Ch10 The Muscular System( Pt1)

Administrative Updates and Course Overview

• Grading and Schedule Adjustments:

  • Student grades have been updated; specifically, previous grades that were changed resulted in improvements.

  • The discussion board was regraded as of the past Sunday.

  • A correction was made regarding Chapter 5 lectures, which were mistakenly listed under Chapter 9.

  • The current study focus is Chapter 10: The Muscular System.

  • Lectures for Chapter 10 will occur on Tuesday and Thursday.

  • The Chapter 10 lecture quiz is due this coming Sunday.

  • A lecture exam is scheduled for one week from today.

  • In the lab, the focus is starting on appendicular muscles, with a quiz on appendiculars scheduled for Thursday.

  • The Chapter 10 lecture material consists of $91$ slides; the goal is to cover approximately $45$ to $46$ slides in the current session.

• Study Strategy:

  • There are significant similarities between the Nervous System and the Muscular System. It is recommended to create a comparison/contrast note to avoid relearning identical concepts twice.

  • The instructor suggests using AI to generate reviews for these similarities.

General Functions and Distribution of Muscle Tissue

• Distribution: Muscle tissue is distributed throughout the entire body.

• Primary Functions:

  • Propulsion: Moves food through the digestive tract and propels the egg in the female reproductive system.

  • Elimination: Responsible for the elimination of unprocessed or unutilized food through the anus.

  • Respiration: The contraction and expansion of the diaphragm determines air flow through the lungs.

  • Circulation: The heart pumps blood through body tissue. However, since there is only one heart, it is less effective at returning blood from the lower extremities (feet and legs).

  • Venous Return: Blood in the limbs is moved when veins are squeezed by the contraction of surrounding skeletal muscles. This movement pushes blood from the feet back up toward the heart.

Skeletal Muscle Overview and Statistics

• Classifications: There are three types of muscle tissue, with skeletal muscle being the primary focus of this chapter.

• Prevalence: Skeletal muscle constitutes the majority of muscle in the body.

• Quantitative Data:

  • It composes approximately $40\%$ to $50\%$ of total body weight.

  • There are over $700$ individual skeletal muscles in the human body.

Physiology and Functional Roles of Skeletal Muscle

• Body Movement: Because muscles cross and articulate with bones, they allow for movement. This includes facial expressions, speaking, breathing, and swallowing.

• Support and Posture: Muscles help maintain body position and posture. The adage that poor posture can cause the body to "stick" in a slumped position is considered $100\%$ true due to muscle adaptation.

• Organ Protection: Muscles protect and support internal organs. The abdominal organs, which lack bone protection, are guarded by three layers of abdominal muscles oriented in different directions. The development of these muscles (e.g., a "six-pack") determines the amount of impact or trauma the abdomen can sustain.

• Regulation of Elimination: Muscle sphincters (circular muscles) regulate the orifices of the urethra and the anus.

  • Internal Sphincter: Involuntary; signals the need to urinate or defecate.

  • External Sphincter: Voluntary; controlled by the individual to decide when to release material.

  • Control of these sphincters is absent in infants and can be lost during aging.

• Heat Production: Muscle activity produces heat. Shivering is a response to cold that uses muscle friction to raise body temperature.

Characteristics of Muscle Tissue

• Excitability: The ability to respond to internal or external stimuli. This involves changes in the Resting Membrane Potential ($RMP$). Like nerves, once a muscle reaches a specific threshold, it will fire.

• Contractility: The ability of the muscle to shrink or shorten in length.

• Extensibility: The ability of the muscle to be stretched.

• Elasticity: The ability of the muscle to change shape and return to its original shape without sustaining permanent damage.

Gross Anatomy and Organization of Skeletal Muscle

• Skeletal Muscle as an Organ: Each muscle is an organ composed of various tissues working in synchronization: skeletal muscle fibers, connective tissue, blood vessels, and nerves.

• Hierarchical Organization:

  1. Whole Muscle: The largest unit (the organ).

  2. Fascicles: Bundles of many muscle fibers found within the whole muscle.

  3. Muscle Fibers: Individual cells found within fascicles.

Connective Tissue Wrappings and Attachments

• Root Terms: "Myo" is the most common medical term prefix for muscle.

• Three Layers of Connective Tissue:

  • Epimysium: The outer layer that surrounds and holds the entire muscle organ together (meaning "upon" or "above").

  • Perimysium: Surrounds fascicles and many blood vessels (meaning "around").

  • Endomysium: Within the muscle; supports individual muscle fibers. It helps maintain the electrical signal and provides capillary support for blood flow.

• Types of Attachments and Coverings:

  • Tendon: A stiff or dense attachment of muscle to bone.

  • Ligament: Attaches bone to bone (distinguishable from tendons).

  • Aponeurosis: A flat, sheet-like connective tissue attachment.

  • Fascia: Wraps around individual muscles.

  • Superficial Fascia: Separates the muscle from the skin.

  • Fasciitis: Inflammation or irritation of the muscle covering due to overstress, overuse, age, or athletic injury.

Microscopic Anatomy of the Muscle Fiber

• Vascularization and Innervation: Skeletal muscle is highly vascularized to ensure a constant supply of oxygen and nutrients and the removal of waste. It is innervated by somatic motor neurons at the neuromuscular junction ($NMJ$).

• Sarcoplasm: The specific term for the cytoplasm of a muscle cell, containing standard organelles and the nucleus.

• Myoblast: The embryonic cell responsible for producing muscle tissue (similar to how osteoblasts make bone).

• Multi-nucleated: Muscle fibers contain multiple nuclei.

• Sarcolemma: The plasma membrane of the muscle cell. It contains:

  • Voltage-Gated Ion Channels: Control the firing of the muscle via action potentials.

  • Sodium-Potassium Pumps: Maintain the $RMP$.

  • T-tubules: Transverse tubules that allow the electrical signal to travel deep into the cell.

Internal Structures of the Muscle Fiber

• Myofibrils: Thousands of tiny microscopic fibers per cell, primarily made of protein.

• Sarcoplasmic Reticulum ($SR$): Similar to the endoplasmic reticulum. It includes Terminal Cisternae, which act as reservoirs/storage for calcium.

• Triad: A structural unit composed of one T-tubule and two terminal cisternae. This is the site where calcium pumps store calcium and release it when triggered by an electrical signal.

• Myofilaments: Smaller structures within myofibrils, categorized as:

  • Thick Filaments: Composed of Myosin protein strands with Myosin heads (shaped like little claws).

  • Thin Filaments: Composed of Actin protein. Actin contains Myosin binding sites. Two other proteins, Troponin and Tropomyosin, are associated with thin filaments.

The Sarcomere: The Unit of Contraction

• Definition: The individual muscle unit length; the arrangement of overlapping thick and thin myofilaments. Multiple sarcomeres exist along a single muscle fiber.

• Anatomy of a Sarcomere:

  • Z-discs (Z-lines): Anchors on both ends of the sarcomere.

  • I-bands: Light-appearing regions containing only thin filaments and the Z-disc. These disappear during maximum contraction.

  • A-bands: Dark regions in the central part of the sarcomere where thick and thin filaments overlap. Each sarcomere has one A-band and two I-bands.

  • H-zone: Located in the center of the A-band; contains only thick filaments and disappears during maximum contraction.

  • M-line: The center line of the H-zone where thick filaments attach.

Structural and Elastic Proteins

• Connectin: A functional protein that stabilizes thick filaments from the Z-disc to the M-line. It provides the characteristic of elasticity.

• Dystrophin: A structural protein that anchors myofibrils to the sarcolemma.

• Duchenne Muscular Dystrophy: A progressive deteriorating disease caused by a deficiency in dystrophin. Without it, the muscle is damaged every time it contracts and cannot be repaired. Patients historically died in their teens, but may now survive to about age $30$ .

Muscle Metabolism and Energy Sources

• Mitochondria: Abundant in muscle fibers; the "ATP factories" for aerobic respiration (requires oxygen).

• Myoglobin: A protein within the cell that stores oxygen for quick bursts of energy.

• Glycogen: A polysaccharide (complex glucose) stored in the liver and muscles for carbohydrate energy.

• Creatine Phosphate: A molecule that quickly gives up a phosphate group to replenish $ATP$ supplies.

The Neuromuscular Junction ($NMJ$) and Motor Units

• Motor Unit: A single motor neuron and all the muscle fibers it controls.

  • Small Motor Units: Less than $5$ fibers; used for fine, meticulous motor skills (e.g., sewing, picking up a paper clip).

  • Large Motor Units: Thousands of fibers; used for force and strength (e.g., picking up a brick).

• Components of the $NMJ$:

  • Synaptic Knob: The end of the neuron containing vesicles filled with Acetylcholine ($ACH$), the most common neurotransmitter.

  • Synaptic Cleft: The gap between the neuron and the muscle.

  • Motor End Plate: The specialized region of the sarcolemma with receptors for $ACH$.

  • Acetylcholinesterase: An enzyme in the cleft that breaks down $ACH$ to prevent continuous, uncontrolled muscle firing.

Electrophysiology of Muscle Contraction

• Resting Membrane Potential ($RMP$):

  • Nerve $RMP$: $-70\,mV$.

  • Muscle $RMP$: $-90\,mV$.

• Threshold: For a muscle to fire, it must reach a threshold of $-65\,mV$ (moving from $-90\,mV$).

• Steps of Activation:

  1. Action potential reaches the synaptic knob, opening voltage-gated calcium channels.

  2. Calcium enters the knob, causing the release of $ACH$ into the synaptic cleft via vesicles.

  3. $ACH$ drifts across the cleft and binds to receptors on the motor end plate.

  4. Chemically gated ion channels open; Sodium ($Na^+$) moves in and Potassium ($K^+$) moves out, reaching the threshold of $-65\,mV$.

  5. Depolarization and the Action Potential propagate down the sarcolemma and into T-tubules.

The Sliding Filament Model (Crossbridge Cycling)

• Calcium Release: The action potential reaches the Triad ($2$ cisternae, $1$ T-tubule), causing terminal cisternae to release calcium into the cytosol.

• Conformational Change: Calcium binds to Troponin, which changes shape. This moves the Tropomyosin complex, exposing the Myosin binding sites on the actin filament.

• The Cycle:

  1. Crossbridge Formation: Myosin heads bind to the exposed sites on actin.

  2. Power Stroke: The myosin head pulls the actin filament toward the center of the sarcomere, shortening the muscle. This requires $ATP$.

  3. Release: Myosin releases the actin (requires $ATP$).

  4. Reset: Myosin head resets to its original position.

• Requirements: Cycling continues as long as both Calcium and $ATP$ are present. If Calcium is removed, the binding sites are covered again. If $ATP$ is exhausted, the muscle ceases to function.

Clinical Pathologies

• Myasthenia Gravis: An autoimmune disease, more common in women, where the immune system attacks binding sites. It can be a side effect of some pharmaceuticals. Symptoms include drooping eyes (ptosis), sagging jaws, or difficulty swallowing.

• Spastic Paralysis: Caused by toxins from anaerobic bacteria.

  • Tetanus (caused by Clostridium fentanyl [sic]): Often called "lockjaw"; prevented by the tetanus vaccine. Usually contracted via deep wounds (e.g., stepping on a rusty nail).

  • Botulism (caused by Clostridium botanum [sic]): Contracted from poorly canned foods. It is highy deadly but utilized in small doses for Botox (cosmetic) and migraine treatments.

Questions & Discussion

• Question: Does the Calcium only exist in the synaptic knob?

• Answer: In the context of the nerve signal, it is in the knob. In the muscle, it is stored in the terminal cisternae/sarcoplasmic reticulum.

• Discussion on Exams: The professor emphasizes that students must practice the two discussion questions on Blackboard. These are worth $5$ points each ($10$ points total), which is an entire letter grade on the exam.

• Lab Transition: The class ended early to allow more time in the lab to work on histology and physiology models, as the instructor noted some students struggled with the first lab exam.