Advanced Muscle Physiology and the Sarcomere Structure
Osteoarthritis and the Effects of Wear and Tear
Wear and Tear Definition: This refers to the cumulative effects on the body as it is used over time, specifically regarding joints.
Synovial Joints: Frequent use of these joints can lead to a condition that resembles inflammation (an "itis").
Osteoarthritis (): * Characterized as general wear and tear of the joint. * It specifically affects the articular cartilage within synovial joints. * Physiological Response: As the cartilage breaks down, bones begin to touch (bone-on-bone). * Epiphyses Widening: The ends of the bones (epiphyses) tend to widen because osteocytes receive increased stress, which triggers abnormal bone growth in the area.
Cellular Anatomy of Skeletal and Cardiac Muscle
Striped Appearance: Both cardiac and skeletal muscle exhibit a stripy or striated appearance. This is due to a highly organized structural protein arrangement.
Internal Composition: * A single muscle cell is primarily filled with stacks of proteins rather than standard organelles. * Sarcoplasmic Reticulum: A modified organelle appearing as "blue webbing" that wraps around bundles of proteins. * Mitochondria: These are scattered and squished in between the protein bundles. * Nuclei and Other Organelles: These are shoved to the outer edges of the cell to make room for the protein stacks.
Building Muscle: Increasing protein intake provides the body with necessary building blocks (absorbed through the digestive system) to synthesize these internal cellular proteins.
Terminology: * Myofilament: An individual strand of protein. * Myofibril: A bundle of myofilaments.
Contractile and Regulatory Proteins
The Contractile Proteins: These are the proteins responsible for the physical shortening/contraction of the muscle. * Myosin (Thick Filament): * Appears physically thicker than actin. * Structure: A twisted protein filament with "knobby heads" resembling the head of a golf club. * It has a "furry" appearance because it consists of tons of myosin molecules bound together with heads sticking out in all directions. * Actin (Thin Filament): * Recognized as the red component on lab models. * Found in conjunction with regulatory proteins.
The Regulatory Proteins: These control when contraction is allowed to occur. * Tropomyosin: * A ribbon-like or strand-like protein. * Functions by covering the "active binding sites" (the "hug zone") on actin. * It "trips up" myosin, preventing it from binding to actin during relaxation. * Troponin: * Acts like "staples," "tacks," or a "complex of proteins" that hold the tropomyosin strand in place over the binding sites.
The Sarcomere: The Functional Unit of Muscle
Definition: The arrangement of the four proteins (actin, myosin, troponin, and tropomyosin) into an organized, repeating subunit.
Structural Boundaries: * Z-Discs (Z-Lines): The zipper-like boundaries of the sarcomere. A single sarcomere spans from one Z-disc to the next. * M-Line: The "middle line" of the sarcomere.
Protein Anchoring: * Myosin attaches to the M-line ( for Myosin). * Actin attaches to the Z-discs.
Mechanism of Contraction (Sliding Filament Theory): * Muscle contraction occurs when the regulatory proteins (troponin and tropomyosin) are moved, allowing myosin to grab actin and pull it toward the M-line. * Crucial Detail: The proteins do not change their length or shorten. Myosin and actin stay the same length; they simply slide past each other, which changes the overall length of the sarcomere subunit. * Because sarcomeres are bound to the cell membrane, when the thousands of sarcomeres within a cell shorten, the entire muscle cell shortens.
Striations: The "dark zones" and "light zones" in muscle tissue are created by the overlap of these proteins. Areas of overlap appear darker, while areas with fewer layers appear lighter.
Muscle Damage and Soreness
Micro-damage: During intense exercise, muscle cells are not typically torn. Instead, protein components are damaged. For example, myosin heads may be ripped off or actin filaments ripped apart.
Inflammation: The cell responds to protein damage by clearing debris, which triggers a rushing of fluid (inflammation).
Delayed Onset Muscle Soreness (DOMS): * The pain felt after a workout is not from the proteins themselves (as they lack nerves). * The pain results from increased pressure in the tissue due to inflammation. * Bare Nerve Endings: These simple structures in the tissue signal "ouch" when they are squished by the surrounding fluid pressure.
Neuromuscular Interaction and Motor Units
Nervous System Control: Skeletal muscle is voluntary and will not contract without a signal from the central nervous system.
Denervation Atrophy: If the functional nerve to a muscle is damaged, the muscle will never contract again. Over time, the muscle breaks down its internal components and shrinks because it is no longer being used.
Motor Neuron Structure: The signal travels from the cell body () down a long tapered part called the axon to the end terminals ().
Neurotransmitters: Chemicals dropped by the neuron onto the muscle tissue to deliver the message to contract.
Neuromuscular Junction (NMJ): The specific location where the motor neuron and the muscle cell meet (though they do not physically touch; there is a small gap).
Motor Unit Definition: One single motor neuron plus all the individual muscle cells it controls/innervates.
Variations in Motor Units: * Large Motor Units: One neuron may control muscle cells (e.g., in the calf for powerful, gross movements). * Small Motor Units: One neuron may control as few as muscle cells (e.g., in the face or hands for fine, detailed motor control).
Recruitment: The body can increase the strength of a contraction by "using more motor units." Carrying a heavy stack of textbooks requires more motor units than carrying a light bag of markers.
Questions & Discussion
Question: Why do the proteins have a striped appearance? * Response: It is due to the very organized structural protein arrangement and the overlap or spaces in between those proteins.
Question: What are the two main proteins in muscle? * Response: Actin and Myosin.
Question: What is Rigor Mortis? * Response: It is unregulated muscle contraction that happens after death. It will be discussed in further detail after the Sliding Filament Theory is fully covered.
Question: How do hospitals measure heart attacks using these proteins? * Response: In a myocardial infarction (heart attack), heart muscle cells die due to lack of oxygen and nutrients. Troponin is the smallest of the muscle proteins, so it is the first to leak out of the dying cell through small gaps. Doctors measure troponin levels in the blood to judge how much of the heart was affected and how much tissue has died.
Question: Why do muscles twitch, like by the eyelids? * Response: Twitches can be caused by many things: overactive motor neurons, sensitive cell membranes, or the sarcoplasmic reticulum leaking calcium. Eyelid twitches are specifically tied to sleep deprivation and can take up to weeks of consistent sleep to resolve.
Question: Does caffeine affect muscle contraction? * Response: Caffeine is a neurostimulant that affects the nervous system. Energy drinks containing caffeine, guarana, and B vitamins are also stimulants. Regular consumption can lead to cardiac symptoms, especially in young people.
Anecdote regarding Strychnine: The professor noted that strychnine (a poison) is molecularly very similar to caffeine—essentially "caffeine on steroids"—and acts as a massive neurostimulant even at low doses.
Discussion on Summer Microbiology Class: The professor clarified that Microbiology is not running this summer, contrary to rumors among students. The department is currently down one faculty member who retired in December, and there is no instructor available to teach the course during the summer as current professors are already working "overload" and need a break.
Model Cost Detail: The plastic sarcomere model used in class cost approximately six years ago.