Muscular System Bio 211

Muscular System Responsibilities?

Movement of and within the body.

Skeletal muscle?

Moves your bones and is voluntary.

Smooth muscle?

Inside organs and blood vessels; makes up about 2% of total body weight.

Cardiac muscle?

Only in your heart.

Body movement function?

Coordinated and localized movements due to contraction of muscles attached to bones.

Maintenance of posture?

Stabilizes joints and helps maintain the body’s posture.

Protection and support?

Muscles arranged along the walls of the abdominal and pelvic cavities protect internal organs and support their normal position.

Storage and movement of materials?

Sphincters (circular muscle bands) contract and relax to regulate passage of material and allow voluntary expulsion of feces and urine.

Heat production?

Continuously generated by energy required for muscle contraction to maintain body temperature; shiver when cold.

Electrical excitability?

Property where muscles respond to electrical stimulation (action potentials) resulting in contraction.

Conductivity?

Property involving a change in electrical charge across the plasma membrane.

Contractility?

Ability of the muscle to physically shorten.

Extensibility?

Ability of a muscle to stretch without breaking.

Elasticity?

Ability to return to original length and shape after contraction or extension.

Skeletal Muscle Tissue classification?

Voluntary muscle controlled by the somatic nervous system.

Striated appearance reason?

Light (actin) and dark (myosin) protein bands seen under microscopy.

Epimysium?

Layer of connective tissue around the entire muscle that is continuous with the fascia.

Perimysium?

Layer of connective tissue around muscle fascicles that contains blood vessels and nerves.

Endomysium?

Layer of connective tissue around each individual muscle fiber that electrically insulates each fiber.

Tendon?

Dense connective tissue with poor vasculature that connects a muscle to a bone.

Aponeurosis?

A broad and irregular tendon, such as the external oblique aponeurosis which helps form the rectus sheath.

Fascia?

A sheet of connective tissue external to the epimysium that does not stretch; can be deep or superficial (mostly adipose).

Shin Splints?

A clinical correlation associated with injury or inflammation of the fascia and muscle tissue in the lower leg.

How long muscle fibers form?

Myoblasts fuse to form the fiber during development.

Multinucleated state cause?

The direct result of multiple myoblasts fusing together to form a single long skeletal muscle fiber.

Satellite cells?

Myoblasts that do not fuse and remain on "standby" to help with muscle healing if needed.

Muscle fiber count at birth?

We are born with all the muscle fibers we will ever have.

Atrophy?

Muscle fibers becoming smaller due to disuse, such as when an arm is kept in a cast.

Hypertrophy?

Muscle fibers becoming larger through training.

Myofibrils?

Bundles of myofilaments that make up a muscle fiber; hundreds of thousands exist in a single fiber.

Myofilaments?

The protein filaments within myofibrils, consisting of thick filaments (myosin) and thin filaments (actin).

Sarcolemma?

The specialized plasma membrane of a muscle cell or fiber.

T-tubules?

Invaginations of the sarcolemma that extend deep into the cell to reach the sarcoplasmic reticulum.

Sarcoplasmic reticulum (SR)?

A mesh-like structure surrounding each myofibril that serves as the site where calcium lives.

Terminal cisternae?

Sac-like regions of the sarcoplasmic reticulum that hold calcium.

Calcium pumps?

Active transport mechanisms that move calcium into the SR where it binds to calmodulin and calsequestrin for storage.

Calcium channels?

Channels that open to release calcium from the SR into the sarcoplasm to trigger muscle contraction.

Sarcoplasm?

The cytoplasm of a muscle fiber containing a large amount of glycogen used for ATP synthesis.

Myoglobin?

A brown protein found only in muscles that binds oxygen molecules and releases O2 when necessary for ATP production.

Rhabdomyolysis?

Clinical correlation where muscle membrane damage causes myoglobin to leak into the blood and appear in the urine, turning it dark tea-colored.

Sarcomere?

The basic functional unit of a myofibril, consisting of overlapping lines of actin and myosin filaments.

Z-discs (Z-line)?

Structural bands that separate one sarcomere from the next.

I band?

Region containing only thin filaments (actin) that completely disappears during contraction.

H zone?

The midline region containing only thick filaments (myosin) that shrinks with contraction.

A band?

The region spanning the entire length of the thick filament, which does not change length during contraction.

M line?

The middle of the sarcomere that does not move and supports the actin and myosin filaments.

Contractile proteins?

Actin and myosin, which actively generate force.

Regulatory proteins?

Troponin and tropomyosin, which act as on/off proteins to regulate the contraction process.

Structural proteins?

Proteins like connectin and dystrophin that align filaments, provide elasticity, and link myofibrils to the sarcolemma.

Connectin?

A structural protein that stabilizes the thick filament.

Dystrophin?

A structural protein that links muscle proteins to other proteins; its absence causes Muscular Dystrophy.

Muscular Dystrophy?

Hereditary disease where the dystrophin protein is absent, causing calcium to leak through channels, activate destructive enzymes, and cause cell wasting.

Myosin molecule structure?

Twisted like two heavy chains forming a tail, with two light chains forming heads resembling twisted golf clubs.

Myosin head orientation?

Tails point toward the M line (middle) while heads point away from the M line.

Myosin head binding sites?

Contains a binding site for actin and a binding site for ATP (site of ATP to ADP conversion).

Actin filament composition?

Thin filaments composed of twisted G-actin monomers that contain myosin binding sites.

Tropomyosin?

A filamentous regulatory protein located in the groove of actin filaments that covers the myosin binding sites.

Troponin?

A regulatory protein sitting on top of actin that binds to tropomyosin and uncovers binding sites during contraction when calcium attaches.

Troponin-tropomyosin complex?

The combined regulatory unit that controls whether myosin heads can attach to actin binding sites.

Motor Unit?

A single motor neuron plus all the individual muscle fibers it innervates.

Fine control motor unit?

A small motor unit controlling fewer muscle fibers for fine, precise movements.

Gross control motor unit?

A large motor unit controlling many muscle fibers for gross, powerful movements.

Neuromuscular junction?

The region where a motor neuron axon interacts with a motor unit; functions like an electrical plug usually in the center of a fiber.

Acetylcholine (ACh)?

The primary neurotransmitter released at the neuromuscular junction in the presence of calcium.

Synaptic knob?

The expanded end of the motor neuron axon that contains synaptic vesicles and acts as the region of neurotransmitter release.

Motor end plate?

The specialized, folded region of the sarcolemma that acts as the "socket" for receiving the synaptic knob.

Resting Membrane Potential (RMP)?

The constant difference in electrical charge across the membrane due to pumps and channels; roughly –90 mV in muscle tissue.

Action Potential initiation?

An event that changes the electrical charge differences across the membrane, moving the end plate potential toward threshold.

Excitation step 1 (Synaptic knob)?

Action potential reaches the synaptic knob, triggering voltage-gated Ca2+ channels to open.

Excitation step 2 (Calcium influx)?

Calcium diffuses down its concentration gradient into the neuron and binds to proteins on synaptic vesicles.

Excitation step 3 (ACh release)?

Synaptic vesicles undergo exocytosis, releasing ACh into the synaptic cleft where it diffuses across.

Excitation-Contraction Coupling step 1?

ACh binds to chemically gated ion channels (ACh receptors) in the motor end plate.

Excitation-Contraction Coupling step 2?

Chemically gated channels open; Na+ rapidly diffuses in and K+ slowly diffuses out.

End Plate Potential (EPP)?

Local depolarization where the inside of the cell becomes more positive, shifting the potential from –90 mV to –65 mV.

Action Potential propagation?

The EPP triggers sequential opening of voltage-gated Na+ channels (depolarization) and K+ channels (repolarization) down the sarcolemma and T-tubules.

Depolarization?

Opening of voltage-gated Na+ channels causing Na+ to move in, changing membrane potential from –65 mV to +30 mV (inside becomes positive).

Repolarization?

Closure of Na+ channels and opening of voltage-gated K+ channels causing K+ to move out, returning potential from +30 mV to –90 mV (inside becomes negative).

Refractory period?

The brief time interval immediately following an action potential during which a muscle fiber cannot respond to another stimulus.

Botox (Botulinum toxin)?

Lethal toxin that blocks exocytosis of ACh vesicles, preventing contraction; can cause fatal asphyxiation or be used clinically for cosmetic wrinkles and spasms.

Curare?

Poison used on hunting darts that causes paralysis by binding to ACh receptors and blocking Na+ channels from opening.

Sarcoplasmic Reticulum calcium release?

Action potential traveling down the T-tubule triggers voltage-gated Ca2+ channels in the terminal cisternae to open, releasing calcium into the sarcoplasm.

Calcium binding event?

Released calcium binds to troponin, causing a shape change that moves tropomyosin off the myosin-binding sites on actin.

Contraction Cycle Step 1?

Cross bridge formation: Myosin heads attach to exposed myosin-binding sites on actin; uses no energy.

Contraction Cycle Step 2?

Power stroke: Myosin head swivels toward the M line, sliding the attached thin filament past the thick filament; releases ADP and phosphate without using energy.

Contraction Cycle Step 3?

Release of myosin head: ATP binds to the ATP binding site on the myosin head, causing cross-bridge detachment from actin; uses energy.

Contraction Cycle Step 4?

Reset myosin head: ATP is hydrolyzed into ADP and Pi by myosin ATPase, providing the energy to cock the myosin head back.

Contraction cycle continuation?

Steps repeat multiple times if calcium is still present; if nerve signals stop or ACh is removed, calcium returns to the SR and the muscle relaxes.

Three uses for ATP in contraction?

To detach the myosin head, to reset the myosin head via ATPase, and to pump calcium back into the SR at the end of contraction.

Rigor Mortis?

Stiffening of muscles after death caused by running out of ATP (due to no oxygen), leaving myosin heads permanently attached to actin filaments.

Immediate muscle metabolism source?

A very limited amount of free ATP immediately available in the muscle tissue.

Myokinase pathway?

An immediate energy system where phosphate is transferred from one ADP to another, making ATP and AMP; lasts a few seconds.

Creatine phosphate pathway?

An immediate system where creatine kinase transfers a phosphate from creatine phosphate to ADP, forming ATP; lasts a few seconds.

Creatine Kinase (CK) clinical marker?

Skeletal and cardiac muscle contain specific CK; blood levels of CK rise significantly when these muscle tissues are damaged.

Glycolysis pathway?

Anaerobic metabolism in the cytosol producing 2 ATP per glucose molecule; results in lactic acid if no oxygen is present; provides a few minutes of energy.

Cellular Respiration pathway?

Aerobic metabolism in the mitochondria requiring oxygen from hemoglobin or myoglobin to synthesize the greatest amounts of long-term ATP.

Three main muscle fiber types?

Slow oxidative (SO) fibers, fast oxidative (FO) fibers, and fast glycolytic (FG) fibers.

Slow oxidative (SO) fibers?

Endurance fibers containing slow ATPase that produce slower, less powerful contractions over long durations; slow to fatigue and appear red due to high myoglobin.

Fast oxidative (FO) fibers?

Quick-burst intermediate fibers containing fast ATPase with less vascular supply; produce fast, powerful contractions with intermediate fatigue and appear lighter red.

Fast glycolytic (FG) fibers?

The largest and most prevalent skeletal muscle fibers overall; contain fast ATPase for max power, fatigue quickly, and appear white due to almost no myoglobin.

Muscle Tension?

The physical force generated within a muscle when it is stimulated to contract.