When you have completed this section, you should be able to
• name the two types of tissue in the central nervous system and state their locations;
• describe the gross and microscopic anatomy of the spinal cord; and
• name the major conduction pathways of the spinal cord and state their functions.
The spinal cord serves three principal functions:
1. Conduction. The spinal cord contains bundles of nerve fibers that conduct information up and down the cord, connecting different levels of the trunk with each other and with the brain. This enables sensory information to reach the brain, motor commands to reach the effectors, and input received at one level of the cord to affect output from another level.
2. Locomotion. Walking involves repetitive, coordinated contractions of several muscle groups in the limbs. Motor neurons in the brain initiate walking and determine its speed, distance, and direction, but the simple repetitive muscle contractions that put one foot in front of another, over and over, are coordinated by groups of neurons called central pattern generators in the cord. These neuronal circuits produce the sequence of outputs to the extensor and flexor muscles that cause alternating movements of the legs.
3. Reflexes. Reflexes are involuntary stereotyped responses to stimuli. They involve the brain, spinal cord, and peripheral nerves.
The spinal cord (fig. 13.1) is a cylinder of nervous tissue that begins at the foramen magnum and passes through the vertebral canal as far as the inferior margin of the first lumbar vertebra (L1). In adults, it averages about 1.8 cm thick and 45 cm long. Early in fetal development, the spinal cord extends for the full length of the vertebral column. However, the vertebral column grows faster than the spinal cord, so the cord extends only to L3 by the time of birth and to L1 in an adult. Thus, it occupies only the upper two-thirds of the vertebral canal; the lower one-third is described shortly. The cord gives rise to 31 pairs of spinal nerves that pass through the intervertebral foramina. Although the spinal cord is not visibly segmented, the part supplied by each pair of spinal nerves is called a segment. The cord exhibits longitudinal grooves on its ventral and dorsal sides—the ventral median fissure and dorsal median sulcus, respectively.
The spinal cord is divided into cervical, thoracic, lumbar, and sacral regions. It may seem odd that it has a sacral region when the cord itself ends well above the sacrum. These regions, however, are named for the level of the vertebral column from which the spinal nerves emerge, not for the vertebrae that contain the cord itself.
In the inferior cervical region, a cervical enlargement of the cord gives rise to nerves of the upper limbs. In the lumbosacral region, there is a similar lumbar enlargement where nerves to the pelvic region and lower limbs arise. Inferior to the lumbar enlargement, the cord tapers to a point called the medullary cone. The lumbar enlargement and medullary cone give rise to a bundle of nerve roots that occupy the canal of vertebrae L2 to S5. This bundle, named the cauda equina1 (CAW-duh ee-KWY-nah) for its resemblance to a horse's tail, innervates the pelvic organs and lower limbs.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.