Chapter 9 Flashcards
Contractility
ability of muscle to shorten forcefully.
Excitability
the capacity of muscle to respond to a stimulus.
Extensibility
means a muscle can be stretched beyond its normal resting length and still be able to contract.
Elasticity
the ability of muscle to recoil to its original resting length after it has been stretched.
Skeletal muscle
responsible for locomotion, facial expressions, posture, respiratory functions, and many other body movements. The nervous system voluntarily, or consciously, controls the functions of the skeletal muscles.
Smooth muscle
most widely distributed type of muscle in the body. It is found in the walls of hollow organs and tubes, in the interior of the eye, and in the walls of blood vessels, among other areas. Not consciously controlled by the nervous system, controlled involuntarily, or unconsciously, by the endocrine and autonomic nervous systems
Cardiac muscle
ound only in the heart, and its contractions provide the major force for moving blood through the circulatory system; autorhythmic - they contract spontaneously at somewhat regular intervals, and nervous or hormonal
stimulation is not always required for them to contract; involuntary
skeletal muscle fibers
Each skeletal muscle is a complete organ consisting of these cells, associated with smaller amounts of connective tissue, blood vessels, and nerves.
muscle fasciculi
A muscle is composed of numerous visible bundles
perimyseum
Each fasciculus is surrounded by this other, heavier connective tissue layer
epimysium
The entire muscle is surrounded by this layer of connective tissue
Fascia
general term for connective tissue sheets within the body.
Muscular fascia (formerly deep fascia )
located superficial to the epimysium, separates and compartmentalizes individual muscles or groups of muscles. It consists of dense irregular collagenous
connective tissue.
Motor neurons
specialized nerve cells that stimulate muscles to contract.
striated
striped, appearance, as seen in longitudinal section, alternating light and dark bands
myoblasts
Muscle fibers develop from less mature, multinucleated cells
hypertrophy
Enlargement
sarcolemma
plasma membrane of a muscle fiber
two delicate connective tissue layers are located just outside the sarcolemma:
- external lamina: deeper and thinner of the two, consists mostly of reticular (collagen) fibers and is so thin that it
cannot be distinguished from the sarcolemma when viewed under a light microscope.
-endomysium: second layer also consists mostly of reticular fibers, but it is a much thicker layer
transverse tubules or T tubules
the many tubelike invaginations along the surface of the sarcolemma that appear at regular intervals along the muscle fiber and extend inward to it
sarcoplasmic reticulum
The T tubules are associated with this highly organized smooth endoplasmic reticulum
sarcoplasm
Other organelles, such as the numerous mitochondria and glycogen granules, are packed into the cell and constitute the cytoplasm, which in muscles is called the sarcoplasm
myofibrils
bundles of protein filaments found in the sarcoplasm; extends from one end of the muscle fiber to the other
A myofibril contains two kinds of protein filaments, called myofilaments:
- Actin myofilaments: thin myofilaments
- myosin myofilaments: thick myofilaments
sarcomeres
The actin and myosin myofilaments form highly ordered units called sarcomeres which are joined end to end to form the myofibrils
globular actin (G actin) monomers
Each F actin strand is a polymer of approximately 200 small, globular units called globuar actin (G actin) monomers; Each G actin monomer has an active site, to which myosin molecules can bind during muscle contraction.
myosin molecules
Myosin myofilaments are composed of many elongated myosin molecules shaped like golf clubs; consists of two myosin heavy chains wound together to form a rod portion lying parallel to the myosin myofilament and
two myosin heads that extend laterally
The myosin heads have three important properties:
- The heads can bind to active sites on the actin molecules to form cross-bridges
- the heads are attached to the rod portion by a hinge region that can bend and straighten during contraction
- the heads are ATPase enzymes, which break down ATP releasing energy.
Z disk
a filamentous network of protein forming a disklike structure for the attachment of actin myofilaments; Each sarcomere extends from one Z disk to an adjacent Z disk
isotropic band, or I band
includes a Z disk and extends from each side of the Z disk to the ends of the myosin myofilaments.
anisotropic band or A band
Each A band extends the length of the myosin myofilaments within a sarcomere.
H zone
A band is a smaller band called the H zone, where the actin and myosin myofilaments do not overlap and only myosin myofilaments are present.
M line
in the middle of the H zone and consists of delicate filaments that attach to the center of the myosin myofilaments; helps hold the myosin myofilaments in place, similar to the way the Z disk holds actin myofilaments in place
Titin
one of the largest known proteins; attaches to Z disks
and extends along myosin myofilaments to the M line. The myosin myofilaments are attached to the titin molecules, which help hold them in position. Part of the titin molecule in the I band functions as a spring, allowing the sarcomere to stretch and recoil.
sliding filament model
Actin and myosin myofilaments do not change length during contraction of skeletal muscle but, instead, slide past one another, causing the sarcomeres to shorten
action potentials
Electrical signals that travel from the brain or spinal cord along the axons to muscle fibers and cause them to contract.
polarized
The inside of most plasma membranes is negatively charged compared with the outside; therefore, the plasma membrane is polarized, meaning that a voltage difference, or electrical charge difference, exists across each plasma membrane.
resting membrane potential
This charge difference across the plasma membrane of an unstimulated cell
millivolts
The resting membrane potential can be measured in these units
two types of gated ion channels:
Ligand-gated ion channels and Voltage-gated ion channels.
ligand
molecule that binds to a receptor.
receptor
receptor is a protein or glycoprotein that has a receptor site to which a ligand can bind.
Ligand-gated ion channels
open when a ligand binds to a receptor that is part of the ion channel
neurotransmitters
the axons of neurons supplying skeletal muscle fibers
release these which bind to ligand-gated Na + channels in the membranes of the muscle fibers.
Voltage-gated ion channels
These channels are gated membrane channels that open and close in response to small voltage (charge) changes across the plasma membrane, or changes in the membrane potential.
threshold
If the depolarization changes the membrane potential to a value called threshold, an action potential is triggered.
depolarization phase
this phase of the action potential is a brief period during which further depolarization occurs and the inside of the cell becomes positively charged
repolarization phase
the return of the membrane potential to its resting value.
all-or-none principle
Action potentials occur according to this principle. If a stimulus is strong enough to produce a depolarization that
reaches threshold, or even if it exceeds threshold by a substantial amount, all of the permeability changes responsible for an action potential proceed without stopping. If stimulus is too weak weak that the depolarization does not reach threshold, few of the
permeability changes occur. The membrane potential returns to its resting level after a brief period without producing an action potential
propagate
Action potentials can travel, or propagate, across the plasma membrane because an action potential produced at one location in the plasma membrane can stimulate the production of an action potential in an adjacent location
action potential frequency
the number of action potentials produced per unit of time.
neuromuscular junction or synapse
Near the muscle fiber it innervates, each axon branch forms a cluster of enlarged axon terminals that rests in an invagination of the sarcolemma to form this which consists of the axon terminals and the area of the muscle fiber sarcolemma they innervate.
pre-synaptic terminal
Each axon terminal is the pre-synaptic terminal
synaptic cleft
The space between the presynaptic terminal and the muscle fiber
postsynaptic membrane or motor end-plate
the muscle plasma membrane in the area of the junction
synaptic vesicles
Each presynaptic terminal contains numerous mitochondria
and these many small, spherical sacs
acetylcholine
vesicles contain this organic molecule composed of acetic acid and choline, which is a neurotransmitter.
neurotransmitter
a substance released from a presynaptic membrane that diffuses across the synaptic cleft and alters the activity of the postsynaptic cell.
acetylcholinesterase
Acetylcholine released into the synaptic cleft is rapidly broken down to acetic acid and choline by this enzyme
excitation-contraction coupling
The mechanism by which an action potential causes contraction of a muscle fiber; it involves the sarcolemma,
T tubules, the sarcoplasmic reticulum, Ca 2 + , and troponin.
terminal cisternae
Near the T tubules, the sarcoplasmic reticulum is enlarged to form these
triad
A T tubule and the two adjacent terminal cisternae together
cross-bridge cycling
During a single contraction, each myosin molecule undergoes the cycle of cross-bridge formation, movement, release, and return to its original position many times.
power stroke/recovery stroke
Movement of the myosin molecule while the cross-bridge is attached is called the power stroke, whereas return of the myosin head to its original position after cross-bridge release is called the recovery stroke.
spastic paralysis
muscles contract and cannot relax, which is followed by muscle fatigue.
flaccid paralysis
activation of the receptors not allowed, and therefore the muscle is incapable of contracting in response to nervous stimulation
Myasthenia gravis
results from the production of antibodies that bind to acetylcholine receptors, eventually destroying the receptor and thus reducing the number of receptors.
muscle twitch
A single, brief contraction and relaxation cycle in a muscle fiber
lag phase (or latent phase )
The time between the application of the stimulus to the motor neuron and the beginning of contraction
contraction phase
the time during which contraction occurs
relaxation phase
time during which relaxation occurs
motor unit
consists of a single motor neuron and all the muscle fibers
it innervates
graded
The strength of muscle contractions varies from weak to strong. In other words, whole muscles respond to stimuli in a graded fashion.
The force of a contraction is increased in two ways:
- Summation involves increasing the force of contraction of the muscle fibers within the muscle
- recruitment involves increasing the number of muscle fibers contracting.
treppe
A muscle fiber, when stimulated in rapid succession, contracts with greater force with each subsequent stimulus; occurs in a muscle fiber that has rested for a prolonged period.
multiple-motor-unit summation
The relationship between increased stimulus strength and an increased number of contracting motor units is called multiple-motor-unit summation because the force of contraction increases as more and more motor units are stimulated.
subthreshold stimulus
is not strong enough to cause an action potential in any of the axons in a nerve and does not cause a contraction.
threshold stimulus
As the stimulus strength increases, it eventually becomes a threshold stimulus, which is strong enough to produce an action potential in a single motor unit axon, causing all the muscle fibers of the motor unit to contract.
submaximal stimuli
Progressively stronger stimuli, called submaximal stimuli,
produce action potentials in axons of additional motor units.
maximal stimulus
maximal stimulus produces action potentials in the axons
of all the motor units of that muscle. Consequently, even
greater stimulus strengths (called supramaximal stimuli ) have no additional effect.
tetanus
As the frequency of action potentials in a skeletal muscle fiber increases the frequency of contraction also increases until a period of sustained contraction, or tetanus, is achieved.
incomplete tetanus
muscle fibers partially relax between the contractions
complete tetanus
muscle fibers produce action potentials so rapidly that no relaxation occurs between them.
multiple-wave summation
As the frequency of contractions increases, the increased tension produced is called multiple-wave summation
Active tension
the force applied to an object to be lifted when a muscle contracts.
active tension curve
The muscle length plotted against the tension produced by the muscle in response to maximal stimuli
Passive tension
the tension applied to the load when a muscle stretches but not stimulated.
total tension
The sum of active and passive tension
isometric contractions
when the length of the muscle does not change, but the amount of tension increases during contraction.
isotonic contractions
the amount of tension produced by the muscle is constant during contraction, but the length of the muscle changes.
Concentric contractions
isotonic contractions in which tension in the muscle is great enough to overcome the opposing resistance, and the muscle shortens.
Eccentric contractions
isotonic contractions in which tension is maintained in a muscle, but the opposing resistance is great enough to cause the muscle to increase in length.
Muscle tone
the constant tension produced by muscles for long periods of time.
Fatigue
the decreased capacity to do work and the reuced efficiency of performance that normally follows a period of
activity.
Psychological fatigue
most common type, involves the central nervous system. The muscles are capable of functioning, but the individual “perceives” that additional muscular work is not possible.
Muscular fatigue
results from calcium ion imbalances as ATP levels drop.
synaptic fatigue
least common type; occurs in the neuromuscular junction. If the action potential frequency in motor neurons is great enough, the amount of acetylcholine released from the presynaptic terminals is greater than the amount synthesized. As a result, the synaptic vesicles become
depleted, and insufficient acetylcholine is released to stimulate the muscle fibers.
physiological contracture
As a result of extreme muscular fatigue, muscles occasionally become incapable of either contracting or relaxing, which is caused by a lack of ATP within the muscle fibers.
Rigor mortis
development of rigid muscles several hours after death, is similar to physiological contracture.
creatine phosphate
During resting conditions, energy from aerobic respiration is used to synthesize creatine phosphate; it accumulates in muscle fibers, where it stores energy that can be used to synthesize ATP.
creatine kinase
The reaction, catalyzed by this enzyme occurs very
rapidly, and is able to maintain ATP levels as long as creatine phosphate is available in the fiber.
Anaerobic respiration
does not require oxygen and results in the breakdown of glucose to yield ATP and lactic acid. For each molecule of glucose metabolized, two ATP molecules and two
molecules of lactic acid are produced.
glycolysis
The first stages of anaerobic respiration and aerobic respiration share an enzymatic pathway called glycolysis. In glycolysis, a glucose molecule is broken down into two molecules of pyruvic acid. Two molecules of TP are used in this process, but four molecules of ATP are produced
Aerobic respiration
requires oxygen and breaks down glucose to produce ATP, carbon dioxide, and water. Aerobic respiration is much more efficient than anaerobic respiration. Anaerobic
respiration results in a net gain of 2 ATP molecules for each glucose molecule, whereas aerobic respiration can produce up to 36 ATP molecules for each glucose molecule.
oxygen deficit or oxygen debt
the insufficient oxygen consumption relative to increased activity at the onset of exercise creates an oxygen deficit, or oxygen debt.
recovery oxygen consumption
The elevated oxygen consumption that occurs after exercise has ended
Slow-twitch oxidative (SO) muscle fibers (or type I fibers )
contract more slowly, are smaller in diameter, have a better-developed blood supply, have more mitochondria, and are more fatigue- resistant than fast-twitch muscle fibers; respond relatively slowly to nervous stimulation.
myosin ATPase
The enzymes on the myosin heads responsible for the breakdown of ATP
myoglobin
Slow-twitch fibers also contain large amounts of this dark pigment similar to hemoglobin in red blood cells, which binds oxygen and acts as an oxygen reservoir in the
muscle fiber when the blood does not supply an adequate amount; myoglobin enhances the capacity of the muscle fibers to perform aerobic respiration.
Fast-twitch muscle fibers (or type II fibers )
respond rapidly to nervous stimulation, and their myosin heads have a fast form of myosin ATPase, which allows them to break down ATP more rapidly than slow-twitch muscle fibers; less blood supply, less myoglobin and mitochondria, lots of glycogen, anaerobic respiration; rely almost exclusively on anaerobic glycolysis for ATP production.
hypertrophies/atrophies
muscle increases in size; decreases in size
anabolic steroids
synthetic hormones used to increase the size and
strength of their muscles; related to testosterone
shivering
When body temperature declines below a certain level, the nervous system responds by inducing these rapid skeletal muscle contractions that produce shaking rather than coordinated movements.
dense bodies and dense areas
Actin myofilaments are attached to dense bodies, which are scattered through the cell cytoplasm, and to dense areas, which are in the plasma membrane. Dense bodies
and dense areas are considered equivalent to the Z disks in skeletal muscle.
intermediate filaments
Noncontractile intermediate filaments also attach to the
dense bodies. The intermediate filaments and dense bodies form an intracellular cytoskeleton, which has a longitudinal or spiral organization.
caveolae
shallow, invaginated areas that lie along the surface of the plasma membrane.
calmodulin
Calcium ions that enter the cytoplasm bind to this protein. Calmodulin molecules with Ca 2 + bound to them
activate an enzyme called myosin kinase, which transfers a
phosphate group from ATP to light myosin molecules on the heads of myosin molecules.
myosin phosphatase
Relaxation of smooth muscle results because of the activity of this enzyme that removes the phosphate group from the myosin molecules
latch state
smooth muscle is able to sustain tension for long periods and without extensive energy expenditure. This
period of sustained tension is the latch state of smooth
muscle contraction.
Visceral smooth muscle (or unitary smooth muscle)
more common of the two types; occurs in sheets and includes the smooth muscle of the digestive, reproductive, and urinary tracts; has numerous gap junctions, which allow action potentials to pass directly from one to another; sheets of smooth muscle cells function as a unit, and a wave of contraction traverses the entire smooth muscle
sheet; often autorhythmic
Multiunit smooth muscle
occurs in various configurations: sheets, as in the walls of blood vessels; small bundles, as in the arrector pili muscles and the iris of the eye; and single cells, as in the capsule of the spleen; has fewer gap junctions than visceral smooth muscle, and cells or groups of cells act as independent units; contracts when stimulated by nerves or hormones.
pacemaker cells
Certain smooth muscle cells in these organs function as pacemaker cells , which tend to develop action potentials more rapidly than other cells.
smooth muscle tone
a relatively constant tension over a long period and maintains that tension in response to a gradual increase in the smooth muscle length
intercalated disks
Adjacent cells join to form branching fibers by these specialized cell-to-cell attachments, which have gap junctions that allow action potentials to pass from cell to cell.
Duchenne muscular dystrophy (DMD)
caused by mutations in the dystrophin gene on the X chromosome. The dystrophin gene is responsible for producing a protein called dystrophin; progressive muscle weakness
