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PMP-101 > 2. Muscular Contractions - NS > Flashcards

2. Muscular Contractions - NS Flashcards

1
Q

Events during a muscle twitch after single nerve activation:

A

a) Latent period

b) Contraction

c) Relaxation

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2
Q

a) Latent period

A

Motor end-plate depolarisation

Depolarisation (AP) transmitted down T tubules

Ca2+ channels open in SR

[Ca2+] in the sarcoplasm

Ca2+ binds to troponin revealing myosin binding site on actin

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3
Q

b) Contraction

A

Myosin binds to actin, moves (powerstroke, ADP ejected), releases (new ATP binds) and reforms many times causing sarcomeres to shorten.

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4
Q

c) Relaxation

A

Ca2+ actively transported back into SR

Troponin-tropomyosin complex blocks myosin binding

Muscle fibre lengthens passively (relaxation)

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5
Q

SR

A

Sarcoplasmic reticulum

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6
Q

MF

A

Muscle fibre

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7
Q

Motor UNIT

A

Motor unit = 1 motor neuron & its muscle fibres

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8
Q

1 motor neuron branches and contracts ….

A

several muscle fibres

(Number of muscle fibres depends on the muscle)

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9
Q

Fine motor control requires a smaller ratio of…

A

Muscle fibres to nerve fibres
- extraocular muscles (the eye) 1:10
- the gastrocnemius (calf) 1:2000

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10
Q

Recruitment of force depends on the number of active muscle fibres

A

Muscles are made up of many motor units

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11
Q

One nerve impulse on one nerve =

A

activation of 1 motor unit = small contraction over whole muscle

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12
Q

Activation of more motor neurons = more motor units =

A

more muscle fibres = more contractile force

Gradation of force depends on the recruitment of motor units

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13
Q

3 Basic principles

A

1) The all or nothing principle
2) Threshold
3) Recruitment

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14
Q

1) The all or nothing principle

A

The skeletal muscle fibre/motor unit either operates or it does not

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15
Q

2) Threshold

A

If the threshold stimulus for a nerve is reached and the threshold for muscle contraction is reached, the muscle fibre will contract, otherwise it will not

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16
Q

3) Recruitment

A

The greater the force of contraction needed, the more motor units (one nerve and its associated innervated muscle fibres) are required. Each motor unit operates in an all or none fashion

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17
Q

Recruiting motor units by increasing stimulus intensity:

A

> Controls the force of contraction (in absence of internal changes, such as fatigue, fibres will contract ‘fully’ each time)

> The more motor units the bigger the twitch

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18
Q

Is there Maximum for tension at stimulus?

A

YES

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19
Q

Stimulation frequency and contractile force

A

Consider:
Lowest frequencies

Low frequencies (slightly higher than above described)

High frequencies

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20
Q

Lowest frequencies

A

Muscle fibres relax fully before next AP arrives
>

Twitches (tension returns to baseline – not shown)

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21
Q

Low frequencies (slightly higher than LOWER frequencies)

A

Next AP arrives before fibres are fully relaxed

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22
Q

AP

A

Action Potential

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23
Q

High frequencies (other graph)

A

No time for the muscle fibres to relax before the next AP arrives

Tetanus

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24
Q

Calciums role in high frequencies -

A

Ca2+ continually available, enabling MAXIMUM contraction

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25
Q

Skeletal Muscle Contraction Requires a Steady Supply of ATP; NEEDED FOR

A

Contraction (crossbridge forming and release),
Relaxation ( pump Ca2+)
Restore Na+ and K+ levels afer AP

26
Q

Sources for skeletal muscle contraction

A

Phosphocreatine - A source of ATP
Carbohydrates
> Aerobic metabolism : producing about 30
ATP for each molecule of glucose

 > Anaerobic glycolysis :  glucose is 
    metabolized to lactate/lactic acid  with a yield 
    of only 2 ATP per glucose
27
Q

Lack of ATP not thought to contribute to muscle fatigue

A

= comes from other changes in the exercising muscle

28
Q

Two types of skeletal muscle fibre

A

Speed & Fatigue Resistance

29
Q

Control of Contraction force

A

All muscles in 1 motor unit are same type

Muscle as a whole made of multiple motor units of different types

30
Q

Slow twitch fibres (minimal force)

A

As stimulus increases more neurones with higher thresholds begin to fire
> Fast twitch fibres

> Generate more force but fatigue more quickly

31
Q

Muscle fibre types

A

1) Slow-twitch (SO or type I)
2) Fast-twitch (FG or type II)

32
Q

1) Slow-twitch (SO or type I)

A

slow contraction
use aerobic metabolism

fatigue-resistant and well suited for prolonged aerobic exercise

33
Q

2) Fast-twitch (FG or type II)

A

rapid contraction
use anaerobic metabolism

Activated in short-term sprint or any short-lived “burst” activity (stop-go activity).

34
Q

Different types of exercise influence different types of muscles
e.g

A
  • sprinting

-jogging

35
Q

Mechanics of movement

A

a) Flexion moves bones closer together.

b) Extension moves bones away from each other.

36
Q

Antagonistic muscle groups

A

> Move bones in opposite directions

> Contraction can pull on a bone

> Cannot push a bone away (Other groups exist)

37
Q

How do we move?

A

Join movement

38
Q

Steps of join movement:

A

1) Origin (bone): does not move

2) Insertion (bone): the point that moves

3) Bones & joints : levers and fulcrums on which muscles exert force to move or resist a load

4) Return to topic in somatosensory NS reflexes

39
Q

Disorders

A

Myasthenia gravis endplate region of the postsynaptic membrane / weakness

40
Q

Common Problems
(Disorders)

A

Muscle Cramp -
> hyperexcitability of somatic motor neurons -
motor unit go into a state of painful sustained
contraction
Overuse / fatigue
Disuse atrophy

41
Q

Cardiac muscle

A
  • Only found in the heart
  • Striated
  • Organized into sarcomere with same banding
    organization
  • Muscle fibres are shorter usually contain only
    one nucleus
  • Connected by intercalated discs
  • Gap junctions and desmosomes
42
Q

Gap Junctions:

A

channels between adjacent cardiac muscle fibres

43
Q

Gap junctions job =

A
  • allow depolarising current to flow from one
    cardiac muscle cell to the next
  • quick transmission of action potentials and the
    coordinated contraction of the entire heart
  • contract in a wave-like pattern so that the heart
    can work as a pump.
44
Q

Desmosome anchors

A

The ends of cardiac muscle fibres together

> cells do not pull apart during contraction

45
Q

Heart: Pacemaker function

A

1) Contractions of the heart (heartbeats) are c
controlled by specialised cardiac muscle cells
called pacemaker (SAN)

2) Pacemaker cells respond to signals from the
autonomic nervous system (ANS) to speed up
or slow down the heart rate

3) Also responds to various hormones that
modulate heart rate to control blood pressure

4) Hear also has uniquely shaped action potential
(contractile cells)

46
Q

Smooth Muscle

A

More variable than skeletal

Located:
> Blood vessel walls
> Walls of GI tract / associated organs
> Urinary system (walls of bladder and ureters)
> Respiratory system (airway passages)
> Reproductive system (both females and
males), and
> Ocular muscles (eye).

47
Q

Smooth muscle contractions =

A

Some alternate between contraction and relaxation (phasic smooth muscle)
Some continuously contracted (tonically contracted)

48
Q

Smooth muscle fibres

A

Lack striations – tissue appears uniform/bright

Small, spindle-shaped cells with a single nucleus

49
Q

Smooth muscle - FILAMENTS:

A

Have actin and myosin contractile proteins, and generate force through thick and thin filaments

Thin filaments are anchored by dense bodies

Filaments occur in parallel with each other, but run obliquely : therefore get contraction in different directions

50
Q

Smooth Muscle vs Skeletal : SIMILARITIES

A

Force - actin - myosin crossbridge / sliding filaments.

Contraction (cross bridge movements) initiated by an increase in free cytosolic Ca2+

51
Q

Smooth Muscle vs Skeletal : DIFFERENCES

A

Layers of smooth muscle may run in several directions

Contract and relax much more slowly

Less energy to generate amount of force

Controlled by the autonomic nervous system
Most of calcium comes from outside cell
>No T-tubules

No troponin in actin filaments – use calmodulin

In skeletal muscle target for calcium is actin: in smooth muscle target for calcium is myosin

52
Q

Smooth muscle contraction : Step by step

A

6 stages

53
Q
  1. External Ca2+ ions enters cell
A

(opened calcium channels in the sarcolemma released from SR)

54
Q
  1. Bind
A

to calmodulin

55
Q
  1. Ca2+ / calmodulin complex then activates an enzyme called…
A

myosin (light chain) kinase (MLCK)

56
Q
  1. MLCK in turn, activates the myosin heads by phosphorylating them
A

converting ATP to ADP and Pi, with the Pi attaching to the head

57
Q
  1. The heads can then attach to actin-binding sites and pull on the thin filaments
A

Causes fibre to contract

58
Q

Muscle contraction continues until ATP-dependent calcium pumps actively transport

A

Ca2+ out of the cell
> low concentration of calcium remains to maintain muscle tone.-Important around blood vessels

59
Q

Smooth Muscle: Part of Autonomic System

A

Single-unit smooth muscle cells

Multi-unit smooth muscle cells

60
Q

Single-unit smooth muscle cells

A

connected by gap junctions
cells contract as a single unit

Receptors are found all over the cell surface
Series of neurotransmitter-filled bulges called varicosities on axon

Forms loose motor units

Varicosity releases neurotransmitters into the synaptic cleft.

Bind to receptors on smooth muscle

61
Q

Multi-unit smooth muscle cells

A

cell must be stimulated independently

Smooth muscle (hollow organs - except the heart) contains pacesetter cells -
spontaneously trigger action potentials

Triggers for smooth muscle contraction:
neural stimulation by the ANS,
hormones
local factors eg stretch receptors

62
Q

Types of Muscle:

A
  1. Cardiac muscle
  2. Skeletal muscle
  3. Smooth muscle

PMP-101 (53 decks)

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