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Last updated: Fri, Jul 26, 2024
The spinal cord is enclosed in a channel (or “canal”) made up of the bony arches of the spinal vertebral bones. There is a gap in the channel between each pair of vertebrae, and a pair of nerves emerges through each gap, one on each side of the body. These are the spinal nerves. They carry both afferent (to the brain) and efferent (from the brain) signals. They also carry both somatic nerves and autonomic nerves.
Each pair of spinal nerves is named based on the names of the vertebrae it emerges from. The vertebrae of the neck are named C1 through C7. (“C” for “cervical.”) C1 is the first vertebra under the skull. The next twelve vertebrae are named T1 through T12. (“T” stands for “thoracic,” the chest.) These are the vertebrae that have attached ribs. The vertebrae of the lower back are L1 through L5. (“L” is for “lumbar.”) Below the lumbar vertebrae sits the sacrum, which in most people is fused from five sacral vertebrae named S1 through S5. Although the sacral vertebrae are fused, the sacrum contains openings for spinal nerves. Attached to the bottom of the sacrum is the coccyx or tailbone.
The spinal nerves that emerge from the cervical vertebrae are named for the vertebrae that follow them. This allows for spinal nerves C1-C7. Following the C7 vertebra is a spinal nerve named C8 that doesn''t have a matching vertebra. From that point down, each spinal nerve is named for the vertebra just above it. The very last spinal nerves are called the coccygeal nerves.
Somatic nerves travel from the spine into the body (the “soma”). Each of these major nerves contains the fibers of a large number of neurons. The fibers are enclosed in a membrane that separates them from the tissue through which they pass. The nerves branch repeatedly until the fiber of a single neuron emerges. This fiber makes contact with (or comes close to) the tissue that is its target.
Or, from the point of view of an afferent neuron (one that sends its signals towards the brain), individual fibers join together until they make up a large nerve that joins the spinal cord.
Somatic nerves can be classed as cutaneous nerves (those that innervate the skin), deep somatic nerves (those that innervate muscles, bones, tendons, and joints), and visceral nerves (those that innervate the viscera, the organs contained in the chest and abdomen such as the heart, lungs, and digestive organs).
The autonomic nervous system (ANS) is a part of the nervous system that has different functions and whose nerves are routed in a different way than the somatic nerves. The autonomic system includes two subsystems, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). These systems are regulated by the brain in more or less the same way as are the somatic nerves.
Fibers for the sympathetic nervous system leave the spinal cord beginning with spinal nerve T1 (below the first thoracic vertebra) through to spinal nerve L1 (below the first lumbar vertebra). The sympathetic fibers separate from the somatic fibers and travel through a separate chain of sympathetic nerves and nodes or ganglia that run along the front of the spinal column, outside of the spinal channel. Sympathetic fibers that are destined for the viscera (internal organs) continue through the sympathetic nerves to their target organs. Sympathetic fibers that are destined for hair follicles, sweat glands, and blood vessels rejoin the spinal nerves and travel to their targets alongside the somatic fibers.
Most fibers for the parasympathetic nervous system leave the brain stem through one of the cranial nerves and travel through the chest and abdomen in a single large nerve, the vagus nerve. These fibers innervate most of the organs of the chest and abdomen. Additional parasympathetic nerves leave the spinal column at the sacrum. These latter nerves serve the lower large intestines, the bladder, and the genitals.
Almost all of your somatic nerves pass through your spinal cord. It contains roughly 100 million neurons—only one for every thousand neurons in the brain, but still a large number. If each of those neurons formed just 200 synapses, there would still be 10 billion connections in your spine.
You may be mentally comparing these numbers with the specs for your computer or smartphone. The analogy between your nervous system and a digital computer, however, is not very close. Modern personal computers have a few central processors and can do a few computations at once. On the other hand, a large share of the connections in your nervous system are active at any given time. Besides this, a synapse in your nervous system is "smarter" than a binary switch in a computer because it isn't a simple on/off switch.
The spine is more than a big pipe carrying information to and from the brain. Much of your basic motion learning takes place in your spine. There is no program in your brain for the basic motion pattern of walking—that's in your spine. Your brain tells your spine when to walk and where to walk. It adjusts your walk to circumstances and fine tunes the walking motions, but the basic program is in your spine. Your spine is the part of your central nervous system that learns other basic motor skills, like typing or playing a musical instrument. Also in your spine are some basic and important nervous reflexes, including the ones that your doctor tests. These are described later in this section (Spinal Reflexes).
The spine also performs an important function in organizing sensory input from the body, which is the major story of this section. You saw in the last section that your body is encased in and suffused with millions of sensory receptors. The signals from these receptors must be organized: signals from overlapping receptive fields must be made sense of; stimulation must be localized; important signals must be highlighted and less important ones suppressed; edges must be detected; and so on.
The plan of this section mimics that of the last. We'll start by looking at how neurons can combine to make circuits. Next, we'll look at how the spinal cord is laid out and some of the circuits that are known to be at work there. After that, we'll see how this increased knowledge of the cord can explain more pain phenomena. Finally, we'll look at how the spinal cord functions to regulate sensitivity to pain. This is important to an understanding of chronic pain, which is the topic of a following chapter.
Within this section...
Segmental Organization of the Spinal Cord (Last updated: Fri, Jul 26, 2024)
Layers (Laminae) of the Dorsal Horn (Last updated: Sun, Jun 18, 2017)
Afferent Terminations in the Spine (Last updated: Wed, Jun 21, 2017)
Interneurons (Last updated: Wed, Jun 21, 2017)
Ascending Projection Neurons (Last updated: Wed, Jun 21, 2017)
Descending Projection Neurons (Last updated: Fri, Jul 26, 2024)
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Pain Processing in the Spine (Last updated: Wed, Jun 21, 2017)