hypothalamic-pituitary axes
Summary
Hypothalamic-pituitary axes are hormone cascades: the hypothalamus releases hormones that control the anterior pituitary, which releases tropic hormones that act on a target gland. The most testable point is that the target gland's hormone usually feeds back negatively on both upstream levels.
Detail
Meaning: The hypothalamus secretes releasing or inhibiting hormones into the hypophyseal portal system, a short capillary link to the anterior pituitary. The anterior pituitary releases tropic hormones into the blood, which act on a target gland, and that gland's hormone feeds back on the hypothalamus and pituitary. Examples: TRH → TSH → thyroid hormones; CRH → ACTH → cortisol; GnRH → FSH and LH → sex steroids; GHRH → GH. Somatostatin inhibits GH release, and dopamine inhibits prolactin release.
On the MCAT: Passages describe a failed gland, a tumor, or an injected hormone and ask how each level changes. If the target gland fails, feedback is lost, so hypothalamic and pituitary hormones rise. If the pituitary fails, the tropic hormone and the target gland's hormone fall while the releasing hormone rises. Giving the end hormone as a drug suppresses both upstream levels.
Common confusion: The posterior pituitary does not follow this pattern. ADH and oxytocin are made by hypothalamic neurons and released from their axon endings in the posterior pituitary, with no portal system or tropic hormone. Also, feedback in these axes is usually negative, but the mid-cycle estrogen-driven LH surge is a positive feedback exception.
Sources
- Campbell Biology
- Human Physiology: An Integrated Approach
- Principles of Anatomy and Physiology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.