How many vertebral segments are there

Seven processes arise from the vertebral arch: the spinous process, two transverse processes, two superior facets, and two inferior facets. The facet joints of the spine allow back motion. Each vertebra has four facet joints, one pair that connects to the vertebra above superior facets and one pair that connects to the vertebra below inferior facets Fig.

The ligaments are strong fibrous bands that hold the vertebrae together, stabilize the spine, and protect the discs. The three major ligaments of the spine are the ligamentum flavum, anterior longitudinal ligament ALL , and posterior longitudinal ligament PLL Fig.

The ALL and PLL are continuous bands that run from the top to the bottom of the spinal column along the vertebral bodies.

They prevent excessive movement of the vertebral bones. The ligamentum flavum attaches between the lamina of each vertebra. The spinal cord is about 18 inches long and is the thickness of your thumb. It runs from the brainstem to the 1st lumbar vertebra protected within the spinal canal. At the end of the spinal cord, the cord fibers separate into the cauda equina and continue down through the spinal canal to your tailbone before branching off to your legs and feet.

The spinal cord serves as an information super-highway, relaying messages between the brain and the body. The brain sends motor messages to the limbs and body through the spinal cord allowing for movement.

The limbs and body send sensory messages to the brain through the spinal cord about what we feel and touch. Sometimes the spinal cord can react without sending information to the brain. These special pathways, called spinal reflexes, are designed to immediately protect our body from harm. Any damage to the spinal cord can result in a loss of sensory and motor function below the level of injury. For example, an injury to the thoracic or lumbar area may cause motor and sensory loss of the legs and trunk called paraplegia.

An injury to the cervical neck area may cause sensory and motor loss of the arms and legs called tetraplegia, formerly known as quadriplegia. Thirty-one pairs of spinal nerves branch off the spinal cord. Each spinal nerve has two roots Fig. The ventral front root carries motor impulses from the brain and the dorsal back root carries sensory impulses to the brain.

The ventral and dorsal roots fuse together to form a spinal nerve, which travels down the spinal canal, alongside the cord, until it reaches its exit hole - the intervertebral foramen Fig.

Once the nerve passes through the intervertebral foramen, it branches; each branch has both motor and sensory fibers. The smaller branch called the posterior primary ramus turns posteriorly to supply the skin and muscles of the back of the body.

The larger branch called the anterior primary ramus turns anteriorly to supply the skin and muscles of the front of the body and forms most of the major nerves.

The spinal nerves are numbered according to the vertebrae above which it exits the spinal canal. The 8 cervical spinal nerves are C1 through C8, the 12 thoracic spinal nerves are T1 through T12, the 5 lumbar spinal nerves are L1 through L5, and the 5 sacral spinal nerves are S1 through S5.

There is 1 coccygeal nerve. The spinal nerves innervate specific areas and form a striped pattern across the body called dermatomes Fig. Doctors use this pattern to diagnose the location of a spinal problem based on the area of pain or muscle weakness. For example leg pain sciatica usually indicates a problem near the L4-S3 nerves.

The spinal cord is covered with the same three membranes as the brain, called meninges. The inner membrane is the pia mater, which is intimately attached to the cord. The next membrane is the arachnoid mater.

The outer membrane is the tough dura mater Fig. Between these membranes are spaces used in diagnostic and treatment procedures. The space between the pia and arachnoid mater is the wide subarachnoid space, which surrounds the spinal cord and contains cerebrospinal fluid CSF.

These joints are much like the majority of other joints in the body, and are known as synovial joints. They are lined by cartilage and contain lubricating fluid to facilitate movement. However, just as other synovial joints can wear, so can facet joints. The wear and tear process is very similar to that of a worn hip or knee; joint movements can become painful, the joint space becomes reduced and the joint itself can become enlarged. The pain from facet joint degeneration is often most pronounced with the back in extension tilting back or when it is straightening from a flexed position, as this is when there is increased pressure on the facet joints.

The other problem that worn facet joints can cause is their enlargement can encroach on the space for the spinal cord and spinal nerves. This can then lead to the condition of spinal stenosis. The muscles that surround and support the spine are frequently referred to as the core muscles. They can be divided into the muscles at the front of the spine for flexion of the spine including the abdominal muscles and the muscles behind the spine that extend it.

The flexor and extensor muscles have an important role in maintaining the balance and the alignment of the spine, and when weakened, can lead to significant problems with mechanical back pain.

Strong core muscles also help to reduce the strain on the spine itself. Maintaining healthy spinal musculature is therefore key to the overall health of the spine. The other soft tissue components of the spine are the ligaments. These are tough fibrous structures which play an important role in stabilising the spine and keeping the vertebrae correctly aligned. There are three main ligaments which run from the top to the bottom of the spine the anterior and posterior longitudinal ligaments and the ligamentum flavum , as well as further ligaments joining adjacent vertebrae such as the interspinous ligaments.

As part of the degenerative process, ligaments can become enlarged or lengthened, and this can also contribute to the development of spinal stenosis or myelopathy. The neurological part of the spine consists of the spinal cord and the nerves emerging from it the spinal nerves. These are contained deep within the centre of the spine in the canal formed by bony arches of each vertebra.

The enveloping bone and ligaments means that the neurological structures are exceedingly well protected. The spinal cord emerges from the brain cavity where it is continuous with the lower brainstem.

The spinal cord carries a vast amount of information both to and from the trunk and limbs. The motor messages run from the brain downwards, and sensory signals originate peripherally and are then relayed back to the brain via the spinal cord. Considering the number and speed of the electrical messages conveyed, the spinal cord is a remarkably small structure only measuring approximately mm in maximum diameter.

The propriospinal cells are spinal interneurons whose axons do not leave the spinal cord proper. Some of these fibers also are found around the margin of the gray matter of the cord and are collectively called the fasciculus proprius or the propriospinal or the archispinothalamic tract. The prominent nuclear groups of cell columns within the spinal cord from dorsal to ventral are the marginal zone, substantia gelatinosa, nucleus proprius, dorsal nucleus of Clarke, intermediolateral nucleus and the lower motor neuron nuclei.

The axons of its neurons contribute to the lateral spinothalamic tract which relays pain and temperature information to the diencephalon Figure 3.

Substantia gelatinosa is found at all levels of the spinal cord. Located in the dorsal cap-like portion of the head of the dorsal horn, it relays pain, temperature and mechanical light touch information and consists mainly of column cells intersegmental column cells. These column cells synapse in cell at Rexed layers IV to VII, whose axons contribute to the ventral anterior and lateral spinal thalamic tracts.

The homologous substantia gelatinosa in the medulla is the spinal trigeminal nucleus. Nucleus proprius is located below the substantia gelatinosa in the head and neck of the dorsal horn.

This cell group, sometimes called the chief sensory nucleus, is associated with mechanical and temperature sensations. It is a poorly defined cell column which extends through all segments of the spinal cord and its neurons contribute to ventral and lateral spinal thalamic tracts, as well as to spinal cerebellar tracts.

The axons originating in nucleus proprius project to the thalamus via the spinothalamic tract and to the cerebellum via the ventral spinocerebellar tract VSCT. Dorsal nucleus of Clarke is a cell column located in the mid-portion of the base form of the dorsal horn. The axons from these cells pass uncrossed to the lateral funiculus and form the dorsal posterior spinocerebellar tract DSCT , which subserve unconscious proprioception from muscle spindles and Golgi tendon organs to the cerebellum, and some of them innervate spinal interneurons.

The dorsal nucleus of Clarke is found only in segments C8 to L3 of the spinal cord and is most prominent in lower thoracic and upper lumbar segments. The homologous dorsal nucleus of Clarke in the medulla is the accessory cuneate nucleus, which is the origin of the cuneocerebellar tract CCT. Intermediolateral nucleus is located in the intermediate zone between the dorsal and the ventral horns in the spinal cord levels.

Extending from C8 to L3, it receives viscerosensory information and contains preganglionic sympathetic neurons, which form the lateral horn. A large proportion of its cells are root cells which send axons into the ventral spinal roots via the white rami to reach the sympathetic tract as preganglionic fibers.

Similarly, cell columns in the intermediolateral nucleus located at the S2 to S4 levels contains preganglionic parasympathetic neurons Figure 3. Lower motor neuron nuclei are located in the ventral horn of the spinal cord.

The a motor neurons are the final common pathway of the motor system, and they innervate the visceral and skeletal muscles. The distribution of cells and fibers within the gray matter of the spinal cord exhibits a pattern of lamination. The cellular pattern of each lamina is composed of various sizes or shapes of neurons cytoarchitecture which led Rexed to propose a new classification based on 10 layers laminae.

This classification is useful since it is related more accurately to function than the previous classification scheme which was based on major nuclear groups Figure 3. Laminae I to IV, in general, are concerned with exteroceptive sensation and comprise the dorsal horn, whereas laminae V and VI are concerned primarily with proprioceptive sensations.

Lamina VII is equivalent to the intermediate zone and acts as a relay between muscle spindle to midbrain and cerebellum, and laminae VIII-IX comprise the ventral horn and contain mainly motor neurons. The axons of these neurons innervate mainly skeletal muscle. Lamina X surrounds the central canal and contains neuroglia.

Rexed lamina I — Consists of a thin layer of cells that cap the tip of the dorsal horn with small dendrites and a complex array of nonmyelinated axons. Cells in lamina I respond mainly to noxious and thermal stimuli. Lamina I cell axons join the contralateral spinothalamic tract; this layer corresponds to nucleus posteromarginalis.

Rexed lamina II — Composed of tightly packed interneurons. This layer corresponds to the substantia gelatinosa and responds to noxious stimuli while others respond to non-noxious stimuli. The majority of neurons in Rexed lamina II axons receive information from sensory dorsal root ganglion cells as well as descending dorsolateral fasciculus DLF fibers.

High concentrations of substance P and opiate receptors have been identified in Rexed lamina II. The lamina is believed to be important for the modulation of sensory input, with the effect of determining which pattern of incoming information will produce sensations that will be interpreted by the brain as being painful. Rexed lamina III — Composed of variable cell size, axons of these neurons bifurcate several times and form a dense plexus.

Rexed lamina IV — The thickest of the first four laminae. In addition, dendrites of neurons in lamina IV radiate to lamina II, and respond to stimuli such as light touch. The ill-defined nucleus proprius is located in the head of this layer. Some of the cells project to the thalamus via the contralateral and ipsilateral spinothalamic tract. Rexed lamina V — Composed neurons with their dendrites in lamina II. This lamina covers a broad zone extending across the neck of the dorsal horn and is divided into medial and lateral parts.

Many of the Rexed lamina V cells project to the brain stem and the thalamus via the contralateral and ipsilateral spinothalamic tract. Moreover, descending corticospinal and rubrospinal fibers synapse upon its cells. Rexed lamina VI — Is a broad layer which is best developed in the cervical and lumbar enlargements. Lamina VI divides also into medial and lateral parts.

Group Ia afferent axons from muscle spindles terminate in the medial part at the C8 to L3 segmental levels and are the source of the ipsilateral spinocerebellar pathways. Many of the small neurons are interneurons participating in spinal reflexes, while descending brainstem pathways project to the lateral zone of Rexed layer VI.

Rexed lamina VII — This lamina occupies a large heterogeneous region. This region is also known as the zona intermedia or intermediolateral nucleus. Its shape and boundaries vary along the length of the cord. Lamina VII neurons receive information from Rexed lamina II to VI as well as visceral afferent fibers, and they serve as an intermediary relay in transmission of visceral motor neurons impulses.

The dorsal nucleus of Clarke forms a prominent round oval cell column from C8 to L3. The large cells give rise to uncrossed nerve fibers of the dorsal spinocerebellar tract DSCT. Cells in the lateral horn of the cord in segments T1 and L3 give rise to preganglionic sympathetic fibers to innervate postganglionic cells located in the sympathetic ganglia outside the cord.

Lateral horn neurons at segments S2 to S4 give rise to preganglionic neurons of the sacral parasympathetic fibers to innervate postganglionic cells located in peripheral ganglia. Rexed lamina VIII — Includes an area at the base of the ventral horn, but its shape differs at various cord levels.

In the cord enlargements, the lamina occupies only the medial part of the ventral horn, where descending vestibulospinal and reticulospinal fibers terminate. The neurons of lamina VIII modulate motor activity, most probably via g motor neurons which innervate the intrafusal muscle fibers.

Its size and shape differ at various cord levels. Rexed lamina X — Neurons in Rexed lamina X surround the central canal and occupy the commissural lateral area of the gray commissure, which also contains decussating axons. In summary, laminae I-IV are concerned with exteroceptive sensations, whereas laminae V and VI are concerned primarily with proprioceptive sensation and act as a relay between the periphery to the midbrain and the cerebellum.

All visceral motor neurons are located in lamina VII and innervate neurons in autonomic ganglia. Surrounding the gray matter is white matter containing myelinated and unmyelinated nerve fibers. These fibers conduct information up ascending or down descending the cord.

The white matter is divided into the dorsal or posterior column or funiculus , lateral column and ventral or anterior column Figure 3. The anterior white commissure resides in the center of the spinal cord, and it contains crossing nerve fibers that belong to the spinothalamic tracts, spinocerebellar tracts, and anterior corticospinal tracts. Three general nerve fiber types can be distinguished in the spinal cord white matter: 1 long ascending nerve fibers originally from the column cells, which make synaptic connections to neurons in various brainstem nuclei, cerebellum and dorsal thalamus, 2 long descending nerve fibers originating from the cerebral cortex and various brainstem nuclei to synapse within the different Rexed layers in the spinal cord gray matter, and 3 shorter nerve fibers interconnecting various spinal cord levels such as the fibers responsible for the coordination of flexor reflexes.

Ascending tracts are found in all columns whereas descending tracts are found only in the lateral and the anterior columns. Four different terms are often used to describe bundles of axons such as those found in the white matter: funiculus, fasciculus, tract, and pathway. Funiculus is a morphological term to describe a large group of nerve fibers which are located in a given area e. Within a funiculus, groups of fibers from diverse origins, which share common features, are sometimes arranged in smaller bundles of axons called fasciculus, e.

Fasciculus is primarily a morphological term whereas tracts and pathways are also terms applied to nerve fiber bundles which have a functional connotation. A tract is a group of nerve fibers which usually has the same origin, destination, and course and also has similar functions. The tract name is derived from their origin and their termination i. A pathway usually refers to the entire neuronal circuit responsible for a specific function, and it includes all the nuclei and tracts which are associated with that function.

For example, the spinothalamic pathway includes the cell bodies of origin in the dorsal root ganglia , their axons as they project through the dorsal roots, synapses in the spinal cord, and projections of second and third order neurons across the white commissure, which ascend to the thalamus in the spinothalamic tracts.

Ascending tracts Figure 3. The ascending tracts transmit sensory information from the sensory receptors to higher levels of the CNS.