Muscular System Lecture Notes

A comprehensive set of vocabulary flashcards covering the functions, structures, physiology, and specific anatomy of the human muscular system as detailed in the lecture transcript.

Skeletal Muscle

Muscle tissue and associated connective tissue that constitutes approximately $40\%$ of body weight; also known as striated or voluntary muscle.

Contractility

The ability of a muscle to shorten with force.

Excitability

The capacity of a muscle to respond to a stimulus.

Extensibility

The ability of muscles to be stretched to their normal resting length.

Elasticity

The ability of a muscle to recoil to its original resting length.

Epimysium

A connective tissue sheath that surrounds a skeletal muscle; also known as muscular fascia.

Muscle fasciculi

Numerous visible bundles that make up a muscle.

Perimysium

Loose connective tissue that surrounds the muscle fasciculi.

Muscle fibers

Several muscle cells that compose a fasciculus.

Endomysium

Loose connective tissue that surrounds a muscle fiber.

Sarcolemma

The cell membrane of the muscle fiber.

Transverse tubules (T tubules)

Tube-like invaginations that occur at regular intervals along the muscle fiber.

Sarcoplasmic reticulum

Highly organized smooth E.R. that stores and releases calcium ($Ca^{2+}$) for muscle contraction.

Sarcoplasm

The cytoplasm of a muscle fiber.

Myofibrils

Threadlike structures composed of actin and myosin myofilaments.

Actin Myofilaments

Thin filaments made of proteins that make up myofibrils.

Myosin Myofilaments

Thick filaments made of proteins that make up myofibrils.

Sarcomere

The basic structural and functional unit of skeletal muscle, consisting of highly ordered repeating units of actin and myosin myofilaments.

Troponin

Molecules attached at specific intervals along actin myofilaments that serve as binding sites for $Ca^{2+}$.

Tropomyosin

Filaments located along the groove between twisted strands of actin that cover attachment sites.

Myosin heads

Structures that resemble golf club heads; they bind to actin attachment sites, bend, straighten, and break down ATP.

Z disk

A network of protein fibers forming an attachment site for actin myofilaments.

I band

Consists of actin myofilaments and spans each Z disk.

A band

The darker, central region of a sarcomere extending the length of the myosin myofilaments.

H zone

The second light zone consisting of myosin myofilaments.

M line

Dark-staining bands within a sarcomere.

Neuromuscular Junction (NMJ)

The site where a motor neuron’s terminal meets the muscle fiber; the site where functional activation for contraction occurs.

Aerobic respiration

The breakdown of glucose in the presence of oxygen ($O_2$) to produce carbon dioxide, water, and ATP, providing approximately $95\%$ of ATP for resting or active muscles.

Creatine phosphate

A molecule that can be converted to ATP to provide energy for about $15$ seconds as contraction starts.

Prime Movers (Agonists)

Muscles that are primarily concerned with a specific movement.

Antagonists

Muscles working in reverse of a particular movement, preventing the prime mover from overextending.

Synergists

Muscles that help prime movers by providing extra force and stabilizing joints against dislocation.

Fixators

Synergists that immobilize a muscle’s origin bone so the prime mover can be more effective.

Motor Units

A group of muscle fibers that all receive signals from the same, single motor neuron.

Slow Oxidative (SO) fibers

Muscle fibers that contract relatively slowly and use aerobic respiration to produce ATP.

Fast Oxidative (FO) fibers

Muscle fibers with fast contractions that primarily use aerobic respiration but can switch to anaerobic respiration, causing them to fatigue faster than SO fibers.

Fast Glycolytic (FG) fibers

Muscle fibers with fast contractions that primarily use anaerobic glycolysis and fatigue most quickly.

Isometric contractions

Contractions where the muscle length does not change, but the amount of tension increases, such as in postural muscles.

Isotonic contractions

Contractions where the amount of tension produced is constant, but the length of the muscle changes.

Concentric contraction

An isotonic contraction in which muscle tension increases as the muscle shortens.

Eccentric contraction

An isotonic contraction in which tension is maintained while the muscle lengthens due to opposing resistance.

Muscle tone

The constant tension produced by body muscles over long periods, responsible for posture and preventing the abdomen from bulging.

Dense body

A structure in smooth muscle fibers anchored to the sarcolemma that is analogous to the Z-discs of skeletal muscle.

Calveoli

Membrane indentations in smooth muscle fibers involved in the sequestration of calcium ions from extracellular fluid.

Pacemaker cells

Specialized cardiac muscle cells that directly control heart rate and respond to the autonomic nervous system ($ANS$) and hormones.

Circular fascicle arrangement

Fascicles arranged in a circle around an opening; act as sphincters (e.g., Orbicularis oris).

Convergent fascicle arrangement

Broadly distributed fascicles that converge at a single tendon (e.g., Pectoralis major).

Pennate fascicle arrangement

Fascicles originating from a tendon that runs the length of the entire muscle; includes unipennate, bipennate, and multipennate patterns.

Fusiform fascicle arrangement

Fascicles parallel to the long axis where the belly of the muscle is larger in diameter than the ends (e.g., Biceps brachii).

Buccinator

Muscle that retracts the angle of the mouth and flattens the cheek.

Orbicularis oculi

Muscle that closes the eye.

Orbicularis oris

Muscle that closes the lip.

Masseter

Muscle that originates on the zygomatic arch and inserts on the lateral side of the mandibular ramus to elevate and protract the mandible.

Sternocleidomastoid

Neck muscle that individually rotates the head or together flexes the neck.

Diaphragm

Muscle of the thorax responsible for inspiration by depressing the floor of the thorax.

Rectus abdominis

Abdominal muscle that flexes the vertebral column and compresses the abdomen.

Levator ani

Muscle of the pelvic floor that elevates the anus and supports pelvic viscera.

Gastrocnemius

Lower limb muscle that plantar flexes the foot and flexes the leg; inserts through the calcaneal (Achilles) tendon.