Muscular System Notes

Types of Muscle

• Cardiac
• Smooth
• Skeletal
• The muscular system primarily refers to skeletal muscle.

Functions of Muscles

• Movement
  • Includes externally visible movements (e.g., lifting arm).
  • Includes internal movements (e.g., breathing, moving contents within the digestive tract, pumping blood).
• Stability
  • Prevents unwanted movement.
  • Maintains posture.
  • Enables movement of one bone while keeping another one still.
• Control of body openings and passages
  • Ring-shaped sphincter muscles regulate movement of content from one area to the next (e.g., digestive tract).
• Heat generation
  • Skeletal muscle produces up to 30% of body heat at rest and up to 40 times as much during exercise.
• Glycemic control
  • Regulation of blood glucose.
  • Skeletal muscle plays a significant role in stabilizing blood sugar levels by absorbing a large share of it.

Skeletal Muscle Fibers

• Skeletal muscle is primarily voluntary (subject to conscious control).
• Skeletal muscle is striated, featuring alternating light and dark bands.
• Striations result from the overlapping arrangement of internal proteins that enable contraction.

Structure of the Muscle Fiber

• Skeletal muscle cells are called muscle fibers due to their long, slender shape.
• Muscle fibers have multiple nuclei.
• Myofibrils are bundles of contractile proteins within muscle cells (ranging in number from 7 to 1000+).
• Numerous mitochondria, a network of smooth ER, glycogen, and red oxygen-binding pigment (myoglobin) are packed between bundles.
• The plasma membrane is called the sarcolemma.
• Sarcolemma has tunnel-like infoldings called transverse (T) tubules that penetrate through the fiber and emerge on the other side.
  • Function: carry an electrical current from the surface of the cell to the interior when the cell is stimulated.
• Smooth ER of muscle fiber is called sarcoplasmic reticulum (SR).
  • Forms a web around each myofibril and T tubules.
  • Have dilated sacs called terminal cisterns.
  • SR is a reservoir for calcium ions.
  • Releases flood of calcium into the cytosol to activate the contraction process.

Myofilaments and Striations

• Myofibrils are packed with contractile proteins called myofilaments.
  • Two main kinds:
    • Thick filaments
    • Thin filaments

Thick Filaments

• Made of several hundred proteins called myosin.
• Myosin head is shaped like a golf club.
  • Two polypeptides intertwined to form a shaft-like tail.
  • Double globular head projecting at an angle.
  • Think of thick filaments as a bundle of 200 to 500 “golf clubs” with the head pointed outward in a helical array.

Thin Filaments

• About half as wide as thick filaments.
• Composed mainly of intertwined strands of a protein called actin.
  • Looks like a string of globular subunits.
• Also has two proteins called tropomyosin and troponin.
  • Act as a molecular “switch” that either allows or inhibits muscle contraction.

Striations

• Patterns of myofilament overlap give muscle fibers striations.
  • Dark bands are called A bands where thick and thin filaments overlap.
  • Light bands are called I bands consisting of only thin filaments.
    • Intersected by Z discs made of proteins that anchor the thin filaments.
  • The segment of a myofibril from one Z disc to the next is called a sarcomere.
    • Muscle shortens when contracting because sarcomeres shorten and pull Z discs closer to each other.
    • The sarcomere is the functional unit of muscle.

The Nerve-Muscle Relationship

• Skeletal muscle must be stimulated by a nerve to contract.
• Nerve cells that stimulate skeletal muscles are called motor neurons, located in the brain and spinal cord.
  • Their axons transmit electrical signals to the muscles.
  • Each motor neuron stimulates all the muscle fibers of a group of fibers, such as in a muscle, to contract at once.
  • One motor neuron + all muscle fibers supplied by it are called a motor unit.
• Where an axon meets another cell is called a synapse.
• Where an axon meets a skeletal muscle fiber, the synapse is called a neuromuscular junction.
  • Axons ends look bulbous and are called axon terminals nestled in a depression on the muscle fiber.
  • Two cells don’t actually touch, separated by a gap called the synaptic cleft.

Neurotransmitters

• The Axon terminal contains membrane-bound sacs called synaptic vesicles which contain a signaling molecule called acetylcholine (ACh).
  • ACh is a neurotransmitter (a chemical signal sent by NS to a cell).
  • When a neuron releases ACh, it diffuses across the synaptic cleft and binds to ACh receptors on the surface of the muscle fiber.
  • This binding stimulates muscle fiber to contract.
• Along with ACh receptors, muscle fiber also has acetylcholinesterase (AChE) which breaks down ACh.
  • Stops stimulation of the muscle causing it to relax.
  • AChE also found in the synaptic cleft.

Muscle Excitation

• Excitation is the process of converting an electrical nerve signal to an electrical signal in the muscle fiber.
• Occurs in a 3-step process:
  • Step 1: The nerve signal arrives at the synapse and stimulates synaptic vesicles to release acetylcholine (ACh) into the synaptic cleft.
  • Step 2: ACh binds to ACh receptors in the sarcolemma.
    • Each receptor is a gated channel that opens in response to ACh.
    • Sodium ions rush into muscle fiber, and potassium exits, driven by concentration and electrical gradients.
  • Step 3: Ion movement electrically excites the sarcolemma and initiates a wave of electrical changes called an action potential.
    • Action potential spreads in all directions away from the neuromuscular junction (like ripples spreading out in a pond).
    • The muscle fiber is now “excited.”
    • 3 events must happen before contraction.

Preparing for Contraction

• Rapid cyclic interactions between myosin and actin of thin and thick filaments drive contraction.
• In relaxed muscle, regulatory proteins block actin and myosin interaction.
• Excitation initiates a chain of events that allows myosin and actin to interact.
• 3 step process:
  • 1) Excitation of T tubules opens calcium channels in the sarcoplasmic reticulum.
    • Calcium floods the cytosol of muscle fiber.
  • 2) Calcium binds to troponin molecules attached to thin filaments.
  • 3) Causes the associated tropomyosin to shift position, exposing the myosin-binding active sites on the actin.

Contraction

• Contraction: muscle fiber develops tension and may shorten due to the sliding filament model.
• Thick and thin filaments slide across each other, shortening the muscle fiber.
• Occurs in 4 steps:
  • Step 1: Each myosin head binds an ATP molecule and hydrolyzes it into ADP and a phosphate group, causing the head to move from flexed position to extended (high-energy position).
  • Step 2: Each myosin head binds to an active site on the thin filament. This link between a myosin head and actin filament is called a cross-bridge.
  • Step 3: Myosin then releases the ADP and phosphate. The release moves myosin head back to the original low-energy flexed position and pulls thin filament along with it (power stroke). The head remains bound to actin until a new ATP binds.
  • Step 4: When new ATP binds, the myosin head releases actin. The head is now ready to repeat the same process. Myosin hydrolyzes ATP to ADP and phosphate and moves to high-energy extended position (recovery stroke). It attaches to a new site farther down the thin filament and produces another power stroke.

Contraction Details

• The cycle repeats at a rate of about 5 strokes per second and each stroke consumes one ATP.
• As the thin filament is pulled along the thick filament, it pulls the Z disc along with it.
• Thin filaments anchored to Z discs mean sarcomeres shorten.

Relaxation

• When the nerve stops stimulating it, a muscle fiber relaxes and returns to resting length.
• Occurs in 4 steps:
  • 1) Nerve signal stops – axon terminal stops releasing ACh.
  • 2) AChE breaks down ACh – stimulation to muscle fiber stops.
  • 3) The sarcoplasmic reticulum reabsorbs calcium using active-transport pumps (ATP is required for muscle contraction and relaxation).
  • 4) Without calcium, the troponin-tropomyosin complex shifts back and blocks myosin from binding to actin.
• Muscles usually don’t fully relax but are usually in a state of partial contraction called muscle tone.
• Keeping muscles firm at rest helps stabilize joints.

Whole-Muscle Contraction

• Minimum contraction exhibited by a muscle cell is called a muscle twitch.
  • A single cycle of contraction and relaxation (very brief, 7ms to 0.1s).
  • Too brief and weak to do muscular work such as moving a joint.
• Work requires the summation of multiple twitches that occur when numerous nervous stimuli occur in rapid succession.
• Rapid enough nervous stimuli allows the muscle to relax only partially between twitches, resulting in smooth contraction called incomplete tetanus.
• Motor units work in shifts to achieve this.

Isometric vs. Isotonic Contraction

• Contraction does not always mean shortening.
• Isometric contraction: tension rises but muscle does not shorten (e.g., lifting something heavy).
• Isotonic contraction: tension rises enough to move heavy object and muscle shortens while maintaining constant tension.

Isotonic Contractions

• Concentric: muscle shortens as it maintains tension (e.g., biceps brachii contracts and flexes the elbow during a bicep curl).
• Eccentric: muscle maintains tension as it lengthens (e.g., as you lower weight in a bicep curl).
  • Maintain control.
  • Injuries during weight lifting are usually during the eccentric phase when sarcomeres and connective tissue of muscle are pulling in one direction while weight is pulling muscle in opposite direction.

Muscle Metabolism

• All muscle contraction requires ATP.
• Two ways of generating ATP:
  • Anaerobic fermentation
  • Aerobic respiration

Anaerobic Fermentation

• Pathway in which glucose is converted to lactate.
• For each glucose molecule, 2 ATP are produced.
• Does not use oxygen.
• Advantage: A way for muscle to produce ATP when demand is high but oxygen cannot be delivered fast enough to meet the needs of aerobic respiration during short bursts of exercise.
• Disadvantage: ATP yield is low.

Aerobic Respiration

• Glucose is metabolized to pyruvate which is oxidized by mitochondria to CO2 and H2O.
• This produces 30 ATP per one glucose.
• Advantage: Far more efficient than anaerobic fermentation.
• Disadvantage: Requires oxygen and cannot keep up with demand during intense exercise.

Example of Shift During Intense Brief Exercise

• Uses Aerobic respiration to start.
• Muscle fiber can convert some ADP back to ATP using phosphate storage molecule phosphate creatine (this is called the phosphagen system).
• Supplies enough ATP for about 6s of sprinting or about 1 minute of brisk walking.
• When ATP sources are exhausted, shifts to anaerobic fermentation.
• Eventually, faster heart rate and breathing provides enough oxygen to match demand and shift back to aerobic.

Fatigue and Endurance

• Muscle fatigue: progressive weakness that results from prolonged muscle use.
• Numerous causes of fatigue:
  • Depletion of glycogen and blood glucose.
  • Leakage of calcium from sarcoplasmic reticulum.
  • Accumulation of K^+ in ECF reduces excitability of the muscle fiber.
• Endurance also depends on several factors:
  • Muscles supply of myoglobin and glycogen.
  • Density of blood capillaries.
  • Number of mitochondria.
  • One's maximal rate of oxygen uptake.
  • Will power.

Types of Muscle Fibers

• Two primary types:
  • Slow twitch
  • Fast twitch

Slow-Twitch

• Respond slowly but are relatively resistant to fatigue.
• Well adapted for aerobic respiration with many mitochondria, myoglobin, and blood capillaries.

Fast-Twitch

• Respond quickly but also fatigue quickly.
• Well adapted for anaerobic respiration, rich in enzymes for anaerobic fermentation, SR releases and reabsorbs Ca^{++} quickly.

Muscular Strength and Conditioning

• Resistance training: such as weight lifting
  • Muscle cells do not undergo mitosis – do not increase in number.
  • Preexisting muscle cells instead increase in size.
  • Does not significantly improve fatigue resistance.
• Endurance (aerobic) exercise: such as running
  • Improves fatigue resistance.
  • Increases glycogen stores, mitochondria numbers, and density of capillaries.
  • Promote more efficient ATP production.
  • Does not significantly increase strength.
• Optimal performance comes from cross-training.