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I.
Row, Row, Row Your Boat
A. The McCagg sisters have genetically determined characteristics (tall bodies, long thigh bones) that contribute to
their amazing rowing ability.
B. But they must also endure rigorous training to develop the bone and muscle mass that is necessary for their
sport.
II. Basic Bone Functions and Structure.
1.
The bones are moved by muscles; thus the whole body is movable.
2.
Bones protect vital organs such as brain and lungs.
3.
The bones support and anchor muscles.
4.
Bone tissue acts as a depository for calcium, phosphorus, and other ions.
5.
Parts of some bones are sites of blood cell production.
6.
There are four types of bones: long (arms), short (wrist), flat (skull), and irregular (vertebrae).
7. Bone is a connective tissue with living cells (osteocytes) and collagen fibers distributed throughout a
ground substance that is hardened by calcium salts.
III. Bone Development:
A. How Do Bones Develop?
1.
Osteoblasts secrete material inside the shaft of the cartilage model of long bones.
2.
Calcium is deposited; cavities merge to form the marrow cavity.
3. Eventually osteoblasts become trapped within their own secretions and become osteocytes (mature bone
cells).
4. In growing children, the epiphyses (ends of bone) are separated from the shaft by an epiphyseal plate
(cartilage) which continues to grow under the influence of growth hormone until late adolescence.
IV. How the Skeleton Grows and Is Maintained
A. How Bone Remodeling Works
1. Bone is renewed constantly as minerals are deposited (by osteoblasts) and withdrawn (by osteoclasts)
during the "remodeling" process.
2.
Bone turnover helps to maintain calcium levels for the entire body.
3. Parathyroid hormone causes bone cells to release enzymes that will dissolve bone tissue and release
calcium to the interstitial fluid and blood; calcitonin stimulates the reverse.
B. Bone Remodeling Over Time
1.
Before adulthood, bone turnover is especially important in increasing the diameter of certain bones.
2. Osteoporosis (decreased bone density) is associated with decreases in osteoblast activity, sex hormone
production, exercise, and calcium uptake.
Overview of the Skeleton
The 206 bones of a human are arranged in two major divisions (axial and appendicular).
Bones are attached to bones by ligaments; bones are connected to muscles by tendons.
V. Joints
A. Synovial joints are the most common and move freely; they include the ball-and-socket joints of the hips and
the hingelike joints such as the knee.
1.
They are stabilized by ligaments.
2.
A capsule of dense connective tissue surrounds the bones of the joint.
3.
The capsule produces synovial fluid that lubricates the joint.
4.
In osteoarthritis, the cartilage at the end of the bone degenerates.
5. In rheumatoid arthritis, the synovial membrane becomes inflamed, the cartilage degenerates, and bone is
deposited into the joint.
B. Cartilaginous joints (such as between the vertebrae ) have no gap, but are held together by cartilage and can
move only a little.
C. Fibrous joints also have no gap between the bones and hardly move; flat cranial bones are an example.
VI. Science Comes to Life: Replacing Joints
VII. The Muscular System and How It Interacts With the Skeleton
A. All types of muscle are capable of contracting in response to stimulation and returning to the original resting
position.
1.
Skeletal muscle responds to nervous system signals and interacts with the skeleton to cause movement.
2.
Cardiac (heart) muscle contracts intrinsically.
3.
Smooth muscle (intestine) responds to stimulation by nerves, hormones, and can contract intrinsically.
B. How Skeletal Muscles and Bones Interact
1.
The human body's skeletal muscles (600 of them) are arranged in pairs or groups.
2. The origin end of the muscle is designated as being attached to the bone that moves relatively
little; whereas, the insertion is attached to the bone that moves the most.
3. Because most muscle attachments are located close to joints, only a small contraction is needed to
produce considerable movement of some body part (leverage advantage). Some work together
synergistically; others operate antagonistically.
4. Reciprocal innervation dictates that only one muscle of an antagonistic pair (e.g. biceps and triceps) can
be stimulated at a time.
VIII.
A Closer Look at Muscles
A. Skeletal muscle is organized in units.
1. Muscles are composed of individual muscle cells (fibers), each of which is composed of many
myofibrils, divided into contractile units called sarcomeres.
2.
Myofibrils are composed of thin (actin), and thick (myosin) filaments.
a) Each actin filament is actually two beaded strands of protein twisted together.
b) Each myosin filament is a protein with a head (projecting outward) and a long tail (which
is bound together with others).
B. The action of muscle is described by the sliding-filament model.
1. Within each sarcomere there are two sets of actin filaments, which are attached on opposite sides of the
sarcomere; myosin filaments lie suspended between the actin filaments.
2. During contraction, the myosin filaments physically slide along and pull the two sets of actin
filaments toward each other at the center of the sarcomere; this is called the sliding-filament
mechanism of contraction.
a) Cross-bridges form between the heads of myosin molecules and actin filaments.
b) When a myosin head is energized, it attaches to an adjacent actin filament and tilts in a
power stroke toward the sarcomere's center.
c)
Energy from ATP drives the power stroke as the heads pull the actin filaments along.
d) After the power stroke the myosin heads detach and prepare for another attachment
(power stroke).
e)
ATP supplies the energy for both attachment and detachment.
f)
A single contraction involves multiple power strokes.
g) At death, there is no ATP to cause the heads to detach, and the body enters rigor mortis.
IX. Control of Muscle Contraction
A. Skeletal muscles contract in response to signals from the nervous system that trigger action potentials along the
plasma membrane and into the interior of the muscle cell.
1. Eventually the signal reaches the sarcoplasmic reticulum (internal tubes), which responds by releasing
stored calcium ions that will bind to troponin, which is associated with another protein, tropomyosin, both of
which are parts of the actin filaments.
2. When calcium binds to troponin, the conformation of actin changes allowing myosin cross-bridges to
form.
3. When nervous stimulation stops, calcium ions are actively taken up by the sarcoplasmic reticulum and
the changes in filament conformation are reversed; the muscle relaxes.
B. Junctions Between Nerves and Muscles
1. At neuromuscular junctions impulses from the branched endings of motor neurons pass to the muscle
cell membranes by acetylcholine.
2. When the neuron is stimulated, calcium channels open allow calcium ions to flow inward, causing a
release of acetylcholine into the synapse.
C. Sources of Energy for Contraction
1. During periods (few seconds) of intense muscle activity, creatine phosphate is the source of phosphate to
remake ATP.
2. During intense and prolonged muscle action, anaerobic lactate fermentation produces low amounts of
ATP and leads to a buildup of lactate.
3. When muscle action is moderate, most of the ATP is provided by aerobic electron transport
phosphorylation, which is dependent on oxygen supply and number of mitochondria present.
X. Properties of Whole Muscles
A. Muscle Tension
1. The cross-bridges that form during contraction exert muscle tension–a mechanical force that can perform
work.
2.
An isometrically contracting muscle develops tension but does not shorten.
3.
An isotonically contracting muscle shortens and moves a load.
B. How Motor Units Function
1.
A single, brief stimulus to a motor unit causes a brief contraction called a muscle twitch.
2.
A second stimulus that quickly follows the first results in temporal summation.
3.
Repeated stimulation without sufficient interval causes a sustained contraction called tetany
4.
Individual muscle cells contract according to the all-or-none principle.
5. The number of motor units that are activated determines the strength of the contraction: Small
number of units = weak contraction; large number at greater frequency = stronger contraction.
6. Muscle tone is the continued steady, low-level of contraction that stabilizes joints and maintains
general health.
C. "Fast" and "Slow" Muscle
1.
Humans have two general types of skeletal muscle cells:
a) "Slow" muscle is more red in color due to myoglobin and blood capillaries; its
contractions are slower but more sustained.
b) "Fast" or "white" muscle cells contain fewer mitochondria and less myoglobin but can
contract rapidly and powerfully for short periods.
2. When athletes train, one goal is to increase the relative size and contractile strength of fast (sprinters)
and slow (distance swimmers) muscle fibers.