Endocrine System

Selective forces:

  • Endocrine System is an integrative system that controls an animal’s activities

  • Animals must coordinate activities

  • Many responses coordinated through the use of hormones

Chemical coordination

  • Endocrine glands: small (smaller than other organs), well-vascularized ductless glands

    • Pituitary, Thyroid, pancreas, adrenals, testes, ovaries

  • Hormones: a chemical messenger that initiates a physiological response. 

    • Can be local or circulating

      • Local: working in a very defined, small space

        • Released in small amounts

        • Does not enter the circulatory system, never enters into the blood vessels

        • Effective for minutes

        • Target cell uses up the small amount of hormone really quickly

      • Circulating: has to go to some distant location

        • Released in large amounts

        • Enters the CVS (cardiovascular system/circulatory)

        • Effective until removed/used up, can hang around longer

        • Target cell uses it, but whatever it doesn’t use up gets removed by the kidneys

  • Target cells: cells affected by the release of a hormone

    • Has ability to interact with many hormones

    • Any given cell may have receptors for a wide variety of hormones

  • Receptor: integral proteins present on the surface of a target cell

    • “Lock and key” interaction- the specific interaction between a hormone and receptor. The shape has to match in order to bind

  • Up regulation: adding active receptors to a target cell surface when hormone levels drop. Adds receptors so that the hormone is more likely to bind to ensure that the process takes place

  • Down regulation: removing active receptors from a target cell surface when hormone levels are high. Decreasing rate of contact to maintain optimal rate of binding (prevent burn out of the cell).

What are the effects of hormones?

  1. Change enzymatic activity/alter cellular metabolism

    1. Like trophic cascades, called tropic cascade

  2. Change membrane permeability. 

    1. Any hormone involved in water conservation or secretion

  3. Cause other glands to release their own hormones

Differences between the nervous and endocrine systems

  • Response time:

    • NS: can generate immediate change, incredibly rapid signal

    • ES: hormone must diffuse, time lag, takes longer for change to be instigated

  • Duration of effect:

    • NS: lasts as long as the action potential (AP) (nerve impulse)

    • ES: lasts as long as the hormone is present, could be hours or days

Mechanisms of hormone action:

  • Chemical Classes of hormones

    • Water-soluble hormones

      • Protein based and produced in the RER (hormone is a protein)

      • Too large or too polar to diffuse through membrane, stuck on the outside where it binds to the receptor

      • Must bind to transmembrane protein that acts as receptor

      • Hormone and receptor trigger a cascade of events

      • Second-messenger concept: the hormone must rely on a second molecule to alter target cell activity

        • Hormone is the 1st messenger and activates the 2nd messenger system in the cytoplasm

          • Typically the 2nd messenger is cAMP (cyclic adenosine monophosphate). Once it is activated, there is a cascade effect

      • Steps: Protein (hormone) travels through the blood vessel, leaves vessel, protein binds to the receptor, activates cAMP, cAMP goes around activation other protein kinases which does other mechanisms (transcription, translation, phosphorylation) which changes activities of the cell

    • Lipid-soluble hormones

      • Cholesterol-based and produced in the SER

      • Must bind to a transport protein to travel through blood, cannot move through the plasma by itself because it is hydrophobic. ex) of transport protein: albumin

      • Transported protein brings the hormone to the target cell where it binds to the receptor

      • Single-messenger concept: the hormone is directly involved in altering the target cell’s activity

        • Hormone diffuses through the membrane and binds to nuclear receptors (to alter transcription, translation, phosphorylation, etc.)

  • Control of secretion rates

    • Negative feedback: a physiological change initiates the release of a hormone that causes a change in the opposite direction. Brought back to homeostasis after you deviated. Most common mechanism in living things. Controlling minor fluctuations in homeostasis.

      • Relies on antagonistic hormones: one works to increase value, other works to bring it back down, work together to maintain the range

      • Any deviation in a set point is immediately corrected. Any deviation results in disease in that organism.

    • Autonomic nervous system: signals generated by the ANS either increases or decreases hormone production

      • Sympathetic: increase production of the hormone

      • Parasympathetic: decrease production of the hormone

    • Tropic cascade: the release of a hormone from one gland triggers the release of another hormone from a second gland

      • ex) hypothalamus

    • Positive feedback: a physiological change in the body causes the release of hormones that amplifies that change. Going away from homeostasis on purpose

      • Requires a shutoff mechanism

      • Potentially dangerous

      • ex) childbirth (not a lot of other examples): oxytocin targets the uterus to contract, baby’s head hits receptors which causes the pituitary gland to release more oxytocin. Only way to shut off the mechanism is when the baby is actually born. Dangerous if mom loses too much blood

Hypothalamus

  • Master program, center for homeostasis, knows what is normal for every aspect of the body and is continuously checking to make sure it is in that range

  • Major integrating center: takes in multiple stimuli and puts it together to make an appropriate response

  • Contains neurons that receives/interprets sensory info. Called a neuroendocrine organ

  • Part of the thalamus, right next to pituitary gland

  • Secretes releasing (prompts the release of something else) or inhibiting (inhibits the activity of that gland) hormones

Pituitary gland

  • Part of the diencephalon (part of the brain)

  • Controlled by the hypothalamus (either directly or by the hormones)

  • Connected to the hypothalamus by the infundibulum (little bridge connecting them)

  • “Master gland”- it produces a ton of hormones that regulate a lot of different aspects of the anatomy and physiology

  • Anterior pituitary gland

    • Largest portion, 75% of the mass of the pituitary gland, produces more hormones than the posterior

    • Endocrine tissue, very vascular capillary bed

    • Under tropic control of hypothalamus (releasing and inhibiting hormones target this gland)

    • Lacks a true connection to the brain (all endocrine tissue)

    • Large blood supply (so all hormones can easily diffuse into the capillaries and enter the circulatory system)

  • Posterior Pituitary gland

    • Under direct control of hypothalamus by ANS

    • Dominated by nervous tissue, direct connection of neurons from the hypothalamus

    • Doesn’t make its own hormones, stores the hormones that the hypothalamus makes

Hormones of the anterior pituitary gland (released from pituitary gland but stimulated by hypothalamus)

  • Thyroid-stimulating hormone (TSH)

    • Target: thyroid gland

    • Action: stimulate production of thyroid hormones

      • TSH and the thyroid hormones has negative feedback

  • Follicle-stimulating hormone (FSH)

    • Target: gonads (in both males and females)

    • Action:

      • Females: stimulated egg production, causes maturation of oocyte within the follicle

      • Males: stimulates sperm production, plays a role in spermatogenesis

  • Luteinizing hormone (LH)

    • Target: gonads

    • Action:

      • Females: stimulates/promotes ovulation, corpus luteum production, and secretion of female sex steroids (estrogen and progesterone). Related to menstrual cycle and early fetal development

      • Males: stimulates/promotes production of testosterone

  • Adrenocorticotropic hormone (ACTH)

    • Target: adrenal cortex

    • Action: increases production of glucocorticoids (all about carbohydrate metabolism). Plays important role in mobilizing resources when needed (fight or flight)

  • Growth hormone (GH), aka somatotropin

    • Target: cartilage, bone, skeletal muscle, liver

    • Action: stimulates metabolism and cell division directly in the cartilage, bone, and skeletal muscle

      • Stimulates secretion of insulin-like growth factor (when it targets the liver) which promotes resources being dumped into the bloodstream, like glycogen which breaks down into glucose to be released in the bloodstream which also stimulates metabolism and cell division. Liver stuff fuels the growth of the other things

  • Prolactin (released of it is from oxytocin initially)

    • Target: mammary glands

    • Action: promotes and maintains lactation

      • In fishes it plays a role in mucus secretion and osmoregulation

      • In amphibians is plays a role in metamorphosis and development

      • In birds is plays a role in brooding behavior

  • Melanocyte-stimulating hormone (MSH)

    • Target: melanocytes

    • Action:

      • Fish, amphibians, nonavian reptiles: promotes dispersion of pigment (darkens skin)

      • Birds and mammals: physiological function unclear

Hormones of the posterior pituitary gland

  • Oxytocin

    • Target: smooth muscle of the uterus (in all verts), mammary glands (mammals)

    • Action:

      • Stimulates uterine contractions

      • Eject milk in response to suckling with prolactin (mammals), but once baby is out of body, prolactin takes over

  • Antidiuretic Hormone (ADH), aka vasopressin

    • Target: kidneys (loop of henle and collecting duct)

    • Action: increase water reabsorption (makes it so that you don’t pee as much)

      • Concentrated urine/restricted flow

Pineal Gland

  • Part of the diencephalon (neuroendocrine) in the thalamus

  • Activity regulated by light levels

  • In ectothermic verts- acts as photoreceptive sensory organ

    • Referred to as “third eye” in some (parietal foramen)

      • Can be homologous to the lens of an eye in some species

  • Birds and mammals - entirely glandular structure, evolutionarily has changed

  • Melatonin

    • Targets: global, targets every single cell in your body (every cell has receptors for it)

    • Action: regulates sleep cycle

      • When light levels are high (during the day), there are low levels of melatonin and vice versa

  • Nonmammalian verts: pineal gland maintains circadian rhythms

  • Mammals: circadian rhythms regulated by part of the hypothalamus but the pineal gland/melatonin reinforces it. Why we are able to do things like night shifts

  • Birds and mammals have photoperiod which regulates seasonal rhythms in reproduction because days get shorter in the winter

Thyroid gland

  • Large endocrine gland in the neck of all verts

  • Highly vascular

  • Divided into halves (in most verts) with a small connection point in between them

  • Tissue composed of sphere-like units (follicles)

    • Follicles produce the hormones

  • Triiodothyronine (T3)

    • Target: global

    • Action: 

      • Promote growth/development of NS

      • Regulate metabolism by regulating cell’s ability to use oxygen

    • Iodine is a very significant structure in thyroid gland

  • Thyroxine (T4)

    • Target: global

    • Action:

      • Promote growth/development of NS

      • Regulate metabolism by regulating cell’s ability to use oxygen

      • Synergist with T3

  • Thyroid gland contains ½ of the iodine in the body, iodine deficiencies cause metabolism problems

  • Under secretion of thyroid hormones

    • Hypothyroidism: slows metabolic activities

      • In fish, birds, mammals under secretion drastically impairs growth and nervous system development

      • Malfunction from a very early age causes cretinism in humans (truncated growth)

      • Weight gain if acquired later in age

  • Oversecretion of thyroid hormones

    • Hyperthyroidism: increases metabolic activities

      • Causes precocious development in all verts, ramping development up, growth and development takes place too quickly, early maturation, goes through puberty too soon

        • Effect most prominent in frogs and fish

  • Thyroid hormones and adaptation

    • Thyroid gland promotes adaptation to cold climates (due to heat production)

    • Cold adapted animals eat more food in winter than summer

    • T3 and T4 drop during hibernation (can be 40% drop) to slow metabolism to make sure the food lasts longer.

  • Calcitonin

    • Target: osteoclasts (bone carving cells, break down bone)

    • Action: decreases blood Ca2+ 

      • Shuts the osteoclasts down, which activates the osteoblasts (bone building). The osteoblasts grab/remove all the calcium in the bloodstream (decreases calcium levels)

Parathyroid glands

  • Closely associated with the thyroid gland (may even be within it)

  • Number of them is highly variable (humans have 2, some reptiles have 1, birds have 4, 2 is most common though)

  • In birds and mammals- removal of gland rapidly decreases blood calcium

  • Parathyroid Hormone (PTH)

    • Fish are the only verts that do not secrete PTH

    • Target: osteoclasts

    • Action: increases blood Ca2+ levels (opposite of calcitonin)

      • Promotes osteoclast activity, inhibits osteoblasts, osteoclasts release calcium into the bloodstream

  • Antagonist to thyroid gland to maintain calcium homeostasis, negative feedback

Adrenal gland

  • Paired glands that is composed of unrelated tissue types in mammals

    • Unrelated tissue types: have cortex and inner medulla

  • Sit right on top of the kidneys

  • Cortisol (glucocorticoid)- in the adrenal cortex

    • Target: global

    • Action: 

      • Regulate food metabolism (glucocorticoid mobilizes glucose)

      • Reduces inflammation

      • Helps body store fat (promotes fat storage)

    • Primarily mammalian and aves hormone

  • Aldosterone (mineralocorticoid)- in the adrenal cortex

    • Target: kidneys

    • Action: regulates water balance/urinary output by regulating sodium levels (mineralocorticoid metabolisms minerals (sodium in this case))

      • Promotes the reabsorption of sodium and plays a role in urination because water diffuses with the increase of sodium concentration

  • Androgens- in the adrenal cortex

    • Target: hair follicles

    • Action: stimulates hair growth, associated with sexual maturation

  • Epinephrine and norepinephrine - in the adrenal medulla

    • Target: global

    • Action: activates flight or fight response

Pancreas

  • Endocrine and exocrine gland

  • Contains endocrine cells (islets of langerhans)- little hubs of hormone production, have different cells within them (but that is beyond our scope)

  • Insulin

    • Target: liver, skeletal muscle, adipose tissue

    • Action: stimulates cells to take in glucose (by glycogen) and store it, works to reduce blood glucose levels

  • Glucagon

    • Target: liver, adipose (NEVER skeletal muscle)

    • Action: stimulates cells to breakdown glycogen and release glucose into the blood

    • Antagonistic hormones because it does the opposite of insulin

Hormones and the reproductive cycle

  • Typically seasonal or cyclic because they have an estrous cycle

    • Females are only sexually receptive during brief periods of “heat” (estrus)

    • Endometrium (inner epithelial layer of the uterus) is not shed, instead it reverts to original state (in nonoriginal state it swells, fills with blood so that it can support an implanted zygote)

  • Menstrual cycle

    • Characteristic of anthropoids (primarily) and elephant shrews and some bats 

    • Females are receptive throughout cycle

    • Ends with the breakdown and discharge of the endometrium

  • Menopause is the permanent cessation of ovulation, due to a decline in estrogen and progesterone

    • Only occurs in a few groups of mammals

    • Usually because it caps lifetime fitness- so the “grandmother hypothesis” is one possible explanation for its evolution

      • Grandmothers help raise grandchildren, helping daughters raise grandchildren indirectly helps fitness, kin selection

    • ex) orcas, japanese macaques, humans, etc.

Fight or flight response

  • Aka: “General adaptation syndrome”

  • Activated when exposed to extreme or long term stress

  • What it does:

    • Prepares the body to deal with a stressor (fighting or getting away)

    • Allows the body to work outside of homeostasis for a short period

  • Stage 1: alarm reaction

    • Something horrifying just happened

    • Initiated by hypothalamus when stress is perceived

    • Sympathetic NS is activated: 

      • Adrenal medulla releases epinephrine (causes increase in blood glucose)

      • Pancreas

        • Increases glucagon and decreases insulin to increase blood glucose

      • Heart contractions increase, vasodilation in coronary and skeletal muscles, vasoconstriction in the kidneys

    • All non-essential organ system functions are inhibited by vasoconstriction (kidneys, digestive system, etc.)

    • Increases resources to the brain, heart, and muscles. Increases oxygen and glucose

  • Stage 2: resistance reaction

    • Extended releases of epinephrine causes release of cortisol. For sustained stimulus

      • Stimulates glucose metabolism to continue powering the flight and use additional resources

      • Pain receptors suppressed

      • Inflammation reduced (to allow animal to continue fighting by preventing stressors that would prevent the animal from doing the task)

  • Stage 3: exhaustion

    • Resistance reaction cannot be maintained any longer (resources have been depleted)

    • Prolonged exposure to cortisol causes

      • Wasting of muscles

      • Suppression of immune system

      • Storage of fat (adipose cells) why weight gain is a common response to stress