Samenvatting
Dt2 Biologie van dieren (jaar 1, UU) samenvatting Campbell
Een samenvatting van de benodigde hoofdstukken uit Campbell biology voor deeltoets 2 van Biologie van Dieren, verplicht vak jaar 1 Biologie UU.
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11 Hormonale regulatieHoofdstuk 41 Chemische signalen
41.1 Hormones bind to target receptors and trigger response pathways
Hormone: secreted molecule that circulates and stimulates specific target cells.
Chemical signalling by hormones is the function of the endocrine system, the other
control system is the nervous system, these two can overlap.
A hormone can produce different responses in different target cells if they have
different receptor types, like with epinephrine.
Cells communication is classified by type of secretion and route taken by the signal:
Endocrine signalling: hormones are secreted into extracellular fluid and
diffuse into the blood or hemolymph and trigger responses in target cells
anywhere in the body. This type of signalling is used for homeostasis, growth
and development, and responses to environmental stimuli.
Paracrine and autocrine signalling involves local regulators: molecules
that act locally and move by diffusion. They play a role in blood pressure,
nervous system function, and reproduction. With paracrine neighbouring cells
are the target cells and with autocrine the secreting cells themselves are the
target. A group of local regulators called prostaglandins promote
inflammation and pain sensation. They are modified fatty acids, but local
regulators can also be polypeptides, like cytokines which enable immune cell
communication and growth factors, or gases, like NO which causes
vasodilation.
Synaptic signalling: neurotransmitters diffuse across synapses and trigger
cells.
Neuroendocrine signalling: neurohormones diffuse into the blood from
synapses.
Pheromones are released into the external environment that triggers other
members of the species.
There are three classes of hormones: polypeptides like insulin, steroid hormones
like cortisol (all derived from cholesterol, contain 4 fused rings), and amines like
epinephrine (synthesised from a single amino acid).
Polypeptides and most amines are water soluble. This means they are secreted
through exocytose, travel through the bloodstream, and bind to cell surface
receptors. Lipid-soluble amines and steroid hormones exit the cell through diffusion,
bind to transport proteins, diffuse into target cells, and bind to receptors in the
cytoplasm or nucleus.
Epinephrine is a water soluble hormone that triggers an intracellular signal
transduction: in the liver it binds a membrane GPCR, which triggers the production
of cAMP by adenylyl cyclase as a second messenger, which activated protein kinase
A, which leads to activation of an enzyme for the breakdown of glucose. Each
enzyme-catalysed step in this cascade provides an opportunity for signal
amplification.
Intracellular receptors for lipid-soluble hormones perform the entire signal
transduction. Most often cytosolic receptors move into the nucleus after binding a
hormone, where it can interact with DNA-binding protein or response elements to
alter transcription. Estradiol is an oestrogen so a steroid hormone, which binds to a
,cytosolic receptor in the liver which leads to the transcription of the vitellogenin
protein that's transported to the reproductive system to produce egg yolk. Thyroxine,
vitamin D, and other lipid-soluble hormones that aren’t steroids have receptors in the
nucleus.
Endocrine cells can be found in organs part of another organ system, but most often
they are found in ductless organs called endocrine glands: thyroid, parathyroid,
adrenal (cortex and medulla), ovaries, testes, pancreas, pituitary (anterior and
posterior), pineal, and hypothalamus.
41.2 Feedback regulation and coordination with the nervous system
In a simple endocrine pathway cells respond directly to a stimulus by secreting a
hormone, like S cells in the duodenum that secrete secretin into the blood when
detecting a low pH. Secretin then reaches the pancreas that responds by releasing
bicarbonate.
This feedback loop involves negative feedback: the response reduces the initial
change.
In a simple neuroendocrine pathway the stimulus is received by a sensory neuron,
which in turn stimulates a neurosecretory cell to secrete a neurohormone into the
bloodstream. Like suckling, which stimulates sensory neurons that send signals to
the hypothalamus. This input triggers the secretion of the neurohormone oxytocin
from the posterior pituitary gland.
This feedback loop involves positive feedback: the stimulus is reinforced, driving a
process to completion.
Endocrine organs integrate endocrine function with the nervous system:
In invertebrates, moulting is controlled by neurosecretory cells in the brain that
produce PTTH, a polypeptide neurohormone. PTTH reaches the prothoracic gland
which then releases ecdysteroid which triggers each successive moult. Ecdysteroid
also controls metamorphosis. Whether ecdysteroid causes moulting or
metamorphosis is controlled by juvenile hormone JH, secreted by endocrine glands
in the brain. When JH levels are high, ecdysteroid stimulates moulting, when JH
drops it induces the formation of a pupa.
In vertebrates endocrine coordination relies on the hypothalamus, which receives
signals from nerves and initiates neuroendocrine signalling in response.
Signals from the hypothalamus travel to the pituitary gland, which has two parts:
The posterior pituitary is an extension of neural tissue of the hypothalamus, since
hypothalamic axons that reach the posterior pituitary secrete neurohormones made
in the hypothalamus. There are 2 posterior pituitary hormones: antidiuretic
hormone ADH or vasopressin (increases water retention in kidneys) and oxytocin
(reproduction, maternal care, bonding).
The anterior pituitary is an endocrine gland that secretes hormones in response to
hypothalamus releasing or inhibiting hormones, which are secreted at the base of the
hypothalamus and drain into portal vessels which divide into a second capillary
network in the anterior pituitary. Many of the anterior pituitary hormones regulate
other endocrine glands, these are called tropic, like FSH and LH (gonadotropins),
GH, TSH, and ACTH. Nontropic hormones include MSH and prolactin: stimulates
milk production.
, Thyroid hormone regulated bioenergetics in mammals (blood pressure, heart rate,
muscle tone, digestive and reproductive functions. If the thyroid hormone level drops,
the hypothalamus releases thyrotropin-releasing hormone TRH, which causes the
anterior pituitary to release thyrotropin TSH, which stimulates the thyroid gland to
secrete thyroid hormone that increases metabolic rate. Thyroid hormone then exerts
negative feedback on the hypothalamus and anterior pituitary.
Thyroid hormone is a pair of tyrosine derived molecules with 3 T3 or 4 T4 iodine
atoms. If people don’t have enough iodine to produce thyroid hormone, the pituitary
receives no negative feedback. A continued elevated TSH level causes the thyroid
gland to enlarge.
Issues with thyroid hormone secretion can arise in the thyroid gland or along the
control pathway. TSH causes hypertrophy (enlargement) of the thyroid gland, a
condition known as goiter. Both hyper and hypothyroidism can be associated with
goiter.
Hyperthyroidism can lead to an increased oxygen consumption and metabolic heat
production, protein catabolism, hyperexcitable reflexes and psychological
disturbances, and over stimulation of beta-adrenergic receptors in the heart causing
an increased heart rate and contraction force. Hyperthyroidism can be caused by
Graves disease: antibodies called TSI mimic TSH and stimulate T3 and T4
secretion. Negative feedback inhibits TRH and TSH, but not TSI. Graves disease is
often accompanied by exophthalmos (bug-eyes).
Hypothyroidism leads to the opposite symptoms of hyperthyroidism. It can cause
cretinism (decreased mental capacity) in infants and bradycardia (slow heart rate). It
is caused by a lack of iodine to form T3 and T4. An absence of negative feedback
causes a rise in TSH.
Growth hormone GH is secreted by the anterior pituitary and stimulates growth both
tropic and nontropic. The liver responds to GH by releasing insulin-like growth
factors IGFs which stimulate bone and cartilage growth. GH also exerts metabolic
effects that raise blood glucose level, thereby opposing insulin.
An abnormal GH production can lead to disorders. Hypersecretion during childhood
can lead to gigantism and in adulthood the overgrowth of extremities called
acromegaly, because of stimulated bone growth. Hyposecretion during childhood
slows long-bone growth and can lead to pituitary dwarfism.
41.3 Endocrine glands respond to diverse stimuli
Another simple hormone pathway is the regulation of Ca2+ ion concentration:
Parathyroid glands, four small structures on the posterior side of the thyroid,
release parathyroid hormone PTH when the blood Ca2+ level falls below 10
mg/100 mL. PTH raises Ca2+ levels directly by causing mineralised matrices in
bones to break down and stimulating reabsorption of Ca2+ in the kidneys. It also
indirectly raises Ca2+ by promoting vitamin D production, which in turn stimulates
Ca2+ uptake from food in the intestines. As the Ca2+ level rises, a negative-
feedback loop inhibits further release of PTH.
To maintain homeostasis, an opposing hormone is needed: the thyroid gland
releases calcitonin if the Ca2+ level rises above a set point. Calcitonin inhibits bone
breakdown and enhances Ca2+ excretion by the kidneys.
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