2 General Spine Anatomy and Long Tract Pathways

10.1055/b-0039-166411

2 General Spine Anatomy and Long Tract Pathways

Jacob V. DiBattista, Ankur S. Narain, Fady Y. Hijji, Philip K. Louie, Daniel D. Bohl, and Kern Singh

2.1 Topographical Anatomy

2.1.1 General Vertebral Column Anatomy

Overview of the vertebral column ( Table 2.1, Fig. 2.1 ).
Column classification:
- Vertebral column divided into three anatomical classifications ( Table 2.2, Fig. 2.2 ).
- Moderate reliability for determining clinical degree of stability.
Surface landmarks ( Table 2.3, Fig. 2.3 ).

**Fig. 2.1** Vertebral column. (Reproduced with permission from An HS, Singh K, eds. Synopsis of Spine Surgery. 3rd ed. New York, NY: Thieme; 2016.)

**Fig. 2.2** Column classification. (Reproduced with permission from Baaj AA, Mummaneni PV, Uribe JS, Vaccaro AR, Greenberg MS, eds. Handbook of Spine Surgery. 2nd ed. New York, NY: Thieme; 2016.)

**Fig. 2.3** Palpable surface landmarks to determine vertebral level. **(a)** Anterior. **(b)** Posterior.

**Table 2.1** Regions of the vertebral column
Region	Vertebral levels	Curvature ^a	Function
Cervical	C1–C7	Lordosis, 20–40 degrees	Supports and moves head Transmits spinal cord and vertebral vessels between head and neck
Thoracic	T1–T12	Kyphosis, 20–40 degrees	Supports and protects thorax
Lumbar	L1–L5	Lordosis, 40–60 degrees	Supports abdomen
Sacral	S1–S5 (fused)	Kyphosis, fused	Transmits weight to lower limbs through the pelvic bones Forms boundary and framework of posterior pelvis
Coccyx	Co	Kyphosis	Vestigial, no apparent function
^aKyphosis: concave anteriorly and convex posteriorly. Lordosis: convex anteriorly and concave posteriorly.

**Table 2.2** Column classification
Classification	Anterior boundary	Posterior boundary
Anterior column	Anterior longitudinal ligament (ALL)	Anterior two-thirds of vertebral body and intervertebral disk
Middle column	Posterior one-third of vertebral body and intervertebral disk	Posterior longitudinal ligament (PLL)
Posterior column	Immediately posterior to PLL	Ligamentum nuchae (C1–C7) or supraspinous ligament (inferior to C7)

**Table 2.3** Palpable surface landmarks and association with vertebral level
Anterior		Posterior
Vertebral level	Surface landmark	Vertebral level	Surface landmark
C2/C3	Body of mandible	C7	Most prominent
T2/T3	Jugular notch	T3	Spine of scapula (medial portion)
T9/T10	Xiphoid process	T7	Inferior tip of scapula
L3/L4	Umbilicus	L4	Superior aspect of iliac crests
S1	Anterior superior iliac spine (ASIS)	S2	Posterior superior iliac spine (PSIS)

2.1.2 General Spinal Cord Anatomy

Vertebral canal:
- Spans from foramen magnum (cranial) to the sacral hiatus (caudal).
- Formed by the vertebral foramina of each vertebra.
- Contents ( Table 2.4, Fig. 2.4 ).
Spinal cord regions and spinal nerves:
- Five divisions and 31 spinal nerves ( Fig. 2.5 ):
  - Cervical: C1–C8 spinal nerves.
  - Thoracic: T1–T12 spinal nerves.
  - Lumbar: L1–L5 spinal nerves.
  - Sacral: S1–S5 spinal nerves.
  - Coccyx: coccygeal nerve.
- Spinal nerves exit vertebral canal through intervertebral foramina.
- Spinal nerve numbering:
  - C1–C7 spinal nerves exit above their respective vertebrae.
  - C8 spinal nerve exits below C7 vertebrae (C7/T1 intervertebral foramen).
  - All remaining spinal nerves exit below their respective vertebrae.
General features of the spinal cord:
- Adult spinal cord is two-thirds the length of the vertebral column:
  - Origin: medulla (brainstem) at the foramen magnum.
  - Termination: conus medullaris (L2).
- Thecal sac comprises a dura-surrounded sac that extends from the spinal cord and contains cerebrospinal fluid (CSF), nerve roots, and the cauda equina.
- Features ( Table 2.5, Figs. 2.5, 2.6 ).
Spinal cord anatomy ( Tables 2.6–2.7 ).
Regional variation in spinal cord white and gray matter composition ( Figs. 2.7, 2.8 ):
- Cervical region:
  - Abundant gray matter for innervation of upper extremities.
  - Region of most abundant white matter.
  - Presence of both the fasciculus gracilis and cuneatus.
- Thoracic region:
  - Reduced size of gray matter (no innervation to extremities).
  - Less white matter than cervical region.
  - Intermediolateral (IML) cell column:
    - Small outpocketing of gray matter from lateral horn containing cell bodies of preganglionic sympathetic neurons.
  - Fasciculus cuneatus only present superior to T6, while fasciculus gracilis is present over the entire length.
- Lumbar region:
  - Abundant gray matter for innervation of lower extremities.
  - Less white matter than superior regions.
  - Presence of the fasciculus gracilis only.
  - Presence of cauda equina.
- Sacral region:
  - Comparatively little white or gray matter.
  - Contains an IML cell column:
    - Cell bodies of parasympathetic preganglionic neurons.
  - Presence of cauda equina.

**Fig. 2.4 (a,b)** Contents of the vertebral canal. (Reproduced with permission from Baaj AA, Mummaneni PV, Uribe JS, Vaccaro AR, Greenberg MS, eds. Handbook of Spine Surgery. 2nd ed. New York, NY: Thieme; 2016.)

**Fig. 2.5** Spinal cord regions, features, and spinal nerve. (Reproduced with permission from Albert TJ, Vaccaro, AR, eds. Physical Examination of the Spine. 2nd ed. New York, NY: Thieme; 2016.)

**Fig. 2.6** Terminal ending of the spinal cord.

**Fig. 2.7** Divisions of spinal cord white matter and gray matter. (Reproduced with permission from Fehlings MG, Vaccaro AR, Maxwell Boakye M, Rossignol S, Ditunno DF, Burns AS, eds. Essentials of Spinal Cord Injury: Basic Research to Clinical Practice. New York, NY: Thieme; 2012.)

**Fig. 2.8** Functional organization of spinal cord gray matter. (Reproduced with permission from Albertstone CD, Benzel EC, Najm IM, Steinmetz M, eds. Anatomic Basis of Neurologic Diagnosis. New York, NY: Thieme; 2009.)

**Table 2.4** Contents of the vertebral canal
Structure	Description
Superficial
Extradural (epidural) space	Contains adipose tissue and internal vertebral venous plexus
Internal vertebral venous plexus (anterior and posterior divisions)	Drains spinal cord and connects with external vertebral plexus. Lack of valves can lead to bidirectional blood flow
Meningeal layers
Dura mater (most superficial)	Composed of fibrous tissue. Continuous with the epineurium of spinal nerves
Subdural space	Potential space between dura and arachnoid mater that can open secondary to trauma (i.e., subdural hematoma)
Arachnoid mater (middle)	Adherent to dura mater. Avascular and translucent
Subarachnoid space	Between arachnoid and pia mater. Contains cerebrospinal fluid (CSF)
Pia mater (deepest)	Composed of thin fibrous tissue. Impermeable to CSF
Denticulate ligaments	Reflections of pia mater that attach to the arachnoid and dura mater. 21 pairs, spanning craniovertebral junction to T12, provide stability to the spinal cord
Spinal cord
Deep

**Table 2.5** General features of the spinal cord
Feature and vertebral level	Description
Cervical enlargement (C4–T1)	Overabundance of nerve fibers for the innervation of upper extremities
Lumbosacral enlargement (L1–L3)	Overabundance of nerve fibers for the innervation of lower extremities
Cauda equina (T12/L1–Co)	Collection of lumbar, sacral, and coccygeal nerve roots that arise from the conus medullaris Travel inferiorly within vertebral canal to exit at their respective vertebral levels
Termination of dural sac (S2)	Termination of dura mater
Filum terminale	Continuation of pia mater from conus medullaris (L2) to the coccyx. Helps anchor spinal cord in place
Internum (L2–S2)	Filum terminale within the dural sac
Externum (S2–Co)	Filum terminale outside the dural sac

**Table 2.6** Spinal cord anatomy: surface structures
Structure	Function
Root
Ventral	Carries motor information from the spinal cord. Contains general somatic efferent (GSE) and general visceral efferent (GVE) fibers
Dorsal	Carries sensory information to the spinal cord. Contains general somatic afferent (GSA) and general visceral afferent (GVA) fibers
Rootlet
Ventral	Branched portion of ventral root that attaches to spinal cord
Dorsal	Branched potion of dorsal root that attaches to spinal cord
Dorsal root ganglion	Site of GSA and GVA neuron cell bodies
Spinal nerve	Aggregation of ventral and dorsal roots (GSA, GVA, GSE, and GVE fibers). 31 pairs, which exit vertebral canal at intervertebral foramina
Ramus
Ventral	Anterior division of a spinal nerve. Innervates the ventral trunk and upper and lower limbs. Much larger than the dorsal mini
Dorsal	Posterior division of a spinal nerve. Most levels divide into medial and lateral branches to innervate skin and muscles of back (dorsal trunk)
Meningeal branches	Branch from spinal nerve prior to rami. Re-enter intervertebral foramen to innervate vertebrae and vertebral canal structures
Anterior median fissure	Runs length of ventral spinal cord, dividing it into right and left halves. Creates a groove where the anterior spinal artery lies
Anterolateral sulcus	Site where ventral rootlets exit the spinal cord
Posterior median sulcus	Runs length of dorsal spinal cord, dividing it into right and left halves. Less prominent than anterior median sulcus
Posterior intermediate sulcus	Small furrow that separates the fasciculus gracilis (medial) from the fasciculus cuneatus (lateral). Only located superior to T6
Posterolateral sulcus	Site where dorsal rootlets enter the spinal cord

**Table 2.7** Spinal cord anatomy: cross section
White matter: myelinated neurons. Contains ascending and descending tracts (pathways).
Structure ^a	Function (tracts)
Anterior funiculus	Ascending: anterior spinothalamic Descending: anterior corticospinal, medial reticulospinal, tectospinal, and medial vestibulospinal
Lateral funiculus	Ascending: lateral spinothalamic and anterior and posterior spinocerebellar Descending: lateral corticospinal, lateral reticulospinal, rubrospinal, and lateral vestibulospinal
Posterior funiculus	Ascending: dorsal column (fasciculus gracilis and cuneatus) Descending: none
Anterior white commissure	Site where certain fibers cross in the midline. Located between the anterior median fissure and gray matter commissure. Connects two halves of spinal cord
Gray matter: unmyelinated neurons. Contains interneurons and cell bodies.
Structure	Function
Ventral horn	Site of GSE cell bodies (lower motor neurons)
Lateral horn	Site of GVE (autonomic) cell bodies
Dorsal horn	Site of interneuron cell bodies (receive GSA and GVA fibers)
Gray commissure	Surrounds central canal. Connects two halves of spinal cord
Other structures
Structure	Function
Central canal	Cerebrospinal fluid–filled space within the center of the spinal cord. Continuous with the ventricular system of the brain. Gradually occludes with age
^aA fasciculus refers to a bundle of parallel axons in the central nervous system (carry same information/modality), while a funiculus refers to a grouping of fasciculi. Both are located within spinal cord white matter.