Module 10: Muscle Tissue

M line

Region in center of H zone that contains proteins that hold thick filaments together at center of sarcomere.

α-actinin

Structural protein of Z discs that attaches to actin molecules of thin filaments.

Motor unit

A somatic motor neuron plus all of the muscle fibers it stimulates.

Clostridium botulinum

Bacterium that produces a toxin that blocks exocytosis of synaptic vesicles at the NMJ.

Clostridium tetani

Bacterium that produces a toxin that results in powerful muscle contractions.

Fast Oxidative-Glycolytic Fibers

Fast speed fibers that can generate considerable ATP by aerobic respiration, which gives them a moderately high resistance to fatigue. They also generate ATP by anaerobic glycolysis - contribute to activities such as walking and sprinting.

Synaptic cleft

Gap between somatic motor neuron and skeletal muscle cell.

Slow oxidative (SO) fibers

Slow speed, fatigue-resistant muscle fibers that are adapted for maintaining posture and for aerobic, endurance-type activities such as running a marathon - contain a large amount of myoglobin and mitochondria

Motor end plate

The region of the sarcolemma opposite the synaptic end bulbs - contains receptors for acetylcholine.

Neuromuscular junction

The region where communication occurs between a somatic motor neuron and skeletal muscle cell.

Glycogen

polysaccharide made up of glucose monomers used for a source of energy within the sarcoplasm of muscle cells.

Anticholinesterases

A family of chemical agents that have the property of slowing the enzymatic activity of acetylcholinesterase, thus slowing removal of ACh from the synaptic cleft and promoting muscle contraction.

Aerobic respiration

A series of oxygen-requiring reactions (the Krebs cycle and the electron transport chain) that produce ATP, carbon dioxide, water, and heat.

Neostigmine

Anti cholinesterase used to treat myasthenia gravis - disease that results in muscle weakness.

Voltage-gated calcium channels

Channels found at the synaptic end bulb - open to cause exocytosis of acetylcholine from motor neuron.

Myosin

Contractile protein of thick filaments that functions as a motor protein.

Actin

Contractile protein that is the main component of thin filament.

A band

Dark, middle part of sarcomere that extends entire length of thick filaments and includes those parts of thin filaments that overlap thick filaments.

Acetylcholinesterase

Enzyme that breaks down acetylcholine into acetyl and choline, products that cannot activate the ACh receptor.

Fast Glycolytic Fibers

Fast speed fibers that have a low myoglobin content and few mitochondria. They contain large amounts of glycogen and generate ATP mainly by glycolysis - adapted for intense anaerobic movements of short duration, such as weight lifting or throwing a ball, but they fatigue quickly.

Calcium

Ion that binds to troponin during muscle contraction.

Sodium

Ion that enters muscle cell through acetylcholine receptors - triggers muscle action potential.

Titin

Large protein that connects a Z disc to the M line of the sarcomere, thereby helping stabilize the position of the thick filament - accounts for much of the elasticity and extensibility of myofibrils.

I band

Lighter, less dense area of sarcomere that contains thin filaments but no thick filaments - a Z disc passes through center.

Synaptic vesicles

Membrane-enclosed sacs containing acetylcholine in the synaptic end bulb of somatic motor neurons.

Creatine Phosphate

Molecule that transfers a high-energy phosphate group to ADP, forming ATP during muscle contraction - provides energy for 15 seconds.

Large motor units

Muscles responsible for large-scale and powerful movements (muscles of the thigh and leg) consist of many of these types of motor units.

Small motor units

Muscles that control precise movements (muscles in fingers, eye muscles) consist of many of these types of motor units.

H zone

Narrow region in center of each A band that contains thick filaments but no thin filaments - shortens when muscle contracts.

Anaerobic glycolysis

Net production of 2 ATP molecules from the breakdown of glucose into pyruvic acid - provides energy for 2 minutes.

Somatic motor neurons

Neurons that stimulate skeletal muscle fibers to contract.

Acetylcholine

Neurotransmitter that stimulates skeletal muscle contraction.

Curare

Plant derivative used by South American Indians on arrows and blowgun darts, causes muscle paralysis by binding to and blocking ACh receptors.

Troponin

Regulatory protein that is a component of thin filament; changes shapes when calcium ions (Ca2+) bind to it resulting in a conformational change that moves tropomyosin away from myosin-binding sites on actin molecules.

Tropomyosin

Regulatory protein that is a component of thin filament; when skeletal muscle fiber is relaxed, tropomyosin covers myosin-binding sites on actin molecules, thereby preventing myosin from binding to actin.

Isometric contraction

Type of contractions during which muscle tension increases but does not exceed the load -the muscle does not change its length.

Eccentric contraction

Type of contractions during which the muscle lengthens while it is generating work (putting down a tray of food).

Concentric contraction

Type of contractions during which the muscle shortens and does work (picking up a tray of food).

Poliovirus

Virus that attacks somatic motor neurons.

Perimysium

a layer of dense irregular connective tissue surrounding groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles.

Electrical excitability

ability of muscle cells (and nerve cells) to respond to certain stimuli by producing electrical signals called action potentials (impulses).

Sarcomere

contractile unit that extends from Z disc to Z disc.

Sarcoplasmic reticulum

fluid-filled system of membranous sacs, similar to the smooth endoplasmic reticulum, responsible for the storage of calcium ions within the muscle cell.

Transverse (T) tubules

invaginations of the sarcolemma that tunnel in from the surface toward the center of each muscle fiber.

Endomysium

layer of connective tissue surrounding each individual muscle cell.

Myofibrils

long threads, made up of thick and thin myofilaments, that extend the entire length of a muscle cell - often called the contractile organelle of a muscle cell.

Thin myofilaments

myofilaments made up of the protein actin, troponin, and tropomyosin.

Thick myofilaments

myofilaments made up of the protein myosin.

Myoglobin

protein, found only in muscle, binds oxygen molecules that diffuse into muscle fibers from interstitial fluid.

Contractility

the ability of muscular tissue to contract forcefully when stimulated by an action potential.

Elasticity

the ability of muscular tissue to return to its original length and shape after contraction or extension.

Extensibility

the ability of muscular tissue to stretch, within limits, without being damaged.

Epimysium

the outer layer, encircling the entire muscle - consists of dense irregular connective tissue.

Sarcolemma

the plasma membrane of a muscle cell.

Myomesin

Structural protein that forms M line of sarcomere.

Dystrophin

Structural protein that links thin filaments of sarcomere to membrane proteins in sarcolemma, which are attached in turn to proteins in connective tissue matrix.