Lecture 5 Flashcards
Describe hierarchical organization of a skeletal muscle
- 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
Describe sarcomere organization
- 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
Describe sarcomere banding
• 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
Describe banding pattern during contraction of skeletal muscle
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.
Describe sliding mechanism events
- Arrival of action potential at terminal end of nerve fiber
- Opening of voltage-gated calcium channels on nerve fiber ending
- Release of neurotransmitter (Ach) from synaptic vesicles into synaptic cleft
- Opening of ligand-gated sodium channels of sarcolemma
- Generation of action potential on sarcolemma
- Voltage-gated channels on T tubules (DHP ─ dihydropyridine ─ channels)
interact with ryanodine receptors on SR membrane - Opening of ryanodine-sensitive calcium ion release channels
- Increase in calcium ion concentration in cytosol
- Activation of sliding filament mechanism
- Released calcium ions bind to troponin.
- Tropomyosin uncovers myosin binding sites on actin.
- ATPase heads of myosin molecules split ATP and bind to actin.
- Stored energy in myosin head causes deformation such that thick and thin filaments slide past one another.
- A second ATP binds to myosin and causes it to release actin.
- Process is repeated over and over.
- Contraction stops when ATP-dependent calcium pump sequesters calcium ions back into SR.
Explain where ATP is required in the contraction of a sarcomere
- ATPase heads of myosin split ATP and bind to actin
- 2nd ATP binds to myosin and causes it to release actin
Describe role of T-tubules and SR in muscle contraction
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
Describe role of Ca2+ in muscle contraction
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
Describe function of SERCA and calsequestrin
• 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
Explain function and location of DHP receptors
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.
Explain function and location of ryanodine channels
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
Define preload and describe results
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.
Define afterload and describe results
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
Describe cross-bridge cycling and role of ATP
- 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
Describe muscle length-tension
(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
Describe where ATP is required during muscle contraction
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
List sources of rephosphorylation
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.
Compare isotonic and isometric contractions
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.
Define myofibers
- 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.
Describe fast fibers
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
Describe slow fibers
- 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
Define motor unit
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.
Describe summation
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
Describe tetany
Tetany:
• If the frequency of spikes is fast enough, there is no time for relaxation between spikes.
• The muscle remains at maximal contraction.
