heart and atrial walls

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3
Q

Outline the role of Skeletal Muscle

A

Voluntary

Muscle connected to bone for movement

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4
Q

Describe features of smooth muscle

A

Involuntary, spindle like cells, found in hollow and visceral organs (GI tract & blood vessels)

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5
Q

Why is it important to understand skeletal muscle structure and function?

A

Understanding muscle function to aid treatment for patients with depleted muscle function

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6
Q

Describe the muscle structure

A

Various layers of complexity

Fasciculi 
Sarcolemma
Myofibrils 
Sarcoplasm
Sarcoplasmic reticulum
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7
Q

What is the Fasciculi?

A

150 skeletal muscle fibres in bundles surrounded by perimysium - type of connective tissue

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8
Q

What is the sarcolemma?

A

Specialised muscle fibre membrane which surrounds striated muscle fibre cells

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9
Q

What are myofibrils?

A

Muscle fibre: 100s- 1000s

long filaments that run parallel to each other to form muscle (myo) fibres

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10
Q

What is the sarcoplasm?

A

The cytoplasm of a myocyte (muscle fiber, muscle cell) contains glycogen, fat, enzymes, mitochondria

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11
Q

What is the sarcoplasmic Reticulum?

A

Membrane-bound structure found within muscle cells (similar to ER) main function is Ca2+ storage and release

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12
Q

What is the sarcomere?

A

The repeating contractile unit (in myofibrils) between two Z lines

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13
Q

What is the Anisotropic band?

A

High density (thick) darker bands - myosin filaments

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14
Q

What is the Isotropic bands?

A

(better known as I bands)

These are the lighter bands of skeletal muscle cells - actin filaments

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15
Q

What is titin?

A

A giant molecular spring connected to myosin; prevents sarcomere from over stretching

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16
Q

Explain what occurs to the light and thick bands of the sarcomere during contraction

A

Size of thick filaments remains the same

Actin filaments slide between thick myosin filaments

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17
Q

What action causes a contraction of the sarcomere?

A

Actin and myosin interact and draw actin filaments across the myosin to shorten sarcomere to provide a contraction

Sarcomere contraction (myosin) requires ATP to facilitate the sliding action

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18
Q

Describe the myosin filaments structure and its role in the Ca2+ stimulated contraction

A

Thick filament (myosin) have long tails and myosin heads. The myosin head interacts with the actin (thin) filaments requiring ATP

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19
Q

Outline the Calcium stimulated muscle contraction

A
  1. ATP is hydrolysed at the myosin head → ADP + pi
  2. Cocks myosin head into high energy state
  3. Myosin head binds to actin binding sites on actin
    filaments
  4. Moves along actin filament using a power stroke
  5. ATP breaks myosin-actin bond restarting cycle
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20
Q

Explain what causes rigor mortis in the deceased

A

ATP required to break actin-myosin bonds e.g. Rigor mortis
Pumps in cells which normally extrude Ca2+ fail causing influx of Ca2+ causing contraction - no ATP forming so myosin head remains attached to actin causing continuous contraction (for upto days)
Eventually relaxes due to cell breakdown

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21
Q

Explain how Ca2+ causes contraction

A

Ca2+ stored in SR
When synapses form and action potential reaches SR, Ca2+ is released into sarcoplasm and binds to a troponin complex associated with tropomyosin (bound around actin filaments). Causes tropomyosin to undergo a conformational change exposing binding sites on the troponin.
In absence of Ca2+tropomyosin and troponin block Ca2+ binding sites

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22
Q

What is the role of the somatic nervous system?

A

Part of the PNS

Provides voluntary control over skeletal muscle

23
Q

What is the somatic nervous system?

A

Efferent Neurons that innervate muscle - motor neurons

24
Q

How does the somatic nervous system innervate the whole body?

A

Upper motor neurons in the brain, brainstem and spinal cord and remain in these regions but synapse (glutamate neurotransmitter) with lower motor neurons in the spinal cord and form synapses with individual muscle cells - signal to muscle (Ach neurotransmitter)

25
Q

What is a motor unit composed of?

A

Motor neuron and the skeletal muscle fibers innervated by that motor neuron’s axonal terminals

Single motor neuron innervates all the muscle fibres it controls

26
Q

How does the motor unit create its effect?

A

Single fibre contracts completely or not at all

Motor neuron can innervate a few to hundred fibers

27
Q

How is the strength of contraction mediated by the motor unit?

A

Can regulate strength of muscle contraction by varying how many activated motor units

28
Q

How are contractions initiated at neuromuscular junctions?

A

Action potential goes through plasma membrane into the T tubules activating release of Ca2+ stored within the interior of the SR into the cytosol.

Involutions of plasma membrane within T tubules in muscle associated with SR

29
Q

Outline how an action potential causes contraction

A
  1. Muscle motor neuron is synapsed to a muscle
  2. Releases Ach causing an action potential to move
    through the plasma membrane into the T tubules
  3. Ca2+ released from SR stores.
  4. Ca2+ binds to troponin causing conformational changes
    in tropomyosin
  5. Myosin binding sites on actin revealed - allows actin and
    myosin to bind using ATP
  6. A powerstroke created to move the myosin heads along
    the actin filaments
30
Q

How far does one actin-myosin cycle move the sarcomere?

A

1 cycle moves the sarcomere 1% of its length = 5.3nm

31
Q

What are the 2 types of cholinergic receptors?

A

Muscarinic (primarily CNS, GPCR, slow)

Nicotinic (neuronal / neuromuscular junctions, fast)

32
Q

What causes the activation of a nicotinic AchR?

A

2 Ach molecules bind to a nicotinic Achr causing conformational changes in the receptor → open ion pore

33
Q

How does the binding to AchR cause contraction?

A

Consequently : rapid increase in Na+/Ca2+ and membrane depolarisation occurs leading to muscle contraction

34
Q

What is the effect of increased Ach synapse release?

A

Increasing the frequency of synapse release can cause a graded response

35
Q

How much movement does a single action potential cause?

A

A single action potential will produce a twitch lasting 100 msec - single twitch
Not enough to confer tension to elastic tendons and bone

36
Q

What causes the summation of action potentials to occur?

A

If a 2nd arrives before the muscle has relaxed => summation of two = greater tension

37
Q

What is tetanus?

A

Rate of action potentials gets so high that the muscle doesn’t relax between stimuli - sustained contraction

38
Q

What effect does an action potential have on axon?

A

An action potential in a motor neuron causes the axon to release ACh → depolarises the muscle cell membrane (sarcolemma)

39
Q

What structures are responsible for the depolarisation of the SR?

A

Transverse tubules depolarise sarcoplasmic reticulum

40
Q

What part of troponin does Ca2+ bind to?

A

Calcium is released and binds to the troponin TnC complex causing tropomyosin to shift and unblock the cross-bridge binding sites

41
Q

What is Duchenne Muscular Dystrophy ?

A

DMD most common severe form of childhood muscular dystrophy (1:5000 males) caused due to a mutated Dystrophin gene

42
Q

What muscle is affected by DMD?

A

Skeletal and Cardiac
Unable to walk by 10-12yrs
Death by early to mid 20s (heart failure)

43
Q

How does DMD cause muscle damage?

A

Connects actin filaments to the sarcolemma - required for mechanical stability

Lack of dystrophin causes dysfunction of sarcolemma stretch = ion pores open and increased intracellular Ca2+

Degradation of structural proteins
CK lost from the cell into blood (indication of muscle damage) - CK required for ATP

44
Q

What is motor neuron disease?

A

MND is a group of disorders that selectively affect motor neurons, the cells that control voluntary muscular activity including speech, mobility and swallowing

45
Q

What disorders are categorised in MND?

A
  • Amyotrophic Lateral
  • Sclerosis (ALS)
  • Progressive muscular atrophy
  • Primary lateral sclerosis
  • Progressive Bulbar palsy
  • Pseudobulbar palsy
  • Other
46
Q

What is Amyotrophic Lateral Sclerosis?

A

ALS is a neuron disease affecting motor neurons, causes severe disability leading to death from ventilatory failure

47
Q

What is the occurrence of ALS?

A 
Affects 1:200,000
5-10% familial 
30-40% C9orf72
15-20% SOD1
5% TARDBP
5% FUS

Sporadic probably caused by a combination of environmental and genetic factors - largely unknown

48
Q

What is myasthenia gravis?

A

Chronic autoimmune NMD causes degrees of skeletal muscle weakness and fatigue

49
Q

Who is affected by Myasthenia Gravis?

A

1-7 in 10,000
Young women (20-30yrs) and
Older men (50-60yrs)
3:2 (F:M)

50
Q

How does AchR contribute to myasthenia gravis?

A

Body makes antibodies against AchRs at neuromuscular junctions

  • Blocks AchRs
  • Increases AChR degradation
  • Impaired signal transduction
51
Q

What is the treatment available for myasthenia gravis?

A

Treatment with acetyl cholinesterase inhibitors or immunosuppressants (prevent AchR degradation)

52
Q

What muscular disorder leads to respiratory failure?

A

ALS is a motor neuron degenerative disorder ultimately causing death by respiratory failure

53
Q

What muscular dystrophy is due to a mutated gene?

A

DMD caused by dystrophin gene mutation required for fibre integrity

54
Q

What disorder disrupts the AchR function?

A

MG autoimmune disease blocks and destroys AchRs at NMJs leading to muscle weakness

MSS & Skin (6 decks)

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