Hockenbury Discovering Psychology 5th txtbk

The Neuron49Once the action potential is started, it is self-sustaining and continues to theend of the axon. In other words, there is no such thing as a partial action potential.Either the neuron is sufficiently stimulated and an action potential occurs,or the neuron is not sufficiently stimulated and an action potential doesnot occur. This principle is referred to as the all-or-none law.Following the action potential, a refractory period occurs during which theneuron is unable to fire. This period lasts for about a thousandth of a second orless. During the refractory period, the neuron repolarizes and reestablishes thenegative-inside/positive-outside condition. Like depolarization, repolarizationoccurs progressively at each segment down the axon. This process reestablishesthe resting potential conditions so that the neuron is capable of firing again. Thegraph in Figure 2.3 depicts the complete sequence from resting potential to actionpotential and back to resting potential.For clarity, we’ve simplified some of the details involved in the action potential, butthis is basically how information is communicated within the neuron. Remember, actionpotentials are generated in mere thousandths of a second. Thus, a single neuroncan potentially generate hundreds of neural impulses per second. Given these minuteincrements of time, just how fast do neural impulses zip around your body?The fastest neurons in your body communicate at speeds of up to 270 miles perhour. In the slowest neurons, messages creep along at about 2 miles per hour. Thisvariation in communication speed is due to two factors: the axon diameter and themyelin sheath. The greater the axon’s diameter, the faster the axon conducts actionpotentials. And, as we said earlier, myelinated neurons communicate faster thanunmyelinated neurons. In myelinated neurons, the sodium ion channels are concentratedat each of the nodes of Ranvier where the myelin is missing. So, in myelinatedneurons the action potential “jumps” from node to node rather than progressingdown the entire length of the axon.Electrical charge in millivolts+300–60–70–80–900 1Ions crossmembraneStimulusthresholdResting potentialAction potentialMillisecondsRefractoryperiodRestingpotential2 3Figure 2.3 Electrical Changes During anAction Potential This graph shows thechanging electrical charge of the neuronduring an action potential. When the neurondepolarizes and ions cross the axonmembrane, the result is a brief positive electricalimpulse of 30 millivolts—the actionpotential. During the refractory period, theneuron reestablishes the resting potentialnegative charge of 70 millivolts and thenis ready to activate again.Communication Between NeuronsBridging the GapKey Theme• Communication between neurons takes place at the synapse, the junctionbetween two adjoining neurons.Key Questions• How is information communicated at the synapse?• What is a neurotransmitter, and what is its role in synaptic transmission?• What are five important neurotransmitters, and how do psychoactivedrugs affect synaptic transmission?The primary function of a neuron is to communicate information to other cells,most notably other neurons. The point of communication between two neurons iscalled the synapse. At this communication junction, the message-sending neuron isreferred to as the presynaptic neuron. The message-receiving neuron is called thepostsynaptic neuron. For cells that are specialized to communicate information,neurons have a surprising characteristic: They don’t touch each other. The presynapticand postsynaptic neurons are separated by a tiny, fluid-filled space, called thesynaptic gap, which is only about five-millionths of an inch wide.The transmission of information between two neurons occurs in one of twoways: electrically or chemically. When communication is electrical, the synaptic gapis extremely narrow, and special ion channels serve as a bridge between the neurons.Electrical communication between the two neurons is virtually instantaneous. Lessthan one percent of the synapses in the brain use chemical transmission.all-or-none lawThe principle that either a neuron is sufficientlystimulated and an action potential occursor a neuron is not sufficiently stimulatedand an action potential does not occur.synapse(SIN-aps) The point of communicationbetween two neurons.synaptic gap(sin-AP-tick) The tiny space between theaxon terminal of one neuron and thedendrite of an adjoining neuron.