Skeletal Muscle

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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.

What are the multiple levels of control?

Cerebrum - Motivation
Cerebellum - Coordination

What do transverse tubules conduct?

Action potential

Where do transfers tubules bring action potentials?

Into the interior of the muscle fiber.

What do sarcoplasmic reticulum store?

Ca2+

What do dihydropyridine receptors match up with?

Ryanodine receptors

How do dihydropyridine receptors respond to an action potential?

By triggering their adjacent ryanodine receptors to release calcium.

What does increased cytosolic calcium trigger?

More ryanodine receptors to open; positive feedback

When is muscle contraction stopped?

When Ca2+ leaves

How is a muscle contraction sustained?

A single action potential causes only a brief muscle twitch

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 motor units are coordinated by the brain in order to prevent fatigue?

Asynchronous recruitment

What molecule is used as a storage molecule for phosphates and can anaerobically donate PO4 to ADP to create ATP?

Phosphocreatine

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 influences muscle size and strength?

Hormones

What promotes synthesis of myosin and actin filaments?

Testosterone and growth hormone (IGF-1)

What is a negative regulator of muscle growth?

Myostatin

What increases muscle efficiency but not in muscle mass?

Endurance exercise

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 fibers cannot change?

Slow-oxidative

The act of unused muscles losing mass and strength

Disuse atrophy

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 primary motor cortex:

Fine, discrete movements of hands and fingers

Function of the brain stem:

Regulation of overall body posture involving involuntary movements of trunk and limbs; Part of multi neuronal (extrapyramidal) motor system

Detect muscle stretch

Spindle fibers

Detects muscle tension

Golgi apparatus

What must spindle fibers maintain?

Tension

What do primary efferent endings sense?

They sense stretch and contraction speed

What do secondary efferent endings sense?

They sense stretch only

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 depolarization due to cyclic changes in membrane permeability.

Pacemaker potentials

Regular swings in membrane potential may or may not reach threshold and produce a series of action potentials

Slow-Wave potentials

For smooth muscle contraction, what does Ca2+ bind?

Calmodulin

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 is the only muscle found in the heart?

Cardiac muscle

What are similarities between cardiac muscle and skeletal muscle?

Striated. Length-tension relationship. Abundance of mitochondria and myoglobin. Transverse tubules and sarcoplasmic reticulum.

What are similarities between cardiac and smooth muscle?

Self-excitation. Interconnected by gap junctions. Innervated by the autonomic nervous system.

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