Synaptic Transmission In The Central Nervous System

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The study of synaptic transmission in the central nervous system (CNS) is both an opportunity to learn more about the diversity and richness of mechanisms underlying this process and an opportunity to learn how some of the fundamental signaling properties of the nervous system, such as action potentials and synaptic potentials, work together to process information and generate behavior. One of the simplest behaviors controlled by the central nervous system is the knee-jerk or stretch reflex. The tap of a neurologist's hammer to a ligament elicits a reflex extension of the leg, illustrated in Fig. 20. The brief stretch of the ligament is transmitted to the extensor muscle and is detected by specific receptors in the muscle and ligament. Action potentials initiated in the stretch receptors are propagated to the spinal cord by afferent fibers. The receptors are specialized regions of sensory neurons with somata located in the dorsal root ganglia just outside the spinal column. The axons of the afferents enter the spinal cord and make at least two types of excitatory synaptic connections. First, a synaptic connection is made to the extensor motor neuron. As the result of its synaptic activation, the motor neuron fires action potentials that propagate out of the spinal cord and ultimately invade the terminal regions of the motor axon at neuromus-cular junctions. There, ACh is released, an EPP is produced, an action potential is initiated in the muscle cell, and the muscle cell is contracted, producing the reflex extension of the leg. Second, a synaptic connection is made to another group of neurons, interneurons (nerve cells interposed between one type of neuron and another). The particular interneurons activated by the afferents are inhibitory interneurons, because activation of these interneurons leads to the release of a chemical transmitter substance that inhibits the flexor motor neuron. This inhibition tends to prevent an uncoordinated (improper) movement (i.e., flexion) from occurring.

spinal cord via afferent fibers (sensory neurons , SN). The afferents make excitatory connections with extensor motor neurons (E). Action potentials initiated in the extensor motor neuron propagate to the periphery and lead to the activation and subsequent contraction of the extensor muscle. The afferent fibers also activate interneurons (I) that inhibit the flexor motor neurons (F).

spinal cord via afferent fibers (sensory neurons , SN). The afferents make excitatory connections with extensor motor neurons (E). Action potentials initiated in the extensor motor neuron propagate to the periphery and lead to the activation and subsequent contraction of the extensor muscle. The afferent fibers also activate interneurons (I) that inhibit the flexor motor neurons (F).

Mechanisms for Excitation, Inhibition, and Integration of Synaptic Potentials

The stretch reflex provides a suitable model system to study the properties of excitatory and inhibitory synaptic transmission in the CNS and to illustrate how these properties are used to process information and generate simple behavior. Figure 21A illustrates procedures that can be used to examine experimentally some of the components of synaptic transmission in the reflex pathway for the stretch reflex. Intracellular recordings are made from one of the sensory neurons, the extensor and flexor motor neurons, and an inhibitory inter-neuron. Normally, the sensory neuron is activated by stretch to the muscle, but this step can be bypassed by simply injecting a pulse of depolarizing current of sufficient magnitude into the sensory neuron to elicit an action potential. The action potential in the sensory neuron leads to a potential change in the motor neuron known as an excitatory postsynaptic potential (EPSP) (Fig. 21B). The potential is excitatory because it increases the probability of firing an action potential in the motor neuron; it is postsynaptic because it is a potential that is recorded on the receptive (postsynaptic) side of the synapse.

Postsynaptic potentials (PSPs) in the CNS can be divided into two broad classes based on mechanisms and duration of these potentials. One class arises from the direct binding of a transmitter molecule with a receptor-channel complex; these receptors are ionotro-pic. The resulting PSPs are generally short lasting and hence are sometimes called fast PSPs; they have also been referred to as classical because these were the first synaptic potentials that were recorded in the CNS. Fast EPSPs also resemble the synaptic potentials at the neuromuscular junction (i.e., the EPP). One typical feature of the classical synaptic potentials is their time course. Recall that the duration of an EPP is about 20 msec. An EPSP, or an inhibitory postsynaptic potential (IPSP), recorded in a spinal motor neuron is of approximately the same duration.

The second class of PSPs arises from the indirect effect of a transmitter molecule binding with a receptor. For these PSPs, one common coupling mechanism is an alteration in the level of a second messenger. The receptors that produce these PSPs are metabotropic. The responses can be long lasting and are therefore referred to as slow PSPs. The mechanisms for fast PSPs mediated by ionotropic receptors will be considered first.

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