Neurons form Gray Matter

• HORN ORGANIZATION- Various sized neurons in combination with glia form the cytoarchitectonic patterns of the spinal cord.

• The ventral horn contains motor neurons that project to somite muscles (GSE) of the trunk or limbs. This horn has a somatotopic organization that follows medial to lateral with more distal muscles of the extremities being located most lateral and the most medial in the ventral horn projecting to the vertebral column musculature.

• The lateral horn emerges just lateral to the central canal and contains the preganglionic sympathetic motoneurons to smooth and cardiac muscle and glands. These neurons project axons to a chain of second level sympathetic ganglia positioned alongside of the vertebral column or collateral ganglia in the abdominal cavity. The targets are blood vessels, sweat glands, bronchials, cardiac muscle, iris dilator, bladder and other visceral organs.

• Neurons of the dorsal horn receive inputs from the dorsal root ganglion cells (DRG neurons) that lie outside the spinal cord. These central projections of DRG neurons enter the spinal cord along a line just over the dorsal horn. DRG’s all lie in the inter-vertebral foramen at respective segmental levels of their exiting nerve. The largest of the dorsal root ganglion cells project directly to motoneurons as stretch receptor inputs for nerve-muscle reflexes. Medium size DRG neurons connect to the nucleus proprius for other spinal cord reflexes and ascending pathways. The pain path has dorsal roots from small DRG’s with small axons that synapse on dendrites formed by combination of substantia gelatinosa and nucleus proprius neurons. An unconscious proprioceptive path from the nucleus dorsalis (Clarkes column) sends their axons to the cerebellum carrying unconscious proprioceptive signals for muscle control.

• Finally, DRG neurons connect visceral and somatic sensory inputs to the autonomic system where they synapse on intermediate neurons of the dorsal horn. These serve to operate autonomic reflexes targeting viscera and peripheral vasculature, sweat glands and hair follicle erection muscle.