Skeletal Muscle: Structure and Contraction II

Interval Training

Alternating periods of high intense exercise interspersed with periods of rest (aerobic and anaerobic training methods)

Electrodiagnostic Medicine

A branch of medicine concerned with the diagnosis of neuromuscular disorders. Nerve conduction velocity (NCV) tests measure the speed of nerve impulses conducted through nerves outside the brain and spinal cord; for example, those of your limbs.

Muscle Tone

A small amount of tautness or tension in the muscle due to weak, involuntary contractions of its motor units.

Creatine Phosphate

A small, amino acid-like molecule that is synthesized in the liver, kidneys, and pancreas and then transported to muscle fibers. An energy storage molecule used by muscle tissue. The phosphate from this can be removed and attached to an ADP to generate ATP quickly.

What is the second step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

ACh binds to receptors in the motor end plate, ultimately triggering a muscle action potential.

What is the first step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

A nerve action potential in a somatic motor neuron triggers the release of acetylcholine (ACh)

Curare

A neurotoxin that causes paralysis by blocking acetylcholine receptors in muscle.

Myogram

A chart of the timing and strength of a muscle's contraction

Hypotonia

A condition in which there is diminished tone of the skeletal muscles

What is the third step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

Acetylcholinesterase destroys ACh so another muscle action potential does not arise unless more ACh is released from the somatic motor neuron.

What is the fifth step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

Ca 2+ binds to troponin on the thin filament, exposing the myosin-binding sites on actin.

What is the seventh step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

Ca 2+ release channels close and Ca 2+ ATPase pumps use ATP to restore low levels of Ca 2+ in the sarcoplasm.

Central Fatigue

Caused by the CNS and psychological mechanisms and manifests as feeling "tired" or not wanting to go on

Spasticity of Muscles

Characterized by increased muscle tone (stiffness) associated with an increase in tendon reflexes and pathological reflexes

Motor Units

Consists of a somatic motor neuron plus all of the skeletal muscle fibers it stimulates. Whole muscles that control precise movements consist of many small motor units. For instance, muscles of the larynx (voice box) that control voice production have as few as two or three muscle fibers per motor unit, and muscles controlling eye movements may have 10 to 20 muscle fibers per motor unit.

What is the sixth step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

Contraction: myosin head binds to actin, undergo power strokes, and release; thin filaments are pulled towards the center of the sarcomere.

Relaxation Period

During this time, calcium ions are actively transported back into the sarcoplasmic reticulum, myosin-binding sites are covered by tropomyosin, myosin heads detach from actin, and tension in the muscle fiber decreases

Contraction Period

During this time, calcium ions bind to troponin, myosin-binding sites on actin are exposed, and cross-bridges form

Anaerobic Glycolysis

Energy-yielding conversion of glucose to lactic acid in various tissues, notably muscle, when sufficient oxygen is not available.

How plentiful is creatine phosphate?

It is three to six times more plentiful than ATP in the sarcoplasm of a relaxed muscle fiber.

What are the three ways muscle fibers can produce ATP?

From creatine phosphate, via anaerobic glycolysis, or by aerobic respiration

Aerobic Respiration

If sufficient oxygen is present, the pyruvic acid formed by glycolysis enters the mitochondria, where it undergoes this, a series of oxygen-requiring reactions (the Krebs cycle and the electron transport chain) that produce ATP, carbon dioxide, water, and heat

What is the ninth and final step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

Muscle relaxes.

Concentric Isotonic Contraction

If the tension generated in this is great enough to overcome the resistance of the object to be moved, the muscle shortens and pulls on another structure, such as a tendon, to produce movement and to reduce the angle at a joint.

Creatine Supplementation

Increases the creatine content of muscles by approximately 20%, but there is a saturation limit

Fused Tetanus

No evidence of relaxation before the following contractions The result is a sustained muscle contraction

Wave Summation

This occurs when a second stimulus is received before the muscle fiber has relaxed, creating a second contraction that is stronger than the first

What is the eighth step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

Tropomyosin slides back into position where it blocks the myosin-binding sites on actin.

Unfused Tetanus

Type of wave summation with partial relaxation observed between twitches

Fast Oxidative-Glycolytic Fibers (FOG)

Typically the largest fibers. Like slow oxidative fibers, they contain large amounts of myoglobin and many blood capillaries. Thus, they also have a dark red appearance. These fibers can generate considerable ATP by aerobic respiration, which gives them a moderately high resistance to fatigue

Rigidity of Muscles

Presents as stiffness

Hypertonia

Refers to increased muscle tone and is expressed in two ways: spasticity or rigidity

Oxygen Debt

Refers to the added oxygen, over and above the resting oxygen consumption, that is taken into the body after exercise. This extra oxygen is used to "pay back" or restore metabolic conditions to the resting level in three ways: (1) to convert lactic acid back into glycogen stores in the liver, (2) to resynthesize creatine phosphate and ATP in muscle fibers, and (3) to replace the oxygen removed from myoglobin.

Half of all of the fibers within a skeletal muscle are what?

Slow Oxidative Fibers

Anabolic Steroids

Synthetic variations of testosterone, the sex hormone responsible for the development of male sexual characteristics. Common names for anabolic steroids are "raids," "juice," "gear," and "stackers." Specifically, these increase muscle size by increasing protein synthesis in muscles, which increases strength.

Twitch Contraction

The brief contraction of all muscle fibers in a motor unit in response to a single action potential in its motor neuron

Frequency of Stimulation

The number of impulses per second

Isotonic Contraction

The tension (force of contraction) developed in the muscle remains almost constant while the muscle changes its length. These are used for body movements and for moving objects. The two types of these are concentric and eccentric.

Eccentric Isotonic Contraction

The tension exerted by the myosin cross-bridges resists movement of a load and slows the lengthening process. As consequence, a contraction occurs in which the muscle lengthens.

Isometric Contraction

The tension generated is not enough to exceed the resistance of the object to be moved, and the muscle does not change its length. An example would be holding a book steady using an outstretched arm.

Refractory Period

The time following an action potential during which a new action potential cannot be initiated

Why does rigor mortis occur?

There is a complete loss of ATP, which is required to cause separation of the cross-bridges during relaxation.

Fast Glycolytic Fibers (FG)

These have low myoglobin content, relatively few blood capillaries, and few mitochondria, and appear white in color. They contain large amounts of glycogen and generate ATP mainly by glycolysis. Due to their ability to hydrolyze ATP rapidly, these fibers contract strongly and quickly.

What causes a muscle to become flaccid?

When the motor neurons serving a skeletal muscle are damaged or cut

At what length does maximum tension during contraction occur?

When the sarcomere length is 2.0-2.4 µm. A muscle fiber develops its greatest tension when there is an optimal zone of overlap between thick and thin filaments.

Recovery Oxygen Uptake

The preferable term for oxygen debt.

Motor Unit Recruitment

A mechanism for increasing tension (contractile length) in a muscle by activating more motor units.

What is the fourth step in the chain of events during the contraction and relaxation of a skeletal muscle fiber?

A muscle action potential traveling along a transverse tubule triggers a change in the voltage-gated Ca 2+ channels that causes the Ca 2+ release channels to open, allowing the release of calcium ions into the sarcoplasm.

Slow Oxidative (SO) Fibers

Appear dark red because they contain large amounts of myoglobin and many blood capillaries. Because they have many large mitochondria, these fibers generate ATP mainly by aerobic respiration, which is why they are called oxidative fibers. These fibers are said to be "slow" because the ATPase in the myosin heads hydrolyzes ATP relatively slowly, and the contraction cycle proceeds at a slower pace than in "fast" fibers. As a result, these fibers have a slow speed of contraction

Aerobic Training

Builds endurance for prolonged activities.

Anaerobic Training

High-intensity, intermittent bouts of exercise such as weight training; plyometric drills; and speed, agility, and interval training.

Muscle Fatigue

Inability of muscle to maintain its strength of contraction or tension; may be related to insufficient oxygen, depletion of glycogen, and/or lactic acid buildup