Muscle Tissue, Physiology, and Related Pathologies

Comprehensive practice flashcards detailing muscle histology, sarcomere structure, sliding filament mechanism, membrane potential electrophysiology, excitation-contraction coupling, metabolism, motor control, smooth muscle mechanisms, and muscle pathologies.

Epimysium

A connective tissue layer that surrounds the entire skeletal muscle.

Perimysium

The connective tissue layer that surrounds each individual fascicle within a muscle.

Endomysium

A layer of loose areolar connective tissue that surrounds an individual muscle cell (muscle fiber).

Transverse (T-) Tubules

Inward foldings or invaginations of the sarcolemma that wrap around myofibrils at regular intervals with ring-shaped structures.

Triad

The three-structure interaction consisting of a central T-tubule flanked on either side by terminal cisternae (lateral sacs) of the sarcoplasmic reticulum.

Tropomyosin

A regulatory protein that winds around the actin strands and covers the active/binding sites on actin when a muscle is not contracting.

Troponin

A regulatory protein complex holding tropomyosin in place that contains three binding sites: one for actin, one for tropomyosin, and one for $Ca^{2+}$.

Titin

An abundant elastic protein shaped like a spring or slinky that anchors and holds thick filaments (myosin) in place within the sarcomere.

Sarcomere

The functional and structural repeating unit of muscle contraction spanning from one Z-line to the next Z-line within a myofibril.

A-Band

The dark region of the sarcomere containing the entire length of the thick filaments (myosin), including both the zone of overlap and the H-zone.

H-Zone

The central, brighter region of the A-band (from the German word 'heller') containing only thick filaments (myosin) without overlapping thin filaments.

I-Band

The light region of the sarcomere containing only thin filaments (actin) and bisected in the middle by a Z-disc.

Sliding Filament Mechanism

Hugh Huxley's model demonstrating that during contraction, myofilaments slide past one another without changing length, causing the I-band to shorten, the H-zone to narrow, and Z-lines to move closer together while the A-band remains constant in width.

Resting Membrane Potential of a Myocyte

The electrical potential across the sarcolemma of an unstimulated muscle cell, typically measured at $-90\,\text{mV}$ due to an unequal distribution of ions maintained largely by leak $K^+$ channels and the $Na^+/K^+$ pump.

Sodium-Potassium-ATPase Pump

An active transport pump that consumes ATP to export $3\,Na^+$ ions out of the cell and import $2\,K^+$ ions into the cell, helping sustain resting membrane potential.

Equilibrium Potential

The electrical membrane potential at which the chemical concentration gradient and the opposing electrostatic gradient for a specific ion balance, resulting in equal and opposite ionic flux ($-89\,\text{mV}$ for $K^+$ and $+60\,\text{mV}$ for $Na^+$).

Inactivation Gate

An intracellular amino acid residue forming a 'ball-in-chain' structure on voltage-gated $Na^+$ channels that swings into the pore shortly after opening to block ion conduction.

Threshold Potential

The critical membrane voltage (approximately $-75\,\text{mV}$ in myocytes, or $15\,\text{mV}$ depolarized from resting potential) at which voltage-gated $Na^+$ channels open to trigger an action potential.

Refractory Period

A span of time during and immediately after an action potential in which a second action potential cannot be initiated because voltage-gated $Na^+$ channels are inactivated.

Motor End Plate

The specialized, folded region of the muscle fiber sarcolemma situated directly beneath the axon terminal of a motor neuron, rich in functional folds and acetylcholine receptors.

End-Plate Potential

A local, graded depolarization of the motor end plate caused by $Na^+$ influx through ligand-gated acetylcholine receptors.

DHP Receptors

Voltage-gated proteins embedded within the T-tubule membrane that undergo conformational changes in response to an action potential, mechanically tugging open adjacent ryanodine receptors.

Ryanodine Receptor

A $Ca^{2+}$ release channel embedded in the sarcoplasmic reticulum membrane that is physically linked to the DHP receptor and releases stored $Ca^{2+}$ into the cytosol upon activation.

Power Stroke

The event during cross-bridge cycling where an inorganic phosphate ($P_i$) detaches from the energized myosin head, causing it to pivot and pull the actin filament toward the center of the sarcomere before releasing ADP.

Rigor Mortis

The sustained post-mortem contraction and rigidity of skeletal muscles caused by cytosolic $Ca^{2+}$ elevation combined with the complete absence of ATP, which prevents myosin heads from detaching from actin.

Creatine Kinase

The enzyme that transfers a phosphate group from creatine phosphate to ADP to rapidly regenerate ATP during the first few seconds of muscle contraction.

Carb Loading

A dietary regimen performed $24\text{--}72\,\text{hours}$ before endurance events (with $36\text{--}48\,\text{hours}$ being optimal) designed to maximize glycogen stores for moderate-to-high intensity exercise exceeding $90\,\text{minutes}$.

Myogram

A graphical recording of muscle tension generation plotted over time.

Latent Period

A brief delay of approximately $2\,\text{ms}$ between the arrival of an action potential and the onset of tension generation in a twitch contraction, during which excitation-contraction coupling occurs.

Unfused Tetanus

A sustained muscle contraction produced by rapid, repetitive stimuli where the muscle fiber exhibits partial relaxation between stimuli, characteristic of muscles maintaining posture.

Fused Tetanus

A continuous, peak-tension muscle contraction produced at high stimulation frequencies with no intermediate relaxation between stimuli.

Type 1 Muscle Fiber

A slow oxidative skeletal muscle fiber characterized by red color, high myoglobin, extensive blood supply, numerous mitochondria, low myosin ATPase activity, low fatigability, and functions in standing and sitting.

Type 2a Muscle Fiber

A fast oxidative glycolytic muscle fiber characterized by light red color, intermediate diameter, intermediate mitochondria and myoglobin, intermediate fatigue resistance, and functions in walking and writing.

Type 2x Muscle Fiber

A fast glycolytic muscle fiber characterized by large diameter, light pink to white color, limited blood supply, few mitochondria, high glycogen, highest myosin ATPase activity, high fatigability, and functions in heavy lifting and sprinting.

Motor Unit

A single motor neuron along with all the individual skeletal muscle fibers that it innervates.

Dense Bodies

Structures anchored to the sarcolemma and throughout the sarcoplasm of smooth muscle cells that serve as attachment points for actin and intermediate filaments in place of Z-discs.

Calmodulin

The cytosolic calcium-binding protein in smooth muscle cells that binds $Ca^{2+}$ to form a complex which activates inactive myosin light chain kinase.

Myosin Light Chain Kinase (MLCK)

An enzyme activated by the $Ca^{2+}$-calmodulin complex that phosphorylates the light chains on smooth muscle myosin heads, enabling cross-bridge formation and contraction.

Myosin Light Chain Phosphatase

The enzyme in smooth muscle that dephosphorylates myosin heads, causing detachment from actin and muscle relaxation.

Duchenne Muscular Dystrophy

An X-linked genetic disorder caused by a mutation in dystrophin, leading to muscle cell membrane breakdown (lysis), cell death, and replacement of muscle tissue with fat (pseudohypertrophy).

Botulinum Toxin

A neurotoxin produced by Clostridium botulinum that causes flaccid paralysis by cleaving SNARE proteins in axon terminals, thereby preventing exocytosis and release of acetylcholine.

Tetanus Toxin

A neurotoxin produced by dormant endospore-forming Clostridium tetani that blocks inhibitory neurotransmitter release onto motor neurons, causing uncontrolled motor neuron firing and spastic/tetanic paralysis.

Acetylcholinesterase

An enzyme present within the sarcolemma and synaptic cleft that rapidly hydrolyzes acetylcholine into acetate and choline to terminate synaptic transmission.

Sarin Nerve Gas

A potent acetylcholinesterase inhibitor that prevents the breakdown of acetylcholine in the synaptic cleft, causing continuous receptor activation and tetanic paralysis.

Black Widow Spider Venom

A venom that forms pore-like channels in motor axon terminals, driving massive $Ca^{2+}$ influx and exhaustive acetylcholine release, producing tetanic paralysis followed by flaccid paralysis once neurotransmitter stores deplete.

Myasthenia Gravis

An autoimmune disorder characterized by antibodies directed against acetylcholine receptors at the neuromuscular junction (often linked to an overactive thymus), typically beginning with asymmetric left facial weakness before progressing bilaterally.

Amyotrophic Lateral Sclerosis (ALS)

Also known as Lou Gehrig's disease, a progressive neurodegenerative disease characterized by the death of motor neurons and lateral spinal cord sclerosis, causing severe muscle atrophy while sparing extraocular eye muscles.

Rhabdomyolysis

A condition involving the rapid breakdown of skeletal muscle fibers from extreme overexertion, releasing myoglobin into the bloodstream and producing characteristic Coca-Cola-colored urine (myoglobinuria) that can cause acute kidney injury.

Dermatomyositis

An autoimmune inflammatory condition affecting both the skin and skeletal muscle simultaneously, marked by autoantibodies, a chest rash, an upper back 'shawl sign', and a heliotrope rash around the eyes.