Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 148 . Most axons in the PNS, whether myelinated or unmyelinated, are shielded from contact with interstitial fluids by Schwann cells. 13.7 Transmembrane Potential The transmembrane potential is the electrical potential of the interior of the cell relative to its surroundings. All living cells have a transmembrane potential that varies from moment to moment depending on the activities of the cell. The resting potential is the transmembrane potential of a resting cell. All neural actions begin with an alteration in the resting potential of a neuron. A typical stimulus creates an interim, localized change in the resting potential. The effect is called a graded potential, and it decreases with distance from the stimulus. If the graded potential is sufficiently large, it triggers an action potential in the membrane of the axon. An action potential is an electrical impulse that is propagated along the surface of an axon and does not decrease as it travels away from its source. The impulse moves along the axon to one or more synapses. Synaptic activity then creates graded potentials in the plasma membrane of the postsynaptic cell. The activity typically includes the release of neurotransmitters, such as ACh, by the presynaptic cell. These compounds bind to receptors on the postsynaptic plasma membrane, changing its permeability. The mechanism is comparable to that of the neuromuscular junction. The response of the postsynaptic cell ultimately depends on what the stimulated receptors do and what other stimuli are influencing the cell at the same time. The integration of stimuli at the level of the individual cell is the simplest form of information processing in the nervous system. The intracellular fluid (cytosol) and extracellular fluid differ markedly in ionic composition. The extracellular fluid (ECF) contains large concentrations of sodium ions (Na+) and chloride ions (Cl–), whereas the cytosol contains high concentrations of potassium ions (K+) and negatively charged proteins. If the plasma membrane were freely permeable, diffusion would continue until all the ions were evenly distributed across the membrane and a state of equilibrium existed. An even distribution does not occur because cells have selectively permeable membranes. Ions cannot freely cross the lipid portions of the plasma membrane; they can enter or leave the cell only through membrane channels. Many kinds of membrane channels exist, each with different properties. At the resting potential (the transmembrane potential of an undisturbed cell), ion movement occurs through leak channels —membrane channels that are always open. Active transport mechanisms also move specific ions into or out of the cell. The cell’s passive and active transport mechanisms do not guarantee an equal delivery of charges across its membrane because membrane permeability varies of 368 Neuroglia of the Peripheral Nervous System As previously noted, the cell bodies of neurons in the PNS are clustered in masses called ganglia . Neuronal cell bodies and most axons in the PNS are completely insulated from their surroundings by the processes of neuroglia. The two types of neuroglia in the PNS are called satellite cells and Schwann cells. Satellite cells , or amphicytes, surround neuron cell bodies in ganglia. They regulate the environment around the neurons. Schwann cells , or neurilemmal cells, form a sheath around peripheral axons. Wherever a Schwann cell covers an axon, the outer surface of the Schwann cell is called the neurilemma