Table of Contents
Neurons and their processes group together in different ways to form patterns or layouts. These patterns (cytoarchitecture) can be seen under the microscope if the brain tissue section is stained by certain stains such as aniline dyes and Nissl stain. Silver stains selectively label the myelin coating of axons (myeloarchitecture). Immunochemical techniques identify the place of neurtransmitters, their enzymes and molecules within neurons (chemoarichtecture).
Each function of the human brain is a result of activity in a certain brain nerve circuit. These circuits form as a result of more than one process during development. First, each neuron extends its axon along a distinct pathway, guided by molecular cues from its environment, and ends at synapses with certain target neurons. This may occur even before the neuron migrates to its final location in the nervous system. Some axons produce an excessive number of branches (collaterals) and contact a big number of target neurons. During development, the connections are pruned and focused on the appropriate tragets. Timing of this pruning and removal of inappropriate axons and synapses is unique for each region of the brain.
Connections among neurons follow imprortant principles. First, Many but not all connections are both ways, so each region tends to recieve input from those regions to which they send their input (Reciprocal Connections). This may be not straightforward. some axons directly innervate the reciprocating neurons in another region, in other cases there is a local circuit interneurons come between that neuron and the reciprocal neurons. So, some connections may pass through one or more brain region and synapses before arriving at theur final destination.
The second principle is that neuron connections are divergent or convergent. Divergent connections form between either one neuron or a small group which send information to a much larger number of destination neurons in different areas of the brain. For example a small group of neurons in the area of brain stem known as Locus Ceruleus (The dark blue center) send axons to the entire cerebral cortex. The neurons of Locus Ceruleus are rich in the neurotramsmitter norepinephrine (notadrenaline) and drugs which affect this chemical release between neurons (for example antidepressants) can have a widespread effect on the cerebral cortex. A convergent system forms of output from multiple brain regions directed toward a single area, for example neurons from multiple association areas (processing areas of the cortex which are neither motor nor sensory) send their axons to a region on the inner (medial) side of the temporal cortex called entorhinal region.
Third, the connections may be organized in parallel or in serial order (ranking in higher and lower levels), or both ways. For example, vision passes through several groups of neurons in retina of the eye on different levels then to lateral geniculate nucleus. From there neurons travel to area of corext responsible for vision and later prgress to multiple association areas of the cortex. Within that hierarchical scheme, different visual information like form and mtoion may be processed in parralel through different areas of the visual system.
Fourth, different regions of the brain have special functions. For example if there is damage to a certain area on the left side of the lower part of the frontal lobe called Broca's area, the person cannot speak normally. However, speech depends on processing information across a number of brain regions through divergent and convergent serial and parallel interconnections. Thus, the role of broca's area in speech has to be viewed within the context of connections of this area with other brain regions.