Excitatory postsynaptic potentials, showing that simultaneous firing of only a few synapses will not cause sufficient summated potential to elicit an action potential, but that simultaneous firing of many synapses will raise the summated potential to threshold for excitation and cause a superimposed action potential.
the EPSP becomes great enough, the threshold for firing will be reached and an action potential will develop spontaneously in the initial segment of the axon. This is demonstrated in Figure 45-10. The bottom postsynaptic potential in the figure was caused by simultaneous stimulation of 4 synapses; the next higher potential was caused by stimulation of 8 synapses; finally, a still higher EPSP was caused by stimulation of 16 synapses. In this last instance, the firing threshold had been reached, and an action potential was generated in the axon.
This effect of summing simultaneous postsynaptic potentials by activating multiple terminals on widely spaced areas of the neuronal membrane is called spatial summation.
Each time a presynaptic terminal fires, the released transmitter substance opens the membrane channels for at most a millisecond or so. But the changed post-synaptic potential lasts up to 15 milliseconds after the synaptic membrane channels have already closed. Therefore, a second opening of the same channels can increase the postsynaptic potential to a still greater level, and the more rapid the rate of stimulation, the greater the postsynaptic potential becomes. Thus, successive discharges from a single presynaptic terminal, if they occur rapidly enough, can add to one another; that is, they can "summate." This type of summation is called temporal summation.
Simultaneous Summation of Inhibitory and Excitatory Postsy-naptic Potentials. If an IPSP is tending to decrease the membrane potential to a more negative value while an EPSP is tending to increase the potential at the same time, these two effects can either completely or partially nullify each other. Thus, if a neuron is being excited by an EPSP, an inhibitory signal from another source can often reduce the postsynaptic potential to less than threshold value for excitation, thus turning off the activity of the neuron.
"Facilitation" of Neurons
Often the summated postsynaptic potential is excitatory but has not risen high enough to reach the threshold for firing by the postsynaptic neuron. When this happens, the neuron is said to be facilitated. That is, its membrane potential is nearer the threshold for firing than normal, but not yet at the firing level. Consequently, another excitatory signal entering the neuron from some other source can then excite the neuron very easily. Diffuse signals in the nervous system often do facilitate large groups of neurons so that they can respond quickly and easily to signals arriving from other sources.
Special Functions of Dendrites for Exciting Neurons
Large Spatial Field of Excitation of the Dendrites. The den-
drites of the anterior motor neurons often extend 500 to 1000 micrometers in all directions from the neu-
ronal soma. And these dendrites can receive signals from a large spatial area around the motor neuron. This provides a vast opportunity for summation of signals from many separate presynaptic nerve fibers.
It is also important that between 80 and 95 per cent of all the presynaptic terminals of the anterior motor neuron terminate on dendrites, in contrast to only 5 to 20 per cent terminating on the neuronal soma. Therefore, the preponderant share of the excitation is provided by signals transmitted by way of the dendrites.
Most Dendrites Cannot Transmit Action Potentials—But They Can Transmit Signals Within the Same Neuron by Electrotonic Conduction. Most dendrites fail to transmit action potentials because their membranes have relatively few voltage-gated sodium channels, and their thresholds for excitation are too high for action potentials to occur. Yet they do transmit electrotonic current down the dendrites to the soma. Transmission of electrotonic current means direct spread of electrical current by ion conduction in the fluids of the dendrites but without generation of action potentials. Stimulation (or inhibition) of the neuron by this current has special characteristics, as follows.
Decrement of Electrotonic Conduction in the Dendrites—Greater Excitatory (or Inhibitory) Effect by Synapses Located Near the Soma. In Figure 45-11, multiple excitatory and inhibitory synapses are shown stimulating the dendrites of a neuron. On the two den-drites to the left, there are excitatory effects near the tip ends; note the high levels of excitatory postsynap-tic potentials at these ends—that is, note the less negative membrane potentials at these points. However, a
Stimulation of a neuron by presynaptic terminals located on dendrites, showing, especially, decremental conduction of excitatory (E) electrotonic potentials in the two dendrites to the left and inhibition (I) of dendritic excitation in the dendrite that is uppermost. A powerful effect of inhibitory synapses at the initial segment of the axon is also shown.
large share of the excitatory postsynaptic potential is lost before it reaches the soma. The reason is that the dendrites are long, and their membranes are thin and at least partially permeable to potassium and chloride ions, making them "leaky" to electric current. Therefore, before the excitatory potentials can reach the soma, a large share of the potential is lost by leakage through the membrane. This decrease in membrane potential as it spreads electrotonically along dendrites toward the soma is called decremental conduction.
The farther the excitatory synapse is from the soma of the neuron, the greater will be the decrement, and the less will be excitatory signal reaching the soma. Therefore, those synapses that lie near the soma have far more effect in causing neuron excitation or inhibition than those that lie far away from the soma.
Summation of Excitation and Inhibition in Dendrites. The uppermost dendrite of Figure 45-11 is shown to be stimulated by both excitatory and inhibitory synapses. At the tip of the dendrite is a strong excitatory post-synaptic potential, but nearer the soma are two inhibitory synapses acting on the same dendrite. These inhibitory synapses provide a hyperpolarizing voltage that completely nullifies the excitatory effect and indeed transmits a small amount of inhibition by elec-trotonic conduction toward the soma. Thus, dendrites can summate excitatory and inhibitory postsynaptic potentials in the same way that the soma can. Also shown in the figure are several inhibitory synapses located directly on the axon hillock and initial axon segment. This location provides especially powerful inhibition because it has the direct effect of increasing the threshold for excitation at the very point where the action potential is normally generated.
Relation of State of Excitation of the Neuron to Rate of Firing
"Excitatory State." The "excitatory state" of a neuron is defined as the summated degree of excitatory drive to the neuron. If there is a higher degree of excitation than inhibition of the neuron at any given instant, then it is said that there is an excitatory state. Conversely, if there is more inhibition than excitation, then it is said that there is an inhibitory state.
When the excitatory state of a neuron rises above the threshold for excitation, the neuron will fire repetitively as long as the excitatory state remains at that level. Figure 45-12 shows responses of three types of neurons to varying levels of excitatory state. Note that neuron 1 has a low threshold for excitation, whereas neuron 3 has a high threshold. But note also that neuron 2 has the lowest maximum frequency of discharge, whereas neuron 3 has the highest maximum frequency.
Some neurons in the central nervous system fire continuously because even the normal excitatory state is above the threshold level. Their frequency of firing can usually be increased still more by further increasing their excitatory state. The frequency can be decreased, or firing can even be stopped, by superimposing an inhibitory state on the neuron. Thus, different neurons respond differently, have different thresholds for excitation, and have widely differing maximum frequencies of discharge. With a little imagination, one can readily understand the importance of having different neurons with these many types of response characteristics to perform the widely varying functions of the nervous system.
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