Intercellular Communication Flashcards
What is the endocrine system?
- A blood-borne, long distance biochemical communication system
- Aims to control and regulate a huge number of physiological processes = maintain homeostasis
Exocrine
External secretion (saliva, sweat…)
Endocrine
Internal secretion = blood
Endocrine organs
- Pituitary gland
- Pineal gland
- Thyroid gland
- Parathyroid glands
- Adrenal glands (cortex and medulla)
Endocrine cells
- Pancreas
- Thymus
- Gonads
- Hypothalamus
What is an endocrine gland?
Ductless glands, comprised of endocrine cells - these glands secrete hormones directly into the blood
Are endocrine hormones long or short distance messengers?
Long distance
Acting on target cells - endocrine
Endocrine hormones act on target cells that contain specific receptors for a particular hormone
Target cells - receptors
Receptors are selective for the specific molecules that bind to them
- If there is no receptor binding, then no physiological effect results = antagonism
Where are receptors found in?
- Surface (plasma membrane)
- Cytoplasm
- Nucleus
What is cortisol?
As your body perceives stress, your adrenal glands make and release the hormone cortisol into your bloodstream. Often called the “stress hormone,” cortisol causes an increase in your heart rate and blood pressure. It’s your natural “flight or fight”
Function of hormones:
- fetal development
- cell growth and cancer
- metabolism
- cardiovascular function
- renal function
- skeletal function
- reproductive function
- immune function
- CNS function
- maintain homeostasis
What is homeostasis?
Homeostasis refers to an organism’s ability to regulate various physiological processes to keep internal states steady and balanced
Example of negative feedback homeostasis
Body temp exceeds 37C —> nerve cells in skin and brain —> temp regulatory center in brain —> sweat glands throughout the body
Order if negative feedback -
- Stimulus
- Sensor
- Control
- Effector
What is negative feedback loop?
It is responsible for reversing the stimulus by activating the opposite responses. Thus, the productivity of the stimulus is reduced. This type of feedback mechanism stabilizes biological systems maintained under homeostatic conditions, important for a constant internal environment
Examples of negative feedback
Regulation of body temperature, blood pH, hormone levels, the oxygen/carbon dioxide balance, blood sugar levels, blood pressure, acid/base balance, water balance (osmoregulation), calcium levels, and energy balance
What is positive feedback loop
Positive feedback homeostasis is a type of feedback mechanism in biological systems, reinforcing a particular stimulus in the body. Therefore, this type of feedback mechanism promotes the change to proceed further. Thus, the initial change amplifies until the removal of the stimulus
ESSENTIALLY PUSHING THE BODY OUT OF HOMEOSTATIC STATUS
Positive feedback - childbirth
Childbirth is one of the most precise examples of positive feedback mechanisms. During childbirth, the pressure on the cervix due to the pushing of the child’s head downwards induces the release of oxytocin, which in turn stimulates further contractions of the cervix. Subsequently, these contractions stimulate the release of oxytocin until the baby is born
Examples of positive feedback
- Lactation = Breastfeeding stimulates milk production, which causes further feeding. And, this continues until the baby stops feeding.
- Ovulation = The dominant follicle inside the ovary releases estrogen, which stimulates the release of FSH and LH. These hormones stimulate further growth of the follicle.
- Blood clotting = The release of clotting factors by the activated platelets stimulates the aggregation of more platelets at the site of injury.
- Fruit ripening = The ripened fruits release ethylene, which stimulates the ripening of the nearby fruits
Is childbirth homeostasis?
No
Hormone classification
- Steroid hormones
- Peptide hormones
- Amino acid derivatives
Steroid hormones
- Derived from cholesterol steroid ring structure
- Produced by the gonads, the adrenal cortex and the kidneys
- Cannot be stored in the vesicles in the endocrine cells that produce them
- Receptors are located inside target cell
Peptide hormones
- Comprised of chains of amino acids
- Can be stored in vesicles in endocrine cells
- Do not readily pass through cell membranes
- Water soluble
- Receptors are found on the cell surface of their target organs
Amino acid derivatives
Tyrosine derivatives Catecholamines: - epinephrine (adrenaline) - norepinephrine (noradrenaline) - dopamine Thyroid hormone: - T4 (thyroxine) - T3 (triidothyroine)
Tryptophan derivatives:
- melatonin
- serotonin
What do steroid hormones and thyroid hormones bind to?
Intracellular receptors, whilst all other hormones bind to extracellular
Mechanisms of hormone release
- Humoral
- Neural
- Hormonal (axis)
Humoral hormone release
Stimulation from changing levels of ions or nutrients in the blood
- parathyroid hormone (PTH)
- PTH controls calcium homeostasis, responds to Ca2+ changes
Neural hormone release
Stimulation by nerves
- the nervous system directly controls the release of the hormone
- sympathetic stimulation (presynaptic) –> chromaffin cells adrenal gland (postsynaptic) –> epinephrine/norepinephrine (neurohormone)
Hormonal hormone release
Stimulation from other hormones
- a hormone controls the release of a different hormone
Neurotransmitters
Released into the blood, secreted by chromaffin cells (neuroendocrine cells found in the medulla of the adrenal glands)
Hormones produced by neurons
Oxytocin and vasopressin, produced in the hypophysis (hypothalamus) - secreted by the posterior pituitary
Releasing hormones
Produced by neurons that will stimulate a gland instead a target organ
- They are transported along neural axons stored and released into the hypophyseal portal system. They then rapidly reach the anterior pituitary where they exert their hormonal action
Why can a steroid hormone not be stored in a vesicle?
Because they are lipophilic, they cannot be stored in vesicles from which they would diffuse easily and are therefore synthesized when needed as precursors
How do negative feedback mechanisms control hormone secretion?
In negative feedback systems, a stimulus elicits the release of a substance; once the substance reaches a certain level, it sends a signal that stops further release of the substance
The pituitary gland
- a pea sized gland
- sits in a bony hollow base of the skull (sella turcica), underneath the brain and behind of the nose
- also known as the hypophysis
Processes of the pituitary gland
Sometimes called the ‘master’ gland’, controls many different processes:
- Blood pressure
- Sexual maturation and reproduction
- Metabolism
- Growth…
Structure of the pituitary gland (hypophysis)
- Anterior pituitary = adenopophysis
2. Posterior pituitary = neuropophysis
Posterior hypophysis position
Attached to the and innervated by the hypothalamus, which controls its activity. The attachment is the stalk or infundibulum
Anterior hypophysis
Synthesises and secretes hormones, non-nerve origin. Under the control by hormones released by the hypothalamus
Hypophysis portal system
Carries blood from the hypothalamus to the anterior pituitary and between the two sides - composed of six distinctive types of secretory cells (one for each hormone)
Posterior hypophysis
Stores and secretes, but does not synthesise hormones, nervous tissue origin. Peptide hormones are synthesised in the cell bodies of hypothalamic neurons - taken to the terminal and stored.
The hormones are released at the terminal into the bloodstream
Hypothalamus
- Works with the pituitary = main regulators of the endocrine system
- Receives input from the cortex, thalamus and limbic system
Hypophysis vasculature
Carries hormones from the hypothalamus to the anterior pituitary :
- hypothalamus secretes releasing hormones
- controls secretion of anterior pituitary hormones
Carries hormones out of the pituitary:
- anterior = endocrine cells
- posterior = from hypothalamus
Pituitary gland - hormones
The pituitary secrets 8 hormones Anterior: - TSH = thyroid stimulating hormone - ACTH = adrenocorticotropic hormone - FSH = follicle stimulating hormone - LH = luteinizing hormone - GH = growth hormone - PRL = prolactin
Posterior:
- ADH = anti-diuretic hormone (vasopressin)
- Oxytocin
ACTH - Adrenocorticotrophic hormone
- stimulates the adrenal gland to release corticosteroids
- important for responding to stress
TSH - Thyroid stimulating hormone
- stimulates the thyroid to release hormones
- thyroid hormones involved in regulating metabolism
FSH - Follicle stimulating hormone
- stimulates follicle growth and oestrogen production from ovary
- sperm production
LH - Luteinizing hormone
- plays role in ovulation
- stimulates androgen (males) secretion by the testis
GH - Growth hormone
- growth and puberty
- stimulates protein synthesis = cell production
- levels decrease throughout lifespan
PRL - Prolactin
- breast development and milk production
- normally levels are low until pregnancy
- found in males as well as females
ADH - Anti-diuretic hormone
- water retention and vessel constriction