Skeletal Muscle Notes

Lab and Exam Information

• Lab hours are from 8:00 AM to 12:00 PM.

• A muscle sheet will be provided for identifying muscle origins, insertions, and actions, and is allowed for use during the muscle practical exam.

• Only your brain and a pen can be used during the bone practical exam.

• Next week there will be no lecture.

• On Monday and Tuesday of next week, the lab will be open from 8:00 AM to 2:30 PM.

• The bone and muscle practical exams will be on Wednesday.

Practical Exam Structure

• The bone and muscle practicals will be conducted separately.

• The bone practical will be first, followed by a 15-20 minute break, and then the muscle practical.

• The week following the practicals (week seven) will involve brain and eyeball dissection in the lab.

• The week after dissection is review and the final exam.

• The final exam will be a take-home exam over several days.

• All assignments should be completed and submitted by Wednesday of the final week.

• The nervous system and special senses will be covered after the exams.

Introduction to Skeletal Muscle

• Skeletal muscles facilitate body movement.

• They enable facial expressions, speech, breathing, and swallowing.

• The heart, a muscle, pumps blood throughout the body.

• Muscles maintain posture and stabilize joints.

• They protect and support internal organs.

• Smooth muscle in the alimentary tract aids in moving food and waste.

• Muscle contractions, such as shivering, generate heat to maintain body temperature.

Characteristics of Skeletal Muscles

• Excitability: Muscles respond to stimuli, specifically electrical impulses.

• Conductivity: Seen particularly in the heart, an electrical impulse spreads from cell to cell.

• Contractility: Muscle filaments slide, shortening the muscle.

• Extensibility: Muscles can be stretched.

• Elasticity: Muscles return to their original length after stretching or shortening.

• Muscles can only contract; relaxation involves ceasing contraction.

• Another muscle is required to extend a muscle back to its original length.

• Antagonistic muscles work in opposing directions (e.g., biceps flexes, triceps extends).

• Muscles must be extended to contract again, with a finite range of contraction.

Skeletal Muscle as an Organ

• A skeletal muscle is an organ composed of multiple tissue types.

• This includes muscle fibers, connective tissue, blood vessels, and nerves.

• Muscle fibers are bundled within fascicles, and multiple fascicles form the entire muscle.

• A muscle fiber is a muscle cell.

• Connective tissue layers include:

  • Epimysium: Dense irregular connective tissue around the entire muscle.

  • Perimysium: Dense irregular connective tissue around each fascicle, containing blood vessels and nerves.

  • Endomysium: Areolar connective tissue wrapping each muscle fiber, providing electrical insulation.

Connective Tissue Components

• Muscles attach to bones or skin via tendons.

• Tendons are cord-like structures of dense regular connective tissue, connecting muscle to bone.

• Ligaments connect bone to bone.

• Aponeuroses are thin, flattened sheets of dense irregular tissue.

  • Examples include the plantar aponeurosis on the sole of the foot and the cranial aponeurosis connecting the frontal and occipital muscles.

• Deep fascia is an irregular connective tissue layer outside the epimysium, separating muscles and grouping those with similar functions.

• Superficial fascia, consisting of areolar and adipose tissue, lies above the deep fascia and separates muscles from the skin.

Blood Supply and Innervation

• Skeletal muscles are highly vascularized to supply oxygen and nutrients and remove waste.

• (O_2) and nutrition needed for muscle contraction and waste removal.

• They are innervated by somatic motor neurons.

• * Soma means body.

• Neurons have axons that connect to muscles or glands.

• Neurotransmitters released at the axon stimulate muscle contraction.

• Skeletal muscle is generally voluntary, requiring conscious thought to contract, with exceptions like deep tendon reflexes.

Muscle Cell Components

• Sarcoplasm: Cytoplasm of a muscle cell.

• Muscle cells contain typical organelles like Golgi apparatus, mitochondria, lysosomes, and endoplasmic reticulum.

• Many muscle cells have multiple nuclei.

• Myoblasts (muscle-building cells) fuse to form muscle cells.

• Satellite cells are undifferentiated myoblasts that aid in muscle cell production.

• Sarcolemma: Plasma membrane of a muscle cell with channels for electrical signal conduction.

• Calcium ions are essential for stimulating muscle contraction through voltage-gated channels.

• T-tubules are invaginations that allow calcium to get down to each myofibril.

Myofibril Structure

• Muscle fibers contain myofibrils, which are composed of myofilaments.

• Myofilaments are the contractile units of muscle made of sarcomeres.

• The sarcoplasmic reticulum wraps around the myofilaments, and T-tubules facilitate calcium delivery.

• Muscle tissue appears striated due to the arrangement of sarcomeres.

• Sarcomeres are delineated by Z discs and include I bands (thin filaments only) and A bands (thick and thin filaments).

• T-tubules have Calcium, Sodium, and Potassium channels to balance electricity inside and outside of cell.

Myofilaments

• Myofilaments consist of interdigitating thick and thin filaments.

• Thick filaments are composed of myosin protein molecules.

• Thin filaments are composed of F-actin strands, with G-actin subunits containing myosin binding sites.

• Tropomyosin and troponin regulate the binding of myosin to actin.

Sarcomere Structure

• $Z$ discs delineate sarcomeres.

• $I$ bands contain only thin filaments (actin).

• $A$ bands contain both thick (myosin) and thin filaments.

• $H$ zone contains only thick filaments (myosin).

Muscle Contraction Mechanism

• Myosin heads on thick filaments bind to actin filaments.

• $ATP$ binding sites on myosin heads provide energy for flexing.

• Myosin heads extend and attach to myosin binding sites on actin, pulling the actin filaments closer to the center.

• The filaments slide, shortening the sarcomere.

• Each muscle fiber contains numerous sarcomeres, so small movements add up.

• During contraction, the muscle shortens and thickens.

• The muscle relies on gravity or antagonistic muscles for extension.

• Muscles can be stretched to the point where actin and myosin overlap.

• Over-stretching a muscle reduces its contractile strength.

Muscle Disorders

• Muscular dystrophy results from defective or insufficient dystrophin, which anchors myofibrils to the sarcolemma, leading to muscle weakness.

• Not being able to anchor myofibrils there causes a lack of control to the muscle

• Without exercise, muscles weaken and atrophy.

• Muscular dystrophy is currently incurable.

• Individuals with serve MS often live to 30 years of age.

Energy Metabolism

• Muscles require Glucose and $O_2$ for energy (ATP production). Oxygen allows the cell to function properly through the Krebs cycle.

• Muscles store glycogen for quick fuel.

• Initially, muscles use glycolysis to break down glycogen.

• Creatine phosphate helps replenish $ATP$ supplies.

• For extended activity, muscles switch to aerobic respiration.

Motor Units

• A motor unit consists of a single motor neuron and all the muscle fibers it controls.

• Spinal nerves contain both afferent (sensory) and efferent (motor) fibers.

• One motor neuron can connect to few or many muscle fibers, depending on the location and function of the muscle.

• Fine motor control (e.g., fingers) involves one neuron connecting to few muscle fibers.

• Large motor units are used in thigh muscles, where precise control is not needed.

• Small motor units:<5 muscle fibers

• Large units: 1 neuron connects to 1000s of fibers

Neuromuscular Junction

• The neuromuscular junction is the area where the nerve innervates a muscle.

• The nerve does not touch the muscle fiber; the gap is called the synaptic cleft.

• Neurotransmitters, such as acetylcholine, transmit the impulse from the neuron to the muscle fiber.

• Most skeletal muscles contain the neurotransmitter acetylcholine and respond to it.

• Electrical signals travel down the nerve to the terminal endplate, stimulating the release of acetylcholine into the synaptic cleft.

• Acetylcholine diffuses across the synaptic cleft and binds to the receptors on the nerve fiber.

• Binding to the receptor sites changes the muscle fiber permeability

• The change in permeability starts the electrical signal

• This triggers a voltage-gated calcium channel.

Neurochemical Transmission

• Cardiac muscle transmits because muscle cells bud up to each other allowing electrical transmission

• In skeletal muscles the neurotransmitters attach to the receptors and send messages down through endomycium to each myofibril.

• Calcium ions then flow into the muscle fibrils, causing the interaction of myosin and actin and leading to sarcomere shortening.

• In heart muscles, cells are arranged so the electrical signal travels through the membrane.

Sarcomere Contraction

• Calcium causes myosin heads to extend and cock.

• Myosin head rises and attaches to the actins receptors

• The myosin heads attach to receptors on actin, flex back, and pull the actin closer to the center, shortening the sarcomere.

• Muscles contract and is kind of like ratcheting together.

• This ratcheting then has a bridge go across from the myosin to the actin to shorten the muscle.

• This process repeats until the myosin heads meet the Z discs, reaching maximum contraction.

• The process requires that you have enough Calcium and ATP.

• The more muscle fibers we have contracting causes greater stregnth.

• Medications and toxic substances can block neuromuscular junctions and stop muscle contractions.

Muscle Relaxation

• Muscle relaxation either through acetylcholinesterase or by acetylcholine.

• Muscles need time to replenish their body.

• When muscles rest stores of glycogen are released from the liver to muscle.

Muscle Disorders (Continued)

• Myasthenia gravis: Antibodies bind to acetylcholine receptors at the neuromuscular junction, which prevents acetylcholine from binding to muscle fiber which stimulates reaction to cause contraction.

• This leads to rapid muscle fatigue and weakness, often affecting facial and eye muscles first. It tends to work its way down the body.

• Myasthenia is an autoimmune condition

Excitation-Contraction Coupling

• Calcium flows into the muscle fibrils.

• That leads to contraction of Myosin and Actin that causes the sarcomere tot shorten.

• Tetanus blocks the release of this neurotransmitter, and that means the acetylcholine is going to keep stimulating those muscles, and those muscles keep contracting. Can be life threatening.

• Botulism, similar to tetanus, causes muscle paralysis and is sometimes used cosmetically to reduce wrinkles.

• They inject botulinum toxin under their skin to get rid of wrinkles.

• What's causing the wrinkles is contraction of little muscles under the skin.

• They may also take butt out of their fat and shooting it in there, you know, to fill up the space, you know, to get rid of wrinkles.

Cross-Bridge Cycling

• Extension: Myosin heads extend and attach to actin.

• Attachment: Flex contraction pulls in.

• Actins release head, others are still connected (contraction)

• Reset and Restart

Supplying Energy

• The muscles need a high supply of ATP.

• If muscles cannot get Glycolysis there will be lactic acid but mostly irritation and pain in the fiber.

• The body is made up again and again/constantly supplied

• ATP = energy source

Anaerobic Respiration and Oxygen Debt

• Lactate formation is low (O_2) so then anaerobic muscles will work.

• Lactate turns into lactic acid so body has to go back again and again for recovery (which makes the debt take longer).

• Aerobic Cellular Respiration: When there's not enough, pyruvate gets in to lactate because there's not enough oxygen available.

• Oxygen Debt is after exercising to fill back the oxygen and repay the debt.

• Used for glucose mainly from liver also from the muscles.

Skeletal Muscle Fiber Types

• Power, speed, and microscope can show what type of muscle is firing/twitching fast.

• Type One - slow oxidative: Red in color w/myoglobin, a thin endurance; High endurance, slow speed, small diameter

• Type 2A : fast oxidative aerobic or respiration: Aerobic Contraction: fast and powerful; a lighter red color

• 2X: fast glycolytic fibers; White in color

  • Speed event: sprinters. Larger in size for diameter.

• Athletes have biopsy taken to see what type.

• There's a bias of runners; one is good for close, fast, and short periods and another for distance due to ATP.

• Mix between all these muscles but varying proportions.

• Genetics is a big factor, you cannot overcome.

• Also training is a high important factor too.

Muscle Tension

• Muscle tension is used when muscles stimulate for contraction.

• Take frog leg and use power on it.

• Latent= electricity spread out

• Contracts

• Relaxes (pull of the force)"latent period- contraction period-relaxation period"

• More unit activated = more to pick up

Contractions

• Depend on voltage=stimulus intensity; all or none

• The stronger the stimulus; the more contraction the higher it goes- will go up to maximum.

Wave Summation

• 20 impulses pre second, time of relaxation, up a little each higher until it reaches the top.

• Produce Tetany or incomplete.

  • 40 or 50 beats= complete contraction.Continuous.Cannot relax

• Not enough energy= fatigue.

• Muscle tension will decrease- will wear out

• Muscle Tone = tension in muscles normally.

• Not enough tension can't mover has to think or it starts a status quo

• While you sleep there still must tension going-not too much but some. This drops in deep sleep.