1
Q
stabilizes their structure somewhat like two strings of pearls twisted together.
A
nebulin
2
Q
Each muscle fiber is made up of rodlike contractile elements called____
A
myofibrils
3
Q
- The__ ___ consists mainly of myosin
A
thick myofilaments
4
Q
- The end of the axon (axon terminal) and the sarcolemma of the muscle fiber in a neuromuscular junction are separated by a gap of only 1-2 nm (nanometers) called the __ __
- Axon terminals release packets of neurotransmitters into this
A
synaptic cleft
5
Q
- This cell stays in the embryonic state (meaning it does not fuse with the others, nor does it differentiate), and becomes a __ __
- are stem cells that can become active under certain circumstances. For example, if the muscle fiber becomes damaged, they can help to replace the injured tissue and repair the muscle cell.
A
satellite cell
6
Q
- The other two molecules found in thin filaments are regulatory proteins.
- Two strands of ___ (green in the diagram above) spiral around the actin and block the myosin binding sites on G-actin (yellow) when the muscle is relaxed. This prevents formation of cross-bridges between the myosin heads and the active sites on actin.
A
tropomyosin
7
Q
- Another filament found in the I-band is an elastic filament composed of a huge protein called __
- The __ extends from Z-disc to the thick filament and attaches to the M-line which consists of proteins that link the thick filaments together.
A
titin
8
Q
- is the region where two excitable cells make chemical or electrical contact with each other.
A
Synapse
9
Q
- each fascicle is surrounded by CT called the ____
- divides muscle into bundles of muscle fibers called fascicles.
A
perimysium
10
Q
this theory states that the thin filaments slide over the thick filaments toward the center of each sarcomere. In other words, during muscle contraction the thin filaments slide past the thick ones so that the actin and myosin filaments overlap more. In a relaxed sarcomere (top diagram) the thick and thin filaments overlap only at the ends of the A-band. But when the muscle is stimulated by the nervous system, the zone of overlap increases as the thin filaments are pulled over the thick (bottom diagram).
As each sarcomere gets shorter, each myofibril gets shorter, then each muscle fiber gets shorter, causing the fibers in a fascicle to get shorter, and finally the entire muscle shortens.
A
Sliding filament model of muscle contraction
11
Q
- As mentioned previously, each muscle cell is formed from the fusion of embryonic mesodermal cells called ___.
- fuse together forming the mature muscle cell.
- If a muscle cell was made from 100 ___ during development, then this cell will have 100 nuclei.
A
Myoblasts
12
Q
- Each individual muscle is surrounded by a connective tissue sheath called the epimysium, and consists of hundreds to thousands of individual muscle cells that are arranged in bundles called ___
- consist of muscle cells
A
fascicles
13
Q
___ (brown) consists of 3 polypeptides: TnI is inhibitory , TnT binds to tropomyosin, and TnC binds to calcium ions. Together ___ and tropomyosin control the interaction between actin and myosin during contraction and relaxation.
A
Troponin
14
Q
Forms cross bridges with thin filaments during contraction.
A
Globular head
15
Q
- The sarcoplasm contains the myofibrils and a very specialized version of the endoplasmic reticulum called the ___ ___
- In muscle, the __ __ consists of smooth ER which surrounds each myofibril and ultimately connects to the sarcolemma via transverse or T-tubules
- Instead of the typical role of smooth ER, the __ __ in skeletal muscle sequesters calcium ions. It is so good at doing this that the total calcium concentration inside the membranes (or cisternae) of the SR can be 40,000 x more than that of the sarcoplasm.
- We will see later, that calcium is the “go” signal for muscle contraction. The T-tubules will carry a wave of depolarization from the sarcolemma down to the SR which will then release its calcium into the sarcoplasm. This results in an interaction between the thick and thin myofilaments which causes the muscle to contract. More on this later after we discuss all the molecular players that are involved.
A
sarcoplasmic reticulum
16
Q
- Each fascicle (which means stick, by the way) consists of numerous muscle cells.
- Muscle cells are long and cylindrical and are referred to as ____ but they are true cells with nuclei and other organelles, not extracellular fibers like collagen and elastin.
- multinucleated cylinder ranging from about 10-100 micrometers in diameter to as much as 30 cm (12 inches) in length!
- is formed during embryological development by the fusion of precursor cells called myoblasts.
- As mentioned previously, each __ __ is formed from the fusion of embryonic mesodermal cells called myoblasts.
A
muscle fibers
17
Q
- It is responsible for anchoring the thin filaments which are mainly made of actin.
A
alpha actinin
18
Q
The outermost epimysium blends with the ___ ___ that lies between neighboring muscles or the superficial fascia deep to the skin. It also extends beyond the muscle to form tendons (ropelike arrangements of dense regular CT) or aponeuroses (flat sheets of CT).
A
deep fascia
19
Q
- Each muscle fiber is made up of rodlike contractile elements called myofibrils. These in turn are composed of ___ which are the structural/functional unit of skeletal muscle.
- The ___ are made up of myofilaments of two types—thick and thin.
- When viewed under the light microscope, skeletal muscle has a banded or striated appearance. This is a result of the arrangement of the myofilaments contained in each myofibril. The myofilaments are organized into repeating units called __ which are the structural/function unit of striated muscle (skeletal and cardiac muscle are striated). Each myofibril consists of about 10,000 ____, and each ___ consists of precisely arranged myofilaments.
- a ____ is the region of a myofibril between two successive Z-lines.
A
sarcomeres
20
Q
- Looking more closely at the molecular make up of the sarcomere and the I and A bands, we can see that multiple proteins are involved .
- The __ __ is composed of discs of a protein called alpha actinin
A
Z-line
21
Q
- consist mainly of actin.
- consist of four protein (Tropomyosin
Troponin, F actin
Nebulin) but contain mainly actin.
A
thin myofilaments
22
Q
- Each individual muscle is surrounded by a connective tissue sheath called the _____, and consists of hundreds to thousands of individual muscle cells that are arranged in bundles called fascicles.
- Dense CT surrounding an entire muscle
A
epimysium
23
Q
- the plasma membrane or plasmalemma is called the ____
A
sarcolemma
24
Q
contains thick filament only.
A
the H-zone
25
* the cytoplasm is called the \_\_\_; it is similar in composition to that of other cells, but it contains granules of stored glycogen and myoglobin
* Myoglobin is a reddish pigment that stores oxygen similar to hemoglobin of RBCs
* muscle fibers contain numerous, densely packed cylindrical rods called myofibrils (each about 1-2 micrometers in diameter, accounting for about 80% of the volume of the cell). There are so many myofibrils that all other organelles such as mitochondria are crammed between them (and the nuclei are pushed to the periphery under the sarcolemma as previously discussed). The myofibrils contain the myofilaments which are the contractile elements of the cells.
sarcoplasm
26
* The SR is mesh-like (blue in the diagram above) except at the A & I-band junctions where it becomes enlarged forming channels called ___ \_\_\_(they always occur in pairs).
terminal cisternae
27
* Each thick filament consists of about 300 molecules of the contractile protein __ , and each ___ molecule is comprised of a long tail (two interwoven helical strands of polypeptides), a flexible hinge region, and a globular head that will form a connection or cross-bridge with actin during muscle contraction.
* In addition to binding with actin, the ___ heads contain ATPase enzymes that will generate energy to power muscle contraction by splitting ATP.
myosin
28
* The G-actin units form long filaments of actin called ___ \_\_
* Two strands of __ \_\_ are twisted together, and the molecule nebulin stabilizes their structure somewhat like two strings of pearls twisted together.
F-actin
29
The part of the muscle fiber’s sarcolemma found in a neuromuscular junction is highly folded to increase surface for millions of ACh receptors. This specialized part of the sarcolemma is called the __ \_\_ \_\_
motor end plate
30
* Contraction occurs when muscle fibers are activated by impulses from neurons of the central nervous system.
* When a nerve sends an electrical impulse (excitation) to the muscle it controls, the myofilaments slide past one another resulting in muscle contraction.
Excitation-Contraction Coupling
31
* ___ \_\_\_ are a unique modification of the sarcolemma.
* are located at the junction of each A-band and I-band. At these junctions, the sarcolemma dives into the interior of the cells forming these
* unique to muscle
* Membrane tubules continuous with the sarcolemma that extend into the sarcoplasm at right angles to the cell surface.
* Conduct electrical impulses (action potentials) into the cell resulting in contraction of the fiber. The intersection of __ \_\_ with SR is called a triad.
Transverse or T-tubules
32
* Each strand of actin is made up of subunits called __ \_\_.
* These have active sites to which the myosin heads attach during contraction.
* The __ \_\_ units form long filaments of actin called F-actin (for fibrous or filamentous actin).
globular or G-actin
33
* which consists of proteins that link the thick filaments together.
M-line
34
* The dark bands of striated muscle are called __ \_\_
* each __ \_\_ contains a region of overlap between thin and thick filaments, and has a light region in its midline called the H-zone (H for helle which mean bright).
* consists of mainly of thick filaments, the M-line, H-zone and a zone of overlap between thick and thin filaments.
A-bands
35
* The muscle fibers are surrounded by thin CT called\_\_\_ which also contains a stem cell called a satellite cell.
* surrounds individual muscle cells = muscle fibers.
endomysium
36
The axon terminal is contain membrane sacs or vesicles of a chemical neurotransmitter called \_\_\_
acetylcholine (ACh)
37
* the light bands are __ \_\_
* contain only thin myofilaments, and have a Z-line (or Z-disc) running right through their middle. Each sarcomere extends from one Z-line to the next. In other words, a sarcomere is the region of a myofibril between two successive Z-lines.
I-bands
38
* Skeletal muscle fibers are controlled by action potentials carried by the axons of motor neurons. The cell bodies of motor neurons reside in the brain or spinal cord. Their axons are bundled up by connective tissue sheaths forming nerves. The axon of each motor neuron divides as it enters the muscle and each axon ending gives off several short branches. Each branch ends at a ___ \_\_\_ ; each muscle fiber has only one __ \_\_ located about halfway along its length.
* The synapse between nerve and muscle is called a __ \_\_
neuromuscular junction
39
* The T-tubules (yellow) join the terminal cisternae of the SR forming a __ (two terminal cisterns + 1 T-tubule = 3).
triad
40
The arrival of an electrical impulse (action potential) from an axon to the axon terminal of a neuromuscular junction opens voltage-gated calcium channels which causes calcium to rush in from the extracellular fluid. This influx of calcium causes the synaptic vesicles to fuse with the axonal membrane and release ACh into the synaptic cleft via exocytosis. ACh diffuses across the cleft and binds to its receptors on the motor end plate of the sarcolemma. This opens chemically-gated sodium and potassium ion channels. More sodium flows in than potassium flows out, so the inside of the sarcolemma becomes less negative.
depolarization
41
The sudden inrush of sodium ions causes the muscle sarcolemma to generate an action potential that spreads across the sarcolemma and ultimately down the T-tubule to the sarcoplasmic reticulum. This results in the release of calcium into the sarcoplasm. The release is short lived, only lasting about 0.03 second due to the amazing ability of ATP-dependent calcium pumps in the SR membranes, but the concentration of calcium loose in the sarcoplasm increases 100x compared to when the muscle is at rest (unstimulated). The calcium facilitates the interaction between the thick and thin myofilaments (actin and myosin) by binding to the regulatory protein, troponin. This results in the shortening of the sarcomere, etc., etc., etc. This is the upshot of \_\_\_-\_\_\_ \_\_\_\_, and the triad is the location of the coupling.
excitation-contraction coupling
42
After ACh binds to its receptors it is broken down by an enzyme located in the synaptic cleft called \_\_\_. The destruction of ACh prevents continued muscle contraction without the proper “command” from the central nervous system. Interestingly, caffeine inhibits the action of acetylcholine esterase and this is why our hands become a little shaky after drinking too much coffee or other highly caffeinated beverage.
acetylcholinesterase
43
Until now, we’ve been discussing how sarcomeres and myofibrils get shorter when they contract as if neither end of the fiber were attached. That’s what occurs in isolated muscle fibers, but that’s not what happens in the body. Typically, muscles are attached to bone at both ends. Each muscle crosses at least one joint. The end of the muscle that is fixed in position is called the \_\_\_
origin
44
When the muscle contracts, the movable bone moves toward the less movable or fixed bone. The end of the muscle that moves toward the fixed end is called the ___ . In the picture above, notice the difference between the myofibril of a muscle fiber that has neither end fixed (a) versus the more realistic situation (b).
insertion
45
The force exerted by a contracting muscle on an object is called __ \_\_
muscle tension
46
the opposing force exerted on the muscle by the weight of the object to be moved is called the \_\_\_. Muscle fibers are either producing tension or they are relaxed. Each muscle is controlled by at least one motor nerve (remember a nerve consists of a bundle of hundreds of axons of motor neurons).
load
47
A motor neuron and all the muscle fibers it innervates is called a __ \_\_. When a motor neuron sends an action potentials, all the muscle fibers it innervates contract simultaneously. The number of muscle fibers per motor unit varies tremendously from as few as 3-4 in muscles where we need fine control (eye muscles, intrinsic muscles of the larynx, finger muscles) to hundreds in muscles that support the weight of the body or produce large locomotor movements.
motor unit
48
The response of a motor unit to a single action potential of its motor neuron is called a \_\_\_. In other words, a ___ is the result of a single stimulus that produces a single contraction and ends with relaxation of a muscle fiber. The muscle fibers contract quickly and then relax.
twitch
49
Muscle contraction can be studied in the lab using isolated muscle or in living animals (or people) by inserting small electrodes in the muscle and supplying an electrical current. The pattern produced is projected on to the computer screen and the resulting curve is referred to as a \_\_\_. Each twitch is divided up into three phases: the latent period, contraction phase, and relaxation phase. A single twitch may last a little as 7 milliseconds (eye muscles) to about 100 milliseconds (soleus) depending on muscle. Twitches are used by the body to generate heat by shivering, but otherwise they are not useful for normal activity because twitches are too brief.
myogram
50
The ___ \_\_\_ is the first 2-3 milliseconds following stimulation by the axon. This is the excitation part of “excitation-contraction” coupling. What’s occurring is that the action potential is spreading across the sarcolemma and down the T-tubules and calcium is being released from the sarcoplasmic reticulum.
latent period
51
The ___ \_\_ takes on average about 15 milliseconds (10-100 msec depending on the muscle). As calcium levels in the sarcoplasm rise, calcium binds to troponin, more and more active sites are exposed and an increasing number of cross-bridges are formed. Tension increases to a peak as the number of cross-bridges increase.
contraction phase
52
The third and final part of a twitch is the ___ \_\_. This lasts on the average about 25 milliseconds (ranging from 10-100 msec). During relaxation, calcium levels in the sarcoplasm are declining as calcium is being actively pumped back into the SR; the number of cross-bridges decrease as well until there are no more. At this point, the myogram has returned back to baseline or the resting level.
relaxation phase
53
* Increasing ____ of motor neuron action potentials (wave summation and tetanus).
* how rapidly the stimuli are applied to the muscle
frequency
54
Increasing the ___ of the stimulus which increases the number of stimulated motor units (recruitment)
strength
55
To begin to understand how whole muscles work, let’s look at what happens when a second stimulus arrives to a muscle immediately following the relaxation phase of a twitch. In this diagram, the green arrows represent when a stimulus was applied to the muscle. Note that repeating the stimulation immediately following the relaxation phase of the first twitch results in a slightly greater increase in tension. If another stimulus (of the same strength) is applied, the next twitch is greater, and so on.
This “step-wise” increase in tension is called ___ (which means stairs in German). What’s happening is that the muscle is getting “primed” for more productive contraction. There’s a slight increase in calcium in the sarcoplasm, and increased activity of enzymes and pumps. Essentially, the muscle is getting warmed-up for greater demands that may be placed on it. The step-wise increase in twitch tension occurs for up to 30-50 stimulations, after that there is no further increase in peak tension. It is thought that when athletes warm-up for an event, they are taking advantage of the treppe phenomenon.
treppe
56
If we leave the strength of the stimulus the same, but increase the frequency of stimulation we can observe the phenomenon of __ \_\_. The nervous system can increase muscle tension by increasing the firing rate of motor neurons. If the rate increases such that a second stimulus arrives before the relaxation phase of the previous twitch has ended, the second twitch will “ride” or add onto the first twitch. This creates greater tension because the second contraction occurs before the muscle has completely relaxed, so the muscle is already partially contracted. What’s happening at the molecular level is that the calcium does not have time to return completely to the SR before the next stimulus arrives. Therefore, the concentration of calcium in the sarcoplasm increases which means that more cross bridges can be formed. The more cross-bridges, the greater the tension. This phenomenon is called __ \_\_ because the individual “waves” of the twitches add on to each other.
wave summation
57
If the muscle is stimulated at an increasingly faster rate (increasing the frequency which decreases the time between applied stimuli), the relaxation time between twitches becomes shorter and shorter and the concentration of calcium in the sarcoplasm rises higher and higher. Summation continues with tension getting greater and greater until a sustained but quivering contraction is reached. This is called __ \_\_. Note that the dashed line represents the maximum possible tension that can be achieved by this muscle. Although incomplete tetanus has caused the muscle to reach a peak value that is about four times greater than that of reached by treppe, it is still below the maximum tension achievable by this muscle.
unfused or incomplete tetanus
58
As stimulation frequency is increased even more, the relaxation phase of the twitch is completely eliminated. That means that no calcium can return to the SR, and the calcium levels are so high that contraction is continuous. At this point, the muscle reaches\_\_ __ .
maximal tension
59
Because the relaxation phase has been eliminated, the contractions fuse into a smooth, sustained plateau of contraction. This is called __ (because the individual twitches have __ together) or complete tetanus (the word tetanus comes from tetan which means rigid or tense).
fused
60
After a while, a muscle working a maximal or near maximal tension (as in vigorous exercise), will exhibit ___ . At this point, even if the stimuli continued, the muscle would not be able to maintain the contraction and the tension would eventually drop to zero.
61
The Muscular System
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