Hormones can be categorized into two primary groups based on their chemical structure: amino acid-based hormones and steroid hormones. This summary focuses on steroid hormones, particularly their mechanism of action within cells, which involves intracellular receptors and direct gene action. Notably, steroid hormones, along with thyroid hormones, are lipid-soluble, allowing them to pass through the lipid bilayer of cell membranes. This characteristic differentiates them from most amino acid-based hormones, which bind to receptors located on the cell surface.

When a steroid hormone, such as estrogen, circulates in the bloodstream, it is typically bound to a transport protein due to its lipid solubility. Upon reaching a target cell, the hormone dissociates from the transport protein and enters the cell. The first step in the process is the hormone passing through the cell membrane. Once inside, the hormone binds to a receptor protein in the cytoplasm, forming a receptor-hormone complex.

This receptor-hormone complex then translocates into the nucleus of the cell. It is important to note that sometimes the receptor is already located within the nucleus, while in other instances, the complex forms in the cytoplasm before entering the nucleus. Once inside the nucleus, the complex binds to specific regions of DNA, triggering a cellular response by affecting gene expression. This process is referred to as direct gene action, as the steroid hormone directly influences the transcription of genes into messenger RNA (mRNA), which is subsequently translated into proteins.

The effects of steroid hormones are generally slower compared to the rapid signaling of the nervous system. This is because the process of gene expression involves several steps: transcription of RNA, translation into proteins, and the subsequent physiological changes that these proteins induce. The entire process can take minutes to hours, with effects that may last for extended periods.

In summary, steroid hormones can enter cells and bind to intracellular receptors, forming a receptor-hormone complex that directly influences gene expression. This mechanism highlights the slower, yet sustained, effects of steroid hormones compared to other hormonal signaling pathways.

Struggling with Anatomy & Physiology?