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The spinal cord is the communication gateway between the brain and spinal nerves, which innervate the trunk and limbs. The cord is a long, thin tube of nervous tissue, enclosed in 3 membranes of the meninges which, in turn, are protected within the bones of the vertebral column. The 31 pairs of spinal nerves arise from the cord and emerge from the vertebrae. The spinal cord extends from the brainstem to the level of upper lumbar vertebrae. In the lower lumbar and sacral regions, nerve roots descend within the spinal canal before exiting, forming the cauda equina.
In cross section, two types of nervous tissue can be seen in the cord: a butterfly-shaped central core of gray matter, and a surrounding white matter. The gray matter contains cell bodies and dendrites of neurons. This is where neurons synapse and transmit information to each other. The white matter, on the other hand, is made of bundles of axons, and serves to conduct information up and down the cord. These bundles are organized into specific groups with specific functions, forming the so-called spinal tracts. Spinal tracts are essentially high-speed cables, each carries a certain type of information, in a one-way traffic, between the spinal cord and a certain area in the brain. All tracts occur on both sides, left and right, of the cord. Ascending tracts conduct sensory information up to the brain, while descending tracts convey motor instructions down the cord. Some tracts cross over to the other side of the cord, before they reach the brain. They convey sensory information from one side of the body to the other side of the brain. When this happens, the information is said to be transmitted contralaterally. Tracts that stay on the same side all the way are said to conduct information ipsilaterally.
Spinal nerves are mixed nerves, they contain both sensory and motor fibers. These fibers are separated shortly before they reach the spinal cord. Sensory fibers enter the cord via the dorsal root, while motor fibers exit via the ventral root.
A sensory pathway typically involves 3 neurons:
– First-order neurons detect stimuli and transmit signals to the spinal cord. The axons of these neurons form sensory fibers that enter the cord via the dorsal root of spinal nerve.
– Inside the cord, first-order neurons synapse with second-order neurons, which ascend a specific tract to the brainstem, or further up to the thalamus. In some pathways, first-order neurons ascend the tract to the brainstem, where they synapse with second-order neurons, which continue to the thalamus.
– Third-order neurons conduct the information the rest of the way to the sensory cortex.
A motor pathway usually involves 2 neurons: an upper motor neuron starts in the motor cortex or brainstem, and a lower motor neuron continues from the brainstem or spinal cord. They conduct motor instructions down, along a specific descending tract. The axons of lower motor neurons exit the cord via the ventral root of spinal nerve, where they continue as motor fibers to effector organs.
The spinal cord is also responsible for fast, involuntary responses of skeletal muscles, called somatic reflexes. Reflexes are essentially automatic and do not require input from the brain, although the brain is informed and aware, usually after-the-fact. A somatic reflex involves a reflex arc composed of a somatic receptor, a sensory neuron, an interneuron, a motor neuron, and an effector muscle. Some reflexes are however more complex, and require multiple pathways, as well as central coordination from the brain. For example, when someone steps on something sharp and lifts their injured leg, the other leg also must react to keep balance or the person would fall over. This involves multiple muscles and require contralateral pathways at several levels of the cord, as well as movement coordination from the brain.
