Muscle Tissue: Functions and Types

• "myo-," "mys-," and "sarco-" are prefixes related to muscle tissue.

Muscle Functions

• Muscles facilitate movement by manipulating the environment and internal passageways.
• Muscles ensure stability by maintaining posture and stabilizing joints.
• Heat production is a significant byproduct of muscle activity.

Types of Muscle Tissue

• Skeletal muscle is voluntary, attached to bones, and responsible for body movements.
• Cardiac muscle is involuntary and is found in the heart.
• Smooth muscle is involuntary and located in hollow organs.

Characteristics of Muscle Tissue

• Excitability is the ability to respond to electrical stimuli.
• Contractility enables muscles to shorten forcefully when stimulated.
• Extensibility refers to the ability to be stretched.
• Elasticity allows muscle tissue to recoil to its resting length after stretching.
• Muscles have dedicated nerve and blood vessel supplies, with skeletal muscles having nerves connected to every fiber.
• Muscles need large amounts of oxygen to function.

Skeletal Muscle Attachments

• An insertion is the point where a muscle attaches to a more movable bone.
• An origin is the point where a muscle attaches to a less movable bone.
• Direct attachment involves the muscle fusing directly to the bone.
• Indirect attachment involves a connection via a tendon or aponeurosis.

Connective Tissue Sheaths

• The epimysium is the outermost layer that surrounds the entire muscle.
• The perimysium is the middle layer that surrounds fascicles (bundles of muscle fibers).
• The endomysium is the innermost layer that surrounds individual muscle fibers.

Muscle Fiber Microanatomy

• The sarcolemma is the plasma membrane, and the sarcoplasm is the cytoplasm of a muscle cell.
• Myofibrils are densely packed, rod-like organelles within muscle fibers, making up about 80% of the cell volume.
• Muscle fibers show striations, which are repeating bands of the myofilaments.
• A sarcomere is the functional unit of muscle, located between Z discs.
• Myofilaments consist of actin (thin filaments with troponin and tropomyosin) and myosin (thick filaments with tails and globular heads).
• The sarcoplasmic reticulum is a smooth ER network that surrounds each myofibril and stores calcium.
• T-tubules are infoldings of the sarcolemma that carry action potentials deep into the muscle fiber.
• A triad includes a T-tubule and two terminal cisternae of the sarcoplasmic reticulum and contains calcium channel proteins.
• Membrane proteins include leakage channels that are always open and gated channels.
• Chemically gated channels open or close in response to a chemical binding.
• Voltage-gated channels open or close in response to changes in action potential.
• Mechanically gated channels open or close in response to a physical shape change.

Muscle Contraction at Neuromuscular Junction

• Action potentials arrive at the axon terminal, triggering voltage-gated calcium channels to open.
• Calcium entry causes vesicles to release acetylcholine (ACh) into the synaptic cleft.
• ACh binds to receptors on the sarcolemma, opening sodium channels which allows influx into the muscle.
• Acetylcholinesterase degrades ACh to stop the signal.

Action Potential Generation Along the Sarcolemma

• At rest, the membrane is polarized due to unequal concentrations of Na+ and K+ ions.
• End plate potential occurs when sodium influx makes the sarcolemma less negative.
• Depolarization occurs as enough sodium enters to meet the threshold, spreading the action potential.
• Repolarization happens when sodium channels close and potassium channels open.
• During the refractory period, another contraction cannot occur until repolarization is complete.

Excitation-Contraction Coupling

• An action potential travels down the sarcolemma, into the T-tubules, causing calcium channels to open.
• Calcium is released from the sarcoplasmic reticulum to facilitate cross-bridge formation.

Cross-Bridge Cycling

• Calcium binds to troponin on actin, which moves tropomyosin to expose myosin-binding sites.
• Myosin heads bind to actin, forming cross-bridges.
• A power stroke occurs as myosin heads pivot, causing the sarcomere to shorten.
• Detachment occurs when ATP binds to myosin, returning the myosin head to resting position, and the cycle repeats until calcium is removed from troponin.

Energy for Muscle Contraction

• Immediate energy is generated via direct phosphorylation, doesn't require oxygen, and lasts about 15 seconds.
• Creatine phosphate donates a phosphate group to ADP to create ATP, a process assisted by creatine kinase.
• Short-term energy is generated via anaerobic pathway, doesn't require oxygen, and lasts about 60 seconds.
• Glycolysis breaks down glucose into 2 pyruvic acid molecules, generating 2 ATP, which leads to lactic acid formation, where pyruvic acid is converted to lactic acid.
• Long-term energy is generated via aerobic (cellular) respiration and produces ~90% of ATP.
• The process requires oxygen and can last over 30 minutes; glucose and oxygen are used to produce water, carbon dioxide, and 32 ATP.
• Muscle fatigue arises from ionic imbalances or depletion of ATP, oxygen, and glucose (glycogen) stores.
• Endurance exercise increases muscle capillaries and mitochondria, improving endurance.
• Resistance exercise increases the number of mitochondria and size of myofilaments, increasing muscle size and strength.

Muscle Responses

• Graded responses are smooth, coordinated movements influenced by frequency and strength of stimulation.
• Individual twitches lead to temporal summation, unfused tetanus, and fused tetanus.
• Tetanus is continuous muscle contraction.
• Recruitment involves sending stimuli to multiple fibers based on size, starting with the smallest, to increase precision or strength.
• Stimulus needs to be strong enough to reach the threshold for contraction to occur.
• Muscle tone is a constant, slightly contracted state that keeps muscles healthy and alert.
• Muscle tension is the force exerted on an object, but does not always shorten the muscle.
• Isotonic contractions occur when tension exceeds the load. - Concentric contractions: muscles shorten to do work - Eccentric contractions: muscles lengthen
• Isometric contractions increase tension without changing muscle length, myosin heads "spin" on actin-binding sites.
• A muscle twitch is the simplest contraction, resulting from a single action potential.
• Larger muscles contract more slowly and hold the contraction longer.
• Slow-twitch fibers contract slowly, have high endurance, but are not very strong, with high amounts of mitochondria, myoglobin, and capillaries.
• Fast-twitch fibers contract rapidly, fatigue quickly, and are much stronger with a limited blood supply.

Factors Affecting Muscle Contraction Force

• Recruitment (number of fibers stimulated): More fibers lead to greater force.
• Size of fibers: Larger muscles develop more tension.
• Frequency of stimulation: Higher frequency equals greater force.
• Degree of muscle stretch: 80-120% of resting length is ideal.
• Slow oxidative (aerobic) fibers: for low intensity, low endurance activities, such as maintaining posture.
• Fast oxidative (aerobic) fibers: for medium intensity activities, such as walking.
• Fast glycolytic (anaerobic) fibers: for short-term, intense/powerful movements, such as hitting a baseball.

Smooth Muscle Tissue

• Smooth muscle is located in hollow organs, is uninucleate, and has only endomysium.
• Most organs have opposing muscle layers (longitudinal and circular) for rhythmic contractions (peristalsis).
• Smooth muscle does not have neuromuscular junctions, instead containing varicosities that release neurotransmitters into a diffuse junction.
• It is innervated by the autonomic nervous system (ANS) and is controlled by nerves, hormones, or local chemical changes.
• Smooth muscle has a less elaborate SR, no T-tubules, and calcium primarily comes from extracellular sources.
• The sarcolemma contains caveolae with many calcium channels.
• Gap junctions in smooth muscle allow depolarization to spread from cell to cell.
• It contains the regulatory proteins tropomyosin and calmodulin.
• Some smooth muscle cells are self-excitatory, and most maintain moderate contraction without fatiguing.
• Smooth muscle can stretch and recoil 50-200% of its resting length.
• Unitary smooth muscle is typical in most hollow organs.
• Multiunit smooth muscle consists of independent fibers innervated by the ANS.
• Multiunit smooth muscle is found in the lungs, large arteries, iris of the eye, and arrector pili muscles.