What is the structure of a neuron?

Maybe many people are not familiar with the term "neuron". We often hear the term "nervous system". The nerves in the human body are a very complex system. The nervous system in the human body is composed of many parts, and neurons are the most basic components of the entire nervous system. Neurons include many parts. The following will introduce the structure of neurons.

The structure of a neuron:

Neurons, also known as nerve cells, are the components of the structure and function of the nervous system.

Basic unit.

Neurons are cells with long synapses (axons) that consist of a cell body and cell processes. The long axon is covered with a sheath to form a nerve fiber, and the tiny branches at the end are called nerve endings. The cell bodies are located in the brain, spinal cord, and ganglia, and the cell processes can extend to organs and tissues throughout the body. The cell body is the part of the cell that contains the nucleus. Its shape and size vary greatly, with a diameter of about 4 to 120 microns. The nucleus is large and round, located in the center of the cell, with little chromatin and a prominent nucleolus. The cytoplasm contains patchy extranuclear chromatin (formerly known as Niel bodies) and many neuronal fibers. Cell processes are long and thin parts extending from the cell body, which can be divided into dendrites and axons. Each neuron can have one or more dendrites, which can receive stimulation and transmit excitement to the cell body. Each neuron has only one axon, which carries impulses from the cell body to another neuron or to other tissues, such as muscles or glands.

The cell bodies (soma) of neurons are located in the gray matter and ganglia of the brain and spinal cord. Their shapes vary, and the most common ones are star-shaped, cone-shaped, pear-shaped, and spherical. The cell bodies vary in size, with diameters ranging from 5 to 150 μm (micrometers). The cell body is the metabolic and nutritional center of the neuron.

The structure of the cell body

The structure of the cell body is similar to that of ordinary cells, with a cell membrane, cytoplasm and nucleus.

(l) Cell membrane: The cell membrane of the cell body and the membrane on the surface of the process are continuous and complete cell membranes. Except for the cell membrane at the synaptic site which has a specific structure, most of the cell membrane is a unit membrane structure. The characteristic of the nerve cell membrane is that it is a sensitive and easily excitable membrane, on which there are various receptors and ion channels, each of which is composed of different membrane proteins. The cell membrane that forms the part of the synapse thickens. The receptors on the membrane can bind to the corresponding chemical substances called neurotransmitters. When the receptor binds to acetylcholine transmitter or γ-aminobutyric acid transmitter, the membrane's ion permeability and the potential difference between the inside and outside of the membrane change, and the cell membrane produces corresponding physiological activities: excitation or inhibition.

(2) Cell nucleus: mostly located in the center of the nerve cell body, large and round, with little heterochromatin, mostly located on the inner side of the nuclear membrane, and more euchromatin, scattered in the middle of the nucleus, so it is lightly colored, with 1 to 2 nucleoli, which are large and obvious. When cells degenerate, the nucleus often moves toward the periphery and becomes dislocated.

(3) Cytoplasm: Located around the nucleus, also known as the perinuclear body, it contains a well-developed Golgi complex, smooth endoplasmic reticulum, abundant mitochondria, Nissl bodies and neurofibrils, as well as lysosomes, lipofuscin and other structures. Neurons with secretory functions also contain secretory granules in their cytoplasm, such as some neurons located in the hypothalamus.

l) Nissl bodies: also known as chromatin, are an alkaliphilic substance in the cytoplasm. In general staining, they are stained with alkaline dyes and are mostly plaque-shaped or granular. It is distributed in the perikarya and dendrites, but not in the axon hillock of the axon initial segment or in the axon. The number, shape and distribution of Nissl bodies vary depending on the type of neuron and different physiological states. A typical example is the anterior horn motor neurons of the spinal cord, which have the largest number of Nissl bodies. They are plaque-like and scattered between the neurofibrils, resembling tiger-skin spots, so they are also called tiger bodies. In the cytoplasm of spinal ganglion neurons, Nissl bodies are granular and scattered.

Under the electron microscope, Nissl bodies are composed of many well-developed parallel-arranged rough endoplasmic reticulum and the free ribosomes between them. The large amount of proteins required for neural activity are mainly synthesized in Nissl bodies and then flow to the nucleus, mitochondria and Golgi complex. When neurons are damaged or poisoned, the number of Nissl bodies can decrease or even disappear. If the damage is recovered and the harmful factors are removed, the Nissl bodies can be restored. Therefore, the morphological structure of Nissl bodies can be used as a marker to determine the functional state of neurons.

2) Neurofibrils: In the cytoplasm of nerve cells, there are filamentous fiber structures with a diameter of about 2 to 3 μm. The silver-stained sections can clearly show brown-black filamentous structures, which are neurofibrils. They are interwoven into a network in the perinuclear body and extend to the dendrites and axons, reaching the undissolved parts of the protrusions. Under electron microscope, neurofibrils are composed of neurofilaments and neural microtubules gathered into bundles. Neurofilaments, also known as neurofilaments, are long, thin tubular structures with a diameter of about 10 nm. They are a type of intermediate filament, but they are different from the intermediate filaments in other cells. Observe at high magnification under an electron microscope. It can be seen that nerve filaments are extremely fine tubular structures with a transparent lumen in the middle and a wall thickness of 3nm. They are extremely long and mostly clustered into bundles. It is dispersed in the cytoplasm and also extends into the processes of neurons. The physiological function of neurofilaments is to participate in the pathway of metabolite and ion transport flow within neurons. Neural microtubules are thin, long, round tubes with a diameter of about 25 nm and a wall thickness of 5 nm. They can extend into the processes of neurons and are arranged in bundles with neurofilaments in the cytoplasm.

, interwoven into a web.

Its physiological function is mainly to participate in the material transport activities in the cytoplasm. The flow rate of various substances close to the surface of the microtubule is the largest. There is motor protein on the surface of the microtubule, which has the function of ATP enzyme. In the presence of ATP, it can make the microtubule slide, thus giving the microtubule a transport function. In addition, there are shorter and more dispersed microfilaments. Microfilaments are the thinnest filamentous structures, with a diameter of about 5nm, varying in length, clustered into bundles, interwoven into networks, and widely distributed in the cytoplasm and processes of neurons. Their main function is contraction, adapting to morphological changes in the physiological activities of neurons. Neurofilaments, microtubules, and microfilaments, these three types of fibers, constitute the cytoskeleton of neurons and are involved in the transport of substances. Under a light microscope, only neurofibrils formed by neurofilaments and neural microtubules are displayed.

3) Lipofuscin: It is often located on one side of large neurons without perinuclear bodies and is brown-yellow granular. It increases with age.

Electron microscopy and histochemistry confirmed that they were residual bodies formed by secondary lysosomes, and their contents were substances remaining during lysosomal digestion, mostly foreign matter, lipid droplets or degenerated organelles.

Some neurons, such as the hypothalamus, have endocrine functions. The brain contains secretory granules with a diameter of 100 to 300 nm, which contain peptide hormones (such as vasopressin, oxytocin, etc.).

4) Protrusion

The neuronal process is the extension of the neuronal cell body. Due to the differences in morphology, structure and function, it can be divided into dendrites and axons.

Dendrite

It is one or more processes that radiate from the cell body. Soma initiation

Some parts are thicker, and become thinner through repeated branching, shaped like tree branches. The structure of dendrites is similar to that of the brain body. The cytoplasm contains Nissl bodies, mitochondria and parallel neurofibrils, but no Golgi complex. In special silver-stained specimens, many spine-like protrusions can be seen on the surface of dendrites, which are about 0.5 to 1.0 μm long and 0.5 to 2.0 μm thick. They are called dendritic spines and are the sites where synapses are formed. Under a normal electron microscope, dendritic spines contain several flat vesicles called spine apparatus. Dendritic branches and dendritic spines expand the surface area of ​​the neuron that receives stimulation. Dendrites receive stimuli and transmit impulses to the cell body.