Chapter 9: Muscle Tissue

Types of Muscle Tissue

Muscle tissue is classified into three main types based on cell shape, number of nuclei, presence of striations, and control mechanisms.

• Cell Shape:

  • Skeletal: long, cylindrical

  • Cardiac: short, branched

  • Smooth: spindle-shaped

• Number of Nuclei:

  • Skeletal: multinucleate

  • Cardiac: uninucleate

  • Smooth: uninucleate

• Striations:

  • Skeletal: striated

  • Cardiac: striated

  • Smooth: no striations

• Control:

  • Skeletal: voluntary

  • Cardiac: involuntary

  • Smooth: involuntary

Functions of Muscle Tissue Beyond Movement

Muscle tissue serves several functions apart from movement:

• Unique Properties: contractility, excitability, extensibility, elasticity

• Body Movements: gross & fine movements, chemical to mechanical energy conversion

• Posture: maintains body position

• Storage: regulates movement of substances (blood, lymph, food, etc.)

Muscle Tissue Organization

Muscle tissue is organized hierarchically from the organ level to molecular level:

• Muscle organ (entire muscle)

• Fascicle: bundle of muscle fibers

• Muscle fiber: single muscle cell

• Myofibril: contractile unit within muscle fiber

• Myofilaments: actin & myosin (contractile proteins)

Comparison of Contractile Myofilaments

Muscle contraction is driven by two main types of myofilaments:

• Actin: thin filament, forms the backbone of the sarcomere

• Myosin: thick filament, motor protein with heads that bind to actin

Key Differences:

• Actin filaments are anchored to Z discs

• Myosin filaments are anchored at the M line

• Myosin heads form cross-bridges with actin during contraction

Role of Troponin and Tropomyosin in Muscle Contraction

Troponin and tropomyosin regulate the interaction between actin and myosin:

• Tropomyosin: covers myosin binding sites on actin in relaxed muscle

• Troponin: binds calcium ions, causing tropomyosin to move and expose binding sites

Connective Tissue Layers in Muscle

Muscles are surrounded and supported by connective tissue layers:

• Fascia: surrounds groups of muscles, organs, nerves, and joints

• Epimysium: covers entire muscle

• Perimysium: covers fascicles

• Endomysium: covers individual muscle fibers

Sarcomere Organization

The sarcomere is the basic contractile unit of muscle:

• A band: length of thick filaments (myosin)

• I band: thin filaments only (actin)

• Z disc: anchors thin filaments

• M line: center of sarcomere, anchors thick filaments

Cellular Specializations of Muscle Tissue

Muscle cells have unique organelles and adaptations:

• Sarcolemma: plasma membrane of muscle fiber

• Sarcoplasmic reticulum: stores and releases calcium ions

• T tubules: transmit action potentials into the cell interior

• Sarcoplasm: cytoplasm, contains myoglobin for oxygen storage

Cross Bridge Cycle and ATP Usage

The cross bridge cycle describes the interaction between actin and myosin during contraction:

1. Myosin heads hydrolyze ATP and become energized

2. Myosin heads bind to actin, forming cross-bridges

3. Myosin heads rotate, pulling actin filaments (power stroke)

4. Myosin heads release ADP, new ATP binds, cross-bridge detaches

ATP is essential for: energizing myosin, detaching cross-bridges, and pumping Ca2+ back into the sarcoplasmic reticulum.

Sliding Filament Theory

Muscle contraction occurs as actin and myosin filaments slide past each other:

• Thick filaments slide past thin filaments, shortening the sarcomere

• Zone of overlap increases as contraction proceeds

Neuromuscular Junction (NMJ)

The NMJ is the synapse between a motor neuron and a skeletal muscle fiber:

• Synaptic vesicles: contain acetylcholine (ACh)

• ACh receptors: located on the muscle cell membrane

• Action potential: triggers ACh release, leading to muscle contraction

Steps of Synaptic Transmission at the NMJ

1. Action potential arrives at axon terminal

2. ACh is released into synaptic cleft

3. ACh binds to receptors on muscle cell membrane

4. Muscle cell depolarizes, generating an action potential

5. Action potential travels down T tubules, triggering Ca2+ release from SR

Excitation-Contraction Coupling

Excitation-contraction coupling links the action potential to muscle contraction:

• Action potential travels along sarcolemma and T tubules

• Ca2+ released from SR binds to troponin

• Tropomyosin moves, exposing myosin binding sites on actin

• Cross-bridge cycle initiates muscle contraction

Sources of ATP in Muscle

Muscle cells use several sources of ATP for contraction:

• Creatine phosphate: rapid ATP regeneration for short bursts

• Anaerobic respiration: glycolysis, produces ATP without oxygen

• Aerobic respiration: uses oxygen, produces most ATP during prolonged activity

Chapter 11: Nerve Physiology (Action Potential)

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the plasma membrane of a cell at rest.

• Typical value:

• Charge separation: specific ions (Na+, K+)

• Negative inside: due to selective permeability and ion pumps

Creation and Maintenance of Resting Membrane Potential

• Created by: unequal distribution of ions, selective permeability

• Maintained by: sodium-potassium pump ()

Ion Channels and Membrane Potential

• Voltage-gated channels: open/close in response to voltage changes

• Leak channels: always open, contribute to resting potential

Graded Potentials vs. Action Potentials

Threshold and Action Potential Generation

• Threshold: minimum voltage required to trigger action potential (about )

• All-or-none principle: action potential occurs fully or not at all

Phases of Action Potential

• Depolarization: Na+ channels open, Na+ enters cell

• Repolarization: Na+ channels open, K+ leaves cell

• Hyperpolarization: K+ channels remain open, membrane potential becomes more negative

Propagation of Action Potential

• Unidirectional propagation: due to Na+ channel inactivation

• Saltatory conduction: action potential jumps between nodes of Ranvier in myelinated axons

Refractory Periods

Myelin Sheath and Conduction

• Myelin sheath: insulating layer around axons, increases speed of conduction

• Nodes of Ranvier: gaps in myelin where action potentials are regenerated

• Saltatory conduction: rapid transmission by jumping from node to node

Summary Equations

• Resting membrane potential:

• Sodium-potassium pump:

Example: Action Potential Propagation

When a neuron is stimulated above threshold, voltage-gated Na+ channels open, causing rapid depolarization. The action potential travels along the axon, jumping between nodes of Ranvier in myelinated fibers, resulting in fast signal transmission.

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.