Learning Modules - Medical Gross Anatomy Spinal Cord and Spinal Nerve - Page 2 of 14

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2 Spinal Cord and Spinal Nerve - Page 2 of 14 The spinal cord is well-protected by the vertebral column, which extends from the skull to the coccyx, enclosing the entire cord. The vertebral column comprises 33 vertebrae: 7 cervical vertebrae, 12 thoracic vertebrae, 5 lumbar vertebrae, 5 sacral vertebrae, and 4 coccygeal vertebrae. In adults, the 5 sacral vertebrae are fused to form the sacrum and the 4 coccygeal vertebrae are fused to form the coccyx. The vertebrae are separated by tough fibrocartilage intervertebral discs that make up about one-fourth of the length of the vertebral column. While the structure of the vertebrae varies from one region to another, the spinal cord runs through the vertebral foramen in each vertebra. The vertebral foramen is bordered anteriorly by the vertebral body, laterally by the pedicles and posteriorly by the laminae. Thus, the spinal cord is entirely protected by bone. Nerves enter and exit the spinal cord through intervertebral foramina, which are enclosed by the inferior vertebral notch on the superior vertebra and the superior vertebral notch on the inferior vertebra.

3 Spinal Cord and Spinal Nerve - Page 3 of 14 The spinal cord is further protected by three membranes, collectively called the meninges. The outer membrane surrounding the spinal cord is the dura mater (from Latin, meaning durable mother). The dura, made of a dense fibrous material, forms the dural sac, which surrounds the spinal cord and cauda equina (to be discussed later) and terminates at the level of the second sacral vertebra. The dura is separated from the vertebrae by the epidural fat in the epidural space (epi, meaning upon). The next layer is the delicate arachnoid mater which is thin and has web-like filaments connecting to the underlying pia mater (arachnoid is Greek for spider). Beneath the arachnoid mater is the subarachnoid space, which is filled with cerebrospinal fluid. Abbreviated CSF, cerebrospinal fluid bathes the brain and spinal cord as well as the cauda equina, providing protection, nourishment, and a medium for exchange of nutrients and waste. The innermost membrane surrounding the spinal cord is the vascular pia mater (Latin, meaning tender or devoted mother), which is very closely apposed to the spinal cord. The pia mater has paired specializations called denticulate ligaments, which extend laterally from the surface of the spinal cord and pierce the arachnoid to attach to the inner aspect of the dura mater at 21 pairs of denticulations (dentate means tooth-like - these are toothlike lateral projections). The denticulate ligaments run longitudinally between the dorsal and ventral roots of the spinal cord and serve to suspend the spinal cord from side to side in the dural sac. The pia continues inferiorly from the end of the spinal cord (at the level of the second lumbar vertebra) as the filum terminale internum. As we said, the dural sac ends at the level of the second sacral vertebra (S2). Caudal to the end of the dural sac is a specialization of meninges called the coccygeal ligament (or filum terminale externum) that attaches the meninges, and consequently the spinal cord, to the coccyx. The coccygeal ligament is composed of specializations of all three layers of meninges.

4 Spinal Cord and Spinal Nerve - Page 4 of 14 The spinal cord itself begins as a continuation of the medulla oblongata of the brain at the level of the foramen magnum in the skull. Along its course, the spinal cord gives rise to 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. In general, the spinal nerves emerge below the pedicle of the vertebra for which they are named. For example, spinal nerve T3 exits through the intervertebral foramen between the 3rd and 4th thoracic vertebrae. In the cervical region, this rule does not apply. While the cervical region of the spine has 7 vertebrae, there are 8 cervical spinal nerves. Spinal nerve C1 exits above vertebra C1, spinal nerve C2 exits through the intervertebral foramen between the 1st and 2nd cervical vertebrae, etc. That pattern holds until spinal nerve C8 which exits between vertebrae C7 and T1. The nerve exiting below vertebra T1 is spinal nerve T1. This relationship is rather confusing in words, but is fairly clear when studied in a diagram. The spinal cord terminates at the level of the first or second lumbar vertebrae as the conus medullaris. Caudal to the conus medullaris are the nerve roots of the more caudal spinal nerves which are collectively called, because of their appearance, the cauda equina (horse's tail).

5 Spinal Cord and Spinal Nerve - Page 5 of 14 While it seems somewhat counterintuitive that the spinal cord would end at the level of the first or second lumbar vertebra, (as is often the case in anatomy) the explanation lies in the embryological development of the spinal cord. In embryos, the spinal cord occupies the entire length of the vertebral canal, but during development the vertebral column grows faster than the spinal cord. Consequently, in an adult, not all spinal cord segments are found at the level of the vertebra of the same number. Because of the discrepancy between the spinal cord segment and the vertebral level, the spinal nerve roots of the more caudal spinal cord segments are rather long and descend for a significant distance before reaching their respective intervertebral foramina. It is these spinal nerve roots that compose the cauda equina beyond L1/L2. The fact that the spinal cord ends at L1/L2 is very useful in clinical practice in that it allows for spinal taps to be performed to sample CSF without the risk of puncturing the spinal cord. Below L1/L2 but above S2, there is still CSF contained in the dural sac (in what is called the lumbar cistern), but instead of a spinal cord there are only nerve roots, which float away from the positive pressure of a needle entering the space. Extreme caution must be taken in performing a spinal tap on an infant or a young child whose spinal cord may extend as far as vertebra L4 because their vertebral column has not reached its maximum length.

6 Spinal Cord and Spinal Nerve - Page 6 of 14 Another important feature of the spinal cord, which relates directly to its function, is its external shape. The spinal cord is shaped something like a glass soda-bottle. It has a cervical enlargement which begins at roughly C4 and extends to roughly T1 and a lumbar enlargement that extends from roughly the T11 vertebra through the L1 vertebral level. The cervical enlargement is the site of the cell bodies of the motor neurons that innervate the upper limbs, as well as the site where the sensory nerves from the upper limbs synapse. The lumbar enlargement is the site of the cell bodies of the motor neurons that innervate the lower limbs and the site where the sensory nerves from the lower limbs synapse. These segments of spinal cord are enlarged because of the extensive sensory input from the limbs, especially from the hands and fingers, as well as the complex and fine musculature of the limbs which requires significantly more innervation to control.

7 Spinal Cord and Spinal Nerve - Page 7 of 14 A cross section of the spinal cord reveals the characteristic butterfly shape of the gray matter of the spinal cord surrounded by white matter. The gray matter changes shape throughout the spinal cord depending on the neural requirements of a given region. For example, the cervical and lumbar enlargements that provide innervation to the limbs have much more gray matter than a segment of spinal cord taken from the thoracic region that supplies a segment of the trunk. Regardless of location, the cord consists of a pair of dorsal horns and a pair of ventral horns of gray matter. The dorsal horns are made of the cell bodies upon which the axons carrying sensory information from the periphery synapse. The ventral horns are composed of the cell bodies of the motor neurons innervating skeletal muscles. A useful mnemonic for remembering the relationships in the spinal cord is: SAME-DAVE (sensory-afferent, motor-efferent; dorsal-afferent, ventral-efferent). In the thoracic region, the upper lumbar region and in segments of the sacral region of the spinal cord, there is also a lateral horn of gray matter. In the thoracic and lumbar regions of the spinal cord, the lateral horn contains autonomic cell bodies for sympathetic nervous system innervation to smooth muscles, cardiac muscles and glands. In the sacral region of the spinal cord, the lateral horn contains autonomic cell bodies for parasympathetic innervation to pelvic organs. The role of the lateral horn and autonomic innervation will be discussed in another module, however, it is helpful to recognize that the lateral horns exist at this point.

8 Spinal Cord and Spinal Nerve - Page 8 of 14 We have mentioned that the spinal cord gives rise to 31 spinal nerves. The spinal nerves, along with the cranial nerves, are the means by which the CNS communicates with the periphery. Together, the spinal nerves and the cranial nerves are referred to as peripheral nerves. The cranial nerves take origin from the brain and will be discussed in another module. The spinal nerves take origin from the spinal cord (as we have discussed) and will be the topic for the remainder of this module.

9 Spinal Cord and Spinal Nerve - Page 9 of 14 Spinal nerves are formed by the union of rootlets that originate from the dorsal and ventral horns of the spinal cord. The dorsal horn gives rise to several dorsal rootlets per section of spinal cord, and the ventral horn gives rise to several ventral rootlets per section of spinal cord. These rootlets converge to form the dorsal roots and the ventral roots, one from each spinal cord segment. Keep in mind that the dorsal horns are the location of the cell bodies upon which the afferent nerves from the periphery synapse. Hence, the dorsal roots and rootlets contain afferent fibers carrying sensory information from the periphery to the CNS. The ventral horns, on the other hand, are the home of the cell bodies for the motor neurons that carry motor information from the CNS to the periphery. Therefore, the ventral rootlets and roots contain efferent fibers.

10 Spinal Cord and Spinal Nerve - Page 10 of 14 Emerging from the dorsal and ventral horns of the spinal cord are dorsal and ventral rootlets, respectively. These rootlets carry nerve fibers both toward and away from the spinal cord. The dorsal rootlets carry sensory (afferent) fibers to the spinal cord, and ventral rootlets carry motor (efferent) fibers away from the spinal cord. Laterally, near each intervertebral foramen, a number of dorsal rootlets coalesce into a single dorsal root, and a number of ventral rootlets coalesce into a single ventral root. These roots then leave the vertebral canal to lie within the intervertebral foramen. There, the dorsal and ventral roots fuse to form the spinal nerve, which is a mixed nerve carrying both sensory and motor fibers. Along the dorsal roots, lateral to the spinal cord and just medial to fusing with ventral roots, are the dorsal root ganglia, tucked in the intervertebral foramina. Recall that a ganglion is a collection of neuron cell bodies in the peripheral nervous system. The dorsal root ganglia (sometimes called the spinal ganglia) contain the cell bodies for the peripheral sensory nerve fibers. The sensory peripheral neurons have an unusual and unique structure. Sensory peripheral neurons are called pseudounipolar neurons because the cell body sits above the neuronal process, which extends in both directions (see diagram). It is very difficult to define the nerve process as an axon or dendrite because the cell is not arranged as a typical neuron. Some texts refer to the nerve process of a pseudounipolar neuron as an "axonodendrite," however usually you will hear it called simply a nerve process. The receptors for the pseudounipolar neurons are located in the skin, subcutaneous tissue, deep tissue or viscera and the nerve process synapses in the dorsal horn of the spinal cord. The cell bodies sit in the dorsal root ganglia and support the neurons. It is very important to recognize that there are NO synapses in the dorsal root ganglia! This is a very common point of confusion, but if you realize that the nerve process runs straight from the receptor all the way into the dorsal horn and that the cell body is sitting in the dorsal root ganglia solely to support the neuron, you should stay out of trouble. Again, there are no synapses in the dorsal root ganglia!

11 Spinal Cord and Spinal Nerve - Page 11 of 14 The ventral roots are formed by the axons of motor (efferent) neurons found in the ventral and lateral horns of the spinal cord. The lateral horns are related to the autonomic nervous system and will be covered in that learning module. The axons of the ventral horn cells (sometimes referred to as anterior horn cells) exit the spinal cord as multiple ventral rootlets that coalesce to form the ventral roots of the spinal nerves. The spinal nerves then distribute regionally by way of dorsal and ventral primary rami. The ventral horn cell axons traveling in the dorsal ramus innervate the skeletal muscle forming the true (paraspinal) back muscles. Those axons traveling in the ventral ramus innervate the skeletal muscle of the limbs and trunk.

12 Spinal Cord and Spinal Nerve - Page 12 of 14 Just distal to the dorsal root ganglia, the dorsal and ventral roots unite to form the 31 pairs of spinal nerves. Therefore, spinal nerves are mixed nerves, carrying both afferent fibers and efferent fibers. This is the closest mixed nerve to the spinal cord; the dorsal roots contain only afferent fibers and the ventral roots contain only efferent fibers. The spinal nerves are extremely short. They project laterally for only about 1-2mm before branching into dorsal and ventral primary rami as they exit the intervertebral foramina. The membranes covering the spinal cord become continuous with the layers surrounding the spinal nerves. The dura extends into the intervertebral foramina along the dorsal and ventral roots to form dural sleeves which eventually blend with the epineurium of the spinal nerves. The spinal nerves are a critical component of the nervous system, acting as the only path of communication between the peripheral nerves and the spinal cord.

13 Spinal Cord and Spinal Nerve - Page 13 of 14 Shortly after exiting the intervertebral foramina, the spinal nerves branch into dorsal and ventral primary rami. Be careful! Dorsal primary rami and ventral primary rami are NOT the same as dorsal roots and ventral roots! Think of the spinal nerve as the trunk of a tree - the word ramus means branch in Latin, so the primary rami are the branches of a spinal nerve, whereas the roots unite to form the spinal nerve. These terms are very easy to confuse. The primary rami are mixed nerves carrying both motor and sensory fibers. The primary rami branch many, many times to provide innervation to almost the entire body. Nerves that branch from the dorsal primary rami supply the deep back muscles as well as the skin over the back. Nerves from the ventral primary rami supply the limbs and the rest of the trunk.

14 Spinal Cord and Spinal Nerve - Page 14 of 14 In summary, the spinal cord acts principally as a highway for information to travel between the brain and the periphery. The spinal cord begins at the foramen magnum and ends at L1/L2 as the conus medullaris. Caudal to the conus medullaris is the cauda equina, which consists of spinal nerve roots from the more caudal segments of the spinal cord. The spinal cord and most of the cauda equina is enclosed by the dural sac which ends at S2 and is connected to the coccyx by the coccygeal ligament. The 31 spinal nerves, which exit the spinal cord through the intervertebral foramina, are responsible for communicating between the spinal cord and the rest of the body. Dorsal rootlets from the dorsal horns and ventral rootlets from the ventral horns converge to form dorsal and ventral roots. The dorsal roots carry only sensory information and the ventral roots carry only motor information. Inside the intervertebral foramina are the dorsal root ganglia, the cell bodies for the peripheral sensory nerve fibers, which are pseudounipolar neurons. Distal to the dorsal root ganglia, the dorsal and ventral roots unite to form spinal nerves which are mixed nerves. The spinal cord gives rise to 31 spinal nerves, named for the vertebral segment where they exit the spinal cord. Spinal nerves are very short, branching shortly after exiting the intervertebral foramina into dorsal and ventral primary rami. The primary rami subsequently branch to supply the body.