A typical neuron consists of a cell body (soma), dendrites, and a single axon. The soma is usually compact. The axon and dendrites are filaments that extrude from it. Dendrites typically branch profusely and extend a few hundred micrometers from the soma. The axon leaves the soma at a swelling called the axon hillock (<em><u>the axon hillock also has the greatest density of voltage-dependent sodium channels. This makes it the most easily excited part of the neuron and the spike initiation zone for the axon</u></em>) and it branches but usually maintains a constant diameter. At the farthest tip of the axon's branches are axon terminals, where the neuron can transmit a signal across the synapse to another cell.
<em>In general, dendrites receive the electrical signal from other neurons, and this signal is transmitted over long distances from the axon.</em>
Like all animal cells, the cell body of every neuron is enclosed by a plasma membrane, a bilayer of lipid molecules with many types of protein structures embedded in it. A lipid bilayer is a powerful electrical insulator, but in neurons, many of the protein structures embedded in the membrane are electrically active. These include ion channels that permit electrically charged ions to flow across the membrane and ion pumps that chemically transport ions from one side of the membrane to the other to generate the action potential that generates the electrical signal. They also have a highly developed synthetic apparatus for the production of neurotransmitters
Epigeal germination is a botanical term indicating that the germination of a plant takes place above the ground. An example of a plant with epigeal germination is the common bean. The opposite of epigeal is hypogeal.
