Nervous system and nerve cells - anatomy

Martina Feichter studied biology with an elective subject pharmacy in Innsbruck and also immersed herself in the world of medicinal plants. From there it was not far to other medical topics that still captivate her to this day. She trained as a journalist at the Axel Springer Academy in Hamburg and has been working for since 2007 - first as an editor and since 2012 as a freelance writer.

More about the experts All content is checked by medical journalists.

Power cables, telephone lines and internet connections help the organism "society" to communicate and function. In the human body, this is done by our nervous system: nerve cells or neurons are the communication channels through which signals are exchanged. Sensory stimuli can be perceived, body reactions such as muscle contractions can be controlled and thought processes can be set in motion.

Central and peripheral

The human nervous system consists of a central and a peripheral part. The central nervous system (CNS) includes the brain and spinal cord; from the latter, nerve pathways lead to all regions of the body - they form the peripheral nervous system. From a functional point of view, this can be divided into two areas, the vegetative (autonomous) and the somatic nervous system.

The autonomic nervous system works independently of the human will, i.e. autonomously. For example, it ensures that meals are digested and hormones are released. In contrast, the body can be controlled at will via the somatic nervous system. For example, people can consciously put one leg in front of the other or frown.

Two halves of the brain in a team

There are two halves of the brain that work together: right controls left and vice versa - the two parts of the brain work in a mirror-inverted manner, so to speak. The left hemisphere is responsible for the right half of the body, the right brain section for the left side. The two hemispheres are connected by a bridge (corpus callosum) through which information can be exchanged. This is absolutely necessary because no hemisphere of the brain can act entirely on its own. Teamwork is a must!

Register, process, and transmit stimuli

All stimuli that come from outside (e.g. cold) or arise in the body itself (e.g. toothache) are registered by nerves and passed on to the brain in the form of electrical impulses. In the head office, the information can be evaluated and linked with one another if necessary. An example: From a sensation of heat in the hand that - as the eyes report - is holding a cup, the brain combines the fact that the coffee in the cup is still very hot.

Finally, the brain also sends out electrical signals, for example to trigger body movements (e.g. winking, raising hands) or to regulate the function of the internal organs (such as the release of gastric juice). And don't forget: Thinking, laughing, reading, learning - all this and much more keeps the brain constantly on its toes and causes the neurons to shoot countless impulses through the network every millisecond - an endless fireworks display.

Neurons - building blocks of the nervous system

The brain consists of around 100 billion neurons, some experts estimate the number to be up to 1 trillion (1,000,000,000,000)! However, there are no space problems in the head, the individual nerve cell bodies are ultimately only 150 micrometers (µm) in size. For comparison: 1 µm is a millionth of a meter.

• Cell bodies with appendages

The cell body (soma) of a nerve cell usually has various extensions: several short dendrites and a more or less long axon. The dendrites are comparable to receiving antennas - they pick up electrical signals from neighboring cells. Conversely, impulses can be passed on to other cells via the axon - it functions as a transmission mast, so to speak, and can be over a meter long.

• myelin sheath

So that the information is not transmitted too slowly at this length, the axon is enclosed in sections by so-called myelin sheaths - special cells that wrap around the axon several times and electrically isolate it. The axon and sheath together form a (medullary) nerve fiber.

The non-isolated narrow gaps between the individual myelin sheaths of an axon are called Ranvier rings. When the impulses are transmitted along the axon, the electrical impulses "jump" from ring to ring (the areas in between are, as mentioned, electrically isolated by the myelin sheaths).This significantly accelerates the conduction of excitation; it is around 100 meters per second - compared to 10 meters per second for nerve cells without a myelin layer.

The isolation of the axons can be defective due to various diseases: In the autoimmune disease multiple sclerosis (MS), for example, the misdirected immune system attacks the myelin sheaths and destroys them in places. As a result, the transmission of information along the affected axon no longer works smoothly - symptoms such as paralysis, sensory and visual disturbances occur.

• Synapses

As a newborn, humans have about as many nerve cells as in adulthood. As they grow up, however, the neurons are increasingly interconnected - for good reason: the closer the nerve cell network, the more efficient the brain is. The contact points between the individual neurons are called synapses. They transfer the information stimuli from one cell to the next. By the way, synapses also exist between nerve cells and muscle cells. For example, nerve impulses can "command" the biceps in the upper arm to contract - so that the hand can bring the coffee mug to the mouth.

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