The Human Nervous System

by Ulrika Kahl, PhD

The human nervous system consists of around a hundred billion nerve cells - or neurons. Roughly and based on anatomical features the nervous systems can be divided into two parts – the central nervous system (CNS) and the peripheral (PNS).
While the brain and the spinal cord constitute the central nervous system, the so-called cranial- and spinal nerves form parts of the peripheral. The peripheral nerves connect the central nervous system with the sense organs, i.e. the organs for vision, hearing, smell, taste and perceptional touch, and other effector organs like muscles and glands.

The nervous system can of course be divided into additional subdivisions, but we will stick to this gross classification for now. The anatomic boundaries also lose their importance to some extent, considering that most cells stretch through several anatomic regions. A neuron can for instance have its cell body in the central nervous system, meanwhile the terminal is found in some peripheral part. See the text about “Signaling between Nerve Cells” for a description of how a typical nerve cell looks.

The Autonomic and Somatic Nervous Systems

Another way of dividing the nervous system is to look at it from a functional point of view. Here one usually separates the anatomic nervous system from the somatic.

The autonomic nervous system is in connection with glands and internal organs like the heart, lungs, stomach, intestines, liver, kidneys and adrenal glands, bladder and reproductive organs, to mention a few. These functions stand above our will end awareness, and is instead taken care of automatically by the nervous system itself.

In contrast, the somatic nervous system handles the muscles in our extremities, and joints and the outer skin, in other words functions that to a large extent is governed by our own will.
Another difference between the autonomic and the somatic nervous system is that while the central cell bodies of somatic neurons extend all the way out to the peripheral target organs, the autonomic neurons have to be reconnected in so called ganglia on the way to the effector organ.

Figure Schematic drawing of the human brain in the skull, as seen from the side if divided in two halves (not to scale).

The Subdivisions of the Brain

The actual brain is usually divided into five anatomical subdivisions; the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon. Depending on what literature you look at, the details of this classification may however vary.

Telencephalon
Telencephalon consists of the two hemispheres, which together form the largest part of the brain. The hemispheres are in contact with each other through a massive bundle of nerve fibers, the corpus callosum. Each hemsiphere is covered by a thin layer of nerve fibers - the cerebral cortex - which is heavily folded. The folds are called gyri, and the grooves between them sulci - or fissures if they are unusually deep. In the interior of each hemisphere there is a centrally placed cavity called the lateral ventricle, and a large mass of nerve fibers, referred to as the basal ganglia.

Diencephalon
Diencephalon is divided into two halves by a long and thin longitudinal cavity - the third ventricle - which is continuous with the lateral ventricle in the telencephalon. Thalamus and hypothalamus are the two major parts of the diencephalon. Thalamus is a collection of several nuclei with various and mostly autonomic functions. The hypothalamus too consists of various groups of cells and pathways, and is also important for the regulation of autonomic and endocrine functions.

Mesencephalon
Mesencephalon is a short segment of the brain between the disencephalon and the metencephalon. Its internal structure is complicated, but the gross anatomic features are simple. It is crossed traversely by a narrow canal - the cerebral aqueduct - which connects the third ventricle in the disencephalon with the fourth ventricle in the metencephalon.

Metencephalon
Metencephalon consists of cerebellum and pons. Cerebellum, which is primarily involved in coordination of movements, is composed of two hemispheres that are joined in the midline by a narrow worm-like portion - the vermis. Like the cerebral cortex the cerebellum is covered by a layer of cells - the cerebellar cortex, which is characterized by a large number of parallel folds - folia - separated by deep fissures. Pons is located in front of the cerebellum and looks like a bridge over the canal-like structure, the brain stem.

Myelencephalon
Myelencephalon - or medulla oblongata - is as hinted in the name a continuation of the spinal cord. To the myelencephalon the forth ventricle stretches from the metencephalon.
The medulla oblongata, pons and mesencephalon together form the brain stem, and contain many important nuclear groups including the cranial nerve nuclei. It also contains important pathways connecting the spinal cord and the telencephalon.

If you are interested in reading more about the brain and nervous systems, click on any of the links below

The Neuron
Signaling between Nerve Cells
Neurotransmitters – The Messengers of the Brain
What Happens at the Postsynapse?