The spinal cord is a bundle of nerve tissue that extends from the base of the brain to the lower back. Together with the brain, it forms the Central Nervous System, or CNS, and connects the brain with the Peripheral Nervous System, or PNS, which is the rest of the nerve tissue in the body. The transmission of information by the spinal cord to and from the brain governs all our conscious control of movement and our awareness of sensation. The spine also governs spinal reflexes, or fast automatic responses, which do not involve the brain, such as the withdrawal reflex to when we touch something hot, or the tendon reflex when the knee is tapped sharply.
The spine is enclosed and protected by the spinal column (also known as the backbone), a stack of twenty-four connected pieces of bone called vertebrae, and by three membranes called the dura mater, the arachnoid, and the pia mater, known collectively as the meninges.
The nerves that make up the spinal cord consist of thousands of bundles of special cells called neurons. A single neuron looks something like the image at left (courtesy of NASAexplores).
The dendrites are small fibers that extend from the body of the neuron. They receive information from the surrounding area and transmit the information to the cell body. The axon is another fiber, usually larger and longer than the dendrites, and is responsible for sending chemical messages from the cell to the surrounding area. In many neurons a white fatty substance called myelin surrounds the axon is surrounded by wand hich improves the speed at which the neuron can conduct information. Myelination, the surrounding of the axon with myelin, is done by two different types of cells: oligodendrocytes myelinate axons in the CNS, and Schwann cells myelinate axons in the PNS.
Electrical or chemical stimulation of a single neuron can cause the neuron to release messenger chemicals called neurotransmitters from the axon terminal. This neurotransmitter release is called a synapse. The released neurotransmitters may then attach to a receptor on another neuron, altering the way that neuron functions and creating communication between the releasing (or presynaptic) neuron and the receiving (or postsynaptic) neuron. The postsynaptic neuron may then stimulate another neuron, creating a chain of communication. A neuron can also synapse on a muscle, where the release of a specific neurotransmitter, acetylcholine, will cause the muscle fibers to contract, moving the muscle. Synapses between neurons are responsible for all the communication in the nervous system. A synapse is represented in image at below, from the Columbia University Psychology Department webpage.
As stated above, the primary job of the spinal cord, other than the execution of spinal reflexes, is to transmit information between the brain and the rest of the body. Different types of nerves transmit different types of information. Motor nerves bring the ´commands═ of the brain to the muscles, releasing acetylcholine and causing the muscles to contract. Somatosensory nerves gather sensory information from the body and conduct it to the brain. The activity of these nerves allows us to feel what is going on around us, such as the temperature of things we are touching, or the pressure of things against our skin.
The activity of nerves in a given part of the body is called innervation. Different areas of the spinal cord innervated different areas of the body. The spinal cord is divided up into four major regions of innervation: cervical, thoracic, lumbar, and sacral. These regions are further divided up and identified by numbers. Each carries a tag with a number and a letter (C, T, L, or S) standing for one of the regions. The diagram at right, courtesy of The Why Files, shows the regions and their tags, and names the specific areas innervated by the motor nerves. The same tags are also used to name the dermatomes, the specific areas of skin in which information is collected by a given region of the spinal cord. The dermatomes are outlined in the diagram at left, from the CHI online index.
Therefore, region C8, for example, receives sensory information and transmits motor information for the outer edge of the hand and arm.
The spinal cord is arranged as a butterfly-shaped area of gray matter (composed of the cell bodies of neurons) surrounded by white matter (composed of the axons of neurons; it appears white because of the myelin surrounding the axons).
In the PNS, the two different types of neurons (motor and sensory) run together in bundles called mixed nerves. As the mixed nerve nears the spine, it separates into its motor and sensory nerve components. The axons of the sensory neurons extend to the dorsal root ganglion, where the cell bodies of all sensory neurons are located. From the dorsal root ganglion, the axon of the sensory neuron crosses over to the opposite side of the spinal cord, where it synapses at the dorsal horn (the area of gray matter which is closer to the back). If the sensory information relates to temperature or pain, it travels to the brain via the spinothalamic tract; if the information relates to other types of sensation, such as vibration or proprioception (awareness of body position), it goes to be brain by the dorsal columns. Motor neurons travel in the opposite direction, carrying commands via the corticospinal tract from the brain to the ventral horn of the spinal cord (the area of gray matter closer to the stomach). Here the tract synapses with a motor nerve, which carries the motor command to the correct region of the body.
Below is a cross section diagram of the spinal cord, from CHI online index with labels of the parts mentioned above.
Nerves in the PNS are able to regenerate; generally, when we injure a limb or another part of our body, we regain all or most of the sensation and motor control of that area (so long as the damage is not too extensive). Neurons in the CNS, which includes those in the spinal cord, cannot regenerate. (The section on Regeneration covers neuronal growth.) Although the spinal column and the meninges can protect the spinal cord from many possible sources of damage, the spinal cord can still suffer physical damage. If the spinal cord is damaged, the area of the body innverated by the injured region can no longer exchange information with the brain, and thus injury to it is a serious problem.