Chapter 10
The sliding filament mechanism
A model that describes muscle contraction in which thin filaments slide past thick ones so that the filaments overlap, causing shortening of a sarcomere, and thus shortening of muscle fibers and alternately shortening of the entire muscle.
motor unit
A motor neuron together with the muscle fibers (cells) it stimulates.
How muscle fiber contraction results in body movement
As the contraction cycle continues, movement of myosin heads applies the force that draws the Z discs toward each other, and the sarcomere shortens. During a maximal muscle contraction, the sarcomere can shorten by as much as half the resting length. The Z discs, in turn, pull on neighboring sarcomeres, shortening the whole muscle fiber, which ultimately leads to shortening of the entire muscle. As the fibers of a skeletal muscle start to shorten, they first pull on their connective tissue coverings (endomysium, perimysium, and epimysium) and tendons. The coverings and tendons become taut, and the tension passed through the tendons pulls on the bones to which they are attached. The result is movement of a part of the body.
T tubule
Small, cylindrical invaginations of the sarcolemma of striated muscle fibers (cells) that conduct muscle action potentials toward the center of the muscle fiber. This arrangement ensures that an action potential excites all parts of the muscle fiber at essentially the same instant.
Oxygen debt
The added oxygen, over and above the oxygen consumed at rest, that is taken into the body after exercise.
twitch contraction
The brief contraction of all the muscle fibers in a motor unit in response to a single impulse in its motor neuron.
Excitation-contraction coupling
the steps that connect excitation (a muscle action potential propagating along the sarcolemma and into the T tubules) to contraction (sliding of the filaments). An increase in Ca2+ concentration in the sarcoplasm starts muscle contraction; a decrease stops it. When a muscle fiber is relaxed, the concentration of Ca2+ in its sarcoplasm is very low. However, a huge amount of Ca2+ is stored inside the sarcoplasmic reticulum. When a muscle action potential propagates along the sarcolemma and into the T tubules, it causes Ca2+ release channels in the SR membrane to open. As these channels open, Ca2+ flows out of the SR into the sarcoplasm around the thick and thin filaments. The released Ca2+ combines with troponin, moving the troponin-tropomyosin complex away from the myosin-binding sites on actin. Once these binding sites are exposed, myosin heads bind to them, and the contraction cycle begins.
Muscular Junction
the synapse between a motor neuron and a skeletal muscle fiber
The contraction cycle
1) ATP splits: The myosin head includes an ATP-binding site and ATPase, an enzyme that splits ATP into ADP (adenosine diphosphate) and P (a phosphate group). This splitting reaction reorients and energizes the myosin head. Notice that ADP and a phosphate group are still attached to the myosin head. 2) Myosin attaches to actin: The energized myosin head attaches to the myosin-binding site on actin and releases the phosphate group. 3) Power stroke occurs: Binding of the myosin head to actin triggers the power stroke of contraction. During the power stroke, the myosin head rotates or swivels and releases the ADP. The myosin head generates force as it rotates toward the center of the sarcomere, sliding the thin filament past the thick filament toward the M line. 4) Myosin detaches from actin: At the end of the power stroke, the myosin head remains firmly attached to actin until it binds another molecule of ATP. As ATP binds to the ATP-binding site on the myosin head, the myosin head detaches from actin.
Sources of energy
1) Creatine phosphate: an energy-rich molecule that is found only in muscle fibers.Creatine phosphate is the first source of energy when muscle contraction begins.
Three ways ATP is produced
1) Creatine phosphate: formed from ATP while the muscle is relaxed, then releases ATP as the muscle contracts. 2) anaerobic cellular respiration: Muscle glycogen is broken down into glucose, which glycolysis converts to pyruvic acid and ATP. Without sufficient oxygen, pyruvic acid is converted to lactic acid. 3) aerobic cellular respiration: If adequate oxygen is available, mitochondria use pyruvic acid, fatty acids, and amino acids to produce additional ATP.
Sarcomere
A contractile unit in a striated muscle fiber extending from one Z disc to the next Z disc.
muscular dystrophy
A group of inherited muscle-destroying diseases that cause progressive degeneration of skeletal muscle fibers.
Sarcoplasmic reticulum
A network of saccules and tubes surrounding myofibrils of a muscle fiber (cell), comparable to endoplasmic reticulum; functions to reabsorb calcium ions during relaxation and to release them to cause contraction.
Synapse
A region where communication occurs between a neuron and another cell, and in this case between a motor neuron and a skeletal muscle fiber.
aponeurosis
A sheetlike fibrous connective tissue that resembles a flattened tendon that serves as a fascia to bind muscles together or as a means of connecting muscle to bone.
Muscle tone
A sustained, partial contraction of portions of a skeletal or smooth muscle in response to activation of stretch receptors or a baseline level of action potentials in the innervating motor neurons.
Myofibril
A threadlike structure, extending longitudinally through a muscle fiber (cell), consisting mainly of thick filaments (myosin) and thin filaments (actin, troponin, and tropomyosin). They are the contractile organelles of skeletal muscle.
skeletal muscle fiber relaxation and the resulting relaxation of an entire skeletal muscle
Calcium ion release channels in the SR close and calcium ion active transport pumps use ATP to restore low levels of calcium ions in the sarcoplasm, the troponin-tropomyosin complex slides back into position where it blocks the myosin-binding sites on the actin molecules, then the muscle relaxes. As the fibers of a skeletal muscle lengthen, their connective tissue coverings and tendons become slack, and tendons decrease their pull on the bones to which they are attached.
The order in which the energy sources are utilized as muscle contraction continues.
Creatine phosphate, anaerobic cellular respiration, aerobic cellular respiration
Muscle tension
Force exerted on an object by contracting or lengthening muscle
Cardiac muscle tissue
Found only in the heart, where it forms most of the heart wall. Like skeletal muscle, cardiac muscle is striated, but its action is involuntary—its alternating contraction and relaxation cannot be consciously controlled. It is regulated by the autonomic (involuntary) division of the nervous system and by hormones released by endocrine glands.
wave summation
If a second stimulus occurs before a muscle fiber has relaxed, the second contraction will actually be stronger than the first.
muscle Spasm
Involuntary contraction of a muscle or group of muscles, which become tightened, develop a fixed pattern of resistance, and result in a diminished level of functioning.
Cramp
Involuntary, painful muscle contraction caused by fatigue or strain
muscle cells
Myocytes or muscle fibers
the main protein component of thick filaments and thin filaments
Myosin is the main component of thick filaments, is a contractile protein that pushes or pulls various cellular structures to achieve movement by converting the chemical energy in ATP to the mechanical energy of motion, that is, the production of force. Thin filaments' main component is the contractile protein actin. Individual actin molecules join to form a thin filament that is twisted into a helix.
Z disc
Narrow, zigzag-shaped regions of dense protein material that separate one sarcomere from the next.
Functions and properties of muscle tissue
Produces body movements, stabilizes body positions, Moves substances within the body, and generates heat. Properties include electrical excitability (the ability to respond to certain stimuli by producing electrical signals called action potentials), contractility (the ability of muscle tissue to shorten forcefully when stimulated by an action potential), Extensibility (the ability of muscle tissue to stretch within limits, without being damaged), and Elasticity (the ability of muscle tissue to return to its original length and shape after contraction or extension).
the benefits for your body of aerobic training (aerobic exercise) and strength training (anaerobic training).
Regular, repeated activities such as jogging or aerobic dancing increase the supply of oxygen-rich blood available to skeletal muscles for aerobic cellular respiration. Aerobic training builds endurance for prolonged activities. Activities such as weight lifting rely more on anaerobic production of ATP through glycolysis. Such anaerobic activities stimulate synthesis of muscle proteins and result, over time, in increased muscle size (muscle hypertrophy). Anaerobic training builds muscle strength for short-term feats.
Muscle tension control
The total tension that a single fiber can produce depends on the rate at which impulses arrive at the neuromuscular junction. When considering the contraction of a whole muscle, the total tension it can produce depends on the number of muscle fibers that are contracting in unison.
The fate of acetylcholine following binding to its receptor
When impulses cease in the motor neuron, ACh is no longer released, and AChE rapidly breaks down the ACh already present in the synaptic cleft.
muscle hypertrophy
enlargement of existing muscle fibers
muscle atrophy
lack of muscle activity; reduces muscle size, tone, and power
Smooth muscle tissue
located in the walls of hollow internal structures, such as blood vessels, airways, and most organs in the abdominopelvic cavity. It is also attached to hair follicles in the skin. The action of smooth muscle is usually involuntary, and, like cardiac muscle, some smooth muscle tissue, such as the muscles that propel food through your gastrointestinal tract, has autorhythmicity. It is regulated by the autonomic (involuntary) division of the nervous system and by hormones released by endocrine glands.
Describe the arrangement of muscle fibers and connective tissue in a skeletal muscle
The hypodermis separates muscle from skin. It is composed of areolar connective tissue and adipose tissue, provides a pathway for nerves and blood and lymphatic vessels to enter and exit muscles, serves as an insulating layer that reduces heat loss, and protects muscles from physical trauma. Fascia (bandage) is a sheet or broad band of dense connective tissue that supports and surrounds muscles and other organs of the body. Fascia holds together muscles with similar functions; allows free movement of muscles; carries nerves, blood vessels, and lymphatic vessels; and fills spaces between muscles. Epimysium, the outermost layer of dense connective tissue, encircles the entire muscle. Perimysium is also a layer of dense, irregular connective tissue, but it surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles. Endomysium penetrates the interior of each fascicle and separates individual muscle fibers from one another. The endomysium is a thin sheath of areolar connective tissue.
Muscle fatigue
The inability of a muscle to contract forcefully after prolonged activity.
isotonic contractions
The tension (force of contraction) developed in the muscle remains constant while the muscle changes its length. Isotonic contractions are used for body movements and for moving objects.
Skeletal muscle tissue
the function of most skeletal muscles is to move bones of the skeleton. Skeletal muscle tissue works primarily in a voluntary manner; that is, its activity can be consciously (voluntarily) controlled by the somatic (voluntary) division of the nervous system.
eccentric isotonic contraction
when muscle lengthens
concentric isotonic contraction
when muscle shortens
isometric contractions
Tension increases greatly without a change in muscle length. The tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not shorten.
Motor unit recruitment
The process by which the number of contracting motor units is increased.
Muscle fiber excitation at the neuromuscular junction
1) Release of acetylcholine: An impulse travels from the brain or spinal cord along a motor neuron to the muscle fiber. Arrival of the impulse at the synaptic end bulb stimulates voltage-gated channels to open. Because calcium ions (Ca2+) are more concentrated in the extracellular fluid, Ca2+ flows inward through the open channels. The entering Ca2+ stimulates the synaptic vesicles to undergo exocytosis. During exocytosis, the synaptic vesicles fuse with the motor neuron's plasma membrane, releasing ACh into the synaptic cleft. The ACh then diffuses across the synaptic cleft between the motor neuron and the motor end plate. 2) Activation of ACh receptors: Binding of two molecules of ACh to a receptor on the motor end plate opens an ion channel in the ACh receptor. Once the channel is open, small cations, most importantly Na+, can flow across the membrane. 3) Generation of muscle action potential: The inflow of Na+ (down its electrochemical gradient) makes the inside of the muscle fiber more positively charged. This change in the membrane potential triggers a muscle action potential. The muscle action potential then travels along the sarcolemma and into the T tubules, stimulating the contraction process. 4) Termination of ACh activity: The effect of ACh binding lasts only briefly because ACh is rapidly broken down by an enzyme in the synaptic cleft called acetylcholinesterase (AChE). AChE breaks down ACh into acetyl and choline, products that cannot activate the ACh receptor.
Sarcolemma
The plasma membrane of a muscle fiber. Muscle action potentials travel along the sarcolemma and through the T tubules, quickly spreading throughout the muscle fiber.