Chapter 9

wave summation

this occurs when a second stimulus is received before the muscle fiber has relaxed, creating a second contraction that is stronger than the first

tendon

A band, strap or cord of tough, inelastic dense regular fibrous connective tissue which connects a muscle with its bony attachment or to whatever other structure the muscle moves, e.g., a portion of skin, the eyeball, etc.

fascicle

A bundle of anatomical fibers, as of a muscle or a nerve; each fascicle is bounded by a thin wrapping of irregular fibrous connective tissue.

calmodulin

A calcium-binding second messenger regulatory protein found in all nucleated cells which affects the activity of many calcium-sensitive enzymes, including those involved in muscular contraction; structurally similar to troponin C component.

calsequestrin

A calcium-binding storage protein found within the cisternae of the sarcoplasmic reticulum of striated muscle cells.

fused tetanus

In a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli (a form of wave = temporal summation), when there is no evidence of partial relaxation, a reduction in measured tension, when the contraction is analyzed by electromyography

incomplete tetanus

In a state of continuous muscular contraction, especially when induced artificially by rapidly repeated stimuli (a form of wave = temporal summation), when there is still evidence of partial relaxation, a reduction in measured tension, when the contraction is analyzed by electromyography.

muscle contraction

Interactions between actin and myosin filaments of the sarcomere are responsible for, (physiology) a shortening or tensing of a part or organ (especially of a muscle or muscle fiber)

motor neuron

The type of neuron whose cell body is located in the brain or spinal cord, and whose axon runs along a peripheral nerve to synapse with an individual effector cell or group of cells to stimulate the effector into action (contraction or secretion); somatic motor neurons innervate skeletal muscle and activate contraction of a group of skeletal muscle cells, i.e., a motor unit; visceral or autonomic motor neurons innervate visceral effectors, i.e., smooth muscle, cardiac muscle, endocrine glands or exocrine glands.

electromyography

a diagnostic test that measures the electrical activity within muscle fibers in response to nerve stimulation

myogram

a graphical recording of muscle activity

ATP

a nucleotide derived from adenosine that occurs in muscle tissue

twitch contraction

a quick contraction followed by relaxation.

tetanus

a sustained muscular contraction resulting from a rapid series of nerve impulses

fascia

A gross anatomical term for the layers of dense fibrous connective tissue which cover and connect skeletal muscles and form the boundaries which define specific muscles and groups.

elasticity

The inherent property in tissues by which they recover their former shape and dimensions, after the removal of external pressure or altering force; springiness; tendency to rebound; as, the elasticity of muscle tissue.

contractility

The power possessed by the fibers of living (skeletal, cardiac and smooth) muscle of contracting or shortening in length which is achieved by the interaction of cytoplasmic contractile proteins, e.g., actin and myosin.

terminal cisterns

The slightly expanded edges of the sarcoplasmic reticulum which are found adjacent to the T = transverse tubules in the interior of striated muscle cells; they participate in Ca2+ ion release during exitation-contraction coupling.

sarcoplasmic reticulum

The smooth endoplasmic reticulum found in striated muscle fibers which stores calcium phosphate salts in a gelatinous state until the muscle is stimulated at which point the Ca2+ release channels open, permitting Ca2+ ions to diffuse into the general sarcoplasm where they serve as the second messenger, triggering the sliding filament mechanism of contraction.

desmosome

The specialized cell junction which links two cells by tying their outer cell membranes together with a tuft of intermediate filaments = tonofilaments embedded in a mass of dense anchoring material; desmosomes are particularly prevalent in tissues such as the epidermis and myocardium which have to withstand mechanical stress; nicknamed the "spot weld" junction.

intercalated disc

The specialized intercellular junction between individual cardiac muscle cells which appears as a dark line in microscopic specimens; at the molecular level it contains both electrical synapses provided by gap junctions and structural connections provided by desmosomes and tight junctions.

axon terminal

The swollen or knob-like distal endings to the terminal branches of axons (and axon collaterals) which house the secretory components of the neuron; the site of synthesis, packaging, storage and release of neurotransmitter molecules.

cross-bridges

The temporary covalent linkages between myosin heads and myosin-binding sites on acting during muscle contraction; energy for the covalent bond comes from hydrolysis of the ATP bound to the myosin head.

extensibility

The term used to describe the ability of a muscle to be stretched after a contraction, even to a length greater than the resting length.

hypodermis

The thin layer of loose adipose tissue underlying the dermis of the skin and binding the skin to the parts beneath, e.g., muscle or bone

H zone

The thin pale stripe or light area in the center of each A band observed in striated muscle when examined by a microscope; the H zone is lighter because within the H zone only myosin and no actin fibers are present block the passage of light; the center of each H zone is marked by a darker M line.

aponeurosis

tendon sheath

neurotransmitter

A chemical substance, e.g., acetylcholine, dopamine, etc., a signal molecule which is released from the presynaptic neuron's axon end bulbs upon the arrival of an action potential/nerve impulse to transmit a stimulus across the synapse by exocytosis and diffusion and then binding to a receptor site on a chemically gated ion channel, opening those channels; the influence of this stimulus on the postsynaptic cell may be excitatory or inhibitory.

actin

A contractile protein which is the main constituent of the thin filaments; it is involved in muscle contraction and an important cytoskeletal element in many other cell types; consists of globular = G-actin subunits which combine to form F = filamentous actin strands which then will further combine with troponin and tropmyosin proteins, to form the fully functional thin filament; thin filaments are anchored together at the Z line = Z disc of the sarcomere; its activity is indirectly regulated by Ca++ ions released from the sarcoplasmic reticulum.

tropomyosin

A fibrous protein monomer associated with actin filaments in the sarcomere of striated muscle cells and in the cytoskeleton of most cells; composed of two helical chains that span the length of 6 or 7 G-actin molecules; there is one tropomyosin associated with each troponin protein; tropomyosin helps stabilize the actin fiber and participates in the binding of actin to myosin; its activity is regulated by troponin which is regulated by Ca++ ions released from the sarcoplasmic reticulum.

satellite cell

A low frequency (<1%) striated muscle tissue cell (discovered in 1961) which is an undifferentiated stem cell (myoblast) which is dormant in healthy muscle tissue but can help repair damage by dividing and migrating to an injured area of the muscle; satellite cells are decreased in number with increased age or with certain diseases such as duchenne muscular dystrophy

action potential

A momentary change in electrical voltage potential (a wave of depolarization) on the surface of a nerve or muscle cell or gland cell which takes place when the cell is stimulated at or above its threshold level; the all-or-none action potential is capable of transmitting information (inhibition or excitation) to the next cell(s) in the communication pathway.

involuntary muscle tissue

A muscle that contracts without conscious control; includes smooth muscle found in walls of internal organs such as the stomach and intestines and bladder and blood vessels as well as cardiac muscle in the heart; a few skeletal muscles also operate under involuntary control, such as the muscles for breathing and swallowing, however, those skeletal muscles are subject to conscious control as well., muscle tissues that are controlled automatically by the nervous system

staircase effect

A phenomenon in muscle tissue in which a number of stimuli of the same strength are applied to the muscle after a period of rest in the relaxed state (a form of wave = temporal summation), the first few contractions of the series show a successive increase in amplitude (strength); the increase in efficiency is due to local temperature increase, increased enzyme efficiency, increased calcium ion availability and some increase in structural elasticity; nickname "the warm-up effect."

autorhythmicity

A property of cardiac muscle cells (and some smooth muscle cells); the ability to depolarize and contract at a set rate on their own with no outside stimulus or regulation necessary. aka automaticity.

troponin

A protein monomer associated with the thin actin filaments involved in striated muscle contraction; this protein complex consists of three functional subunits, one portion which has receptors for calcium ions and two portions which bind the protein to the other two protein components, actin and tropomyosin, of the thin filament; there is one troponin associated with each tropomyosin protein; its activity is regulated by Ca++ ions released from the sarcoplasmic reticulum. Nickname: the "switch molecule."

skeletal muscle tissue

A usually voluntary striated muscle made up of elongated, multinucleated, transversely striated muscle fibers, having principally attachments to bones.

myofiber

An alternate term for a muscle cell or muscle fiber; these cells are specialized for contraction using actin and myosin, organized in striated muscle cells in sarcomeres, where they use the sliding filament mechanism of contraction.

titin

An enormous elastic protein (2500-3500 KD) found in the sarcomere of striated muscle cells; these proteins form a scaffolding important for the correct assembly of the sarcomere; each titin molecule spans the distance from M line to Z disc and contributes to the passive tension in muscle cells as well as keeping the thick myosin filaments centered.

acetylcholine receptor

An integral membrane protein which is a gated Na+ channel which opens in response to the reversible binding of the neurotransmitter acetylcholine; this receptor is located in the post-synaptic cell membranes of skeletal muscle cells, cardiac muscle cells, some smooth muscle cells, some glandular cells, all autonomic postganglionic cells, and a variety of of CNS neurons.

gap junction

An intercellular junction which consists of a network of integral membrane protein channels called connexons which facilitate the cell-to-cell passage of small molecules, e.g., ions, small second messengers, etc., because these junctions are relatively non-selective in their permeability, i.e., they provide direct cytoplasmic connections; they are common in cardiac muscle and in some smooth muscle tissues.

myofilament

Any of the ultramicroscopic protein filaments, comprised of actin and myosin molecules, which are the structural components of a myofibril; they form the physical elements acting during the sliding filament mechanism of contraction.

recruitment

At the level of an entire muscle, the main means of increasing tension production, by stimulating a greater proportion of the individual motor units to contract simultaneously.

subcutaneous

Located or placed just beneath the skin, usually within the dermis

sarcomere

One of the segments into which a myofibril, an organelle of striated muscle, is divided; "the unit of contraction;" the sarcomere has dark a central A band split in the middle by the lighter H zone which contains the M line and, at each end, has one half of a lighter I band which is split by the Z line; the borders of the sarcomere are the Z lines.

intermediate filaments

One of three types of cytoskeletal elements (the other two are thin filaments (actin) and microtubules); frequently the three components work together to enhance both structural integrity, cell shape, and cell and organelle motility; intermediate filaments are stable, durable, intermediate in size compared with thin filaments and microtubules; they are prominent in cells that withstand mechanical stress.

synaptic vesicle

Small membranous containers for neurotransmitter substances which are formed by a Golgi apparatus and deliver the neurotransmitter into the synaptic cleft by a process of exocytosis.

thin filaments

The 7-9 nm diameter contractile proteins attached to the Z discs of striated muscle sarcomeres, arranged with opposite polarity in each half sarcomere; composed of the protein subunits F-actin, troponin, and tropomyosin; these contractile proteins are found in the cytoskeleton of smooth muscle and some other tissues as well.

conductivity

The ability of a tissue to receive a stimulus and transmit a wave of excitation (electrochemical activity), an impulse, over the rest of the structure,and perhaps to adjacent cells in the tissue; a property of nervous, muscular, and glandular tissues.

motor end plate

The area on the sarcolemma within the synapse which contains acetyl choline receptors and various gated ion channels and which contains the enzyme cholinesterase which can catabolize the neurotransmitter, acetyl choline; the binding of acetyl choline (released from the motor neuron's axon terminal bulb) to the gated Na+ channels initiates depolarization of the sarcolemma at this postsynaptic communication point

excitation-contraction coupling

The arrival of a nerve impulse (action potential) at the axon terminal of the neuro-muscular junction leads to acetylcholine release; acetylcholine then binds with gated sodium channels to initiate the depolarization of the sarcolemma; that depolarization leads to depolarization of the sarcoplasmic reticulum permitting the second messenger, calcium ions, to diffuse into the sarcoplasm and trigger the sliding filament contraction of the myofibrils; the cause-and-effect chain of motor neuron depolarization, synaptic neurotransmitter release, nerotransmitter binding, sarcolemma depolarization, sarcoplasmic reticulum depolarization, calcium outflow, and myofibril contraction.

latent period

The brief initial time interval observed in a myogram between the initial stimulation of a muscle fiber and the beginning of its actual contraction.

M line

The center of a sarcomere; a dark line splitting the H zone within th A band which appears dark because it contains a meshwork of fibrous proteins which serve as the anchoring points for myosin = thick filaments.

depolarization

The change in membrane voltage potential which triggers the action potential or nerve impulse; the change is due to the opening of gated Na+ channels which allow Na+ ions to enter the cell cytoplasm causing the resting potential of -70 mV to change to +30 mV in the cell interior.

power stroke

The conformational change in a myosin head, using the energy of ATP hydrolysis, after it has covalently bound to actin, which pulls the thin filament a small molecular distance in the direction of the M line at the center of the sarcomere.

myosin

The contractile protein monomer comprising myosin fibers which consists of a pair of intertwined strands forming the "tail" and the active "head" of each strand which is the specific site for both attachment of myosin to actin and for the repeated power strokes, due to its ATPase activity, which achieve the sliding of actin fibers over the myosin fibers during muscle contraction; its activity is regulated by Ca++ ions released from the sarcoplasmic reticulum; other related myosin-family proteins are associated with the cytoskeleton and with internal cytoplasmic transport processes.

dense body

The dark-staining cell organelles (peripheral proteins) within smooth muscle cells which are attached to the sarcolemma, internally to the thin (actin + myosin) intermediate filaments which move these structures during contraction, and, externally to surrounding endomysium.

acetylcholinesterase

The enzyme located at the motor end plate which breaks down the neurotransmitter acetyl choline in the synapse and which is responsible for stimulating terminating the stimulation of the skeletal muscle cell to make it contract.

endomysium

The protective external sheath of loose irregular fibrous connective tissue surrounding each skeletal muscle fiber/cell; it is continuous with the perimysium surrounding each fascicle (bundle) of the muscle and contains the main capillary blood supply for each muscle fiber/cell within the muscle.

motor unit

The functional unit of a skeletal muscle (organ), composed a voluntary motor neuron and the one or more skeletal muscle fibers which it innervates; muscles with large ratio motor units (1 neuron: many muscle fibers) can provide powerful contractions but cannot provide delicate control for precision movements while muscles with small ratio motor units (1 neuron: few muscle fibers) do not typically provide powerful contractions but can provide delicate control for very precise movements.

sarcoplasm

The general cytoplasm of a striated or skeletal muscle fiber containing all of the organelles and resources required for the sliding filament mechanism of contraction.

myofiber

The general name for the individual contractile cells in all types of muscle tissue

thermogenesis

The generation or production of heat by physiological processes; in humans the main source of heat comes from contractions of skeletal muscles (muscle tone and shivering); additional modest increases in general metabolism are regulated by the thyroid hormones, T3 and T4.

triad

The group of three membranous structures seen in the interior of striated muscle cells, consisting of one T = transverse tube and the two adjacent terminal cisterns in electron microscopy.

shivering

The irregular involuntary contractions of skeletal muscles which produce vibrations of the muscles rather than coordinated movements of limbs; triggered in response to cold or fear or in some inflammatory responses such as fevers; a mechanism for thermogenesis

T tubules

The irregular network of tiny tunnel-like extensions of the sarcolemma which penetrate into the interior of striated (skeletal and cardiac) muscle cells and make intimate contact with the membranes of the terminal cisterns of the sarcoplasmic reticulum; when the striated muscle cell is stimulated by neurotransmitter and depolarization occurs, the wave of depolarization is propagated along these membrane passageways and the depolarization is transferred to the membrane of the sarcoplasmic reticulum causing its depolarization and the opening of the Ca2+ release channels.

neuromuscular junction

The junction or connection, the synapse between the end bulbs of the axon terminals of a nerve fiber and the skeletal muscle cell(s) it stimulates to contract using acetyl choline as the excitatory neurotransmitter to open the chemically-gated Na+ channels to depolarize the sarcolemma; the location for the beginning (excitation) of excitation-coupling of muscle contraction.

synaptic cleft

The junction, space, cleft or gap across which a nerve impulse passes through the action of a neurotransmitter released from an axon terminal to stimulate the next excitable cell in the pathway, either another neuron, or a muscle cell or gland cell; transmission may be by an electrochemical depolarization event or by release of neurotransmitter molecules into the space.

thick filaments

The larger diameter contractile proteins anchored to the M lines of striated muscle sarcomeres, arranged with opposite polarity in each half sarcomere; composed of the protein subunit myosin; these contractile proteins are found in the cytoskeleton of smooth muscle and some other tissues as well; their activity is regulated by Ca++ ions released from the sarcoplasmic reticulum.

Z disc

The lateral boundaries of a sarcomere; a dark line splitting the I band which appears dark because it contains a meshwork of fibrous proteins which serve as the anchoring points for actin = thin filaments.

deep fascia

The layer of dense fibrous connective tissue (fascia) which wraps and binds together muscles in muscle groups and also binds together other internal structures.

smooth muscle tissue

The muscle tissue characterized by the absence of transverse stripes or a banding pattern observed in microscopy, due to the diffuse network arrangement of the contractile proteins, actin and myosin, which are not organized into myofibrils; the cytoplasm also lacks the T tubule system and the connective tissue around this muscle tissue never unites to form tendons or aponeuroses; the spindle-shaped, uninucleate, involuntary contractile cells controlled by the autonomic nervous system; it may be autorhythmic; it is the muscle type found in the walls of tubular internal organs, in the iris of the eye, in the arrector pili muscles, uterus, etc.

striated muscle tissue

The muscle tissue characterized by transverse stripes, a banding pattern observed in microscopy, due to the parallel arrangement of the contractile proteins, actin and myosin, within the cell's contractile organelles, the myofibrils, which are comprised of repeating functional units, the sarcomeres; skeletal and cardiac muscle are striated.

voluntary muscle tissue

The muscle tissue, usually striated skeletal muscle, which can be controlled (contracted) consciously by motor commands transmitted within the Somatic NS from the cerebral cortex, even though some control is maintained by lower centers in the CNS = Central Nervous System; this tissue is usually involved in movement of bones at synovial joints, but also for movement of some soft tissue, e.g., for facial expression, swallowing, etc.; this tissue is also the major source for heat production (thermogenesis) in the regulation of body temperature, muscle tissue that can by consciously controlled;skeletal muscle

myofibril

The non-membrane-bound cell organelle of skeletal and cardiac (striated) muscle cells which is a long cylindrical mass of contractile protein fibers (actin + myosin), organized as a series of repeating sarcomeres, these contractile organelles are triggered to contract in an all-or-none fashion during excitation-contraction coupling when Ca2+ ions to diffuse into the general sarcoplasm where they serve as the second messenger, triggering the sliding filament mechanism of contraction.

sarcolemma

The outer lipid bilayer cell membrane or plasmalemma of a skeletal muscle cell; muscle contraction is initiated when the sarcolemma is depolarized in response to a nerve impulse from a motor neuron at the motor end plate.

myosin heads

The pair of active subregions of each myosin protein monomer which have the capacity to bind and to hydrolyze ATP, to form covalent cross-links with actin filaments, and to perform the power strokes which produce the sliding of filaments that achieves contraction of the sarcomere, the unit of contraction in the myofibrils (organelles) of straiated muscle tissues; their activity is regulated by Ca++ ions released from the sarcoplasmic reticulum.

myosin tails

The pair of subregions of each myosin protein monomer which align with other myosin monomer's tail regions to produce the huge polymer which is the myosin fiber or thick filament observed in the myofibrils of striated muscle tissues

acetylcholine

The peripheral excitatory neurotransmitter released at the neuromuscular junction and responsible for stimulating skeletal muscle cells to contract; ACh also serves as a neurotransmitter in the periphery in the autonomic nervous system and in the central nervous system.

excitability

The property manifested by living organisms, and certain of their tissues (e.g., nervous, muscular, and glandular tissues), of responding to a stimulation of some sort, generally a response to some sort of kinetic energy,

perimysium

The protective external sheath of dense irregular fibrous connective tissue surrounding each fascicle (bundle) of a skeletal muscle; it is continuous with the epimysium surrounding the entire muscle and with the endomysium surrounding each muscle fiber/cell within the muscle.

A band

The repeating dark area in the banding pattern observed in striated muscle when examined by a microscope; the A band is darker because within the A band both myosin and actin fibers are present and block the passage of much light; the center of each A band is marked by a lighter H zone.

I band

The repeating lighter area in the banding pattern observed in striated muscle when examined by a microscope; the I band is lighter because within the I band only actin fibers are present and they block the passage of only a small amount of light; the center of each I band is marked by a darker Z line.

myology

The scientific study of muscle anatomy and physiology., the branch of physiology that studies muscles

cross-striations

The series of parallel lines or light and dark bands observed in the cytoplasm of skeletal and cardiac (striated) muscle cells, the bands are perpendicular to the long axis of the muscle cell; the banding pattern is due to the orderly arrangement of contractile myofilaments organized into repeating sarcomeres within each myofibril.

cardiac muscle tissue

The specialized involuntary, striated, uninucleate, contractile tissue of the heart; comprising the myocardium in which the uninucleate cells are connected to each other by intercalated discs; autorhythmic.

sliding filament mechanism

The specific interactions between myosin and actin, triggered by calcium ions and using the energy of ATP hydrolysis, which include a repetitive covalent binding of the myosin head to the actin filament, the power stroke of the myosin head, the movement of the actin filament toward the M line of the sarcomere, and then the breaking of covalent link between actin and myosin so that the myosin head may bind to the next actin in the "ratchet action" which ultimately shortens each sarcomere and thus causes the muscle cell to contract; this "unit action" is repeated thousands of times since each sarcomere contains many myosin and actin filaments.

myosin-binding site

The specific location on each G-actin molecule where a myosin head can covalently bond to the actin filament during muscle contraction; at rest the myosin-binding site is covered by the tropomyosin molecule, but during contraction, the presence of calcium ions causes a conformational change in the tropomysoin molecule so that it shifts position and exposes the myosin-binding site.

relaxation period

The time interval (usually 10-100 msec) observed in a myogram after the contraction period and following development of peak tension, when the sliding filament mechanism is inactived at the molecular level, actin-myosin cross-bridges are broken and tension (force of contraction) is reduced, until the resting state is restored or another contraction is initiated.

contraction period

The time interval (usually 10-100 msec) observed in a myogram after the latent period following the initial stimulation of a muscle fiber when the sliding filament mechanism is active at the molecular level, actin-myosin cross-bridges are being formed, and tension develops, rising to a peak; if the tension is sufficient to overcome the resistance of a load, the muscle fiber will shorten.

refractory period

The time interval, after a muscle fiber has been stimulated and a contraction has been achieved, which has to pass before the muscle cell can be stimulated to contract again.

all-or-none response

The type of response by any regulated system, upon stimulation to either perform a complete response or to have a total lack of response or effect; e.g., a muscle cell contraction or a nerve impulse (action potential) generated by a weak stimulus, just minimally above the threshold limit, is just as strong as one generated by a strong stimulus well beyond the threshold value.

axon

The usually long process of a nerve fiber that conducts impulses (action potentials) away from the cell body of the nerve cell and to the next cell in the control pathway