Second messengers are the molecules in a signal transduction pathway that will activate an intracellular response. Epinephrine is a hormone that will bind a receptor on the exoplasmic face of the cell, making is a first messenger. G subunits interact with adaptor proteins that will then stimulate the production of second messengers. Receptor tyrosine kinases are examples of receptor proteins that will bind first messengers. cAMP, however, is a widely used second messenger that is involved in the activation of many pathways and signal amplification in the cytosol.

A second messenger response is created in cells that use signal transduction, meaning that the signaling molecules attach to a receptor on the outside of the cell. Steroid hormones are largely nonpolar, and can enter the cell in order to affect cellular processes at the level of transcription. As a result, they do not need to rely on second messenger pathways in order to elicit a response.

The peak of an action potential signals the inactivation of sodium channels. This effectively prevents more sodium from entering the cell and halts the depolarization that was previously occurring, resulting in a maximum depolarization value. Potassium channels remain open, and are the cause for the membrane potential to start dropping (positive charge is leaving the cell). The sodium-potassium pump does not stop during this process. In fact, its continued function is essential for eventually restoring the resting membrane potential.

Neurotransmitters will bind their respective receptors on the post-synaptic membrane, which is a muscle in this case. This binding causes changes to other proteins on that membrane, which results in an opening of ion channels. The muscle then depolarizes due to the influx of positively charged ions, and this can be measured as a positive change in the muscle membrane potential. 

The sodium-potassium pump maintains the resting membrane potential by utilizing 1 ATP to transport 2 potassium ions into the cell, and pumping 3 sodium ions out, which makes the inside of the cell negative relative to the outside of the cell. 

Depolarization, also known as the rising phase, occurs when the membrane potential goes from being negative to positive very quickly. This is instigated by the influx of ions through the open voltage gated channels, and the positive ions make the cell more positive relative to the resting potential.

The correct answer is elicit innate immune responses by recognizing microbial associated molecular patterns. TLRs are pattern recognition transmembrane receptors that recognize bacterial components, such as lipopolysaccharides and flagellin, and viral components such as single stranded DNA. Upon recognition by the leucine rich region, TLRs dimerize to facilitate signal transduction (via the toll-interleukin region domain) to downstream pathways to promote inflammation and recruitment of macrophages. 

G protein coupled receptors are by far the largest class of cell surface receptors. They can respond to a large variety of extracellular signaling molecules, and can elicit a great deal of responses inside cells. With such diversity, it should come as no surprise that not all GPCRs will increase cAMP levels in the cell. In fact, many can display an inhibitory role, and decrease cAMP levels by preventing its production.

The sodium-potassium pump is a type of active transport that brings sodium out of the cell and potassium into the cell. This is in the opposite direction of their natural gradient. The fact that it is going in the opposite direction requires this pump to need energy, or, ATP.

When MAP Kinase signaling is activated, the receptors auto-phosphorylate to activate the signal transduction, which leads to the binding of GTP to Ras. Ras is activated when bound to GTP. Ras then activates downstream MAP Kinases, which lead to a phosphorylate cascade that eventually uses ATP to phosphorylate transcription factors. The phosphorylated transcription factors then go on to alter gene expression in the cell.

Therefore, the correct answer in this question is that transcription factors are phosphorylated using ATP molecules.