Nerves begin with a single nerve cell called a neuron. Nerves contain the fibers of many neurons. These nerve fibers are bundled together with blood vessels and wrapped in connective tissue.
Nerves are located outside the central nervous system (CNS) in what is termed as the peripheral nervous system (PNS).
Within the CNS, bundles of nerve fibers are called tracts.
Nervous tissue is composed of two types of cells: neurons and neuroglia.
Neurons have many shapes and sizes that transmit electrical signals to and from the brain and the spinal cord.
- Three parts of a neuron:
- Dendrite cells, which receive information from other neurons and then transmits that information toward the cell body. One neuron may have thousands of dendrite cells.
- Cell body, which contains the nucleus, and is essential to the life of the cell.
- Axon, which transmits information awayfrom the cell body. The end of the axon undergoes extensive branching to form hundreds to thousands of axon terminals, where chemical neurotransmitters are stored. An axon has several unique structures:
- Myelin sheath is a layer of white fatty material that encases axons of the PNS. It is different from that of the CNS – those being formed by a type of glial cell called oligodendrocytes.
- Neurilemma is the nuclei and cytoplasm of Schwann cells, that lie outside the myelin sheath.
- Nodes of Ranvier are areas of the axon not covered by myelin.
- Three types of neurons:
- Sensory neurons (afferent neurons) carry information from the periphery to the CNS (eg. found in the skin and internal organs)
- Motor neurons (efferent neurons) carry information from the CNS toward the periphery (eg. muscles and glands).
- Interneurons (mixed) are found only in the CNS and form connections between sensory and motor neurons. In the brain, they play a role in thinking, learning, and memory.
Neuroglia, or simply glia, are cells that support and take care of neurons. The word ‘glia’ means glue-like, referring to their ability to hold together the vast network of neurons. They do not transmit any electrical signals.
- Four cell types of neuroglia:
- Astrocytes are star-shaped cells present in blood-brain barrier. They anchor or bind blood vessels to nerves for support and are the most abundant of the glial cells.
- Ependymal cells line the ventricles of the brain as part of the choroid plexus. They are involved in the formation and circulation of cerebrospinal fluid.
- Microglia cells play a protective role in phagocytosis of pathogens and damaged tissue.
- Oligodendrocytes produce the myelin sheath for neurons in the CNS. (The myelin sheath in the PNS is produced by Schwann cells).
