Chapter 11 Muscular Tissue

Comprehensive vocabulary terms and definitions covering muscle tissue types, cellular components, the mechanism of contraction, and whole-muscle physiology based on the lecture notes.

Excitability (responsiveness)

The ability of muscle cells to respond to chemical signals, stretch, and electrical changes across the plasma membrane.

Conductivity

The process where local electrical excitation sets off a wave of excitation that travels along the muscle fiber.

Contractility

The characteristic of muscle cells to shorten when stimulated.

Extensibility

The capability of muscle cells to be stretched between contractions.

Elasticity

The ability of a muscle cell to return to its original rest length after being stretched.

Striations

Alternating light and dark transverse bands resulting from the arrangement of internal contractile proteins.

Endomysium

A thin sleeve of loose connective tissue that surrounds each individual muscle fiber.

Perimysium

A thicker layer of connective tissue that wraps muscle fibers into bundles called fascicles.

Epimysium

A fibrous sheath surrounding the entire muscle that blends with fascia and deeper connective tissues.

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber.

Myofibrils

Long protein cords occupying most of the sarcoplasm.

Myoglobin

A red pigment in muscle cells that provides some of the oxygen needed for muscle activity.

Sarcoplasmic reticulum (SR)

The smooth ER of a muscle fiber that forms a network around each myofibril and acts as a reservoir for $Ca^{2+}$.

Terminal cisterns

Dilated end-sacs of the sarcoplasmic reticulum which cross the muscle fiber and release calcium to activate contraction.

Transverse (T) tubules

Tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side.

Thick filaments

Myofilaments made of several hundred myosin molecules, each shaped like a golf club with a double globular head.

Thin filaments

Myofilaments composed of fibrous (F) actin, tropomyosin, and troponin.

Tropomyosin

A regulatory protein that blocks the active sites on actin when a muscle fiber is relaxed.

Troponin

A small, calcium-binding protein located on each tropomyosin molecule.

Titin

A huge, springy protein that composes elastic filaments, anchoring thick filaments to the Z disc and M line.

Contractile proteins

The proteins myosin and actin, which perform the work of shortening the muscle fiber.

Regulatory proteins

The proteins tropomyosin and troponin, which act like a switch to determine when a fiber can contract.

Sarcomere

The functional contractile unit of a muscle fiber, measured as the segment from one Z disc to the next Z disc.

Somatic motor neurons

Nerve cells with cell bodies in the brainstem and spinal cord that lead to the skeletal muscles.

Motor unit

One nerve fiber and all the muscle fibers innervated by it, behaving together as a functional unit.

Neuromuscular junction (NMJ)

The functional connection or synapse between a nerve fiber and a muscle fiber.

Acetylcholine (ACh)

The neurotransmitter contained within synaptic vesicles that is released into the synaptic cleft to stimulate a muscle fiber.

Acetylcholinesterase (AChE)

An enzyme found in the sarcolemma and basal lamina that breaks down ACh to allow for muscle relaxation.

Resting membrane potential (RMP)

The stable negative charge across the plasma membrane of an unstimulated, polarized cell.

Action potential

The quick up-and-down voltage shift from the negative RMP to a positive value and back to negative again.

End plate potential

The rapid fluctuation in membrane voltage at the motor end plate caused by the binding of ACh to its receptors.

Power stroke

The step in contraction where myosin releases ADP and $P_i$ and flexes, pulling the thin filament along with it.

Length–tension relationship

The principle that the amount of tension generated by a muscle depends on how stretched or shortened it was before stimulation.

Rigor mortis

The hardening of muscles and stiffening of the body that occurs after death due to the lack of ATP required for muscle relaxation.

Muscle twitch

A quick cycle of contraction and relaxation when a muscle is directly stimulated with an electrode.

Latent period

The delay of about $2\text{ ms}$ between the onset of a stimulus and the onset of the twitch.

Recruitment (Multiple motor unit summation)

The process of bringing more motor units into play by stimulating the nerve with higher voltages.

Incomplete tetanus

A state of sustained fluttering contraction caused by high-frequency stimuli where the muscle only partially relaxes between twitches.

Isometric contraction

Contraction in which the muscle produces internal tension but stays the same length because external resistance prevents movement.

Isotonic contraction

Contraction with a change in muscle length but no change in tension.

Concentric contraction

A form of isotonic contraction where the muscle shortens as it maintains tension.

Eccentric contraction

A form of isotonic contraction where the muscle lengthens as it maintains tension.

Phosphagen system

The combination of ATP and creatine phosphate which provides nearly all the energy used for short bursts of intense activity.

Slow-twitch (slow oxidative) fibers

Muscle fibers well adapted for endurance and fatigue resistance, characterized by many mitochondria and high myoglobin levels.

Fast-twitch (fast glycolytic) fibers

Muscle fibers adapted for quick, powerful responses, utilizing glycolysis and anaerobic fermentation for energy.

Cardiomyocytes

Individual cardiac muscle cells which are joined by intercalated discs containing gap junctions.