Muscle Physiology Lecture Notes

These flashcards cover the key concepts from the lecture notes on muscle physiology, including skeletal, smooth, and cardiac muscle.

What is the function of skeletal muscle?

Supporting and moving the skeleton.

What nervous system division initiates skeletal muscle contraction?

The somatic motor division of the peripheral nervous system.

What are the functions of smooth muscle?

Propelling luminal contents, regulating internal flow by changing tube diameter, making hairs stand up, and changing pupil diameter.

How is smooth muscle contraction regulated?

Autonomously or by signals from the autonomic nervous system, hormones, and autocrine or paracrine signals.

What regulates cardiac muscle?

The autonomic nervous system, hormones, and autocrine or paracrine signals; it can also undergo spontaneous contractions.

What causes the striated appearance of skeletal muscle?

Alternating light and dark bands perpendicular to the long axis of the muscle fiber.

What is a skeletal muscle cell also referred to as?

A muscle fiber.

How are skeletal muscle fibers formed?

During development by the fusion of myoblasts into a single, cylindrical, multinucleated cell.

What are satellite cells?

Undifferentiated stem cells that participate in the repair of damaged muscle fibers.

What is hypertrophy?

An increase in the size of muscle fibers.

How are skeletal muscles attached to bones?

By tendons consisting of collagen fibers.

What are myofibrils?

Cylindrical bundles of thick and thin filaments within a muscle fiber.

What protein primarily comprises thick filaments?

Myosin.

What is the function of cross-bridges?

Contacting the thin filament and exerting force during muscle contraction.

What proteins primarily comprise thin filaments?

Actin, nebulin, troponin, and tropomyosin.

What is a sarcomere?

One unit of the repeating pattern of thick and thin filaments in skeletal muscle.

What defines the limits of a sarcomere?

Two successive Z lines.

What is the sarcoplasmic reticulum?

A sleevelike series of segments around each myofibril that's homologous to the endoplasmic reticulum.

What are transverse tubules (T-tubules)?

Tubular structures that lie between and are intimately associated with the terminal cisternae of the sarcoplasmic reticulum.

What is the function of calsequestrin?

Allows the storage of a large quantity of Ca2+ without having to transport it against a large concentration gradient.

What is the role of alpha motor neurons?

Innervate skeletal muscle fibers and initiate action potentials in this type of muscle.

What is a motor unit?

A motor neuron and all the muscle fibers it innervates.

What is the motor end plate?

The region of the muscle fiber plasma membrane that lies directly under the terminal portion of the axon, where receptors for acetylcholine are located.

What is the neuromuscular junction?

The junction of an axon terminal with the motor end plate.

What neurotransmitter is released at the neuromuscular junction?

Acetylcholine (ACh).

What is an end-plate potential (EPP)?

A local depolarization of the motor end plate caused by the influx of Na+ being greater than the outflux of K+.

What enzyme breaks down acetylcholine?

Acetylcholinesterase.

What happens when curare binds to nicotinic ACh receptors?

ACh cannot bind to the receptor, resulting in no EPP and no contraction.

How do organophosphates affect neuromuscular transmission?

They inhibit acetylcholinesterase, leading to a maintained depolarization and eventual desensitization of the motor end plate.

How does botulinum toxin affect neuromuscular transmission?

Blocks the release of acetylcholine from axon terminals.

What is excitation-contraction coupling?

The sequence of events by which an action potential in the plasma membrane activates the force-generating mechanisms.

What is the role of tropomyosin in muscle contraction?

Partially covers the myosin-binding site on each actin monomer, preventing cross-bridge binding.

What is the role of troponin in muscle contraction?

Holds tropomyosin in its blocking position and regulates access to myosin-binding sites on actin.

How does Ca2+ initiate cross-bridge cycling?

It binds to troponin, causing tropomyosin to move away from the myosin-binding site on actin.

How is cytosolic Ca2+ lowered to allow relaxation?

Ca2+-ATPases pump calcium ions from the cytosol back into the lumen of the sarcoplasmic reticulum.

What is the sliding-filament mechanism?

Overlapping thick and thin filaments in each sarcomere move past each other, propelled by movements of the cross-bridges.

What happens during a cross-bridge cycle?

A myosin cross-bridge binds to a thin filament, moves, detaches, and is set to repeat the process.

What are the four steps of muscle contraction with cross-bridges?

1. Attachment of energized myosin cross-bridge to thin filament. 2. Power stroke: movement of the cross-bridge and release of Pi and ADP. 3. Binding of ATP to myosin, causing detachment of cross-bridge. 4. Hydrolysis of ATP, re-energizing the cross-bridge.

What functions does ATP perform related to muscle fiber contraction and relaxation?

Na+/K+-ATPase maintains Na+ and K+ gradients for action potentials, Ca2+-ATPase in sarcoplasmic reticulum transports calcium ions into the reticulum, myosin-ATPase energizes the cross-bridges, ATP binding to myosin dissociates cross-bridges bound to actin.

What is rigor mortis?

The gradual stiffening of skeletal muscles that begins several hours after death due to lack of ATP.

What is muscle tension?

The force exerted on an object by a contracting muscle.

What is the load?

The force exerted on the muscle by an object.

What is an isometric contraction?

The muscle generates tension but does not change length.

What is an isotonic contraction?

The muscle changes length while the load on the muscle remains constant.

What is a concentric contraction?

Tension exceeds the load, and shortening occurs.

What is an eccentric contraction?

An unsupported load is greater than the tension generated by cross-bridges, and the load pulls the muscle to a longer length.

What is a twitch?

The mechanical response of a muscle fiber to a single action potential.

What is the latent period?

The interval of a few milliseconds following the action potential before the tension in the muscle fiber begins to increase.

What is summation?

Increase in muscle tension from successive action potentials occurring during the phase of mechanical activity.

What is tetanus?

A maintained contraction in response to repetitive stimulation.

What accounts for the tension decline at lengths less than L0 (optimal length)?

Overlapping sets of thin filaments from opposite ends of the sarcomere may interfere with binding, and Z lines collide with the ends of thick filaments, creating internal resistance.

What are the three ways a muscle fiber can form ATP?

Phosphorylation of ADP by creatine phosphate (CP), oxidative phosphorylation of ADP in the mitochondria, and phosphorylation of ADP by the glycolytic pathway in the cytosol.

How does creatine phosphate contribute to ATP homeostasis?

Provides a very rapid means of forming ATP at the onset of contractile activity. Acts as a phosphate donor in a reversible reaction.

Define muscle fatigue

Decline in muscle tension as a result of previous contractile activity.

What are the three types of skeletal muscle fibers?

Slow-oxidative fibers (type 1), fast-oxidative-glycolytic fibers (type 2A), and fast-glycolytic fibers (type 2X).

What causes the difference in maximal shortening velocity between fiber types?

Different myosin enzymes with high or low ATPase activities, giving rise to fast and slow fibers.

In muscle physiology, define 'recruitment'.

The process of increasing the number of motor units that are active in a muscle at any given time, achieved by activating excitatory synaptic inputs to more motor neurons.

What is the order in which motor units are recruited?

Slow-oxidative motor units, then fast-oxidative-glycolytic motor units, then fast-glycolytic motor units (only during very strong contractions).

What is denervation atrophy?

When denervated muscle fibers will become progressively smaller in diameter and the amount of contractile proteins they contain will decrease.

How does exercise affect muscle fibers?

Increased amounts of contractile activity—in other words, exercise—can produce an increase in the size (hypertrophy) of muscle fibers as well as changes in their capacity for ATP production and the subtype of myosin they express.

Define antagonists in relation to muscle contraction.

Groups of muscles that produce oppositely directed movements at a joint.

Define flexion and extension.

Flexion refers to the bending of a limb at a joint, whereas extension is the straightening of a limb.

Describe muscle cramps.

Involuntary tetanic contraction of skeletal muscles produced by high action potential rates.

Define hypocalcemic tetany.

Excessive muscle contractions caused when extracellular Ca2+ decreases below normal, spontaneously opening Na+ channels of nerve and muscle.

In general, what causes muscular dystrophy?

Genetic disorder that results from defects of muscle-membrane-stabilizing proteins such as dystrophin.

Describe myasthenia gravis

Autoimmune disorder in which destruction of ACh receptors of the motor end plate causes progressive loss of the ability to activate skeletal muscles.

List 2 Characteristics common to all smooth muscles.

1)lack the cross-striated banding pattern found in skeletal and cardiac fibers and 2) the nerves to them are part of the autonomic division of the nervous system rather than the somatic division

What are dense bodies?

cytoplasmic structures functionally similar to Z lines in skeletal muscle fibers; anchor the thin filaments

What is The Latch State

muscle in this state can maintain tension in an almost rigorlike state without movement.

What generates actions potentials in the plasma membrane of some smooth muscle cells?

When an excitatory input is superimposed and slow waves are depolarized above threshold.

What are Single-unit smooth muscles?

undergo synchronous activity, both electrical and mechanical or respond to stimulation as a single unit.

What are multiunit smooth muscles?

Have no or few gap junctions so each cell responds independently to actitivy on cells around it.

Where is Cardiac muscle found?

Only in the heart.

Describe excitation–contraction coupling in cardiac muscle

small amount of Ca2+ through L-type Ca2+ channels triggers opening of ryanodine receptors that release a larger amount of Ca2+ from the sarcoplasmic reticulum.