THE SKELETAL-MUSCULAR SYSTEM The Muscles you are RESPONSIBLE for:
You are responsible to know the following list of muscles. In addition you must know the location of the muscle on a model and at least 2 actions of each of these muscles.
ANTERIOR MUSCLES:
Orbicularis oris Pectoralis major External oblique Sternocleidomastoid Biceps brachii Deltoid
Vastus lateralis Frontalis (Epicranius) Rectus femoris Sartorius
Gracilis
Adductor Group Fibularis longus Temporalis
Orbicularis oculi Zygomaticus
Masseter
Vastus medialis Tibilalis anterior Transversus abdominus Rectus abdominis
POSTERIOR MUSCLES:
Adductor muscles Gluteus maximus Gluteus medius Gastrocnemius Latissimus dorsi Deltoid Semitendinosus Semimembranosus Soleus
Biceps femoris Triceps brachii External oblique Trapezius
Remember to review the characteristics of Skeletal Muscle Tissue under Histology! There are four general functions of skeletal muscle:
1. Movement: When skeletal muscle contracts, it produces movement of the attached bones and joints.
2. Maintain Posture: Our muscles, especially our back muscles are always partially contracted to help give our body frame shape! Our arms and legs and back, and stomach…..all of our body looks the way it does because different muscle groups help maintain the posture of that particular body part!
3. Stabilize Joints: Muscle tendons cross over, lay lateral and medial to joints to help provide added support at that joint.
4. Generate Heat: Every time a muscle contracts it does work, and that work generates heat. This heat generated by muscles helps us maintain our body temperature.
Like all specialized cells skeletal muscle cells have some “different” names for cell structures that you are already familiar with. Before we begin with these structures it is important to know that a muscle cell is also called a muscle fiber or simply a fiber. All three have the same meaning!
1. Sarcolemma: The outer membrane around a muscle fiber. It provides a limit or boundary to the cell and some control of entry.
2. Sarcoplasm: This is the cytoplasm of the muscle cell. It contains all of your typical organelles.
3. Sarcoplasmic Reticulum: The “endoplasmic reticulum” of the muscle fiber. It houses calcium until required for contraction.
4. Mitochondria: Have the same function as in any cell, to produce ATP, except they are extremely active in muscle cells because muscle cells are constantly contracting and therefore need a lot of energy!
5. Myofibrils: Are longitudinally arranged units of the fiber. They are cross banded.
6. Myofilaments: Protein strands, longitudinally arranged inside myofibrils. Two types of filaments exist, thick and thin. You will learn more about them when we talk about molecular structure.
7. T-Tubules: Tubules are separate from sarcoplasmic reticulum and help
communicate with the cells interior. They are the avenue for passage of impulses into the fiber.
Two general categories of filaments exist, THICK and THIN
1. The thick filament protein is called Myosin. Each myosin molecule has a distinctive structure. It has a rod-like tail terminating in two globular heads. These heads are sometimes called cross bridges. They are the “business end” of myosin because they link the thick and thin myofilaments together during contraction. Each thick filament within the sarcomere contains about 200 myosin molecules. Besides bearing actin
binding sites, there heads of myosin contain ATP binding sites and ATPase enzymes that split ATP to generate the energy for muscle contraction.
2. There are three thin filament proteins, Actin, Troponin, and Tropomyosin. The thin filaments are composed chiefly of actin. The polypeptide subunits of actin contain active sites to which the myosin cross bridges attach during contraction. Tropomyosin is a regulatory protein of actin. Two strands of tropomyosin spiral around actin and help stiffen it. In a relaxed muscle fiber the tropomyosin filaments block the myosin binding sites on actin. The other major protein in the thin filament is troponin. It is actually a three polypeptide complex. One polypeptide is an inhibitory subunit that binds to actin. An other binds to tropomyosin and helps to position it on actin. The third binds calcium ions. Both troponin and tropomyosin help to control the myosin-actin interactions involved in contraction.
STEPS INVOLVED IN A MUSCLE CONTRACTION:
*Please note there are a number of ways to lists the steps involved in a muscle contraction, the following has thus far been the most favorable by other students that have taken the course and so I have adopted the following:
1. Nerve impulse (stimuli) moves towards the neuromuscular junction (space between the nerve and the muscle cell).
2. The axon end of the nerve releases a neurotransmitter (chemical substance) called acetylcholine.
3. Acetylcholine moves over the entire sarcolemma and then deep inside the muscle cell through the T-tubules.
4. This electrical impulse causes the sarcoplasmic reticulum to release calcium. 5. Calcium combines with troponin on actin.
6. Myosin and actin can now interact.
7. Muscle cell contracts and therefore the muscle cell shortens. This takes ATP. 8. Calcium returns to the sarcoplasmic reticulum. This takes ATP.
9. Troponin interferes with the actin/myosin complex again. 10. Muscle cell relaxes again.
Keep in mind this whole process happens to each and every muscle cell activated. Remember, it takes more muscle cells to pick up your book bag than it does to pick up a pencil or pen!
Each muscle fiber/cell is enclosed in a delicate connective covering called the ENDOMYSIUM.
Several covered muscle fibers are then wrapped by a more ridged fibrous membrane called a PERIMYSIUM to form a bundle of fibers called a FASICLE.
Many fasicles are bound together by an even tougher layer of connective tissue called an EPIMYSIUM, which covers the entire muscle.
The epimysia blend into strong cord-like TENDONS, or into sheet like APONEUROSES which connect muscles indirectly to bones or to other connective tissue coverings.
NOTE, A DIAGRAM WOULD BE VERY HELPFUL IN GOING OVER THE FOLLOWING:
MYOFIBRILS or simply FIBRILS are contractile organelles found in the cytoplasm of muscle cells. Alternating light (I) and dark (A) bands along the length of the perfectly aligned myofibrils give the muscle cell as a whole its striped appearance. A closer look at the banding pattern reveals that the light I band has a midline interruption, a darker area called the Z disk, and the dark A band has a lighter central area called the H zone. It is this banding pattern that reveals the working structure of myofibrils. The myofibrils are actually chains of tiny contractile units called sarcomeres, which are aligned end-to- end along the length of the myofibrils. AND, it is the arrangement of even smaller structures, myofilaments…that you already know about…myosin and actin etc..within the sarcomere that actually produces the banding pattern.
Thin filaments are anchored to the Z disk. The light I band includes parts of the two adjacent sarcomeres and contains only thin filaments. Although the thin filaments overlap the ends of the thick filaments, they do not extend into the middle of the relaxed sarcomere and thus this middle or central zone is the H zone which does not contain actin filaments.
In a fully contracted sarcomere, the light H zone in the center of the A band has disappeared, the Z disks are closer to the thick filaments, and the I bands have nearly disappeared. The A bands move closer together but do not change in length.
ENERGY USE FOR MUSCLE CONTRACTION: HOW MANY ATP’S DOES EACH OF THE FOLLOWING METHODS GIVE US TO USE FOR ENERGY!!!!
1. Direct phosphorelation of ADP by reaction with creatine phosphate (CP): Energy Source= CP
Oxygen Use=NONE
Products=1 ATP per CP, creatine Duration of Energy= 15 seconds of energy
2. Anaerobic mechanism (glycolysis and lactic acid formation): Energy Source=glucose
Oxygen Use=NONE
Products=2 ATP per glucose, lactic acid
Duration=30 to 60 seconds of energy
3. Aerobic mechanism (oxidative phosphorylation)
Energy Source: glucose; pyruvic acid; free fatty acids from adipose tissue; amino acids from protein catabolism
Oxygen Use=REQUIRED
Products=36 ATP per glucose, CO2, H2O Duration=Hours of energy!!!!
