Neurons never function in isolation; they are organized into ensembles or circuits that process specific kinds of information. Although the arrangement of neural circuits varies greatly according to the intended function, some features are characteristic of all such ensembles. The synaptic connections that define a circuit are typically made in a dense tangle of dendrites, axons terminals, and glial cell processes that together constitute neuropil (the suffix –pil comes from the Greek word pilos, meaning “felt”; see Figure 1.3). Thus, the neuropil between nerve cell bodies is the region where most synaptic connectivity occurs. The direction of information flow in any particular circuit is essential to understanding its function. Nerve cells that carry information toward the central nervous system (or farther centrally within the spinal cord and brain) are called afferent neurons; nerve cells that carry information away from the brain or spinal cord (or away from the circuit in question) are called efferent neurons. Nerve cells that only participate in the local aspects of a circuit are called interneurons or local circuit neurons. These three classes—afferent neurons, efferent neurons, and interneurons—are the basic constituents of all neural circuits.
Neural circuits are both anatomical and functional entities. A simple example is the circuit that subserves the myotatic (or “knee-jerk”) spinal reflex (Figure 1.5). The afferent limb of the reflex is sensory neurons of the dorsal root ganglion in the periphery. These afferents target neurons in the spinal cord. The efferent limb comprises motor neurons in the ventral horn of the spinal cord with different peripheral targets: One efferent group projects to flexor muscles in the limb, and the other to extensor muscles. The third element of this circuit is interneurons in the ventral horn of the spinal cord. The interneurons receive synaptic contacts from the sensory afferent neurons and make synapses on the efferent motor neurons that project to the flexor muscles. The synaptic connections between the sensory afferents and the extensor efferents are excitatory, causing the extensor muscles to contract; conversely, the interneurons activated by the afferents are inhibitory, and their activation by the afferents diminishes electrical activity in motor neurons and causes the flexor muscles to become less active (Figure 1.6). The result is a complementary activation and inactivation of the synergist and antagonist muscles that control the position of the leg.
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A more detailed picture of the events underlying the myotatic or any other circuit can be obtained by electrophysiological recording (Figures 1.6 and 1.7). There are two basic approaches to measuring electrical activity: extracellular recording where an electrode is placed near the nerve cell of interest to detect activity, and intracellular recording where the electrode is placed inside the cell. Such recordings detect two basic types of signals. Extracellular recordings primarily detect action potentials, the all-or-nothing changes in the potential across nerve cell membranes that convey information from one point to another in the nervous system. Intracellular recordings can detect the smaller graded potential changes that serve to trigger action potentials. These graded triggering potentials can arise at either sensory receptors or synapses and are called receptor potentials or synaptic potentials, respectively. For the myotatic circuit, action potential activity can be measured from each element (afferents, efferents, and interneurons) before, during, and after a stimulus (see Figure 1.6). By comparing the onset, duration, and frequency of action potential activity in each cell, a functional picture of the circuit emerges. As a result of the stimulus, the sensory neuron is triggered to fire at higher frequency (i.e., more action potentials per unit time). This increase triggers in turn a higher frequency of action potentials in both the extensor motor neurons and the interneurons. Concurrently, the inhibitory synapses made by the interneurons onto the flexor motor neurons cause the frequency of action potentials in these cells to decline. Using intracellular recording (see Chapter 2), it is possible to observe directly the potential changes underlying the synaptic connections of the myotatic reflex circuit, as illustrated in Figure 1.7.
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