Axonal projection

Although mast cells and basophils probably account for >90% of stored histamine in the body, histamine is also present in platelets, enterochromaffin-like cells, endothelial cells, and neurons. Histamine can act as a neurotransmitter in the brain. Histaminergic nerves have their cell bodies within a very small area of the brain (the magnocellular nuclei of the posterior hypothalamus) but have axons in most areas of the forebrain. There is also evidence for axons projecting into the spinal (Fig. 1) cord. Finally, there is evidence that histamine synthesis can be induced in tissues undergoing rapid tissue growth and repair. In certain neonatal tissues (e.g. liver), the rate of synthesis of this unstored diffusable histamine (termed nascent histamine) is profound and may point to a role for histamine is cell proliferation. 

The nigrostriatal tract is one of the four main dopaminergic pathways in the central nervous system. About 75% of the dopamine in the brain occurs in the nigrostriatal pathway with its cell bodies in the substantia nigra, whose axons project in the corpus striatum. Degeneration of the dopaminergic neurons in the nigrostriatal system results in Parkinsons disease. 

Figure 8.2 The distribution of noradrenergic neurons in the brain. The cell bodies are clustered in nuclei (A1 A7) in the pons/medulla regions of the brainstem and their axons project both rostrally and caudally to most regions of the neuraxis. The major nucleus is the locus coeruleus...

Imai, H. Steindler, D.A. and Kitai, S.T. The organization of divergent axonal projections from the midbrain raphe nuclei in the rat. J Comp Neurol 243 363-380, 1986. 

Ebrahimi F.A. and Chess A. (2000). Olfactory neurons are interdependent in maintaining axonal projections. Curr Biol 10, 219-222. 

Rodriguez I., Feinstein P. and Mombaerts P. (1999). Variable patterns of axonal projections of sensory neurons in the mouse vomeronasal system. Cell 97, 199-208. 

The axonal projections from the DDC-expressing cells in the ventral ganglion also show tendencies to follow common pathways. The projections from the ventral lateral serotonin cells extend medially to fuse with axons projecting from the contralateral serotonin cells. At the midline, this projection is met by an axonal projection from the medial dopamine cell. 

The main projections of the DRN and the MRN ascend through the forebrain within the median forebrain bundle. The basal ganglia, substantia nigra, and amygdala are mainly innervated by the DRN, whereas MRN axons project densely to the basal forebrain, septal nuclei, thalamus, posterior hypothalamus, LDT, and DRN. The hippocampus receives an overlapping innervation from the DRN and the MRN (Hensler et al, 1994 Vertes et al, 1999) (Table 9.1). 

Comparison of hypocretm/orexin and melanin-concentrating hormone neurons and axonal projections in the embryonic and postnatal rat brain. J. Chem. Neuroanat. 27, 165-81. 

Locus coeruleus A collection of noradrenergic neuronal cell bodies located in the pons, from which the axons project to a number of forebrain structures. 

Axons of antennal ORCs project through the antennal nerve to enter the brain at the level of the ipsilateral antennal lobe (AL) of the deutocerebrum (52). ORC axons project from the flagellum to targets in the AL, but axons from antennal mechanosensory neurons bypass the AL and project instead to an "antennal mechanosensory and motor center" in the deutocerebrum posteroventral (with respect to the body axis of the animal) to the AL (52, 58, 64). In moths and certain other insect groups, sex-pheromonal information is processed in a prominent male-specific neuropil structure in each AL called the macroglomerular complex (MGC) (16, 52, 64, 65). 

Imbalanced Neurotransmission. Most brain regions are innervated by axonal projections secreting multiple neurotransmitters. When the system is healthy, the activity of the two (or more) neurotransmitters is held in a delicate balance. Some illnesses result from an imbalance in transmission of multiple neuro-... 

A typical neuron consists of a cell body and an axonal projection through which information in the form of an action potential passes from the cell body to the axonal terminal. Information is received by the cell body via a complex array of dendrites which make contact with adjacent neurons. The structural complexity and the number of dendritic processes vary according to the type of nerve cell and its physiological function. For example, the granule cells in the dentate gyrus of the hippocampus (a region of the brain which plays a role in short-term memory) receives and integrates information from up to 10 000 other cells in the vicinity. 

The majority of the inputs to the granule cells are excitatory, each of which provides a small depolarizing current to the membrane of the cell body. The point of contact between the axonal projection from the neuron and an adjacent cell is termed the s)mapse which under the electron microscope appears as a swelling at the end of the axon. Most synapses are excitatory and are usually located along the dendritic branches of the neuron. The contributions of the individual excitatory s)mapses are additive and, as a result, when an excitatory stimulus occurs a wave of depolarizing current travels down the axon to stimulate the adjacent cell body. However, some synapses are inhibitory, usually fewer in number and strategically located near the cell body. These synapses, when activated, inhibit the effects of any excitatory currents which may travel down the dendritic processes and thereby block their actions on the neuron (Figure 2.1). 

Noradrenaline Most regions long axons project Oil receptors - inhibitory Depression... 

Figure 1. A. SimpMed diagram of the rodent hippocampal formation illustrating the major glutamatergic circuitry. The principal neuronal helds granule cells (GC) of the dentate gyrus and pyramidal cells of CAl and CA3 in Ammon s horn are shown. The main excitatory connections are also indicated the perforant path from entorhinal cortex to the granule cells, from there the mossy hbre (mf) axonal projections to CA3 and then the Schaffer collaterals (Sch) from CA3 to ipsilateral CAl and commissural (Comm) to contralateral CAl cells. Evoked responses in (B) were obtained by stimulating the afferent pathway from entorhinal cortex, the medial perforant path (Med), and recording the granule cell (GC) response in the hilus of the dentate gyrus.

Glutamatergic neurons are widely distributed throughout the entire brain. Most glutamatergic neurons are so-called projection neurons their axon projects into distant brain regions. Prominent glutamatergic pathways are the connections between different regions of the cerebral cortex (cortico-cortical projections), the connections between thalamus and cortex, and the projections from cortex to striatum (extrapyramidal pathway) and from cortex to brain stem/spinal chord (pyramidal pathway). 

Markakis, E.A., Gage, F.H. (1999). Adult-generated neurons in the dentate gyrus send axonal projections to field CA3 and are surrounded by synaptic vesicles. J Comp Neurol, 406, 449-60. 

Stanfield, B.B., Trice, J.E. (1988). Evidence that granule cells generated in the dentate gyrus of adult rats extend axonal projections. Exp Brain Res, 72, 399-406. 

FIGURE 8—12. Overactivity of norepinephrine neurons is associated with anxiety and may mediate the autonomic symptoms associated with anxiety, such as tachycardia, dilated pupils, tremor, and sweating. Shown here are hyperactive norepinephrine neurons with their axons projecting forward to the cerebral cortex from their cell bodies in the locus coeruleus. 

---