Migrations of nerve cells

The important point here is that the production and migration of nerve cells are provoked by local biochemical events easily affected by the local prenatal environment, and the same is true for axon guidance processes that connect nerve cells to other nerve cells—connections of critical importance for later cognitive and emotional behavior. 

As mentioned above, Schwann cells are indispensable not only for the process of axonal regeneration but also for the subsequent myelination and maintenance of physiological function. Similar molecular interactions as with neurons should therefore allow migration of Schwann cells from the host into the nerve bridge. 

Extension of neurites and astrocytic processes occurs at the margin of explants within 48-72 h. This is followed by the outward migration of astrocytes. Many small dark neuruns follow in rapid succession. Astrocytes continue to proliferate and migrate and soon cover the peripheral portions of the explants. The pulsating tips of neurites anchor themselves to the processes or cytoplasm of astrocytes and to other neurites as well. While thus securely fastened, translocation of nerve cell somata is effected by movement of their nuclei into the extended neuiitic processes and by subsequent withdrawal toward the cell body of the trailing neurites. Neurons or groups of neurons frequently follow the same... 

Cell adhesion molecules (CAMs) play critical roles in all facets of nervous system development and maintenance. Important phenomena in which CAMs are involved include initial formation of the neural tube and the neural crest, migration of all neurons and glial cells, axonal outgrowth and guidance, target selection, synaptic stabilization and plasticity, myelination and nerve regeneration after injury (see Chs 4,24,28-30 and 53). Adhesion molecules interact with each other and with nonadhesive cell-surface and/or cytoplasmic molecules, and, in the two... 

The nervous system consists of two main units the central nervous system (CNS), which includes the brain and the spinal cord and the peripheral nervous system (PNS), which includes the body s system of nerves that control the muscles (motor function), the senses (the sensory nerves), and which are involved in other critical control functions. The individual units of the nervous system are the nerve cells, called neurons. Nenrons are a nniqne type of cell becanse they have the capacity to transmit electrical messages aronnd the body. Messages pass from one nenron to the next in a strnctnre called a synapse. Electric impnlses moving along a branch of the nenron called the axon reach the synapse (a space between nenrons) and canse the release of certain chemicals called neurotransmitters, one of which, acetylcholine, we described earlier in the chapter. These chemicals migrate to a nnit of the next nenron called the dendrites, where their presence canses the bnild-np of an electrical impnlse in the second nenron. 

The coexistence of lipid and water solubility in the same molecule is essential for the action of a local anaesthetic drug. Lipophilicity permits the migration of drug across the phospholipid membrane of the nerve cell hydrophilicity is essential for the ionisation of the drug within the nerve. It follows that lipid and water solubility are the external and internal facilitators of local anaesthetic action in the nerve cell. Both within and without the nerve cell the unionised and ionised forms coexist in dynamic equilibrium. Outside the nerve, the active species is the unionised tertiary amine form. Conversely, inside the cell the ionised form predominates. The lower intracellular pH induces a shift in the equilibrium in favour of ionisation (Figure 5.5). 

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