The Spinal Cord as well as the spinal nerves contain neural circuits that control some of our most rapid reactions to environmental changes. If you pick up something hot, the muscles grasping the object may relax, causing you to drop the object even before you are consciously aware of the extreme heat or pain. This is an example of a spinal reflex, a super quick, automated response to certain kinds of stimuli, that is made possible through the neurons only in the spinal nerves and spinal cord.
The spinal cord is elongated and almost cylindrical in shape. This part of the central nervous system is suspended in the spinal column and surrounded by the meninges and cerebrospinal fluid.
Note the spinal cord does not extend the entire length of the spinal column.
In Lumbar Puncture (Spinal Tap) procedures a specimen of cerebrospinal fluid is taken from the subarachnoid space at a point bellow the end of the spinalcord. Which means it is taken from a point bellow the 2nd lumbar vertebra.
Apart from the cranial nerves, the spinal cord is the only nervous link between the brain and the rest of the body. All impulses from the brain to the various organs is carried by nerve tissue that descends through the spinal cord and then leaves at the appropriate levels to the structures they supply. The also applies to sensory nerves from organs and tissues enter and pass upwards towards the brain through the spinal cord.
The spinal cord is located within the spinal cavity of the spinal column. The spinal cavity is formed as the vertebral foramen of all the vertebrae are stacked on top of the other. The surrounding vertebrae provide a strong shelter for the enclosed spinal cord. The vertebral ligaments, meninges, and cerebrospinal fluid all provide additional protection to the spinal cord.
The meninges are three connective tissue sheathings 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 is the the dura mater (which is latin for tough mother), 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 dura mater 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 dura mater and the wall of the vertebral cavity.
The middle meninx is an avascular sheath called the arachnoid mater (which translate to ‘similar to a spider’ in latin) because of its spider’s web arrangement of delicate collagen fibers and some elastic fibers. It is deep to the dura mater and is continuous with the arachnoid mater of the brain. Between the dura mater and the arachnoid mater is a thin subdural space, which contains interstitial fluid.
The innermost meninx is the pia mater (delicate), which is a thin transparent connective tissue layer that adheres to the surface of the spinal cord and brain. It consists of interlacing bundles of collagen fibers and some fine elasti 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.
The three spinal meninges also cover the spinal nerve roots until they exit the spinal column through the intervertebral foramina. Spinal nerve roots connect the 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 are called denticulate ligaments (small tooth) and are thickenings of the pia mater. They fuse with the arachnoid mater and inner surface of the dura mater between the anterior and posterior nerve roots of spinal nerves on either side. The denticulate ligaments also extend all along the length of the spinal cord and protect the spinal cord against sudden movements or displacement.
External Anatomy of the Spinal Cord
Although the spinal cord is roughly cylindrical, some areas areas are flattened slightly such as the anteriorly and posteriorly surfaces. The spinalcord 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.
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 narrowing, 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 dura mater 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 (C1–C8), 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. 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 inter- nal 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 the Spinal Cord
The spinal cord is made up of a inner core of gray matter, which is surround by a regions of white 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. While the posterior median sulcus is a narrow furrow on the posterior (dorsal) side.
The gray matter of the spinal cord is shaped like a butterfly and resembles the letter H. 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:
- Anterior (ventral) white columns
- Posterior (dorsal) white columns
- Lateral white columns
Each column in turn contains distinct bundles of axons having a common origin or destination and carrying similar in- formation. 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.
The spinal cord has two principal functions in maintaining homeostasis: nerve impulse propagation and integration of information. The white matter tracts in the spinal cord are highways for nerve impulse propagation. Sensory input travels along these tracts toward the brain, and motor output travels from the brain along these tracts toward skeletal muscles and other effector tissues. The gray matter of the spinal cord receives and integrates incoming and outgoing information.
More information on spinal cord physiology will be added shortly.