Muscle Tissue and Contraction

Muscle Contraction Cycle

The muscle contraction cycle describes the sequence of molecular events that enable muscle fibers to contract and relax. This process is essential for all voluntary and involuntary muscle movements.

• Active-site exposure: Calcium ions bind to troponin, causing tropomyosin to move and expose active sites on actin filaments.

• Cross-bridge formation: Myosin heads bind to the exposed active sites on actin, forming cross-bridges.

• Myosin head pivoting (power stroke): The myosin head pivots, pulling the actin filament toward the center of the sarcomere.

• Cross-bridge detachment: ATP binds to the myosin head, causing it to detach from actin.

• Myosin reactivation: ATP is hydrolyzed, re-cocking the myosin head for another cycle.

Example: The contraction of skeletal muscle during movement, such as lifting an object.

Types of Muscle Tissue

Muscle tissue is classified based on structure, function, and control mechanisms.

• Smooth muscle: Involuntary, non-striated muscle found in the walls of hollow organs (e.g., intestines, blood vessels). Responsible for peristalsis and movement of substances.

• Skeletal muscle: Voluntary, striated muscle attached to bones; responsible for body movement.

• Cardiac muscle: Involuntary, striated muscle found only in the heart; responsible for pumping blood.

Example: Smooth muscle moves food through the digestive tract via peristalsis.

Regulation of Muscle Contraction

Muscle contraction is regulated by proteins that control the interaction between actin and myosin.

• Tropomyosin: A protein that covers the active sites on actin in resting muscle fibers, preventing myosin binding.

• Troponin: Binds calcium ions and moves tropomyosin away from actin's active sites during contraction.

• Myosin: The motor protein that binds to actin and generates force.

• Titin: A structural protein that provides elasticity and stability to the sarcomere.

Example: In resting muscle, tropomyosin blocks actin's active sites, preventing contraction.

Sarcomere Structure

The sarcomere is the basic contractile unit of striated muscle fibers.

• Myosin filaments (thick filaments): Composed of myosin molecules.

• Actin filaments (thin filaments): Composed of actin, tropomyosin, and troponin.

• Z-discs: Define the boundaries of each sarcomere and anchor thin filaments.

• Glycogen stores: Not a structural component of the sarcomere; serve as an energy reserve in muscle fibers.

Example: The sliding filament model describes how actin and myosin filaments slide past each other during contraction.

Muscle Shapes and Adaptations

Muscles are classified by their shapes, which influence their function and adaptability.

• Convergent muscles: Broad at origin and converge to a single tendon; can change the direction of pull depending on which fibers are active.

• Parallel, pennate, and circular muscles: Other shapes with distinct functional properties.

Example: The pectoralis major is a convergent muscle that can pull the arm in different directions.

Muscle Compartments and Examples

Muscles are grouped into compartments based on location and function.

• Biceps femoris: Located in the posterior compartment of the thigh; involved in knee flexion.

• Other examples: Flexor digitorum brevis (foot), gastrocnemius (posterior leg), adductor longus (medial thigh).

Example: The posterior compartment of the thigh contains muscles that flex the knee.

Levers in the Human Body

Levers are rigid structures that move around a fixed point (fulcrum) when a force is applied.

• First-class lever: Fulcrum is between the effort and the load (e.g., tipping the head backward).

• Second-class lever: Load is between the fulcrum and the effort (e.g., standing on toes).

• Third-class lever: Effort is between the fulcrum and the load (e.g., biceps curl).

Example: The atlanto-occipital joint acts as a first-class lever when nodding the head.

Muscle Origins, Insertions, and Actions

Each muscle has a specific origin (attachment to stationary bone), insertion (attachment to movable bone), and action (movement produced).

• Biceps brachii: Origin: scapula; Insertion: radius; Action: elbow flexion, forearm supination.

• Vastus intermedius: Origin: femur; Insertion: tibial tuberosity; Action: knee extension.

• Rectus abdominis: Origin: pubis; Insertion: sternum and costal cartilages; Action: vertebral column flexion.

• Masseter: Origin: zygomatic arch and maxilla; Insertion: mandible; Action: jaw elevation (not hip extension).

Example: Incorrect pairing: Masseter does not insert on orbicularis oris nor act in hip extension.

Muscle Coordination: Agonists, Antagonists, Synergists, and Fixators

Muscle groups work together to produce smooth and controlled movements.

• Agonists (prime movers): Main muscles responsible for a movement.

• Antagonists: Muscles that oppose the action of agonists; relax and stretch when the agonist contracts.

• Synergists: Assist the agonist in performing a movement.

• Fixators: Stabilize the origin of the agonist.

Example: During elbow flexion, the biceps brachii is the agonist, and the triceps brachii is the antagonist.

Nervous System: Neurons and Action Potentials

Membrane Potential and Its Changes

Neurons maintain a resting membrane potential, which is essential for the generation of action potentials.

• Resting membrane potential: The inside of the neuron is more negative than the outside, typically around -70 mV.

• Depolarization: A shift in membrane potential toward a more positive value, usually due to sodium influx.

• Hyperpolarization: Membrane potential becomes more negative than resting potential.

• Repolarization: Return of the membrane potential to resting value after depolarization.

Equation:

where is the membrane potential, is the potential inside the cell, and is the potential outside the cell.

Action Potential Initiation and Propagation

An action potential is a rapid, temporary change in membrane potential that travels along the neuron.

• Initiation site: Usually starts at the axon hillock, where the density of voltage-gated sodium channels is highest.

• Key ion: Sodium (Na+) influx is primarily responsible for the initiation and rising phase of the action potential.

• Propagation: In myelinated axons, action potentials jump between nodes of Ranvier (saltatory conduction); continuous propagation cannot occur due to myelin increasing resistance to ion flow.

Example: The rapid transmission of nerve impulses along myelinated axons in the peripheral nervous system.

Specialized Cardiac Cells and Automaticity

Cardiac muscle cells can generate action potentials without external nervous input due to specialized pacemaker cells.

• Pacemaker cells: Located in the sinoatrial (SA) node; initiate and regulate the heartbeat.

• Other specialized cells: Bundle of His, Purkinje fibers, and AV node cells help conduct impulses but do not initiate them.

Example: The SA node sets the pace of the heart by spontaneously generating action potentials.

Resting State of Neurons

At rest, the inside of a neuron is more negative than the outside due to the distribution of ions across the membrane.

• Resting potential: Maintained by the sodium-potassium pump and selective permeability of the membrane.

Example: The resting membrane potential is essential for the excitability of neurons.

Refractory Periods

After an action potential, neurons experience periods during which they are less responsive to stimuli.

• Absolute refractory period: The neuron is completely insensitive to a second stimulus; occurs during the depolarization and early repolarization phases when sodium channels are inactivated.

• Relative refractory period: A stronger-than-normal stimulus is required to initiate another action potential.

Example: The absolute refractory period ensures unidirectional propagation of action potentials.

Summary Table: Key Muscle and Nervous System Concepts