A ganglion is a cluster of nerve cell bodies in the peripheral nervous system, which includes all nerves outside of the brain and spinal cord. These structures act as relay stations where nerve signals are transferred between neurons. Ganglia are composed of the cell bodies of neurons, called somata, and their associated dendritic structures, all encapsulated by a protective layer of connective tissue.
Sensory Ganglia
Sensory ganglia are specialized clusters of neurons dedicated to transmitting sensory information from the body’s periphery toward the central nervous system. The most prominent examples are the dorsal root ganglia, which are associated with the spinal nerves. Each of the 31 spinal nerves has a dorsal root ganglion, appearing as a small swelling on the nerve root just as it exits the intervertebral foramina—the small openings between your vertebrae. The neurons within dorsal root ganglia are responsible for detecting stimuli like touch, pressure, vibration, temperature, and pain from the skin, muscles, and joints.
When you touch a hot surface, for instance, sensory receptors in your skin generate a signal that travels to the neuron’s cell body inside the dorsal root ganglion. From there, the signal is relayed into the spinal cord and ultimately to the brain for interpretation. These ganglia contain unipolar neurons, a specific type of nerve cell where the cell body has a single process that splits into two branches. One branch extends out to the periphery to detect sensory information, and the other extends into the central nervous system to transmit it. Supporting these neurons are satellite glial cells, which help regulate the chemical environment and are thought to play a role in modulating pain signals.
Autonomic Ganglia
The autonomic nervous system manages involuntary processes like heart rate, digestion, and respiratory rate. Its ganglia are part of this regulation and are divided into two opposing categories: sympathetic and parasympathetic. Nerve fibers running from the central nervous system to these ganglia are preganglionic, while fibers from the ganglia to the target organ are postganglionic.
Sympathetic Ganglia
Sympathetic ganglia are responsible for the body’s “fight-or-flight” response, preparing it for high-alert situations. Most of these ganglia are located in a chain-like structure called the sympathetic trunk, which runs parallel to the spinal cord on both sides. When the body perceives a threat, preganglionic neurons from the spinal cord activate neurons within these ganglia. These postganglionic neurons then trigger widespread physiological changes, such as increasing heart rate, redirecting blood flow to muscles, and slowing down digestion.
Parasympathetic Ganglia
In contrast, parasympathetic ganglia manage “rest-and-digest” functions that conserve the body’s energy. These ganglia are located very close to or within the walls of the organs they innervate, allowing for specific control. For example, after a meal, parasympathetic ganglia near the stomach and intestines receive signals that activate neurons to increase digestive secretions and muscle contractions, aiding food breakdown and absorption.
The Basal Ganglia Misnomer
The term “basal ganglia” often causes confusion because, despite their name, these structures are not true ganglia. The basal ganglia are a group of interconnected nuclei found deep within the cerebral hemispheres of the brain, making them a part of the central nervous system. This naming is a historical artifact that has persisted, though some anatomists advocate for the more accurate term “basal nuclei.”
The function of the basal ganglia is also distinct from that of peripheral ganglia. Instead of relaying sensory or autonomic signals, they play a part in motor control, helping to initiate and smooth out voluntary muscle movements. They are also involved in more complex functions such as learning, habit formation, and emotional processing. The basal ganglia work in concert with the cerebral cortex, thalamus, and brainstem to regulate movement and behavior.
Associated Medical Conditions
A common related issue is a ganglion cyst, a benign, fluid-filled lump that appears near joints, often on the wrists or hands. Despite the name, these cysts are not related to the neural structures of ganglia. They are harmless sacs of synovial fluid that have leaked from a joint, though they can cause pain if they press on a nearby nerve.
In contrast, some conditions directly involve the nerve ganglia. The virus that causes chickenpox, varicella-zoster, can lie dormant for decades within the cell bodies of dorsal root ganglia. If the virus reactivates later in life, it causes shingles, a condition characterized by a painful, blistering rash that follows the path of the nerve associated with the infected ganglion. The pain from shingles can sometimes persist long after the rash has cleared, a complication known as postherpetic neuralgia.
Less common are tumors that arise from ganglionic cells. A ganglioneuroma is a benign tumor that grows from the nerve cells of the autonomic ganglia, often found in the chest, abdomen, or along the sympathetic chain. While non-cancerous, these tumors can sometimes grow large enough to compress surrounding structures, leading to symptoms that require medical intervention.