MUSCLES. FUNCTIONS OF MUSCULAR SYSTEM. Body movement Maintain posture Respiration Produce body heat Communication Constriction of organs and blood vessels Heartbeat . Connective Tissue Sheaths in Skeletal Muscle. Figure 10.1a. Connective Tissue Sheaths.
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Produce body heat
Constriction of organs and blood vessels
The fascicles are parallel. They are long fibers, which can contract to 75% of their length. They contract a long way, but they are relatively weak, because there are relatively few fascicles. E.g. Sternocleidomastoid.
PENNATE (means “feather shape”) MUSCLES: three types:
CONVERGENT MUSCLE has more fibers than parallel, but contracts a greater distance than pinnate. E.g. Pectoralis major.
CIRCULAR MUSCLE (Sphincter) is arranged in a circle, with a small area of tendon on the sides. It allows closure of the eyes, mouth, etc. They are not very strong, but they don’t need to be.
In skeletal muscle fibers, there are light and dark stripes called striations, which can be seen under a microscope.
A cross section of skeletal muscle looks like bundles of circles because you are looking at cut fascicles.
These striations (stripes) are caused by dark and light bands.
The dark band is called an A band. (There is an “A” in dark)
The light band is called an I band. There is an “I” in light)
dark band + light band is one sarcomere
In the center of each light I band is a Z disc
One sarcomere is the area from one Z disc to the next Z disc.
So, each sarcomere extends from the middle of one light band to the middle of the next light band. In the center of the dark band is a lighter colored area called the H zone. It is the area of the myosin without heads.
The A band consists of both actin and myosin.
Sarcomere model video 1
Sarcomere model video 2
MUSCLE FASCICLE: a group of muscle fibers, surrounded by perimysium.
MUSCLE FIBER: a single muscle cell
MYOFIBRIL: a long organelle inside a muscle fiber, contains actin and myosin myofilaments.
MYOFILAMENTS: these are proteins, and there are two types: actin (with troponin and tropomyosin) and myosin. The myofilament is the lowest level of organization that is composed of actin, myosin, troponin, and tropomyosin proteins.
Therefore, a myofilament is part of a myofibril, which is inside a muscle fiber, which is inside a muscle fascicle.
The Sliding Filament Theory
T tubules are in yellow
The sarcoplasmic reticulum stores a lot of calcium ions, which are released when the muscle is stimulated to contract.
The calcium diffuses out of the sarcoplasmic reticulum and land on the actin filaments, where they trigger the sliding filament mechanism of contraction.
After the contraction, the calcium ions are pumped back into the sarcoplasmic reticulum for storage.
In a muscle fiber, an action potential results in muscle contraction. How does this happen?
The action potential continues to travel along the sarcolemma (cell membrane of the muscle).
Part of this electrical impulse breaks away from the sarcolemma and travels down the T-tubules, while the rest of the electrical impulse continues longitudinally down the muscle cell to the next sarcomere and T-tubule.
TROPOMYOSIN is a single long protein strand like a piece of yarn that winds around the actin filament.
TROPONIN is a globular complex of three proteins, and is found in clumps around the tropomyosin protein.
Both troponin and tropomyosin cover the actin filament when the muscle is relaxed.
This is an illustration of an actin molecule. You can see the thready tropomyosin and the globular troponin proteins wrapping around the double-stranded actin.
When calcium binds to the globular troponin, it moves, taking the tropomyosin thread with it. This exposes the myosin binding site on the actin.
A new ATP molecule must bind to the myosin before the cross-bridge can be release. When ATP is not available after a person dies, the cross-bridges that have formed are not released, causing muscle to become rigid (rigor mortis)
NOTE: Sarcomeres lengthen during muscle relaxation only if gravity or an opposing muscle pulls the sarcomere back to its original length.
The action potential reaches the cell membrane
The action potential reaches the T-tubules
The ion channels in the sarcoplasmic reticulum open
Calcium ions move along their concentration gradient
Actin forms cross-bridges to myosin
The actin myofilaments move closer to each other, causing contraction of the sarcomere.
NOTE: A muscle fiber will not respond to a stimulus until that stimulus reaches the threshold level.
A muscle TWITCH is one single muscle fiber contraction.
A MOTOR UNIT is a single neuron and all of the muscle fibers on which it synapses.
If one neuron sends a signal, only its muscle fibers contract (the motor unit). This allows for strength variations in lifting a chair vs. an eraser. For full strength, all the motor units contract. For half strength, half of the motor units contract.
There are 3 motor units in this diagram; that allows for 3 different levels of contraction. The more motor units there are, the more precisely the muscle can respond.
The action potential continues from one motor neuron to the next motor neuron until the last neuron lands on its target cells; in this case, skeletal muscle fibers.
A single motor neuron and all the skeletal muscle fibers it innervates constitute a motor unit.
A muscle in your tongue may only have a few muscle fibers innervated by a neuron to allow for precise movement. However, large thigh muscles may have as many as 1000 muscle fibers per neuron, since precision is not necessary.
A muscle twitch has three phases
The lag phase is the time between the application of a stimulus and the beginning of contraction.
The contraction phase is the time of contraction.
The relaxation phase is the time during which the muscle relaxes.
The refractory period is the time between muscle twitches.
The strength of muscle contraction can vary from weak to strong. For example, the force generated by muscles to lift a feather is much less than the force required to lift a 25 pound weight.
The force of contraction produced by a muscle is increased in two ways:
Summation, which involves increasing the force of contraction of the muscle fibers within the muscle
Recruitment, which involves increasing the number of muscle fibers contracting
The force of contraction of individual muscle fibers is increased by rapidly stimulating them.
Stimulus frequency is the number of times a motor neuron is stimulated per second.
When the stimulus frequency is low, there is time for complete relaxation of muscle fibers between twitches.
As stimulation frequency increases, there is not enough time between contractions for muscles to completely relax.
Thus, one contraction summates, or is added onto, a previous contraction. As a result, the overall force of contraction increases.
Tetanus is the condition in which a muscle remains contracted between stimuli without relaxing.
The bacteria that make this toxin live deep in the soil and cannot survive in air.
If you step on something that imbeds soil deeply into your tissues (like a rusty nail), you might contract the bacteria.
You will need a tetanus shot before the toxins accumulate.
A toxin caused by a certain bacteria can cause muscle to remain contracted (in tetanus).
It quickly results in death because the diaphragm and other respiratory muscles cannot function properly, and the person suffocates.
In recruitment, the strength of contraction of the muscle is increased by increasing the number of motor units stimulated.
When only a few motor units are stimulated, a small force of contraction is produced, because only a small number of muscle fibers are contracting.
As the number of motor units stimulated increases, more muscle fibers are stimulated to contract, and the force of contraction increases.
Maximum force of contraction is produced in a given muscle when all the motor units of that muscle are stimulated, or recruited.
Motor unit recruitment allows muscles to have slow, smooth sustained contractions so our movements are not jerky.
If all the motor units in a muscle could be stimulated simultaneously, a quick, jerky motion would occur.
Because the motor units are recruited gradually so that some are stimulated and held in tetanus while additional motor units are recruited, slow, smooth, sustained contractions occur.
Muscle contractions are classified as either isometric or isotonic. Most muscle contractions are a combination.
Two types of isotonic contractions:
CONCENTRIC CONTRACTIONS are isotonic contractions in which the muscle tension increases as the muscle shortens. Most movements performed by muscle contractions are of this type.
ECCENTRIC CONTRACTIONS are isotonic contractions in which tension is maintained as the muscle lengthens. An example is when a person lets down a heavy weight slowly. Substantial force is produced in the muscles and injuries can occur from repetitive eccentric contractions, such as in the hamstring muscles when a person runs downhill.
Even when muscles are relaxed, some of their fibers are still contracting, giving the muscle some tone.
Therefore, the normal state of a muscle, with some contraction, is called muscle tone. This is important in posture so you can stand upright but mostly relaxed.
Muscle tone refers to the constant tension produced by muscles of the body over long periods of time. It is responsible for keeping the back and legs straight, the head held in an upright position, and the abdomen from bulging. it declines during REM sleep.
Upper motor neuron
Lower motor neuron
What fuel does a car use?
What fuel does a candle use?
What fuel do humans use?
Where do we get ATP?
We can make a little ATP in the cytoplasm of our cells, but not enough to live on.
Most of our ATP is made by the mitochondria inside our cells.
Mitochondria are like little protozoa (animals) that live in our cells. Each cell has hundreds of them. Muscle cells have thousands of them.
What is their fuel?
Oxygen and glucose
THAT is why we need to inhale oxygen and consume sugars….to feed our mitochondria so they can make ATP for us!
In order for the muscle mitochondria to produce enough ATP, they need oxygen (for their own aerobic respiration) and sugars that are in storage.
Mitochondria can only perform aerobic respiration.
What can we do to make ATP if our muscle cells run out of oxygen?
Start performing anaerobic respiration.
We can do this ourselves in the cytoplasm of our cells.
Takes place in the mitochondria
Breaks down glucose to produce ATP
Waste products are CO2 and H2O (we exhale them)
The good thing about making ATP from our mitochondria is that we can make a LOT of it.
The bad things are that it takes longer to make it, and it requires oxygen, and a muscle cell may have used up all the oxygen during a sprinting run.
Takes place in the cytoplasm
Does not require oxygen
Breaks down glucose to produce ATP
Waste product is lactic acid
The good thing about making ATP this way is that we can make it FAST.
The bad thing is that it does not make much ATP, and we deplete the reserves quickly.
The waste product of aerobic respiration is carbon dioxide and water. These are not a problem…we eliminate them by exhaling.
The waste product of anaerobic respiration is lactic acid, which can irritate muscle fibers, causing muscle pain (stitch in your side) and muscle cramps.
We deactivate lactic acid by adding oxygen to it. Therefore, breathing heavily adds the oxygen to our system to deactivate lactic acid, and the muscle pains go away.
What do we do when we run out of ATP?
Muscle fibers cannot stockpile ATP in preparation for future periods of activity.
However, they can store another high energy molecule called creatine phosphate, which is the storage form of ATP.
Creatine phosphate is made from the excess ATP that we accumulate when we are resting.
During short periods of intense exercise, the small reserves of ATP existing in a cell are used first.
Then creatine phosphate is broken down to produce ATP.
Why does sprinting require anaerobic respiration?
We use up all of the ATP faster than we can make it.
When we run out of ATP, we break down creatine phosphate to make more ATP.
When we run out of ATP and creatine phosphate, we start using anaerobic respiration to make more ATP.
When we run out of glucose, or too much lactic acid is built up, we have to stop and rest.
Anaerobic metabolism is ultimately limited by depletion of glucose and buildup of lactic acid within the muscle fiber.
Why do sprint runners tire out during the last part of a fast run?
Sprinting is an anaerobic activity…the oxygen requirement is quickly exceeded, so the muscle has to use anaerobic respiration to continue to contract. This requires a lot of glucose and also results in a buildup of lactic acid.
Once the sprint-runner has used up the available glucose, or has produced too much lactic acid, the muscles fatigue.
Anaerobic respiration produces lactic acid, which causes the painful cramps because it creates an oxygen debt.
The amount of oxygen needed to replenish the supply following aerobic demand is called the oxygen debt.
When you continue to breathe heavily after exercising, it means you have an oxygen debt.
Muscles can do without oxygen for a while pretty well, unlike the brain.
To pay back a minor oxygen debt, you just have to breathe heavily for a while.
This heavy breathing brings in oxygen, which is used to convert lactic acid to glucose, replenish the depleted ATP and creatine phosphate stores in the muscle fibers, and to replenish oxygen stores in the lung, blood, and muscles.
After the oxygen debt has been paid back, breathing returns to normal.
People who are in good physical condition can carry out both aerobic and anaerobic activities efficiently, and do not suffer from an oxygen debt for very long.
1. Improved muscular strength, endurance, flexibility
2. Improved cardio-respiratory endurance
3. Increased bone density and strength
4. Relief from depression
5. Increased HDLs
Fun Fact: -You use 200 muscles to take one step.
Lack of use causes muscle ATROPHY. This happens quickly. Astronauts can lose 40% of their muscle in two weeks! It is regained quickly, too.
Atrophy is a decrease in muscle size because of the decrease in myofilaments within muscle fiber.
Casting a broken limb also leads to temporary atrophy.
Severe atrophy involves the permanent loss of skeletal muscle fiber and the replacement of those fibers by connective tissue.
Damage to the nervous system, or a severed motor nerve can cause atrophy. The muscle becomes flaccid (having no tone) .
More info on Muscle Atrophy
This refers to a group of inherited muscle disorders in which skeletal, cardiac, and smooth muscle tissue degenerates and the person experiences progressive weakness and other symptoms, including heart problems.
The muscle is replacement by fat and other connective tissue.
This is a genetic lack of a protein called DISTROPHIN. It causes the muscle tissue to harden, inhibiting contraction, causing progressive paralysis.
Tendonitis is an inflammation of the tendon or its attachment point. It usually occurs from overuse of the muscle to which the tendon is attached.
A strain is a tear in a muscle. Remember, a sprain is a tear in a ligament.
A muscle strain will heal faster than a torn ligament because muscles have good blood supply and ligaments do not.
Muscle stimulator to relieve muscle spasms or to prevent muscle atrophy in casts
Common disorder in adults, especially women
Painful muscles, debilitating fatigue, sleep disturbance, and joint stiffness
Many trigger points: painful lumps in muscles
Treatment includes anti-inflammatory medicines, physical therapy, acupuncture, and exercise.
Muscle stimulators help
Ganglion cysts arise as outpouchings from fluid filled areas such as the fluid around tendon sheaths.
When the fluid, called synovial fluid, leaks out from these spaces, it can become a cystic structure.
Treatment is to drain the fluid with a needle, but the fluid can be jelly-like and difficult to remove, and they frequently grow back.
If conservative treatments fail to correct the cyst, an operation can be done to excise the cyst.
Then you do a surgery to scoop out the whole cyst, find the stalk and tie it off.
A Baker's Cyst, or popliteal cyst, is a collection of fluid in the back of the knee joint.
A Baker's cyst is usually a symptom of another problem, or it may be an incidental finding with no significant meaning.
Most often in adults the Baker's cyst is found in conditions where there is chronic swelling or fluid accumulation in the knee joint.
These conditions include knee arthritis, meniscus injuries, and ligamentous injuries.
Treatment of a Baker's cyst that is the result of a problem within the knee consists of treating the underlying problem. These treatments may include anti-inflammatory medications and cortisone injections.
With aging, fibrous connective tissue replaces some muscle fibers, causing decreased strength.
As people age, the number of muscle fibers decreases, and new ones cannot be added.
The strongest humans can lift about 3 times their own body weight, but the average gorilla can lift 10 times its own body weight! Gorillas can lift 4,600 pounds. By the way, they don’t drink water. They get it by eating 50 pounds of plants a day.
The rhino beetle can carry 800 times its own weight.
And, pound for pound, the African Crowned Eagle can carry more than a cargo plane, because it can fly carrying up to 4 times it's own weight. Something that would keep a cargo plane grounded.
But the strongest creature is the ant. If you had the strength of an ant, you could lift over your head and carry 6,600 pounds.
There are no distinct sarcomeres
They contract more slowly than skeletal muscle…their twitch time is very long = several seconds
It doesn’t get tired (“I’m too tired to urinate!”)
They contract in response to neurons as well as hormones and changes in local environment (amount of oxygen, lactic acid, etc).
They may be autorhythmic (self-exciting); they can contract spontaneously without being stimulated (like cardiac muscle).
They do not develop oxygen debt.
Only found in the heart.
The cells are involuntary (like smooth muscle) and striated (like skeletal muscle).
They have intercalated discs which join each cell.
Cardiac cells are long, striated, and branching, with one nucleus per cell.
The actin and myosin myofilaments are organized into sarcomeres, but not as uniformly as in skeletal muscle.
As a result, cardiac muscle cells are striated, but not as distinctly as skeletal muscle.
Cardiac muscle is involuntary and does not fatigue.
Cardiac muscle cells are connected to one another by intercalated discs which facilitate action potential conduction between themselves.
This allows them to function as a unit and they all contract together.
Contraction of cardiac cells is influenced by hormones, such as epinephrine.
As one cell contracts, the action potential goes through all the cells, and they all contract as a unit. That’s why the heart contracts all at once.
It has an intrinsic beat. The cells contract on their own, without a signal.
Even if you chop a heart up, each piece will beat by itself!