1. Neurons have four distinct anatomical regions. 2. Neuronal membranes contain a resting electrical membrane potential. 3. The resting membrane potential is the result of three major determinants. 4. The resting membrane potential can be changed by synaptic signals from a presynaptic cell. 5. Action potentials begin at the axon’s initial segment and spread down the entire length of the axon. Axons branch near their ends into several specialized endings called presynaptic terminals (or synaptic boutons). When the action potential rapidly arrives, these presynaptic terminals transmit a chemical signal to an adjacent cell. The site of contact of the presynaptic terminal with the adjacent cell is called the synapse, shown in the inset in Figure 4-1. It is formed by the presynaptic terminal of one cell (presynaptic cell), the receptive surface of the adjacent cell (postsynaptic cell), and the space between these two cells (the synaptic cleft). Presynaptic terminals contain chemical transmitter–filled synaptic vesicles that can release their contents into the synaptic cleft. The presynaptic terminals of an axon usually contact the receptive surface of an adjacent neuron or muscle cell, usually on the neuron’s dendrites, but sometimes this contact is made on the cell body or, occasionally, on the presynaptic terminals of another cell (e.g., for presynaptic inhibition). On many neurons, presynaptic terminals often synapse on small protrusions of the dendritic membrane called dendritic spines (Figure 4-2 and see Chapter 5). The receptive surface of the postsynaptic cell contains specialized receptors for the chemical transmitter released from the presynaptic terminal. The origins of the resting electrical membrane potential are complicated, particularly in a quantitative way. In qualitative terms, however, the resting membrane potential is the result of the differential separation of charged ions, especially sodium (Na+) and potassium (K+), across the membrane and the resting membrane’s differential permeability to these ions as they attempt to move back down their concentration and electrical gradients (see Chapter 1). Even though the net concentration of positive and negative charges is similar in both the intracellular and extracellular fluids, an excess of positive charges accumulates immediately outside the cell membrane, and an excess of negative charges accumulates immediately inside the cell membrane (Figure 4-4). This makes the inside of the cell negatively charged with respect to the outside of the cell. The magnitude of the resulting electrical difference (or voltage) across the membrane varies from cell to cell, ranging from about 40 to 90 millivolts (mV), and is usually about 70 mV in mammalian neurons. Because the extracellular fluid is arbitrarily considered to be 0 mV, the resting membrane potential is –70 mV, more negative on the inside than on the outside. Three major factors cause the resting membrane potential. • The Na+, K+ pump. Cell membranes have an energy-dependent pump that pumps Na+ ions out of the cell and draws K+ ions into the cell against their concentration gradients. This maintains the differential distribution of each of these charged ion species across the membrane that underlies their ability to produce a voltage across the membrane. The pump itself makes a small, direct contribution to the resting membrane potential because it pushes three molecules of Na+ out for every two molecules of K+ drawn into the cell, thus concentrating positive charges outside the cell. • An ion species will move toward a dynamic equilibrium if it can flow across the membrane. Using K+ as an example, the concentration difference across the membrane actively maintained by the Na+, K+ pump produces a concentration gradient, or chemical driving force, that attempts to push the ion passively across the membrane from high concentration inside the cell toward low concentration outside. If K+ can flow across ion channels in the membrane, exiting K+ leaves behind unopposed negative charge (often from negatively charged protein macromolecules trapped inside the cell) that builds an electrical gradient, or electrical driving force, pulling K+ back inside the cell. These opposing gradients eventually produce a dynamic equilibrium, even though there may still be more K+ inside than outside, as well as a charge imbalance across the membrane. This uneven distribution of charge at dynamic equilibrium produces a voltage across the membrane called the equilibrium potential for that ion. When an ion species can flow across a channel in the membrane, it flows toward its equilibrium state, and it drives the voltage across the membrane toward its equilibrium potential. • Differential permeability of the membrane to diffusion of ions. The resting membrane is much more permeable to K+ than to Na+ ions because there are vastly more K+ leak channels than Na+ leak channels in the membrane. This greater membrane permeability to K+ means that K+ ions can more closely approach their dynamic equilibrium state, and equilibrium potential, compared with Na+ ions, which have difficulty moving across the membrane. Therefore the equilibrium potential for the more permeant K+ ions (about –90 mV in many mammalian neurons) will have the predominant influence on the value of the resting membrane potential compared with the equilibrium potential of the vastly less permeant Na+ ions (about +70 mV in many mammalian neurons). Therefore, as noted earlier, the resting membrane potential of many mammalian neurons is about –70 mV, close to the equilibrium potential for K+.
The Neuron
Neurons Have Four Distinct Anatomical Regions
Neuronal Membranes Contain a Resting Electrical Membrane Potential
The Resting Membrane Potential Is the Result of Three Major Determinants
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The Neuron
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