LECTURE 7
MUSCULAR SYSTEM:
Muscles have only one function; to contract.
Bend your arm (biceps- is contracting). What happens if I relax? Nothing.
To extend the arm, I need another muscle. Therefore, you need lots of muscles for all the complex movements of the body.
CROSS SECTION OF A MUSCLE
The MUSCLE FASCIA is loose fibrous connective tissue on the outside of the muscle. It creates a slippery surface for muscles to rub against each other. Deep to the fascia is the
EPIMYSIUM, (dense irregular fibrous connective tissue), and which eventually becomes the tendon (which is connected to bone). The epimycium extends into the muscle belly to form compartments called FASICLES. This tissue surrounding the fascicles is now called the PERIMYCIUM. Each fascicle contains MUSCLE FIBERS, which are individual muscle cells, each one surrounded by ENDOMYCIUM. When you eat steak and find it’s stringy, each string is a fascicle, and the fat around the whole outside of the slice of meat is near where the fascia is.
TYPES OF MUSCLE PATTERNS: Depends on the pattern of the fascicles.
1. PARALLEL MUSCLE is when 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.
2. PENNATE MUSCLES: three types:
- UNIPENNATE: the fascicles are short, but there are more of them. They are stronger, but do not have the same length contraction ability of the parallel muscles.
- BIPENNATE are fascicles that insert into the tendon from both sides; they are stronger than unipennate.
c. MULTIPENNATE are the strongest (biceps femoris). The fascicles are in multiple bundles inserting onto one tendon
3. CONVERGENT MUSCLE has more fibers than parallel, and contract a greater distance than pinnate. E.g. Pectoralis major.
4. 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.
TERMS:
ORIGIN = The region which usually doesn’t move when the muscle contracts. Look at the biceps brachii; does the shoulder move when I bend my arm? No; the shoulder = origin.
INSERTION= The point of attachment that moves; bend arm, radial tuberosity = attachment.
AGONIST = The main muscle for a particular action; bend arm, biceps = agonist.
SYNERGIST = The muscle that helps the agonist. There are several muscles that assist when the arm is bent.
ANTAGONIST = Does the opposite action; bend elbow, antagonist extends. Every muscle in the body has to have an antagonist.
TYPES OF MUSCLE CELLS
1. Skeletal
2. Smooth
3. Cardiac
1. SKELETAL MUSCLE
These are very long fibers (biceps muscle can be 8-10 cm).
They have lots of nuclei (1,000’s of them).
They originate from stem cells, called MYOBLASTS, thousands of which fuse together to form one muscle fiber. That’s why one skeletal muscle fiber has a lot of nuclei.
· Myoblasts exist in adults, so damaged muscle is easily replaced; muscle tears heal well.
· A muscle cell torn in half can regenerate two cells, like a worm.
· There are almost no muscle diseases, because muscle can heal.
HISTOLOGY
• Skeletal muscle has striations (like cardiac muscle)
• It is voluntary (unlike smooth or cardiac muscle)
The plasma membrane of muscles is called a SARCOLEMMA.
The cytoplasm of muscle cells is called SARCOPLASM.
MYOFILAMENTS WITHIN SARCOMERES
In skeletal muscle fibers, there are light and dark stripes called striations, which can be seen under a microscope. These striations result from the internal structure of SARCOMERES within the sarcoplasm. The sarcomere is the basic unit of contraction in skeletal muscle. Every dark band + light band is one sarcomere. Sarcomeres consist of two types of myofilaments made out of protein: thin (ACTIN) filaments and thick (MYOSIN) filaments. Both ends of a thick filament are studded with knobs called myosin heads.
MECHANISM OF CONTRACTION: The Sliding Filament Theory
Contraction results as the myosin heads of the thick filaments attach like hooks to the thin actin filaments at both ends of the sarcomere and pull the thin filaments toward the center of the sarcomere. The myosin head is like a hook with a hinge. After a myosin head pivots at its hinge, it draws the actin closer, then lets go, springs up again to grab the actin filament again, pulls it closer, and it keeps repeating this until the entire actin filament has been drawn in as far as it can go. The complete process takes only a fraction of a second. The thick and thin filaments do not shorten; they merely slide past each other.
The energy required is ATP. This sliding filament mechanism begins whenever calcium ions bind to the thin filament. Where does the calcium come from?
SARCOPLASMIC RETICULUM AND T TUBULES
Within the cytoplasm of all body cells is an endoplasmic reticulum. The endoplasmic reticulum in muscle cells is called the SACROPLASMIC RETICULUM. It surrounds each sarcomere like the sleeve of a loosely crocheted sweater. Most of the “yarn fibers” of this “knit sweater” that run longitudinally, but some run perpendicular to them and surround structures called T tubules.
The sarcoplasmic reticulum stores a lot of calcium ions, which are released when the muscle is stimulated to contract. The calcium diffuses through the sarcoplasmic reticulum to the thin 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.
For contraction to take place, you need two things: nerve signal and calcium
• For skeletal muscle to contract, the synaptic knob of a neuron must first release a chemical called ACETYLCHOLINE onto the region where it sits on the muscle cell, known as the ENDPLATE.
• Calcium is also needed for muscle contraction.
• The nerve signal is called an ACTION POTENTIAL.
• It causes a release of calcium from the muscle fiber, which causes contraction.
Muscle contraction begins when a nerve triggers an impulse that travels along the sarcolemma (cell membrane of the muscle). Part of this nerve impulse breaks away and travels down the T tubules, while the rest of the nerve impulse continues longitudinally down the muscle cell to the next sarcomere and T tubule (“T” stands for “transverse”). T TUBULES are continuations of the sarcolemma (cell membrane) which invaginate to the deepest regions of the muscle cell. Since the T tubules conduct the nerve impulse throughout the muscle cell, all the sarcomeres of that cell contract at the same time.
The action potential of the nerve goes down the T-tubules and causes calcium to leak out of the sarcoplasmic reticulum. The calcium causes the muscle fibers to contract. After a while, the calcium gets pumped back where it came from, the muscle fibers move back into position, and the muscle will relax. How does the calcium cause the muscle fibers to contract?
TROPOMYOSIN is a single long protein strand 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. Tropomyosin blocks the attachment site for the myosin head, so the myosin “hook” has nothing to grab onto, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the myosin heads can attach to actin and produce muscle contraction. When the calcium gets pumped back where it came from, the tropomyosin protein blocks the myosin head again so it can no longer get its hook into the actin filament, and the muscle will relax.
Troponin is found in both skeletal muscle and cardiac muscle, but not in smooth muscle.
Both cardiac and skeletal muscles are controlled by changes in the intracellular calcium concentration. When calcium rises, the muscles contract, and when calcium falls the muscles relax.
http://www.youtube.com/watch?v=CepeYFvqmk4
http://www.youtube.com/watch?v=WRxsOMenNQM&feature=related
http://www.youtube.com/watch?v=InIha7bCTjM&NR=1
TWITCH = single muscle fiber contraction (takes 1/20th of a second).
How is it that I can pick up and hold a chair if the fiber only contracts for 1/20th second?
There are lots of fibers per muscle (tens of thousands), each one contracting at different intervals, so contraction is maintained, just like tug-of-war. One person in ten can drop the rope and get a better grip because the others are maintaining the tension. In a muscle, there are 1,000s of fibers.
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 muscles of the back are larger motor units (more muscle fibers per neuron), but there are fewer motor units present = strength, but less precision. The muscles that move the eye are smaller motor units (10 fibers per neuron), but there are many motor units present = less strength, more precision.
Muscles also respond to forces:
Exercise à HYPERTROPHY (growth in size); can happen in two ways
1. Increase in number of fibers
2. Increase in size of individual fibers
Lack of use à ATROPHY. This happens quickly. Astronauts can lose 40% of their muscle in two weeks! It is regained quickly, too. If a neuron is cut, muscle atrophies.
Eating protein also does not increase muscle. The average person only needs one ounce of protein a day, two if you work out. Two ounces is like one mini hamburger. Most people eat too much meat.
Fun fact: You use 200 muscles to take one step.
MUSCLE TONE
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.
ENERGY FOR MUSCLE CONTRACTION
What do you think is the energy molecule needed? ATP.
In order for the mitochondria to produce enough ATP, it needs oxygen and the sugars that are in storage.
MUSCLE SPASMS are sudden and involuntary muscle contractions usually caused from overexertion. Avoid them by stretching before and after activities. You need heat and massage to increase circulation. A portable muscle stimulator that clips to your belt will help a great deal.
OXYGEN DEBT
The amount of oxygen needed to replenish the supply following anaerobic 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. But for marathon runners to pay back an extreme oxygen debt, they have to eat a lot of carbohydrates over a two-day period. That’s why they load up on pasta before a marathon.
EXERCISE
There are many physiological benefits of exercise:
1. improved muscular strength, endurance, flexibility
2. improved cardio-respiratory endurance
3. increased bone density and strength
4. relief from depression and increased HDLs
MUSCLE PROBLEMS:
MUSCULAR DYSTROPHY
This is a genetic lack of a protein called DISTROPHIN. It causes the muscle tissue to harden, inhibiting contraction, causing progressive paralysis.
MUSCLE STRAINS
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.
Treatments for injuries: RICE
o Rest
o Ice - 20 minutes on, 20 minutes off. Use ice pack or frozen bag of peas!
o Compression- Ace wrap from distal to proximal. Don’t leave any openings while wrapping.
o Elevation – Above the heart.
Treatments for Injuries:
o Ice for the first 72 hours (NO heat!)
o Anti-inflammatory medicines
o Ibuprofin, 600 mg TID (3x a day)
o Over the counter (OTC) pills are 200mg
o Heat and massage are needed after the third day.
o Can try a muscle stimulator too…works pretty well!
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 they frequently grow back. Then you do a surgery to scoop out the whole cyst, find the stalk and tie off!
Baker’s Cysts, or popliteal cyst, is a collection of fluid in the back of the knee joint. It 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 a 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. The cyst can be drained with a needle, but the fluid can be jelly-like and difficult to remove. If conservative treatments fail to correct the cyst, an operation can be done to excise the cyst.
2. SMOOTH MUSCLE CELLS
Found around internal organs (intestines, uterus, etc). They are involuntary and not striated. When smooth muscle contracts around the intestines, the movement is called PERISTALSIS.