Muscles!

Web resources from Kevin: Check these out to see if they are of any use (but don't copy them to this site due to copyright issues).

Blackwell Publishing
McGraw Hill
There are others you can find. Two of my links from last year are no longer good, so see what you can dig up.



Types of Muscle

• Smooth Muscle - Involuntary
  • Located in GI tract, uterus, blood vessels.
  • Plays a role in regulating blood pressure by regulating flow of blood through capillaries.
    • Thicker in arteries than in veins.
  • One nucleus per cell.
  • Controlled by ANS.
• Cardiac Muscle - Involuntary
  • Located only in the heart
  • Does not need nervous stimulation to beat, has its own intrinsic rhythmic system
    • The rate of the beat is influenced by the ANS, but not controlled by it
    • One single cell will beat intrinsically
  • Contains intercalated discs between cells
    • These discs connect the cells electrically, so depolarization can spread from one cell to another. This makes it so the ventricles and atria can contract together, keeping the rhythm of the heart even. When these discs fail to move together, you'll experience heart fibrillations
  • These cells are much shorter than skeletal muscle cells
• Skeletal Muscle - Voluntary
  • Skeletal muscle makes up approximately 40% of our body weight
  • These muscles are attached to bones
    • The origin of the muscle is the (relatively) unmovable part. (For the bicep, it is the coracoid process of the scapula and the interrubercular groove of the humerus)
    • The insertion of the muscle is the moveable part. (For the bicep, it is the radial tuberosity)
  • Each muscle cell has its own nervous stimulation. This does NOT mean each muscle cell has its own neuron, since one neuron can have many branching ends.
    
    • The ratio of nervous stimulation to muscle cells is higher in muscles needing finer motor control. For example, your eyes have a higher ratio of nervous stimulation to muscle cell than do your quadriceps.
      
    • A group of muscle cells innervated by ONE neuron is called a MOTOR UNIT.
      
  Skeletal muscle cells are longer cells and have multiple nuclei.

How do muscles work?
Skeletal muscles are about 40% of body weight and another 10% in smooth muscle.

Contracting a Muscle:
The thick and thin filaments do the actual work of a muscle, the way they do this is pretty cool. Thick filaments are made of a protein called myosin. At the molecular level, a thick filament is a shaft of myosin molecules arranged in a cylinder. Thin filaments are made of another protein called actin. The thin filaments look like two strands of pearls twisted around each other.
During contraction, the myosin thick filaments grab on to the actin thin filaments by forming crossbridges. The thick filaments pull the thin filaments past them, making the sarcomere shorter. In a muscle fiber, the signal for contraction is synchronized over the entire fiber so that all of the myofibrils that make up the sarcomere shorten simultaneously. There are two structures in the grooves of each thin filament that enable the thin filaments to slide along the thick ones: a long, rod-like protein called tropomyosin and a shorter, bead-like protein complex called troponin. Troponin and tropomyosin are the molecular switches that control the interaction of actin and myosin during contraction. During contraction, the thin filaments slide past the thick filaments,
shortening the sarcomere.

While the sliding of filaments explains how the muscle shortens, it does not explain how the muscle creates the force required for shortening. To understand how this force is created, let's think about how you pull something up with a rope:

1. Grab the rope with both hands, and then arms will extended.
2. Loosen your grip with one hand, let's say the left hand, and maintain your grip with the right.
3. With your right hand holding the rope, change your right arm's shape to shorten its reach and pull the rope toward you.
4. Grab the rope with your extended left hand, and then release your right hand's grip.
5. Change your left arm's shape to shorten it and pull the rope, returning your right arm to its original extended position so it can grab the rope.
6. Repeat steps 2 through 5, alternating arms, until you finish.

Muscles create force by cycling myosin crossbridges.
To understand how muscle create force, let's apply the rope example. Myosin molecules are golf-club shaped. For our example, the myosin clubhead (along with the crossbridge it forms) is your arm, and the actin filament is the rope:

1. During contraction, the myosin molecule forms a chemical bond with an actin molecule on the thin filament (gripping the rope). This chemical bond is the crossbridge. For clarity, only one cross-bridge is shown in the figure above (focusing on one arm).
2. Initially, the crossbridge is extended (your arm extending) with adenosine diphosphate (ADP) and inorganic phosphate (Pi) attached to the myosin.
3. As soon as the crossbridge is formed, the myosin head bends (your arm shortening), thereby creating force and sliding the actin filament past the myosin (pulling the rope). This process is called the power stroke. During the power stroke, myosin releases the ADP and Pi.
4. Once ADP and Pi are released, a molecule of adenosine triphosphate (ATP) binds to the myosin. When the ATP binds, the myosin releases the actin molecule (letting go of the rope).
5. When the actin is released, the ATP molecule gets split into ADP and Pi by the myosin. The energy from the ATP resets the myosin head to its original position (re-extending your arm).
6. The process is repeated. The actions of the myosin molecules are not synchronized -- at any given moment, some myosins are attaching to the actin filament (gripping the rope), others are creating force (pulling the rope) and others are releasing the actin filament (releasing the rope).

Isotonic vs. Isometric ContractionThe shortening of the fibers creates mechanical force, or muscle tension. Whether the muscle itself changes length (same-force or isotonic contraction) or not (same-length or isometric contraction) depends upon the load attached to the muscle. For example, your biceps muscle is attached to your shoulder blade at one end and to your ulna in your forearm at the other end. When the biceps contracts, it shortens and pulls the ulna toward the shoulder blade (the ulna is attached to the elbow joint). This movement allows you to lift your forearm and a given load. In contrast, if you are carrying a heavy load, such as a full suitcase, that makes you unable to lift your forearm, then the biceps does not shorten significantly. But the force that the muscle generates is helping you carry the suitcase.

The contractions of all muscles are triggered by electrical impulses, whether transmitted by nerve cells, created internally (as with a pacemaker) or applied externally (as with an electrical-shock stimulus).







Anatomy and Vocabulary of Skeletal Muscle


-A MUSCLE is bound in FASCIA
-Fascia comes together to form a TENDON which attaches the muscle to bone
-A MUSCLE is bound into compartments called FASCICLES
-Fascicles contain many MUSCLE CELLS or MUSCLE FIBERS. These are multinucleated cells and are very long (as long as the muscle)
-Muscle cells contain many MYOFIBRILS, which are bundles of MYOFILAMENTS (ACTIN and MYOSIN).
-Myofibrils are divided into segments known as sarcomers
-SARCOMERES are the basic unit of contraction, and there are many along each myofibril. A sarcomere is the distance from one Z disc to another. Each sarcomere will shorten during contraction, thus shortening the entire cell (and the entire muscle).


This diagram compares the striations seen on the muscle fiber to the arrangement of the actin and myosin filaments which account for these striations. The proper name for each band of striations is shown for both the myofibril and the sarcomere. Note that the I band crosses two sarcomeres, so it is only labelled on the myofibril section (½ of the I band is in each of two sarcomeres). Skeletal muscles are organized stripes or bands. The wider dark band is the A band and consists of myosin filaments (with some actin overlap). The lighter band is the I band, which is the region with only actin filaments (which are thinner than myosin). The thin dark stripe is the Z band and is the boundary of the sarcomere.


Sliding Filament Theory

1- An action potential reaches the axon of a motor neuron
2- This action potential opens voltage-gated calcium ion channels, allowing calcium to rush in
3- The calcium then allows ACh to be released into the cleft
4- ACh binds to receptors on the muscle cell, which open Na+ channels, thus depolarizing the muscle cell.
5- The action potential travels along the sarcolemma (cell membrane) of the muscle cell and down T-tubules (transverse extensions of the sarcolemma).
6- The action potential on the T-tubules triggers the release of calcium ions from the sarcoplasmic reticulum
7- This free calcium binds to the troponin protein, which is found on the tropomyosin, a long molecule that covers the binding sites on actin. When the troponin and calcium bind, the troponin moves the tropomyosin away, exposing a binding site for myosin.
8- The myosin heads bind to the binding sites on the actin, forming cross-bridges. The cross bridge then ratchets, causing a contraction which shortens the cell by approximately 1%
9- ATP then binds to the myosin head of the cross-bridge, breaking the actin-myosin bond. The myosin head will cleave the ATP, which cocks it into position again. If the binding sites are still uncovered it will again bind and ratchet, bind and ratchet, as long as the binding sites are exposed and ATP is present.
10- The ratcheting pulls the actin filaments towards the center of the sarcomere, shortening the muscle.
11- During this process, calcium is constantly being pumped into the sarcoplasmic reticulum, so if there is not continuous nervous stimulation (which results in calcium ion release), the levels of calcium will fall and the tropomyosin will again block the binding sites on the actin filament. The result will be relaxation of the muscle.

Short narrative version: ACh is released into the neuromuscular junction, binding to ligand-gated receptors. These receptors open the gates for the sodium ion channels. This in turn allows the action potential to travel along the sacroplasmic reticulum, through the T tubules, and to the sarcolemma. The Ca+ ions them come out, and the ions diffuse into the sacroplasm. The Ca+ ions then bind to troponin, which changes shape and uncovers a binding site on the actin. With this binding site exposed, the myosin head binds to the actin by forming a cross bridge. The cross bridge binds and pulls on the actin filament, allowing for contraction.

Slow-and-Fast-Twitch Fibers

Slow twitch fibers have a rich capillary supply, numerous mitochondria and aerobic respiratory enzymes, and a high concentration of myoglobin. Myoglobin is a red pigment that improves the delivery of oxygen to the slow-twitch fibers. So people who physical train their body to have high endurance will have a higher percentage of slow-twitch fibers. Fast-twitch fibers are thicker, have fewer capillaries and mitochondria than slow-twitch fibers and not as much myoglobin. These have more of a white pigment and are adapted to respire anaerobically by a large store of glycogen. World-class sprinters and weight lifters will have a high percentage of fast-twitch fibers because their muscles are used only for a short period of time with but with a great deal of force.

DISEASES

Atrophy-

Atrophy is the partial or complete wasting away of a part of the body. Some causes of atrophy include poor nourishment poor circulation, loss of hormonal support, loss of nerve supply to the target organ, disuse or lack of exercise or disease intrinsic to the tissue itself.
Disuse atrophy of muscles and bones, with loss of mass and strength, can occur after prolonged immobility, such as extended bed rest, or having a body part in a cast (living in darkness for the eye, bedridden for the legs, etc). This type of atrophy can usually be reversed with exercise unless severe. Astronauts must exercise regularly to minimize atrophy of their limb muscles while they are in microgravity.

Dystrophy-

Muscular dystrophy refers to a group of genetic, hereditary muscle diseases that cause progressive muscle weakness. Muscular dystrophy's are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.

Botulism-

Botulism is a rare, but serious paralytic illness caused by a nerve toxin, botulin, (produced by the bacterium Clostridium botulinum). Botulinic toxin is one of the most powerful known toxins: about one microgram is lethal to humans. It acts by blocking nerve function and leads to respiratory and musculoskeletal paralysis.
Botox is Botulinic toxin used in minute doses both to treat painful muscle spasms, and as a cosmetic treatment in some parts of the world.

Tetanus-

Tetanus is a medical condition that is characterized by a prolonged contraction of skeletal muscle fibers.



Review Questions
1. What is the main way to maintain muscle tone and body temperature?
a. Muscle twitch
b. regular exercise
c. healthy diet
d. all of the above

2. What percent of your weight is skeletal muscle tissue?
a. 60%
b. 20%
c. 90%
d. 40%

3. What are the three types of muscle tissue?
a. smooth
b. cardiac
c. rough
d. skeletal
e. a,b and d

4. Which is not an example of where you would find smooth muscle tissue?
a. heart
b. uterus
c. esophagus
d. eye

5. Which of these muscles have motor units with the highest innervation ration?
a. leg muscles
b. arm muscles
c. muscles that move the fingers
d. muscles of the trunk

6. Spastic paralysis may occur when there is damage to __________.
a. the lower motor neurons.
b. the upper motor neurons.
c. either the lower or the upper motor neurons.

7. What causes myosin to release from actin once a muscle fiber has finished contracting?
a. ATP
b. ADP
c. Calcium
d. Glucose
e. Nothing, it lets go once it has finished.

8. Fast-twitch fibers have less ________ than slow-twitch fibers?
a. Melanin
b. Mitochondria
c. Myoglobin
d. A and C
e. B and C

9. The sarcolemma in skeletal muscle is analogous to the:
a. Cytoplasm
b. Endoplasmic Reticulum
c. Nucleus
d. Mitochondria
e. Cell Membrane

10. Within a muscle the fibers are divided into larger bundles called ____, each surrounded by its own connective tissue sheath.
a. sarcomeres
b. sarcolemmas
c. myofibers
d. fascicles


11. The term ____ is synonymous with muscle cell.
a. myofiber
b. sarcomere
c. myofibril
d. fascicle

12. Skeletal muscle cells are different from other cells in that they
a. lack smooth endoplasmic reticulum
b. have no mitochondria
c. have multiple nuclei
d. depend entirely on anaerobic respiration

13. Skeletal muscle cells are striated that is, they have alternating dark and light bands called ____, respectively.
a. A and I bands
b. H and M bands
c. Z and M lines
d. I and H bands

14. Contraction in a muscle occurs because the
a. thin filaments get shorter
b. thick filaments get shorter
c. thin filaments slide between the thick filaments
d. titin proteins pull on opposite ends of the sarcomere

15. The region of the resting sarcomere where the thin and thick filaments are overlapping is seen in the
a. narrow dark line know as the Z line
b. dark color of the I bands
c. lighter region of the I bands
d. dark color of the A bands

16. What is the definition of a Motor Unit?
a. somatic motor neuron
b. a group of muscle fibers in the arm
c. somatic motor neuron, together with all the muscle fibers this neuron innervates.
d. an autonomic nerve in colon, together with all the muscle fibers it innervates.
e. either c or d

17. ATP attaches to the myocin head releasing it from actin. At this point what happens if calcium ions still are present?
a. nothing because the myocin is power hungry.
b. the ATP combines with a phosphate attaching to the actin and contracts.
c. ATP is hydronized into ADP and a phosphate and myocin moves up to actin dropping P and with the power stored from ADP causes filaments to slide.
d. b or c

18. A muscle cell contains an Endoplasmic Reticulum (ER). True or False.
a. True, all cells have an ER
b. False, A muscle cell contains a Sarcoplasmic Reticulum not an ER.

19. What organ in the body has the unique ability to stretch several times greater than its original size for an extended period of time and then return to normal size without any harm to the organ?
a. colon
b. stomach
c. uterus
d. bladder

20. A 1 microgram dose of what bacterium is lethal for humans, but today is used in microscopic units for therapeutic and cosmetic treatments.
a. Penicillin
b. Salmonella
c. Clostridium Botulinum
d. Botox