A neuron may receive input from thousands of presynap-tic neurons simultaneously. Some incoming nerve fibers may produce EPSPs while others produce IPSPs. Whether or not the neuron fires depends on whether the net input is excitatory or inhibitory. If the EPSPs override the IPSPs, threshold may be reached and the neuron will fire; if the IPSPs prevail, the neuron will not fire. Summation is the process of adding up postsynaptic potentials and responding to their net effect. It occurs in the trigger zone.
Suppose, for example, you are working in the kitchen and accidentally touch a hot cooking pot. EPSPs in your motor neurons might cause you to jerk your hand back quickly and avoid being burned. Yet a moment later, you might nonchalantly pick up a cup of tea that is even hotter than the pot. Since you are expecting the teacup to be hot, you do not jerk your hand away. You have learned that it will not injure you, so at some level of the nervous system, IPSPs prevail and inhibit the motor response.
It is fundamentally a balance between EPSPs and IPSPs that enables the nervous system to make decisions. A postsynaptic neuron is like a little cellular democracy acting on the "majority vote" of hundreds of presynaptic cells. In the teacup example, some presynaptic neurons are sending messages that signify "hot! danger!" in the form of EPSPs that may activate a hand-withdrawal reflex, while at the same time, others are producing IPSPs that signify "safe" and suppress the withdrawal reflex. Whether the postsynaptic neurons cause you to jerk your hand back depends on whether the EPSPs override the IPSPs or vice versa.
One action potential in a synaptic knob does not produce enough activity to make a postsynaptic cell fire. An EPSP may be produced, but it decays before reaching threshold. A typical EPSP has a voltage of 0.5 mV and lasts only 15 to 20 msec. If a neuron has an RMP of —70 mV and a threshold of —55 mV, it needs about 30 EPSPs to reach threshold and fire. There are two ways in which EPSPs can add up to do this, and both may occur simultaneously.
1. Temporal summation (fig. 12.22a). This occurs when a single synapse generates EPSPs at such short time intervals that each is generated before the previous one decays. This allows the EPSPs to add up over time to a threshold voltage that triggers an action potential (fig. 12.23). Temporal summation
Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition
12. Nervous Tissue
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470 Part Three Integration and Control
470 Part Three Integration and Control
(b) Spatial summation
Figure 12.22 Temporal and Spatial Summation. (a) In temporal summation, a single presynaptic neuron stimulates the postsynaptic neuron so intensely that its EPSPs add up to threshold and make it fire. (b) In spatial summation, multiple inputs to the postsynaptic cell each produce a moderate amount of stimulation, but collectively they produce enough EPSPs to add up to threshold at the trigger zone and make the cell fire.
can occur if even one presynaptic neuron intensely stimulates the postsynaptic neuron.
2. Spatial summation (fig. 12.22b). This occurs when EPSPs from several different synapses add up to threshold at the axon hillock. Any one synapse may admit only a moderate amount of Na+ into the cell, but several synapses acting together admit enough Na+ to reach a threshold. The presynaptic neurons cooperate to induce the postsynaptic neuron to fire.
Facilitation is a process in which one neuron enhances the effect of another one. In spatial summation, for example, one neuron acting alone may be unable to induce a post-synaptic neuron to fire. But when they cooperate, their combined "effort" does induce firing in the postsynaptic cell. They each enhance one another's effect, or facilitate each other.
- Resting membrane potential
Figure 12.23 Summation of EPSPs. If enough EPSPs arrive at the trigger zone faster than they decay, they can build on each other to bring the neuron to threshold and trigger an action potential.
Presynaptic inhibition is the opposite of facilitation, a mechanism in which one presynaptic neuron suppresses another one. This mechanism is used to reduce or halt unwanted synaptic transmission. In figure 12.24, we see three neurons which we will call neuron S for the stimulator, neuron I for the inhibitor, and neuron R for the responder. Neuron I forms an axoaxonic synapse with S (that is, I synapses with the axon of S). When presynaptic inhibition is not occurring, neuron S releases its neuro-transmitter and triggers a response in R. But when there is a need to block transmission across this pathway, neuron I releases the inhibitory neurotransmitter GABA. GABA prevents the voltage-regulated calcium gates of neuron S from opening. Consequently, neuron S releases less neu-rotransmitter or none, and fails to stimulate neuron R.
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