Hypothalamospinal tract

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The hypothalamospinal tract is a mostly unmyelinated nerve pathway that starts in the hypothalamus and runs down to the brainstem and spinal cord to control preganglionic autonomic neurons (both sympathetic and parasympathetic). It provides direct autonomic output, though most hypothalamic signals go through other relay areas. The tract mainly comes from the paraventricular nucleus, with smaller inputs from other hypothalamic regions, and it also receives pain-related input from the spinohypothalamic tract. It descends through the periaqueductal gray and the dorsal longitudinal fasciculus, then travels in the lateral tegmentum of the brainstem and down the spinal cord in the dorsolateral part of the lateral funiculus. In the brainstem it connects with preganglionic neurons, and in the spinal cord it reaches T1–L3 for sympathetic output and S2–S4 for parasympathetic output. From T1 it synapses with second-order neurons that go to the superior cervical ganglion and then to the eye and face (eyelids, pupil, skin). Some fibers also reach the ciliospinal center to help regulate pupil dilation. Some axons carry oxytocin. If this tract is damaged above T1, it can cause Horner’s syndrome on the same side, featuring ptosis (droopy eyelid), a small pupil (miosis), and lack of sweating on the face (anhidrosis).