Action potential, in neurons

Phase 0 begins when the membrane potential reaches threshold (-40 mV). Recall that the upstroke of the action potential in neurons is due to increased permeability of fast Na+ channels, resulting in a steep, rapid depolarization. 

The concentrations of sodium, potassium (and chloride) ions in the body are high and make the largest contribution to the electrical charge of cells hence they are known as electrolytes. They have two important roles maintenance of the total solute concentration in the cell which prevents excessive movement of water into or out of cells through osmosis and the controlled movement of these ions across cell membranes acts as a signalling mechanism (e.g. the action potential in neurones and muscle. Chapter 14). Severe disruption of sodium or potassium levels in the body interferes with this signalling mechanism and with osmotic balance in cells. 

Because three Na+ ions move outward for every two K+ ions that move inward, the process is electrogenic—it creates a net separation of charge across the membrane. The result is a transmembrane potential of —50 to —70 mV (inside negative relative to outside), which is characteristic of most animal cells and essential to the conduction of action potentials in neurons. The central role of the Na+K+ ATPase is reflected in the energy invested in this single reaction about 25% of the total energy consumption of a human at rest ... 

Allows signaling via rhodopsin linked to cAMP phosphodiesterase in vertebrate eye Creates action potentials in neuronal signal transmission... 

Most of the very successful work in cellular neurobiology was conducted under the protective umbrella of Sherrington s reflex doctrine, which provided sufficient information to help work out how neuronal circuits were organized. The reflex circuit model could explain the following the spinal reflexes involved in posture and movement the encoding of stimuli in sequences of action potentials in neurons, which could lead to sensation and even the coordination of the sensory and motor systems needed to account for movement (motor) behaviour. 

Information is also encoded within the frequency of Ca + oscillations that occur in the cytosol. Oscillations can derive either from fluctuations of the entry of external calcium or of the release from internal stores. The former occur primarily in excitable cells, after the periodic opening of plasma membrane Ca + channels, such as induced, for example, by the rhythmic changes of the plasma membrane potential of the heart or by bursts of action potential in neurons. In nonexcitable cells, the predominant mechanism of [Ca +]c elevation is from the activation of plasma membrane receptors coupled to G-proteins... 

In this section, we first Introduce some of the key properties of neurons and action potentials, which move down the axon very rapidly. We then describe how the voltagegated channels responsible for propagating action potentials In neurons operate. In the following section, we examine how arrival of an action potential at the axon terminus causes secretion of chemicals called neurotransmitters. These chemicals, in turn, bind to receptors on adjacent cells and cause changes in the membrane potential of these cells. Thus electric signals carry Information within a nerve cell, while chemical signals transmit Information from one neuron to another or from a neuron to a muscle or other target cell. 

The essential physiological processes, which enable us to see, hear, move, sense, think, and experience emotion are completely dependent on the voltage-gated Na (Nay) channels. Nay channels are the major and most important group of Na channels. The Nay channels are involved in the generation of the rapid upstroke of the action potential in neurons (Figure 16.2), cardiac and skeletal muscle, and neuroendocrine cells. The classic... 

These days, stimulus devices are being used in various biomedical applications such as cochlear implant, deep-brain stimulation, muscle stimulation, and retinal prosthesis. These devices are connected between two electrodes and deliver stimulation pulses to induce a contraction in muscle tissues or action potentials in neurons. 

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