Many elements are transported along the axon, such as organelles. Neurotransmitters are usually released in these enlarged regions. Axon colaterals are usually very thin, but they can get thicker in some regions, looking like beads, and in the terminal end to form the presynaptic element (see below). However, it is not a passive element during the transmission of information because it can integrate and process information too. The information integrated in the dendritic tree and in the soma is transported by the axon to other neurons. The complete axonal processes is referred to as axonal tree. The axon usually branches many times, and each branch is called axon collateral. The axon may length is variable, from less than 1 mm to several meters (Figures 2 and 4). The region where the axon sprouts out is known as axon hillock. This short segment is the axon initial segment. The axon emerges as a very thin process from the soma or from a primary dendrite. Thick sections of nervous tissue after Golgi's technique. Dendrites contain mitochondria, multivesicular bodies, endosomes, and cytoskeleton components like microtubules, intermediate filaments and actin filaments.įigure 4. These dendrites are referred as spinous dendrites, whereas smooth dendrites lack spines. Spines are major structures for incoming information. The dendrites of many neurons have small protruding specializations known as spines, which are the postsynaptic elements (see below). All these dendritic features are relevant because they influence how the information is received and integrated by the neuron.
The number, shape, length,and branching pattern of dendrites are variable depending on the neuronal type. The disposition of primary dendrites and the branching pattern conform the general shape of the dendritic tree. Commonly, there are more than one primary or main dendrite in a neuron, which are those that directly emerge from the soma.
Because of this branching-like pattern, the whole set of dendrites of a neuron is referred to as dendritic tree (Figures 1, 2, 3 and 4). The word dendrite comes from "dendron", the Greek word for tree. Dendrites and axon rise from the soma.ĭ endrites are the main neuronal structures for receiving information from other neurons. It contains the nucleus, usually in a central position, endoplasmic reticulum, Golgi apparatus, mitochondria, endosomes, cytoskeleton and other cytoplasmic components. The average size of the soma is about 20 µm, although it can be larger or smaller depending on the neuronal type. The shape of the neuronal soma is variable: pyramidal, rounded, star-like and fusiform. A Golgi impregnated neuron has been placed on top. Rat cerebral cortex after general staining. Neuron of rat cerebral cortex after the Golgi's staining. The size and shape of the soma, the branching pattern and density of dendrites, as well as the length and branching of the axon are different for each type of neuron type.įigure 2. A neuron is divided in three domains: soma, dendrite and axon (Figures 2 and 3). Neurons show the most complex and diverse cellular morphology of the body. Image from the cerebral cortex and hippocampus of the a rat brain.
In humans, the majority of neurons are found in the cerebellum, nearly 70000 millions, and in the cerebral cortex with around 15000 millions.įigure 1. In the mouse encephalon, the total number of neurons is around 71 millions (reviewed in Heculano-Houzel 2009 see Figure 1). There are more in the spinal cord and in the peripheral nervous system. The estimated total number of neurons in the human encephalon is around 86000 millions. In the central nervous system there are numerous neuronal circuits that connected between each other. Some specialized neurons communicate with muscle cells by complex synapses known as motor plates. Neurons communicate with each other through specialized structures called synapses, which allow the formation of such circuits. Neurons are specialized in the reception, processing and sending information by chemical and electrical mechanisms that are fundamentally associated to their plasma membranes.Īll these functions are carried out by ensembles of neurons connected with one another forming complex neuronal circuits. The nervous system allows animals to communicate with the external environment and internal environments by sensing a broad variety of stimuli, processing the information, and sending orders, usually a muscle contraction that moves some parts of the organism or the whole body. Neurons, together with glial cells, are the cells that form the central and peripheral nervous systems.
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