Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 79

The most obvious feature of most neurons is their extensive dendritic branching patterns. Most neurons have several primary dendritic branches that leave the cell body. These typically divide numerous times (out to the tenth order or more) forming a tree-like structure often called the dendritic tree or dendritic arborization. Figure 2-4 shows two cells from a rabbit retina with very different arborizations.

Figure 2-4: Two neurons from a rabbit retina: a starburst amacrine cell (left), and a directionally selective ganglion cell (right).

The dendritic arborization is where neurons receive most of their synaptic input — typically hundreds or thousands of synapses. These synapses are almost always a mixture of excitatory and inhibitory inputs. In many cases, the excitatory inputs are on dendritic spines, which are little mushroom-like appendages on the dendrites.

In any region of the nervous system are usually a finite number of distinct dendritic neuronal forms, between 10 and 30. Neurons are anatomically classified by the types of branching patterns using parameters such as total size, branching density, and the location of branches relative to other neurons. If you use a microscope to look at unstained tissue sections, these branching structures are usually not evident. Stains are necessary to reveal the structure of individual neurons.

The location of the dendritic branches with respect to nearby neurons determines some of what inputs may synapse on the neuron. The form of the branching structure determines how those inputs interact. Picture thousands of excitatory and inhibitory inputs going on and off in complicated patterns, producing a complex pattern of excitation and inhibition in the postsynaptic neuron, which is constantly changing. This neuron may itself synapse on hundreds of other neurons. Multiply this by 80 to 100 billion neurons and you have a circuit more complicated than any computer we have today!

Neurons also have an axon, their typical output structure. Usually a single process leaves the cell body and branches before forming synapses with other neurons. The axon conducts action potentials, millisecond-long electrical pulses that move from the cell body to the synaptic terminals of the axon, where they cause neurotransmitter to be released onto postsynaptic neurons.