Intro | Amygdala | Brainstem | Cerebellum | Cerebrum | Corpus Callosum | Reticular Formation | Hippocampus | Hypothalamus | Medulla | Pituitary Gland | Pons | Spinal Cord | Thalamus
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Return to main tutorial page
The Spinal Cord lies within the vertebral column or backbone, beginning just outside the skull and extending down to just below waist level. This structure connects higher centers of the central nervous system with the rest of the body via the somatic and autonomic nerves of the peripheral nervous system. These two divisions of the peripheral nervous system convey both sensory information from all receptors to the brain (afferent nerve fibers) and motor control information from the brain to muscles of the body and internal organs (efferent nerve fibers). Damage the axon tracts found in the spinal cord will usually result in serious sensory and/or motor dysfunction (sensory loss and paralysis). In addition, the simple motor reflexes are supported by the spinal cord. These reflexes are built into the nervous system and do not need conscious thought to take effect. The knee jerk is an example of such a reflex. When the knee is tapped, the sensory nerve conveys an impulse to the spinal cord, where it is relayed to the motor nerve. The quadriceps muscle at the front of the thigh contracts and the leg jerks upward. The "hot stove" reflex is somewhat more complex, but still restricted to spinal cord control.
The spinal cord is highly organized into 31 segments, each with one sensory and one motor nerve on either side. Sensory spinal nerves enter the spinal cord on the back (dorsal) side, whereas motor spinal nerves exit the spinal cord on the front (ventral) side. The cell bodies of sensory nerves lie outside the spinal cord within the dorsal root ganglia, whereas the cell bodies of motor nerves lie within the spinal cord. Starting at the top of the spinal cord there are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. Each motor nerve innervates a particular region of the body. The regions innervated by sensory nerves are called dermatomes. For example, the sensory nerve entering the spinal cord at the 4th cervical level conveys touch information from a dermatome consisting of the top of the shoulders and the skin lying between on both the front and back of the trunk. The motor nerves emerging from the 4th cervical level innervate the muscles found in this same body region. The sensory dermatomes overlap one another up to a third of their width.