Glossary terms will appear here

Acetylcholine- An acetic acid ester that is important as a synaptic transmitter in most species of animals, and many different types of neurons.

Actin- A protein that participates in muscle contraction and other forms of cellular motility.

Action Potential- An all-or-none membrane depolarization propagated along a nerve fiber without loss of amplitude.

ADP- A nucleotide formed by hydrolysis of ATP; with the release of one high-energy bond.

ATP- An energy-rich nucleotide used as a common energy currency by all cells

Axon Terminal- The end of each axon, which typically is the site where signals are passed to another neuron or other cell.

Calmodulin- A tropin like calcium-binding regulatory protein found in essentially all tissues.

Cross-bridges-Spirally arranged projections from myosin thick filaments that bind to sites on actin thin filaments during muscle contraction.

Cytosol- The unstructured aqueous phase of the cytoplasm between the structured organelles.

Glycolysis- The metabolic pathway by which hexose and triose sugars are broken down to simpler substances, especially pyruvate or lactate. This pathway produces 2 molecules of ATP.

Graded potential- A nerve impulse proportional to the intensity of the stimulus that produces it and declining thereafter. Compare with action potential.

H Zone- The light zone in the center of the resting muscle sarcomere, where myosin filaments od not overlap with actin filaments; the region between actin filaments.

Intercalated disk- The junctional region between two connected cardiac muscle cells.

Kreb’s Cycle-The metabolic cycle responsible for the complete oxidation of the acetyl portion of the acetyl coenzyme A molecule

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Mitochondria- Membrane-enclosed organelle where ATP is produced during aerobic metabolism.

Motor end plate- the infolding part of the exterior of the sarcolemma where the axon terminal transfers an action potential.

Motor unit- The unit of motor activity consisting of a motor neuron and the muscle fibers it innervates; important in producing graded muscle force.

Multi-unit muscle- A smooth muscle in which individual muscle fibers contract only when they receive excitatory input from neurons; contraction of these muscles is neurogenic.

Myoplasm- The cytosol in a muscle cell.

Myosin-The protein that makes up the thick filaments and cross bridges in muscle fibers.

Oxidative Phosphorylation- The formation of high-energy phosphate bonds via phosphorylation of ADP to ATP, accompanied by the transport of electrons to oxygen from the substrate.

Phosphagen System- A system by which phosphocreatine donates a high energy phosphate group to ADP, thereby regenerating ATP.

Rate modulation- The proportionate increase in the contractile force of a muscle as the rate of nerve impulses increases. Compare with motor unit.

Recruitment- The pattern in which neurons with increasingly high thresholds become active as intensity increases. This pattern can occur in sensory neurons (range fractionation) or in motor neurons.

Sarcolemma- The surface membrane of muscle fibers.

Sarcomere- The contractile unit of a myofibril, it is bounded by two Z disks (lines).

Sarcoplasmic reticulum (SR)- A smooth, membrane-limited network surrounding each myofibril. Calcium is stored in the SR and released as free Calcium during muscle excitation-contraction coupling.

Single-unit muscle- A smooth muscle in which individual fibers are coupled through gap junctions, allowing excitation to spread through the muscle independent of neuronal activity; contraction in these muscles in often myogenic, driven by internal pacemaker cells.

Skeletal muscle- The striated muscle whose contraction is responsible for moving the bodies of animals. Contraction of these fibers in neurogenic; that is, contraction occurs only when the fibers are excited by synaptic input from motor neurons.

Smooth muscle- Muscle without sarcomeres and hence without striations. Myofilaments are nonuniformly distributed within small, mononucleated, spindle-shaped cells.

Striated muscle- Characterized by sarcomeres aligned in register. Skeletal and cardiac muscle are striated.

Summation- Convergence of several impulses arriving simultaneously in order to release enough neurotransmitter molecules to trigger an electrical impulse in the next neuron; cummulative effect of successive action potentials which causes stronger contractions.

T tubules- Branching membrane-limited, inter-communicating tubules that are continuous with the surface membrane and are closely apposed to the terminal cisternae of the sarcoplasmic reticulum.

Tetanus- An uninterrupted muscular contraction caused by high frequency motor impulses. Also the name of a neurotoxin that is retrogradely (toward the cell body) transported in axons and that causes prolonged excitation of muscle fibers, causing tetanic contraction.

Terminal cisternae- The closed spaces that make up part of the sarcoplasmic reticulum on both sides of the Z line, making close contact with the T tubules.

Tropomyosin- A long protein molecule located in the grooves of actin filaments of muscle; inhibits muscle contraction by blocking the interaction of myosin cross bridges with actin filaments.

Troponin- A complex of globular calcium-binding proteins associated with actin and tropomyosin in the thin filaments of muscle. When troponin binds calcium, it undergoes a conformational change, allowing tropomyosin to reveal myosin-binding sites on the actin filament.

Z disk (Z line, Z band)- A narrow zone at either end of a muscle sarcomere, consisting of a latticework into which the actin thin filaments are anchored.