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


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.