Muscular system
Skeletal Muscle Fibers
Skeletal muscles are composed of striated subunits called sarcomeres, which are composed of the myofilaments actin and myosin.
LEARNING OBJECTIVES
Outline the structure of a skeletal muscle fiber
KEY TAKEAWAYS
Key Points
Muscles are composed of long bundles of myocytes or muscle fibers.
Myocytes contain thousands of myofibrils.
Each myofibril is composed of numerous sarcomeres, the functional contracile region of a striated muscle. Sarcomeres are composed of myofilaments of myosin and actin, which interact using the sliding filament model and cross-bridge cycle to contract.
Key Terms
sarcoplasm: The cytoplasm of a myocyte.
sarcoplasmic reticulum: The equivalent of the smooth endoplasmic reticulum in a myocyte.
sarcolemma: The cell membrane of a myocyte.
sarcomere: The functional contractile unit of the myofibril of a striated muscle.
Skeletal Muscle Fiber Structure
Myocytes, sometimes called muscle fibers, form the bulk of muscle tissue. They are bound together by perimysium, a sheath of connective tissue, into bundles called fascicles, which are in turn bundled together to form muscle tissue. Myocytes contain numerous specialized cellular structures which facilitate their contraction and therefore that of the muscle as a whole.
The highly specialized structure of myocytes has led to the creation of terminology which differentiates them from generic animal cells.
Generic cell > Myocyte
Cytoplasm > Sarcoplasm
Cell membrane > Sarcolemma
Smooth endoplasmic reticulum > Sarcoplasmic reticulum
Myocyte Structure
Myocytes can be incredibly large, with diameters of up to 100 micrometers and lengths of up to 30 centimeters. The sarcoplasm is rich with glycogen and myoglobin, which store the glucose and oxygen required for energy generation, and is almost completely filled with myofibrils, the long fibers composed of myofilaments that facilitate muscle contraction.
The sarcolemma of myocytes contains numerous invaginations (pits) called transverse tubules which are usually perpendicular to the length of the myocyte. Transverse tubules play an important role in supplying the myocyte with Ca+ ions, which are key for muscle contraction.
Each myocyte contains multiple nuclei due to their derivation from multiple myoblasts, progenitor cells that give rise to myocytes. These myoblasts asre located to the periphery of the myocyte and flattened so as not to impact myocyte contraction.
Myocyte: Skeletal muscle cell: A skeletal muscle cell is surrounded by a plasma membrane called the sarcolemma with a cytoplasm called the sarcoplasm. A muscle fiber is composed of many myofibrils, packaged into orderly units.
Myofibril Structure
Each myocyte can contain many thousands of myofibrils. Myofibrils run parallel to the myocyte and typically run for its entire length, attaching to the sarcolemma at either end. Each myofibril is surrounded by the sarcoplasmic reticulum, which is closely associated with the transverse tubules. The sarcoplasmic reticulum acts as a sink of Ca+ ions, which are released upon signalling from the transverse tubules.
Sarcomeres
Sarcomere: The sarcomere is the functional contractile region of the myocyte, and defines the region of interaction between a set of thick and thin filaments.
Myofibrils are composed of long myofilaments of actin, myosin, and other associated proteins. These proteins are organized into regions termed sarcomeres, the functional contractile region of the myocyte. Within the sarcomere actin and myosin, myofilaments are interlaced with each other and slide over each other via the sliding filament model of contraction. The regular organization of these sarcomeres gives skeletal and cardiac muscle their distinctive striated appearance.
Myofilaments (Thick and Thin Filaments)
Myofibrils are composed of smaller structures called myofilaments. There are two main types of myofilaments: thick filaments and thin filaments. Thick filaments are composed primarily of myosin proteins, the tails of which bind together leaving the heads exposed to the interlaced thin filaments. Thin filaments are composed of actin, tropomyosin, and troponin. The molecular model of contraction which describes the interaction between actin and myosin myofilaments is called the cross-bridge cycle.
Structure and Function of the Muscular System
The muscular system controls numerous functions, which is possible with the significant differentiation of muscle tissue morphology and ability.
LEARNING OBJECTIVES
Describe the three types of muscle tissue
KEY TAKEAWAYS
Key Points
The muscular system is responsible for functions such as maintenance of posture, locomotion, and control of various circulatory systems.
Muscle tissue can be divided functionally (voluntarily or involuntarily controlled) and morphologically ( striated or non-striated).
These classifications describe three distinct muscle types: skeletal, cardiac and smooth. Skeletal muscle is voluntary and striated, cardiac muscle is involuntary and striated, and smooth muscle is involuntary and non-striated.
Key Terms
myofibril: A fiber made up of several myofilaments that facilitates the generation of tension in a myocyte.
myofilament: A filament composed of either multiple myosin or actin proteins that slide over each other to generate tension.
myosin: A motor protein which forms myofilaments that interact with actin filaments to generate tension.
actin: A protein which forms myofilaments that interact with myosin filaments to generate tension.
striated: The striped appearance of certain muscle types in which myofibrils are aligned to produce a constant directional tension.
voluntary: A muscle movement under conscious control (e.g. deciding to move the forearm).
involuntary: A muscle movement not under conscious control (e.g. the beating of the heart).
myocyte: A muscle cell.
The Musculoskeletal System
The muscular system is made up of muscle tissue and is responsible for functions such as maintenance of posture, locomotion and control of various circulatory systems. This includes the beating of the heart and the movement of food through the digestive system. The muscular system is closely associated with the skeletal system in facilitating movement. Both voluntary and involuntary muscular system functions are controlled by the nervous system.
The muscular system: Skeletal muscle of the muscular system is closely associated with the skeletal system and acts to maintain posture and control voluntary movement.
Muscle is a highly-specialized soft tissue that produces tension which results in the generation of force. Muscle cells, or myocytes, contain myofibrils comprised of actin and myosin myofilaments which slide past each other producing tension that changes the shape of the myocyte. Numerous myocytes make up muscle tissue and the controlled production of tension in these cells can generate significant force.
Muscle tissue can be classified functionally as voluntary or involuntary and morphologically as striated or non-striated. Voluntary refers to whether the muscle is under conscious control, while striation refers to the presence of visible banding within myocytes caused by the organization of myofibrils to produce constant tension.
Types of Muscle
The above classifications describe three forms of muscle tissue that perform a wide range of diverse functions.
Skeletal Muscle
Skeletal muscle mainly attaches to the skeletal system via tendons to maintain posture and control movement. For example, contraction of the biceps muscle, attached to the scapula and radius, will raise the forearm. Some skeletal muscle can attach directly to other muscles or to the skin, as seen in the face where numerous muscles control facial expression.
Skeletal muscle is under voluntary control, although this can be subconscious when maintaining posture or balance. Morphologically skeletal myocytes are elongated and tubular and appear striated with multiple peripheral nuclei.
Cardiac Muscle Tissue
Cardiac muscle tissue is found only in the heart, where cardiac contractions pump blood throughout the body and maintain blood pressure.
As with skeletal muscle, cardiac muscle is striated; however it is not consciously controlled and so is classified as involuntary. Cardiac muscle can be further differentiated from skeletal muscle by the presence of intercalated discs that control the synchronized contraction of cardiac tissues. Cardiac myocytes are shorter than skeletal equivalents and contain only one or two centrally located nuclei.
Smooth Muscle Tissue
Smooth muscle tissue is associated with numerous organs and tissue systems, such as the digestive system and respiratory system. It plays an important role in the regulation of flow in such systems, such as aiding the movement of food through the digestive system via peristalsis.
Smooth muscle is non-striated and involuntary. Smooth muscle myocytes are spindle shaped with a single centrally located nucleus.
Types of muscle: The body contains three types of muscle tissue: skeletal muscle, smooth muscle, and cardiac muscle, visualized here using light microscopy. Visible striations in skeletal and cardiac muscle are visible, differentiating them from the more randomised appearance of smooth muscle.
Slow-Twitch and Fast-Twitch Muscle Fibers
Skeletal muscle contains different fibers which allow for both rapid short-term contractions and slower, repeatable long-term contractions.
LEARNING OBJECTIVES
Describe the different types of skeletal muscle fibers and their respective functions
KEY TAKEAWAYS
Key Points
Slow-twitch fibers rely on aerobic respiration to fuel muscle contractions and are ideal for long term endurance.
Fast-twitch fibers rely on anaerobic respiration to fuel muscle contractions and are ideal for quick contractions of short duration.
Key Terms
aerobic: A combination of glycolysis and the Krebs cycle, an efficient but slow way of producing ATP.
anaerobic: Glycolysis alone, an inefficient but quick way of producing ATP with pyruvate converted to lactate.
glycolysis: The breakdown of glucose (or other carbohydrates) by enzymes, generating ATP and pyruvate.
slow-twitch: Type I fibers characterized as muscles with long contraction duration, associated with endurance.
Krebs cycle: A sequence of reactions which converts pyruvate into carbon dioxide and water, generating further
adenosine triphosphate (ATP).fast-twitch: Type II fibers which are characterized by fast muscle contractions of short duration.
Skeletal muscle fibers can be further subdivided into slow and fast-twitch subtypes depending on their metabolism and corresponding action. Most muscles are made up of combinations of these fibers, although the relative number substantially varies.
Slow Twitch (Type 1)
Slow-twitch fibers are designed for endurance activities that require long-term, repeated contractions, like maintaining posture or running a long distance. The ATP required for slow-twitch fiber contraction is generated through aerobic respiration (glycolysis and Krebs cycle), whereby 30 molecules of ATP are produced from each glucose molecule in the presence of oxygen. The reaction is slower than anaerobic respiration and thus not suited to rapid movements, but much more efficient, which is why slow-twitch muscles do not tire quickly. However, this reaction requires the delivery of large amounts of oxygen to the muscle, which can rapidly become rate-limiting if the respiratory and circulatory systems cannot keep up.
Due to their large oxygen requirements, slow-twitch fibers are associated with large numbers of blood vessels, mitochondria, and high concentrations of myoglobin, an oxygen-binding protein found in the blood that gives muscles their reddish color. One muscle with many slow-twitch fibers is the soleus muscle in the leg (~80% slow-twitch), which plays a key role in standing.
Fast Twitch (Type II)
Fast-twitch fibers are good for rapid movements like jumping or sprinting that require fast muscle contractions of short duration. Unlike slow-twitch fibers, fast twitch-fibers rely on anaerobic respiration (glycolysis alone) to produce two molecules of ATP per molecule of glucose. While much less efficient than aerobic respiration, it is ideal for rapid bursts of movement since it is not rate limited by need for oxygen. Lactate (lactic acid), a byproduct of anaerobic respiration, accumulates in the muscle tissue reducing the pH (making it more acidic, and producing the stinging feeling in muscles when exercising). This inhibits further anaerobic respiration. While this may seem counter-intuitive, it is a feedback cycle in place to protect the muscles from over-exertion and resultant damage.
As fast-twitch fibers generally do not require oxygenation, they contain fewer blood vessels and mitochondria than slow-twitch fibers and less myoglobin, resulting in a paler color. Muscles controlling eye movements contain high numbers of fast-twitch fibers (~85% fast-twitch).
Determination and Alteration of Muscle Type
While there is evidence that each person has a unique proportion of fast-twitch versus slow-twitch muscles determined by genetics, more research is required. Regardless, repeated exercise that prioritizes one type of muscle fiber use over the other can lead to improvements in an individual’s ability to perform that activity through alterations in the number and composition of fibers associated with improvements in the respiratory and circulatory systems.
Sliding Filament Model of Contraction
In the sliding filament model, the thick and thin filaments pass each other, shortening the sarcomere.
LEARNING OBJECTIVES
Describe the sliding filament model of muscle contraction
KEY TAKEAWAYS
Key Points
The sarcomere is the region in which sliding filament contraction occurs.
During contraction, myosin myofilaments ratchet over actin myofilaments contracting the sarcomere.
Within the sarcomere, key regions known as the I and H band compress and expand to facilitate this movement.
The myofilaments themselves do not expand or contract.
Key Terms
I-band: The area adjacent to the Z-line, where actin myofilaments are not superimposed by myosin myofilaments.
A-band: The length of a myosin myofilament within a sarcomere.
M-line: The line at the center of a sarcomere to which myosin myofilaments bind.
Z-line: Neighbouring, parallel lines that define a sarcomere.
H-band: The area adjacent to the M-line, where myosin myofilaments are not superimposed by actin myofilaments.
Movement often requires the contraction of a skeletal muscle, as can be observed when the bicep muscle in the arm contracts, drawing the forearm up towards the trunk. The sliding filament model describes the process used by muscles to contract. It is a cycle of repetitive events that causes actin and myosin myofilaments to slide over each other, contracting the sarcomere and generating tension in the muscle.
Sarcomere Structure
To understand the sliding filament model requires an understanding of sarcomere structure. A sarcomere is defined as the segment between two neighboring, parallel Z-lines. Z lines are composed of a mixture of actin myofilaments and molecules of the highly elastic protein titin crosslinked by alpha-actinin. Actin myofilaments attach directly to the Z-lines, whereas myosin myofilaments attach via titin molecules.
Surrounding the Z-line is the I-band, the region where actin myofilaments are not superimposed by myosin myofilaments. The I-band is spanned by the titin molecule connecting the Z-line with a myosin filament.
The region between two neighboring, parallel I-bands is known as the A-band and contains the entire length of single myosin myofilaments. Within the A-band is a region known as the H-band, which is the region not superimposed by actin myofilaments. Within the H-band is the M-line, which is composed of myosin myofilaments and titin molecules crosslinked by myomesin.
Titin molecules connect the Z-line with the M-line and provide a scaffold for myosin myofilaments. Their elasticity provides the underpinning of muscle contraction. Titin molecules are thought to play a key role as a molecular ruler maintaining parallel alignment within the sarcomere. Another protein, nebulin, is thought to perform a similar role for actin myofilaments.
Model of Contraction
The molecular mechanism whereby myosin and acting myofilaments slide over each other is termed the cross-bridge cycle. During muscle contraction, the heads of myosin myofilaments quickly bind and release in a ratcheting fashion, pulling themselves along the actin myofilament.
At the level of the sliding filament model, expansion and contraction only occurs within the I and H-bands. The myofilaments themselves do not contract or expand and so the A-band remains constant.
The sarcomere and the sliding filament model of contraction: During contraction myosin ratchets along actin myofilaments compressing the I and H bands. During stretching this tension is release and the I and H bands expand. The A-band remains constant throughout as the length of the myosin myofilaments does not change.
The amount of force and movement generated generated by an individual sarcomere is small. However, when multiplied by the number of sarcomeres in a myofibril, myofibrils in a myocyte and myocytes in a muscle, the amount of force and movement generated is significant.
ATP and Muscle Contraction
ATP is critical for muscle contractions because it breaks the myosin-actin cross-bridge, freeing the myosin for the next contraction.
LEARNING OBJECTIVES
Discuss how energy is consumed during movement
KEY TAKEAWAYS
Key Points
ATP prepares myosin for binding with actin by moving it to a higher- energy state and a “cocked” position.
Once the myosin forms a cross-bridge with actin, the Pi disassociates and the myosin undergoes the power stroke, reaching a lower energy state when the sarcomere shortens.
ATP must bind to myosin to break the cross-bridge and enable the myosin to rebind to actin at the next muscle contraction.
Key Terms
M-line: the disc in the middle of the sarcomere, inside the H-zone
troponin: a complex of three regulatory proteins that is integral to muscle contraction in skeletal and cardiac muscle, or any member of this complex
ATPase: a class of enzymes that catalyze the decomposition of ATP into ADP and a free phosphate ion, releasing energy that is often harnessed to drive other chemical reactions
ATP and Muscle Contraction
Cross-bridge muscle contraction cycle: The cross-bridge muscle contraction cycle, which is triggered by Ca2+ binding to the actin active site, is shown. With each contraction cycle, actin moves relative to myosin.
Muscles contract in a repeated pattern of binding and releasing between the two thin and thick strands of the sarcomere. ATP is critical to prepare myosin for binding and to “recharge” the myosin.
The Cross-Bridge Muscle Contraction Cycle
ATP first binds to myosin, moving it to a high-energy state. The ATP is hydrolyzed into ADP and inorganic phosphate (Pi) by the enzyme ATPase. The energy released during ATP hydrolysis changes the angle of the myosin head into a “cocked” position, ready to bind to actin if the sites are available. ADP and Pi remain attached; myosin is in its high energy configuration.
The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin. As soon as the actin-binding sites are uncovered, the high-energy myosin head bridges the gap, forming a cross-bridge. Once myosin binds to the actin, the Pi is released, and the myosin undergoes a conformational change to a lower energy state. As myosin expends the energy, it moves through the “power stroke,” pulling the actin filament toward the M-line. When the actin is pulled approximately 10 nm toward the M-line, the sarcomere shortens and the muscle contracts. At the end of the power stroke, the myosin is in a low-energy position.
After the power stroke, ADP is released, but the cross-bridge formed is still in place. ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site. ATP can then attach to myosin, which allows the cross-bridge cycle to start again; further muscle contraction can occur. Therefore, without ATP, muscles would remain in their contracted state, rather than their relaxed state.
Control of Muscle Tension
Muscle tension is influenced by the number of cross-bridges that can be formed.
LEARNING OBJECTIVES
Describe the factors that control muscle tension
KEY TAKEAWAYS
Key Points
The more cross-bridges that are formed, the more tension in the muscle.
The amount of tension produced depends on the cross-sectional area of the muscle fiber and the frequency of neural stimulation.
Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere; less tension is produced when the sarcomere is stretched.
If more motor neurons are stimulated, more myofibers contract, and there is greater tension in the muscle.
Key Terms
tension: condition of being held in a state between two or more forces, which are acting in opposition to each other
Control of Muscle Tension
Neural control initiates the formation of actin – myosin cross-bridges, leading to the sarcomere shortening involved in muscle contraction. These contractions extend from the muscle fiber through connective tissue to pull on bones, causing skeletal movement. The pull exerted by a muscle is called tension. The amount of force created by this tension can vary, which enables the same muscles to move very light objects and very heavy objects. In individual muscle fibers, the amount of tension produced depends primarily on the amount of cross-bridges formed, which is influenced by the cross-sectional area of the muscle fiber and the frequency of neural stimulation.
Muscle tension: Muscle tension is produced when the maximum amount of cross-bridges are formed, either within a muscle with a large diameter or when the maximum number of muscle fibers are stimulated. Muscle tone is residual muscle tension that resists passive stretching during the resting phase.
Cross-bridges and Tension
The number of cross-bridges formed between actin and myosin determine the amount of tension that a muscle fiber can produce. Cross-bridges can only form where thick and thin filaments overlap, allowing myosin to bind to actin. If more cross-bridges are formed, more myosin will pull on actin and more tension will be produced.
Maximal tension occurs when thick and thin filaments overlap to the greatest degree within a sarcomere. If a sarcomere at rest is stretched past an ideal resting length, thick and thin filaments do not overlap to the greatest degree so fewer cross-bridges can form. This results in fewer myosin heads pulling on actin and less muscle tension. As a sarcomere shortens, the zone of overlap reduces as the thin filaments reach the H zone, which is composed of myosin tails. Because myosin heads form cross-bridges, actin will not bind to myosin in this zone, reducing the tension produced by the myofiber. If the sarcomere is shortened even more, thin filaments begin to overlap with each other, reducing cross-bridge formation even further, and producing even less tension. Conversely, if the sarcomere is stretched to the point at which thick and thin filaments do not overlap at all, no cross-bridges are formed and no tension is produced. This amount of stretching does not usually occur because accessory proteins, internal sensory nerves, and connective tissue oppose extreme stretching.
The primary variable determining force production is the number of myofibers (long muscle cells) within the muscle that receive an action potential from the neuron that controls that fiber. When using the biceps to pick up a pencil, for example, the motor cortex of the brain only signals a few neurons of the biceps so only a few myofibers respond. In vertebrates, each myofiber responds fully if stimulated. On the other hand, when picking up a piano, the motor cortex signals all of the neurons in the biceps so that every myofiber participates. This is close to the maximum force the muscle can produce. As mentioned above, increasing the frequency of action potentials (the number of signals per second) can increase the force a bit more because the tropomyosin is flooded with calcium.
Types of Muscle Tissue: Skeletal, Smooth, and Cardiac
Skeletal
aka "striated", aka "voluntary" - attached to bones and under conscious, willful control. Has the ability to contract (shorten) and thereby bring about movement
I. Muscle and Muscle Fiber Structure:
A muscle is composed of many muscle fibers (muscle fiber = muscle cell). The individual muscles are separated from each other and held in place by a covering called the FASCIA. This fascia also forms TENDONS and APONEUROSES connecting muscles to bones and muscles to muscles (tendons & ligaments)
A muscle also contains 3 different layers of connective tissue:
Epimysium - outermost layer, surrounds entire muscle
Perimysium - separated and surrounds the FASCICLES (bundles) of muscle fibers
Endomysium - surrounds each individual muscle fiber
Skeletal muscle fibers contain numerous nuclei and mitochondria --> Energy
The muscle fiber membrane is called the SARCOLEMMA and the cytoplasm is called the SARCOPLASM. Within the sarcoplasm are many parallel fibers known as MYOFIBRILS
Each myofibril is made of many protein filaments called MYOFILAMENTS. There are two types:MYOSIN - thick filamentsACTIN - thin filamentsStructure of a SARCOMERE (functional unit of a muscle)Actin and Myosin filaments are arranged in an overlapping pattern of light ("I" bands) and dark ("A" bands). In the middle of each "I" band is a line called a "Z" line. The section of a myofibril from one Z-line to the next Z-line is the SARCOMERE.*** The arrangement of these sarcomeres next to each other produces the STRIATIONS of the skeletal muscle fibers.Each Myofibril is surrounded by a network of channels called SARCOPLASMIC RETICULUM.
TRANVERSE TUBULES pass through the fibers to spread the cell membrane throughout the muscle.
II. How Muscles Work with the Nervous SystemNEUROMUSCULAR JUNCTION - where a NERVE FIBER and muscle fiber come together. A.K.A. Myoneural junction. MOTOR NEURON ENDINGS - nerve fiber caries the impulse that stimulates the muscle fibersMOTOR END PLATE - specialized part of muscle fiber membrane (sarcolemma) located at the neuromuscular junction, has many foldsSYNAPTIC CLEFT - An actual "gap" or cleft which exits between the motor neuron endings and the motor end plate.SYNAPTIC VESICLES - numerous vesicles in motor neuron endings, where neurotransmitters are stored before being released into the synaptic cleft.NEUROTRANSMITTER - substance that is released from nerve endings into synaptic cleft. Stimulates an impulse. In this case, a "muscle impulse". One of the major neurotransmitters is ACETYLCHOLINE. This brings about muscle contractions. CHOLINESTERASE is an enzyme that breaks down acetylcholineIII. Events in Muscle Contraction:
Nerve impulse stimulates the release of a neurotransmitter (acetylcholine) from synaptic vesicles into synaptic cleft ' stimulates muscle impulse ' impulse spreads across sarcolemma and into fiber along the T-tubules ' this impulse causes an increase in the cisternae's permeability to calcium ions. The S.R. has a high conc. of CA++ Calcium ions diffuse into the sarcoplasm ' the CA++ causes the formation of "cross bridges"
between the actin and myosin filaments ' the filaments slide between each other ' this shortens the myofibrils which in turn shorten the muscle fibers, which shortens the muscles
"Calcium Pump" returns CA++ into the S.R. (requires energy- ATP)
Enzyme Cholinesterase stops action of Acetylcholine
IV. ENERGY SOURCE:
Provided by ATP around myofibrils. ATP is produced by cellular respiration which occurs in the mitochondria (requires O2 and glucose)
* Creatine Phosphate provides energy for the regeneration of ATP
* Only 25% of energy produced during cellular respiration is used in metabolic processes - the rest is in the form of HEAT. This is what produces our body heat and maintains body temperature. More muscle activity = more heat
ATP = adenosine triphosphate | ADP = adenosine diphosphate
V: Other Terms
1. Threshold Stimulus
2. All-or-None Response
3. Motor Unit
4. Recruitment
5. Muscle Tone
6. Muscular Hypertrophy
7. Muscular Atrophy
8. Muscle Fatigue
9. Muscle Cramp
LABEL PRACTICEFascicle
Endomysium (2)
Bone
Muscle Fiber
Epimysium
Perimysium
Tendon
1. Name and describe the three different layers of connective tissue in a muscle.
2. Myofibrils are composed primarily of two protein filaments called _______________________ and __________________________
3. What is a motor unit?
4. Name and describe the parts of a neuromuscular junction.
5. What do acetylcholine and cholinesterase do?
6. Bundles of muscular fibers are called ________________________
7. The more active your muscles are, the more body heat they release. True or False?
8. Name the following structures
A. ______________________ Network of membranous channels that surround and run parallel to the myofibrils
B. ______________________Thick protein filaments within the A-bands
C. ______________________Protein filament that slides inward, toward the middle of a sarcomere, during a muscle contraction
D. ______________________ Conducts a muscle impulse deep into a sarcoplasm, to the cisternae
E. ______________________The segment of a myofibril between two Z-lines
9. What causes muscles to appear striated?
10. Neurotransmitters are stored in vesicles found in the ____________________________
11. Creatine phosphate serves to do what? (It supplies energy for what?)
12. What causes muscle fatigue and muscle cramps?
14. What is a threshold stimulus?
15. It is the function of the _________________________ in muscle contractions to supply energy for muscle fiber contractions.
16. What does the "all-or-none response" mean?
17. A "partial, but sustained, contraction" describes what?
18. The ________________________ appearance of skeletal muscles results from the arrangement of sarcomeres.
19. ____________________________ has the function of transmitting a muscle impulse into the interior of the cell.
20. ___________% of energy released by cellular respiration is available for use by metabolic processes - the rest is lost as body heat.
21. A substance called a ______________________ crosses the synaptic cleft and stimulates the muscle fiber to contract.
22. A _______________________ can be described as cordlike and connecting muscles to bones, while an ___________________ would be described as a fibrous sheet of connective tissue connecting muscles to muscles.
23. Describe the events of a muscle contraction in order. What is the sliding filament model?
24. __________________________is the shrinkage of a muscle due to lack of use.
25. What molecules pass in and out of the sarcoplasmic reticulum as a muscle fiber contracts and releases? _________________________
26. The A bands are [ light / dark ] whereas the I bands are [ light / dark ]
27. What are the three types of muscles? Which type(s) are voluntary? Which type(s) are striated?
28. Where do you find the motor end plate?
29. What is recruitment?
30. Muscle fibers are made of individual ___________________________ which are composed of ___________________ and ____________________