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Ch.18 Skeletal Muscle: Structure and Function/Ch.19 Neural Control of…

- - - - The point of distal muscle attachment to the moving bone represents the insertion.
        
        A fascia of fibrous connective tissue, the epimysium, surrounds the entire muscle.
        
        The perimysium surrounds a bundle of up to 150 fibers called a fasciculus.
        
        The endomysium wraps each muscle fiber. Fibers are multinucleated.
        
        The sarcolemma, a thin, elastic membrane, covers the surface of each muscle fiber.
        
        A single muscle fiber contain smaller functional units called fibrils or myofibrils
        
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        Muscle fiber numbers probably remain largely fixed by the second trimester of fetal development.
    - - Tendons connect both ends of the muscle to the periosteum, the bone's outermost covering.
        
        The smallest molecule is the tropocollagen.
        
        Five tropocollagen molecules form the microfibril.
        
        Microfibrils unite to form collagen fibers.
        
        An endotendon encloses a bundle of fibers
        
        An epitendon sheath, known as a fascicle, surrounds a group of endotendons.
        
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    - - Within is the sarcoplasmic reticulum, a longitudinal latticelike network of tubular channels and vesicles. This highly specialized system provides structural integrity to the cell.
        
        It allows the wave of depolarization to spread rapidly from the fiber's outer surface to its inner environment through the T-tubule system to initiative muscle action.
        
        The sarcoplasmic reticulum that surrounds each myofibril contains biologic "pumps" that take up Ca2+ from the fiber's sarcoplasm.
        
        The produces a calcium concentration gradient between the sarcoplasmic reticulum (higher [Ca2+]) and the sarcoplasm surrounding the filaments (lower [Ca2+]).
  - - - The remaining 5% contains salts and other substances, including high-energy phosphates; urea; lactate; minerals calcium, magnesium and phosphorus; various enzymes; sodium, potassium and chloride ions; amino acids; fats and carbohydrates.
        
        The most abundant muscle proteins include titan, the largest protein in the body (10% of muscle mass), myosin (60% of muscle protein), actin and tropomyosin.
        
        Each 100g of muscle tissue contains about 700mg of the oxygen-binding, conjugated protein myoglobin.
  - - - These vessels divide into numerous arterioles, capillaries and venules to form a diffuse network in and around the endomysium.
        
        Flow decreases during the muscle's contraction phase and increases during relaxation to provide an auxiliary "milking action" that moves blood through the muscles and propels it via the venous system back to the heart.
        
        When a muscle generates about 60% of its force-generating capacity for several seconds, elevated intramuscular pressure occludes local blood flow during the contraction.
        
        With sustained high-force contraction, the intramuscular high-energy phosphates and glycolytic anaerobic reactions provide the main energy source for muscular effort.
        
        Trained muscles' increased capillary-to-muscle fiber ratio helps to explain improved exercise capacity with endurance training.
        
        An enhanced capillary microcirculation expedites removal of heat and metabolic byproducts from active tissues in addition to facilitating delivery of oxygen, nutrients and hormones.
        
        A positive association also exists between VO2max and the average number of muscle capillaries.
        
        Vascular stretch and shear stress on the vessel walls from increased blood flow during exercise stimulate capillary development with intense aerobic training.
  - - - Differences in sarcomere alignment and length strongly affect a muscle's force and power generating capacity.
        
        Fusiform or spindle-shaped fibers run parallel to the muscle's long axis (eg, biceps brachii) and taper at the tendinous attachment.
        
        Pennate or fan-shaped fibers' fasciculi lie at an oblique pennation angle that varies up to 30 degrees.
        
        No pennated fibers run the full length of the muscle.
        
        In pennate muscle, the complex arrangement of connective tissue, tendons and relatively short fibers creates a larger cross-sectional area than fusiform fibers because more sarcomeres "pack" into a given volume of muscle.
        
        An unusually large pennation angle of 30 degrees results in only a 13% loss in an individual fiber's force capacity, this makes for a huge increase in total fiber-packing ability.
        
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        The term physiologic cross-sectional area (PCSA) refers to the total cross-sectional areas of all fibers within a particular muscle.
        
        A fusiform fiber has no pennation, so the fiber's cross-sectional area represents the true anatomic cross section.
        
        Fusiform muscle fibers run parallel to a muscle's long axis.
        
        In this case, fiber length equals muscle length, and a fiber's force generation transmits directly to the tendon.
        
        This arrangement facilitates rapid muscle shortening.
        
        The complex parallel muscle, also called series-fibered muscle features individual fibers that run parallel to the muscle's line of pull.
        
        Unlike simple fusiform arrangements where a fiber runs the entire muscle length, the complex parallel arrangement features muscle fibers that terminate in the muscle's mid belly and taper to interact with the connective tissue matrix or adjacent muscle fibers.
        
        The quadriceps and plantar flexors exhibit high force production from their low fiber length-to-muscle length (FL:ML) ratios and relatively large physiologic cross-sectional areas (PCSA).
        
        In contrast, the hamstrings and dorsiflexor muscles show architecture designed for high contractile velocity from their relatively high FL:ML and long FL.
        
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- - - - The theory proposes a muscle shortens or lengthens because thick and thin filaments slide past each other without changing length. The myosin cross bridges cyclically attach, rotate, and detach from the actin filaments with energy from ATP hydrolysis and provide the molecular motor to drive fiber shortening.
        
        During muscle shortening there is no change in the width of the A band (which is the length of a single myosin thick filament).
        
        The A band widens in an eccentric muscle action as the fiber lengthens during force generation.
        
        The cyclic, oscillating to-and-fro motion of crossbridges move like oars knifing through water.
        
        But unlike oars, crossbridges do not all move synchronously. If they did, muscle action would produce a series of uneven and jerky actions instead of finely graded, smoothly modulated movements and force outputs.
        
        The optimal sarcomere length for maximum isometric tension is where the maximum actin and myosin filament interaction.
        
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- - - - The fast-twitch fiber's intrinsic speed of shortening and tension development ranges three to five times faster than slow-twitch fibers.
        
        Studies show that human skeletal muscle contains type I, type IIa, type IIx and a new type IIb subtype.
        
        Type IIa fiber exhibits fast shortening speed and a moderately well-developed capacity for energy transfer from both aerobic and anaerobic sources.
        
        Type IIa represent the fast-oxidative-glycolytic (FOG) fibers.
        
        Type IIb fiber possesses the greatest anaerobic potential and most rapid shortening velocity; it represents the "true" fast-glycolytic (FG) fiber.
        
        A type IIx fiber falls midway between its a and b counterparts in physiologic and metabolic characteristics .
  - - - Slow-twitch fibers receive their characteristic red pigmentation from a rich mitochondrial supply and accompanying iron-containing cytochromes combined with high myoglobin levels.
        
        The fibers have been labeled SO, or slow-oxidative, to describe their slow shortening speed and reliance on oxidative metabolism.
        
        Muscle glycogen depletion patterns indicate that prolonged, intense aerobic activity demands almost exclusive reliance on slow-twitch muscle fibers.
        
        Even after exercising for 12hr, the limited glycogen remaining in active muscle exists mostly in the relatively "unused" fast-twitch fibers.
        
        Differences in oxidative capacity between the two fiber types also determine the magnitude of blood flow through muscle, with slow-twitch fibers receiving the largest quantity.
  - - - Generally, the trend in one's muscle fiber type distribution remains consistent among the body's major muscle groups.
        
        Successful endurance athletes possess predominantly slow-twitch fibers in the major muscles activated in their sport. In contrast, fast-twitch fibers predominate for elite sprint athletes.
        
        As might be expected, men and women who perform in middle-distance events display approximately equal percentages of the two fiber types. The same distribution also occurs in power athletes; throwers, jumpers and high jumpers.
        
        There is a correlation between higher VO2max and greater percentages of ST fibers.
        
        Larger muscle fibers in males athletes and a larger total muscle mass characterizes the principal gender differences in muscle morphology.
- - - - For example, physical force generated within a muscle fiber may release the transcription factor serum response factor (SRF) from the structural muscle protein titan, leading to increased muscle growth.
  - - - Titin plays an important role in muscle force regulation, particularly for eccentric or active lengthening muscle actions.
- - - - Brain
        
        Following adolescence, the plasticity of neuronal addition and formation of new circuits slows but does not stop, even into older age.
        
        Regular physical activity contributes to the maintenance and development of optimal neural circuitry with aging.
        
        The cerebellum functions by means of intricate feedback circuits to monitor and coordinate other areas of the brain and spinal cord involved in motor control.
        
        Movement tasks first learned by trial and error, like riding a bicycle or swinging a golf club at a particular cadence, remain coded as coordinated patterns in the cerebellar memory banks.
        
        The hypothalamus, situated below the thalamus, regulates metabolic rate and body temperature. The hypothalamus also influences autonomic nervous system activity.
      - The Spinal Cord
        
        Encased by 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 coccygeal).
        
        The spinal cord core contains principally three types of neurons: motor neurons, sensory neurons and interneurons.
        
        Ascending Nerve Tracts
        
        Ascending nerve tracts in the spinal cord forward sensory information from peripheral receptors to the brain for processing.
        
        Peripheral sensory nerve endings serve as specialized receptors to detect conscious and subconscious sensory information.
        
        The "conscious" receptors show sensitivity to kinesthesia (detection of body position, weight, or movement of muscles, tendons, and joints) and proprioception (sense of the relative position of body parts and magnitude of effort applied in movement), temperature and sensations of light, sound, smell, taste, touch and pain.
        
        Receptors also monitor subconscious changes in the body's internal environment; the include chemoreceptors that respond to changes in blood gas tension and pH and baroreceptors that react almost instantaneously to any change in arterial blood pressure.
        
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        Descending Nerve Tracts
        
        Axons from the brain move downward through the spinal cord along two major pathways....
        
        The pyramidal (lateral) tract activates the skeletal musculature in voluntary movement under direct cortical control. And ....
        
        The extrapyramidal (ventromedial) tract, controls posture and muscle tone via the brainstem.
        
        The tracts of the extrapyramidal tract control posture and provide a continual background level of neuromuscular tone.
        
        The tracts of the pyramidal tract eventually excite the alpha motor neurons that control and modulate the fine and gross properties of skeletal muscles during all purposeful movements.
        
        One tract, the corticospinal tract has two-thirds of its axons originating from the motor cortex.
        
        The Reflex Arc
        
        Afferent neurons transmit sensory input from peripheral receptors. These neurons interconnect or synapse in the cord through interneurons that relay information to different cord levels.
        
        The impulse then passes over the motor root pathway via anterior motor neurons to the effector organ, the muscle.
        
        An example of a simple reflex occurs when one suddenly but unexpectedly touches a hot object. Stimulation of pain receptors located in the skin of the fingers transmits sensory information over afferent fibers to the spinal cord.
        
        This activates efferent motor fibers to elicit an appropriate muscular response by immediately jerking the hand away.
        
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    - - The peripheral nervous system contains 31 pairs of spinal nerves and 12 pairs of cranial nerves.
        
        The peripheral nervous system includes afferent neurons that relay sensory information from receptors in the periphery toward the CNS and...
        
        Efferent neurons (somatic and autonomic) that transmit information away from the brain to peripheral tissues.
        
        Somatic nerve fibers, also called motor neurons or motoneurons, innervate skeletal muscle.
        
        Autonomic nerves, also called visceral, involuntary, or vegetative nerves, activate cardiac muscle, sweat and salivary glands, some endocrine glands, and smooth muscle cells, also called involuntary muscle, in the intestines and walls of blood vessels.
        
        Whereas tissues of the heart and viscera display considerable autonomic excitability, conscious control also affects these tissues.
        
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- - - - Each muscle fiber generally receives input from only one neuron, yet a motor neuron may innervate many muscle fibers because the terminal end of an axon forms numerous branches.
        
        The motor neuron pool describes the collection of alpha motor neurons that innervate a single muscle.
        
        There are four different nerve fiber groups that exist based on size and thus transmission velocity (myelinated fibers transmit conduction faster and the thicker the myelination the faster the transmission.): 1) A-alpha, 2) A-beta, 3) A-delta, 4) C-nerve (from thickest/fastest to thinnest/slowest).
        
        The neuromuscular junction (NMJ) or motor endplate represents the interface between the end of a myelinated motor neuron and muscle fiber.
        
        The phenomenon of neural facilitation known as disinhibition affects neurons within the CNS rather than electro-chemical events at the NMJ because the NMJ does not release inhibitory neurotransmitters.
        
        Three factors produce neuronal facilitation: 1) Decreased sensitivity of the motor neuron to inhibitory neurotransmitters, 2) reduced quantity of inhibitory neurotransmitter substances transported to the motor neuron, 3) Combined effect of both mechanisms
        
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  - - - Motor units are classified based on three physiologic and mechanical properties of the muscle fibers they innervate: 1) twitch characteristics, 2) tension characteristics, 3) fatigue resistance
        
        The three common categories based on twitch type are: 1) Fast twitch, high force and fast fatiguable, 2) Fast twitch, moderate force, and fatigue resistant (type IIa), 3) Slow twitch, low force and fatigue resistant (type I)
        
        "Phasic" motor neurons fire rapidly with short bursts while "tonic" motor neurons fire slowly but continuously.
        
        With prolonged aerobic training, fast-twitch fibers become almost as fatigue resistant as their slow-twitch counterparts.
        
        Surgically innervating fast-twitch muscle fibers with the neuron from a slow-twitch motor unit eventually alters the twitch characteristics of the fast-contracting fibers.
        
        A stimulus strong enough to trigger an action potential in the motor neuron activates all of the accompanying muscle fibers in the motor unit to contract synchronously.
        
        A motor unit does not exert a force gradation, either the impulse elicits an action or it does not.
        
        All fibers of the motor units react simultaneously ("all or none" principle)
        
        The force of graded muscle action varies from slight to maximal via two mechanisms: 1) increased number of motor units recruited, 2) increased frequency of motor unit discharge
        
        Motor unit recruitment describes adding motor units to increase muscle force.
        
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        Fatigue represents the decline in muscle tension or force capacity with repeated stimulation or during a given time period.
        
        Voluntary muscle actions exhibit four main components listed in the following order of nervous system hierarchy:
        
        1) Central nervous system; exercise-induced alterations in levels of CNS neurotransmitters, ACh in various brain regions, neuromodulators (which alter one's psychic or perceptual state to disrupt physical ability.)
        
        2) Peripheral nervous system; Reduced glycogen content of active muscle fibers relates to fatigue during prolonged intense activity. This "nutrient fatigue" occurs with sufficient oxygen available for aerobic pathways. Depletion of PCr and adenine nucleotide pool (ATP, ADP, AMP) also accompanies the fatigue state in prolonged submaximal effort.
        
        3) Neuromuscular junction; lack of oxygen and increased level of blood and muscle lactate relate to muscle fatigue in short-term, maximal exertion. The dramatic increase in H+ in the active muscle dramatically disrupts the intracellular environment. (There are other factors that alter contractile function win anaerobic physical activity.)
        
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- - - - Proprioception allows continual monitoring of the progress of any sequence of movements and serves to modify subsequent motor behavior.
  - - - When the muscle stretches, the spindles also stretch.
        
        Muscles that perform complex movements have more spindles per gram of muscle than those that perform gross movements.
        
        The spindle, covered by a sheath of connective tissue, contains two specialized types of muscle fiber called intramural fibers.
        
        The ends of the intrafusal fibers contain actin and myosin filaments and exhibit shortening capability .
        
        Two sensory afferent fibers and one motor efferent fiber innervate the spindles.
    - - The stretch reflex consists of three main components: 1) Muscle spindle that responds to stretch, 2) Afferent nerve fiber that carries the sensory impulse from the spindle to the spinal cord, 3) Efferent spinal cord motor neuron that activates the stretched muscle fibers.
  - - - When stimulated by excessive tension, the GTO receptors transmit signals to the spinal cord to elicit reflex inhibition of the muscles they supply.
        
        Excessive change in muscle tension increases the GTO sensor's discharge to depress motor neuron activity and reduces force output.