FEATURES OF SPINAL CORD: Structural arrangements, meninges (features & functions), external anatomy of spinal cord & internal anatomy of spinal cord.

PROTECTIVE STRUCTURES

Two types of connective tissue coverings—bony vertebrae and tough, connective tissue meninges—plus a cushion of cerebrospinal ﬂuid (produced in the brain) surround and protect the delicate nervous tissue of the spinal cord.

Vertebral Column

The spinal cord is located within the vertebral canal of the vertebral column. The vertebral foramina of all the vertebrae, stacked one on top of the other, form the vertebral canal. The surrounding vertebrae provide a sturdy shelter for the enclosed spinal cord. The vertebral ligaments, meninges, and cerebrospinal ﬂuid provide additional protection.

Meninges

The meninges are three connective tissue coverings that encircle the spinal cord and brain. The spinal meninges surround the spinal cord and are continuous with the cranial meninges, which encircle the brain. The most superﬁcial of the three spinal meninges, the duramater, is composed of dense, irregular connective tissue. It forms a sac from the level of the foramen magnum in the occipital bone, where it is continuous with the duramater of the brain, to the second sacral vertebra. The spinal cord is also protected by a cushion of fat and connective tissue located in the epidural space, a space between the duramater and the wall of the vertebral canal. The middle meninx (singular form of meninges) is an avascular covering called the arachnoid mater because of its spider’s web arrangement of delicate collagen ﬁbers and some elastic ﬁbers. It is deep to the duramater and is continuous with the arachnoid mater of the brain. Between the duramater and the arachnoid mater is a thin subdural space, which contains interstitial ﬂuid. The innermost meninx is the pia mater, a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. It consists of squamous to cuboidal cells within interlacing bundles of collagen ﬁbers and some ﬁne elastic ﬁbers. Within the pia mater are many blood vessels that supply oxygen and nutrients to the spinal cord. Between the arachnoid mater and the pia mater is the subarachnoid space, which contains cerebrospinal ﬂuid that serves as a shock absorber and suspension system for the spinal cord and brain.

All three spinal meninges cover the spinal nerve roots up to the point where they exit the spinal column through the intervertebral foramina. As you will see later in the chapter, spinal nerve roots are structures that connect spinal nerves to the spinal cord. Triangular-shaped membranous extensions of the pia mater suspend the spinal cord in the middle of its dural sheath. These extensions, called denticulate ligaments are thickenings of the pia mater. They project laterally and fuse with the arachnoid mater and inner surface of the duramater between the anterior and posterior nerve roots of spinal nerves on either side. Extending all along the length of the spinal cord, the denticulate ligaments protect the spinal cord against sudden displacement that could result in shock.

EXTERNAL ANATOMY OF SPINAL CORD

The spinal cord, although roughly cylindrical, is ﬂattened slightly anteriorly and posteriorly. In adults, it extends from the medulla oblongata, the inferior part of the brain, to the superior border of the second lumbar vertebra. In newborn infants, it extends to the third or fourth lumbar vertebra. During early childhood, both the spinal cord and the vertebral column grow longer as part of overall body growth. Elongation of the spinal cord stops around age 4 or 5, but growth of the vertebral column continues. Thus, the spinal cord does not extend the entire length of the adult vertebral column. The length of the adult spinal cord ranges from 42 to 45 cm (16–18 in.). Its diameter is about 2 cm (0.75 in.) in the mid thoracic region, somewhat larger in the lower cervical and mid lumbar regions, and smallest at the inferior tip. When the spinal cord is viewed externally, two conspicuous enlargements can be seen. The superior enlargement, the cervical enlargement, extends from the fourth cervical vertebra to the ﬁrst thoracic vertebra. Nerves to and from the upper limbs arise from the cervical enlargement. The inferior enlargement, called the lumbar enlargement, extends from the ninth to the twelfth thoracic vertebra. Nerves to and from the lower limbs arise from the lumbar enlargement.

Inferior to the lumbar enlargement, the spinal cord terminates as a tapering, conical structure called the conus medullaris, which ends at the level of the intervertebral disc between the ﬁrst and second lumbar vertebrae in adults. Arising from the conus medullaris is the ﬁlum terminale, an extension of the pia mater that extends inferiorly and blends with the arachnoid mater and duramater and anchors the spinal cord to the coccyx. Spinal nerves are the paths of communication between the spinal cord and speciﬁc regions of the body. The spinal cord appears to be segmented because the 31 pairs of spinal nerves emerge at regular intervals from intervertebral foramina). Indeed, each pair of spinal nerves is said to arise from a spinal segment. Within the spinal cord there is no obvious segmentation but, for convenience, the naming of spinal nerves is based on the segment in which they are located. There are 8 pairs of cervical nerves, 12 pairs of thoracic nerves (T1–T12), 5 pairs of lumbar nerves (L1–L5), 5 pairs of sacral nerves (S1–S5), and 1 pair of coccygeal nerves (Co1). Because the spinal cord is shorter than the vertebral column, nerves that arise from the lumbar, sacral, and coccygeal regions of the spinal cord do not leave the vertebral column at the same level they exit the cord.

The roots of these spinal nerves angle inferiorly in the vertebral canal from the end of the spinal cord like wisps of hair. Appropriately, the roots of these nerves are collectively named the cauda equina, meaning “horse’s tail”. Two bundles of axons, called roots, connect each spinal nerve to a segment of the cord by even smaller bundles of axons called rootlets. The posterior (dorsal) root and rootlets contain only sensory axons, which conduct nerve impulses from sensory receptors in the skin, muscles, and internal organs into the central nervous system. Each posterior root has a swelling, the posterior (dorsal) root ganglion, which contains the cell bodies of sensory neurons. The anterior (ventral) root and rootlets contain axons of motor neurons, which conduct nerve impulses from the CNS to effectors (muscles and glands).

INTERNAL ANATOMY OF SPINAL CORD:

A freshly dissected section of the spinal cord reveals regions of white matter that surround an inner core of gray matter. The white matter of the spinal cord consists primarily of bundles of myelinated axons of neurons. Two grooves penetrate the white matter of the spinal cord and divide it into right and left sides. The anterior median ﬁssure is a wide groove on the anterior (ventral) side. The posterior median sulcus is a narrow furrow on the posterior (dorsal) side. The gray matter of the spinal cord is shaped like the letter H or a butterﬂy; it consists of dendrites and cell bodies of neurons, unmyelinated axons, and neuroglia. The gray commissure forms the crossbar of the H. In the center of the gray commissure is a small space called the central canal; it extends the entire length of the spinal cord and is ﬁlled with cerebrospinal ﬂuid. At its superior end, the central canal is continuous with the fourth ventricle (a space that contains cerebrospinal ﬂuid) in the medulla oblongata of the brain. Anterior to the gray commissure is the anterior (ventral) white commissure, which connects the white matter of the right and left sides of the spinal cord. The gray matter on each side of the spinal cord is subdivided into regions called horns.

The posterior (dorsal) gray horns contain cell bodies and axons of interneurons as well as axons of incoming sensory neurons. Recall that cell bodies of sensory neurons are located in the posterior (dorsal) root ganglion of a spinal nerve. The anterior (ventral) gray horns contain somatic motor nuclei, which are clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles. Between the posterior and anterior gray horns are the lateral gray horns, which are present only in thoracic and upper lumbar segments of the spinal cord. The lateral gray horns contain autonomic motor nuclei, which are clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands. The white matter of the spinal cord, like the gray matter, is organized into regions.

The anterior and posterior gray horns divide the white matter on each side into three broad areas called columns:

(1) Anterior (ventral) white columns,

(2) Posterior (dorsal) white columns, and

(3) Lateral white columns. Each column in turn contains distinct bundles of axons having a common origin or destination and carrying similar information. These bundles, which may extend long distances up or down the spinal cord, are called tracts. Recall that tracts are bundles of axons in the CNS, whereas nerves are bundles of axons in the PNS. Sensory (ascending) tracts consist of axons that conduct nerve impulses toward the brain. Tracts consisting of axons that carry nerve impulses from the brain are called motor (descending) tracts. Sensory and motor tracts of the spinal cord are continuous with sensory and motor tracts in the brain.