The force generated by a muscle when it contracts involves muscle proteins within muscle cells, namely actin and myosin. Beginning with the arrival of an action potential from the motor neuron’s axon, muscles generate force through a cascade of electrical and biochemical events. The release of acetylcholine at the presynaptic membrane into the synaptic cleft is caused by the action potential which opens calcium channels. Temporary binding of neurotransmitter at the postsynaptic membrane with the muscle’s acetylcholine receptors leads to depolarization of the postsynaptic membrane and opening of calcium channels. Twisting of tropomyosin to expose myosin attachment sites on actin is the result of calcium released from the sarcoplasmic reticulum and binding to troponin molecules. two strands of protein, myosin and actin, attach to each other by forming a cross-bridge which allows them to slide relative to each other to shorten the muscle and generate force. When depolarization ends, is pumped back into the sarcoplasmic reticulum and actin- myosin cross-bridges can no longer form resulting in relaxation.

When a motor neuron is electrically stimulated with a single impulse, a muscle innervated by that neuron produces a force called a twitch. Whereas the impulse might be 1 to 3msec in duration, the twitch is 10 to 100msec long. This is because it takes a long time for the to be pumped back into the sarcoplasmic reticulum. When the rate of impulses is low, the twitches have time to relax (Figure 1A). When the rate of simulation is high, the twitches fuse and the force in the muscle sums (Figures 1B and 1C). Maximal tension in the muscle, a condition known as tetanus (Figure 1D), is generated when the frequency of action potential is raised to the point when all cross- bridge binding sites are continuously activated and force output no longer shows any ripples.

Figure 1

Myasthenia gravis (MG) is a disease in which the number of acetylcholine receptors at the postsynaptic neuromuscular junctions becomes greatly reduced. What is the expected difference between contraction of the muscle of the MG patient and that of a healthy person in response to stimulation by a neuron?

Which of the following is true regarding a transmembrane receptor?

A type III secretion system is a mechanism several bacteria use to evade the immune system. They insert a syringe-like structure into a nearby host cell and secrete effector proteins that kill the host cell. What term best describes this kind of signaling?

The cellular membrane is a very important structure. The lipid bilayer is both hydrophilic and hydrophobic. The hydrophilic layer faces the extracellular fluid and the cytosol of the cell. The hydrophobic portion of the lipid bilayer stays in between the hydrophobic regions like a sandwich. This bilayer separation allows for communication, protection, and homeostasis.

One of the most utilized signaling transduction pathways is the G protein-coupled receptor pathway. The hydrophobic and hydrophilic properties of the cellular membrane allows for the peptide and other hydrophilic hormones to bind to the receptor on the cellular surface but to not enter the cell. This regulation allows for activation despite the hormone’s short half-life. On the other hand, hydrophobic hormones must have longer half-lives to allow for these ligands to cross the lipid bilayer, travel through the cell’s cytosol and eventually reach the nucleus.

Cholesterol allows the lipid bilayer to maintain its fluidity despite the fluctuation in the body’s temperature due to events such as increasing metabolism. Cholesterol binds to the hydrophobic tails of the lipid bilayer. When the temperature is low, the cholesterol molecules prevent the hydrophobic tails from compacting and solidifying. When the temperature is high, the hydrophobic tails will be excited and will move excessively. This excess movement will bring instability to the bilayer. Cholesterol will prevent excessive movement.

Which of the following molecules can be found inside of a cell?

I. Cyclic adenosine monophosphate (cAMP)

II. Protein kinase A

III. Protein kinase C

Which of the following is false regarding cellular signaling?

I. Transmembrane receptors are found on both plasma membranes and nuclear membranes

II. Transmembrane receptors are always ion channels

III. A ligand can be polar or nonpolar

Which organelle is primarily responsible for ATP production in eukaryotic cells?

Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males.

Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.

Erections occur when nitric oxide is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions.

Nitric oxide is which of these types of signals?

Listed below are events that occur during a signal transduction pathway.

I. The plasma membrane receptor interacts with an effector protein

II. Second messenger molecules are released

III. Ligand binds to the plasma membrane receptor

Which of the following lists these events in the correct order?

Sildenafil (commonly called Viagra) is a common drug used to treat erectile dysfunction and pulmonary arterial hypertension. Sildenafil's effect comes from its ability to cause vasodilation in smooth muscle cells. For this problem, we're only going to consider its effects on erections in males.

Erectile dysfunction is a common medical problem in older men. Its most significant effect is the prevention of erections. Erections occur when there is an increase in blood flow via enlargement of an artery (vasodilation). Understanding the mechanism by which vasodilations occur is important in order to treat erectile dysfunction.

Erections occur when nitric oxide is released from an area in the penis and binds to guanylate cyclase in other cells of the penis, which creates cyclic guanosine monophosphate (cGMP) from GTP. cGMP causes a relaxation of the arterial wall in order to increase blood flow to the region, thereby causing an erection. cGMP is broken down over time by cGMP-specific phosphodiesterase type 5 (PDE5) into GTP, which reverses the effect and causes vasoconstriction on the arterial wall. Combatting this effect is the major method by which Viagra functions.

Which of the following is not a possible mechanism by which Sildenafil treats erectile dysfunction?

Type 1 diabetes is a well-understood autoimmune disease. Autoimmune diseases result from an immune system-mediated attack on one’s own body tissues. In normal development, an organ called the thymus introduces immune cells to the body’s normal proteins. This process is called negative selection, as those immune cells that recognize normal proteins are deleted. If cells evade this process, those that recognize normal proteins enter into circulation, where they can attack body tissues. The thymus is also important for activating T-cells that recognize foreign proteins.

As the figure below shows, immune cells typically originate in the bone marrow. Some immune cells, called T-cells, then go to the thymus for negative selection. Those that survive negative selection, enter into general circulation to fight infection. Other cells, called B-cells, directly enter general circulation from the bone marrow. It is a breakdown in this carefully orchestrated process that leads to autoimmune disease, such as type 1 diabetes.

Unlike T-cells and B-cells, macrophages use phagocytosis and digestion as their principal functions. Macrophages are directed to the site of infection by chemical mediators, such as chemokines and cytokines. These mediators react with surface proteins on macrophages and induce intracellular changes, driving the macrophages to the site of infection. Which of the following is likely true of this form of cell signaling?

I. It is mediated by an intracellular second messenger

II. It exclusively mediates changes in gene expression in the macrophage

III. It drives intracellular changes to occur over several days or weeks

IV. It is an example of autocrine signaling