FIGURE 21 Excitatory and inhibitory postsynaptic potentials in spinal motor neurons. (A) Intracellular recordings are made from a sensory neuron (SN), interneuron, and extensor (E) and flexor (F) motor neurons. (B) An action potential in the sensory neuron produces a depolarizing response in the motor neuron (MN). This response is called an excitatory postsynaptic potential (EPSP). (C) An action potential in the interneuron produces a hyperpolarizing response in the motor neuron. This response is called an inhibitory postsynaptic potential (IPSP).
Postsynaptic Mechanisms Produced by Ionotropic Receptors
Mechanisms responsible for fast EPSPs mediated by ionotropic receptors in the CNS are fairly well known. Specifically, for the synapse between the sensory neuron and the spinal motor neuron, the ionic mechanisms for the EPSP are essentially identical to the ionic mechanisms at the skeletal neuromuscular junction. Transmitter substance released from the presynaptic terminal of the sensory neuron diffuses across the synaptic cleft, binds to specific receptor sites on the postsynaptic membrane, and leads to a simultaneous increase in permeability to Na+ and K+, which makes the membrane potential move toward a value of 0 mV.
Although this mechanism is superficially the same as that for the neuromuscular junction, two fundamental differences exist between the process of synaptic transmission at the sensory neuron-motor neuron synapse and the motor neuron-skeletal muscle synapse. First, these two different synapses use different transmitters. The transmitter substance at the neuromuscular junction is ACh, but the transmitter substance released by the sensory neurons is an amino acid, probably glutamate. (Although beyond the scope of this chapter, many different transmitters are used by the nervous system— up to 50 or more, and the list grows yearly. Thus, to understand fully the process of synaptic transmission and the function of synapses in the nervous system, it is necessary to know the mechanisms for the synthesis, storage, release, and degradation or uptake, as well as the different types of receptors for each of the transmitter substances. The clinical implications of deficiencies in each of these features for each of 50 transmitters cannot be ignored. Fortunately, most of the transmitter substances can be grouped into four basic categories: ACh, monoamines, amino acids, and the peptides.)
A second major difference between the sensory neuron-motor neuron synapse and the motor neuron-muscle synapse is the amplitude of the postsynaptic potential. Recall that the amplitude of the postsynaptic potential at the neuromuscular junction was approximately 50 mV and that a one-to-one relationship existed between an action potential in the spinal motor neuron and an action potential in the muscle cell. Indeed, because the EPP must only depolarize the muscle cell by approximately 30 mV to initiate an action potential, there is a safety factor of 20 mV. In contrast, the EPSP in a spinal motor neuron produced by an action potential in an afferent fiber has an amplitude of only 1 mV.
The small amplitude of the EPSP in spinal motor neurons (and other cells in the CNS) poses an interesting question. Specifically, how can an ESP with an amplitude of only 1 mV drive the membrane potential of the motor neuron (i.e., the postsynaptic neuron) to threshold and fire the spike in the motor neuron that is necessary to produce the contraction of the muscle? The answer to this question lies in the principles of temporal and spatial summation.
When the ligament is stretched (see Fig. 20), many stretch receptors are activated. Indeed, the greater the stretch, the greater the probability of activating a larger number of the stretch receptors present; this process is referred to as recruitment. Activation of multiple stretch receptors is not the complete story, however. Recall the principle of frequency coding in the nervous system (see Chapter 4). Specifically, the greater the intensity of a stimulus, the greater the number per unit time or frequency of action potentials elicited in a sensory receptor. This principle applies to stretch receptors as well. Thus, the greater the stretch, the greater the number of action potentials elicited in the stretch receptor in a given interval and, therefore, the greater the number of EPSPs produced in the motor neuron from that train of action potentials in the sensory cell. Consequently, the effects of activating multiple stretch receptors add together (spatial summation), as do the effects of multiple EPSPs elicited by activation of a single stretch receptor (temporal summation). Both these processes act in concert to depolarize the motor neuron sufficiently to elicit one or more action potentials, which then propagate to the periphery and produce the reflex.
Temporal Summation. Temporal summation can be illustrated by considering the case of firing action potentials in a presynaptic neuron and monitoring the resultant EPSPs. For example, in Fig. 22A and B, a single action potential in SN1 produces a 1-mV EPSP in the motor neuron. Two action potentials in quick succession produce two EPSPs, but note that the second EPSP occurs during the falling phase of the first, and the depolarization associated with the second EPSP adds to the depolarization produced by the first. Thus, two action potentials produce a summated potential that is 2 mV in amplitude. Three action potentials in quick succession would produce a summated potential of 3 mV. In principle, 30 action potentials in quick succession would produce a potential of 30 mV and easily drive the cell to threshold. This summation is strictly a passive property of the cell. No special ionic conductance mechanisms are necessary.
A thermal analog is helpful to understand temporal summation. Consider a metal rod that has thermal properties similar to the passive electrical properties of
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