Mini Guyton Fisiologia Pdf
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The neural pathways of the autonomic nervous system continue to play a pivotal role during prolonged orthostatic stress. Sustained elevation of efferent sympathetic activity is documented by increases in plasma noradrenaline concentration and spillover (Ziegler et al. 1977) and by directly recorded sympathetic nerve activity to skeletal muscle (Burke et al. 1977; Iwase et al. 1987; Joyner et al. 1990). During prolonged orthostatic stress additional reflex activation of the neurohumoral system reinforces the action of the cardiovascular reflexes through additional constriction of blood vessels. It also minimizes the loss of body water during prolonged orthostatic stress by activation of the renal blood volume controlling systems. The humoral responses are of importance for cardiovascular adjustment during hypotensive orthostatic stress, but cannot supplant the functions of the neural system (Rowell, 1993).
In dogs arteriolar vasoconstriction plays a role in determining venous pooling in skeletal muscle by reducing blood flow and post-capillary pressures (i.e. passive changes in venous volume that depend on changes in flow) (Hainsworth et al. 1983). In accordance, in humans it has been shown that increase in vasoconstriction due to increased sympathetic outflow to legs during orthostasis may lead to a reduction in calf volume (Wallin & Sundlof, 1982; Iwase et al. 1987; Vissing et al. 1989; Hopman et al. 1993; Shamsuzzaman et al. 1998). However, the magnitude of the effect of changes in blood flow on venous blood volume in human skeletal muscle is not known yet.
Factors which augment cutaneous vascular capacity, such as varicose veins, are associated with symptoms of orthostatic intolerance in healthy subjects (Chapman & Asmussen, 1942) and may aggravate orthostatic hypotension in autonomic failure as well. Similarly, local heating of the skin (e.g. exposure to a hot environment) or increased heat production (e.g. fever or exercise) leads to increased skin blood flow and to an additional sequestration of blood in the skin of the lower body. This can cause orthostatic symptoms at standing (Horvath & Botelho, 1949). Due to unrestrained cutaneous vasodilatation (Mosely, 1969; Johnson et al. 1973) skin blood flow and venous blood volume are likely to increase even more in patients with peripheral adrenergic failure. Furthermore, autonomic failure patients may be more susceptible to heat stress due to impaired sweating, which results in a diminished ability to dissipate heat (Bannister et al. 1967).
End plate potentials (EPPs) are the voltages which cause depolarization of skeletal muscle fibers caused by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction. They are called "end plates" because the postsynaptic terminals of muscle fibers have a large, saucer-like appearance. When an action potential reaches the axon terminal of a motor neuron, vesicles carrying neurotransmitters (mostly acetylcholine) are exocytosed and the contents are released into the neuromuscular junction. These neurotransmitters bind to receptors on the postsynaptic membrane and lead to its depolarization. In the absence of an action potential, acetylcholine vesicles spontaneously leak into the neuromuscular junction and cause very small depolarizations in the postsynaptic membrane. This small response (~0.4mV) is called a miniature end plate potential (MEPP) and is generated by one acetylcholine-containing vesicle. It represents the smallest possible depolarization which can be induced in a muscle.
EPP are caused mostly by the binding of acetylcholine to receptors in the postsynaptic membrane. There are two different kinds of acetylcholine receptors: nicotinic and muscarinic. Nicotinic receptors are ligand gated ion channels for fast transmission. All acetylcholine receptors in the neuromuscular junction are nicotinic. Muscarinic receptors are G protein-coupled receptors that use a second messenger. These receptors are slow and therefore are unable to measure a miniature end plate potential (MEPP). They are located in the parasympathetic nervous system such as in the vagus nerve and the gastrointestinal tract. During fetal development acetylcholine receptors are concentrated on the postsynaptic membrane and the entire surface of the nerve terminal in the growing embryo is covered even before a signal is fired. Five subunits consisting of four different proteins from four different genes comprise the nicotinic acetylcholine receptors therefore their packaging and assembly is a very complicated process with many different factors. The enzyme muscle-specific kinase (MuSK) initiates signaling processes in the developing postsynaptic muscle cell. It stabilizes the postsynaptic acetylcholine receptor clusters, facilitates the transcription of synaptic genes by muscle fiber nuclei, and triggers differentiation of the axon growth cone to form a differentiated nerve terminal. Substrate laminin induces advanced maturation of the acetylcholine receptor clusters on the surfaces of myotubes.
When a vesicle releases its neurotransmitters via exocytosis, it empties its entire contents into the synaptic cleft. Neurotransmitter release from vesicles is therefore stated to be quantal because only whole numbers of vesicles can be released. In 1970, Bernard Katz from the University of London won the Nobel Prize for Physiology or Medicine for statistically determining the quantal size of acetylcholine vesicles based on noise analysis in the neuromuscular junction. Using a book on mechanical statistics[clarification needed], he was able to infer the size of individual events going on at the same time.
The synaptic vesicles of acetylcholine are clear core synaptic vesicles with a diameter of 30 nm. Each acetylcholine vesicle contains approximately 5000 acetylcholine molecules. The vesicles release their entire quantity of acetylcholine and this causes miniature end plate potentials (MEPPs) to occur which are less than 1mV in amplitude and not enough to reach threshold.
When an action potential causes the release of many acetylcholine vesicles, acetylcholine diffuses across the neuromuscular junction and binds to ligand-gated nicotinic receptors (non-selective cation channels) on the muscle fiber. This allows for increased flow of sodium and potassium ions, causing depolarization of the sarcolemma (muscle cell membrane). The small depolarization associated with the release of acetylcholine from an individual synaptic vesicle is called a miniature end-plate potential (MEPP), and has a magnitude of about +0.4mV. MEPPs are additive, eventually increasing the end-plate potential (EPPs) from about -100mV up to the threshold potential of -60mV, at which level the voltage-gated ion channels in the postsynaptic membrane open, allowing a sudden flow of sodium ions from the synapse and a sharp spike in depolarization. This depolarization voltage spike triggers an action potential which propagates down the postsynaptic membrane leading to muscle contraction. It is important to note that EPPs are not action potentials, but that they trigger action potentials.In a normal muscular contraction, approximately 100-200 acetylcholine vesicles are released causing a depolarization that is 100 times greater in magnitude than a MEPP. This causes the membrane potential to depolarize +40mV (100 x 0.4mV = 40mV) from -100mV to -60mV where it reaches threshold.
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