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Muscle contraction process
Neural impulse. Neuromuscular junction. Transverse tubules and Sarcoplasmic reticulum. Calcium spark. Actin and Myosin.
How is motor neuron activity kept in balance?
By relative degrees of inhibitory and excitatory inputs.
How do dihydropyridine receptors respond to an action potential?
By triggering their adjacent ryanodine receptors to release calcium.
What happens if a muscle fiber is re-stimulated after it has completely relaxed?
The second twitch is the same magnitude as the first twitch.
What happens if a muscle fiber is re-stimulated before it has completely relaxed?
The second twitch is added onto the first twitch; summation.
What happens if a muscle fiber is stimulated so rapidly that it does not have an opportunity to relax at all between stimuli?
A maximal sustained contraction occurs; tetanus.
How are stronger muscle contractions produced?
Motor units are stimulated to contract; motor unit recruitment.
Which muscles have few muscle fibers per motor unit?
Muscles that provide precise, delicate movements; external eye
Which muscles have many muscle fibers per motor unit?
Muscles that are used for powerful, coarsely controlled movement; leg muscles
What molecule is used as a storage molecule for phosphates and can anaerobically donate PO4 to ADP to create ATP?
What molecule is a Ca 2+ binding protein within the sarcoplasmic reticulum and helps regulate free Ca 2+?
Calsequestrin; Important determinant of contraction speed
What protein spring lends elasticity to muscles?
Titin (connectin); Critical for maintaining muscle resting length
Type 1 skeletal muscle fiber:
Slow-oxidative. 60-100 m/sec to peak tension. Lower myosin-ATPase activity. High resistance to fatigue.
Type 2a skeletal muscle fiber:
Fast-oxidative. 20-40 m/sec to peak tension. Higher myosin-ATPase activity. Intermediate resistance to fatigue.
Type 2b, 2d, or 2x:
Fast-glycolytic. 20-40 m/sec to peak tension. Higher myosin-ATPase activity. Low resistance to fatigue.
What increases in diameter of fast-glycolytic fibers due to addition of actin and myosin filaments?
Strength training; little improvement to endurance
What takes place between fast-glycolytic and fast-oxidative fibers?
Interconversion; Involves alteration of innervation/impulse frequency
What does increased body temperature allow?
More rapid ATP synthesis and increases activity of Ca2+ pumps.
What contraction is used within the flight of birds and large insects?
Synchronous muscle contractions
How do asynchronous muscle contractions function?
Triggered by stretch and deactivated by shortening in the presence of elevated myoplasmic Ca2+
Why are spinal reflexes of afferent neurons important?
They maintain posture and basic protective movements.
Function of the brain stem:
Regulation of overall body posture involving involuntary movements of trunk and limbs; Part of multi neuronal (extrapyramidal) motor system
Similarities between skeletal and smooth muscles:
Actin and Myosin. Activated by Ca2+. ATP used for cross bridge formation.
Differences between skeletal and smooth muscles:
Arrangement of contractile units. Excitation and activation mechanisms. Contraction speed. No transverse tubules. Seldom sarcoplasmic reticulum.
How are smooth muscles excited?
Neurogenic - Action potential from neurons.
Myogenic - Contractions triggered internally by muscle tissue.
Regular swings in membrane potential may or may not reach threshold and produce a series of action potentials
For smooth contraction, what does Ca2+-calmodulin complex bind and activate?
Myosin light chain kinase (MLC kinase)
For smooth muscle contraction, what does MLC kinase phosphorylate and allow?
Myosin light chains. Allows myosin heads to interact with actin; Cross-bridge cycling begins
Pros and cons of smooth muscle:
Contracts more slowly, and uses less energy than skeletal muscle. Lower myosin ATPase activity results in slower contraction. Slower Ca2+
What maintains tension for long periods with very low ATP consumption?
Latch state (vertebrates), catch state (non-vertebrates)
How does phasic smooth muscle contract?
Contracts in bursts triggered by action potentials that cause increase cytosolic Ca2+
How does tonic smooth muscle contract?
Partially contracted at all times; Varies its contraction according to cytosolic Ca2+ level.
Multi-unit smooth muscle:
Separately stimulate by nerves to contract. Contractile activity is neurogenic and phasic. Can be initiated by the autonomic nervous system.
Single-unit smooth muscle:
Self-excitable and contract as a single unit. Gap junctions neighboring cells (functional syncytium). Contractile activity is myogenic and may be phasic (pacemaker potentials) or tonic (slow-wave potentials). Modified by autonomic nervous system.
What are similarities between cardiac muscle and skeletal muscle?
Striated. Length-tension relationship. Abundance of mitochondria and myoglobin. Transverse tubules and sarcoplasmic reticulum.
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