BIOL2251 Muscle Tissue and Muscular System

Excitability

Ability of a cell to respond to a stimulus, causing a local change in resting membrane potential

Conductivity

The ability to propagate an electrical signal along the plasma membrane

Contractility

Exhibited when contractile proteins within skeletal muscles slide past one another

Extensibility

Ability to be stretched or lengthened

Elasticity

Ability of a muscle cell to return to its original length following shorting or lengthening

Connectin

Contractile spring-like protein that releases tension allowing for elasticity

Skeletal muscle fibers

Another name for skeletal muscle cells or myofibers

Skeletal muscle

Organ composed of skeletal muscle fibers, connective tissue layers, blood vessels, and nerves

Fascicle

A discrete bundle of muscle cells/fibers

Epimysium

Layer of dense irregular connective tissue surrounding entire skeletal muscle

Perimysium

Layer of dense irregular connective tissue surrounding each fascicle

Endomysium

Delicate covering of areolar connective tissue surrounding each muscle fiber

Tendon

Cord of dense regular connective tissue connecting muscle to bone

Aponeurosis

Thin, flattened sheet of dense regular connective tissue attaching a muscle to a skeletal component or to fascia

Visceral (deep) fascia

Additional expansive sheet of dense irregular connective tissue external to the epimysium

Superficial fascia

Areolar connective tissue and adipose connective tissue that separates muscle from skin; also called subcutaneous layer

Vascularized

Skeletal muscles are ____ by an extensive network of blood vessels

Innervated

Skeletal muscle is ____ by motor neurons of the somatic nervous system

Somatic motor neurons

Neurons extending from the brain and spinal cord to skeletal muscle fibers

Axon

Process of a neuron that propagates nerve signals away from the cell body

Neuromuscular junction

A small gap of about 30nm between the motor neuron axon and muscle fiber

Voluntary muscle

Skeletal muscle fibers able to be consciously controlled by the nervous system

Myoblast

Undifferentiated muscle cell with the potential of becoming a muscle fiber

Multinucleated

Skeletal muscle fibers are ____ (i.e. having numerous nuclei)

Muscle satellite cells

Myoblasts remaining in adult skeletal muscle as adult stem cells to assist with repair and regeneration

Sarcolemma

Plasma membrane of a skeletal muscle cell

T-tubules

Deep invaginations of the sarcolemma that extend into the skeletal muscle fiber as a network of narrow, membranous tubules to the SR

Myofibrils

Long, cylindrical bundles of myofilaments within skeletal muscle fibers

Myofilaments

Intracellular contractile protein filaments

Sarcoplasmic reticulum

Internal membrane complex fitted around each myofibril; stores calcium ions required for contraction

Terminal cisternae

Blind sacs at either end of the sarcoplasmic reticulum

Triad

Structure composed of two terminal cisternae and a centrally located T-tubule

Voltage-sensitive Ca2+ channels

Channels within the membrane of T-tubules responsive to electrical signals

Ca2+ release channels

Channels contained in the terminal cisternae membrane of the SR

Ca2+

Ions housed within the lumen of the terminal cisternae of the sarcoplasmic reticulum; its release initiates muscle contraction

Calmodulin and calsequestrin

Specialized Ca2+ binding proteins

Thick filaments

Bundles of myosin protein molecules

Globular head

Strand of myosin containing a binding site for actin and a catalytic ATPase site

Elongated tail

Tails of two strands of a myosin molecule are intertwined and point toward the center of the filament

Thin filaments

Filaments primarily composed of two strands of actin twisted around each other to form a helical shape

Actin

Contractile protein forming the major part of the thin filaments in a sarcomere

G-actin

Globular subunit of F actin with an active site for binding a myosin head

Myosin binding site

The myosin head binds to the ____ of an actin molecule during muscle contraction

Tropomyosin

Short, thin, twisted filament; consecutive molecules cover small regions of actin strands, including the myosin binding sites in non-contracting muscle

Troponin

Globular protein attached to tropomyosin with a binding site for Ca2+

Sarcomere

Smallest contractile and functional unit of skeletal muscle; repeating microscopic, cylindrical units

Z discs

Delineates each end of a sarcomere; composed of specialized proteins positioned perpendicular to myofilaments and serve as anchors for thin filaments

I band

Light region containing only thin filaments and is bisected by the Z-disc

A band

Central, dark-appearing region of a sarcomere containing entire thick filament with partial thin filament overlap

H zone

Most central portion of the A-band in a resting sarcomere with no thin filament overlap

M line

Thin transverse protein meshwork structure in the center of the H zone; serves as attachment site for thick filaments

Striations

Unique striped patterns within a skeletal muscle fiber formed by repeating light and dark bands of overlapping myofilaments

Titin

Protein extending from the Z-discs to the M-line through the core of each thick filament for stabilization and alignment (AKA connectin)

Dystrophin

Part of a protein complex that anchors myofibrils adjacent to the sarcolemma

Muscular dystrophy

Collective term for hereditary diseases in which the skeletal muscles degenerate

Myoglobin

Reddish, globular protein that binds oxygen when the muscle is at rest and releases it during muscle contraction

Creatine phosphate

Molecule providing muscle fibers with very rapid means of supplying ATP

Motor unit

A single motor neutron and the skeletal muscle fibers it controls

Synaptic knob

Expanded tip of a motor neuron's axon that is enlarged and flat to cover a relatively large surface area of sarcolemma

Synaptic vesicles

Small membrane sacs filled with molecules of neurotransmitters; repelled from synaptic knob plasma membrane when motor neuron is at rest

Acetylcholine (ACh)

Neurotransmitter responsible for stimulating skeletal muscle

Motor end plate

Specialized region of sarcolemma of a skeletal muscle fiber at the end of a motor neuron; contains a vast number of ACh receptors

Synaptic cleft

Extremely narrow, fluid-filled space separating the synaptic knob and motor end plate

Acetylcholinesterase (AChE)

Enzyme that breaks down acetylcholine following their release into the synaptic cleft

Resting membrane potential

Electrical difference at plasma membrane when a cell is at rest (-90mV in skeletal muscle fibers with more Na+ outside the cell and more K+ inside the cell)

Muscle fiber excitation

First physiologic event of skeletal muscle contraction; event occurs at neuromuscular junction resulting in release of ACh that will bind to receptors

Nerve impulse

Electrical signal propagated down a motor neuron's axon that triggers opening of voltage gated Ca2+ channels

Release of ACh from synaptic knob

Binding of Ca2+ to synaptic vesicles results in exocytosis of ACh into the synaptic cleft

Binding of ACh at motor end plate

ACh diffuses across synaptic clef and binds with receptors, causing excitation of skeletal muscle fibers

Excitation-contraction coupling

Second physiologic event of muscle contraction that links events of skeletal muscle stimulation at the NMJ to the events caused by sliding myofilaments within sarcomeres

Development of end-plate potential

ACh receptors (chemically gated ion channels) open temporarily by ACh binding, allowing more Na+ to diffuse into the muscle fiber than K+ which diffuses out, creating a positive charge inside the muscle fiber (changes RMP from -90mV to -65mV)

End-plate potential (EPP)

Minimum voltage change (threshold) in motor end plate that can trigger opening of voltage-gated channels to initiate an action potential

Action potential

Electrical change involving depolarization and repolarization

Depolarization

A reversal in polarity at the sarcolemma due to the influx of Na+ down its concentration gradient into the muscle fiber

Repolarization

Return of the inside of the sarcolemma to RMP (-90mV) due to outward flow of K+ down its concentration gradient outside the muscle fiber

Refractory period

Period of time that includes depolarization and repolarization where the muscle cannot be restimulated

Release of Ca2+ from the SR

When the electrical signal reaches a triad a conformational change occurs in Ca2+ release channels allowing Ca2+ to diffuse into the cytosol where it can interact with myofilaments

Ca2+ binding and crossbridge cycling

Third physiological event in muscle contraction

Calcium binding to troponin

Initiates crossbridge cycling by inducing a conformational change that causes the troponin-tropomyosin complex to move and exposes the myosin binding sites of actin

Crossbridge cycling

Four step process of repeated formation, power stroke, release, and resetting

Crossbridge formation

Myosin heads, in cocked position, attach to exposed myosin binding sites of actin, forming a bridge between thin and thick filaments

Power stroke

The swiveling of a myosin head pulls a thin filament past the thick filament toward the center of the sarcomere; ADP and Pi are released and ATP binding sites become available

Release of myosin head

ATP binds to ATP binding site of myosin head, causing release of the head from the binding site on actin

Resetting of myosin head

Myosin ATPase splits ATP into ADP and Pi, providing the energy to reset the myosin head in its cocked position

Sarcomere shortening

The repetitive actions of crossbridge cycling in the presence of Ca2+ results in _____.

Sliding filament theory

A description of the repetitive movement of thin filaments sliding past thick filaments

Termination of nerve signals

First step in skeletal muscle relaxation; no additional release of ACh occurs and any remaining ACh in the synaptic cleft is hydrolyzed by AChE; both EPPs at the motor end plate and electrical signals cease

Tetanus

Form of spastic paralysis that involves overstimulation of skeletal muscles and excessive contractions

Botulism

Muscular paralysis caused by a toxin preventing release of ACh at synaptic knobs

Glycolysis

Metabolic pathway involving breakdown of glucose into two pyruvate molecules, producing 2 net ATP; occurs in cytosol and is nonoxidative

Aerobic cellular respiration

Process of oxidizing nutrients (such as pyruvate) occurring in mitochondria producing more ATP but at a slower rate than glycolysis

Lactate formation

Process occurring under low oxygen availability (demands for aerobic respiration not met) where pyruvate molecules are converted into lactate molecules

Lactate dehydrogenase

An enzyme that catalyzes the reaction converting pyruvate into lactate

Junctional folds

Infoldings of sarcolemma containing millions of ACh receptors

Muscle tension

Force exerted by a contracting muscle on an object

Load

Opposing force exerted on the muscle by the weight of the object to be moved

Isometric contraction

Muscle contracts but stays the same length (shortening does not occur); muscle tension increases but does not exceed load

Isotonic contraction

Muscle shortens because muscle tension exceeds load

Muscle twitch

Simplest contraction resulting from a muscle fiber's response to a single action potential from a motor neuron

Latent period of muscle twitch

Events of excitation-contraction coupling with no muscle tension