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2. Describe the spine and the relationships of its parts. Detail the various joints of the spine and supportive ligaments.

The spine (aka vertebral column) consists of thirty-three vertebrae that span the distance between the occipital bone of the skull ending distally with the coccyx. The spine has five regions, each comprising a distinct type of vertebrae, some with interleaving intervertebral discs (joints). They include:

• Cervical spine - 7 cervical vertebrae

  • Neck

• Thoracic spine - 12 thoracic vertebrae

  • Articulates with ribs

• Lumbar spine - 5 lumbar vertebrae

• Sacrum - 5 fused (co-ossified) segments

  • Mostly immovable

  • Articulates with coxal (hip) bones at sacro-iliac (SI) joints

    • Slightly movable

• Coccyx - 4 fused (co-ossified) segments.

  • ‘Tailbone’

  • Mostly immovable

Joints (articulations):

The most superior joints of the spine are the atlanto-occipital joints, synovial joints between the occipital condyles and the superior facets of the atlas (C1). 

The joints below are the atlanto-axial joints, synovial joints between the inferior facets of the atlas (C1) and the superior facets of the axis (C2), as well as a synovial pivot joint with the dens (odontoid process) of C2 forming the axis of the pivot. 

Moving inferiorly, all vertebrae have three types of joints:

• Anteriorly, a symphysis (a midline, cartilaginous joint) including the body of the superior vertebrae, an intervertebral disc, and the body of the inferior vertebrae

  • Anulus fibrosus: outer layer; consists of collagen and fibrocartilage and surrounds the nucleus pulposus

  • Nucleus pulposus: inner layer; a remnant of the notochord

  • The inferior-most intervertebral disc sits between the body of L5 and the articular surface of the sacrum at the lumbosacral junction. 

• Posteriorly, two synovial facet joints

Supportive ligaments:

Providing direct support to the bodies of the vertebrae and intervertebral discs are the anterior & posterior longitudinal ligaments.

• Posterior longitudinal ligament (PLL) supports the stacked vertebral bodies and intervertebral discs posteriorly.  The PLL extends from the axis (C2) to the sacrum, and fuses with the anulus fibrosus of the intervertebral discs.

  • Most anterior feature of the vertebral canal

The posterior ligamentous complex -- consisting of the supraspinous & interspinous ligaments, ligamenta flava, and facet joint capsules -- support the laminae, spinous processes, and facet joints.

• Supraspinous ligaments are typically only present from C7 to L4, and are frequently sparse in areas. Supraspinous ligaments tend to connect 3-4 apices of serial spinous processes.

• Interspinous ligaments connect one spinous process to another from the root of the spinous process to the apex, and are best visualized from a lateral view. Not present or less robust in certain areas

• Ligamenta flava (sing. = ligamentum flavum) means yellow ligament and is composed predominantly of yellow elastic tissue. These span the distances between adjacent laminae within the vertebral canal 

3. Describe the morphology of a typical vertebrae. Compare the distinguishing features of cervical, thoracic, and lumbar vertebrae. 

3D Vertebrae

Cervical vertebrae                        Thoracic and Lumbar vertebrae                          Sacrum   

Most vertebrae consist of two major parts: a body and a vertebral arch with a vertebral foramen between these structures.

Body: Substantial, cylindrical structure that is the anterior-most portion of the vertebra.

Vertebral arch: comprised of two regions: 

• Pedicles, which attach to the posterior aspect of the body

Laminae, which extend postero-medially from the pedicles to meet at the posterior midline.

Extending from the arch are three processes:

• Spinous process: projects posteriorly along the midline

  • Palpable

• Transverse processes: extend posterolaterally

  • In thoracic region - articulates with ribs

The space between the superior articular process and the body is the superior vertebral notch, whereas the space between the inferior articular process and the body is the inferior vertebral notch. 

A pair of superior and inferior vertebral notches together form an intervertebral foramen, which is a lateral opening from the vertebral canal through which a spinal nerve is transmitted.

The vertebral arch and body surround a space, the vertebral foramen, which transmits spinal meninges, the spinal cord, spinal nerves, and associated structures.

4. Describe the meninges: their structure, distribution, specializations, functions, and relationships to osteological and neurological structures.

The central nervous system (CNS) is surrounded and protected by three layers of meninges. The meninges surrounding the brain are slightly more specialized than those surrounding the spinal cord, but the cranial and spinal meninges are contiguous. The meninges include:

• Pia mater ("delicate mother")

  • is a delicate investment that is closely applied to the brain and spinal cord

  • enmeshes blood vessels on the surface of the CNS.

  • Specializations:

    • Filum terminale internum: can be observed at the tip of the conus medullaris (~L2), traveling inferiorly through the dural sac

      • Enveloped by the filum terminale externum from ~S2 to the coccyx

      • Opaque white compared to the surrounding cauda equina

    • Denticulate ligaments: paired, extensions of pia mater

      • Located between the dorsal and ventral rootlets and roots of spinal nn.

      • ~20-22 denticulate ligaments

      • Opaque white compared to the rootlets and roots of spinal nn.

• Arachnoid mater ("spidery mother")

  • is a filmy, transparent, layer that is adhered to the dura mater and connected to the pia mater via trabeculae.

  • is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid (CSF).

• Dura mater ("tough mother")

  • outermost covering,

  • two layers in the cranium; one in the spinal canal

  • Specializations:

    • Dural sac: extension of dura past the termination of the spinal cord

      • The spinal cord ends at ~L2 at the conus medullaris but nerve roots continue inferiorly (cauda equina) and still protected by meninges and bathed in cerebrospinal fluid (CSF)

      • Dura (and the other meninges) continue inferiorly to ~S2 as the dural sac

    • Filum terminale externum: dural part of filum terminale

      • Often referred to as coccygeal ligament

      • After the dural sac terminates at S2, the externum envelops the internum to attach to the coccyx

      • Serve to anchor the dural sac to the coccyx

Spaces (actual & potential) - from superficial to deep:

• Epidural space - actual (in spine region); potential (in cranial region)

  • Between bone and dura mater

  • Filled with epidural fat in spine region

• Subdural space - potential

  • Between dura mater and arachnoid mater

• Subarachnoid space - actual

  • Between arachnoid mater and pia mater

  • Cerebrospinal fluid (CSF) flows within

Epidural block (also referred to as ‘an epidural’) is the injection of an anesthetic agent into the epidural space. Common areas of injection include ~L3-L4 or at the level of the sacral hiatus or sacral foramina (caudal epidural block). This type of block will be discussed in more detail during the reproductive anatomy sessions.

6. Define the two major structural systems of the nervous system. Describe the major structures of the central nervous system (CNS) and peripheral nervous system (PNS).

The nervous system is structured into a central nervous system (CNS) and peripheral nervous system (PNS). These systems are described separately but are functionally and anatomically continuous.

Terminology important to both CNS and PNS:

• Neuron: main unit of the nervous system

  • Composed of 3 main structures:

    • Cell body

    • Dendrite(s): process(es) that carry impulses to the neuronal cell body

    • Axon: process that carries impulses away from the neuronal cell body

      • Nerve fiber: composed of axon, neurolemma, and connective tissue

Gray matter: composed of cell bodies

White matter: composed of systems of interconnection (processes such as axons)

Central Nervous System

• Composed of: brain and spinal cord

• Terminology:

  • Nucleus: collection of cell bodies of neurons in CNS

  • Tract: nerve axon (fiber) collection in CNS

Peripheral Nervous System

• Composed of: nervous tissue outside of CNS

  • 12 pairs of cranial nerves and 31 pairs of spinal nerves

  • Spinal nerves typically form complex associations, called plexuses, from which named nerves arise

• Terminology:

  • Nerve: nerve fiber bundles, associated connective tissue, vasculature

  • Ganglion: collection of cell bodies of neurons in PNS

7. Distinguish between the parts of a spinal nerve, and explain how spinal nerves are named.

Spinal nerves arise as ventral and dorsal rootlets from the ventral and dorsal horns of gray matter, respectively. Based on their origin and composition, ventral rootlets are entirely efferent (motor), and dorsal rootlets are entirely afferent (sensory).

Rootlets coalesce to form roots. The ventral root is entirely motor, whereas the dorsal root is entirely sensory.

A notable (and grossly identifiable structure) in this area are the dorsal root ganglia (DRG). These are located in association with the dorsal root proximal to the trunk of spinal nerve, and are collections of cell bodies outside of the PNS.

The ventral & dorsal roots join to form the trunk of the spinal nerve, often referred to as just spinal nerve. At this point, the afferent (from the dorsal root) and efferent (from the ventral root) fibers combine, making the trunk of the spinal nerve a mixed nerve (having both afferent and efferent nerve fibers).

The trunk of the spinal nerve exits the vertebral canal laterally via an intervertebral foramen and almost immediately bifurcates into a dorsal primary ramus (DPR) and a ventral primary ramus (VPR). Both the DPR and VPR are mixed nerves. Primary ramus means 1st branch, and so think of the DPR and VPR as the first branches of the spinal nerve. Each DPR and VPR will typically have numerous distal branches (some of which are named).

The dorsal primary rami (DPR) efferently and afferently serve the deep back (including vertebral joints and deep back muscles) as well as cutaneous innervation for the back. DPR branches rarely form plexuses (networks of nerve fibers from different spinal cord levels).

The ventral primary rami innervate a substantial area including all structures not supplied by DPRs and cranial nerves, including the upper and lower limbs. VPR branches frequently form plexuses (cervical, brachial, lumbar, & sacral).

Spinal nerves are named for the spinal cord segment from which it originates and the vertebral level that the nerve exits the vertebral column.

There are 31 spinal nerve pairs:

• 8 cervical spinal nerve pairs (C1-C8)

• 12 thoracic spinal nerve pairs (T1-T12)

• 5 lumbar spinal nerve pairs (L1-L5)

• 5 sacral spinal nerve pairs (S1-S5)

• 1 coccygeal spinal nerve pairs (Co1)

8. Illustrate at what levels spinal nerves exit intervertebral foramina, and describe how knowledge of this can allow a better understanding of the clinical presentation of herniated discs.

Spinal nerves typically exit the vertebral column through intervertebral foramina inferior to the same-numbered vertebra.

For example, the T7 spinal nerve exits the column between the T7 & T8 vertebrae. Another example includes the L5 spinal nerve exits the vertebral column between the L5 & S1 vertebrae.

The main exception is in the cervical region. There are 8 cervical spinal nerves and only 7 cervical vertebrae. Cervical spinal nerves exit the vertebral column superior to the same-numbered vertebra. For example, the C1 spinal nerve exits the vertebral column between the skull & C1, C2 spinal nerve exits between C1 & C2 vertebrae, and C8 spinal nerve exits between C7 & T1 vertebrae.

Herniations of the IV disc tend to occur posterolaterally.

•Example: herniation of IV disc between L3 & L4 would likely NOT affect L3 spinal nerve as is exits too superiorly in IV foramen, but it MAY affect L4 spinal nerve.