Nerve cells (Neurons)

A neuron (nerve cell) is the structural and functional unit of your nervous system. Billions of these neurons form a functional mosaic via countless intercellular connections, enabling the perception and interpretation of sensations (sensory stimuli), the storage of experiences (memory), and reactions (behavior).

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

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

Together, the CNS and the ANS form two systems for controlling the human body. In addition, there is the endocrine system (hormone system), which forms another instrument for transmitting information and control and is managed by—or considered part of—the autonomic nervous system.

All of these systems share the goal of transmitting information. They ensure that your tissues and organs can communicate with each other!

The difference is that the nervous system (NS) transmits signals electrically via neurons. This is relatively fast and can be switched on and off easily.

With the hormonal system, it is a chemical transmission of signals via the bloodstream, which, in contrast to the NS, is rather slow and cannot be switched on and off quite as easily or quickly.

Structure of a Nerve Cell

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

A neuron consists of a cell body (soma), which has numerous protrusions of the nerve membrane. These protrusions form, on the one hand, the so-called dendrites (there are very, very many of them, much like branches on a tree), which receive (afferent) signals from other (nerve) cells. On the other hand, there is an axon (which can sometimes form collaterals). This is a relatively long line ranging from a few millimeters up to 1.5 meters (note: in giraffes or large mammals, these can naturally be even longer!). Via the axon, the electrical signal is transmitted to the end bulb, the so-called synapse, of the nerve cell. The synapses represent the contact points with other cells and transfer the signal to dendrites of the next nerve cell, to skeletal muscle cells, gland cells, or to organs, where an action is then triggered.

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

Nerve cells are (along with liver cells) the most metabolically active cells in your body. A neuron is also a very flexible unit and—especially in the area of dendrites—is capable of diverse growth and transformation processes. It can be trained!

The axon is surrounded by connective tissue cells (so-called glial cells: Schwann cells in the periphery, or astrocytes and oligodendrocytes in the CNS). These provide the axon with structure and support and form a more or less thick insulating layer (myelin layer or myelin sheath), which influences conduction speed and makes electrical signal transmission possible in the first place. Together with the axon, this sheath forms the nerve fiber.

Because of this insulating layer, your 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 myelin, due to the accumulation of myelinated axons; the gray color represents the unmyelinated axons or the nerve cell bodies! That is why the cerebral cortex is gray, as it is well known that many cell bodies reside there. And inside the spinal cord, a gray butterfly shape is visible (where the cell bodies are located), which is surrounded by white matter (the pathways, meaning axons).

Spinal cord cross section User:Polarlys, Medulla spinalis – Querschnitt – 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 in the spinal cord (2nd motor neuron). To perform a movement, the signal travels from the brain via descending pathways to the spinal cord, where it is switched (in the anterior horn) to another motor neuron and sent directly from there to your skeletal muscles.

To perform a voluntary movement, you therefore need 2 nerve cells, with the signal being relayed once.

Sensory Neurons (Efferent Signals): A Sensation

Sensory sensations are conducted from free nerve endings or receptors via the axon to the posterior horn of the spinal cord. However, the cell body of this 1st sensory neuron lies outside the spinal cord, specifically in the spinal ganglia formed by them. In the spinal cord, an initial relay occurs via interneurons to another (second sensory) neuron, which ascends via ascending pathways toward the brain. In front of the cerebrum lies the thalamus (diencephalon), where another relay to a 3rd sensory neuron takes place.

Sensory signals are thus conducted to your brain via at least 3 neurons, needing to be relayed twice. As a result, efferent signals can also be modulated—that is, altered and distributed—more extensively and in diverse ways.

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

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

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Cover Image Credit

Microscopic image of a mouse's cerebral cortex. 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