Biology Chapter 6: The Muscular System

Chapter 6: The Muscular System

• Muscles are responsible for all types of body movement
• Three basic muscle types are found in the body: skeletal, smooth, and cardiac
• Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber)
• Muscle contractions are due to the movement of microfilaments
• All muscles share terminology, including prefixes like "mys-" referring to muscle and "prefix" referring "flesh".
• Most skeletal muscle fibers are attached to bones.
• Skeletal muscle cells are multinucleated and cigar-shaped.
• Skeletal muscle is also known as striated muscle because of its obvious stripes
• Skeletal muscle is also known as voluntary muscle because it's the only muscle tissue subject to conscious control
• Connective tissue surrounds and bundles skeletal muscle cells.
• The epimysium encloses a single muscle fiber.
• The perimysium wraps around a fascicle (bundle) of muscle fibers.
• The epimysium covers the entire skeletal muscle.
• The epimysium is on the outside of the epimysium.
• The epimysium blends into connective tissue attachments, forming cordlike structures (tendons) or sheetlike structures (aponeuroses).
• Tendons and Aponeuroses attach muscles to bones, cartilages, or connective tissue coverings.
• Smooth muscle has no striations and is not under conscious control.
• Smooth muscle is commonly found in the walls of hollow organs (such as the stomach, urinary bladder, and respiratory passages).
• Smooth muscle fibers are spindle-shaped.
• Smooth muscle contractions are sustained.
• Cardiac muscle has striations and is found only in the walls of the heart.
• Cardiac muscle cells are uninucleate (one nucleus) and branching.
• Cardiac muscle cells are joined by intercalated discs.
• Cardiac muscle contracts at a rate set by the pacemaker.
• Skeletal muscle has three important roles: maintaining posture and body position, stabilizing joints, generating heat.

Microscopic Anatomy of Skeletal Muscle

• Myofibrils, specialized plasma membranes, and organelles are inside muscle cells

• Light and dark bands give skeletal muscle its striated appearance

• I band = light band (contains only thin filaments); Z disc is a midline interruption

• A band = dark band (contains the entire length of thick filaments); H zone is a lighter central area; M line is in the center of H zone

• Sarcomere is the contractile unit of muscle fibers; it’s the structural and functional unit of skeletal muscle

• Myofilaments produce banding (striped) patterns; thick filaments are myosin filaments; thin filaments are actin filaments

• Thick filaments consist of the protein myosin; contain ATPase enzymes which split ATP to release energy for muscle contractions

• Myosin heads act as cross bridges when they link thick and thin filaments during contraction

• Thin filaments consist of the contractile protein actin; actin is anchored to the Z disc; at rest, the H zone within the A band lacks actin filaments

• Sarcoplasmic reticulum (SR) is a specialized smooth endoplasmic reticulum which surrounds myofibrils and stores and releases calcium

Stimulation and Contraction of Skeletal Muscle Cells

• Functional properties of skeletal muscle include responsiveness, conductivity, contractility, and elasticity.
• Responsiveness (also called irritability) is the ability to receive and respond to a stimulus.
• Conductivity is the ability to propagate an electrical signal over the muscle cell membrane.
• Contractility is the ability to forcibly shorten.
• Elasticity is the ability to recoil to resting length after shortening or stretching.

The Nerve Stimulus and Action Potential

• Skeletal muscles must be stimulated by a neuron to contract.

• The association site of the axon terminal of the motor neuron and the sarcolemma of a muscle is known as the neuromuscular junction.

• ACh (acetylcholine) is a chemical released by nerve impulse upon arrival at the axon terminal, stimulating skeletal muscle.

• The synaptic cleft is the space between the nerve and muscle.

• When a nerve impulse reaches the axon terminal of the motor neuron, calcium channels open and calcium enters the terminal.

• Calcium ions causes synaptic vesicles to release acetylcholine (ACh).

• ACh diffuses across the synaptic cleft and attaches to the sarcolemma of the muscle cell.

• If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium ions (Na+).

• More Na+ ions enter than K+ ions leave, establishing an electrical gradient (depolarization), opening more Na+ channels.

• An action potential is created.

• The action potential is conducted along the sarcolemma from one end of the cell to the other.

• Acetylcholinesterase (AChE) breaks down ACh into acetic acid and choline, ending muscle contraction

• A single nerve impulse produces only one contraction.

• Cell returns to resting state when potassium (K+) ions exit the cell; sodium-potassium pump moves sodium (Na+) and potassium (K+) ions back to their original positions.

Mechanism of Muscle Contraction: The Sliding Filament Theory

• Calcium (Ca2+) binds regulatory proteins on thin filaments, exposing myosin-binding sites.
• Myosin heads attach to thin filaments, causing them to slide toward the center of the sarcomere.
• Contraction occurs as the cell shortens.
• A cross-bridge attaches and detaches several times during a contraction.
• ATP provides energy for the sliding process, continuing as long as calcium is present.

Contraction of Skeletal Muscle as a Whole

• Graded responses are different degrees of skeletal muscle shortening produced by changing the frequency of stimulation or the number of muscle cells being stimulated.

• Muscle fibers contract to their maximum when adequately stimulated.

• Within a whole skeletal muscle, not all fibers may be stimulated during the same interval.

• Different of muscle fiber contractions may give differing responses.

• Graded responses produce different degrees of skeletal muscle shortening.

• Muscle response to increasingly rapid muscle stimulation results in muscle contractions becoming “summed,” where one contraction is immediately followed by another.

• When stimulations become more frequent, contractions get stronger and smoother.

• Muscle now exhibits unfused tetanus.

• Fused (complete) tetanus is achieved when the muscle is stimulated so rapidly that there is no evidence of muscle relaxation, resulting in smooth and sustained contractions.

• Muscle force depends on the number of fibers stimulated.

• Contraction of more fibers results in greater muscle tension.

• When all motor units are active, the muscle contraction is as strong as it can get.

Providing Energy for Muscle Contraction

• ATP is the only energy source that powers muscle contraction directly.
• Muscle fibers store a small amount of ATP, which is quickly used up.
• Other pathways must produce ATP after the initial ATP is depleted.
• Three pathways to regenerate ATP include direct phosphorylation of ADP by creatine phosphate, aerobic pathway, and anaerobic glycolysis and lactic acid formation.

Types of Muscle Contractions

• Isotonic contractions: myofilaments slide past each other during contractions; the muscle shortens and movement occurs (e.g., bending the knee, lifting weights, smiling).
• Isometric contractions: muscle filaments try to slide, but the muscle is pitted against an immovable object; tension increases, but muscles do not shorten (e.g., pushing palms together).

Muscle Tone

• Muscle tone is the state of continuous partial contractions of muscles.
• It results from different motor units being stimulated systematically.
• Muscle tone maintains firmness and readiness for action.

Effect of Exercise on Muscles

• Aerobic exercise increases muscle size, strength, and endurance, making the body's metabolism more efficient and improving digestion and coordination. Resistance exercise increases muscle size and strength through individual muscle fibers enlarging.
• Muscle fatigue and oxygen deficit occur when muscle activity is strenuous and prolonged; ion imbalances, oxygen deficit, and lactic acid accumulation contribute to muscle fatigue.