61 The Spinal Cord

Overview of the Spinal Cord

The spinal cord runs along the inside of the vertebral column and serves as the signaling conduit between the brain and the periphery.

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the medulla oblongata of the brain to the level of the lumbar region. The brain and spinal cord together make up the central nervous system (CNS). The spinal cord, protected by the vertebral column, begins at the occipital bone and extends down to the space between the first and second lumbar vertebrae. The spinal cord has a varying width, ranging from 0.5 inch thick in the cervical and lumbar regions to 0.25 inch thick in the thoracic area. The length of the spinal cord is approximately 45 cm (18 in) in men and about 43 cm (17 in) long in women.

Layers and Regions of the Spinal Cord

The spinal cord is protected by three layers of tissue called meninges and divided into three regions.

Spinal Cord Tissue Layers

The dura mater is the outermost layer of spinal cord tissue, forming a tough protective coating. The space between the dura mater and the surrounding bone of the vertebrae is called the epidural space. The epidural space is filled with adipose tissue and contains a network of blood vessels. The middle layer is called the arachnoid mater. The pia mater is the innermost protective layer and is tightly associated with the surface of the spinal cord. The space between the arachnoid and pia maters is called the subarachnoid space and is where the CSF is located. It is from this location at the level of the lumbar region that CSF fluid is obtained in a spinal tap.

Spinal Cord Regions

In cross-section, the peripheral region of the cord displays neuronal white matter tracts containing sensory and motor neurons. Internal to this peripheral region is the gray, butterfly-shaped central region made up of nerve cell bodies. This central region surrounds the central canal, which is an anatomic extension of the spaces in the brain known as the ventricles and like the ventricles, contains cerebrospinal fluid.

The spinal cord is divided into cervical, thoracic, and lumbar regions. The cervical region is divided into eight levels that are related to different motor and sensory functions in the neck and the arms. The spinal nerves of the thoracic region supply the thorax and abdomen. The nerves of the lumbosacral spinal cord supply the pelvic region, legs, and feet.

Spinal Cord Nerve Branches

Thirty-one pairs of spinal nerves (sensory and motor) branch from the human spinal cord. Each spinal nerve is formed from the combination of nerve fibers from its posterior and anterior roots. The posterior root is the sensory (afferent) root that carries sensory information to the brain from other areas of the body. The anterior root is the motor (efferent) root that carries motor information to the body from the brain.

The Spine

The spine encases the spinal cord for protection and support.

Number of Vertebrae

In human anatomy, the vertebral column (backbone or spine) usually consists of 24 articulating vertebrae and nine fused vertebrae in the sacrum and the coccyx. Situated in the dorsal aspect of the torso and separated by intervertebral discs, it houses and protects the spinal cord in its spinal canal. There are normally 33 vertebrae in humans, including the five that are fused to form the sacrum, the four coccygeal bones that form the tailbone, and the others separated by intervertebral discs. The upper three regions comprise the remaining 24, and are grouped as cervical (seven vertebrae), thoracic (12 vertebrae) and lumbar (five vertebrae).

Vertebral Shape

A typical vertebra consists of the vertebral body and vertebral arch. These parts together enclose the vertebral foramen that contains the spinal cord. The vertebral arch is formed by a pair of pedicles and a pair of laminae. Two transverse processes and one spinous process are posterior to (behind) the vertebral body. The spinous process projects toward the posterior direction, while one transverse process projects to the left and the other to the right. The spinous processes of the cervical and lumbar regions can be felt through the skin. Facet joints are located above and below each vertebra. These restrict the range of movement. Between each pair of vertebrae are two small openings called intervertebral foramina through which the spinal nerves exit.

Vertebral Curvature

When viewed laterally, the vertebral column presents several curves corresponding to the different regions of the column: cervical, thoracic, lumbar, and pelvic.

Cervical and Thoracic Curves

The cervical curve convexes forward and begins at the apex of the odontoid (tooth-like) process. It ends at the middle of the second thoracic vertebra. The thoracic curve convexes dorsally, begins at the middle of the second thoracic vertebra, and ends at the middle of the 12th thoracic vertebra.

Lumbar and Pelvic Curves

The lumbar curve, which is more pronounced in women than in men, begins at the middle of the last thoracic vertebra and ends at the sacrovertebral angle. It is convex anteriorly with the lower three vertebrae much more convex than the upper two. This curve is described as a lordotic curve. The pelvic curve begins at the sacrovertebral articulation and ends at the point of the coccyx; its concavity is directed downward and forward.

Primary and Secondary Curves

The thoracic and sacral curvatures are termed primary curves because they are present in the fetus and remain the same in the adult. As the child grows, lifts the head, and begins to assume an upright position, the secondary curves (cervical and lumbar) develop. The cervical curve forms when the infant is able to hold up his or her head (at three or four months) and sit upright (at nine months). The lumbar curve forms between twelve to eighteen months when the child begins to walk.

Spinal Cord Grey Matter and Spinal Roots

The grey matter of the spinal cord contains neuronal cell bodies, dendrites, axons, and nerve synapses.

The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The spinal cord is much shorter in length than the bony spinal column. The human spinal cord extends from the foramen magnum of the occipital bone of the skull and continues to the conus medullaris near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale.

Spinal Cord Topography and Roots

The spinal cord is compressed dorsoventrally, giving it an elliptical shape. The cord has grooves in the dorsal and ventral sides. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side.

Each segment of the spinal cord is associated with a pair of ganglia called dorsal root ganglia, situated just outside of the spinal cord. These ganglia contain cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.

The grey matter, in the center of the cord, is shaped like a butterfly and consists of cell bodies of interneurons and motor neurons, as well as neuroglia cells and unmyelinated axons. Projections of the grey matter (the “wings”) are called horns. Together, the grey horns and the grey commissure form the H-shaped grey matter.

Dorsal and Ventral Roots

The dorsal root ganglia lie along the vertebral column by the spine. The dorsal root ganglia develops in the embryo from neural crest cells, not the neural tube. Hence, the spinal ganglia can be regarded as grey matter of the spinal cord that became translocated to the periphery.

The axons of dorsal root ganglion neurons are known as afferents. In the peripheral nervous system, afferents refer to the axons that relay sensory information into the central nervous system. These neurons are of the pseudo-unipolar type, meaning that they have an axon with two branches that act as a single axon, often referred to as distal and proximal processes. Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral foramina as they become spinal nerves.

The nerve endings of dorsal root ganglion neurons have a variety of sensory receptors that are activated by mechanical, thermal, chemical, and noxious stimuli. In these sensory neurons, a group of ion channels thought to be responsible for somatosensory transduction has been identified. Compression of the dorsal root ganglion by a mechanical stimulus lowers the voltage threshold needed to evoke a response and causes action potentials to be fired. This firing may even persist after the removal of the stimulus.

Impulse Transmission

The dendrite receives information from another neuron’s axon at the synapse, and the axon sends information to the next neuron’s dendrites. Unlike the majority of neurons found in the CNS, an action potential in a dorsal root ganglion neuron may initiate in the distal process in the periphery, bypass the cell body, and continue to propagate along the proximal process until reaching the synaptic terminal in the dorsal horn of the spinal cord.

The distal section of the axon may either be a bare nerve ending or encapsulated by a structure that helps relay specific information to nerve. For example, a Meissner’s corpuscle or a Pacinian corpuscle may encapsulate the nerve ending, rendering the distal process sensitive to mechanical stimulation, such as stroking or vibration.

Ion Channels

Two distinct types of mechanosensitive ion channels have been found in the dorsal root ganglia, broadly classified as either high-threshold (HT) or low-threshold (LT). As their names suggest, they have different thresholds as well as different sensitivities to pressure. These are cationic channels whose activity appears to be regulated by the proper functioning of the cytoskeleton and cytoskeleton-associated proteins. The presence of these channels in the dorsal root ganglion gives reason to believe that other sensory neurons may contain them as well.

High-threshold channels have a possible role in nociception. These channels are found predominantly in smaller sensory neurons in the dorsal root ganglion cells and are activated by higher pressures, two attributes that are characteristic of nociceptors. Also, the threshold of HT channels was lowered in the presence of PGE2 (a compound that sensitizes neurons to mechanical stimuli and mechanical hyperalgesia), which further supports a role for HT channels in the transduction of mechanical stimuli into nociceptive neuronal signals.

Spinal Cord White Matter

The white matter of the spinal cord is composed of bundles of myelinated axons.

White matter is one of the two components of the central nervous system. It consists mostly of glial cells and myelinated axons and forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. It is the tissue through which messages pass between different areas of grey matter within the nervous system.

Composition of White Matter

White matter is composed of bundles of myelinated nerve cell processes (or axons). The axons connect various grey matter areas (the locations of nerve cell bodies) of the brain to each other and carry nerve impulses between neurons. The axonal myelin acts as an insulator and increases the speed of transmission of all nerve signals. White matter does not contain dendrites, which are only found in grey matter along with neural cell bodies and shorter axons.

In a freshly cut brain, the tissue of white matter appears pinkish white to the naked eye because myelin is composed largely of lipid tissue that contains capillaries. In nonelderly adults, 1.7-3.6% of the white matter is blood. Myelin is found in almost all long nerve fibers and acts as electrical insulation. This is important because it allows the messages to pass quickly from place to place.

Spinal Cord Columns

The spinal cord white matter is subdivided into columns. The dorsal columns carry sensory information from mechanoreceptors (cells that respond to mechanical pressure or distortion). The axons of the lateral columns ( corticospinal tracts ) travel from the cerebral cortex to contact spinal motor neurons. The ventral columns carry sensory pain and temperature information and some motor information.

Function of White Matter

Long thought to be passive tissue, white matter actively affects how the brain learns and functions. While grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions. The brain in general (and especially a child’s brain) can adapt to white-matter damage by finding alternative routes that bypass the damaged white-matter areas; therefore, it can maintain good connections between the various areas of grey matter. Using a computer network as an analogy, the grey matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together.

Axon Tracts

Within white matter, there are three different kinds of tracts or bundles of axons that connect one part of the brain to another and to the spinal cord:

1. Projection tracts extend vertically between higher and lower brain and spinal cord centers. They carry information between the cerebrum and the rest of the body. The corticospinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord.
2. Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.
3. Association tracts connect different regions within the same hemisphere of the brain. Among their roles, association tracts link perceptual and memory centers of the brain.

White Matter-Grey Matter Interactions

White matter forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. Aggregates of grey matter, such as the basal ganglia and brain stem nuclei, are spread within the cerebral white matter. The cerebellum is structured in a similar manner as the cerebrum, with a superficial mantle of cerebellar cortex, deep cerebellar white matter (called the “arbor vitae”), and aggregates of grey matter surrounded by deep cerebellar white matter (dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus). The fluid-filled cerebral ventricles (lateral ventricles, third ventricle, cerebral aqueduct, and fourth ventricle) are also located deep within the cerebral white matter.