Craig C. Freudenrich
Some might consider the spinal cord as a mere conduit that carries nerve impulses from the body to the brain. Yes, it relays information between the body and the brain, but it also processes information.
The spinal cord is a projection of the brain stem that extends down through the spine's vertebrae. Like the brain, the spinal cord is bathed in cerebrospinal fluid and protected by tough coverings called meninges. Butterfly-shaped gray matter sits inside each segment of the spinal cord. The gray matter contains the cell bodies of motor neurons and connecting interneurons. Surrounding the gray matter is the white matter, which contains the white, myelin-covered axons of sensory neurons and matter neurons. The white matter is organized into several nerve highways called tracts. In the various tracts, axons either run to the brain or descend from the brain.
Axons of sensory neurons enter from the top of dorsal roots into the dorsal horn of the gray matter; the cell bodies of these sensory neurons lie within the vertebrae in the dorsal ganglia. Axons of motor neurons leave the spinal cord through the bottom, or ventral, roots. Finally, a collection of nerves of the sympathetic nervous system runs within the vertebrae below and parallel to the cord.
All of this intricate anatomy works well in concert so that when you touch a hot pot, you automatically pull your hand away without even thinking about it. This is because your spinal cord has "made the decision" in a simple reflex pathway. The hot pot stimulates a sensory neuron. The sensory neuron's axon is wired through the gray matter to other neurons: First, it contacts a motor neuron. The motor neuron sends an impulse out of the spinal cord to a muscle in your hand or arm. The muscle contracts and pulls your hand away. The sensory neuron also may send a signal through an interneuron to inhibit the motor neuron of the opposing muscle (muscles usually work in pairs). Finally, it sends a signal through the tracts to inform the brain that something was going on, but the brain was not involved in the reaction.
The reflex pathway processes information quickly. Spinal reflex pathways can be inhibited by descending impulses from the brain. In this way, the spinal cord processes and relays information.
The spinal cord reaches through the openings in the vertebrae of your back. It contains gray matter (nerve cell bodies) and white matter (axons or nerve processes) running to and from the brain and out toward the body. Within these vertebrae, the nerves each separate into dorsal roots, which are sensory nerve cell processes and cell bodies, and ventral roots, which are motor nerve cell processes. The autonomic nerve cell bodies lie along a chain that is inside the vertebrae and runs parallel to the spinal cord. The axons exit via the spinal nerve sheaths.
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