Edited by Dr. Stefano Casali
The Neurons
They are the cells responsible for receiving and transmitting nerve impulses to and from the CNS. Neurons can be divided into three zones:
- A cell body or soma;
- Of the extensions called dendrites;
- A single extension called neuritis or axon.
Neurons are classified into four types based on their shape:
- unipolar neurons (they have a single extension and are very rare in vertebrates);
- bipolar neurons (they have a single axon and a single dendrite. They are found in the olfactory epithelium of the nasal mucosa);
- pseudounipolar neurons (they have a single extension that starts from the soma, after a short distance it forks into two branches arranged in a T shape, one that enters the CNS and the other that reaches the periphery);
- multipolar neurons (with several extensions one of which is the axon and the others the dendrites).
They can also be classified on the basis of their function:
- sensory (afferent) neurons, are specialized in receiving sensory impulses on their dendritic termination and in transmitting them to the CNS for processing;
- motor neurons or motor neurons (efferent), originate from the CNS and carry impulses to various organs and cells, muscle, glandular and other nerve cells.
- interneurons: they are found in the CNS and have the function of connecting and integrating sensory and motor nerve cells to form a network of nerve circuits. Their number has been increased by the evolution of the nervous system.
The nerves
Nerve fibers consist of neuronal axons wrapped in particular sheaths of ectodermal origin. Groups of nerve fibers make up the bundles of the brain and spinal cord and peripheral nerves. There are differences in the sheaths that surround the axons depending on whether the fibers are part of the CNS or the PNS. In adult nervous tissue, most axons are enveloped by single or multiple folds of a sheath cell, represented by the Schwann cell in the SNP fibers and by the oligodendrocyte in the CNS fibers. In invertebrates and minor vertebrates, axons can regenerate after a traumatic rupture. In mammals the phenomenon is less common and is restricted to the peripheral nerves. Schwann cells are most responsible for this regeneration.
The metabolic and supportive function of neurons is carried out by neurologic cells also called glial cells. They are able to recover the ions and metabolic products of neurons, such as potassium, glutamate and more that accumulate around neurons. They participate in the energy metabolism of neurons by releasing glucose from their glycogen stores. The astrocytes of the peripheral areas of the CNS form a continuous cell layer around the blood vessels probably constituting the blood-brain barrier. The blood-brain barrier is semi-permeable, it allows some substances to pass through, but not others. In most parts of the body, the smallest blood vessels, the capillaries, are covered only by endothelial cells. Normally, there are small spaces between the endothelial cells that allow many substances to move easily through the capillary wall. But, in the brain, the endothelial cells are very attached to each other (junction complexes) and the various substances cannot cross the capillary wall. Glial cells (astrocytes) arrange themselves to form a continuous layer around the brain capillaries. It seems, however, that astrocytes are not essential to constitute the blood-brain barrier, but would be important for the transport of ions from the brain to the blood. The barrier e.e. has the following functions:
- Protect the brain from "foreign substances" present in the blood, which could damage it;
- Protect the brain from hormones and neurotransmitters released to act in other parts of the body;
- Maintain a constant environment for the brain.
General properties of the blood brain barrier:
- Large molecules do not pass through the barrier;
- Poorly soluble lipid molecules do not penetrate the brain. Lipid-soluble molecules (such as barbiturates and alcohol), on the other hand, cross the barrier very well;
- Molecules with high electrical charge are slowed down.
The blood-brain barrier can be canceled or reduced by the following causes:
- Hypertension;
- Development: the barrier is not fully formed at birth;
- Hyperosmolarity: a substance present in the blood with a high concentration can cross it;
- Microwave;
- Radiation;
- Infections;
- Trauma, Ischemia, Inflammations.
Other articles on "Neurons, Nerves, and the Blood Brain Barrier"
- nervous system
- nerve cells and synapses
