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Physiology 1 > Lecture 5 > Flashcards

Lecture 5 Flashcards

1
Q

Describe hierarchical organization of a skeletal muscle

A
  • Epimysium: Connective tissue surrounding entire muscle
  • Muscle: Made up of multiple fascicles
  • Perimysium: Connective tissue surrounding individual fascicle
  • Fascicle: A bundle of myofibers
  • Endomysium: Delicate connective tissue around each myofiber
  • Sarcolemma (= plasmalemma): Cell membrane of muscle fiber
  • Myofiber (= muscle cell): Individual multinucleated muscle cell
  • Myofibril: A chain of sarcomeres within a myofiber
  • Myofilament: Actin and myosin filaments that make up a sarcomere
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2
Q

Describe sarcomere organization

A
  • Sarcolemma
    • = Plasmalemma
    • T-tubules
      • Invaginations of sarcolemma
      • Lie close to cisternae of sarcoplasmic reticulum
      • Form triads with cisternae
      • Two per sarcomere
  • Sarcoplasmic reticulum
    * = Endoplasmic reticulum
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3
Q

Describe sarcomere banding

A

• Z discs (Z lines):
- Anchor actin filaments
- Located at each end of a sarcomere
• I bands:
- Composed entirely of actin
- Width changes during contraction
• A bands:
- Composed of actin and myosin
- Width does not change during contraction
• H bands:
- Composed entirely of myosin
- Width changes during contraction

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4
Q

Describe banding pattern during contraction of skeletal muscle

A

Actin filaments:
- Form the I bands which become narrower in width.
A band:
- Is equivalent to the length of the myosin filaments and does not change width.
H band:
- Is the part of the A band that is not overlapped by actin filaments, it becomes narrower.

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5
Q

Describe sliding mechanism events

A
  1. Arrival of action potential at terminal end of nerve fiber
  2. Opening of voltage-gated calcium channels on nerve fiber ending
  3. Release of neurotransmitter (Ach) from synaptic vesicles into synaptic cleft
  4. Opening of ligand-gated sodium channels of sarcolemma
  5. Generation of action potential on sarcolemma
  6. Voltage-gated channels on T tubules (DHP ─ dihydropyridine ─ channels)
    interact with ryanodine receptors on SR membrane
  7. Opening of ryanodine-sensitive calcium ion release channels
  8. Increase in calcium ion concentration in cytosol
  9. Activation of sliding filament mechanism
  10. Released calcium ions bind to troponin.
  11. Tropomyosin uncovers myosin binding sites on actin.
  12. ATPase heads of myosin molecules split ATP and bind to actin.
  13. Stored energy in myosin head causes deformation such that thick and thin filaments slide past one another.
  14. A second ATP binds to myosin and causes it to release actin.
  15. Process is repeated over and over.
  16. Contraction stops when ATP-dependent calcium pump sequesters calcium ions back into SR.
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6
Q

Explain where ATP is required in the contraction of a sarcomere

A
  • ATPase heads of myosin split ATP and bind to actin

- 2nd ATP binds to myosin and causes it to release actin

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7
Q

Describe role of T-tubules and SR in muscle contraction

A

Sarcolemma Action Potential —-> Depolarization of Ttubules —-> Conformational Change in DHP Receptors —-> Conformational Change in Ryanodine Receptors —-> Opening of Ryanodine Receptor Calcium Channels —-> Release of Calcium from Sarcoplasmic Reticulum

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8
Q

Describe role of Ca2+ in muscle contraction

A

Binding of Calcium to Troponin C —-> Conformational Change in Troponin —-> Tropomyosin is Pulled away from Active Sites on Actin —-> Exposure of Active Sites on Actin —-> Binding of Myosin Heads to Actin Active Sites

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9
Q

Describe function of SERCA and calsequestrin

A

• SERCA* uses ATP to pump calcium back into the SR
- *(Sarcoplasmic Reticulum Calcium ATPase)
• Calsequestrin in the SR maintains an optimum calcium concentration gradient to facilitate return of calcium to SR

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10
Q

Explain function and location of DHP receptors

A

Dihydropyridine (DHP) receptors:
• Voltage-sensitive L-type calcium channels arranged in quadruplets
• Located on the sarcolemma T-tubules
• Cause a conformational change in the ryanodine receptors
• A minute amount of calcium flows into the cytosol via these channels.

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11
Q

Explain function and location of ryanodine channels

A 
Ryanodine receptors (RyRs or Ca2+ - release channels):
• Located on the cisternae of the sarcoplasmic reticulum
• Open in response to conformational change in DHP receptors
• Allow calcium into the cytosol from the SR
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12
Q

Define preload and describe results

A

Definition:
• Load on a muscle in the relaxed state (before it contracts)
Results:
• Preload stretches the muscle which stretches the sacromere.
• Preload generates passive tension in the muscle.
• The muscle resists the tension applied to it.
• The force of the resistance is measured as passive tension.
• The greater the preload, the greater the passive tension in the muscle.

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13
Q

Define afterload and describe results

A

Definition:
• Load the muscle works against.
Results:
• If the muscle generates more force than the afterload, an isotonic contraction occurs.
• If the muscle generates less force than the afterload, an isometric contraction occurs.
Types of tension:
• Passive: produced by the preload
• Active: produced by cross-bridge cycling
• Total: sum of active and passive tension

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14
Q

Describe cross-bridge cycling and role of ATP

A
  • Cross-bridge cycling starts when free calcium is available and attaches to troponin.
  • Contraction is the continuous cycling of cross-bridges.
  • ATP is not required to form the cross-bridge linking to actin but is required to break the link with actin.
  • Cross-bridge cycling continues until:
  • Withdrawal of calcium ion
  • ATP is depleted
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15
Q

Describe muscle length-tension

A 
(look at slides 32-25)
D = no tension 
• Actin filament pulled out
all the way with no overlap.
• Sarcomere length = 3.5 μm

C = max. tension
• Actin filament has overlapped all the cross bridges.
• Sarcomere length = 2.2 μm

B = max tension
• Actin filaments touch.
• Sarcomere length = 1.65 μm

A = tension drops towards 0
• Actin filaments overlap.
• Sarcomere length

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16
Q

Describe where ATP is required during muscle contraction

A

Where is ATP required for muscle contraction:
• Most is used for the sliding filament mechanism.
• Pumping calcium ions from sarcoplasm back into sarcoplasmic reticulum.
• Pumping sodium and potassium ions through the sarcolemma to reestablish resting potential.
Concentration of ATP in muscle fiber:
• About 4 mmol.
• Enough to maintain contraction for 1-2 seconds

17
Q

List sources of rephosphorylation

A

Phosphocreatine:
• Releases energy rapidly
• Reconstitutes ATP
• ATP + phosphocreatine provides enough energy for 5-8 seconds of contraction.
Glycolysis:
• Lactic acid build-up
• Can sustain contraction for 1 minute.
Oxidative metabolism:
• Provides more than 95% of all energy needed for long-term contraction.

18
Q

Compare isotonic and isometric contractions

A

Isometric:
• An isometric contraction occurs when there is an increase in tension but not in length.
Isotonic:
• Muscle length changes in an isotonic contraction.
- Eccentric:
• An eccentric contraction occurs when the muscle lengthens.
- Concentric:
• A concentric contraction occurs when the muscle shortens.

19
Q

Define myofibers

A
  • The myofiber type is determined by the innervating neuron.
  • Fiber types are classified mainly on endurance (resistance to fatigue) and speed of contraction.
  • Muscles usually have a mix of fiber types.
  • Some muscles are almost entirely of one fiber type or another:
  • Muscles predominantly composed of dark fibers:
  • Soleus.
  • Muscles predominantly composed of light fibers:
  • Gastrocnemius.
20
Q

Describe fast fibers

A

Light, fast fibers (white fibers):
• Fast twitch fibers contract rapidly but have less endurance.
• Characteristics include:
• Fewer mitochondria
• Primarily use anaerobic respiration resulting in a buildup of pyruvic and lactic acids
• Little myoglobin
• Larger concentration of ATPase

21
Q

Describe slow fibers

A
  • Dark, slow fibers (red fibers):
  • Slow twitch fibers contract more slowly but have more endurance.
  • Characteristics include:
  • More mitochondria
  • Primarily use aerobic respiration
  • More myoglobin
  • Smaller concentration of ATPase
22
Q

Define motor unit

A

A single nerve cell (neuron) may innervate from
a few to several hundred myofibers.
• A neuron and the myofibers it innervates
constitute a motor unit.
• When a neuron fires, all the myofibers in the
motor unit contract.
• All-or-none really refers to a motor unit.

23
Q

Describe summation

A

Summation
• Electrical events occur faster than mechanical events:
• An additional spike can occur before the previous calcium ions have been returned to the SR.
• This increases the total amount of calcium ion in the cytosol and increases the rate of cycling between the myosin and actin cross-bridges.
• This increases muscle tension.
• Each additional spike adds to the effects of the previous spikes

24
Q

Describe tetany

A

Tetany:
• If the frequency of spikes is fast enough, there is no time for relaxation between spikes.
• The muscle remains at maximal contraction.

25
Q

Muscles as levers

A

Lever systems are classified according to the position of the fulcrum in relation to the in-force and the out-force:
• First-class; fulcrum is in the middle:
- Example = raising chin using sternocleidomastoids or similar muscles
(fulcrum = atlas/axis complex)
- In-force and out-force move in opposite directions.
• Second-class; Resistance (out-force) is in the middle:
- Example: Raising the body on the ball of the foot.
- Fulcrum = ball of foot.
- Both in and out forces are on the same side of the fulcrum.
• Third-class; effort (in-force) is in the middle:
- Example: Lifting a weight in the palm of your hand
- Both in and out forces are on the same side of the fulcrum.
- Both forces move in same direction.

Physiology 1 (11 decks)

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