Shorts Notes on Depolarization and Re-polarization, Hormone and its effect on target cells and Unique hormones in birds


Depolarization and Re-polarization:
When a stimulus is appeared to a point along the resting plasma membrane the permeability to Na+ ions causes the temperature to go from -70mV toward 0. This loss in membrane polarity is called depolarization. When depolarization reaches certain level, special Na+ channels (voltage gated) that are sensitive to changes in membrane potential quickly open and more Na+ ions can rush to the inside of the neuron.
Re-polarization: Shortly after Na+ ions move into the cell, Na+ ions rapidly diffuse outward. The movement of K+ ions out of the cell builds up positive charge outside the cell again and membrane becomes re-polarized.

Hormone and its effect on target cells:
A hormone specialized chemical messenger that an endocrine gland produces and secretes. Hormones circulate through body fluid and affect the metabolic activity of target cells in a specific way. Target cells has receptors to which chemical messenger either selectively bind or on which they have an effect only rarely does a hormone operates independently one hormone influences, depends on and balances another hormone in a controlled feed back network.
Biochemical reactions: In target cell hormones help control biochemical reactions in three ways: (i) A hormone can increase the rate of other substances enter or leave the cell; (ii) It can stimulate a target cell to synthesize enzymes proteins or other substances (iii) It can promote a target cell to activate or suppress existing cellular enzymes.

Unique hormones in birds:
In some birds like Pigeons and doves, the pituitary gland secretes the hormone protactin. Protactin stimulates the production of “Pigeon’s milk” by desquamation (sloughing of cells) in pigeon’s crop. Protactin also stimulates and regulates broodiness and certain other kinds of parental behaviour and along with estrogen, stimulates full development of thebrood (incubation) patch. Brood patch helps keep the eggs at a temperature between 33 and 37°C. Birds thyroid gland produces hormone thryoxine. It regulates the normal development of feathers and the molt cycle and plays a role in the onset of migratory behaviour. In male birds the teats produce the hormone testosterone. It controls secondary sexual characters of male, such as bright plumage color comb and spurs all of which strongly influence sexual behaviour. Ultimobrnachial glands are small, paired structures in the neck just below parathyroid glands. They secrete hormone calcitonium which is involved in regulating blood calcium concentrations. Bursa fabricuis is a sac that lies just dorsal to the cloaca and eupties into it. Its tissues produce secretions that are responsible for maturation of white blood cells which play important role in immunological reactions.

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