Nerves, electric potential

Acetylcholine acts as a chemical relay between the electrical potentials of nerve and muscle 714... 

The human eye is a more or less spherical organ located in the orbit, a cavity in the skull of the anterior head. The main task of the eye is the dioptric function, focusing the ambient light on the retina to evoke electric potentials which can be transmitted via the optic nerve to the brain to create visual impressions (Fig. 1). 

The chemical connection between brain cells is by means of neurotransmitters—chemicals such as dopamine that convey nerve signals. In normal brain connections a neuron fires (discharges electrical potential). This in turn releases a small quantity of the neurotransmitter, which attaches itself to a receptor molecule in an adjacent neuron, causing it to fire in turn. Eventually the dopamine is returned to the cells until the ne.xt signal comes along. 

In most instances the arrival of a nerve signal at the presynaptic end of a neuron causes the release of a transmitter substance (neurohormone). Tire transmitter passes across the 10-50 nm (typically 20 nm) synaptic cleft between the two cells and induces a change in the electrical potential of the postsynaptic membrane of the next neuron (Fig. 30-10).149 401 Excitatory transmitters usually cause depolarization of the membrane. By this we mean that the membrane potential, which in a resting neuron is -50 to -70 mv (Chapter 8), falls to nearly zero often as a consequence of an increased permeability to Na+ and a resultant inflow of sodium ions. The resulting postsynaptic... 

Action potentials are waves of depolarization and repolarization of the plasma membrane. In a resting nerve cell, the electric potential gradient (At//) across the plasma membrane is about —70 mV, inside negative. This potential difference is generated mainly by the unequal rates of diffusion of K+ and Na+ ions down concentration gradients maintained by the Na+-K+ ATPase. 

At the neuromuscular junction, the electrical changes following the action of the neurotransmitter substance acetylcholine on the muscle cell were investigated by Katz and Fatt in 1951. It was reasonable to suppose that the ionic currents passed through channels that were activated by acetylcholine. Both here and with the nerve action potential only the currents produced by flow through some hundreds or thousands of channels at once could be measured. 

Here binding of the ligand again causes a conformational change in the protein but this time such that a specific ion channel is opened (Fig. 3). This allows a certain ion to flow through that subsequently alters the electric potential across the membrane. For example, at the nerve-muscle junction the neurotransmitter acetylcholine binds to specific receptors that allow Na+ ions to flow into and K+ ions out of the target cell (see Topic N3). 

When a sound pressure wave impinges on the ear, it is amplified by the external auditory meatus and causes the tympanic membrane to vibrate in a characteristic manner. This vibration is transformed by the auditory ossicles of the middle ear into movements of the stapedial footplate. These movements create pressure waves in the fluids of the inner ear which displace the basilar membrane of the cochlear duct and cause the hair cells located on the top of the basilar membrane to generate electrical potentials. This potential elicits impulses in the auditory nerve. After the auditory nerve, the nerve impulses are transmitted through the cochlear nuclei, the trapezoid body, the... 

The 1952 Hodgkin-Huxley model for membrane electrical potential is perhaps the oldest and the best known cellular kinetic model that exhibits temporal oscillations. The phenomenon of the nerve action potential, also known as excitability, has grown into a large interdisciplinary area between biophysics and neurophysiology, with quite sophisticated mathematical modeling. See [103] for a recent treatise. 

We badly need a small sense organ for detecting wireless frequencies, eyes for infra-red, ultra-violet and x-rays, ears for supersonics, detectors of high and low temperatures, of electrical potential and current, and chemical organs of many kinds. We may perhaps be able to train a greater number of hot and cold and pain receiving nerves to take over these functions.16... 

Finally, in the case of the role of membrane potential in nervous conduction, a traveling wave of an electrical potential gradient (voltage) is used to relay signals over relatively long distances along the cell membrane ie as in axonal linkages between cells (see e.g. 5). This latter manifestation of a membrane potential as part of the nerve impulse is perhaps the most... 

Action potential—transient change in the electrical potential across a membrane which results in the generation of a nerve impulse. 

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