The superior cervical ganglion (SCG) is a nerve bundle within the autonomic nervous system. Specifically, the SCG belongs to the sympathetic nervous system, a division responsible for the body’s “fight or flight” response to dangerous or stressful situations.
Anatomical Location and Structure
The superior cervical ganglion is the largest and most superior ganglion of the sympathetic chain on each side of the neck. It is formed from the fusion of the top four cervical sympathetic ganglia (C1-C4). This elongated, spindle-shaped structure is typically about 1 to 3 centimeters in length. It is situated deep within the neck at the level of the second and third cervical vertebrae (C2-C3).
This location places it behind the major carotid artery and in front of the longus capitis muscle. Nerves originating from the thoracic region of the spinal cord ascend to this ganglion, where they transmit signals that will be distributed throughout the head and neck.
Key Functions of the Ganglion
Innervation of the Head and Neck
The superior cervical ganglion is the sole ganglion in the sympathetic nervous system that provides innervation to the head and neck. From the ganglion, postganglionic fibers emerge and travel along major blood vessels, such as the internal and external carotid arteries, forming complex networks called plexuses. This distribution method allows the sympathetic signals to reach many target structures throughout the facial region, cranial cavity, and neck.
Eye Control
The SCG plays a direct role in controlling specific muscles within the eye. It is responsible for dilating the pupil, a response known as mydriasis, which allows more light to enter the eye during a “fight or flight” situation. Nerve fibers from the ganglion achieve this by activating the dilator pupillae muscle in the iris. Additionally, these nerves innervate the superior tarsal muscle, which assists in raising the upper eyelid.
Glandular Regulation
Sympathetic signals from the superior cervical ganglion also regulate the activity of various glands in the head. It stimulates the sweat glands on the face and scalp, leading to perspiration. Concurrently, it inhibits the function of the salivary glands and the lacrimal glands, which produce tears. This reduction in saliva and tear production conserves resources during high-stress events.
Blood Vessel Constriction
Another function of the ganglion is the constriction of blood vessels, a process called vasoconstriction. Nerves extending from the SCG target the smooth muscle within the walls of arteries in the skin of the face. This action narrows the vessels, reducing blood flow to the skin. These nerves also extend to blood vessels within the brain, helping to regulate cerebral blood flow.
Pineal Gland and Heart Rate
The influence of the SCG extends to deeper structures, including the pineal gland. This small gland in the brain is responsible for producing melatonin, a hormone that regulates sleep-wake cycles. Sympathetic innervation from the ganglion can affect its output. The ganglion also sends fibers downward to connect with the cardiac plexus, a network of nerves near the heart, where it contributes to an increase in heart rate.
Associated Medical Conditions
When the superior cervical ganglion or the sympathetic nerve pathway connected to it is damaged, it can lead to Horner’s syndrome. This syndrome is not a disease but a collection of signs that point to an underlying medical issue from the disruption of sympathetic nerve signals. The most common signs of Horner’s syndrome include ptosis, which is a mild drooping of the upper eyelid, and miosis, a constricted or smaller pupil. A third sign is anhidrosis, which is a decrease or absence of sweating on the affected side of the face. The affected eye may also appear slightly sunken.
Damage to the nerve pathway can occur for various reasons. Physical trauma to the neck, a stroke affecting the brainstem, or a tumor can all cause this condition. One cause is a Pancoast tumor, a type of lung cancer at the top of the lung that can compress the nearby sympathetic nerves. Surgical procedures in the neck or chest area can also inadvertently lead to nerve damage and the subsequent development of Horner’s syndrome.
The Superior Cervical Ganglion Block Procedure
A superior cervical ganglion block is a medical procedure involving the injection of a local anesthetic near the SCG to temporarily interrupt its signal flow. This is a therapeutic intervention for specific medical conditions. By numbing the ganglion, the block can help modulate an overactive sympathetic nervous system response.
This procedure has been explored as a treatment for Post-Traumatic Stress Disorder (PTSD), where the “fight or flight” response can become chronically overactive. An SCG block is thought to help “reset” this system by turning off nerve traffic. This provides relief from symptoms such as hypervigilance and anxiety.
The block is also used to manage certain chronic pain syndromes that affect the head, face, and neck, such as Complex Regional Pain Syndrome (CRPS). For these pain disorders, interrupting the sympathetic nerve signals can reduce pain and associated symptoms like abnormal swelling or skin temperature changes. The procedure is performed using ultrasound or fluoroscopic guidance to ensure the anesthetic is delivered precisely to the target area.