Tumors of the Spinal Canal (eBook)

2 Anatomy of the Spinal Cord and Nerve Roots

Jack Zakrzewski and Douglas A. Cotanche

Summary

This chapter offers an extensive overview of the anatomy of the spinal column and spinal cord, the development of the spinal cord, the internal anatomy of the gray matter and the ascending and descending columns in the white matter, the anatomy of the meninges, and the anatomy of the arterial and vascular supply of the spinal cord.

Keywords: vertebral column, spinal cord, spinal nerve, spinal vasculature, development of the spine, meninges, ascending and descending spinal pathways

2.1 Introduction

The spinal cord is one of the two components of the central nervous system (CNS), with the other being the brain. The spinal cord’s major function is to provide access to the CNS for the nerves controlling the structures of the neck, body, and limbs of a human being. The spinal cord is composed of a central core of gray matter that contains the neurons and interneurons that communicate with the body. On the periphery of the central gray matter is the white matter, a series of myelinated neuronal processes that communicate with regions up and down the spinal cord and with major tracts to and from the brain. In this chapter we will describe in detail the anatomy of the spinal column and spinal cord, its embryonic development, the organization of the gray matter, and the ascending and descending columns in the spinal cord. We will also describe the meninges in the spinal cord, the arterial and venous supply to the spinal cord, and the anatomy of the spinal nerves. This chapter is intended to serve as an anatomical overview of the vertebral column and spinal cord and will serve as a foundation for the remaining chapters on tumors of the spinal cord.

2.2 Anatomy of the Vertebral Column and Vertebrae

The vertebral column consists of the curved assemblage of the individual vertebrae that make up the spine (Fig. 2.1). The individual vertebrae are connected by synovial facet joints (sometimes referred to as zygapophysial joints), ligaments, muscles, and fascia. There are three main functions of the spine: support the trunk and posture, protect the enclosed spinal cord and nerves, and provide attachment sites for various muscles.1 The vertebrae also serve as sites of hematopoiesis. The vertebrae of the column receive blood via branches of the intersegmental somatic arteries. Each of these arteries is named based on the level of the column it is found in.

Between each of the vertebrae is an intervertebral disk (Fig. 2.2). Each disk is made of an outer fibrous ring called the anulus fibrosus and an inner gel-like center called the nucleus pulposus. The outer ring is made mostly of type I cartilage, while the nucleus pulposus consists mainly of type II cartilage and water.2 The intervertebral disks allow for limited amount of vertebral movement, while also acting as quasi-ligaments that hold the column together. Most importantly, they act as shock absorbers that distribute forces along the column.

The anterior aspect of the vertebral column is mostly made up of the vertebral bodies and the intervertebral disks, which are then covered by the anterior longitudinal ligament. This ligament forms a plane with the prevertebral and endothoracic fascia, as well as with the subperitoneal areolar tissue of the posterior abdominal wall, which can act as a route for the spread of pathogens and cancer. The lateral aspect of the column consists of the articular processes in the cervical and lumbar regions and the transverse processes in the thoracic region. The oval intervertebral foramina are found here, behind the bodies themselves and between the pedicles. These foramina allow for interaction between the lumen of the vertebral canal and the paravertebral soft tissues. It is the route of exit of the individual spinal nerves. In addition, it can also act as a route of tumor spread. The posterior aspect of the vertebral column is made of the posterior aspect of the laminae and the spinous processes, as well as the facet joints. It is covered by ligaments and the deep muscles of the back.

2.2.1 The Vertebrae: General Features

Each vertebra consists of three main parts: the vertebral body, the dorsal vertebral arch (sometimes called the neural arch), and the vertebral foramen (Fig. 2.3). The foramen contains the spinal cord, the meninges, and the various vessels that feed and drain the vertebral canal. The vertebrae vary in size and shape depending on the level of the spine they are located in. This section will give a broad overview of vertebral characteristics, as the details are beyond the scope of this text.

The pedicles are located on each side of the posterior surface of the vertebrae and form the lateral walls of the vertebral arch. They are short, thick, and narrower than the rest of the arch, and are continuous with the laminae. The spinous process projects dorsally and caudally from the laminal junction. Its main purpose is to serve as a lever for the antigravity muscles that control posture and movement. The articular processes are paired structures: one superior and one inferior. They originate from the vertebral arch and contribute to the facet joint, allowing for some movement between the vertebrae. The transverse processes project laterally from the laminae, although their exact location varies between spinal levels. Like the spinous process, these projections act as levers for the various back muscles and ligaments.

2.2.2 The Vertebral Canal

The vertebral canal is a continuous series of foramina that is located posterior to the vertebral bodies. It begins at the foramen magnum and terminates in the sacral hiatus. Although it is quite stationary in the thoracic region, it does move slightly in the cervical and lumbar regions. The canal is larger and triangular in shape in the cervical and lumbar regions and smaller and more circular in the thoracic region.

2.2.3 The Intervertebral Foramina

The intervertebral foramina serve as the main route into and out of the vertebral canal. They are found between the posterolateral aspect of the superior vertebral pedicle, the compact bone of the inferior vertebral pedicle, and the ventral aspect of the fibrous capsule of the facet joint. Each foramen is covered by a protective fibrous tissue. The region in which the spinal nerve canal is found is called the true foramina. It also contains the spinal nerves and sheaths, the meningeal nerves, the spinal arteries, and the venous plexus. It is the main site of nerve compression.

2.3 Development of the Spinal Cord

2.3.1 Early Development

The tissues that will become the brain and spinal cord arise early in human embryonic development, shortly after the formation of the trilaminar embryo. Gastrulation is completed by day 18. On day 19, the central region of the ectodermal epithelium begins to thicken and form the pear-shaped neural plate (Fig. 2.4). One day later, the lateral edges of the neural plate begin to elevate and become the neural folds. The neural groove lies centrally between the two neural folds. The neural folds continue to elevate and curl inward toward one another. As they do, the neural groove deepens to become a trench. By day 22, the two neural folds contact one another and begin to fuse into a neural tube, first in the cervical regions and then continuing to zipper up and down the neural plate (Fig. 2.5). The epithelial tissues at the very tip of the neural folds do not become incorporated into the neural tube, but instead separate away from where the folds are merging and migrate out into the mesoderm region between the neural tube and the overlying ectoderm. These tissues are the neural crest cells and they will be discussed below in the development of the sensory components of the spinal nerve and in the development of the meninges.