How do neurofibrils differ from nerve fibers
Synonyms: neuron, neuronum
English: neuron, nerve cell
Neurons are specialized cells that are responsible for the reception of stimuli as well as the transmission and processing of nerve impulses (conduction of excitation).
The classification of nerve cells can be made on the basis of the observation of individual cells or on the basis of the properties assigned to the nerve cells in cell associations (e.g. according to the line speed according to Erlanger / Gasser). In the case of the individual nerve cells, their morphology and associated neurotransmitters (e.g. acetylcholine in so-called cholinergic cells) are used for classification.
2.1 ... according to morphology
During the embryonic period, there are also apolar nerve cells (neuroblasts) that do not yet have any appendages.
2.2 ... by function
In the central nervous system (CNS), nerve cells are the essential components of the parenchyma of the spinal cord and brain. In the peripheral nervous system (PNS) bundles of thousands of nerve fibers with their surrounding sheath and supply layers are called nerves. The cranial nerves, which originate from structures in the CNS, are also part of the PNS.
Fibers running from the receptors in the sensory organs to the CNS are called afferent, nerve fibers running from the CNS to the effectors (e.g. muscles, glands) are called efferent. Here, efferent and afferent fibers can be attached to one another and take a common course. Within the CNS, "afferent" stands for toconductive, "efferent" for fromconductive.
Different sections can be differentiated on the nerve cell:
- The dendrites are the finest plasmatic ramifications of the cell body, which establish contact with thousands of other nerve cells via synapses and receive excitations from them.
- Soma or perikaryon is the name given to the cell body of a nerve cell, the plasmatic area around the cell nucleus, without dendrites and axons.
- The axon (also called neurite) is a long extension of the nerve cell, which is used to transmit nerve impulses. The action potentials are passed on to other nerve cells or muscle cells via the axon. Inside the axon is the axoplasm, which makes up more than 90% of the cytoplasm of the nerve cell.
- The axon hillock at the transition from the soma to the axon generates a sequence of action potentials when the depolarization threshold is exceeded.
- The synaptic end button at the end of the axon transmits the incoming signal to the dendrites of the downstream cell by means of chemical excitation transmission with the help of neurotransmitters.
Like other cells, nerve cells have an extensive cytoskeleton. Neurofibrils ensure the consistency of the cell's shape, while microtubules play an important role in axonal transport (see below).
3D illustration of a neuron
5.1 Excitation conduction
The information transmitted via nerve cells is encoded by changes in the electrical potential of the cell membrane. This mechanism is the basis of the conduction of excitation. By opening ion channels in the cell membrane, ions can flow in or out and thus change the cell's charge. This change in charge is passed on passively or through action potentials and transferred to other nerve cells at the synapses.
5.2 Axonal transport
Material movements within the sometimes very long axons are made possible by special cellular processes that are summarized under the term "axonal transport".
Neurotransmitters are specialized messenger substances in nerve cells. Chemically, they have very different structures, but what they have in common is the transfer of excitation to another neuron. It takes place at chemical synapses. Neurotransmitters are released presynaptically and enter the synaptic gap between the nerve cells. Post-synaptically, they are recognized by receptors on the cell membrane and reversibly bound to them.
The bond leads to a temporary opening of ion channels in the membrane, which triggers ion currents and thus a change in the membrane potential. An exciting (EPSP) or an inhibiting (IPSP) postsynaptic potential can be triggered.
This principle of excitation transmission applies not only to connections between nerve cells, but also to neuromuscular synapses. Here, the impulses of excited nerve cells are transmitted to muscle fibers by means of acetylcholine (ACh) at motor end plates.
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