The human muscular system: structure and function

Muscular System

Function of action potentials?

The function of action potentials is to rapidly communicate information within a neuron, coupling the neurons "input," either synaptic, sensory or intrinsic stimulation with its output, neurotransmitter secretion.

Cell electrical properties are the result of?

Cells use atoms that have become charged as a result of gaining, or losing, valency shell electrons. Cells are wet circuits that operate in a salty, conductive, medium.

How is the outside surface of the plasma membrane different from the inside surface?

The outer and inner surface of the plasma membrane of quite different. They form two separate water interacting surfaces, and proteins coat the outer surface. Although proteins extend through the membrane, they are only exposed on the outer surface. By means of selective permeability, the outer surface has a higher positive charge than its inner surface

Know the terms depolarization, repolarization, hyperpolarization, hypopolarization.

Depolarization: This occurs when the inside of the plasma membrane becomes less negative, which is indicated by movement of the curve upward toward zero. Repolarization is the return of the membrane potential to its resting value. Hyperpolarization is the event a neuron undergoes when its membrane potential grows more negative with respect to the extracellular solution. Hyperpolarization can be caused by the flow of positively charged ions (such as potassium) out of the cell, or by the influx of negatively charged ions (such as chloride). Hypopolarization is similar to deplorization.

5. What is responsible for the resting membrane potential? What is a resting membrane potential?

Plasma membranes are polarized, which means there is a voltage difference, or electrical charge difference, across the membrane before action potentials can be generated. This charge difference is called the resting membrane potential.

6. Know these terms: all-or-none response, absolute refractory period, relative refractory period, latent period, graded response.

The all-or-none principle states that if a stimulus is strong enough to generate a nerve action potential, the impulse is conducted along the entire neuron at maximum strength, unless conduction is altered by conditions such as toxic materials in cells or fatigue. The absolute refractory period is that period immediately following the discharge of a nerve impulse during which the cell cannot be induced to fire again. Relative refractory period is that period immediately following the discharge of a nerve impulse during which the cell cannot be induced to fire again. The latent period is the interval between stimulus and response. And a graded response is the gradual response to a stimulant.

7. What are gap junctions?

Gap junctions are an intercellular network of protein channels that facilitates the cell-to-cell passage of ions, hormones, and neurotransmitters. They allow action potentials to pass directly from one cell to another

8. What does it mean when the membrane is said to be polarized? Depolarized?

A plasma membrane is said to be polarized when there is a voltage difference across the membrane before action potentials can be generated. A membrane is said to be depolarized when the inward movement of Na+ makes the inside of the membrane more positive.

9. Know the structure of a chemical synapse and the sequence of events that occur during synaptic transmission, e.g., NMJ, roles of Ach and acetylcholinesterase.

Each synaptic vesicles contain Ach, an organic molecule composed of acetic acid and choline, which functions as a neurotransmitter. When an action potential reaches the presynaptic terminal, it causes voltage-gated calcium ion (Ca2+) channels in the plasma membrane of the axon to open, and as a result Ca2+ diffuse into the cell. Once inside the cell, the ions cause the contents of a few synaptic vesicles to be secreted by exocytosis from the presynaptic terminal into the synaptic cleft. The acetylcholine molecules released from the synaptic vesicles then diffuse across the cleft and bind to receptor molecules located within the postsynaptic membrane. This causes ligand-gated Na+ channels to open, increasing the permeability of the membrane to Na+. Na+ then diffuse into the cell causing depolarization. In skeletal muscle, each action potential in the motor neuron causes a depolarization that exceeds threshold, resulting in the production of an action potential in the muscle fiber.

10. How is the strength of a stimulus relayed to the next cell so that it responds accordingly?

Ach released into the synaptic cleft is rapidly broken down to acetic acid and choline by the enzyme acetylcholinesterase. Acetylcholinesterase keeps Ach from accumulating within the synaptic cleft, where it would act as a constant stimulus at the post synaptic terminal. The release of Ach and its rapid degradation in the synaptic cleft ensures that one presynaptic action potential yields only one postsynaptic action potent molecules are actively reabsorbed by the presynaptic to be then combined with the acetic acid produced within the Ach.

11. Know the terms contractility, extensibility, elasticity, excitability.

Contractility is the capability or quality of shrinking or contracting, especially by muscle fibers. Extensibility is the quality of being extensible; the capacity of being extended; as, the extensibility of a fiber. Elasticity is the condition or property of being elastic; flexibility. Excitability is the capacity of muscle to respond to stimulus. All are properties of muscle.

12. Properties and functions of skeletal muscle?

The properties of muscle are: contractility, excitability, extensibility, and elasticity. The major functions are: body movement, maintenance of posture, respiration, production of heat, communication, constriction of organs and vessels, and heart beat.

13. Similarities and difference between the three types of muscle?

Smooth muscle is the most widely distributed muscle in the body, and it has the greatest variety of functions. These include: propelling urine, mixing food in the stomach and intestines, constricting the pupils and regulating the flow of blood. Cardiac muscles are found only in the heart. They provide the major force for moving blood through the circulatory system. Skeletal muscles are composed of skeletal muscle fibers associated with smaller amounts of connective tissue, blood vessels, and nerves. Skeletal muscle fibers are skeletal muscle cells. Each is a single cylindrical cell containing several nuclei located around the periphery of the fiber near the plasma membrane.

14. Hypertrophy of skeletal muscle?

Hypertrophy of muscles is due mainly to an increase in muscle fiber size, rather than a substantial increase in number, and typically occurs in response to exercise.

15. Know the terms: epimysium, perimysium, endomysium, fasciculus, muscle fiber, sarcolemma, sarcotubular system, myofibrils, transverse tubules.

A muscle consists of many fasciculi grouped together and surrounded by a third and heavier layer, the epimysium, which is composed of dense, collagenous connective tissue and covers the entire surface of the muscle. A bundle of muscle fibers with their endomysium is surrounded by another, heavier connective tissue layer called the perimysium. The endomysium is a delicate network of loose connective tissue with numerous reticular fibers, and surrounds each muscle fiber outside the external lamina. Each bundle ensheathed by perimysieum is a muscle fasciculous. The sarcolemma is the plasma membrane of the muscle fiber. Muscle fibers are a single cylindrical cell containing several nuclei located around the periphery of the fiber near the plasma membrane. Myofibrils are a threadlike structure approximately 1 -- 3 p.m. In diameter that extends from one end of the muscle fiber to the other. Closely packed myofibrils, variable numbers of mitochondria and an elaborate network of membranous tubules and cisternae constitute the sarcotubular system. Transverse tubules play a critical role in excitation-contraction coupling. They are regularly arranged tubelike invaginations along the surface of the sarcolemma.

16. Components of the sarcomere? Lines, bands, zones?

Each sarcomere extends from one Z. disk to an adjacent Z. disk. The arrangement of the actin myofilaments and myosin myofilaments gives the myfibril a banded, or striated, appearance when viewed longitudinally. Each isotropic, or I band, includes a Z. disk and extends from either side of the Z. disk to the ends of the myosin myofilaments. When seen in longitudinal and cross sections, the I band on either side of the Z. disk consists only of actin myofilaments. Each anisotropic, or A band, extends the length of the myosin myofilaments within a sarcomere. The actin and myosin myofilaments overlap for some distance at both ends of the A band. In a cross section of the A band in the area where actin and myosin myofilaments overlap, each myosin myofilament is visibly surrounded by six actin myofilaments. In the center of each A band is a smaller band called the H. zone, where the actin and myosin myofilaments do not overlap and only myosin myofilaments are present. A dark band called the M. line is in the middle of the H. zone and consists of delicate filaments that attach to the center of the myosin myofilaments. The M. line helps to hold the myosin myofilaments in place similar to the way the Z. disk holds actin myofilaments in place. The numerous myofibrils are oriented within each muscle fiber so that A bands and I bands of parallel myofibrils are aligned and thus produce the striated pattern seen.

17. Components and functions of the thick and thin myofilaments?

Thin myofilaments consist predominantly of the protein actin and thick myofilaments consists of myosin. They are arranged spatially into sarcomeres. Thin filaments run between and parallel to the thick filaments within the A band. It is this placement that allows for the sliding filament model.

18. Location of active sites? Location of ATPase?

ATPases are a class of enzymes that catalyze the decomposition of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and a free phosphate ion. This dephosphorylation reaction releases energy, which the enzyme (in most cases) harnesses to drive other chemical reactions that would not otherwise occur. ATPase is in the head of the myosin myofilament.

19. Is ATP needed for contraction and relaxation of skeletal muscle?

Yes, the energy from one ATP molecule is required for each cycle of cross-bridge formation, movement, and release. After a cross-bridge has formed and movement has occurred, release of the myosin head from actin requires ATP to bind to the head of the myosin molecule.

20. What is rigor mortis?

Rigor mortis is the development of rigid muscles several hours after death and is similar to physiologic contracture. ATP production stops shortly after death, and ATP levels of thin muscle fibers decline.

21. What is the sliding filament hypothesis? How does it work? What happens during muscle contraction?

The sliding filament model of muscle contraction includes all the events that result in actin myofilaments sliding over myosin myofilaments to shorten the sarcomeres of muscle fibers. Actin and myosin myofilaments do not change length during contraction of skeletal muscle. Instead, the actin and myosin myofilaments slide past one another in a way that cause the sarcomeres to shorten. The shortening of sarcomeres is responsible for the contraction of the skeletal muscles. When sarcomeres shorten the myofibrils, which consist of sarcomeres joined end to end, shorten. They myofibrils extend the length of the muscle fibers, and when they shorten the muscle fibers shorten. Muscle bundles are made up of muscle fibers and muscles are made up of muscle bundles. Therefore, when sarcomeres shorten, myofibrils, muscle fibers, muscle bundles, and muscles shorten to produce muscle contractions.

22. How does curare cause muscle paralysis?

Organic poisons, such as curare, bind to the acetylcholine receptors, preventing acetylcholine from binding to them. Curare does not allow activation of the receptors, and therefore the muscle is incapable of contracting in response to nervous stimulation.

23. How does the sarcotubular system function to cause skeletal muscle contraction?

A cycle of events resulting in contraction proceeds very rapidly when the heads of the myosin molecules bind to actin. The heads of myosin molecules move at their hinged region, forcing the actin myofilament, to which the heads of the myosin molecules are attached, to slide over the surface of the myosin myofilament. After movement, each myosin head releases from the actin and returns to its original position.

24. Sources of energy for muscle contraction?

Muscle contractions receive their source of energy from ATP.

25. What does the action potential from a motor neuron cause to happen in a skeletal muscle cell to bring about a contraction?

An action potential is propogated to the presynaptic terminal of the motor neuron. The action potential causes the permeability of the presynaptic terminal to increase. Ca2+ diffuse into the presynaptic terminal, causing acetylcholine contained within several synaptic vesicles to be released by exocytosis into the synaptic cleft. Acetylcholine released from the presynaptic terminal diffuses across the synaptic cleft and binds to acetylcholine receptor molecules in the postsynaptic membranes of the sarcolemma. The binding of acetylcholine to its receptor site causes ligand-gated Na+ channels to open, and the postsynaptic membrane becomes more permeable to Na+. Na+ diffuse into the muscle fiber, causing a local depolarization that exceeds threshold and produces an action potential. Acetycholine is rapidly degraded in the synaptic cleft to acetic acid and choline by acetylchoinesterase, thus limiting the length of time acetylcholine is bound to its receptor site. The action potential produced in a muscle fiber is propaged from the postsynaptic membrane near the middle of the fiber toward both ends and into the T-tubules. The depoloarization that occurs in the T-tubule in response to the action potential causes voltage-gated Ca2+ channels of the membrane of the sarcoplasmic reticulum to open, and the membrane of the sarcoplasmic reticulum becomes very permeable to Ca2+. Ca2+ diffuse from the sarcoplasmic reticulum into the sarcoplasm. Ca2+ bind to troponin; the troponin-tropomyosin complex changes its position and exposes the active site on the actin myofilaments, initiating the contraction.

26. Know the components of a muscle twitch.

Lag phase, contraction phase, relaxation phase are the components.

27. What happens when a stimulus reaches threshold?

A threshold stimulus produces an action potential and results in contraction of the muscle cell.