The Skeletal System: bone, bone formation, bone diseases ...



Chapter 6: Muscular System

Lecture Notes taken predominantly from:

Marieb,E.N. 2009. Essentials of Human Anatomy and Physiology. PBC

The muscular system allows us to move our bodies around (usu. skeletal muscle) and to move stuff inside our bodies (usu. smooth muscle). 3 types of muscle tissue exist:

1. Skeletal muscle – is multinucleate (many nuclei in one cell), long (some over 0.5m!), striated (striped), and typically under voluntary control (we must think to make it contract). Skeletal muscle can also contract over at variable speeds from very fast to quite slowly. Aside from voluntary movement, skeletal muscles also generate body heat, maintain posture, and stabilize joints. Most are connected to bones by tendons.

2. Cardiac muscle – cells have 1 nucleus, branched, striated (like skeletal), and controls the pimping of the heart involuntarily. Cardiac muscle is arranged in spiral sheets around the chambers of the heart and is only found here. Contraction speed is relatively slow compared to rapid skeletal contractions.

3. Smooth muscle – cells w/ 1 nucleus, slender cells with pointy ends (fusiform), usually come in 2 crossed sheets (circular and longitudinal) around hollow organs. Usually contract slowly to move urine, empty bowels, move food, etc.

All muscle cells (also called muscle fibers) come surrounded by a layer of connective tissue known as the endomysium. Skeletal muscle also contains larger bundles within a second layer of connective tissue known as perimysium (= muscle fasicles), and entire skeletal muscles are covered with an epimysium (=fascia) also of connective tissue. These layers of connective tissue fuse at the ends of muscles to form tendons (connect muscle to bone) and aponeuroses (sheet-like tendons). Tendons resist heavy strain and often must cross over joints.

A closer look at Skeletal Muscle:

The cell membrane of the muscle fiber/cell = sarcolemma - specialized to maintain a voltage across the cell membrane. Each skeletal muscle cell contains 100s of contractile organelles known as myofibrils. The arrangement of proteins within each myofibril accounts for the striations of skeletal muscle. The I-band = light bands w/ only thin actin protein fibers and the A-band = dark bands with thick myosin protein fibers (+ actin fibers). The thin actin fibers are held in place by a special membrane known as the z-disc that can be found in the center of each I-band. The space between 2 z-disc membranes is known as the sarcomere. So, each muscle fiber/cell has hundreds of myofibrils, and each myofibril can have lots of sarcomeres from one end to the other. Each sarcomere = the fundamental unit of contraction within each muscle.

How do sarcomeres contract?

1. A nerve cell (motor neuron) sends an electrical signal to the end of its fiber and stimulates the release of acetylcholine into a tiny gap between the end of the nerve cell and the sarcolemma (cell membrane) of the muscle cell. This tiny gap is known as the neuromuscular junction. Acetylcholine diffuses across the neuromuscular junction and binds with receptors on the sarcolemma.

2. The binding of acetylcholine causes the sarcolemma to open Na+ channels and allow Na+ ions to diffuse rapidly across the cell membrane and into the cell. The voltage across the sarcolemma drops to zero (even a little negative) as a result.

3. The voltage drop stimulates the sarcoplasmic reticulum (the smooth endoplasmic reticulum of a muscle cell) to release Ca2+ ions.

4. These Ca2+ ions act to pull the trigger on small projections (myosin heads) found along the length of thick myosin fibers. The myosin heads bind to the thin actin fibers and then spring toward the center of each sarcomere, pulling the actin fibers from each side of the sarcomere closer together.

5. If ATP is present (usually is) the myosin heads are ‘re-cocked’ by ATP so that if Ca2+ is still around or is released again, the muscles (i.e. sarcomeres within each myofibril within each muscle cell within each muscle) will contract more fully or again.

Muscle will continue to contract as long as there is a supply of ATP. Stored ATP just free in the muscle cell only maintains a contraction for 2 seconds. Creatine Phophate (CP) molecules can quickly regenerate ATP (by donating its phosphate to ADP), but that only lasts for up to 15 seconds. If you need more out of your muscles than that, aerobic respiration or anaerobic respiration is required.

Aerobic respiration makes ATP from glucose (sugars) and O2. This can generate ATP for hours as long as there is enough O2 available (can bicycle, jog, walk for long whiles). Unfortunately, the process is rather slow (in part because it takes time to replenish O2).

If aerobic respiration is too slow for our energy demands (weight lifting, sprinting), anaerobic respiration can keep us moving for a minute or so without O2. However, anaerobic respiration requires lots of fuel (glucose) and creates a build up of lactic acid that interferes with making more ATP.

Muscle fatigue = the depletion of ATP. If ATP manufacture cannot meet demands, you can run out of ATP. No matter how hard you try, your muscle will not respond.

Muscle tone = the involuntary and constant contraction of muscle fibers that help keep your muscle ready for action.

Exercise and muscles:

Anaerobic exercise (weight-lifting, sprinting, for ex.) increase muscle size and strength. Aerobic exercise (biking, running for ex.) increase strength and endurance. Aerobic exercise also improves metabolism, digestion, coordination, brain power, etc. Get 30-60 minutes, almost everyday to meet the most common ultimate health recommendations.

5 Rules of Skeletal Muscle Activity:

1. Skeletal muscles typically cross a joint.

2. Bulk of muscle lies proximal to the joint that is crossed.

3. All muscle have at least 2 attachments (origin – part that remains still; insertion – part that moves).

4. Skeletal muscles only pull (i.e. contract) – they NEVER push.

5. During contraction, insertion moves toward origin.

Body movements have particular names:

flexion = bend, or decrease of angle between bones (perpendicular to frontal plane); extension = opposite of flexion, extend to 180( (make even with frontal plane); hyperextension = extend beyond 180(.

Rotation = to rotate around a central axis, like when shaking your head ‘no’.

Abduction = move laterally away from the midline; Adduction = move back toward midline.

Circumduction = combines flexion, extension, abduction, and adduction to trace a circle with the distal end of a limb (typical of ball and socket joints).

Special movements associated with particular parts of the body include: dorsiflexion (bringing toe toward the shin/tibia) and plantar flexion (pointing toes); inversion (sole of foot turned inward/medially) and eversion (sole turned laterally); supination (palms face outward, anteriorly) and pronation (palms turned backward, posteriorly); and opposition (thumb tips touching fingertips of same hand).

Muscles can be categorized by function: Prime mover = the muscle responsible for a particular movement; Synergist = helper muscle that stabilizes joins or aids in coordinating a movement; Fixator = type of synergist that keeps a bone in place so that a movement can be made (like holding the scapula in place when you punch). Antagonist = the muscle that would contract to perform the opposite movement. For eg. The biceps brachii are the prime mover when the elbow is flexed; the triceps brachii = antagonist (extends elbow).

Naming muscles: Muscles have some funky Latin names, but their names are quite descriptive if you learn the Latin meanings. For example, Rectus femoris = straight (rectus) muscle found over the femur (femoris). Size (maximus, medius), location (temporalis), number of origins (triceps, biceps), shape (deltoid = triangular), and action (flexor, extensor) are all meaningful descriptors of the muscles in the body. There are OVER 600 muscles! So, memorizing a little Latin can go a very long way in memorizing muscle names and locations.

Know the names, insertion points (parts that are moved), origin points (places that remain fixed during contraction), and the movements that are caused by the following muscles:

Head and Neck Region: Orbicularis Oculi; Orbicularis Oris; Masseter; Temporalis; Sternocleidomastoid

Trunk Muscles: Pectoralis Major; Intercostals; Rectus Abdominus, Trapezius; Latissimus Dorsi; Deltoid

Arm Muscles: Biceps Brachii; Triceps Brachii

Leg Muscles: Gluteus Maximus; Gluteus Medius; Hamstring = Biceps Femoris, Semimembranosus, and Semitendinosus; Quadriceps group = Rectus Femoris and Vastus Muscles; Gastrocnemius.

Muscular Dystrophy = an inherited homeostatic imbalance (disorder) in which muscle fibers deteriorate abnormally. The most severe form, Duchenne’s muscular dystrophy, only effects boys and is typically diagnosed between 2-7 years old. Death occurs in early 20s (respiratory failure).

Muscle tissue declines with age. By age 80, muscle strength is about 50% of peak that occurs in mid-twenties.

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