Corpus: Neuron

A nerve cell or neuron is a specialised cell that is responsible for the reception, transmission and processing of nerve impulses (excitation conduction).

The categorisation of nerve cells can be based on the observation of individual cells or on the properties assigned to the nerve cells in cell clusters (e.g. according to the conduction velocity according to Erlanger/Gasser). In the case of individual nerve cells, their morphology and associated neurotransmitters (e.g. acetylcholine in so-called cholinergic cells) are primarily used for classification.

...according to morphology[Bearbeiten]

• unipolar nerve cell
• bipolar nerve cell
• pseudounipolar nerve cell
• multipolar nerve cell
• apolar nerve cell
• anaxonic nerve cell

...by function[Bearbeiten]

• motor nerve cells: somatomotor; vegetative: visceromotor (sympathetic, parasympathetic)
• Sensory nerve cells: somatosensitive; vegetative: viscerosensitive (sympathetic, parasympathetic)
• Interneurones

...by neurotransmitter[Bearbeiten]

A distinction is made according to the neurotransmitters to which the neurones preferentially react:

• adrenergic nerve cells
• cholinergic nerve cells
• dopaminergic nerve cells
• GABAergic nerve cells
• glutaminergic nerve cells
• histaminergic nerve cells
• purinergic nerve cells
• serotonergic nerve cells

...by transcriptome[Bearbeiten]

Nerve cells can be further differentiated into superclusters, clusters and subclusters according to their transcriptome, i.e. according to the genes that are translated into mRNA in a nerve cell. These distinctions are currently (2024) still the subject of basic research. However, initial results show that there are thousands of forms of epigenetically specialised neurons in the CNS that fulfil different tasks.^[1]

In the central nervous system (CNS), nerve cells are the main components of the parenchyma of the spinal cord and brain. In the peripheral nervous system (PNS), bundles of thousands of nerve fibres with their respective surrounding sheath and supply layers are referred to as nerves. The cranial nerves, which originate in structures of the CNS, are also part of the PNS.

Fibres running from the receptors in the sensory organs to the CNS are called afferent, nerve fibres running from the CNS to the effectors (e.g. muscles, glands) are called efferent. Efferent and afferent fibres can be attached to each other and follow a common course. Within the CNS, "afferent" stands for "leading to" and "efferent" for "leading from".

Various sections of the nerve cell can be differentiated:

• The dendrites are the finest plasmatic ramifications of the cell body, which establish contact with thousands of other nerve cells via synapses and receive excitation from them.
• The soma or perikaryon is the cell body of a nerve cell, the plasmatic area around the cell nucleus, without dendrites and axon.
• The axon (also known as the neurite) is a long extension of the nerve cell that serves to transmit nerve impulses. The action potentials are transmitted via the axon to other nerve cells or muscle cells. Inside the axon is the axoplasm, which makes up more than 90 % of the nerve cell's cytoplasm.
• The axon hillock at the transition from the soma to the axon generates a sequence of action potentials when the depolarisation threshold is exceeded.
• The synaptic terminal button at the end of the axon transmits the incoming signal to the dendrites of the downstream cell through 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 representation of a neuron

Some nerve cells have their own names due to their specific morphology, localisation or function, e.g:

• Anterior horn cells
• Pyramidal cells
• Purkinje cells
• Renshaw cells
• Betz cells
• Lugaro cells
• Basket cells
• Rosehip cells
• Tufted cells

Excitation conduction[Bearbeiten]

Information transmitted via nerve cells is encoded by changes in the electrical potential of the cell membrane. This mechanism is the basis of excitation conduction. By opening ion channels in the cell membrane, ions can flow in or out and thus change the charge of the cell. This change in charge is transmitted passively or through action potentials and transferred to other nerve cells at the synapses.

Excitatory or inhibitory signals arriving via synapses are processed by the nerve cell. Excitatory stimuli that arrive almost simultaneously add up in their effect and lead to a summation potential building up at the axon hillock. If the summation potential at the axon hillock reaches the threshold potential of the nerve cell, an action potential is triggered along the axon as a reaction.

Axonal transport[Bearbeiten]

The movement of substances within the sometimes very long axons is made possible by special cellular processes that are summarised under the term "axonal transport".

Neurotransmitters are specialised messenger substances of the nerve cell. They have very different chemical structures, but what they have in common is the transmission of excitation to another neuron. This takes place at chemical synapses. Neurotransmitters are released presynaptically and enter the synaptic cleft between the nerve cells. Postsynaptically, they are recognised by receptors on the cell membrane and reversibly bound to them.

The binding leads to a temporary opening of ion channels in the membrane, which triggers ion currents and thus a change in the membrane potential. This can trigger an excitatory (EPSP) or inhibitory (IPSP) postsynaptic potential.

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 fibres via acetylcholine (ACh) at motor end plates.

• The construction of a neuron

• human nerve cell by www.sciepro.com; CC BY-ND 4.0 DE
• Image source podcast: © canacrtrk / Pexels