Synonyms
Brain, CNS (central nervous system), nerves, nerve fibers Medical: Neuron, ganglion cell Greek: Ganglion = node
Definition
Nerve cells (neurons) are cells whose primary function is to transmit information by means of electrical excitation and synaptic transmission. The totality of nerve cells and other cells directly related to their function are called the nervous system, whereby a distinction is made between the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), mainly consisting of peripheral nerves. The human brain contains between 30 and 100 billion nerve cells.
Like other cells, the nerve cell has a cell nucleus and all other cell organelles, which are localized in the cell body (soma or perikaryon). A stimulus that encounters a nerve cell causes an excitation that spreads in the cell membrane of the neuron (depolarization of the cell membrane) and is transmitted via long cell extensions, the neurites or axons. This excitation is called action potential.
The neurites (axons) can reach a length of up to over 100 cm. The excitation can therefore be propagated in a directed manner over a long distance, e.g. by moving your big toe. Each nerve cell has only one axon.
Structure
Nerve cells are divided into different parts. Each cell has a nucleus with surrounding cytoplasm and cell organelles. This central area of the cell is called soma.
The soma of the nerve cell has one or more thin extensions, which can be divided into dendrites and axons. Dendrites make contact with other nerve cells (synapses) and can passively transmit electrical excitation. If this excitation exceeds a certain threshold, an action potential is triggered in the axon by opening voltage-dependent sodium channels that pass on this excitation along the entire length of the axon.
In this way, a signal can be transmitted over large distances within a short time. Axons can grow to over one meter in length (e.g. motor fibers from the spinal cord to the foot muscles), so that excitatory nerve cells are among the largest cells in the body. The axon either makes a single synapse to another nerve cell (e.g.
in sensitive nerves), or it branches out and makes contact with several cells (e.g. in nerves that innervate muscles). At these synapses there are so-called transmitter vesicles in the cytoplasm of the cell, small membrane-enveloped vesicles which contain high concentrations of neurotransmitters. If required, these can be released into the synaptic cleft and trigger a signal at the cell membrane of the post-synapse – the target cell.
Cytoskeleton elements such as microtubules run through nerve processes. These are tube-like protein building blocks which, like rails, serve as a pathway for transport proteins (dynein and kinesin) that transport biological loads such as large proteins, vesicles and even entire cell organelles. In this way, the supply of distant axon elements can be ensured.
Many nerve cells are also surrounded by extensions of other cells to achieve better electrical properties (myelinization). As a result, the nerve fibers increase in diameter, but can pass on excitation much faster. For example, motor fibers that form skeletal muscles are particularly well encased, but also pain fibers that are supposed to cause a protective reaction.