A neuron or nerve cell is an electrically excitable cell that transmits and processes information in the nervous system. Neurons communicate with each other through connections called synapses. There are around 100 billion neurons in the human brain . Although neurons do not have any consciousness on their own, consciousness, emotions and thoughts emerge as billions of cells communicate with each other through trillions of connections.

Axon and Dendrites

A typical neuron consists of a cell body and threadlike extensions. There are two types of extensions. Dendrites are numerous, short, fringe-like structures. The axon is usually single and longer than the dendrites. Signals from other neurons to a neuron are received by special structures called synapses in dendrites and cell bodies. The signal generated by the neuron is transmitted along its axon, and is carried to the synapses in the dendrites and bodies of other neurons by branches at the tip of the axon. It has been calculated that there are 4 km of neural network connections per cubic millimeter of gray matter of the brain.

Action Potential

In the brain, signals are transmitted by electrical pulses. The electrical impulse that occurs for signal transmission in nerve cells is called an action potential. Each electrical pulse has a voltage of about 0.1 volts, lasts for a thousandth or two of a second, and is transmitted along axons at speeds of up to 430 kilometers per hour. When the electrical pulse reaches the synapse, it triggers the release of chemicals called neurotransmitters into the synaptic cleft. Neurotransmitters can have an excitatory or suppressive effect on the receiving neuron.

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Keeping neurons ready to generate electrical signals at any moment requires a great deal of energy. Although our brain is only two percent of our body’s weight, it carries out twenty percent of all energy consumption.

Sensory, Motor and Interneurons

Neurons are divided into subtypes according to their properties. Sensory neurons are sensitive to sensory stimuli such as touch, sound, and light, and they carry these stimuli to the central nervous system. Motor neurons transmit movement signals produced in the brain and spinal cord to the muscles. Interneurons (interneurons) connect with other neurons in the brain or spinal cord. A group of interconnected neurons is also called a “neural circuit”.

Neuron Development

Neurons arise from neural stem cells during the embryo and infancy when the brain develops . The formation of new neurons (neurogenesis) in the adult brain largely stops. However, a considerable amount of new neuron formation in the hippocampus and olfactory bulb also occurs in adulthood.

Glial Cells

The normal functioning of neurons depends on the correct balance of the intercellular environment. The support cells of the nervous system, called glia cells, provide this environment. The name glia is derived from the Greek word meaning glue. Thus, it was emphasized that they hold the nervous system together like a glue.

Although old information is that there are about 10 glia cells for each neuron in our brain, new studies suggest that this ratio is around 1. The formation of neuron connections during the developmental period, feeding of neurons in the adult brain, keeping them in place, isolating the axons so that the signals can be carried faster, cleaning the cell residues, and protecting the brain from infection are all done with the help of glia cells. The glia cells in the central nervous system are oligodendrocytes, astrocytes, ependymal cells, and microglia. In the peripheral nervous system, which consists of nerves other than the brain and spinal cord, there are Schwann cells and satellite cells.

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Astrocytes are the most abundant glial cells. They are involved in the formation of the blood-brain barrier. They regulate the balance of the intercellular environment by attracting excess potassium ions and clearing the neurotransmitters released during synaptic impulses. They can also control the widening and narrowing of blood vessels.

Oligodendrocytes form an insulating myelin sheath around axons in the central nervous system. Thanks to the myelin sheath, electrical signals are carried faster and more efficiently.

Ependymal cells line the surface of the cavities in the interior of the brain and spinal cord. The cerebrospinal fluid (CSF) is located in these spaces called the ventricular system. Ependymal cells assist in the production and circulation of CSF.

Microglia cells are the central nervous system-specific forms of macrophages, which are immune system cells. They can become activated and inflamed due to brain damage or infection . Microglia may have a role in problems such as Alzheimer’s disease , Parkinson’s disease and ALS .

Schwann cells form the myelin sheath in the peripheral nervous system.

Satellite cells are support cells in sensory, sympathetic and parasympathetic nerve nodes outside the central nervous system. They help maintain the intercellular chemical balance in these nodes.

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