Unusual Characteristics
Among the other cells in the body, neurons are unusual. Firstly, neurons are self excitable or that they can respond to a stimulation. The type of stimulation that is received depends on the location that is. Most neurons respond to neurotransmitters released by other neurons. Secondly, neurons send electric signals. Third, as a response to the electrical signals, neurons secrete neurotransmitters. They can act as an exciter or an inhibitor. Fourth, the neurons that a person is born with will usually last until death. Lastly, neurons do not divide. When a person is still a fetus, their neurons lose the ability.
Even though neurons lack the ability to divide, this does not mean that more neurons can be made. Researchers have found that there are stem cells in the hippocampus that can form neurons.
Even though neurons lack the ability to divide, this does not mean that more neurons can be made. Researchers have found that there are stem cells in the hippocampus that can form neurons.
Structures
Neurons come in different shapes and sizes, but below is what the neurons all have in common.
- The cell body, also called the soma, is surrounded by a plasma membrane and like regular cells, they have cytoplasm, or the perikaryon, and a nucleus. The cell body sends electrical signals, either created by cell body or is received by the dendrites, down the axon. The cell body also supplies the cell with nutrients and creates the chemicals and proteins for message transmission.
- Dendrites usually are short projections from the cell body that receive the electrical signals from other neurons. It then sends it to the cell body for processing. Some neurons can have one while others can have many and the more that a neuron has, the more input it can receive.
- Axons, sometimes called nerve fibers, come from the cell body and come in contact with other neurons, muscle cells, or gland cells. The triangular area that comes directly off the cell body is called the axon hillock. The plasma membrane for the axon is called the axolemma and the cytoplasm is called axoplasm. The axon also has some branches called axon collaterals. At the end of the axon is branching extensions called axon terminals or telodendria. There is a small expansion at the end of the axon terminal called a synaptic knob and within these knobs are synaptic vesicles which contain neurotransmitters.
- The place where a neuron sends information to another neuron is called a synapse. Between the knob of the presynaptic (chemical producer) and postsynaptic (chemical receiver) neurons is a fluid filled gap called the synaptic cleft. There are two types of synapses: chemical and electrical synapses. A chemical synapse is made from a presynaptic neuron and postsynaptic. This synapse can be from an axon to the dendrite, cell body or another axon, as long as it is not on the myelin sheath (see below).
Neuron TypesThere are four types of neurons in the body. The most common is the multipolar neuron. This neuron has many dendrites, one axon and can have multiple nerve processes. Next is the bipolar neuron, which has two processes: a dendrite and an axon. Bipolar neurons are typically found in the retina or the eye. After that is the unipolar neuron or pseudounipolar. It is called this because it starts out as a bipolar neuron and then the processes fuse into one, looking like a "T." One end is called the dendrites or central process, while the other is the axon or peripheral process. Lastly is the anaxonic neuron, which is only has dendrites. Usually it produces a local signal.
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