The superior cervical ganglion (SCG) is part of the autonomic nervous system (ANS); more specifically, it is part of the sympathetic nervous system, a division of the ANS most commonly associated with the fight or flight response. The ANS is composed of pathways that lead to and from ganglia, groups of nerve cells. A ganglion allows a large amount of divergence in a neuronal pathway and also enables a more localized circuitry for control of the innervated targets. The SCG is the only ganglion in the sympathetic nervous system that innervates the head and neck. It is the largest and most rostral (superior) of the three cervical ganglia. The SCG innervates many organs, glands and parts of the carotid system in the head. Structure Location The SCG is located opposite the second and third cervical vertebrae. It lies deep to the sheath of the internal carotid artery and internal jugular vein, and anterior to the Longus capitis muscle. The SCG contains neurons that supply sympathetic innervation to a number of target organs within the head. The SCG also contributes to the cervical plexus. The cervical plexus is formed from a unification of the anterior divisions of the upper four cervical nerves. Each receives a gray ramus communicans from the superior cervical ganglion of the sympathetic trunk. Morphology and physiology and its neurons The superior cervical ganglion is reddish-gray color, and usually shaped like a spindle with tapering ends. Sometimes the SCG is broad and flattened, and occasionally constricted at intervals. It formed by the coalescence of four ganglia, corresponding to the upper four cervical nerves, C1-C4. The bodies of these preganglionic sympathetic neurons are specifically located in the lateral horn of the spinal cord. These preganglionic neurons then enter the SCG and synapse with the postganglionic neurons that leave the rostral end of the SCG and innervate target organs of the head. There are a number of neuron types in the SCG ranging from low threshold to high threshold neurons. The neurons with a low threshold have faster action potential firing rate, while the high threshold neurons have a slow firing rate. Another distinction between SCG neuron types is made via immunostaining. Immunostaining allows the classification of SCG neurons as either positive or negative for neuropeptide Y (NPY), which is found in a subgroup of high-threshold neurons. Low threshold, NPY-negative neurons are secretomotor neurons, innervating salivary glands. High threshold, NPY-negative