• Receptor: protein binds specifically w/ other mol; 
• Specificity:  hormones must bind to specific receptors to inlfuence target cell fn
• Affinity: strength of binding betw. hormone & receptor

1. amino acids, peptides, proteins
2. steroid hormones (derivatives of cholesterol)
3. eicosanoids (from arachadonic acid)

• hormones are potent
• blood level of hormone depends on: rate of synthesis, rate of degradation/ clearance from blood
• half life: persistence of a hormone in the blood (usu. < 1 min-30min)
• time to onset of hormone action variable: enzyme activation - rapid (minutes); enzyme synthesis- hours to days
• some hormones secreted as prohormones; activated once target cell reached
• duration of hormone also variable (hours to days)

• usually neg. feedb. (setpoint); sometimes pos. feedb. (goal)
• 3 types of stimuli: humoral, neural, hormonal

• hormone reaction in direct response to change in blood lev. of a nutrient
• eg: parathyroid hormone (PTH) & blood calcium; insulin & blood glucose

• not as common
• eg: sympathetic ns & epinephrine release by adrenal medulla
• eg: hypothalamic neurons & oxytocin release

• 3-tiered system involving hypothalamus, pituitary & target endocrine gland - concept of hypothalamic-pituitary axis

• neural; produces a number of hormones which travel to anterior pituitary via hypophyseal portal system

1. Thyroid releasing hormone (TRH) [hypothalamus]
2. Thryroid stimulating hormone (TSH) [ant. pit.]
3. Thyroid hormones (T3 and T4) [thyroid gland]

2 key hom. regulatory systems

• Autonomic nervous system

sympathetic vs parasymp.

sensory and motor neural pathways

fast response

• Endocrine System

hormones released into ECF and often travel to target organs via bloodstrean

slower response time, but response long-lived

diff. chem. classes of hormones w/ associated mechanisms of action