Chapter 15 - The Spinal Cord: Meninges, Spinal Nerves, and Spinal Tracts

The Spinal Cord

The spinal cord descends within the vertebral canal (bony canal formed by vertebrae)

• Spinal cord and its covering meninges are separated from bony vertebrae by fat-filled epidural space

Spinal cord is surrounded by three layers of meninges:

• Dura mater - outermost layer

• Arachnoid mater

• Pia mater - innermost layer

Dura mater

Fibrous, outermost meningeal layer

• Separated from bony vertebrae by epidural space

Forms continuous tube around spinal cord

• Anchored to margins of foramen magnum of skull - continuous with dura mater of cranial cavity

• Extends laterally to cover each spinal nerve - anchored to bony margins of intervertebral foramen

• Terminates inferiorly by attaching to coccyx

Arachnoid mater

Middle meningeal layer

• Separated from dura mater by very thin subdural space

Subarachnoid space

• Filled with cerebrospinal fluid - provides fluid cushion around spinal cord

• Thin arachnoid trabeculae pass through space (between arachnoid mater and pia mater)

Pia mater

–Innermost meningeal layer

• Separated from arachnoid mater by subarachnoid space

–Tightly adherent to outer surface of spinal cord

–Terminates as filum terminale

• At inferior end of spinal cord, pia mater narrows to thin cord-like filum terminale that continues beyond end of spinal cord

• Filum terminale runs inferiorly to attach to coccyx

• Serves to anchor inferior end of spinal cord

Spinal Cord - Cross Section

Spinal cord is part of central nervous system

–Consists of gray matter and white matter

• Gray matter - located centrally

→ Contains neuron cell bodies

• White matter - located peripherally

→ Consists of neuronal axons

–Spinal cord enlargements

Gray matter

–Forms central, “H”-shaped region

–Divided into dorsal horn and ventral horn

–Dorsal horn

• Located on dorsal (posterior) side of spinal cord

• Receives axons from sensory neurons

–Ventral horn

• Located on ventral (anterior) side of spinal cord

• Contains cell bodies of motor neurons

White matter

–Peripherally located

–Consists of neuronal axons, running vertically (up or down spinal cord)

• Tract - localized bundle of functionally related axons within white matter

• Tracts contain descending motor axons (running downward, from brain to spinal cord) or ascending sensory axons (running upward, from body to brain)

Spinal cord enlargements

–Two regions of the spinal cord are enlarged (have a wider diameter) due to an increased number of neurons

• More neurons required for innervation of the upper or lower limbs

–Cervical enlargement (C5-T1 spinal cord levels)

• Gives rise to the brachial plexus for innervation of the upper limbs

–Lumbosacral enlargement (L1-S4 spinal cord levels)

• Gives rise to the lumbar and sacral plexuses that innervate the lower limbs

Spinal Nerves

Spinal nerves exit vertebral column between adjacent vertebrae via intervertebral foramen

–Nerves are named and numbered according to the vertebral level of their intervertebral foramen:

• C1-C8 cervical spinal nerves

→ C1 spinal nerve exits between skull & C1 vertebra

→ C8 spinal nerve exits between C7 & T1 vertebrae

• T1-T12 thoracic spinal nerves

• L1-L5 lumbar spinal nerves

• S1-S5 sacral spinal nerves

• Co1 - single pair of coccygeal spinal nerves

–Spinal cord region that gives rise to each spinal nerve are also named and numbered in same manner

–Each spinal nerve is formed by the junction of a dorsal root and a ventral root

–Dorsal root

• Structure that carries only axons of sensory (afferent) neurons into the spinal cord

• Sensory neuron cell body located in dorsal root ganglion (associated with dorsal root, outside of the spinal cord)

• Dorsal root located on dorsal (posterior) side of spinal cord

• Sensory axons enter dorsal horn of spinal cord gray matter

–Ventral root

• Structure that carries only axons of motor (efferent) neurons out of the spinal cord

• Motor neuron cell body located in ventral horn of spinal cord gray matter

• Ventral root exits on ventral (anterior) side of spinal cord

Mnemonic for spinal nerves: SAME DAVE

Sensory = Afferent

Motor = Efferent

Dorsal (root) = Afferent

Ventral (root) = Efferent

–Each spinal nerve is formed by the junction of a dorsal root and a ventral root

–Thus, each spinal nerve carries axons from both afferent neurons and efferent neurons

Spinal Cord - Longitudinal

Spinal cord extends inferiorly within vertebral column

• Begins at foramen magnum of skull (continuous with medulla oblongata of brain stem)

Conus medullaris - conical, terminal end of spinal cord

• In adult, spinal cord terminates at L1-L2 vertebral level

• Pia mater continues inferiorly as filum terminale

At birth, spinal cord and vertebral column are same length (spinal cord extends full length of vertebral column)

• During childhood growth, spinal cord reaches adult size early

• Vertebral column continues to grow in length

In adult, spinal cord does not extend full length of vertebral column

• Conus medullaris located at  L1-L2 vertebral level

Each spinal nerve still exits through its intervertebral foramen

• In adult, only upper cervical nerves run laterally (from spinal cord to intervertebral foramen)

• Other spinal nerves must descend before exiting vertebral column

Cauda equina

• Large collection of descending spinal nerves located inferior to termination of spinal cord (below L2 vertebral level)