Interneurons are by far the most numerous type of neuron. This is because of their massive use in the CNS to process and store information. CNS interneurons tend to be small, and highly dendritic. They have diverse shapes depending on their functions and where they are found in the CNS. Some of the best known and most widely studied interneurons are found in the spinal cord controlling motoneurons. Each motoneuron receives many excitatory and inhibitory inputs via interneurons activated through the CNS, and through local reflexes involving spindles and GTOs. An interesting interneuron associated with each motoneuron is the Renshaw cell. A Renshaw cell provides a local negative feedback loop around each motoneuron. The motoneuron sends a recurrent fiber to activate the Renshaw cell, which in turn sends inhibitory signals to the motoneuron, and also to a type Ia inhibitory interneuron that inhibits a motoneuron serving the antagonist muscle (i.e., it inhibits an inhibitor). The direct negative feedback from the Renshaw cell reduces its firing sensitivity and tends to linearize its input/output firing characteristics from its various excitatory inputs.
The Ia inhibitory interneuron is excited by a gamma afferent from a spindle in the agonist (e.g., flexor) muscle, thereby inhibiting the antagonist muscle. The agonist spindle gamma afferent also directly stimulates the agonist of the a-moto-neuron muscle. Output from a GTO force sensor excites a type Ib inhibitory interneuron, which in turn inhibits the agonist of the a-motoneuron muscle. Figure 1.14 illustrates this system. Stretch reflex pathways involving the GTOs, spindles, inhibitory interneurons and motoneurons are collectively called a myotatic unit (see Kandel et al., 1991, Ch. 38).
Because structure and function are intimately related in physiological systems, the CNS offers considerable challenge in understanding its function, given the diverse array of interneuron morphology found in the layered components of the brain (e.g., the cortex, the cerebellum). For example, in the cerebral cortex primates, there are about six layers containing about eight types of interneuron, including pyramidal cells (the output elements of the cortex), basket cells (inhibitory on pyramidal cells), chandelier cells, neurogliaform cells, arcade cells, bouquet cells, double bouquet cells, and long stringy cells (see Kandel et al., 1991, Ch. 50).
The cerebellum is another specialized portion of the CNS with a highly organized structure. The role of the cerebellum is to coordinate motor actions by comparing what happens in terms of movement with what the CNS intended. The cerebellum has only 10% of the volume of the human brain; however, it contains over half the neurons found in the CNS. For example, there are 1011 granular cell neurons in the cerebellum, more than the total number in the cerebral cortex (Ghez, 1991).
The cerebellum has three layers, and five types of interneuron, including Purkinje cells, Golgi cells (type II), basket cells, stellate cells, and granule cells. Figure 1.1-5 shows cerebellum features schematically. The outermost layer is called the molecular layer, the middle layer is the Purkinje layer, and the inner layer is the granular layer. In the molecular layer are parallel fibers from the granular cells, stellate cells, basket cells, the extensive dendrites from the Purkinje cells, and terminal arborizations from climbing fibers (inputs to the cerebellum). The Purkinje layer is relatively thin; it contains the closely packed, large (50 to 80 |im diameter) cell bodies of the Purkinje cells. The innermost granular layer contains granular cell cell bodies, terminal branches from the mossy (major input) fibers. The granular cells send axons to the outer surface of the molecular layer where they "T" into the parallel fibers. The Golgi cells have their cell bodies in the outer granular layer, and send extensive dendritic trees out into the molecular layer to receive excitatory synaptic inputs from the parallel fibers. In the granular layer, the output arborizations of Golgi cells make extensive inhibitory axodendritic synapses with granular cells, suppressing their excitation from mossy fibers. y-Aminobutyric acid (GABA) is the inhibitory neurotransmitter used by Golgi cells. In the molecular layer, the stellate and basket cells act to inhibitor Purkinje cells. To quote Ghez (1991):
Like the Purkinje cells, stellate and basket cells receive excitatory connections from the parallel fibers (granule cell axons). Stellate cells have short axons that contact nearby dendrites of Purkinje cells in the molecular layer, while basket cell axons run perpendicular to the parallel fibers and contact the cell bodies of more distant Purkinje cells. As a result, when a group of parallel fibers excites a row of Purkinje neurons and neighboring basket cells, the excited basket cells inhibit the Purkinje cells outside the beam of excitation [sic]. This results in a field of activity that resembles the center-surround antagonism that we have encountered in sensory neurons [e.g., in the retina].
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