Intrinsic neurons

Figure 1.8 Some basic neuronal systems. The three different brain areas shown (I, II and III) are hypothetical but could correspond to cortex, brainstem and cord while the neurons and pathways are intended to represent broad generalisations rather than recognisable tracts. A represents large neurons which have long axons that pass directly from one brain region to another, as in the cortico spinal or cortico striatal tracts. Such axons have a restricted influence often only synapsing on one or a few distal neurons. B are smaller inter or intrinsic neurons that have their cell bodies, axons and terminals in the same brain area. They can occur in any region and control (depress or sensitise) adjacent neurons. C are neurons that cluster in specific nuclei and although their axons can form distinct pathways their influence is a modulating one, often on numerous neurons rather than directly controlling activity, as with A . Each type of neuron and system uses neurotransmitters with properties that facilitate their role...

Of course, while the identification of these distinct systems may be useful there are many neural pathways that would not fit easily into one of them. Thus some inhibitory pathways, such as that from the caudate nucleus to substantia nigra, utilising GABA, are not intrinsic neurons. The dopamine pathway from the substantia nigra to striatum may start from a small nucleus but unlike other monoamine pathways it shows little ramification beyond its influence on the striatum. The object of the above classification is not to fit all neural pathways and mechanisms into a restricted number of functional categories but again to demonstrate that there are different forms of neurotransmission. 

While aspartic acid (aspartate) is also found in the CNS and has excitatory effects on neurons, little is known of its precise location and action although it may be released from intrinsic neurons and hippocampal pathways. It will not be discussed further. 

Figure 17.5 Possible scheme for the initiation of depolarisation block of DA neurons. In (a) the excitatory effect of glutamate released on to the DA neuron from the afferent input is counteracted by the inhibitory effect of DA, presumed to be released from dendrites, acting on D2 autoreceptors. In the absence of such inhibition due to the presence of a typical neuroleptic (b) the neuron will fire more frequently and eventually become depolarised. At5q)ical neuroleptics, like clozapine, will be less likely to produce the depolarisation of A9 neurons because they are generally weaker D2 antagonists and so will reduce the DA inhibition much less allowing it to counteract the excitatory input. Additionally some of them have antimuscarinic activity and will block the excitatory effect of ACh released from intrinsic neurons (see Fig. 17.7)...

Figure 17.7 Possible mechanism by which atypical neuroleptics with antimuscarinic activity produce few EPSs. Normally the inhibitory effects of DA released from nigrostriatal afferents on to striatal neuron D2 receptors is believed to balance the excitatory effect of ACh from intrinsic neurons acting on muscarinic (M2) receptors (a). Typical neuroleptics block the inhibitory effect of DA which leaves unopposed the excitatory effect of ACh (b) leading to the augmented activity of the striatal neurons and EPSs (see Fig. 15.2). An atypical neuroleptic with intrinsic antimuscarinic activity reduces this possibility by counteracting the excitatory effects of released ACh as well as the inhibitory effects of DA (c). Thus the control of striatal neurons remains balanced...

Other cholinergic pathways in the brain include a network of intrinsic neurons in the striatum, and also various nuclei in the lower brain stem which project to the cerebellum are the origins of the cranial nerves. Striatal cholinergic neurons project mainly to spiny neurons which are the principal locus for the relay of cortical information flowthrough the basal ganglia (Calabresi et al.. 

Huber, M.T., Krieg, J.C., Dewald, M., Voigt, K., and Braun, H.A. Stimulus sensitivity and neuromodulatory properties of noisy intrinsic neuronal oscillators. Biosystems 1998,48 95-104. 

Levey et al. (1993) found that axon terminals immunoreactive for D1 and D2 receptor proteins formed symmetrical synapses exclusively, and primarily with unlabeled dendritic shafts. In cultures, D1 and D2 receptors have been colocalized to terminals of intrinsic neurons (Wong et al., 1999). Functional D1 receptors have also been demonstrated on the terminals of striatal cells in the substantia nigra (Fiorillo and Williams, 1998). Collectively, the results for D1 and D2 receptors suggest their presence on the terminals of intrinsic GABA neurons. 

Costa, M., Patel, Y, Furness, J. B., and Arimura, A (1977) Evidence that some intrinsic neurons of the intestine contain somatostatin. Neurosci. Lett. 6,215-222... 

Goldberg JL (2004) Intrinsic neuronal regulation of axon and dendrite growth. Curr Opin Neurobiol 14 551-557... 

Bonnemann C, Holland A, Meyer DK. 1989. Cholecystokinin in rat olfactory bulb is mainly contained in intrinsic neurons. Neurosci Lett 103 281-286. 

Le Jeune H, Jourdan F. 1994. Acetylcholinesterase-containing intrinsic neurons in the rat main olfactory bulb Cytological and neurochemical features. Eur J Neurosci 6 1432-1444. 

Fig. 7. Schematic drawing of a transverse section through the forebrain depicting pathways likely to use glutamate as a neurotransmitter. I = principal subcortical afferents to the thalamus from. somatosensory relay nuclei and the spinal cord (a), cerebellar nuclei (h). and retina (c) 2 = intrinsic neurons and retinal inputs to the hypothalamus 3 = thalamocortical inputs 4 = corticothalamic inputs 5 = cortical inputs to the basal ganglia and other areas in the brainstem and spinal cord 6 = associational and commi.ssural connections in the cerebral cortex. For further details, see Sections 3.5-3.9.

The ventromedial part of the lateral nucleus consists of smaller cells. Those in the region of the hilus correspond to the small intrinsic neurons described by Chan-Palay... 

The existence of a rest group of neurons that remains unaffected by large lesions of the efferent cerebellar pathways in the kitten has been claimed as evidence in favour of the presence of intrinsic or nucleocortical neurons in the central nuclei (Jansen and Jansen 1955). Many of these neurons were found to be large and to be located in the posterior interposed nucleus. Intrinsic neurons of the cerebellar nuclei have been observed in Golgi preparations of the rat by Chan-Palay (1973a, 1977) as small multipolar neurons in the dentate nucleus. The terminals of these intrinsic, inhibitory neurons on the soma and dendrites of cerebellar nuclear cells were tentatively identified as small... 

Serotonin-like immunoreactivity resides in a fine network of varicose fibers in the neuropil of all cerebellar nuclei. This plexus is most dense in the hilar region of the lateral cerebellar nucleus of the rat (Takeuchi et al., 1982), among the small, intrinsic neurons of this region (Chan-Palay, 1977) and in the caudal and dorsal regions of the central nuclei of the opossum (Fig. 117) (Bishop et al., 1985). 

Chan-Palay V (1973b) Axon terminals of the intrinsic neurons in the nucleus lateralis of the cerebellum An electron microscope study. Z. Anal. Entwickl-Gesch., 142, 187-206. 

Kubota Y, Kawaguchi Y (1993) Spatial distributions of chemically identified intrinsic neurons in relation to patch and matrix compartments of rat neostriatum. J. Comp. Neurol., 332, 499-513. 

I. Some smaller cells in layer Ila lack basal dendrites (Haberly and Price, 1978b) and are reminiscent of dentate granule cells in the hippocampus. Layer III is a thicker but less densely packed cellular layer containing larger pyramidal cells whose dendrites also extend throughout layer I. Layer III also contains large multipolar cells whose dendrites do not enter layer I and other intrinsic neuronal types. Layer III exhibits a superficial... 

---