Neurons cluster together to form ganglia. Usually, one ganglia is larger and more central than others. This ganglia is called the brain. In vertebrates, most cells of the nervous system are found in the brain and spinal cord. This is where most information processing, storage and retrieval occurs.
Neurons vary considerably in size and shape, but have 3 principal components: a cell body and two types of cytoplasmic extensions (an axon and dendrites).
|Cell body||The cell body is the enlarged portion which more closely resembles other cells. It contain a nucleus with a prominent nucleolus and the bulk of the cytoplasm. It is characterized by chromatophilic substances (Nissl bodies), which are specialized layers of rough endoplasmic reticulum which synthesize proteins and microtubuleues which transport material within the cells, and filamentous strands of protein called neurofibrils. Within the CNS, neurons are clustered into nuclei; within the PNS, neurons are clustered into ganglia.|
|Dendrites||Dendrites are branched processes which extend from cytoplasm of cell body. Dendrites respond to specific stimuli and conduct impulses to the cell body. Some dendrites are covred with minute dendritic spindles which enhance their surface area and provide contact points for other neurons. The area occupied dendrites is called the dendritic zone of a neuron.|
|Axon||The axon is the second type of cytoplasmic extension. The axon conducts nerve impulses away from the cell body. The axon is a long and cylindrical, ranging from just a few mm in CNS to over a meter in the spinal cord. The cytoplasm of an axon contains many mitochondria, microtubules and neurofibrils. Nerve fiber often refers to axon or elongated dendrite.The axon hillock (aka axon base) integrates information from dendrites to initiate nerve impulses. The axon terminals form a synapse with the target cell receiving this nerve impulse. Side branches called collateral branches extend a short distance from the axon.|
|Myelin Sheath||Additionally, the axon is coated with myelin by Schwann cells (in the PNS) and oligodendrocytes (in the CNS). Myelin has an important role in neuronal signaling.|