Lecture 12, Endocrinology. I suspect that many people talk about this topic without having any clue (“oh, this is must be due to hormones”). Thus, I am very excited to learn more.
Function of hormones
Everything started with single cell organisms. When multicell organisms appeared, the issue arised - how to communicate across cells? There are 4 main mechanisms:
- Cell-to-cell contact - physically touching, thus, short range. Like passing a note to a neighbor in a class.
- Paracrine (secreting paracrine factors) - local. Like whispering to several neighbors.
- Neuronal - fast. Like texting your friend during class.
- Endocrine (hormones traveling through blood) - long range, but slow. Like sending email to entire class. Enables coordination across cells.
Coordination is useful for
- Multiple coordinated cell transformations (e.g. metamorphosis)
- To respond to environment (e.g. stress or sexual stimuli).
|Made of||Amino acids||Cholesterol|
|Hydrophilic (lypophobic)||Hydrophobic (lypophilic)|
|Soluble in water?||Yes||No|
|Travel||Freely through blood stream||Bound to chaperones|
|Affect||Existing protein activity||Transcription (rate of new protein synthesis)|
|Can travel through cell membrane||No||Yes|
|Examples||insulin, vasopressin, oxytocin||glucocorticoids|
Hormones have subtle chemical differences in structure, but the higher level structure is similar.
There are many endocrine glands in our body. Each secretes certain signal. Some are located in the brain. Often peripheral glands are regulated by brain glands.
The signal flows from brain to hypothalamus, then pituitary gland and finally rest of the body.
Consists of two parts - anterior and posterior. They secret different set of hormones and use different mechanisms. To secrete hormones in anterior part, hypothalamus has to release its hormones. For posterior part hormones come directly from neurons (i.e. there is a direct neural connection).
Anterior releases e.g ACTH, posterior - vasopressin, oxytocin.
The amount of hormones can be controlled through feedback loops, e.g. cortisol negative feedback loop. Cortisol gets secreted through CRH and ACTH. Each of glands producing these have receptors for cortisol. When they detect cortisol in blood stream, they start producing less of CRH and as a result ACTH, thus, lowering cortisol production.
Effect on brain
To affect brain hormones have to travel through blood stream (i.e. cross blood brain barrier). This barrier is located in blood vessels. It regulates what gets inside/outside brain’s bloodstream.
Steroid hormones can simply pass this barrier. Peptide can’t on their own, but have a carrier transport for this. Alcohol also passes blood brain barrier.
In order for a hormone to have an effect in brain, neurons have to have its receptors. Different parts of the brain can be more or less receptive. Some areas just have more receptors. There can also be individual variation in number of receptors.
Level of hormones can affect number of receptors. E.g. if there is too much of some hormone, cells can decrease number of its receptor.