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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Understand the phospholipid structure of neural membranes and the protein complexes therein that compose ion channels and transporters.

  • Know how the Na+/K+ ATP pump creates the ionic basis for the neural resting potential and electrical responses to stimulation.

  • Understand how the Nernst potential calculates the balance between the tendency for ions to traverse the membrane due to concentration differences versus the tendency to move via electrical potential across the membrane.

  • See that the neuron can be modeled as an equivalent electrical circuit composed of batteries derived from ionic concentration differences across the membrane and variable resistors capturing the action of ion channels.

  • Understand how the action potential is created by the opening and closing of voltage-gated Na+ and K+ channels.

  • Understand the absolute and relative refractory periods of the action potential.

  • See why action potentials are necessary for long-distance neural potential propagation.

OVERVIEW

Neurons, like all cells, carry out metabolic, synthesis, and reproductive functions. In their evolution from nonneuronal precursor cells, neurons elaborated on several existing cellular processes to promote intra- and intercellular communication. Three of the most important communication functions considered here are (1) ion channel–mediated membrane potential and (2) membrane receptor recognition of exogenous molecules, and (3) secretion. The existence of a modifiable membrane potential allows intracellular communication between the dendritic tree and the cell soma via the flow of electrical currents. Membrane receptors that recognize exogenous molecules cause currents to flow through the membrane (and sometimes other, more long-lasting effects). Exocytosis of neurotransmitter molecules at synapses, a modified form of secretion, enables specific cell–cell communication in defined functional circuits.

NEURAL MEMBRANES

Virtually all cells, including those of plants, maintain a negative potential inside with respect to outside. Although this potential may have originated merely as a by-product of the predominance of negatively charged proteins inside cells, it enabled the possibility of electrical control of the flow of ions through membrane channels due to the voltage gradient across the membrane.

Single-cell organisms express a variety of receptors that recognize desirable versus toxic substances in their environment. Early in the evolution of multicellular life-forms, some cells began to express receptors for the exocytosis of substances from other cells in the organism rather than from the external environment. These substances may have originally been waste products from other cells, but, eventually, some cell began secreting molecules whose existence and detection constituted intercellular signaling (hormones). Neurons evolved to elaborate hundreds of specific ion channel types whose activation modulates their membrane potential. The summated membrane potential at the neural cell soma or axon initial segment causes action potentials via the action of voltage-gated ion channels. These action potentials travel to the end of the axon where they generate the release of neurotransmitters targeted at other neurons (or muscles or glands) by ...

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