lateral horn

**Anatomy:** In a transverse spinal cord section, gray matter is divided into dorsal (posterior) horn (sensory), ventral (anterior) horn (somatic motor), and, only at certain levels, a lateral horn. The lateral horn is the intermediolateral cell column (IML), located between the dorsal and ventral horns, and exists at T1–L2 (some sources T1–L3). It is the origin of the thoracolumbar (sympathetic) outflow. Sacral levels S2–S4 contain the parasympathetic nucleus (intermediolateral equivalent), but this does not form a true lateral horn bulge.

**Function and pathway:** Cell bodies of preganglionic sympathetic neurons (cholinergic, myelinated B fibers) reside here. Their axons exit via the ventral root, travel through the spinal nerve and white rami communicantes, and enter the sympathetic chain, where they: (1) synapse at the same level, (2) ascend or descend and synapse at another level, or (3) pass through without synapsing as splanchnic nerves (greater T5–T9, lesser T10–T11, least T12; lumbar splanchnics) to synapse in prevertebral ganglia (celiac, superior and inferior mesenteric, aorticorenal). Preganglionic fibers to the adrenal medulla also originate here and synapse directly on chromaffin cells (a modified postganglionic neuron), releasing mainly epinephrine. Preganglionic neurotransmitter is ACh acting on nicotinic receptors at ganglia. Postganglionic fibers are mostly norepinephrine-releasing on adrenergic receptors (exceptions: sweat glands via ACh on muscarinic receptors, and renal vasculature via dopamine).

**Descending control:** Hypothalamic and brainstem (e.g., rostral ventrolateral medulla) pathways descend through the lateral funiculus (and reticulospinal pathways) to synapse on lateral horn neurons. The ciliospinal center of Budge (C8–T2) controls pupillary dilation.

**Clinical significance:** - *Horner syndrome* (ptosis, miosis, anhidrosis, ± enophthalmos): interruption of the oculosympathetic pathway (first-order: hypothalamus to the lateral horn; second-order: lateral horn T1 to the superior cervical ganglion, e.g., Pancoast tumor, T1 root injury, Klumpke palsy; third-order: superior cervical ganglion up the carotid, e.g., carotid dissection, cavernous sinus lesions). Lateral medullary (Wallenberg) syndrome also produces Horner via descending fibers. - *Syringomyelia*: central cavity expansion (classically C8–T1) causes a cape-like bilateral loss of pain and temperature sensation with sparing of light touch/vibration, and may damage the lateral horn/ventral horn, producing Horner syndrome and hand weakness. - *Spinal cord injury above T6*: loss of supraspinal control of lateral horn neurons causes autonomic dysreflexia (noxious stimulus below the lesion causes uncontrolled sympathetic discharge, severe hypertension, with reflex bradycardia and flushing/sweating above the lesion, and headache). Spinal shock can cause neurogenic shock (hypotension, bradycardia). - *Poliomyelitis* and *ALS* primarily affect the ventral horn, not the lateral horn; *Tabes dorsalis* affects the dorsal columns. - Cord segment levels with a lateral horn are tested alongside the autonomic nervous system "thoracolumbar = sympathetic, craniosacral = parasympathetic."

**High-yield mnemonic:** "T1–L2: the lateral horn is the sympathetic horn" — "Sympathetic = Spinal lateral horn (thoracolumbar)." "S2, 3, 4 keep the poop off the floor" for sacral parasympathetic.

**Note:** In neuroscience of Drosophila, "lateral horn" also denotes a brain olfactory center, but for USMLE it refers to the spinal cord structure.