Nervous System: Basics

The Nervous system comprises all the nervous tissue of an organism. Traditionally, it has been divided into central nervous system (CNS) and peripheral nervous system (PNS). The CNS includes the encephalon, contained in the skull, and its continuation, the spinal cord. The PNS includes all the remaining nervous tissue, and it is composed of groups of nervous cell bodies (ganglions), networks of interlacing nerves (plexus), and bundles of nerve fibers having a common origin, termination and function (fiber tracts). In the CNS, nervous cell bodies are grouped in nuclei. The slender projections of neuronal cells often run together from one part of the CNS to another, forming a fascicle or cord.

Between neuronal cells there are junctions, called synapses, through which nervous stimuli are transmitted from one cell to another by physical or chemical mechanisms. In addition to neurons, the CNS also includes neuroglial cells (specialized support cells), and connective tissue.

The nerves emerge in pairs from the encephalon and the spinal cord, one for each side of the organism. Nerves that comes from the encephalon are called cranial nerves, and those that come from the spinal cord are called spinal nerves. Peripheral nerves allow neurons of the CNS to keep in touch and collect information from the entire organism. Nervous fibers that leave from the CNS with the cranial or spinal nerves are called efferent or motor because they conduct nerve impulses to the periphery. Afferent or sensitive nerve fibers are long extensions of the nerve cells of the ganglia. They carry impulses from the periphery to the CNS and reach it via the cranial or spinal nerves. In general, sensitive and motor fibers are mixed in the cranial or spinal nerves.

Spinal Cord

The spinal cord is located inside the spinal canal, and is protected by the vertebral column. Besides, it is surrounded by three membranes of connective tissue called meninges. From the outside in, they are the dura mater, the arachnoid mater and the pia mater. The subarachnoid space, between the arachnoid mater and the pia mater, is filled with cerebrospinal fluid and contributes to protect the spinal cord.

In a transverse section of the spinal cord it is possible to differentiate the grey matter, in the centre, surrounded by the white matter (except when the dorsal horns touch the spinal cord margins). The grey matter is composed of interneurons, cell bodies and dendrites of efferent neurons, fibers of afferent neurons and glial cells. The white matter is composed of groups of myelinated axons.

The human spinal cord comprises 31 spinal nerves: 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves and 1 coccygeal nerve. The cervical nerves control the muscles and glands, and receive sensory information from the neck, shoulder, arm and hand. Thoracic, lumbar and sacral nerves are associated with the chest and abdominal walls, hip and legs, genitals and lower digestive tract, respectively. The coccygeal nerve supplies the skin over the coccyx.

The names of the ascending tracts begin with the prefix spino- and end with the name of the cerebral region where the first synapse is. The names of the motor descending tracts begin with a prefix that points out the cerebral region where the fibers are originated, and ends with the suffix -spinal.

For example: The corticospinal tract begins in the cerebral cortex and descends along the spinal cord; or the anterior spinothalamic tract carries impulses to the thalamus.

References:

Silva N.A., Sousa N., Reis R.L., Salgado A.J. (2014). From basics to clinical: A comprehensive review on spinal cord injury. Prog Neurobiol. 2014 Mar;114:25-57.

Brüel A., Christensen E.I., Tranum-Jensen J., Qvortrup K., Geneser, F. (2015). Geneser Histología. 4th edition. Ed. Médica Panamericana.