A&P Lab 6: Muscle Histology

The smallest contractile unit within skeletal muscle would correspond to which region of the following image?

1 and 7

Resting membrane potential depends on what two factors?

1) presence of sodium and potassium gradients, specifically more sodium outside the neuron than inside, and more potassium inside than outside 2) differential permeability of plasma membrane to sodium and potassium ions. Leak channels allow (membrane contains many more potassium; much more permeable to potassium ions)

The brain contains about how many neurons?

100 billion neurons

What event directly triggers release of neurotransmitters shown in image?

A nerve impulse arrives at the axon terminal triggering the opening of Ca2+ channels, which allows for the diffusion of Ca2+ into the terminal. This in turn leads directly to the release of neurotransmitters by exocytosis

Which event causes cross bridge detachment?

ATP binding to myosin head

Synaptic vesicles at the neuromuscular junction contain __________.

Acetylcholine

How is acetylcholine (ACh) removed from the synaptic cleft?

Acetylcholine (ACh) diffuses away from the ACh receptor and is broken down by the enzyme acetylcholinesterase (AChE), which is located in the synaptic cleft.

The binding of the neurotransmitter to receptors on the motor end plate causes which of the following to occur?

Acetylcholine (the neurotransmitter) binds to an acetylcholine (ACh) receptor which is a chemically gated or ligand gated cation channel that allows sodium to diffuse into the cell and potassium to diffuse out of the cell. Because the influx of sodium is greater than the efflux of potassium, the membrane potential becomes more positive, or depolarized. Recall that a cation is a positively charged ion, so a cation channel is a channel that allows positively charged ions (Na+ and K+) to flow through it.

How/when does the myosin head cock back to store energy for the next cycle?

After the myosin head detaches, energy from ATP hydrolysis is used to re-cock the myosin head.

What is a myofibril?

An organelle within a skeletal muscle fiber that contains thick and thin filaments organized into sarcomeres.

Which of the following interactions must occur first so that the others can take place?

Binding of calcium (B) to troponin removes the blocking action of tropomyosin along the thin myofilament. This allows myosin to bind to actin and form the cross bridge illustrated in this figure

Role of calcium in cross bridge cycle?

Binds to troponin, altering shape (moves away from active sites on actin)

What most directly causes synaptic vesicles to release acetylcholine into the synaptic cleft?

Calcium entry causes synaptic vesicles to release acetylcholine into the synaptic cleft. Each synaptic vesicle contains thousands of acetylcholine molecules.

How do calcium ions initiate contraction in skeletal muscle fibers?

Calcium ions bind to troponin, changing its shape. The shape change in troponin, caused by calcium binding, causes a shift in the position of tropomyosin along the thin filament. This exposes active sites to myosin and allows cross bridges to form. The cycling of cross bridges is what creates tension during contraction.

Which type of muscle contains intercalated discs?

Cardiac

Acetylcholine receptors are best characterized as what type of ion channel?

Chemically gated ion channels open due to the binding of a neurotransmitter, such as acetylcholine. Chemically gated Na+-K+ channels allow the diffusion of Na+ into the muscle fiber and K+ out of the muscle fiber. These channels can also be called chemically-gated cation channels because Na+ and K+ are both cations.

When does cross bridge cycling end?

Cross bridge cycling ends when sufficient calcium has been actively transported back in the sarcoplasmic reticulum so that calcium is no longer bound to troponin.

Which of the following best describes the events of "contraction" in "excitation-contraction coupling"?

Cross bridge formation

Where in the cross bridge cycle does ATP hydrolysis occur?

During cocking of the myosin head

What are the three connective tissue wrappings of skeletal muscle, starting from outermost wrapping and moving inward (deep)?

Epimysium, perimysium, endomysium

What means of membrane transport is used to release the neurotransmitter into the synaptic cleft?

Exocytosis

Skeletal muscle cells are grouped into bundles called __________.

Fascicles

What are intercalated discs?

Intercalated discs are specialized junctions that connect adjacent branched cardiac muscle cells to each other. These intercalated discs contain both mechanical junctions (desmosomes) that hold the cells tightly together, and electrical junctions (gap junctions) that allow electrical signals to travel from one cell to another.

The concentration of ___ is higher inside than outside the cell.

K+

The membrane is more permeable to ___.

K+

The resting membrane potential is maintained by Na+-K+ pumps that actively transport ___ into and ___ out of the cell.

K+; Na+

What prevents the Na+ and K+ gradients from dissipating?

Known as the Na+-K+ pump, or simply the pump, this transporter moves three Na+ out of the cell and two K+ into the cell for every ATP it hydrolyzes. This pumping action prevents the Na+ and K+ gradients from running down as these ions passively move through leak channels.

_____ for Na+ and K+ are ubiquitous (in all cells), and they allow for the diffusion of these ions across plasma membranes and are responsible for generating the resting membrane potential.

Leak channels

What would happen if acetylcholine was not removed from the synaptic cleft?

Multiple action potentials would occur in the muscle fiber. Action potentials will not cease until acetylcholine is removed from the synaptic cleft. Therefore, the constant presence of acetylcholine would cause multiple muscle action potentials and near-constant muscle contraction.

What is another name for a muscle cell?

Muscle fiber

How are the structural components of skeletal muscle arranged, starting from smallest structure to largest structure?

Myofilaments are found within myofibrils, which are organelles within the muscle fiber (cell). Muscle cells are arranged in bundles called fascicles, and groups of fascicles make up a whole muscle. So, from smallest structure to largest: myofilament, myofibril, muscle fiber, fascicle, muscle.

What, specifically, is a cross bridge?

Myosin binding to actin

The concentration of ___ is higher outside than inside the cell.

Na+

The contractile, or functional, unit of a muscle fiber is __________.

Sarcomere

Which type of muscle is composed of large cylindrical cells that have multiple nuclei located in the periphery (edge) of the cell?

Skeletal (large, cylindrical, striated)

Which type of muscle contains thin, elongated cells with a single nucleus in the center of the cell?

Smooth (thin elongated cells that are tapered at each end, and they have a single centrally located nucleus. While skeletal muscle cells can be as long as 20 inches (sartorius muscle), smooth muscle cells are only about 100 um (one tenth of a millimeter) in length. Smooth muscle cells are often tightly packed into layers within the walls of organs)

Which of the following are composed of myosin?

THICK

The sarcolemma of the endplate region contains ACh receptors. The opening of these ACh receptors depolarizes the muscle fiber, which leads to the generation of a muscle action potential. The endplate region is the region of the muscle cell that is innervated by the motor neuron axon terminal at the neuromuscular junction.

Thas all she wrote

When an action potential (nerve impulse) arrives at the axon terminal of a motor neuron, which ion channels open?

The action potential opens up voltage-gated calcium channels and calcium rushes into the axon terminal, leading to the release of the neurotransmitter. Voltage-gated channels are sensitive to changes in membrane voltage. In this case, the depolarization of the membrane during the action potential stimulates the voltage-gated calcium channels to open.

What event most directly triggers the release of calcium from the terminal cisternae?

The action potential propagating down the T tubule causes voltage sensitive tubule proteins to change shape, which opens calcium release channels in the terminal cisternae.

What role does tropomyosin play in the cross bridge cycle?

The displacement of tropomyosin exposes the active sites of actin, allowing cross bridges to form.

On average, the resting membrane potential is -70 mV. What does the sign and magnitude of this value tell you?

The inside surface of the plasma membrane accumulates more negative charge because of the presence of Na+ and K+ gradients and the selective permeability of the membrane to Na+ and K+.

Which protein functions as a motor protein that applies the power stroke during muscle contraction?

The myosin head groups (C) hydrolyze ATP to power molecular movement along the actin subunits of the thin myofilaments.

What forms the neuromuscular junction?

The neuromuscular junction is where the axon terminal of a motor neuron interacts with the special endplate region of the skeletal muscle fiber across a space (or gap) called the synaptic cleft.

What is another name for the plasma membrane of a muscle cell?

The prefix "sarco" refers to muscle. Sarcolemma is another name for the cell membrane or plasma membrane of muscle cells.

The resting membrane potential depends on two factors that influence the magnitude and direction of Na+ and K+ diffusion across the plasma membrane. Identify these two factors.

The presence of concentration gradients and leak channels. The concentration gradient and the large number of K+ leak channels allow for rather robust K+ diffusion out of a cell. In contrast, the concentration gradient and the relatively few Na+ leak channels allow for much less Na+ diffusion into a cell.

What structure in skeletal muscle cells functions in calcium storage?

The sarcoplasmic reticulum

All neural activities begin with a change in _____ of a neuron.

The separation of charges creates a voltage (electrical potential difference), which can be measured using a voltmeter. The resting membrane potential of a neuron averages -70mV (millivolts). All neural activities begin with a change in the resting membrane potential of a neuron

The calcium that initiates skeletal muscle contraction is released from what structure(s)?

The terminal cisternae release calcium, which is the "go" signal for a muscle contraction. The terminal cisternae are the enlarged ends of the sarcoplasmic reticulum.

During contraction, what prevents actin myofilaments from sliding backward when a myosin head releases?

There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching.

The plasma membrane is much more permeable to K+ than to Na+. Why?

There are many more K+ leak channels than Na+ leak channels in the plasma membrane.

In muscle fibers, which regulatory protein blocks the attachment of myosin heads to actin?

Tropomyosin

How does troponin facilitate cross bridge formation?

Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin.

With the stain Hematoxylin & Eosin (H&E for short), nucleic acids like DNA, stain dark blue or purple. Proteins, like collagen (CT) or the myofilament proteins (_____ & _____) found in muscle stain pink or reddish in color.

actin and myosin

The sliding filament model of contraction involves ________.

actin and myosin sliding past each other and partially overlapping

What is the functional role of the T tubules?

allows the muscle action potential to travel deep into the muscle cell The t-tubules are invaginations of the muscle cell sarcolemma which extend deep into the muscle cell and travel near the terminal cisternae of the sarcoplasmic reticulum. The t-tubule propagates the muscle action potential deep into the muscle cell and the depolarization signals the terminal cisternae to release calcium.

Which region in the image below contains myosin protein filaments?

btw 2 and 6

Which of the following is the correct sequence of events for muscle contractions?

motor neuron action potential, neurotransmitter release, muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke, sliding of myofilaments

A resting neuron has a greater _____ charge on the inside of the membrane.

negative

There is a greater _____ charge on the outside of the neuron.

positive

Which of the following best describes the events of "excitation" in "excitation-contraction coupling"?

propagation of the action potential along the sarcolemma

A volt meter can measure

resting membrane potential

ACh receptors are found mainly in the __________.

sarcolemma of the endplate region

When the chemically gated ion channels open, which ion is mainly responsible for depolarizing the sarcolemma?

sodium The inward diffusion of sodium ions, through acetylcholine receptor channels, depolarizes the sarcolemma. Potassium also diffuses through acetylcholine receptors, but less potassium diffuses than sodium, and the outward movement of potassium is not depolarizing. The acetylcholine receptor protein that is located at the neuromuscular junction serves as both a receptor for binding of the neurotransmitter acetylcholine, and as a chemically gated (ligand-gated) ion channel that allows cations (Na+ and K+) to diffuse through the sarcolemma.