SCI Primer/Spinal Tracts










Spinal Cord
Motions and Functions Available at Different Levels of Spinal Cord Injury
The spinal cord lies within the spinal cavity, consisting of the vertebral column, the meninges, spinal nerves, spinal fluid, blood vessels, and a cushion of adipose/fat tissue. The spinal cord has two general functions: 1) It provides the two-way conduction routes to/from (afferent/efferent) the brain; and 2) it serves as the reflex center for all spinal reflexes. Thirty-one pairs of nerves connect to the spinal cord. They have no special names, but are numbered according to the level of the spinal column from which they emerge. There are 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), and 1 coccygeal pairs of spinal nerves.  
The figure below shows a cross-section of the spinal cord at a typical level, illustrating the position of the horns, where the nerves synapse, and the three main tracts:
Spinothalamic tract, carrying pain and temperature impulses to the thalamus and the parietal lobe
Posterior columns, carrying position and pressure sense to the thalamus and the parietal lobe
Corticospinal (or pyrimidal) tract, carrying impulses to initiate muscle activity from the frontal lobe.
Each nerve has two roots connecting to the spinal cord: The posterior (dorsal) is the sensory root; the anterior (ventral) is the motor root. Sensory receptors send information to the spinal cord (and thus to the brain) through the spinal nerve. The cell bodies for these nerve fibers are located in the dorsal root ganglion and enter the spinal cord through the dorsal/posterior root. Some fibers synapse with other neurons in the dorsal/posterior horn, while others continue up to the brain. The cell bodies in the ventral/anterior horn send axons through the ventral/anterior root to muscles to control movement.  
Reflexes
A reflex is an automatic, i.e."involuntary," response of the body to some stimulus, e.g. pupil dilation, to collect more light, at night. Reflexes have afferent (signals to the CNS) and efferent (signals from the CNS) components and may involve one nerve/synapse (monosynaptic reflex) or multiple synapses (polysynaptic reflex). The simplest spinal reflex is the tendon reflex, a monosynaptic reflex. When the tendon is tapped, the sensory receptor in the muscle sends a signal to the spinal cord (afferent system), where it enters through the dorsal/posterior root, then synapses with a sensory nerve axon in the ventral/anterior horn. From there, the receiving axon carries the impulse through the ventral/anterior root back to the muscle (efferent system) whose tendon was tapped. The muscle jerks/ contracts. 
Spinal/Peripheral Nerves
All cervial spinal nerves control movement of the shoulder and arm muscles and receive all sensory input from the skin, muscles, bones, and joints. After leaving the vertebral canal, they merge to form peripheral nerves. The merger of the spinal nerves in the neck region is called the brachial plexis.  
Main Action of the Brachial Plexus Nerves
Modified from: Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education, Rehabilitation Services Administration, 1981, p. 27.
Peripheral Nerve
Action
Axillary
Flex and abduct shoulder
Musculocutaneous
Flex elbow
Radial
Extend elbow, wrist, and fingers
Median
Flex wrist and fingers
Ulnar
Dexterity of fingers
The thoracic spinal nerves do not recombine into plexuses. The anterior roots control the muscles of the abdomen and back; the posterior roots receive information from the abdomen, skin, and muscles and joints of the chest and upper back. The lumbar and sacral nerves, like the cervical, do combine after leaving the vertebral canal. This merger, occuring in the pelvis, is called the lumbosacral plexus, which controls the movement of, and receives information from, the lower extremities, bladder, and bowel.
Main Action of the Lumbosacral Plexus Nerves
Modified from: Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education,  
Rehabilitation Services Administration, 1981, p. 28.
Peripheral Nerve
Action
Femoral
Extend knee