as a result of ach stimulation calcium ions are released from the extracellular fluid.
as a result of ach stimulation calcium ions are released from the extracellular fluid.
as a result of ach stimulation calcium ions are released from the?
the extracellular fluid
the kidney filters 250 mmol of calcium ions a day in pro-urine (or glomerular filtrate), and resorbs 245 mmol, leading to a net average loss in the urine of about 5 mmol/d. the quantity of calcium ions excreted in the urine per day is partially under the influence of the plasma parathyroid hormone (pth) level - high levels of pth decreasing the rate of calcium ion excretion, and low levels increasing it. however, parathyroid hormone has a greater effect on the quantity of phosphate ions (hpo) excreted in the urine. phosphates form insoluble salts in combination with calcium ions. high concentrations of hpo in the plasma, therefore, lower the ionized calcium level in the extra-cellular fluids. thus, the excretion of more phosphate than calcium ions in the urine raises the plasma ionized calcium level, even though the total calcium concentration might be lowered. the kidney influences the plasma ionized calcium concentration in yet another manner. it processes vitamin d into calcitriol, the active form that is most effective in promoting the intestinal absorption of calcium. this conversion of vitamin d into calcitriol, is also promoted by high plasma parathyroid hormone levels.
calcium metabolism
all currently known activity of the olivocochlear system is via a nicotinic class neurotransmitter receptor complex that is coupled with a calcium-activated potassium channel. together, these systems generate an unusual synaptic response to stimulation from the brain. the olivocochlear synaptic terminals contain various neurotransmitters and neuroactive peptides. the major neurotransmitter employed by the olivocochlear system is acetylcholine (ach), although gamma-aminobutyric acid (gaba) is also localized in the terminals. ach release from the olivocochlear terminals activates an evolutionarily ancient cholinergic receptor complex composed of the nicotinic alpha9 and alpha10 subunits. while these subunits create a ligand-gated ion channel that is especially permeable to calcium and monovalent cations the cellular response of the outer hair cells to ach activation is hyperpolarizing, rather than the expected depolarizing response. this comes about due to the rapid activation of an associated potassium channel. this channel, the apamin sensitive, small conductance sk2 potassium channel, is activated by calcium that is likely released into the cytoplasm via calcium-induced calcium release from calcium stores within the subsynaptic cisternae as a response to incoming calcium from the nicotinic complex. however, it has not been ruled out that some incoming calcium through the nicotinic alpha9alpha10 channel may also directly activate the sk channel. electrophysiological responses recorded from outer hair cells following ach stimulation therefore show a small inward current (carried largely by incoming calcium via the acetylcholine receptor) that is immediately followed by a large outward current, the potassium current, that hyperpolarizes the outer hair cell.
olivocochlear system