Muscles and Muscle Tissue

Muscle tissue transforms chemical energy (ATP) into directed mechanical energy to generate force. This study guide covers the structure, function, and physiology of muscle tissue, with a focus on skeletal muscle.

Types of Muscle Tissue

There are three main types of muscle tissue in the human body, each with distinct structural and functional characteristics.

Four Key Functions of Muscle

Muscles perform at least four important functions for the body:

• Produce movement: Responsible for all locomotion and manipulation, such as walking, moving the eyes, and smiling. Smooth muscle propels substances (foodstuffs, urine) through internal body channels, and cardiac muscle pumps blood.

• Maintain posture and body position: Skeletal muscles function almost continuously, making tiny adjustments to counteract gravity and maintain body posture.

• Stabilize joints: Muscles strengthen and stabilize the joints of the skeleton, even as they cause movement.

• Generate heat: Muscles generate heat as they contract, which plays a role in maintaining normal body temperature.

Levels of Organization of Skeletal Muscle

Skeletal muscle is a discrete organ composed of muscle tissue, connective tissue, and nerves. The organization proceeds from macroscopic to microscopic elements:

1. Skeletal Muscle (Organ): The whole muscle is covered externally by the epimysium.

2. Fascicle: A bundle of muscle fibers surrounded by perimysium.

3. Muscle Fiber (Cell): A single muscle cell, surrounded by endomysium.

4. Myofibril: Rodlike contractile elements that occupy about 80% of muscle fiber volume. Composed of chains of sarcomeres aligned end to end.

5. Myofilaments (Filaments): Even smaller structures within the sarcomeres, corresponding to the contractile proteins actin and myosin.

Identification of Microscopic Features of Skeletal Muscle

The specialized structures within a skeletal muscle fiber are critical for contraction.

Key Bands and Lines of the Sarcomere

• A band (dark band): Central region containing the entire length of the thick filaments (myosin).

• I band (light band): Region containing thin filaments only (actin).

• H zone: Lighter region in the middle of the A band, where thick filaments lack myosin heads and the thin filaments do not extend into this area in a relaxed muscle.

• Z disc: Coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another.

• M line: Line of protein myomesin that holds adjacent thick filaments together in the H zone.

Function of Titin (Connectin) in Muscle Tissue

Titin is a giant protein that composes the elastic filament:

• Extends from the Z disc to the thick filament, running within the thick filament to the M line.

• Holds thick filaments in place, maintaining sarcomere organization.

• Helps muscle cell spring back after stretching, acting as an elastic element.

• Resists excessive stretching, preventing sarcomeres from being pulled apart.

The Cross-Bridge Cycle of Muscle Contraction

The cross-bridge cycle is the molecular mechanism by which myofilaments slide relative to each other, resulting in muscle contraction. It occurs when calcium levels are high enough to expose the myosin-binding sites on actin.

1. Cross Bridge Formation: The energized myosin head (with hydrolyzed ATP) attaches to actin, forming a cross bridge.

2. The Power (Working) Stroke: ADP and Pi are released, and the myosin head pivots and bends, pulling the actin filament toward the M line.

3. Cross Bridge Detachment: A new ATP molecule attaches to the myosin head, causing the myosin head to detach from actin.

4. Cocking of the Myosin Head (Re-energizing): The ATPase on the myosin head hydrolyzes ATP to ADP and Pi, re-cocking the myosin head to its high-energy position, ready to bind actin again.

The cycle repeats as long as ATP is available and calcium remains bound to troponin. If ATP is not available, detachment cannot occur, leading to the stiffness observed in rigor mortis.

Events Leading Up to Muscle Cell Contraction

1. Neuromuscular Junction Activation:

   • Calcium entry causes vesicles containing acetylcholine (ACh) to be released into the synaptic cleft by exocytosis.

   • ACh diffuses across the synaptic cleft and binds to ACh receptors on the junctional folds of the sarcolemma.

   • ACh binding opens chemically gated ion channels, leading to a local depolarization called the end plate potential (EPP).

2. Muscle Fiber Excitation (AP Propagation):

   • The EPP spreads to adjacent sarcolemma and triggers an action potential (AP).

   • The AP is generated when voltage-gated sodium channels open, and sodium rushes into the cell.

   • This AP propagates along the entire sarcolemma and travels deep into the cell via the T tubules.

3. Excitation-Contraction (E-C) Coupling:

   • The AP propagation down the T tubules causes voltage-sensitive proteins in the tubule membrane to change shape.

   • This shape change opens the calcium release channels in the terminal cisterns of the sarcoplasmic reticulum (SR).

   • Calcium ions () flow into the cytosol.

4. Contraction Initiation:

   • Ca2+ binds to troponin.

   • Troponin changes shape, moving tropomyosin away from the myosin-binding sites on the actin filaments.

   • Myosin heads bind to actin, and the cross-bridge cycle begins, resulting in contraction.

Importance of Calcium and ATP in Muscle Contraction

• Calcium (Ca2+): The final trigger for contraction. Stored and released by the sarcoplasmic reticulum (SR). When released, Ca2+ binds to troponin, which removes the inhibitory action of tropomyosin, exposing the myosin-binding sites on actin and allowing the cross-bridge cycle to begin.

• ATP (Adenosine Triphosphate): The only energy source used directly for contractile activities. Needed to energize the myosin head by hydrolysis (ATP → ADP + Pi), detach the myosin head from actin, and power the calcium pump in the SR (restoring Ca2+ to the SR for relaxation). Also powers the sodium-potassium pump in the sarcolemma to restore resting membrane potential.

Components of the Neuromuscular Junction

The neuromuscular junction (NMJ), also known as the motor end plate or motor synapse, is the site where a somatic motor neuron innervates a skeletal muscle fiber.

• Axon Terminal: The end of the neuron's axon. Contains synaptic vesicles, which store the neurotransmitter acetylcholine (ACh).

• Synaptic Cleft: A gel-filled space separating the axon terminal and the muscle fiber sarcolemma. Contains the enzyme acetylcholinesterase (AChE), which quickly breaks down ACh, terminating its effects.

• Junctional Folds of the Sarcolemma: Highly folded parts of the muscle fiber membrane, increasing the surface area for ACh receptors.

Events During Muscle Cell Excitation (Effect of Acetylcholine)

1. ACh is released from the axon terminal into the synaptic cleft.

2. ACh binds to receptors on the sarcolemma, opening ion channels.

3. More Na+ enters than K+ exits, making the inside of the sarcolemma less negative (depolarization). This local depolarization is the end plate potential (EPP).

4. The EPP spreads to adjacent areas of the sarcolemma, opening voltage-gated sodium channels. Sodium influx generates the action potential, which spreads along the sarcolemma and down the T tubules.

5. The effects of ACh are quickly terminated by acetylcholinesterase located in the synaptic cleft, which breaks down ACh to prevent continued contraction without further stimulation.

Reactions by Which Skeletal Muscle Fibers Produce ATP

Skeletal muscle fibers must rapidly regenerate ATP using three primary pathways:

1. Direct Phosphorylation of ADP by Creatine Phosphate (CP):

   • Mechanism: A high-energy phosphate group is transferred directly from stored creatine phosphate (CP) to ADP via the enzyme creatine kinase, forming ATP almost instantly.

   • Oxygen Use: None.

   • Duration/Speed: Fastest pathway, provides maximal muscle power for about 15 seconds (e.g., 100-meter dash).

2. Anaerobic Pathway (Glycolysis and Lactic Acid Formation):

   • Mechanism: Glucose (from blood or muscle glycogen) is broken down via glycolysis into two pyruvic acid molecules. When oxygen delivery is impaired, pyruvic acid is converted into lactic acid.

   • Products: 2 ATP per glucose molecule; lactic acid.

   • Oxygen Use: None (anaerobic).

   • Duration/Speed: Faster than aerobic respiration but less efficient. Can sustain strenuous activity for 30–40 seconds.

3. Aerobic Respiration:

   • Mechanism: Pyruvic acid enters mitochondria and is completely broken down to CO2, H2O, and ATP via the Krebs cycle and electron transport chain.

   • Products: 32 ATP per glucose molecule, CO2, and H2O.

   • Oxygen Use: Required (aerobic).

   • Duration/Speed: Slowest pathway, but provides the most ATP. Supports prolonged, moderate activity.

Key Terms and Concepts

• Striated: Muscle tissue with a banded appearance due to the arrangement of actin and myosin filaments.

• Voluntary: Muscle action under conscious control (e.g., skeletal muscle).

• Involuntary: Muscle action not under conscious control (e.g., cardiac and smooth muscle).

• Myofibril: A long, filamentous organelle found within muscle cells that has a banded appearance.

• Sarcomere: The functional contractile unit of muscle fiber, defined as the segment between two Z discs.

• Cross Bridge: The connection formed when a myosin head binds to an actin filament during muscle contraction.

• ATP: The primary energy currency of the cell, required for muscle contraction and relaxation.

• Calcium (Ca2+): The ion that triggers muscle contraction by binding to troponin.