Endocrinology. 50m lectures by William Peterson and Tom McFadden for Human Behavioral Biology
This is a good overview of the endocrine system and the way it works. It starts with communication. Multicellular life requires communication. There are four ways cells communicate:
- cell-cell contact: short range: 1:1
- Paracrine: short range: 1:many
- Neuronal: long-range, fast, specific
- endocrine: long-range, slow, widespread (not specific)
The endocrine system is about chemical messangers (hormones) in the blood. The endocrine system is slow but allows for complex coordination of multiple systems (e.g., metamorphosis of catepillar where every cell transforms in a coordinated way, coordination of response to a given environmental trigger, puberty, pregnancy, sex determination, stress, sexual behavior).
There are two main types of hormones: Peptide & Steroid hormones (there are also other amino acid based hormones).
Peptides are made from amino acids; hydrophilic = water-loving = polar = lipophobic and so water soluble; they travel freely in the blood; receptors are on the cellular membrane which sets off in the cell a secondary messenger cascade (e.g., opening ion channels); quick acting with short effects mainly through protein activity; e.g., insulin, vasopressin, oxytocin, ACTH, CRH, etc.
Steroids are made from cholesterol; hydrophobic = water-hating = nonpolar = lipophillic and so not water soluble; they travel bound to chaperones (protein carriers); as lipids, they can get through the cell membrane and bind to receptors within the cell; slow acting with lasting effects mainly through transcription e.g., glucocorticoids, androgens (testosterone), estrogen, etc.
Note that the evolutionary perspective suggests that receptors being able to distinguish subtle chemical differences is important.
Note that dopamine & epinephrine are neurotransmitters, but can also be released into the blood making them hormones. So some chemicals play different roles at different times and locations in the body.
How the brain controls hormones?
Endocrine glands are specialized structures to release hormones into the bloodstream. The primary brain endocrine glands are the hypothalamus & pituitary gland (also the pineal) which control the peripheral endocrine glands including the thyroid gland, adrenal glands, pancreas, ovary, testis. The hypothalamus links the nervous system to the endocrine system via the pituitary which is underneath it. Anterior pituitary hormones include ACTH, FSH, LH, TSH, prolactin, endorphins, GH. Posterior pituitary hormones include vasopressin, oxytocin. Hormones are stored in vesicles to be ready for release.
An important example which shows how the brain controls the endocrine system: The hypothalmic-pituitary-adrenal (HPA) axis: hypothalamus releases CRH to anterior pituitary which releases ACTH into the blood which signals the adrenal cortex (on kidneys) to release glucocorticoids (stress hormones) such as cortisol. Some receptors are in the pituitary/hypothalamus so they know how much is in the system (feedback). Negative feedback: more cortisol can trigger a reduction in ACTH release. the HPA axis shows how the brain controls (regulates) hormones throughout the body.
How hormones influence the brain
For hormones to influence the brain they need to cross the blood-brain barrier (which is just special epithelial cells lining the blood vessels in the brain to tightly regulate what can get in). The type, location & density of receptors affects the effects and also the level of hormone + qualities of receptors affect the effects.
down-regulation: reducing the number or sensitivity of receptors in response to high hormone levels
up-regulation: increase in the number or sensitivity of receptors
Hormones can have the following actions on neurons: change membrane potential via ion channels; change transcription of genes (including receptors); change protein activity & transport (could affect neurotransmission, formation of new synapses, etc.)
Note hormones can affect networks of neurons to shape behavior at the system level.
Types of behavior and the hormones that are involved:
- Stress: glucocorticoids
- sexual behavior: testosterone, estrogen, vasopressin, oxytocin
- Aggression: testosterone, glucocorticoids, estrogen, epinephrine
- Depression: glucocorticoids, thryoid hormone, estrogen, progesterone, melatonin