Ganglion

Original Editor - lucinda hampton

Top Contributors - Lucinda hampton, Uchechukwu Chukwuemeka and Vidya Acharya  

Introduction

A ganglion is a collection of neuronal bodies found in the somatic and autonomic branches of the peripheral nervous system (PNS).

Ganglia can be thought of as synaptic relay stations between neurons. The information enters the ganglia, excites the neuron in the ganglia and then exits.

Among vertebrate animals there are three major groups of ganglia. These include:

1. Cranial nerve ganglia that contain the neurons of the cranial nerves
2. Dorsal root ganglia or spinal ganglia where the cell bodies of sensory or afferent nerves are located
3. Autonomic ganglia, which contain the cell bodies of the autonomic nervous system. In this system, nerve fibers that run from the central nervous system to the ganglia are called the preganglionic fibers. The nerve fibers originating in the ganglia that connect to effector organs are termed postganglionic fibers.

Image 1: Shows the Autonomic Ganglia (red SNS, blue PNS)

In addition to the ganglion of the peripheral nervous system, there are also parts of the brain that contain a clusters of interconnected nuceli. These reside in the cerebral cortex, thalamus and brainstem and basal ganglia and are associated with several key functions including motor control, emotions, cognition and learning.

Structure

Ganglia are oval in structure and contain neuronal cell bodies (somata), satellite cells (surround neurons in the sensory, sympathetic and parasympathetic ganglia and help regulate the chemical environment, a type of glial cell), and a protective connective tissue layer. Autonomic and sensory ganglia are histologically similar, with the former containing multipolar neurons, and the latter usually containing unipolar or pseudo unipolar neurons.

A dense connective tissue capsule covers the ganglion, with a single layer of flat shaped satellite cells surrounding each neuronal cell body. A basement membrane covers the outer region of the satellite cells.

A spinal ganglion (dorsal root ganglion) is a cluster of nerve bodies positioned along the spinal cord at the dorsal and ventral roots of a spinal nerve. The dorsal root ganglia contain the cell bodies of afferent nerve fibres (those carrying impulses toward the central nervous system); efferent neurons (carrying motor impulses away from the central nervous system) are present in the ventral root ganglia.

Cranial Nerve Ganglion is analogous to the dorsal root ganglion, except that it is associated with a cranial nerve, instead of a spinal nerve (associated with the spinal cord). The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the trigeminal ganglion is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. Like the sensory neurons associated with the spinal cord, the sensory neurons of cranial nerve ganglia are unipolar in shape with associated satellite cells.

Autonomic ganglia belong to the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The sympathetic chain ganglia constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. At the superior end of the chain ganglia are three paravertebral ganglia in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the prevertebral ganglia, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms.

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