Lecture Eleven - Hormonal control Flashcards
What is a hormone?
A chemical signal that is secreted into the circulatory system and communicates regulatory messages within the body.
Hormones may reach all parts of the body (through the blood stream) but only certain types of cells (i.e. target cells) are able to respond.
What does the nervous system do?
Conveys high speed electrical signals along specialised cells called neurons.
These messages control movement of body parts in response to sudden environmental change.
What does the endocrine system do?
Comprises all of an animals hormon-secreting cells.
In contrast to the NS, hormons conordinate slower, but longer acting responses to stimuli (e.g. stress).
Hormones also regulate long term developmental processes (e.g. growth, development of primary and secondary sexual characteristics).
Hormone-secreting organs call endocrine glands secrete horones directly into the extracellular fluid, where they diffuse into the blood.
What is the overlap between the NS and endocrine systems?
Structurally, chemically and functionalls related.
Neurosecretory cells (specialised nerve cells) secrete hormones (known as neurohormones) into the blood.
Some cehmicals (e.g. endorphins) serve as both hormones of the endocrine system and chemical signals in the nervous system.
Nervous system also plays a role in certain sustained responses (e.g. day nigh cycles) by increasing or decreasing secretions from endocrine glands.
What are the control pathways in the endocrine system?
A receptor, or sensor, detects a stimulus and sends information to a control centre.
After comparing the incoming information to a set point (i.e. desired value), the control centre sends out a signal directing an effector to respond.
In endocrine and neuroendocrine pathways, this outgoing signal, called an efferent system, is a hormone or neurohormone, which acts on particular effector tissues and elicit specific physiological or developmental changes.
There are three types of hormonal control pathways:
1) Simple endocrine pathway.
2) Simple neurohormone pathway.
3) Simple neuroendocrine pathway.
What are feedback loops?
A common feature of control pathways is a feedback loop connecting the response to the initial stimulus.
Negitive feedback - effector response reduces the initial stimulus and eventually response ceases.
Positive feedback - effector response increases the initial stinulus.
What are the three major classes of molecules which function as hormones in vertebrates.
1) Proteins and peptides.
2) Amines derived from amino acids.
3) Sterois (not water soluble).
Signalling by any of these molecules involves these key steps:
- Reception of the signal occurs when the molecule binds to a specific receptor protein in or on the target cell. This triggers signal transduction within the target cells, seulting in a response.
How does transmission of hormones which are water soluble?
Receptors are imbedded in the plasma memberane.
Binding of a hormone to its receptor initiates a transduction pathway (series of changes that ceonverts an extracellular chemical signal to an intracellular response).
How does transmission of hormones which are water insoluble (steroid) work?
Receptor is located within the target cell (as the homones can diffuse directly through the membrane).
Hormone forms complexes with receptors inside cell.
These protein-receptor complexes bind onto DNA and induce change in gene expression.
What are the three way in which hormones can interact to affect the same processes?
1) Opposing interaction where effect of one hormone opposes the effect of another.
2) Synergistic interaction where combined actions of two or more hormones is required to produce a particular effect.
3) Permissive interaction where a hormone exerts an effect on a cell only after another hormone has ‘primed’ the target cell.
What is the hypothalamus?
Integrates vertebate endocrine and nervous system.
Recieves information from nerves throughout the body and brain then initiates appropriate endocrine signals.
Hypothalamus contains several sets of neurosecretory cells whos hormonal secretions are stored in, or reguate the acivity of, the pituitary gland.
What is the pituitary gland?
Consists of two parts:
1) Posterior pituitary.
An extension of the hypothalamus.
Stores and secretes ADH and oxytocin that are made by the neurosecretory cells fo the hypothalamus.
Antidiuretic hormone (ADH) enhances water reabsorption in kidneys.
Oxytocin induces uterine contractions and milk ejection.
2) Anterior pituitary.
A discrete structure consisting of endocrine cells that synthesis and secrete several hormones into the blood.
Some of these hormones, known as tropic hormones, control the function of other endocrine glands (e.g. testes, ovary and thyroid).
The anterior pituitary itself is controlled by two kinds of tropic horones (releasing and inhibiting hormones). These are produced by the hypothalamus.
Hormones produced by the anterior pituitary gland include Follicile stimulating hormone (FSH), lutenizing hormones (LH), thyroid stimulating hormone (TSH) and adrenocortocotropic hormone (ACTH).
What are non-tropic hormones?
Prolactin stimulates lactation in mammals but has diverse effects in different animals (e.g. fat metabolism and reproduction in birds, delayed metamorphosis in amphibians).
Endorphins inhibit the sensation of pain.
Melatocyte stimulating hormones (MSH) influences skin pigmentation in some animals and fat metabolism in mammals.
What are the non-pituitary hormones?
These help regulate metabolism, homeostasis, development and behaviour.
What are the thyroid glands?
In mammals, thyroid gland consists of two lobes located on the ventral surface of the trachea.
Produces triiodonthronine (T3) and thyroxine (T4).
Stimulates and maintains metabolic processes.
Thyroid also produces calcitonin which is involved in calcium homeostasis.
What does teh pancreas do?
Two types of cell sin pancreas, alpha and eta, secrete glucagon and insulin respectively.
Antagonistic effects of these two hormones are vital to maintaining glucose homeostasis.
Diabetes caused by decrease in insulin or decreasedresponse to insuline in target cells.
