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

2. Muscular Contractions - NS Flashcards

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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
Skeletal Muscle Contraction Requires a Steady Supply of ATP; NEEDED FOR
Contraction (crossbridge forming and release), Relaxation ( pump Ca2+) Restore Na+ and K+ levels afer AP
26
Sources for skeletal muscle contraction
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
Lack of ATP not thought to contribute to muscle fatigue
= comes from other changes in the exercising muscle
28
Two types of skeletal muscle fibre
Speed & Fatigue Resistance
29
Control of Contraction force
All muscles in 1 motor unit are same type Muscle as a whole made of multiple motor units of different types
30
Slow twitch fibres (minimal force)
As stimulus increases more neurones with higher thresholds begin to fire > Fast twitch fibres > Generate more force but fatigue more quickly
31
Muscle fibre types
1) Slow-twitch (SO or type I) 2) Fast-twitch (FG or type II)
32
1) Slow-twitch (SO or type I)
slow contraction use aerobic metabolism fatigue-resistant and well suited for prolonged aerobic exercise
33
2) Fast-twitch (FG or type II)
rapid contraction use anaerobic metabolism Activated in short-term sprint or any short-lived “burst” activity (stop-go activity).
34
Different types of exercise influence different types of muscles e.g
- sprinting -jogging
35
Mechanics of movement
a) Flexion moves bones closer together. b) Extension moves bones away from each other.
36
Antagonistic muscle groups
> Move bones in opposite directions > Contraction can pull on a bone > Cannot push a bone away (Other groups exist)
37
How do we move?
Join movement
38
Steps of join movement:
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
Disorders
Myasthenia gravis endplate region of the postsynaptic membrane / weakness
40
Common Problems (Disorders)
Muscle Cramp - > hyperexcitability of somatic motor neurons - motor unit go into a state of painful sustained contraction Overuse / fatigue Disuse atrophy
41
Cardiac muscle
- 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
Gap Junctions:
channels between adjacent cardiac muscle fibres
43
Gap junctions job =
- 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
Desmosome anchors
The ends of cardiac muscle fibres together > cells do not pull apart during contraction
45
Heart: Pacemaker function
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
Smooth Muscle
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
Smooth muscle contractions =
Some alternate between contraction and relaxation (phasic smooth muscle) Some continuously contracted (tonically contracted)
48
Smooth muscle fibres
Lack striations – tissue appears uniform/bright Small, spindle-shaped cells with a single nucleus
49
Smooth muscle - FILAMENTS:
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
Smooth Muscle vs Skeletal : SIMILARITIES
Force - actin - myosin crossbridge / sliding filaments. Contraction (cross bridge movements) initiated by an increase in free cytosolic Ca2+
51
Smooth Muscle vs Skeletal : DIFFERENCES
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
Smooth muscle contraction : Step by step
6 stages
53
1. External Ca2+ ions enters cell
(opened calcium channels in the sarcolemma released from SR)
54
2. Bind
to calmodulin
55
3. Ca2+ / calmodulin complex then activates an enzyme called...
myosin (light chain) kinase (MLCK)
56
4. MLCK in turn, activates the myosin heads by phosphorylating them
converting ATP to ADP and Pi, with the Pi attaching to the head
57
5. The heads can then attach to actin-binding sites and pull on the thin filaments
Causes fibre to contract
58
Muscle contraction continues until ATP-dependent calcium pumps actively transport
Ca2+ out of the cell > low concentration of calcium remains to maintain muscle tone.-Important around blood vessels
59
Smooth Muscle: Part of Autonomic System
Single-unit smooth muscle cells Multi-unit smooth muscle cells
60
Single-unit smooth muscle cells
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
Multi-unit smooth muscle cells
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
Types of Muscle:
1. Cardiac muscle 2. Skeletal muscle 3. Smooth muscle

PMP-101 (53 decks)

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