Term
| Describe the different levels of muscle structure, from outer to inner: |
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Definition
| Fascia- Outer covering of muscle; Skeletal muscle- bundle of fascicles; Perimysium- connective tiisue sheath surrounding fascicles; Muscle fiber- bundled together to form a fascicle; Endomysium- thin connective tissue sheath surrounding each muscle fiber within a fascicle; Myofibril- subunit of skeletal muscle fiber, with striations (dark A bands and light I bands); Myofilaments- subunits of myofibril, composed of thick myosin and thin actin filaments |
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Term
| Describe the actions of skeletal muscles: |
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Definition
Extensor- Increases angle at a joint
Flexor- Decreases angle at a joint
Abductor- Moves limb away from midline of body
Adductor- Moves limb toward midline of body
Levator- Moves insertion upward
Depressor- Moves insertion downward
Rotator- Rotates bone along its axis
Sphincter- Constricts an opening |
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Term
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Definition
| Each somatic motor axon branches to innervate numerous muscle fibers; the motor neuron and the fibers it innervates are motor units. |
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Term
| Explain the significance of recruitment of motor units: |
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Definition
| Fine neural control over the strength of muscle contraction is optimal when there are many small motor units involved. (ex. extraocular muscles of the eyes- 1:23 innervation ratio for neurons to fibers- Innervation ratio of the gastrocnemius is between 1:100 and 1:2000. the gastrocnemius is more powerful at the expense of fine neural control). a neuron that innervates a small number of fibers has a smaller cell body, and is stimulated by lower levels of excitatory input than a larger neuron that innervates a greater number of fibers. When contractions of greater strength are required, larger and larger motor units are activated. This is recruitment. |
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Term
| Describe the banding pattern of a myofibril: |
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Definition
| Each myofibril is striated, with dark (A) bands and light (I) bands. in the middle of each I band are Z lines. The A bands are composed of thick myosin filaments, and the edges of each band also contain thin filaments which overlap the thick filaments. The central region of the A bands contain only thick filaments- these regions are the H bands. The I bands contain only thin actin filaments, composed of globular subunits known as G-actin, as well as the protein tropomyosin (located at intervals in the thin filaments). The protein troponin is attached to the tropomyosin. |
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Term
| Explain how the A and I bands change length during a muscle contraction: |
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Definition
| As sarcomeres shorten in length, the A bands do not shorten, but move closer together. The I bands (which represent the difference between A bands of successive sarcomeres) decrease in length. In the process of contraction, The I bannds on either sides of the A bands slide deeper and deeper toward the center, producing increasing amounrs of overlap with the thick filaments. The I bands and H bands get shorter during muscle contraction. |
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Term
| Briefly explain the cross-bridge cycle: |
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Definition
| 1) The myosin head is activated by the splitting of ATP into ADP and Pi. This bonds the myosin head to actin, forming a cross-bridge between the thick and thin filaments. 2) After the Pi group leaves the cross-bridge, the myosin head changes its orientation, producing the power stroke that moves the actin forward. |
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Term
| Explain the cross-bridge cycle: |
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Definition
| 1) In resting fiber, the cross-bridge is not attached to actin. 2) The crossbridge binds to actin. 3) Pi is releasesd from ADP and Pi, which causes conformation al change in myosin. 4) ADP is released. 5) A new ATP binds to myosin head, allowing it to release from actin 6) ATP is hydrolyzed, causing crossbridge to return to its original orientation. |
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Term
| Explain the sliding filament model of contraction: |
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Definition
| 1) Shortening of myofibrils is caused by the shortening of sarcomeres- the distance between z discs is reduced 2) Shortening of sarcomeres is accomplished by sliding of the myofilaments- the length of each filament remains the same during the contraction. 3) Sliding of filaments is accomplished by asynchronous power strokes of myosin cross-bridges, which pull the thin actin filaments over the thick myosin filaments. 4) The A bands remain the same length during contractions, but are pulled toward the origin of the muscle. 5) Adjacent A bands are pulled together as I bands between them shorten 6) The H bands shorten during contraction as thin filaments on the sides of the sarcomeres are pulled toward the middle. |
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Term
| Explain excitation-contraction coupling in skeletal muscles: |
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Definition
| 1) ACh released by somatic motor neurons binds to nicotinic ACh receptors in the sarcolemma, causing a depolarization that stimulates 2) voltage-gated channels, producing action potentials. 3) The conduction of action potentials along with the transverse tubules stimulates the opening of voltage-gated Ca2+ channels. 4) The channels in the transverse tubules are mechanically coupled to Ca2+ channels in the sarcoplasmic reticulum, causing them to open. Ca2+ then diffuses out of the sarcoplasmic reticulum, so that it can bind to troponin and stimulate muscle contraction. |
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Term
| Distinguish between twitch, summation and tetanus: |
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Definition
Twitch- when a muscle is stimulated and produces a single contraction and relaxation.
Summation- When the graded strength of a muscle contraction is produced by a subsequent stimulation- a second twitch, that rides "piggyback" on the first.
Incomplete tetanus- When the stimulus increases the frequency of stimulation, the relaxation time between successive twitches becomes shorter and shorter as the strength increases in amplitude.
Complete tetanus- When there is a "fusion frequency" of stimulation and there is no visible relaxation between successive twitches. If stimulation is continued, muscle will fatigue. |
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Term
| Distinguish between isotonic, isometric, and eccentric contractions: |
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Definition
Isotonic- When the tension produced by the shortening muscle is just greater than the force at each value, causing the muscle to shorten
Isometric- When the force opposing the contraction of the muscle is so great that the muscle can't shorten
Eccentric- When the force exerted on a muscle is greater than the force of muscle contraction. The muscle will lengthen despite its contraction. |
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Term
| Identify the series-elastic component: |
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Definition
| This refers to the elastic compostion of the muscle and its associated structures which must be stretched tight before the tension exerted by the muscle can cause movement. |
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Term
| Explain the length tension relationship in striated muscles: |
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Definition
| The strength of a muscle contraction is dependent upon its resting length. If the muscle is too short or too long prior to stimulation, the filaments in the sarcomeres will not have optimum amount of overlap. At its resting length in vivo (within a living organism, as compared to in vitro, which is outside a living organism such as in a lab), a muscle is at its optimum length for contraction. |
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Term
| Distinguish between the different types of skeletal muscle fibers: |
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Definition
Slow-twitch red fibers- (Type I) Adapted for aerobic respiration and are resistant to fatigue.
Fast-twitch white fibers- (Type II) Adapted for anaerobic respiration.
Intermediate fibers- fast-twitch, but adapted for aerobic respiration, subdivided into type IIA (fast oxidative) and type IIX fibers. |
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Term
| Describe aerobic capacity: |
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Definition
| the maximum rate of oxygen consumption (by areobic respiration) in the body. It is often expressed in the abbreviated form as the VO2 max. |
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Term
| Describe lactate threshold: |
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Definition
| The percentage of the maximal oxygen uptake at which a significant rise in blood lactate levels occur. |
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Term
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Definition
| An exercise-induced reduction in the ability of a muscle to generate force. It may be produced by an accumulation of extracellular K+ as a result of nerve activity. Also can be produced by anaerobic respiration by fast-twitch fibers; production of lactic acid and consequent fall in pH. Depletion of muscle glycogen and other metabolic changes interfere with CA2+release from the sarcoplasmic reticulum, which interfere with excitation-contraction coupling rather than depletion of ATP appears to be responsible for muscle fatigue. Fatigue is often caused by changes in the CNS before the muscles themselves fatigue (central fatigue). This reduces the force of voluntary contractions. |
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Term
| How does exercise training affect skeletal muscles? |
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Definition
| Endurance training increases the aerobic capacity of muscle fibers and their use of fatty acids for energy, so their reliance on glycogen and anaerobic respiration- and thus susceptibility to fatigue- is reduced. Resistance training causes hypertrophy of muscle fibers because of an increase in the size and number of myofibrils. |
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Term
| Describe the components of monosynaptic muscle stretch reflexes: |
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Definition
| 1) Passive stretch of a muscle (produced by tapping its tendon) stretches the spindle (intrafusal fibers). 2) Stretching of a spindle distorts its central (bag or chain) region, which stimulates dendritic endings of sensory nerves. 3) Action potentials are conducted by afferent (sensory) nerve fibers into the spinal cord on the dorsal roots of spinal nerves. 4) Axons of sensory neurons synapse with dendrites and cell bodies of somatic motor neurons located in the ventral horn grey matter of the spinal cord. 5) Efferent nerve impulses in the axons of somatic motor neurons (which form the ventral roots of spinal nerves) are conducted to the ordinary (extrafusal) muscle fibers. These neurons are alpha motoneurons. 6) Release of ACh from the endings of alpha motoneurons stimulates the contraction of the extrafusal fibers, and thus of the whole muscle. 7) Contraction of the muscle relieves the stretch of its spindles, thus decreasing activity in the spindle afferent nerve fibers. |
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Term
| Describe the effects of Golgi tendon organs: |
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Definition
| Golgi tendon organs continuously monitor tension in the tendons produced by muscle contractions, or passive stretching of a muscle. ex. 1) Tension on a tendon activates a sensory neuron, 2) this causes the sensory neuron to stimulate an interneuron. 3) The interneuron then inhibits the alpha motoneuron, 4) which causes tension on the tendon to be reduced. |
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Term
| Explain reciprocal innervation of skeletal muscles: |
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Definition
| This is when the sensory neuron that stimulates the motor neuron of a muscle also stimulates interneurons within the spinal cord via collateral branches. These interneurons inhibit the motor neurons of antagonist muscles via IPSPs. ex. 1) Muscle stretch activates spindle apparatus. 2) The agonst muscle contracts in stretch reflex. 3) This causes the antagonist muscle to relax. |
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Term
| Describe the characteristics of cardiac muscle, and how these compare to skeletal muscle: |
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Definition
| Like skeletal muscles, they are striated and contract by means of the sliding filament mechanism. Long, fibrous skeletal muscle cells are structurally and functioally seperate from each other, myocardial cells are short, branched and interconnected. Myocardial cells are electricaly connected by intercalated discs (gap junctions) and contracts to its full extent as a unit, whereas skeletal muscles are graded, depending on the number of cells stimulated. Skeletal muscles require external stimulation by somatic motor nerves before they produce action potentials and contract, cardiac muscle cells contract automatically with action potentials originating in the pacemaker region, which is regulated by autonomic innervation. Also, in skeletal muscles, there is direct excitation-contraction coupling between the transverse tubules and the sarcoplasmic reticulum, in myocardial cells the voltage-gated Ca2+ channels in the plasma membrane and the Ca2+ release channels do not directly interact. Instead, the Ca2+ that enters the cytoplasm through the voltage-gated Ca2+ channels in the transverse tubules stimulates opening of the the Ca2+ release channles in the sarcoplasmic reticulum. |
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Term
| Describe the structure of smooth muscle: |
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Definition
| They do not contain sarcomeres, and contain a great deal of actin and some myosin (a 16:1 ratio). The thin filaments are very long, and attach to regions of the smooth muscle plasma membrane, or to dense bodies, which are analogous to Z discs. Smooth muscles are arranged in circular layers or longitudinals layers. Myosin proteins are stacked perpendicular to the long axis of the thick filaments, so they can bind to actin all along the length of the thick filament. Depolarizations are graded, and conducted from one smooth muscle to the next. |
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Term
| How are smooth muscle contractions regulated? |
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Definition
| Through excitation-contraction coupling; Ca2+ passes through voltage gated channels in the plasma membrane as it enters the cytoplasm and binds to calmodulin. The calmodulin-Ca2+ complex then binds with myosin light-chain kinase (MLCK) by removing a phosphate group. The activated MLCK in turn, phosphorylates the myosin light-chains, thereby activating the crossbridges to cause contraction. Contraction is ended when myosin phosphatase becomes activated. Upon its activation, myosin phosphatase removes the phosphates from the myosin light-chains and thereby inactivates the crossbridges. |
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