In the human ovarian cycle, ovulation is triggered by a sharp rise in luteinizing hormone (LH). What is the cause of this sharp rise?

The sharp rise in luteinizing hormone (LH) during the human ovarian cycle is caused by the following sequence of events:

1. The first half of the ovarian cycle is known as the follicular phase. During this phase, the levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) increase slowly.

2. The rise in FSH and LH stimulates the growth of follicles in the ovaries. These follicles start to produce estrogen.

3. The increasing levels of estrogen stimulate the hypothalamus, a part of the brain that controls hormone production.

4. In response, the hypothalamus releases gonadotropin-releasing hormone (GnRH).

5. GnRH then stimulates the pituitary gland, another part of the brain, to produce and release more LH.

6. This results in a sharp rise or 'surge' in LH levels, which triggers ovulation. This is when the mature egg is released from the follicle into the fallopian tube.

7. After ovulation, the levels of LH decrease again. The second half of the cycle, known as the luteal phase, begins.