CHAPTER 11 Functional Organization of Nervous Tissue

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2. An increase in membrane permeability to Na + can cause gradeddepolarization, and an increase in membrane permeability to K + or Cl − can result in graded hyperpolarization.3. The term graded potential is used because a stronger stimulus pro-duces a greater potential change than a weaker stimulus.4. Graded potentials can summate, or add together.5. A graded potential decreases in magnitude as the distance from thestimulation increases.

11.6 The Synapse (p. 387)

Electrical Synapses

1. Electrical synapses are gap junctions in which tubular proteins calledconnexons allow local currents to move between cells.2. At an electrical synapse, an action potential in one cell generates alocal current that causes an action potential in an adjacent cell.

Chemical Synapses

Action Potentials

1. An action potential is a larger change in the resting membrane potentialthat spreads over the entire surface of the cell.2. Threshold is the membrane potential at which a graded potentialdepolarizes the plasma membrane sufficiently to produce an actionpotential.3. Action potentials occur in an all-or-none fashion. If action potentialsoccur, they are of the same magnitude, no matter how strong thestimulus.4. Depolarization occurs as the inside of the membrane becomes morepositive because Na + diffuses into the cell through voltage-gated ionchannels.5. Repolarization is a return of the membrane potential toward the rest-ing state. It occurs because voltage-gated Na + channels close and Na + diffusion into the cell slows to resting levels and because voltage-gated K + channels continue to open and K + diffuses out of the cell.6. The afterpotential is a brief period of hyperpolarization followingrepolarization.

1. Anatomically, a chemical synapse has three components. ■ The enlarged ends of the axon are the presynaptic terminals con-taining synaptic vesicles. ■ The postsynaptic membranes contain receptors for theneurotransmitter. ■ The synaptic cleft is a space separating the presynaptic and post-synaptic membranes.2. An action potential arriving at the presynaptic terminal causes therelease of a neurotransmitter, which diffuses across the synaptic cleftand binds to the receptors of the postsynaptic membrane.3. The effect of the neurotransmitter on the postsynaptic membrane isstopped in several ways. ■ The neurotransmitter is broken down by an enzyme. ■ The neurotransmitter is taken up by the presynaptic terminal. ■ The neurotransmitter diffuses out of the synaptic cleft.4. Neurotransmitters are specific for their receptors. A neurotransmit-ter can be stimulatory in one synapse and inhibitory in another,depending on the type of receptor present.5. Neuromodulators influence the likelihood that an action potential ina presynaptic terminal will result in an action potential in the mem-brane of a postsynaptic cell.6. An excitatory postsynaptic potential (EPSP) is a depolarizing gradedpotential of the postsynaptic membrane. It can be caused by an increasein membrane permeability to Na + .7. An inhibitory postsynaptic potential (IPSP) is a hyperpolarizinggraded potential of the postsynaptic membrane. It can be caused byan increase in membrane permeability to K + or Cl − .8. Presynaptic inhibition decreases neurotransmitter release. Presynapticfacilitation increases neurotransmitter release.

Refractory Period

1. The absolute refractory period is the time during an action potentialwhen a second stimulus, no matter how strong, cannot initiate anotheraction potential.2. The relative refractory period follows the absolute refractory periodand is the time during which a stronger-than-threshold stimulus canevoke another action potential.

Action Potential Frequency

1. The strength of stimuli affects the frequency of action potentials. ■ A subthreshold stimulus produces only a graded potential. ■ A threshold stimulus causes a graded potential that reaches thresholdand results in a single action potential. ■ A submaximal stimulus is greater than a threshold stimulus andweaker than a maximal stimulus. The action potential frequencyincreases as the strength of the submaximal stimulus increases. ■ A maximal or a supramaximal stimulus produces a maximum fre-quency of action potentials.2. A low frequency of action potentials represents a weaker stimulus thana high frequency.

Spatial and Temporal Summation

1. Presynaptic action potentials through neurotransmitters produce gradedpotentials in postsynaptic neurons. The graded potential can summateto produce an action potential at the trigger zone.2. Spatial summation occurs when two or more presynaptic terminalssimultaneously stimulate a postsynaptic neuron.3. Temporal summation occurs when two or more action potentialsarrive in succession at a single presynaptic terminal.4. Inhibitory and excitatory presynaptic neurons can converge on apostsynaptic neuron. The activity of the postsynaptic neuron is deter-mined by the integration of the EPSPs and IPSPs produced in thepostsynaptic neuron.

Propagation of Action Potentials

1. An action potential generates local currents, which stimulate voltage-gated Na + channels in adjacent regions of the plasma membrane toopen, producing a new action potential.2. In an unmyelinated axon, action potentials are generated immediatelyadjacent to previous action potentials.3. In a myelinated axon, action potentials are generated at successivenodes of Ranvier.4. Reversal of the direction of action potential propagation is preventedby the absolute refractory period.5. Action potentials propagate most rapidly in myelinated, large-diameteraxons.

11.7 Neuronal Pathways and Circuits(p. 398)

1. Convergent pathways have many neurons synapsing with a few neurons.2. Divergent pathways have a few neurons synapsing with many neurons.3. Reverberating circuits have collateral branches of postsynaptic neu-rons synapsing with presynaptic neurons.4. Parallel after-discharge circuits have neurons that stimulate severalneurons arranged in parallel that stimulate a common output.