Nerve impulse

Excitable media are some of tire most commonly observed reaction-diffusion systems in nature. An excitable system possesses a stable fixed point which responds to perturbations in a characteristic way small perturbations return quickly to tire fixed point, while larger perturbations tliat exceed a certain tlireshold value make a long excursion in concentration phase space before tire system returns to tire stable state. In many physical systems tliis behaviour is captured by tire dynamics of two concentration fields, a fast activator variable u witli cubic nullcline and a slow inhibitor variable u witli linear nullcline [31]. The FitzHugh-Nagumo equation [34], derived as a simple model for nerve impulse propagation but which can also apply to a chemical reaction scheme [35], is one of tire best known equations witli such activator-inlribitor kinetics ... 

Several naturally occurring amines mediate the transmission of nerve impulses and are referred to as neurotransmitters Two examples are epinephrine... 

Aminobutanoic acid known as 7 aminobutync acid (GABA) it is a 7 amino acid and is involved in the transmission of nerve impulses... 

As an activator of the phosphokinases, magnesium is essential in energy-requiring biological processes, such as activation of amino acids, acetate, and succinate synthesis of proteins, fats, coen2ymes, and nucleic acids generation and transmission of nerve impulses and muscle contraction (67). 

Local anesthetics produce anesthesia by blocking nerve impulse conduction in sensory, as well as motor nerve, fibers. Nerve impulses are initiated by membrane depolarization, effected by the opening of a sodium ion channel and an influx of sodium ions. Local anesthetics act by inhibiting the channel s opening they bind to a receptor located in the channel s interior. The degree of blockage on an isolated nerve depends not only on the amount of dmg, but also on the rate of nerve stimulation (153—156). 

Typically, in order for motion to occur, several muscle sets must work together to perform even the simplest movements. The bicep is a two-muscle set the tricep is a three-muscle set. Each set works in tandem. Within each muscle group, muscle fibers obey the aU. or none principle, ie, aU. muscle fibers contract or none contract. Therefore, if the muscle fibers of a muscle group are stimulated enough by nerve impulses to contract, they contract to the maximum. 

Choline functions in fat metaboHsm and transmethylation reactions. Acetylcholine functions as a neurotransmitter in certain portions of the nervous system. Acetylcholine is released by a stimulated nerve cell into the synapse and binds to the receptor site on the next nerve cell, causing propagation of the nerve impulse. 

This makes choline an important nutritional substance. It is also of great physiological interest because one of its esters, acetylcholine [51-84-3] appears to be responsible for the mediation of parasympathetic nerve impulses and has been postulated to be essential to the transmission of all nerve impulses. Acetylcholine and other more stable compounds that simulate its action are pharmacologically important because of their powerful effect on the heart and on smooth muscle. Choline is used clinically in Hver disorders and as a constituent in animal feeds. 

Cochlea A snail-shaped fluid-filled organ of the inner ear, lined on its inner surface with specialized hair cells that convert sound pressure vibrations into nerve impulses. 

Sensory hearing loss Irreversible hearing loss resulting from damage to the inner ear tissue that translates sound pressure into nerve impulses. 

Neurotransmitter (Section 22.4) Substance, usually a naturally occurring amine, that mediates the transmission of nerve impulses. 

Anesthetic, local. A compound that blocks conduction of nerve impulses, thus rendering insensible the area to which it is... 

Baroreceptor. Specialized pressure-sensitive tissue located in carotid arteries. Nerve impulses proportional to arterial blood pressure are conducted from this tissue to the brain which in turn exerts control over the blood pressure. 

Peripheral sympathetic blocking agent. A drug that disrupts the transmission of nerve impulses to sympathetically innervated structures. 

On the basis of this work and other research, in 1849 Helmholtz was appointed professor of physiology at the University of Koonigsberg. There he devoted himself to the physiology of the eye, first explaining the mechanism of lens accommodation. In 1851 Ins invention of the ophthalmoscope, still the basic instrument used by eye doctors to peer at the retina of the eye, immediately made Helmholtz famous. In 1852 he also became the first experimentalist to measure the speed of nerve impulses in the human body. 

Another consequence of the effect of pressure on gas solubility is the painful, sometimes fatal, affliction known as the bends. This occurs when a person goes rapidly from deep water (high pressure) to the surface (lower pressure), where gases are less soluble. The rapid decompression causes air, dissolved in blood and other body fluids, to bubble out of solution. These bubbles impair blood circulation and affect nerve impulses. To minimize these effects, deep-sea divers and aquanauts breathe a helium-oxygen mixture rather than compressed air (nitrogen-oxygen). Helium is only about one-third as soluble as nitrogen, and hence much less gas comes out of solution on decompression. 

The CNS consists of die brain and die spinal cord and receives, integrates, and inteqirets nerve impulses. 

N euro transmitters are chemical substances called neurohormones. These are released at Hie nerve ending that facilitate the transmission of nerve impulses. The two neurohormones (neurotransmitters) of the sympathetic nervous system are epinephrine and norepinephrine Epinephrine is secreted by the adrenal medulla Norepinephrine is secreted mainly at nerve ending of sympathetic (also called adrenergic) nerve fibers (Pig. 22-2). 

Rgure 22-2. Neurotransmission in the central nervous system. Neurotransmitter molecules (eg, norepinephrine), released by the presynaptic nerve, cross the synapse and bind with receptors in the cell membrane of the postsynaptic nerve, resulting in the transmission of the nerve impulse. 

Electron microscopic study reveals an incalculably small space between nerve endings and the effector organ (eg, tlie muscle, cell, or gland) diat is innervated (or controlled) by a nerve fiber. Fbr a nerve impulse to be transmitted from die nerve ending (motor end plate) across die space to die effector organ, a neurohormone is needed. 

The PNS has two neurohormones (neurotransmitters) acetylcholine (ACh) and acetylcholinesterase (ACliE). ACh is a neurotransmitter responsible for die transmission of nerve impulses to effector cells of die parasympathetic nervous system. ACh plays an important role in die transmission of nerve impulses at synapses and myoneural junctions. ACh is quickly... 

Cholinergic blocking dragp inhibit die activity of acetylcholine in parasympadietic nerve fibers (see Chap. 24 for a description of die role of acetylcholine in the transmission of nerve impulses across parasympadietic nerve fibers). When die activity of acetylcholine is inhibited, nerve impulses traveling along parasympadietic nerve fibers cannot pass from die nerve fiber to die effector organ or structure ... 

Only one impulse can pass along a nerve fiber at any given time. After the passage of an impulse, tiiere is a brief pause, or interval, before the next impulse can pass along the nerve fiber. This pause is called the refractory period, which is the period between the transmission of nerve impulses along a nerve fiber. By... 

After receiving lidocaine hydrochloride (Xylocaine HCI), the threshold is raised to a higher level, allowing fewer stimuli to reach the threshold. This results in decreased stimulation of the nerve fiber and prevents conduction of the nerve impulses causing the arrthvthmia. 

Systemic and coronary arteries are influenced by movement of calcium across cell membranes of vascular smooth muscle. The contractions of cardiac and vascular smooth muscle depend on movement of extracellular calcium ions into these walls through specific ion channels. Calcium channel blockers, such as amlodipine (Norvasc), diltiazem (Cardizem), nicardipine (Cardene), nifedipine (Procardia), and verapamil (Calan), inhibit die movement of calcium ions across cell membranes. This results in less calcium available for the transmission of nerve impulses (Fig. 41-1). This drug action of the calcium channel blockers (also known as slow channel blockers) has several effects on die heart, including an effect on die smooth muscle of arteries and arterioles. These drug dilate coronary arteries and arterioles, which in turn deliver more oxygen to cardiac muscle. Dilation of peripheral arteries reduces die workload of die heart. The end effect of these drug is the same as that of die nitrates. 

Magnesium plays an important role in the transmission of nerve impulses. It is also important in the activity of many enzyme reactions, for example carbohydrate metabolism. Magnesium sulfate is used as replacement therapy in hypomagnesemia Magnesium sulfate (MgS04) is used in die prevention and control of seizures in obstetric patients with pregnancy-induced hypertension (PIH, also referred to as eclampsia and preeclampsia). It may also be added to TPN mixtures. 

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