Local Potentials

We now consider the disturbances in membrane potential that occur when a neuron is stimulated. Typically (but with exceptions), the response of a neuron begins at a den-drite, spreads through the soma, travels down the axon, and ends at the synaptic knobs. We consider the process in that order.

Neurons can be stimulated by chemicals, light, heat, or mechanical distortion of the plasma membrane. We'll take as our example a neuron being chemically stimulated on its dendrite (fig. 12.10). The chemical—perhaps a pain signal from a damaged tissue or odor molecule in a breath of air—binds to receptors on the neuron. These receptors are ligand-regulated sodium gates that open and allow Na+ to rush into the cell. The inflow of Na+ neutralizes some of the internal negative charge, so the voltage across the membrane drifts toward zero. Any such case in which membrane voltage shifts to a less negative value is called depolarization. The incoming sodium ions diffuse for short distances along the inside of the plasma membrane and produce a current that travels from the point of stimulation toward the cell's trigger zone. Such a short-range change in voltage is called a local potential.

There are four characteristics that distinguish local potentials from the action potentials we will study shortly (table 12.2). You will appreciate these distinctions more fully after you have studied action potentials.

Large anions that cannot escape cell

Figure 12.9 Ionic Basis of the Resting Membrane Potential.

Note that sodium ions are much more concentrated in the extracellular fluid (ECF) than in the intracellular fluid (ICF), while potassium ions are more concentrated in the ICF. Large anions unable to penetrate the plasma membrane give the cytoplasm a negative charge relative to the ECF. If we suddenly increased the concentration of Cl" ions in the ICF,would the membrane potential become higher or lower than the RMP?

Large anions that cannot escape cell

Saladin: Anatomy & I 12. Nervous Tissue I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 12 Nervous Tissue 457

Chapter 12 Nervous Tissue 457

Compare Local And Action Potentials

Figure 12.10 Excitation of a Neuron by a Chemical Stimulus. When the chemical (ligand) binds to a receptor on the neuron, the receptor acts as a ligand-regulated ion gate through which Na diffuses into the cell. This depolarizes the plasma membrane.

Figure 12.10 Excitation of a Neuron by a Chemical Stimulus. When the chemical (ligand) binds to a receptor on the neuron, the receptor acts as a ligand-regulated ion gate through which Na diffuses into the cell. This depolarizes the plasma membrane.

Table 12.2 Comparison of Local Potentials and Action Potentials

Local Potential

Action Potential

Produced by ligand-regulated gates on the dendrites and soma

May be a positive (depolarizing) or negative (hyperpolarizing) voltage change

Graded; proportional to stimulus strength

Reversible; returns to RMP if stimulation ceases before threshold is reached Local; has effects for only a short distance from point of origin Decremental; signal grows weaker with distance

Produced by voltage-regulated gates on the trigger zone and axon Always begins with depolarization

All-or-none; either does not occur at all or exhibits same peak voltage regardless of stimulus strength Irreversible; goes to completion once it begins Self-propagating; has effects a great distance from point of origin Nondecremental; signal maintains same strength regardless of distance

  1. Local potentials are graded, meaning that they vary in magnitude (voltage) according to the strength of the stimulus. A more intense or prolonged stimulus opens more ion gates than a weaker stimulus. Thus, more Na+ enters the cell and the voltage changes more than it does with a weaker stimulus.
  2. Local potentials are decremental, meaning they get weaker as they spread from the point of stimulation. The decline in strength occurs because as Na+

spreads out under the plasma membrane and depolarizes it, K+ flows out and reverses the effect of the Na+ inflow. Therefore, the voltage shift caused by Na+ diminishes rapidly with distance. This prevents local potentials from having any long-distance effects.

3. Local potentials are reversible, meaning that if stimulation ceases, K+ diffusion out of the cell quickly returns the membrane voltage to its resting potential.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

12. Nervous Tissue


© The McGraw-Hill Companies, 2003

458 Part Three Integration and Control

4. Local potentials can be either excitatory or inhibitory. So far, we have considered only excitatory local potentials, which depolarize a cell and make a neuron more likely to produce an action potential. Acetylcholine usually has this effect. Other neurotransmitters, such as glycine, cause an opposite effect—they hyperpolarize a cell, or make the membrane more negative. The neuron is then less sensitive and less likely to produce an action potential. A balance between excitatory and inhibitory potentials is very important to information processing in the nervous system, and we explore this more fully later in the chapter.

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    What is local potential?
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    What are the characteristics of local potential?
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  • Pentti
    Is local potential self regenerating?
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    What is acute local potential?
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    What is the origin of local potentials at the synapse?
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    What the importance of local potentials?
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  • Emppu
    Are local potentials summative?
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