Skeletal muscle cells have a distinctive 'striped' microscopic appearance called ____. This reflects very defined anatomical relationships between the individual myofibrils which are packed into each muscle cell (fiber). Each myofibril consists of thousands of basic functional units (sarcomeres) joined end to end at ____ ____. In turn, each cell (fiber) is packed with thousands of these ____. Exercise produces bulging muscles not by adding more fibers (cells) but by adding more myofibrils within existing cells.
striation; Z discs; myofibrils
Within the sarcomeres are overlapping thin (____) and thick (____) filaments attached to the Z discs by means of an elastic central filament of the protein ____. The interaction between myosin & actin filaments produces contraction and relaxation.
actin; myosin; titan
In vivo, each muscle fiber receives a single axon terminal from a somatic motor neuron. The motor neuron stimulates the muscle fiber to contract by liberating ____ at the neuromuscular junction. The specialized region of the sarcolemma of the muscle fiber at the neuromuscular junction is known as a ____ ____ ____. The cell body of a somatic motor neuron is located in the ____ ____ of the gray matter of the spinal cord and gives rise to a single axon that emerges in the ____ ____ of a spinal nerve. Each axon, however, can produce a number of ____ ____ to innervate an equal number of muscle fibers. Each somatic ____ ____, together with all of the muscle fibers that it innervates, is known as a motor unit.
acetylcholine; motor end plate; ventral horn; ventral root; collateral branches; motor unit
Whenever a somatic motor neuron is activated, all of the muscle fibers that it innervates are stimulated to contract. In vivo, ____ ____ (where contraction strength is varied) of whole muscles are produced by ____ ____ ____ ____ of motor units that are activated. In order for these graded contractions to be smooth and sustained, different motor units must be activated by rapid, ____ stimulation.
graded contraction; variations in the number; asynchronous
Fine neural control over the strength of muscle contraction is optimal when there are ____ ____ motor units involved. In the extraocular muscles that position the eyes, for example, the innervation ratio (motor neuron:muscle fibers) of an average motor unit is one neuron per ____ muscle fibers. This affords a fine degree of control. The innervation ratio of the gastrocnemius, by contrast, averages one neuron per ____ muscle fibers. Stimulation of these motor units results in more powerful contractions at the expense of finer gradations in contraction strength.
many small; 23; 1000
All of the motor units controlling the gastrocnemius, however, are not the same size. Innervation ratios vary from 1:100 to 1:2,000. A neuron that innervates ____ muscle fibers has a smaller cell body and is stimulated by lower levels of excitatory input than a ____ neuron that innervates a ____ ____ ____ muscle fibers. The smaller motor units, as a result, are the ones that are used most often. When contractions of greater strength are required, larger and larger motor units are activated in a process known as ____ of motor units.
fewer; larger; greater number of; recruitment
When the sarcomere lengths are greater than about ____ µm, the tension produced by the muscle contraction ____ with increasing sarcomere length. This is because there are fewer interactions of myosin cross bridges with actin. When the sarcomeres reach a length of about ____ µm, there is no overlap of thick and thin filaments, and no interactions can occur between myosin and actin. Therefore the muscle produces zero tension.
2.2; decreases; 3.6
When the sarcomere length is shorter than ____ µm, the force generated by muscle contraction declines with decreasing sarcomere length. This is believed to result from interference with cross-bridge action, in part because of increasing distance between thick and thin filaments as the muscle fiber gets shorter and thicker. Skeletal muscles generate ATP through cell respiration and through the use of phosphate groups donated by ____ ____. The aerobic abilities of skeletal muscle fibers differ according to muscle fiber type, which are distinguished by their speed of contraction, color, and major mode of energy metabolism.
2.0; creatine phosphate
Skeletal muscles at rest obtain most of their energy from the aerobic respiration of ____ ____. During exercise, muscle ____ and blood ____ are also used as energy sources (fig. 12.22). Exercise of greater intensity and duration causes increased blood glucose uptake into the exercising muscles (fig. 12.23). This is largely due to the ability of muscle contraction to stimulate the insertion of ____ ____ into the sarcolemma, which is similar to the mechanism of insulin action but involving different signaling molecules.
Skeletal muscles metabolize anaerobically for the first ____ ____ ____ seconds of moderate-to-heavy exercise, because the cardiopulmonary system requires this amount of time to sufficiently increase the oxygen supply to the exercising muscles. If exercise is moderate, ____ ____ contributes the major portion of the skeletal muscle energy requirements following the first ____ minutes of exercise.
fatty acids; glycogen; glucose; GLUT4 carriers; 45 to 90; aerobic respiration; two
The maximum rate of oxygen consumption (by aerobic respiration) in the body is called the ____ ____ ____. Some world-class athletes have maximal oxygen uptakes that are twice the average for their age and sex—this appears to be due largely to ____ factors, but training can increase the maximum oxygen uptake by about ____.
aerobic capacity (V02max); genetic; 20%
The intensity of exercise can also be defined by the ____ ____ ____. This is the percentage of the maximal oxygen uptake at which a significant rise in blood lactate levels occurs. For average healthy people, for example, a significant amount of blood lactate appears when exercise is performed at about ____ of the maximum rate of oxygen consumption (VO2max) .
lactate (or anaerobic) threshold; 50%
During light exercise (at about 25% of the VO2max), most of the exercising muscle's energy is obtained from the ____ ____ ____ ____ ____. These are derived mainly from stored fat in adipose tissue.
During sustained muscle activity, ATP may be used faster than it can be produced through cell respiration. At these times, the rapid renewal of ATP is extremely important. This is accomplished by combining ADP with phosphate derived from another high-energy phosphate compound called ____ ____.
aerobic respiration of fatty acids; creatine phosphate
Within muscle cells, the creatine phosphate concentration is more than ____ times the concentration of ATP and represents a ready reserve of high-energy phosphate that can be donated directly to ADP (fig. 12.24). Production of ATP from ADP and phosphocreatine is so efficient that, even though the rate of ATP breakdown rapidly increases from rest to exercise, muscle ATP concentrations ____ ____ ____ in aerobically adapted muscle.
three; decrease only slightly
The enzyme that transfers phosphate between creatine and ATP is called ____ ____. Skeletal muscle and heart muscle have two different forms (____) of this enzyme. The skeletal muscle ____ is found to be elevated in the blood of people with muscular dystrophy. The plasma concentration of the ____ characteristic of heart muscle is elevated as a result of myocardial infarction (damage to heart muscle), and measurements of this enzyme are thus used as a means of diagnosing heart disease.
creatine kinase; isoenzymes; isoenzyme; isoenzyme
Muscle fatigue during a sustained maximal contraction, when all the motor units are used and the rate of neural firing is maximal—as when lifting an extremely heavy weight—appears to be due to an accumulation of ____ ____. Remember that K+ leaves axons and muscle fibers during the repolarization phase of action potentials. This reduces the membrane potential of muscle fibers and interferes with their ability to produce action potentials. Fatigue under these circumstances lasts ____ ____ ____ ____, and maximal tension can again be produced after less than ____ ____ ____.
extracellular K+; only a short; a minute's rest
Muscle fatigue that occurs during other types of exercise appears to have different causes. Chiefly, there is depletion of muscle ____ and a reduced ability of the ____ ____ to release Ca2+, leading to failure of excitation-contraction coupling. It has been known from the early twentieth century that fatigue occurs when ____ ____ accumulates, and that restoring ____ respiration allows muscle glycogen and contractile ability to recover. This led to the widespread belief that the lowered pH caused by lactic acid interferes with excitation-contraction coupling. Ongoing research, however, suggests that increased concentrations of lactate and H+ (lower pH) have less impact on muscle fatigue than was once thought.
glycogen; sarcoplasmic reticulum; lactic acid; aerobic
When humans exercise, however, we often experience fatigue ____ our muscles themselves have fatigued sufficiently to limit exercise. Put another way, our maximum voluntary muscle force is often less than the maximum force that our muscle is itself capable of producing. This demonstrates ____ ____—muscle fatigue caused by changes in the CNS rather than by fatigue of the muscles themselves. In part, it involves a reduced ability of the "upper motoneurons" (interneurons in the brain devoted to motor control) to drive the "lower motoneurons" (in the spinal cord). Muscle fatigue thus has two major components: a ____ component (fatigue in the muscles themselves) and a ____ component (fatigue in the CNS that causes reduced activation of muscles by motoneurons).
before; central fatigue; peripheral; central
When muscles hypertrophy, or grow larger as a result of increased fiber thickness, the number of nuclei in each fiber must increase in proportion to the larger volume of the fiber. These new nuclei are provided by ____ ____, muscle stem cells. They are also involved in muscle repair. With age there is a decline in the number of satellite cells as well as their ability to proliferate in response to muscle damage and their ability to form new muscle fibers. Scientists demonstrated that satellite cells from young muscles have a reduced ability to proliferate and form new muscle fibers when they are exposed to old muscle fibers. Some studies suggest that elderly people with declining muscle mass produce increased amounts of a different paracrine regulator, ____, a paracrine regulator that is able to inhibit satellite cell function and muscle growth.
satellite cells; myostatin
In order for the nervous system to control skeletal movements properly, it must receive continuous sensory feedback information concerning the effects of its actions. This sensory information includes (1) the tension that the muscle exerts on its tendons, provided by the ____ ____ ____, and (2) muscle length, provided by the ____ ____ ____. The spindle apparatus, so called because it is wider in the center and tapers toward the ends, functions as a length detector. Muscles that require the finest degree of control, such as the muscles of the hand, have the highest density of spindles.
Golgi tendon organs; muscle spindle apparatus
Each spindle apparatus contains several thin muscle cells called ____ fibers packaged within a connective tissue sheath. Like the stronger and more numerous "ordinary" muscle fibers outside the spindles—the ____ fibers—the spindles insert into tendons on each end of the muscle.
In the spinal cord, two types of lower motor neurons innervate skeletal muscles. The motor neurons that innervate the extrafusal muscle fibers are called ____ ____; those that innervate the intrafusal fibers are called ____ ____. The alpha motoneurons are faster conducting (60 to 90 meters per second) than the thinner gamma motoneurons (10 to 40 meters per second). Because only the extrafusal muscle fibers are sufficiently strong and numerous to cause a muscle to shorten, only stimulation by the alpha motoneurons can cause muscle contraction that results in skeletal movements. However, stimulation by gamma motoneurons does result in isometric contraction of the spindles.
intrafusal fibers; extrafusal fibers; alpha motoneurons; gamma motoneurons
Under normal conditions, the activity of gamma motoneurons is maintained at the level needed to keep the muscle spindles under proper tension while the muscles are ____. Undue relaxation of the muscles is prevented by ____ ____ ____ of the spindles, which in turn elicits a ____ ____. This mechanism produces a normal resting muscle length and state of tension, or ____ ____.
relaxed; stretch and activation; reflex contraction; muscle tone
The ____, like the cerebrum, receives sensory input from muscle spindles and Golgi tendon organs. It also re-ceives fibers from areas of the cerebral cortex devoted to vision, hearing, and equilibrium.
There are no direct ____ tracts (to the LMNs of the cord) from the cerebellum. The cerebellum can influence motor activity only indirectly, through its output to the basal nuclei. It is interesting that all output from the cerebellum is ____; these inhibitory effects aid motor coordination by eliminating inappropriate neural activity. Damage to the cerebellum interferes with the ability to coordinate movements with our spatial judgment. Under- or overreaching for an object may occur, followed by ____ ____, in which the limb moves back and forth in a pendulum-like motion.
cerebellum; descending; inhibitory; intention tremor
____ ____ ____ are arranged in ____ layers in the walls of blood vessels and bronchioles (small air passages in the lungs). Both ____ ____ ____ smooth muscle layers occur in the tubular digestive tract, the ureters (which transport urine), the ductus deferentia (which transport sperm cells), and the uterine tubes (which transport ova). The alternate contraction of circular and longitudinal smooth muscle layers in the intestine produces ____ ____, which propel the contents of these tubes in one direction.
Although smooth muscle cells do not contain ____ (which produce striations in skeletal and cardiac muscle), they do contain a great deal of actin and some myosin, which produces a ratio of thin to thick filaments of about ____ ____ ____ (in striated muscles the ratio is 2 to 1). Unlike striated muscles, in which the thin filaments are relatively short (extending from a Z disc into a sarcomere), the thin filaments of smooth muscle cells are quite long. They ____ ____ the sarcoplasm to attach either to regions of the plasma membrane of the smooth muscle cell or to cytoplasmic protein structures called ____ ____, which are analogous to the Z discs of striated muscle. This arrangement causes smooth muscle to 'scrunch' when stimulated.
smooth (visceral) muscles; circular; circular and longitudinal; peristaltic waves; sarcomeres; 16 to 1; criss-cross; dense bodies
In smooth muscle, the myosin proteins of the thick filaments are arranged so that the myosin heads can form cross bridges with actin ____ ____ ____ ____ of the thick filaments which is essential for proper smooth muscle function. Smooth muscles must be able to contract even when ____ ____—in the urinary bladder, for example, the smooth muscle cells may be stretched up to two and a half times their resting length. The smooth muscle cells of the uterus may be stretched up to eight times their original length by the end of pregnancy. Striated muscles, because of their structure, lose their ability to contract when the sarcomeres are stretched to the point where actin and myosin ____ ____ ____; not so in smooth muscle where actin & myosin remain in 'striking distance' even when under extreme stretch.
all along the length; greatly stretched; no longer overlap