Table of Contents
Introduction
Despite the brain’s complexity, we can gain insights into its functioning by focusing on two key aspects: the wiring of individual neurons and the biophysical, biochemical, and electrophysiological characteristics of these neurons. To begin our exploration, let’s delve into the components of the nervous system and how the electrical properties of neurons enable them to process and transmit information.
The Action Potential: An Introduction
Theories on how the nervous system encodes and transmits information date back to the ancient Greek physician Galen, who proposed a hydraulic mechanism involving fluid flow through nerves to activate muscles. This theory persisted for centuries until Luigi Galvani’s groundbreaking discoveries in the late 18th century. Galvani demonstrated that nerve and muscle cells could be stimulated by electrical charges, suggesting that electrical signaling played a role in the nervous system.
However, it wasn’t until the 1930s, when modern electronic amplifiers and recording devices were developed, that the debate about the role of electricity in nerve function was finally settled. Pioneering work by H.K. Hartline using electrodes on the optic nerve of a horseshoe crab provided evidence of electrical signals known as action potentials or nerve impulses. These action potentials are the fundamental events used by nerve cells to transmit information within the nervous system.
Features of Action Potentials
Recording experiments have revealed three crucial features of nerve action potentials. Firstly, they have a short duration of about 1 millisecond. Secondly, action potentials are elicited in an all-or-nothing manner, meaning that they either occur fully or not at all. Lastly, the intensity of the stimulus is encoded by the frequency of action potentials rather than their size. As the intensity of the stimulus increases, more action potentials are generated.
Action potentials play a vital role in the brain, facilitating the transmission of information both within the nervous system and between the central nervous system and the periphery. Therefore, gaining a thorough understanding of their properties is essential. Intracellular recording techniques, which involve measuring the potential difference across a nerve cell membrane using a microelectrode, have proven invaluable in studying how action potentials are initiated and propagated.
Intracellular Recordings from Neurons
Microelectrodes with minute tips that can penetrate cells without causing damage are used to measure the potential difference across a nerve cell membrane. When the electrode is inside the cell, a sharp change in potential is detected. The reading instantly shifts from 0 mV to a potential difference of -60 mV inside the cell compared to the outside. This recorded potential is called the resting potential, which varies among different nerve cells but typically falls between -80 mV and -40 mV.
In addition to studying resting potentials, researchers have also been able to record and analyze action potentials. By inserting two electrodes into a cell, one for recording and the other for stimulation, they can manipulate the membrane potential and observe the resulting changes. When the membrane potential becomes more negative, it is called hyperpolarization, while depolarization refers to a decrease in the polarized state of the membrane.
Components of the Action Potentials
Action potentials consist of several distinct components. The threshold is the membrane potential level at which the all-or-nothing initiation of an action potential occurs. The rising phase of an action potential is known as the depolarizing phase or upstroke, while the region between the 0 mV level and the peak amplitude is called the overshoot. The repolarization phase describes the return of the membrane potential to its resting state. Finally, the undershoot or hyperpolarizing afterpotential refers to the phase where the membrane potential becomes more negative than the resting potential.
Ionic Mechanisms of Resting Potentials
Before delving into the ionic mechanisms of action potentials, it is crucial to understand the ionic mechanisms of resting potentials since the two are closely related. The story of resting potentials began in the early 1900s when Julius Bernstein proposed that the resting potential (Vm) equals the potassium equilibrium potential (EK). This concept depends on the unequal distribution of ions and the selective permeability of cell membranes.
Potassium ions (K+) play a prominent role in resting potentials due to the high membrane permeability to K+ ions. With a high concentration of K+ inside the cell and a low concentration outside, K+ naturally moves from areas of high concentration to low concentration through diffusion. However, as K+ ions leave the inner surface of the membrane, they leave behind negatively charged ions that attract the positive charge of the departing K+ ions, creating an inward-directed electrical force that counterbalances the outward diffusion. Eventually, an equilibrium is established, known as the Nernst Equilibrium Potential.
In addition to K+, sodium ions (Na+) also contribute to resting potentials. The higher concentration of Na+ outside the cell compared to inside creates both a chemical driving force and an electrical driving force for Na+ to enter the cell. The small permeability of the cell membrane to sodium allows for this movement, resulting in a slightly more depolarized membrane potential.
Goldman-Hodgkin and Katz (GHK) Equation
When a membrane is permeable to multiple ions, like Na+ and K+, the Nernst equation no longer accurately determines the membrane potential. Instead, the Goldman-Hodgkin-Katz (GHK) equation is employed. This equation takes into account the ratio of Na+ permeability to K+ permeability and enables a more precise description of the membrane potential.
By applying the GHK equation to experimental data, a better fit is achieved, revealing the significant contribution of K+ permeability relative to Na+ permeability. In summary, the resting potential arises from the high permeability to K+ ions and the slight permeability to Na+ ions.
Membrane Potential Laboratory
Visit the Membrane Potential Laboratory to interactively explore the effects of altering external or internal potassium ion concentration and membrane permeability to sodium and potassium ions. Predictions can be made using both the Nernst and the Goldman-Hodgkin-Katz equations.
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