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This 10 page Class Notes was uploaded by Hayoung Lee on Friday October 7, 2016. The Class Notes belongs to BIO 2430 at Texas State University taught by T. Prabhakaran in Fall 2016. Since its upload, it has received 4 views. For similar materials see Human Anatomy and Physiology in Biomedical Sciences at Texas State University.
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Date Created: 10/07/16
Anatomy and Physiology 09.28.16 MUSCULAR SYSTEM Study of muscle is Myology Branch of medical science that deals with treating disorders of the musculoskeletal system is Orthopedics Primary function of muscles is to bring about movement Muscles help generate body heat Muscles help stabilize joints, and also help maintain body posture Smooth muscle sphincters of visceral organs help in storage function of organs like stomach and urinary bladder Muscle cells are called muscle fibers. o Muscle fibers convert chemical energy derived from food molecules into mechanical energy (energy of motion) o All muscle fibers contract and relax. Contraction is energy dependent (active process), while relaxation is passive, and does not use energy. o The force generated when the muscle fiber contracts is called muscle tension. When muscle tension is greater than the load, movement occurs Myo and Sacro refer to a muscle fiber. o Sarcoplasm means the cytoplasm of the muscle fiber o Sarcolemma means the plasma membrane of a muscle fiber. o Myocardium means heart muscles 3 types of muscle tissues are: o 1. Skeletal muscle tissue (striated) – forms the skeletal muscles, known as the voluntary muscles; muscle fibers are multinucleate, highly elongated, and have plenty of myoglobin o 2. Smooth muscle tissue (non-striated) – forms the smooth or visceral muscles known as the involuntary muscles o 3. Cardiac muscle tissue – forms the heart muscles (myocardium); in appearance it is like skeletal muscle tissue, but functionally it is like the smooth muscle tissue. Cardiac muscle fibers are striated, but are thicker, shorter, branched and interconnected by intercalated disks. Skeletal muscle is an organ with thousands of skeletal muscle fibers, bundled into muscle fascicles. o Each skeletal muscle fiber is covered over by a fine connective tissue called endomysium. o Each muscle fascicle is covered over by a perimysium and the entire muscle is covered over by a thick protective covering (dense irregular connective tissue) called epimysium. Anatomy and Physiology o All these connective tissues are joined to a tendon (dense regular connective tissue made of collagen fibers) which attaches muscle to bone o There will also be a broad band of connective tissue lining the body wall called fascia, which supports the skeletal muscle o Each skeletal muscle fiber has a neuromuscular junction (NMJ). This is a junction between a motor nerve ending and the sarcolemma of a skeletal muscle fiber Electrical signals called action potentials from the motor neuron (spinal cord or brain) reaches the skeletal muscle fiber through the NMJ and initiate the muscle contraction The motor nerve ending called the synaptic end bulb has synaptic vesicle that contain a neurotransmitter called Acetylcholine (Ach). o The gap between the synaptic end bulb and the sarcolemma is called synaptic cleft, which contains synaptic fluid o The sarcolemma at the junction called the motor endplate, is folded up and has receptor for Ach When a nerve signal reaches the NMJ, Ach is released, which passes through the synaptic fluid and binds with the receptor on the motor endplate o The binding action opens up sodium channels and allows sodium ions to enter the skeletal muscle fiber, causing the muscle fiber to generate its own electrical signal (action potential). o This electrical signal travels along the sarcolemma and initiates the contraction of the skeletal muscle contraction As soon as Ach binds with the receptor an enzyme called AChE (Acetycholinestase) degrades (breaks up) Ach Each skeletal muscle fiber has a highly specialized organelle called sarcoplasmic reticulum (SR) o This represents the modified smooth endoplasmic reticulum and it is for storing calcium ions inside the skeletal muscle fiber The sarcolemma of the skeletal muscle fiber forms tube like extensions into the interior of the skeletal muscle fiber, and these are called the t-tubules transeverse tubules) o Electrical signals from NMJ reach the SR through the t-tubules Skeletal muscle fibers are organized into functional groups called motor units in every skeletal muscles o Each motor unit consists of one motor neuron and all the skeletal muscle fibers that it stimulates to initiate muscle contraction Anatomy and Physiology o Some motor units are small and will have only 10 or 20 muscle fibers per motor neurons (like eye muscles that require very precise movements) o Others are large with 2000 to 3000 skeletal muscle fibers per motor neuron (like biceps, quadriceps, which generate powerful contractions) Microscopic structure of a skeletal muscle fiber o Inside each skeletal muscle fiber are rod-shaped structures called myofibrils, that are arranged parallel to one another o Each myofibril has alternating patches of dark and light bands, which give the typical striations to a skeletal muscle fiber 10.03.16 o Each myofibril is made up of myofilaments (protein filaments) called thick and thin filaments Thick filaments occupy the dark band (A-band) Thin filaments occupy the light band (I-band) and extend to the A-band In the middle of the A-band is the H-zone, which has only the thick filament Thin filaments in the I-band are interconnected by a thick protein band called the Z-disk The area between the two z-disks is called a sarcomere, which is the functional unit of contraction of a skeletal muscle fiber o Thick filaments are made of myosin protein Each myosin molecule has a long double stranded tail and two projections called the myosin heads Thin filaments are made of 3 proteins: actin, troponin, and tropomyosin Actin molecules join to form a double stranded, thin actin filament which has myosin binding sites where myosin heads can attach Tropomyosin is a band of protein that blocks the binding sites on actin when the muscle fiber is at rest Troponin molecules hold the tropomyosin in place when the muscle fiber is at rest, but when the muscle fiber is stimulated, troponin combines with calcium ions (Ca2+), and displaces tropomyosin, and exposes the binding sites on actin, leading to cross bride activity Skeletal muscle contraction o Calcium ions and energy from ATP (adenosine triphosphate) are necessary for the muscle fiber to contract o when the nerve signal reaches the NMJ, Ach from the synaptic vesicles bind with the receptor on Motor endplate, producing Anatomy and Physiology action potentials that reach the sarcoplasmic reticulum (SR) through the t-tubules o SR then releases calcium ions into sarcoplasm o Calcium ions then combine with troponin and displaces tropomyosin from the binding sites on actin, leading to cross- bridge activity Myosin head splits ATP to form ADP and Phosphate (P), and using the energy from ATP, attaches to the actin filament. o The cross bridge then rotates toward the H-zone and releases ADP This rotation or swiveling of the cross bridge is called the power stroke as it pulls the actin filament towards the H- zone The thin filament then begins to slide past the thick filament towards the center of the sarcomere Myosin head then picks up another ATP and detaches from the actin filament When the thin filament slides towards the H-zone, the sarcomere becomes shorter, I-band is reduced, and the H-zone disappears, but thick and thin filaments do not change in length When nerve signals stop at NMJ, Ach is degraded by AChE, thereby stopping the action potentials at the motor end plate o Calcium ions are then taken back into SR by active transport o As the calcium ion content in sarcoplasm declines, troponin gives up calcium and takes up the resting position o Tropomyosin then blocks the binding sites on actin, and prevent cross bridge activity; then muscle relaxes SK muscle (organ) muscle fascicle muscle fibers myofibrils myofilaments (thick [myosin filament] and thin [actin filament]) CONTROL OF MUSCLE TENSION A skeletal muscle is capable of generating different levels of force (tension) according to the needs of the body. I f only a few motor units are activated in a muscle, only a small force is generated; if all the motor units are activated at the same tie, the same muscle will produce considerably larger force. This process of incorporating more and more motor units in muscle response is called motor unit recruitment Anatomy and Physiology A second factor involved in graded muscle response is stimulus frequency. o The force of contraction of a muscle increases with increasing frequency of nerve signals reaching the NMJ of a skeletal muscle fiber A muscle twitch (twitch contraction) is a single response of a muscle to a single stimulus o In a myogram tracing of a muscle twitch, 3 phases can be seen: 1. Latent period – a very short period (2-3 millisecond) during which the muscle appears to show no response externally, but internally, it is the time when the nerve signal from the motor end plate reaches the SR, casing Calcium ion release, and the displacement of Tropomyosin from the active binding sites on actin which leads to cross- bridge activity The muscular responds to the stimulus by releasing CA^2+ from SR 2. Period of Contraction: the time of repeated cross- bride activity which results in muscle tension or force. If the tension is greater than the load, the movement occurs, the muscle becomes shorter cross bridge activity, sliding movement, 3. Period of Relaxation: the time when calcium ions are withdrawn (taken back) into SR from sarcoplasm; tropomyosin covers the binding sites and the muscle goes back to the resting stage Ca2+ are withdrawn into SR, tropomyosin will block, biding sit on actin filament o Wave summation and tetanus: If two equal stimuli are applied to a muscle, one after the other very rapidly, the second response (wave) appears to be larger than the first nd This is because the 2 stimulus hits the muscle before it has completed its first relaxation period (ex. The 2nd stimulus is applied before calcium withdrawal into SR is completed) so the 2 ndresponse involves greater cross- bridge activity resulting in greater tension This “adding up” of tension is called wave summation If the stimulus frequency goes up to 20-30 times per second, the muscle shows a wave like pattern of contraction in which each wave is larger than the previous one, known as unfused tetanus (incomplete tetanus) o If the muscle is stimulated at a rate of 80-100 times per second, the muscle remains in a state of sustained contraction Anatomy and Physiology during which the muscle does not relax at all, being known as fused (complete) tetanus o Our skeletal muscles do not work in “twitches,” but they build up the force of tension through incomplete and complete tetanic contractions. Remember, both the number of motor units involved as well as the frequency of stimuli play an equal role in changing the force of contraction of skeletal muscles o When you use a muscle constantly, it gets thicker and stronger. This is because of the increase in the number of myofibrils inside the muscle fiber as the muscle is subjected to greater stress. Skeletal muscle fibers do not increase in number by mitosis, as they are multinucleate fibers Muscle Metabolism o Like all other cells in the body, muscle fibers also rely on ATP molecules for energy When they need energy, ATP is broken down to form ADP and Phosphate (P), releasing usable chemical energy for the cell ATP + (water)(ATPase) ADP + P + chemical energy (ADP is later combined with P, using energy from food molecules to form new ATP o Stored ATP in a skeletal muscle fiber can support muscle contraction only for about 4-6 seconds. But our skeletal muscles very often work for much longer periods and so skeletal muscle fibers re-build ATP molecules from ADP using 3 separate processes: 1. Using Creatine Phosphate (CT) – skeletal muscle fibers have rich supply of CP which is another energy-rich molecule. When the stored ATP is used up, skeletal muscle fiber makes new ATP by using CP: CP + ADP ATP + Creatine (later Creatine will be combined with Phosphate (P) to make new CP molecules. Together, stored ATP and CP can support strenuous muscle action for about 15 seconds, which is long enough for weight lifting, 100- meter race) 10.05.16 2. Anaerobic glycolysis: This is the process by which glucose is broken down to form Pyruvic acid in the cytoplasm (sarcoplasm), releasing a small fraction (5%) of energy, which is used to build up 2 ATP molecules per glucose molecule. All skeletal muscles have plenty of stored Glycogen, and when the skeletal muscles work strenuously for Anatomy and Physiology longer than periods of 15 seconds, they rely on glycolysis for energy. But when oxygen supply decreases, pyruvic acid is converted into lactic acid. This can provide energy for about 40 seconds; together ATP, CP and anaerobic glycolysis can support 60 seconds of strenuous muscle activity as in 400-meter race. 3. Aerobic respiration (oxidative phosphorylation) – oxygen dependent ATP production – takes place in mitochondria, and produces 32-34 ATP per glucose molecules. If there is enough oxygen available, pyruvic acid that enters the mitochondria will be completely oxidized to form carbon dioxide and water, releasing about 95% of the energy in a glucose molecule The energy release is slower than in glycolysis, but the energy yield is much higher than in glycolysis. Activities that last a long time such as Marathon race or endurance exercise rely on aerobic respiration for energy Muscle fatigue: the physiological inability for muscles to contract. This is due to a number of factors: o 1. Declining calcium release from SR o 2. Depletion of CP o 3. Oxygen deficit o 4. Depletion of nutrients o 5. Lactic acid accumulation o 6. Lack of Ach Oxygen debt or recovery oxygen uptake? o After strenuous exercise, heavy breathing continues for sometime to restore the resting state. This post exercise oxygen consumption was called “oxygen debt” before, but now it is referred to as “recovery oxygen uptake” The extra oxygen taken in after exercise is used mainly to replenish ATP in muscle fibers; rebuilding CP, removing lactic acid, rebuilding glycogen, etc. take place very slowly long after strenuous activity, and so “oxygen debt” is not repaid during the heavy breathing following exercise. o Based on the type of energy metabolism the muscle fibers rely on, skeletal muscle fibers are grouped into 3 functional groups, that are present in varying proportions in different muscles: 1. Slow oxidative fibers (SO fibers): also called red fibers as they have large amount of myoglobin; have Anatomy and Physiology many mitochondria and make ATP mainly through aerobic respiration (oxidative phosphorylation) These are fatigue resistant, and capable of flow but prolonged contractions (for endurance exercise, marathon running, jogging) 2. Fast oxidative glycolytic fibers (FOG fibers: large fibers, have myoglobin, and are moderately fatigue resistant, have large amounts of glycogen. These rely on anaerobic glycolysis as well as aerobic respiration for ATP; as they are glycolytic they are fast fibers, contract and relax faster than SO fibers 3. Fast glycolytic fibers (FG fibers): are called “white fibers”: have very little myoglobin, and relatively few mitochondria. Have plenty of glycogen and rely on anaerobic glycolysis for ATP. They are capable of powerful contractions, but only for short durations, as they fatigue very quickly (for weightlifting) o All skeletal muscles have a mixture of all these functional fiber types, but a motor unit will have only 1/3 types. If only a small force is required, then only motor units with SO fibers will be activated; if force is needed motor units of FOG (fast oxidative glycolytic fibers) are recruited; if maximal force is required, then motor units of FG (fast glycolytic fibers) will be activated o Neck muscles that hold the head up have SO (slow oxidative fibers); biceps and triceps have mainly FG (fast glycolytic fibers); leg muscles have both SO (slow oxidative) and FOG (fast oxidative glycolytic) fibers. Skeletal muscles and how they work: o Most muscles work in antagonistic pairs, such as flexors/extensors and adductors/abductors. o Each muscle has a tendon of origin and tendon of insertion. Movement is usually at the tendon of insertion. o The muscle which is the prime mover in any movement is called an agonist; the muscle which opposes the action of the agonist is called an antagonist Ex. In elbow flexion, biceps brachii is the agonist, while triceps barchii is the antagonist Ex. In elbow extension, triceps becomes the agonist and biceps the antagonist o Synergists are muscles that help the agonists. Fixators are them muscles that stabilize the origin of the prime mover. Each muscle can change their role according to the needs of the body. Anatomy and Physiology o Muscle tone: At rest, skeletal muscles are firm and ready to respond immediately when stimulated; this feature is known as muscle tone or tonus. Muscle tone is due to the fat that some motor units in the muscle undergo partial contraction (controlled by spinal reflexes), generating some tension, even when the muscle is at rest. To sustain muscle tone, small motor units in the muscle take turns to contract and relax; these contractions do not result in movement, but they prevent the muscle from becoming weak or flaccid. Muscle tone helps stabilize joins and maintain posture Smooth muscles: o The visceral muscles are made of non-striated, uni-nucleate muscle fibers. They are involuntary in function. o Apart from thick and thin filaments, smooth muscle fibers have intermediate filaments, which are attached to the sarcolemma by thick proteins called dense bodies, which correspond to Z- disks in skeletal muscle fibers. o Smooth muscle fibers have no sarcomere, no troponin, and no SR. The intermediate filaments and dense bodies bring about a cork-screw type spiraling contraction when the muscle fiber is stimulated. o There are two types of smooth muscles: 1. Visceral or single unit smooth tissue – seen in the wall of small arteries, veins, stomach, intestine, uterus, and urinary bladder. The muscle fibers work together as one unit when stimulated, as the fibers have gap junctions through which ions go from one fiber to the next. 2. Multiunit smooth muscle tissue – consists of individual muscle fibers, each with its own motor nerve fiber that causes the contraction of that fiber only. Seen in the wall or aorta, trachea, and in ciliary muscles of the eye o Smooth muscles are aerobic, capable of slow, prolonged contractions. o Smooth muscle fibers are capable of cell division, and they increase in number even in an adult Ex. Uterus during pregnancy NERVOUS SYSTEM – COMMUNICATION, CONTROL, + COORDINATION Nervous system consists of: Anatomy and Physiology o 1. Central Nervous System (CNS): is made of the brain (with 12 pairs of cranial nerves) and spinal cord (with 31 pairs of spinal nerves) o 2. Peripheral Nervous System (PNS): consists of cranial nerves, spinal nerves, plus ganglia (nerve cell [neuron]) groups outside CNS, enteric plexus (neurons in the wall of GI tract), and sensory receptors Two functional groups of PNS are: o 1. Somatic nervous system (SNS) SNS has sensory neurons (afferent neuron), which take impulses from body parts to CNS, and motor neurons (efferent neurons) that take impulses from CNS to body parts (voluntary muscles) o 2. Automatic nervous system (ANS) Has two units of
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