CHEMICAL SIGNALS IN ANIMALS:THE JOURNEY OF HORMONES.

Animals have got two internal communication systems and also their regulation which includes the nervous system and the endocrine system.

Credit: Wikimedia
In animals the slower means of communication usually regulate other biological processes such as the maturation of the butterfly , during which all parts of the body must be informed on how fast to grow and also when to develop the major characteristics to separate the male from the female even the adult to the juvenile species. All this possible information mostly relayed by hormones.
Collectively, all of the animals hormone-secreting cells constitute the endocrine system, Hormone-seecreting organs are called endocrine glands and sometimes it is called ductless glands simply because they secrete their chemical messengers directly into their body fluids.
The Exocrine glands usually supplies chemicals such as sweat, mucus and digestive enzymes into ducts that convey the products to the suitable positions. Even though it is very easy to differentiate between the endocrine and nervous systems, the lines between these two regulatory systems are blurred and the hormones depends heavily on overlap between them.
NEUROSECRETORY CELLS
Most endocrine organs and tissues comprises of specialized nerve cells which are known as neurosecretory cells that supplies hormone, some animals as unique as the insects as well as the vertebrates have this neurosecretory cells in their brain that aids in the secretion of hormones into the blood.
Some chemicals plays the roles of both as hormones of the endocrine system and also for signals in the nervous system, lets take the epinephrine as a case study where by its functions in the vertebrate body as the “fight or fight” hormone which is produced by the adrenal medulla an endocrine gland. Sometimes it works as a neurotransmitter that transfers informations or messages between neurons in the nervous system.
INVERTEBRATE REGULATORY SYSTEMS
I will clearly illustrate the endocrine and nervous system interaction here in a simplified manner, diverse invertebrate hormones function in the homeostatis by simply regulating water balance.
For example, in a hydra one hormone stimulates growth and budding (asexual reproduction) but it prevents sexual reproduction. As for the more complex invertebrates the endocrine and the bervous systems are are basically and generally integrated in the control of reproduction and as well as development.
One well studied example of nerve and hormone interection in controlling both reproductive functions and behavious that involves the hormone that regulates egg laying in the mollusk aplysia supplied by special neurons

REGULATION BY FEEDBACK AND HOMEOSTASIS
The image below shows the feedback mechanisms in which blood concentration controls the secretion of calcitonin and parathyroid hormone (PTH), two hormones with antagonistic action , thereby maintaining the calcium levels close to a physiological set point.

Let me use Crustaceans as an example for growth and reproduction, water balance, movement of pigments in the integument and in the eyes and the regulation of metabolism. In insect and crustaceans but mostly in all arthropods with exoskeletons. Molting is triggered by a hormone called ECDYSONE. In insects, ecdysone is secreted from a pair of endocrine glands called the prothoraic glands which behind the head.
Brain hormone(BH) and ecdysone are balanced by juvenile hormone(JH), the third hormone in this is secreted by a pair of small glands just behind the brain. The adolescent hormone which stimulates the preservation of larval characteristics , this takes place in the presence of a relatively high concentration of adolescent hormone

CHEMICAL SIGNALS AND THEIR MODES OF ACTION
All living things like the animals all exhibit coordination by chemical signals and one of the major responsibility of the hormone is to carry information through the bloodstream to all parts of the body over targeted cells while some other kinds of chemical messengers function in other ways.
Local regulators are chemical signals that acts between cells on a restricted scale, once they go external apart from the cells that created them then the local regulators are then taken up by the said target cells which are broken down by enzymes which takes place within seconds.
Consequently they can only affect the local targets and still other chemical signals known as Pheromones which conveys information between two different species as they are attracted by their mates.
A VARIETY OF LOCAL REGULATORS AFFECT NEIGHBORING TARGET CELL
Chemical messengers which normal affect the targeted cells adjacent to or close to their point of secretion function in the local regulation. This brings me to neurotransmitters which takes information from one neuron to another or most times it happens from one neuron to a muscle or gland as well as targeted cells. Most cells create the gas nitric oxide (NO) which functions as a multiples of local regulators. They are highly reactive and potentially toxic, Nitric oxide normally affects its targets in about few seconds and finally breaks down.
Growth factors are simply the peptides and proteins which functions as local regulators and are present in extracellular arena for other kinds of cells to grow, divide as well as develop normally. The growth factors are generally named for the first function revealed for them.
The action of growth factors are mainly studied in cell cultures but during this period so many experiments show that growth factors performed within the animal body even in cell culture.
An example is injecting one called epidermal growth factor (EGF) into fatalmice accelerates the epidermal development.
CHEMICAL SIGNALS BIND TO A SPECIFIC RECEPTOR PROTEINS

Science have known the number of local regulators to be small fraction of the total number in the animal kingdom. In respect to thses, more than fifty hormones regulate functions of the human body as well as the endocrine systems of the other animals which are similarly expanded.
A type of given chemical signal can equally affect different target cells within an animal differently or even different species separately. For an example the thyroxine which is responsible for the metabolic regulation and plays a diverse roles in the development of animal. The thyroxine which triggers the change in adult frog from the early stage (tadpole) stimulating the tadpole tails and other morphological changes during their metamorphosis.
In sharp contrast to the variety of chemical signals and the varied responses they can elicit, the modes of action of all chemical signals are remarkably similar. Each chemical signal all have a specific shape which is familiar to the signal target. The signal actions begins when it binds to a specific receptor and The receptor protein may be inside the targeted cell. The binding of chemical signal to a receptor protein triggers normally the chemical events within the target cell that results in a change in the behavior.
CHEMICAL SIGNALS BIND TO PLASMA-MEMBRANE PROTEINS INITIATING SIGNAL TRANSDUCTION PATHWAY.

A dramatic instance of hormone action is illustrated by the process when a frog’s skin changes colour and turns lighter or darker. It is an adaptation that aids in the camouflage of the frog as the lighting changes.
Melanocyte stimulating hormone(MSH) is an peptide hormone that is incharge of the colour changing in the frog and it is secreted by the pituitary gland at the base of the brain. Melanocyte specialized skin cells that contains the dark brown pigment melanin in cytoplasmic organelles called melanosomes. The skin of the fron appears light when the melanosomes are clustered tightly around the cell nucleus and then it becomes darker when the melanosomes spread throughout the cell. The addition of MSH to interstitial fluid environment around the pigment with the cells then the melanosomes dissolves and by so doing the direct microinjection of MSH into individual melanocytes does not induce melanosome dispersion.

References :
Chemical signal

Cell signaling

Phases of Signal Transduction

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