Overview of the Spinal Cord
The nervous system is critical to many of our homeostatic feedback loops. In most of these loops, the structures of the nervous system make up more than one component, and carry out more than one function in these loops. For example, specialized nerve endings often act as sensors (receptors), information is carried along nerves and/or tracts of the spinal cord, integration occurs within the CNS, and spinal cord tracts and nerves carry the responding information back out to the effectors. The spinal cord is a nervous system structure dedicated to relaying information from the periphery to the brain and back, as well as carrying out certain levels of integration, such as those found in many reflexes. The structure of the spinal cord aids it in carrying out these relaying and integrative functions.
The spinal cord is a central nervous system structure that extends inferiorly from the brain stem and into the lower back. Throughout its length, it is enclosed within the spinal column, with the cord passing through the vertebral foramen of the vertebrae. In an adult, the spinal cord itself terminates at a point called the medullary cone, at approximately the level of the first lumbar vertebrae (L1). Below the medullary cone, the vertebral canal contains a bundle of nerve roots called the cauda equina.
This figure shows other important features of the spinal cord, many of them related to the spinal cord’s function of relaying information. Starting between the base of the skull and the first cervical vertebrae, and continuing into the sacral region of the spinal column, a pair of spinal nerves extend from the spinal cord (although information is transmitted in both directions on sensory and motor neurons within these mixed nerves). All but the first spinal nerve (C1) pass through the intervertebral foramen of the spinal cord, whereas spinal nerve C1 passes between the occipital bone and vertebrae C1. In all there are 31 pairs of spinal nerves that carry information to and from the spinal cord and the periphery of the body. Note that not all of the spinal nerves arise from the cord at the level of the vertebrae between which they pass. This is most obvious when considering those spinal nerves arising in the lower lumbar and sacral regions. The nerve roots for these nerves arise from the spinal cord at, or near, the medullary cone, which you will recall is near the L1 vertebrae. These roots are contained within the cauda equina until passing out of the spinal column. Because the spinal nerve roots don’t always originate at the level of the vertebrae that they pass through, the segments of the spinal cord are named for the spinal nerve to which they give rise. For example, segment S2 of the spinal cord would be located near the T12 vertebrae.
Because the spinal cord terminates near vertebrae L1, and there is a lot of body tissue that needs to be innervated below this level, there are a significant number of nerves arising from the lower aspect of the spinal cord. This leads to an area of increased spinal cord thickness in the lumbosacral regions of the spinal cord (corresponding to a region associated with the inferior thoracic vertebrae) called the lumbar enlargement. There is a corresponding cervical enlargement in the cervical segments that give rise to nerves innervating the upper limbs.
Spinal Cord Structure
Recall that the central nervous system tissues can generally be divided into white matter and gray matter. White matter is the myelin-containing region composed of axons, which make up the tracts of the CNS. These carry information between different regions and structures in the CNS. Gray matter contains the cell bodies and dendrites and therefore is the site of synaptic transmission.
In the cortex of the brain, gray matter makes up the cortical (outer) regions, while the white matter tracts tend to make up the majority of the deep tissues of the brain, although there are exceptions to the latter, such as the deep basal and thalamic nuclei that are composed of gray matter. In contrast to this general arrangement of the brain, the spinal cord is arranged with the white matter surrounding the central gray matter, indicating that the spinal tracts carry information up and down the cord along the outer aspects, while synaptic transmission tends to occur more centrally.
Cross-sectional view of a spinal cord segment.
In the image above, you can see how the central gray matter is somewhat butterfly shaped, with each side of the “butterfly” containing a posterior (dorsal) horn and an anterior (ventral) horn. Each of the horns is contiguous with the posterior and anterior spinal nerve roots, respectively. The posterior root of the nerve carries sensory information into the posterior horn, often synapsing there. The anterior horn contains the cell bodies of somatic motor neurons, and it sends its axons out the anterior root of the spinal nerve to the muscle cells it innervates. The lateral horn is not found at all levels of the spinal cord, but is limited to thoracic and lumber segments of the cord. This is because the lateral horns contain the neurons of the sympathetic nervous system, which leave the cord only in these segments. Even though the cell bodies are found in the lateral horns, their axons leave via the anterior nerve roots, just like those that control skeletal muscle. The matched horns on each side of the “butterfly” are connected via the gray commissure, which also surrounds the cerebrospinal fluid filled central canal.
The white matter of the spinal cord is divided into columns. Each segment of the cord contains matched posterior, lateral and anterior columns. The anterior columns and posterior columns are partially separated by the anterior median fissure and posterior median sulcus, respectively. Each pair is also connected by a commissure of white matter that runs adjacent to the gray commissure, termed the anterior and posterior commissures. The columns are further divided into tracts that carry sensory information up the spinal cord (ascending tracts) and motor information down the spinal cord (descending tracts).
Cross-sectional view of the length of the spinal cord.
Although each segment of the spinal cord has similar features, there are some differences along its length, as you may be able to determine from the image above. The main difference is that the ratio of gray matter to white matter varies among segments of the spinal cord. At the lower levels of the spinal cord there is a greater ratio of grey matter to white matter. This should make sense, as there are less ascending and descending tracts of whiter matter as you move lower. As previously mentioned, the lateral horns are only found in the thoracic and lumber regions of the spinal cord, where they contain the motor nuclei of the sympathetic nervous system. Finally, the size of the anterior and posterior horns varies, depending on the amount of tissue they are innervating. For example, the thoracic segments have relatively small anterior horns, as there is little skeletal muscle to innervate in the thorax and abdomen, while the cervical and thoracolumbar regions have large anterior horns, used to innervate the skeletal muscles of the arms and legs, respectively.
Spinal Cord Tracts
The white matter of the spinal cord is divided into the paired posterior (dorsal), lateral, and anterior (ventral) columns. These columns are sometimes called funiculi (or funiculus when singular) and are made up of axons that are traveling up (ascending) or down (descending) the spinal cord. The ascending tracts generally carry sensory information from the periphery to the brain, while the descending tracts carry motor signals to muscles and glands.
The columns can be further divided into tracts (sometimes called fasciculi), which is a way of functionally grouping the neurons based on similar origin, destination and function. These tracts are often named for the structures that they connect. For example, the spinothalamic tract indicates that the fibers are carrying information from the spinal cord to the thalamus of the brainstem. You may note from its name that it is an ascending tract, so the information that it carries is sensory.
Some of the tracts cross over (decussate) either in the spinal cord or brainstem, and when this occurs, the relationship between the origin and destination is termed contralateral. Much of our motor control is contralateral. For example, your right arm is mainly controlled by the motor area in your left brain. When the origin and destination of a tract are on the same side of the body, it is referred to as an ispsilateral relationship.
Cross-section of the spinal cord, indicating how the white matter columns can be divided into various tracts.
This table lists the major spinal tracts, indicates if they decussate, and provides a brief description of the types of information that they carry.
Spinal Cord Communication
The spinal cord acts as a conduit for information traveling up and down its length. But because most of this information has to either exit the spinal cord to send signals to peripheral tissues (efferent transmission), or information from peripheral tissues needs to be carried into the spinal cord (afferent transmission), there must be appropriate structures for these types of transmission to occur. It is the 31 pairs of spinal nerves and their related structures that provide the pathways for this interaction.
Sensory and motor fibers enter and exit the cord via rootlets that arise from both the posterior and anterior aspects of the cord. Anterior rootlets carry motor information out of the spinal cord (i.e. they contain efferent fibers) while the posterior rootlets carry sensory information into the spinal cord (i.e. they contain afferent fibers). Several posterior rootlets merge together to form the posterior root, while several anterior rootlets similarly converge to form the anterior root.
Rootlets and the passing of information to and from the spinal cord.
Along the posterior root is a ganglion, where cell bodies of many of the sensory neurons are found. These are unipolar neurons, such that their dendrites extend out to the peripheral tissues, and their axons project into the dorsal horn of the spinal cord, where they synapse. These unipolar peripheral neurons are considered first order neurons in the sensory pathway, while the neurons they synapse with in the posterior horn are considered the second order neurons of the sensory pathway. It is the axons of these second order neurons that make up the various ascending white matter tracts.
The anterior root does not contain a ganglion. This is because motor control is typically a two neuron pathway. It starts with an upper motor neuron, whose cell body is in the cerebral cortex or gray matter of the brainstem. This neuron projects its axon via a descending white matter tract to a point in the spinal cord where it synapses in the ventral horn with a lower motor neuron. The cell body of the lower motor neuron is in the gray matter of the spinal cord, and it projects its axon out one of the anterior rootlets and through the anterior root. Ganglia are only found where neuron cell bodies are outside the CNS.
Distal to the posterior root ganglion, the fibers of the anterior and posterior root merge together and pass through the dura to become the spinal nerve. Because the spinal nerves contain both sensory and motor fibers, they are considered a mixed nerve, as opposed to either a sensory or motor nerve.
Just distal to the intervertebral foramen of the spinal column, the spinal nerve branches into rami (singular: ramus). In general, the posterior ramus communicates with structures posterior to the cord, while the anterior ramus communicates with structures anterior to the cord. In spinal nerves T1-L2, the anterior ramus gives rise to a communicating ramus that communicates with the sympathetic ganglia in the region. The sympathetic motor pathway involves two motor neurons, so this ganglion houses the second motor neuron’s cell body. In various regions of the body, the anterior rami from several spinal nerves join together and then branch again, in a complex network of nerves called a plexus.
A plexus is a network of anterior rami from neighboring spinal nerves that come together in a weblike or tangled network adjacent to the spinal cord, and from which new nerves arise. These nerves contain fibers from several spinal nerves. The four main plexuses are the cervical, brachial, lumbar and sacral. Some people consider the coccygeal plexus a fifth plexus, although it is much smaller than the others.
The plexuses are complex networks, with four or more spinal nerve rami contributing to each of the four main plexuses, and with several nerves arising from each. The following table lists the four main plexuses, the spinal nerves that contribute to each, and some of the main nerves that arise from them.