Essential Information and Problems
Student Performance Objectives - for the lecture
1. List and explain the 4 major functions of the skeletal muscular system.
2. Explain why each skeletal muscle is considered to be an organ.
3. Identify the following structures give a typical muscle of the human body cut in a transverse section: muscle fibers, fascia, tendons, sarcolemma, epineurium, perineurium, endoneurium, fascicles, blood vessels, nerve fibers, lymphatic channels.
4. Explain the relationship of a muscle to its origin, insertion, action and innervation.
5. Explain the structure-function relationship between the connective tissues of a muscle and tendon and periosteum.
6. Identify the following structures in a muscle fiber cut in transverse section: sarcolemma, region of the neuromuscular junction, sarcoplasm, myofibrils, actin myofilaments, myosin myofilaments, nuclei, mitochondria, sarcoplasmic reticulum, t-tubules.
7. Draw a diagram of a neuromuscular junction labeling the following parts: nerve fiber, presynaptic membrane, postsynaptic membrane, synaptic cleft, synaptic vesicles, receptor sites, molecules of neurotransmitter (acetylcholine), and cholinesterase.
8. Explain how a neuromuscular junction works.
9. Explain how a signal is transmitted from the neuromuscular junction to the sarcoplasmic reticulum.
10. Draw a diagram of a muscle fiber showing the relationship between myosin, actin, and connectin in a myofibril.
11. Describe how ATP and calcium are required for muscle contraction.
12. Explain the functions of troponin and tropomyosin in muscle contraction.
13. Starting with a nerve signal reaching the neuromuscular junction, provide a comprehensive explanation for the chain of events that lead to contraction of the muscle fiber followed by its relaxation.
14. Explain the significance of glucose and glycogen in muscle fiber metabolism.
15. Define glycogenesis and glycogenolysis and their control by insulin and epinephrine.
16. Explain the overall significance of the metabolic pathway called glycolysis on the ability of muscle fibers to extract energy from glucose and for the human body to sprint (run at maximum speed) for short distances.
17. Define the following terms and their significance in muscle fiber metabolism: anaerobic metabolism, aerobic metabolism, lactic acid, and oxygen debt.
18. Explain the overall significance of the metabolic pathways called the Kreb's cycle, the electron transport system and oxidative phosphorylation on the ability of muscle fibers to extract energy from glucose and for the human body to jog (run slowly) for long distances.
19. Explain the role of creatine and creatine phosphate in muscle metabolism.
20. Explain the following terms: minimal stimulus, graded contractions, maximal stimulus, tetanus, fatigue, and treppe.
21. Describe the concept of the motor unit as a way of explaining graded contractions in a muscle.
22. Distinguish between the contraction of a muscle fiber and the contraction of a muscle as a whole.
23. Describe the differences between the following pairs of terms: hypertrophy-atrophy, and isotonic contraction-isometric contraction.
24. Define and explain the following terms: shivering, tone, rigor mortis.
25. Explain how the following agents prevent muscles from operating: curare, nerve gas.
Student Required Muscles - for Laboratory Practical Examinations (muscles will be
added or removed at your laboratory instructor's discretion). For each muscle, identify
its position on the laboratory muscle man and yourself, and describe its action. For
muscles labeled with an asterisk, use a skeleton to show the muscle's origin and insertion
in addition to its action. Note: we only have one week to study muscle gross anatomy
and so there are limitations on how many muscles we can cover - we know you would
like to learn them all, but time is a factor.
The following sites can help in identification of muscles for the practical.
A. Functions of the Muscular System
1. Movement - The skeletal muscles pull on the bones causing movements at the joints. The skeletal muscles also pull on soft tissues of the face causing facial expressions. The movement of the diaphragm permits external respiration.
2. Support - The muscles of the body wall support the internal organs. As these muscles lose their tone, the internal organs of the abdominal-pelvic cavity bulge outward as seen in most individuals as they age.
3. Protection - The skeletal muscles, particularly of the body wall, cushion the body's internal organs (e.g., abdominal cavity) from force applied to the exterior of the body.
4. Heat Generation - Heat is a waste product of muscle metabolism and this heat helps to maintain our internal body temperature of 37°C. Shivering is a mechanism to generate heat to warm an overly-cooled body.
B. Skeletal Muscle Organization
1. Overall Arrangement of Fibers: There are 4 arrangements for the fibers in a skeletal muscle - parallel, pennated, circular and convergent.
a. In parallel muscles there are relatively few, long fibers that run parallel to the muscle's mechanical axis and are generally designed for speed rather than power. E.g., in the leg, the long, slender sartorius is a parallel muscle designed for speed
b. In pennated muscles there are many, short fibers that run an an angle to the muscle's mechanical axis and are designed for power rather than speed. E.g. the bulky gastrocnemius muscle of the calf is a pennated muscle designed for power.
c. In circular muscles (or sphincters) the fibers are arranged as a circle around a tube or opening. E.g., the orbicularis oris is a sphincter surrounding the mouth.
d. In convergent muscles, the fibers converge to a common attachment site. E.g., the pectoralis major.
2. Muscle Fiber Length: Muscles are composed of long fibers that do not generally extend the entire muscle length; they extend a partial distance and then overlapping fibers extend for another partial distance, and so on, until reaching the other end.
3. Muscle Connective Tissues:
There are 6 types of connective tissue associated with skeletal muscle: endomysium, perimysium, epimysium, fascia, tendon and aponeurosis.
a. Each fiber is surrounded by a thin layer of connective tissue, the endomysium.
b. The fibers in muscles are arranged in groups called fascicles, with each fascicle surrounded by a slightly thicker layer of connective tissue, the perimysium.
c. All the fascicles of the muscle are surrounded by a thicker layer of connective tissue, the epimysium. http://www.ivy-rose.co.uk/References/glossary_entry644.htm
d. All these connective tissue layers strongly bond overlapping muscle fibers within the muscle as a whole. The result is that the contraction of all the fibers in a line within the muscle is equivalent to the contraction of a single long fiber. All these connective tissue elements converge to form the muscle's tendon or aponeurosis (a broad, flat tendon) which are the connective tissues that attach the muscle to the periosteum of bone, or to softer tissue.
e. A dense connective tissue called fascia invests the muscle outside of the epimysium. Fascia helps to hold the muscle in place in the body and separates it from other muscles and body parts.
4. Skeletal Muscles as Organs: Blood vessels, nerves and lymphatic channels penetrate muscles by passing through the layers of connective tissue, eventually reaching the fibers. Interstitial fluid, derived from blood, flows through the connective tissue matrix moistening the fibers, providing them with oxygen and nutrients, and removing carbon dioxide and other wastes. White blood cells pass among the fibers serving a protective function. Clearly, the skeletal muscles are organs containing many different tissue types.
5. Skeletal Muscle Attachments: The point of attachment of a muscle that remains relatively stationary when the muscle contracts is the muscle's origin. The point of attachment that moves when the muscle contracts is the muscle's insertion. The innervation of a muscle refers to the name of the particular nerve whose signal causes the muscle to contract. Muscle actions on joints (e.g., flexion, extension) are covered in the laboratory portion of this course.
C. Skeletal Muscle Fiber Organization
a. Nuclei: Each long, cylindrical skeletal muscle fiber contains many nuclei generally located just under the sarcolemma, which is the muscle fiber's outer membrane.
b. Sarcoplasmic reticulum: The fibers possess an extensive sarcoplasmic reticulum (roughly equivalent to the endoplasmic reticulum of other cells) that surrounds the groups of contractile elements called myofibrils.
c. Myofibrils: The myofibrils are composed of 3 types of protein: contractile, regulatory, and elastic.
1. Contractile proteins: actin and myosin myofilaments are the actual contractile proteins of the muscle fiber. It is the molecular arrangement of actin and myosin myofilaments within the muscle fibers that give skeletal muscle the name striated muscle. (a) The striations, or stripes, are dark and light regions of the myofibrils.
(b) The dark regions are called A bands. They consist of parallel myosin and actin filaments overlapping except in the middle of the dark zones where actin is absent - this part of the A band is a little lighter than the rest of the A band and is called the H zone.
(c) The light regions are called I bands. They consist only of actin filaments attached to Z discs. The Z discs make up the center of the I bands). See section f - The Sarcomere, below.
2. Regulatory proteins: tropomyosin and troponin are the regulatory proteins that control the interactions of actin and myosin. Tropomyosin is bound to actin and blocks actin's active sites (that can connect to myosin) as long as the muscle fiber is at rest. Each tropomyosin has a troponinmolecule attached to it, like a handle, that can move tropomyosin aside exposing actin's active sites to myosin, when troponin binds to calcium.
3. Elastic protein: connectin (also called titin) serves to anchor both actin and myosin to Z discs within the myofibril and to ultimately transmit the force of contraction of actin and myosin to the sarcolemma resulting in the shortening of the muscle fiber as a whole.
d. Mitochondria: Each muscle fiber possesses hundreds of mitochondria that supply the ATP to power the movements of the myofilaments that cause muscle contraction.
e. Transverse Tubules: Transverse tubules (t-tubules) run from the sarcolemma deep into the interior of the muscle fiber. These tubules conduct electrical signals from the surface sarcolemma to the myofibrils within the muscle fiber.
f. The Sarcomere: this is the functional unit of contraction of a skeletal muscle fiber. It is the portion of a myofibril extending from one Z disc to the next Z disc. Actin myofilaments are anchored directly to the Z discs. Myosin myofilaments are anchored to the Z discs through the elastic protein, connectin.
2. The Neuromuscular Junction -
Skeletal muscle fibers contract when they receive a stimulus from a motor nerve that comes from the spinal cord or brain. The neuromuscular junction (NMJ) is the point of communication between a branch of a motor nerve and an individual muscle fiber. The important regions of the NMJ are:
a. Synaptic knob - the enlarged ending of the tip of the nerve fiber (called the axon terminal).
b. Motor endplate - the slight depression in the sarcolemma in which the synaptic knob is located.
c. Junctional folds - these folds in the sarcolemma increase the functional surface area for the interaction of chemicals released from the synaptic knob and the motor endplate.
d. Synaptic cleft - the physical space separating the synaptic knob from the motor endplate. This distance is very small - around 100 nm (nanometers). To clearly see the synaptic cleft one needs an electron microscope and about 100,000x magnification. An ebola virus is in the range of the size of the synaptic cleft. The cleft is filled with a fine dispersion of large molecules (glycoproteins and collagen) that form a thin gel through which the neurotransmitters must diffuse to reach the motor endplate. This this gel is referred to as the basal lamina.
e. Synaptic vesicles - these organelles in the synaptic knob accumulate the neurotransmittercalled acetylcholine (Ach), which will be released like a fine spray into the synaptic cleft and which will diffuse across the synaptic cleft and bind to receptors on the surface of the junctional folds of the motor endplate.
f. Cholinesterase - this enzyme is found in the basal lamina and continuously breaks down acetylcholine (Ach). For the muscle fiber to be stimulated, enough Ach must be released from the synaptic vesicles to overcome the tendency of this enzyme to break it apart. The presence of cholinesterase insures that once nerve signals stop, all remaining Ach in the synaptic cleft will be destroyed and the muscle will relax.
g. Schwann cell - this specialized connective tissue cell of the nervous system surrounds the NMJ separating it from the interstitial fluids surrounding the rest of the nerve fiber and muscle fiber.
h. Presynaptic membrane and postsynaptic membrane - these two terms are useful in that they denote the membrane of the synaptic knob on the axon terminal as the presynaptic membrane, and the membrane of the motor endplate on the sarcolemma as the postsynaptic membrane. Note that the term synapse refers to the location where a nerve fiber connects with another nerve fiber or, in this case, a muscle fiber. Neuromuscular junction (NMJ) more accurately describes the "synapse" in muscular tissue.
3. Skeletal muscle fiber bioelectrical effects
a. Sodium-Potassium Pump Action:
A pump (called the sodium-potassium pump) in the sarcolemma of a muscle fiber uses the energy of ATP to pump potassium ions into the muscle fiber and pump sodium ions out of the muscle fiber. Chloride ions follow sodium and phosphates tend to pair up with potassium. So, when a muscle fiber is not contracting (resting), it is bathed in a sodium chloride solution on the outside and a potassium phosphate solution on the inside.
b. Permeability of the sarcolemma: the sarcolemma permits the diffusion of relatively large amounts of potassium ions out of the cell and only a small amount of sodium ions into the cell. These diffusive movements are simply due to these ions moving down their concentration gradients.
c. Development of the Resting Potential: the outward diffusion of potassium establishes a net positive charge on the outside of the sarcolemma (potassium carries a single positive electrical charge) and a net negative charge on the inside of the sarcolemma (due to the negative ions left inside when potassium diffused out of the cell). The permeability properties of the membrane do not allow phosphates or proteins to follow potassium when it diffuses out of the cell. This net charge on either side of the membrane is called the resting potential and is measured as about -90 millivolts (mv) in a muscle fiber (as measured from the inside where it is negative).
D. Skeletal Muscle Fiber Contraction: the series of events that follow utilize the
vocabulary and concepts from the previous sections.
a. Calcium's Role: An electrical signal traveling down a nerve fiber reaches the axon terminal causing a diffusion of calcium ions from the surrounding fluid to enter the synaptic knob. The electrical signal itself ends.
b. Role of Synaptic Vesicles: The calcium that enters the synaptic knob causes fusion of some of the synaptic vesicles with the presynaptic membrane which results in release of acetylcholine(Ach) into the synaptic cleft.
c. Role of Receptor Sites: Ach diffuses across the synaptic cleft and attaches to receptors on the postsynaptic membrane (which is the membrane of the junctional folds of the the motor endplate).
d. Concepts of Depolarization and Repolarization at the Motor Endplate: The attachment of Ach to these receptors opens channels in the postsynaptic membrane and sodium and potassium ions briefly diffuse through the membrane - sodium ions flow into the muscle fiber at the motor endplate and then potassium ions diffuse out of the muscle fiber at the motor end plate. The inward diffusion of sodium ions reverses the charge on the membrane - the resting potential is reversed in that now the membrane is negative on the outside and positive on the inside (about +75 mv). This process is known as depolarization. Then when potassium diffuses out of the cell, the charge goes back to the resting potential - this process is called repolarization. Depolarization and repolarization all occur in the span of about 1 millisecond. But this is enough time to trigger the next set of events leading to the contraction of the muscle.
e. Signal Transfer beyond the motor endplate and into the Muscle Fiber: Sequential depolarization and repolarization in the region of the motor endplate is referred to as the endplate potential (EPP). This EPP sets off a wave of similar depolarizations and repolarizations along the sarcolemma moving outward from the motor endplate. Sodium and potassium ion movements at the motor endplate are in response to the opening and closing of ligand-gated ion channels (Ach is the ligand, or chemical, that stimulates opening of the ion gates by binding to the receptor sites on the postsynaptic membrane). In contrast, sodium and potassium ion movements along the sarcolemma (beyond the motor endplate) are in response to the opening and closing of voltage-gated ion channels.
f. The Signal - It should be clear that what is happening is the transmission of a biological signal. It is a bio-electrical signal in that it is the flow of charged particles, ions, that are carrying the signal in the form of depolarizations and repolarizations along the sarcolemma. This same signal was briefly converted to a chemical signal at the neuromuscular junction (NMJ). In other words, the signal from the nervous system was bioelectrical. It was then converted to chemical at the NMJ. It is then reconverted to bio-electrical by the voltage-gated ion channels of the sarcolemma.
g. Signal Transfer from Sarcolemma to Sarcoplasmic Reticulum (SR): The signal travels along the sarcolemma and when it reaches the t-tubules, it passes inward and travels to the end of the t-tubules where the signal is transferred to the sarcoplasmic reticulum (SR).
h. The SR and Calcium: the SR transports calcium into its spaces (cisternae) and stores it there. When the signal from the t-tubules reaches the SR, some of these calcium ions are released out of the SR cisternae into the intracellular fluid. These calcium ions interact with the myofibrils of the muscle fiber.
i. Calcium Interaction with the Myofibrils: calcium released from the SR binds to troponin which acts to shift tropomyosin away from its position blocking actin's active sites.
j. The Initial Interaction between Actin, Myosin and ATP: myosin combines with an ATP molecule, splits it, and uses the energy derived to position myosin's head for a power stroke. The myosin heads, ready for action, can now attach to actin's active sites because troponin has moved tropomyosin out of the way. Attachment occurs (actin-myosin cross bridge forms) and the power stroke is executed - the myosin heads pull (slide) the actin molecules along the myosin myofilaments.
k. The Continuing Interaction between Actin, Myosin and ATP: now the myosin head, still attached to the actin active site, binds another ATP molecule. In so doing, it releases its hold on the actin active site. The new ATP molecules is split, the myosin head is once again positioned, using ATP's energy, for the power stroke, it attaches to actin and pulls it some more. This continues until the degree of contraction desired is achieved.
http://www.getbodysmart.com/ap/muscletissue/contraction/coupling/tutorial.html - excitation-contraction coupling
http://www.getbodysmart.com/ap/muscletissue/contraction/actionpotentials/tutorial.html - action potential
http://www.getbodysmart.com/ap/muscletissue/contraction/multipleheads/tutorial.html - asynchronous myosin-head movement
E. Skeletal Muscle Fiber Relaxation:When the signal from the nervous system ceases because you wish to stop contracting the muscle in question:http://www.getbodysmart.com/ap/muscletissue/contraction/relaxation/tutorial.html
1. Signals (nerve impulses) cease to flow from the brain or spinal cord to the muscle.
2. Ach is no longer released from synaptic vesicles in the neuromuscular junction.
3. Cholinesterase breaks down all remaining Ach in the synaptic cleft.
4. Electrochemical signals cease to flow along the sarcolemma and down the t-tubules.
5. The SR pumps calcium ions back into the SR cisternae thus removing them from contact with troponin.
6. Without calcium, troponin repositions tropomyosin so that actin's active sites are blocked from interaction with myosin heads.
7. Myosin heads, although in position to carry out the power stroke, can not do so because they are not attached to actin. So they just sit there and do nothing.
8. The muscle fiber, and the muscle as a whole, returns to its original length through the pull of elastic connective tissues within the fibers and through being stretched as antagonistic muscles contract.
F. Motor Units
1. The Concept - a motor unit is a motor neuron of the brain or spinal cord and all the muscle fibers within a muscle that it innervates. Different muscles have different numbers of motor units. If a muscle has only a few motor units then each motor unit has many fibers and very fine motor control is not possible (e.g., large, postural muscles of the back). If a muscle has many motor units then each motor unit has only a few muscle fibers and a fine degree of motor control is possible e.g., muscles of the tongue).
2. Examples - The ocular muscles have many motor units and less than 10 fibers per unit; they are capable of bringing about slight, precise movements of the eyeball. The large gastrocnemius muscle of the calf has few motor units and as many as 1000 fibers per unit. It is perfectly suited for plantar flexing the foot in walking and running, but not for fine muscular movements.
3. Relation to the All-or-None Law of Muscle Fiber Contraction - when a motor unit fires, all the fibers of the muscle contract to the maximum that they can. This is a statement of the all-or none law of muscle fiber contraction which states that if the stimulus from a nerve to a muscle fiber goes above the "threshold stimulus", then the muscle contracts fully; there is no such thing as a muscle contracting partially - it contracts to the fullest extent possible, or it does not contract at all. Of course if a muscle was completely empty of ATP, you could stimulate it above threshold as much as you wanted and it would never contract- but the idea of this law is to clarify that contractile extent is not relative to stimulus strength in an individual muscle fiber - as long as you are above threshold, then you get a maximal contraction. So if a muscle that has 500 motor units has 250 of its units contract, then the muscle contracts, as a whole, to 50% of its capacity, but the fibers of the 250 motor units that did fire contracted maximally. Another example: if a muscle has 10 motor units and 2 units fire, than the fibers of the two firing units contract maximally, but the muscle as a whole is only contracting to 20% of its capacity.
G. Skeletal Muscle Metabolism
1. Energy for Contraction: The energy for contraction depends on a steady supply of ATP. Depending on the intensity of the demands (intensity of exercise), the muscle fiber derives its ATP through several means.
a. Extremely intense (maximal effort) activity such as running or swimming full speed can be carried out only for a short period (10 seconds or less) because the ATP must come mostly from:
1. Creatine phosphate (CP) stored in the muscle fiber. CP donates its high energy phosphate group to ADP to make ATP. The muscle fiber enzyme creatine kinase catalyzes this reaction. This mechanism of ATP generation is called the phosphagen system.
2. ATP derived from the transfer of a phosphate from one ADP to another ADP yielding one ATP and one AMP. The enzyme myokinase catalyzes this reaction.
b. Powerful, activity, less intense than maximum, can be carried out for up to about a minute because ATP must come mostly from:
1. Anaerobic fermentation of glucose from the blood to lactic acid with the production of only 2 ATP molecules per glucose molecule fermented. This is the metabolic pathway called glycolysis and it involves the actions of about 10 enzymes acting sequentially. As lactic acid builds up the muscle becomes fatigued.
2. Anaerobic fermentation of glucose from the muscle fiber's stored glycogen with the same consequences as 2a.
c. Long-term, continuous physical activity can be carried out from minutes to hours depending on other factors relating to one's physical shape (cardiovascular-pulmonary efficiency, age, and degree of hydration and electrolyte balance). In this case, muscle metabolism involves:
1. Derivation of oxygen from myoglobin within the muscle as well as from hemoglobin of the blood.
2. The complete oxidation of glucose to carbon dioxide and water utilizing oxygen and the enzyme systems of the mitochondria.
3. The complete oxidation of fatty acids to carbon dioxide and water utilizing mitochondrial enzyme systems.
2. Hypertrophy and Atrophy
a. Hypertrophy is the physical enlargement of muscles. The muscles enlarge because their fibers enlarge due to the development of more myofibrils. The muscles do not enlarge due to formation of additional muscle fibers (see hyperplasia, part c below). Hypertrophy is clearly seen in the leg muscles of runners and dancers or in the muscles of body builders that lift weights. The enlargement of the muscle is also due to increased connective tissue and blood vessels (increased vascularity). The tendons attaching the muscle to bone also thicken in response to increased tension as the strengthening muscle contracts and pulls on it. Some individuals' muscles do not hypertrophy as easily or as symmetrically as do others'. Under ideal circumstances, a body builder can gain about 7 pounds of muscle per year. Taking steroid hormones can result in development of about 35 pounds of muscle per year, but this is at the expense of damage to liver and kidneys, and changes in gonadal and neurological function (e.g., increased aggressiveness).
b. Atrophy is the diminished size of muscles seen when muscles are underexercised. The muscles lose myofibrils. Having a muscle immobilized, as in recovery from a bone fracture with the use of a plaster cast or nylon brace, can result in atrophy. Severe atrophy accompanies loss of nerve impulses flowing into a muscle as occurs in trauma to the spinal cord or to peripheral nerves, or in poliomyelitis where the motor neurons of the anterior horn cells are selectively destroyed by the virus. In severe, prolonged disuse, muscle fibers can be replaced by connective tissue over a period of 6 months to 2 years.
c. Hyperplasia is the enlargement of a tissue due to growth by mitosis of additional cells. This type of growth is not observed in skeletal muscle enlargement but it is seen in some cases of smooth muscle enlargement such as the growth of the smooth muscular layers of the uterus during pregnancy. Epithelial tissues also enlarge through hyperplasia such as the thickening of the skin's epidermis in callous formation and the thickening of the uterine endometrium during a woman's monthly ovulatory-menstrual cycle.
3. Muscle Tone is the constant contraction of a small number of motor units in a muscle that keeps the muscle slightly contracted all the times. The muscle is in a state or readiness to contract by being maintained at the ideal contractile length. Muscles become more toned as they are exercised. Flabby muscles lack tone and are not at the best length to contract most efficiently when required.
4. Isotonic and Isometric Contractions
a. Isotonic contractions are muscular contractions resulting in shortening of the muscle. Lifting a weight of any type and placing the object from point A to point B involves muscles contracting, shortening, relaxing and then lengthening. A weight lifter doing "curls" by repeatedly flexing and then extending the elbow is an example of isotonic contraction.
b. Isometric contractions are muscular contractions resulting in tensing of a muscle, but not shortening. Firmly holding a book in one's arm is an example of isometric contraction of all the muscles required to prevent the book from dropping or moving from the position in which you have placed it.
c. All muscular actions involve some aspects of both isotonic and isometric contraction so that there is hardly a time when a an activity does not result in both types of contraction occurring simultaneously. Since all muscles have some degree of tone (see 3 above) then all muscles undergo isometric "tensing" all the time. The movement of the body from place to place involves a rhythmic shortening and lengthening (contraction and shortening) of some muscles and the bracing of certain joints (contraction and tensing) by other muscles to make the movements efficient and graceful.
a. Explanation - treppe is the increased force of contraction observed in a muscle even though the stimulus intensity remains the same. This phenomenon is observed under laboratory conditions when a muscle is stimulated between 10 and 20 times per second by an external, electrical stimulator. It is hypothesized that either the sarcoplasmic reticulum does not have time to reabsorb all the calcium released from the previous contraction, or that with each successive contraction, slightly more calcium ions are released from the SR into the vicinity of the myofibrils. With either explanation, the result is increased interaction of calcium with troponin and, consequently, an increased interaction of myosin with actin.
b. Application - treppe appears to be the physiological explanation for "warming up" prior to competing in an athletic contest. The warmed-up muscles contract more powerfully than muscles that have been resting for a period of time without use.
6. Shivering is the rapid, repeated and uncontrollable contraction of skeletal muscles resulting in the generation of heat. During muscle contraction only about 25 % of the energy of ATP is actually converted into the work of muscle contraction, the remainder of the energy is released as heat. Shivering can be initiated by chemicals (pyrogens) released from leukocytes (white blood cells) that have phagocytized bacteria during an infection. The resulting increase in body temperature is part of the immune system's defense against infection. Shivering is also initiated when the body temperature drops due to exposure to cold. The heat released from the rapidly contracting muscles is distributed by the blood throughout the body and the body is warmed.
7. Rigor mortis - this refers to the stiffening of the body that begins about 4 hours after death, reaches a maximum after about 12 hours, and then gradually releases over the next 12-60 hours. Two explanations are given for rigor mortis, both of which may be occurring simultaneously:
a. After myosin and actin connect and muscle contraction occurs, the release of actin myofilaments by myosin depends on myosin combining with a new ATP molecule. After death there are no new ATP molecules being made so myosin does not let go of actin and "relaxation" only occurs due to gradual bacterial invasion and decomposition of the muscle fibers and the body as a whole.
b. After death, the muscle fiber's various membranes, including the sarcolemma and the sarcoplasmic reticulum, lose their selective permeability and excessive amounts of calcium ions may leak into the muscle fiber stimulating the attachment of myosin and actin. Eventual release occurs only as explained above in part a.
H. Selected Terms Relating to Skeletal Muscles Studied in the Laboratory
1. Subliminal stimulus - an electrical stimulus that brings about no observable response (contraction) in a muscle.
2. Minimal stimulus - the least intense stimulus required to bring about the first observable response from a muscle. The minimal stimulus can also be called the threshold stimulus. It is the least degree of stimulation that, presumably, opens voltage-regulated gates in the sarcolemma of muscle fibers within the muscle.
3. Graded contractions - the increased degree of contraction observed in a muscle as the intensity of a stimulus is increased. With increased stimulus intensity, more and more fibers within the muscle contract. If the muscle is being stimulated indirectly through the nerve leading to it (e.g., stimulating the sciatic nerve leading to the gastrocnemius muscle), then with increasing stimulus intensity, more and more of the muscle's motor units are signaled to contract resulting in progressively greater and greater observable force of contraction of the muscle.
4. Wave Summation (Temporal Summation) - when electrical stimuli are applied to a muscle at frequencies between 20-40 stimuli/sec, each stimulus producing a contraction is followed by another stimulus to contract before the muscle can fully relax from the first stimulus. The result is that each contractile wave builds from the previous one. Although the stimulus intensity remains the same, the degree of contraction of the muscle progressively increases as the stimulus frequency increases. The term "wave summation" is used to indicate that successive "waves of contraction" are added to each other. The term "temporal summation" is used to indicate that the timed arrival of the stimuli is very close, i.e., the stimuli arrive in very rapid succession.
5. Incomplete Tetanus - the rapid, oscillating contraction-relaxation cycles (fluttering) of a muscle that is undergoing wave summation. See d, above.
6. Complete Tetanus - when a muscle is stimulated by an external signaling device (in the laboratory) at a frequency of 40 -50 stimuli/sec, the muscle fibers that contract from one stimulus never have time to relax before the next stimulus arrives. Therefore the muscle remains fully contracted without any signs of relaxation. Such levels of contraction are not seen physiologically because motor neurons, firing at maximal rates, are able to stimulate muscles at frequencies of only about 25 stimuli/sec.
7. Fatigue - after prolonged use, sustained levels of muscular performance decline as the muscle tires, or undergoes fatigue. The following factors are thought to contribute to fatigue:
a. the neuromuscular junction may run low on acetylcholine
b. the muscle fiber may be running low on glycogen and/or glucose so that muscle fiber ATP availability declines
c. lactic acid buildup raises the acidity (lowers the pH) of the muscle fibers which reduces the efficiency of enzymes required for normal fiber metabolism
d. the fiber's resting membrane potential may change from normal (optimal) levels due to reduced ATP availability to power the sodium-potassium pump.
I. Drug Effects on the Skeletal Muscles - the neuromuscular junction (NMJ) is vulnerable
to many drugs and poisons. The following 4 chemicals are representative of the many
substances affecting the NMJ.
1. Substances that Block Signal Transmission across the NMJ
a. Botulism toxin: this poison is produced by the bacterium Clostridium botulinum, the organism that causes the most serious and deadly form of food poisoning. Improperly canned foods or improperly preserved fish may be contaminated with this organism and its toxin. The botulism toxin inhibits release of acetylcholine (Ach) from synaptic vesicles in the NMJ of both skeletal and smooth muscles. The potential results are flaccid paralysis of the muscles of the large intestine (constipation), pharynx (difficulty swallowing and speaking), and the diaphragm (respiratory paralysis and possibly death).
b. Curare: this is the term used to describe a number of structurally related plant chemicals that were historically used by South American Indians as arrow poisons. Animals struck by curare-coated arrow tips died of respiratory paralysis. Curare binds tightly to Ach receptor sites on the sarcolemma within the NMJ. The result is the inability of Ach released from the presynaptic membrane to attach to the sarcolemma. Consequently signal transmission from nerve fiber to muscle fiber is stopped and the muscles are paralyzed.
2. Substances that Stimulate Electrical Transmission across the NMJ
a. Cholinesterase inhibitors: there are many such substances that go under the heading of nerve gases (weapons of mass destruction), and pesticides used in insect sprays. These chemicals bind the cholinesterase in the NMJ and the result is that Ach is not broken down and accumulates in the NMJ. Ach just keeps working - stimulating the muscle fiber at the sarcolemmal receptor sites- causing uncoordinated muscular contractions, spasms, convulsions, and death by respiratory spasms.
b. Tetanus toxin: this poison is produced by the bacterium Clostridium tetani, the organism that causes the deadly disease, lockjaw. The tetanus toxin blocks the release of a central nervous system inhibitor (glycine) that regulates nerve signal flow to the skeletal muscles. The result is unregulated and excessive overstimulation of the skeletal muscles including the respiratory muscles causing respiratory spasms and death by asphyxiation.
Biomedical Terminology: Define each term.
ligand-gated ion channels
voltage-gated ion channels
Muscular System Problems
1. Choose one of the problems described below.
2. Prepare your solution as a word document.
3. Send it to your professor as an email attachment. You will receive an email response.
Problem #1: A 23 year old man lifts weights and utilizes anabolic steroids to enhance
muscular hypertrophy. In one intensive year of weights and steroid use, he accomplishes
the same muscular growth he would have attained from 5 years of weight lifting without
steroid use. Utilize the Internet to research the pros and cons of anabolic steroid
Your report should include
1. A definition of "hypertrophy" and a detailed explanation of how skeletal muscles undergo hypertrophy .
2. A definition of anabolic steroid and a list of specific steroids available by prescription or over-the-counter.
3. A detailed list and explanation of the physiological effects of short-term and long-term anabolic steroid use.
4. Your decision, based on your research, whether or not the man should continue using steroids to enhance muscular development.
5. An explanation of the similarities and differences in the effects of anabolic steroids on the bodies of women compared with men.
Problem #2: An anatomy class visits a morgue and a student touches the skin over the
biceps brachii of a corpse. The student comments that the "feel" is cold and hard.
The instructor explains that the body is in rigor mortis. Utilize the Internet to
answer the following questions:
1. What is rigor mortis? What tissue of the body is in rigor?
2. How long does it take for the body to enter rigor mortis and when does the body go out of rigor, relative to the time of death?
3. What is the physiological explanation for rigor mortis?
4. What is the usefulness of the phenonenon of rigor mortis in forensic medicine?
5. Assume the body is buried in a casket and exhumed five years later for a medical test. If the biceps brachii muscle was examined microscopically, what might you expect to find and why?