Vl

Spinal cord c-

- Foramen magnum

■ Cervical / enlargement

Spinal cord

Brain stem

Brain stem

-Cervical enlargement c-

Lumbar enlargement

- Conus medullars

-i— Lumbar enlargement

- Conus medullaris

-Filum terminale

Figure

(a) The spinal cord begins at the level of the foramen magnum.

(b) Posterior view of the spinal cord with the spinal nerves removed.

tapers to a structure called the conus medullaris. From this tip, nervous tissue, including axons of both motor and sensory neurons, extends downward to become spinal nerves at the remaining lumbar and sacral levels. Originating from among them, a thin cord of connective tissue descends to the upper surface of the coccyx. This cord is called the filum terminale (fig. 11.5b). The filum terminale and the spinal nerves below the conus medullaris form a structure that resembles a horse's tail, the cauda equina.

Two grooves, a deep anterior median fissure and a shallow posterior median sulcus, extend the length of the spinal cord, dividing it into right and left halves. A cross section of the cord (fig. 11.6) reveals that it consists of white matter surrounding a core of gray matter. The pattern the gray matter produces roughly resembles a butterfly with its wings outspread. The upper and lower wings of gray matter are called the posterior horns and the anterior horns, respectively. Between them on either side is a protrusion of gray matter called the lateral horn. Motor neurons with relatively large cell bodies in the anterior horns (anterior horn cells) give rise to axons that pass out through spinal nerves to various skeletal muscles. However, the majority of neurons in the gray matter are in-terneurons (see chapter 10, p. 370).

A horizontal bar of gray matter in the middle of the spinal cord, the gray commissure, connects the wings of the gray matter on the right and left sides. This bar surrounds the central canal, which is continuous with the ventricles of the brain and contains cere-brospinal fluid. The central canal is prominent during embryonic development, but it becomes almost microscopic in an adult.

The gray matter divides the white matter of the spinal cord into three regions on each side—the anterior, lateral, and posterior columns (or funiculi). Each column consists of longitudinal bundles of myelinated nerve fibers that comprise major nerve pathways called nerve tracts.

Essentials of Human Physiology

Essentials of Human Physiology

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.

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