Intemeurons

GC) axon (Jia et al, 1999). The GC are the main inhibitory intemeurones, while the peri-glomerular cells can alter the probability of transmission at the first synapse. The olfactory inputs to the M/TCs have two ways in which they may be connected, via their primary dendrites, to a particular glomerulus. First, they may supply only one functional type as in MOE input [Fig. 5.14(a)], Second, they may supply two or more functional types [Fig. 5.14(b)]. The single connectivity type is found in the MOB, as the primary OR-M/TC... 

Bulfone A., Wang F., Pevner R., Anderson S., et al. (1998). An olfactory sensory map develops in the absence of normal projection neurons or GABAergic intemeurons. Neuron 21, 1273-1282. 

The third class of neurons includes the intemeurons, which lie entirely within the CNS. Because the human brain and spinal cord contain well over 100 billion neurons, interneurons account for approximately 99% of all the neurons in the body taken together. Intemeurons lie between afferent and... 

Figure 6.1 Types of neurons. Afferent neurons, which transmit impulses toward the CNS and efferent neurons, which transmit impulses away from the CNS, lie predominantly in the peripheral nervous system. Intemeurons, which process sensory input and coordinate motor responses, lie entirely within the central nervous system.

The gray matter is composed of nerve cell bodies and unmyelinated intemeuron fibers. The location of the gray matter in the spinal cord is opposite to that of the brain. In the brain, the gray matter of the cerebrum and the cerebellum is found externally forming a cortex, or covering, over the internally located white matter. In the spinal cord, the gray matter is found internally and is surrounded by the white matter. 

Interneurons are found in all areas of the spinal cord gray matter. These neurons are quite numerous, small, and highly excitable they have many interconnections. They receive input from higher levels of the CNS as well as from sensory neurons entering the CNS through the spinal nerves. Many intemeurons in the spinal cord synapse with motor neurons in the ventral hom. These interconnections are responsible for the integrative functions of the spinal cord including reflexes. 

Synapses between first-order sensory neurons and alpha motor neurons, either directly or by way of intemeurons, result in spinal cord reflexes. Reflexes are discussed in more detail in a subsequent section in this chapter. 

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons a polysynaptic reflex has two or more synapses between these neurons. In this case, intemeurons lie between the sensory and motor neurons. The more intemeurons involved, the more complex the response is.

A reflex is initiated by stimulation of a sensory receptor located at the peripheral ending of an afferent or first-order sensory neuron. This afferent neuron transmits impulses to the spinal cord. Within the gray matter of the spinal cord, the afferent neuron synapses with other neurons. As such, the spinal cord serves as an integrating center for the sensory input. The afferent neuron must ultimately synapse with an efferent or motor neuron. When the afferent neuron synapses directly with the motor neuron, it forms a monosynaptic reflex. An example of this type of reflex is the stretch reflex. When the afferent neuron synapses with an intemeuron that then synapses with the motor neuron, it forms a polysynaptic reflex, e.g., the withdrawal reflex. Most reflexes are polysynaptic. The motor neuron then exits the spinal cord to innervate an effector tissue, which carries out the reflex response. 

The excitatory intemeuron then synapses with the alpha motor neuron that innervates Hie flexor muscles of the right leg. Consequently, stimulation of the excitatory intemeuron leads to stimulation of the alpha motor neuron, which then stimulates the flexor muscles to contract and pick up or withdraw the foot from the painful stimulus. The inhibitory intemeuron synapses with the alpha motor neuron that innervates the extensor muscles of the right leg. Therefore, stimulation of the inhibitory intemeuron leads to inhibition of the alpha motor neuron. As a result, the extensor muscles relax. 

Cossart, R., Esclapez, M., Hirsch, J. C Bernard, C., and Ben Ari, Y. (1998) GluR5 kainate receptor activation in intemeurons increases tonic inhibition of pyramidal cells. Nat. Neu-rosci. 1,470-478. 

Rodriguez-Moreno, A., Lopez-Garcia, J. C and Lerma, J. (2000) Two populations of kainate receptors with separate signaling mechanisms in hippocampal intemeurons. Proc. Natl. Acad. Sci. USA 97,1293-1298. 

Cossart, R., Tyzio, R Dinocourt, C Esclapez, M Hirsch, J. C Ben Ari, Y and Bernard, C. (2001) Presynaptic kainate receptors that enhance the release of GABA on CA1 hippocampal intemeurons. Neuron 29,497-508. 

Frerking, M Malenka, R. C., and Nicoll, R. A. (1998) Synaptic activation of kainate receptors on hippocampal intemeurons. Nat. Neurosci. 1,479M86. 

Semyanov, A. and Kullmann, D. M. (2001) Kainate receptor-dependent axonal depolarization and action potential initiation in intemeurons. Nat. Neurosci. 4,718-723. 

Nusser, Z., Kay, L. M., Laurent, G Homanics, G. E., and Mody, I. (2001) Disruption of GABAa receptors on GABAergic intemeurons leads to increased oscillatory power in the olfactory bulb network../. Neurophysiol. 86,2823-2833. 

Belcher, G., Ryall, R. W., and Schaffner, R. (1978) The differential effects of 5-hydroxytryptamine, noradrenaline and raphe stimulation on nociceptive and non-nociceptive dorsal horn intemeurons in the cat. Brain Res., 151 529-531. 

An important subset of pheromone-specialist PNs in male M. sexta receives input from both component A and component B input channels, described above, but the physiological effects of the two inputs are opposite (72). That is, if antennal stimulation with component A leads to excitation, then stimulation with component B inhibits the intemeuron, and vice versa. Simultaneous stimulation of the antenna with both components A and B elicits a mixed inhibitory and excitatory response in these special PNs. Thus these neurons can discriminate between the two inputs based upon how each affects the spiking activity of the cell. These PNs also respond uniquely to the natural pheromone blend released by the female these pheromone specialist neurons have enhanced ability to follow intermittent pheromonal stimuli occurring at natural frequencies of 10 stimuli per sec (77). 

The Ap fibres are examples of afferents that stimulate inhibitory interneurones (in the substantia gelatinosa (SG)) and, therefore, prevent nociceptive transmission to the CNS. The C fibres are examples of afferents that inhibit inhibitory interneurones and, therefore, enhance nociceptive transmission. Note that both types of fibre stimulate the second-order neurone (2°) directly but it is the intemeurone that modifies the transmission. 

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