Nerve cells (Neurons)

A neuron (nerve cell) is the structural and functional unit of our nervous system. Billions of such neurons create a functional mosaic through countless connections between cells, allowing for the perception and interpretation of sensations (sensory stimuli), the storage of experiences (memory), and reactions (behavior).

The nervous system is divided into the central nervous system (CNS, voluntary, conscious or somatic nervous system) and the autonomic nervous system (ANS, autonomous, involuntary nervous system). The CNS includes all anatomical structures surrounded by our meninges.

The CNS controls conscious, targeted processes such as active movements through skeletal muscles. The ANS takes over the control of unconscious (autonomous) activities such as the digestive system or blood pressure and much more.

The CNS and ANS together form two systems for controlling the human body. Additionally, there is the endocrine system (hormone system), which serves as another instrument for information transmission and regulation, collectively managed by the autonomic nervous system.

All these systems aim to transmit information. They ensure that tissues and organs can communicate with each other!

The difference lies in the nervous system (NS) transmitting signals electrically through neurons. This is relatively fast and can be switched on and off effectively.

In the hormonal system, chemical transmission of signals occurs through the bloodstream, and unlike the NS, it is slower and less efficient in being switched on or off quickly.

Structure of a Nerve Cell

Nerve cells can therefore create stimuli (e.g. pain), transmit, and very importantly and interestingly, modify them within the cell network!

A neuron consists of a cell body (soma) that has many protrusions of the nerve membrane. These protrusions form, on one hand, the so-called dendrites (there are very, very many, similar to branches on a tree), which receive (afferent) signals from other (nerve) cells. On the other hand, there is an axon (which can partially form collaterals). This is a relatively long conduit ranging from a few millimeters to up to 1.5 meters (NB: For giraffes or large mammals, they can naturally have an even greater length!). Through the axon, the electrical signal is passed to the synapse, the nerve cell’s endpoint. Synapses act as contact points with other cells, transmitting the signal to dendrites of the next nerve cell, skeletal muscle cells, gland cells, or organs, where a reaction is thus triggered.

Nerve cells Complete neuron cell diagram de, labeled as public domain, details on Wikimedia Commons

Nerve cells are (along with liver cells) the most metabolically active cells in the body. Furthermore, the neuron is a very flexible unit and is capable—especially in the dendrite area—of manifold growth and transformation processes. It can be trained!

The axon is surrounded by connective tissue cells (the so-called glial cells: Schwann cells in the periphery or astrocytes and oligodendrocytes in the CNS). These give structure and support to the axon and form a more or less thick insulation layer (myellin sheath), which affects conductivity and enables electrical signal transmission. This sheath, together with the axon, forms the nerve fiber.

Due to this insulating layer, the CNS actually consists of 90% connective tissue and only 10% actual nerve cells!

John A Beal, PhD Dep’t. of Cellular Biology & Anatomy, Louisiana State University Health Sciences Center Shreveport, Human brain frontal (coronal) section description 2, CC BY 2.5

The white color comes from myellin, due to the accumulation of myellinated axons; the gray color is the unmyellinated axons or the nerve cell bodies! That is why the brain cortex is gray, as many cell bodies are located there. And inside the spinal cord, you can see a gray butterfly shape (where the cell bodies are located), which is enclosed by white matter (the pathways, i.e., axons).

Spinal cord cross-section User:Polarlys, Medulla spinalis – Cross-section – German and Latin, CC BY-SA 3.0

Motor Neurons (Afferent Signals): A Movement

Simply put, the cell bodies are located in the brain (1st motor neuron) and the spinal cord (2nd motor neuron). To execute a movement, the signal thus comes from the brain via descending pathways to the spinal cord, where it is switched over to another motor neuron in the so-called anterior horn and then directly transferred to the skeletal muscles.

To perform a voluntary movement, 2 nerve cells are needed, where the signal is switched once.

Sensory Neurons (Efferent Signals): A Sensation

Sensory sensations are conveyed by free nerve endings or receptors through the axon to the posterior horn of the spinal cord. However, the cell body of this 1st sensitive neuron is located outside the spinal cord, namely in the so-called spinal ganglia, which are formed through this. In the spinal cord, there’s a first transmission over interneurons to another (second sensitive) neuron, which ascends towards the brain on ascending pathways. Before reaching the cerebrum, the thalamus (the so-called interbrain) is located, where another transition onto a 3rd sensitive neuron occurs.

Sensory signals are transmitted to the brain through at least 3 neurons, with two transmissions needing to occur. This allows efferent signals to be modified, changed, and distributed more and in a variety.

As we will see in a later blog, this offers us several therapeutic possibilities!

Diseases, problems, and therapeutic options will also be discussed in a future blog. Stay tuned.

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Title Image Credit

Microscopic image of the cerebral cortex of a mouse. Wei-Chung Allen Lee, Hayden Huang, Guoping Feng, Joshua R. Sanes, Emery N. Brown, Peter T. So, Elly Nedivi PLoSBiol4.e126.Fig6fNeuron, CC BY 2.5