Spinal ganglia: anatomy, characteristics and function.

Let's see what are the spinal ganglia and what is their function in the peripheral nervous system.

The spinal ganglia are a group of nodules located in the dorsal or posterior roots of the spinal nerves, where the bodies of the neurons of the afferent or sensory pathway are housed. bodies of the neurons of the afferent or sensory pathway of the peripheral nervous system are housed..

In this article we will explain what are the spinal ganglia and their relationship with each of the parts of the peripheral nervous system.

The peripheral nervous system

The peripheral nervous system (PNS) includes the spinal nerves, cranial nerves, and their associated ganglia (groups of nerve cells outside the central nervous system [CNS]). The nerves contain nerve fibers that carry information into (afferent) or out of (efferent) the CNS.

Generally, efferent fibers are involved in motor functions, such as muscle contraction or gland secretion; and afferent fibers transmit sensory stimuli from the skin, mucous membranes and deep structures.

The main task of the PNS is to connect the various stimuli received by our body (external, internal and proprioceptive, or information about the position of one's own muscles) to the central nervous system; and the latter, in turn, connects to the organs and body systems it has to regulate and manage.

The PNS is composed of 12 pairs of cranial nerves, which exit the skull through various openings, and 32 pairs of spinal nerves, each identified by its relationship to the vertebra or vertebral canal from which it exits..

The spinal nerves

The spinal nerves extend from the spinal cord through the spinal muscles to different areas of the body.

Each of the 31 pairs of spinal nerves has a ventral and a dorsal root; each root is made up of 1 to 8 tiny rachillae or bundles of nerve fibers.Each root is made up of 1 to 8 tiny rootlets or bundles of nerve fibers. At the dorsal root of a typical spinal nerve, near the junction with the ventral root, is a dorsal or spinal root ganglion, a bulge containing nerve cell bodies.

The dorsal (or posterior) roots are primarily sensory. Each dorsal nerve root (except, in general, C1) contains afferent (sensory or receptor) fibers from the nerve cells of its ganglion. The dorsal roots contain fibers originating from cutaneous and deep structures.

The nerve fibers can be classified according to their anatomical and physiological basis into: somatic efferent fibers, which innervate skeletal muscles; and somatic afferent fibers, which transmit sensory information from the skin, joints and muscles to the central nervous system.

The cell bodies of the afferent fibers are constituted by unipolar cells (characterized by a single protruding somal extension) in the spinal ganglia, which are interposed in the course of the dorsal roots (dorsal root ganglia).

The peripheral ramifications of these ganglion cells are distributed by the somatic structures; and the central ramifications transmit sensory impulses down the dorsal roots to the dorsal cord of the gray matter and ascending tracts of the spinal cord.

The spinal ganglia

Nerve ganglia are clusters of cells that constitute small nodes located outside the central nervous system that function as relay or intermediate connections between different neurological structures of the body.

They can be divided into two types: vegetative ganglia, consisting of multipolar nerve cells located around the viscera on which they act, receive signals from the central nervous system and send them to the periphery (efferent function); and spinal ganglia or dorsal root ganglia, consisting of abundant distinct neural connections, which are responsible for receiving signals from the periphery to send them to the brain (afferent function).

The spinal ganglia collect and modulate sensory information.The spinal ganglia are functionally the reservoirs of the neuronal somas of the primary afferent fibers of the entire sensory system, having specialized in higher animals as organs located outside the central nervous system.

The group of spinal ganglia includes the spinal ganglia and the trigeminal (or Gasser's), facial (or geniculate), glossopharyngeal (extracranial or Andersch's and intracranial or Ehrenritter's) and vagus (jugular and knuckle) ganglia.

The VIII pair or statoacoustic nerve also has two ganglions, the vestibular or Scarbular.the vestibular or Scarpa's and the cochlear, spiral or Corti's, but its bipolar type neurons correspond to second order neurons of a specialized sensory pathway whose functional significance is not exactly similar to that of the general sensory or spinal ganglia.

Spinal ganglion lesions

The affectation of the spinal ganglia or dorsal roots may be due to several causes.Among the most common we can find the following:

Herpes Zoster infection

It is characterized by the appearance of localized, unilateral, vague pain, which precedes in 3 or 5 days a vesicular eruption (appearance of vesicles or blisters on the skin). It may be accompanied by systemic symptoms such as fever, fatigue or myalgias..

Vertebral tumors

They can produce, in addition to radicular lesions, other manifestations such as lumbar pain, pathological fractures, reduced mobility or deviations of the spine. There are also primary tumors (neurofibroma) and metastatic tumors, such as lymphoma or meningeal sarcomatosis, in which several roots are usually affected.

Spinal ganglia and pain transmission

The sensation of pain appears when specific nerve fibers (called "A delta" and "C") are activated. This activation can be triggered by a malfunction of the musculature and other soft parts (which is what happens in "non-specific syndromes"), or by various structural alterations that have been shown to be the cause of pain in some cases.

When stimulated, these nerve fibers activate nerve cells in the spinal cord that transmit the pain to the brain.. The nerve fibers "A delta" and "C" or capsaicin-sensitive, are thin and very numerous, and arise from the spinal ganglia, where their cell body is located, bifurcating into two extremities.

When these nerve fibers are activated, they release substances (neurotransmitters or neuromodulators) that trigger inflammation of the innervated tissues. This inflammation triggered by the release of substances contained in the nerves (rather than by substances released by cells in the Blood or tissues, as was classically understood to be inflammation) is called "neurogenic inflammation"..

This type of inflammation can induce blood cells (such as macrophages, for example) to release substances that trigger classical inflammation (such as histamine), so that both types of inflammation would reciprocally enhance each other. And in fact, the release of chemical mediators of inflammation could also increase or directly trigger pain.

Literature references:

• Carpenter, M., Sutin, J., Mascitti, T. and Lorenzo, I. (1990). Human neuroanatomy. Buenos Aires: El Ateneo.
• Navarro, X. (2002) Physiology of the autonomic nervous system. Revista Neurología, 35(6): 553-562.