Central Nervous System: The Spinal Cord – Structure, Function, and Nerve Plexuses

Central Nervous System: The Spinal Cord

Introduction

The spinal cord is a major component of the central nervous system (CNS), responsible for transmitting and processing information between the brain and the rest of the body. This section covers the anatomical features, protective coverings, internal structure, and nerve plexuses associated with the spinal cord.

Longitudinal Anatomy of the Spinal Cord

Regions and Structure

The adult spinal cord is approximately eighteen inches long, extending from the foramen magnum to the first lumbar vertebra. It is divided into regions corresponding to the vertebral column: cervical, thoracic, lumbar, and sacral. Two enlargements, cervical and lumbar, accommodate increased neural structures for the limbs.

• Cervical enlargement: C3–T2, nerves to/from arms

• Lumbar enlargement: T7–T11, nerves to/from legs

• Cauda equina: Bundle of nerves resembling a horse’s tail, extending beyond the spinal cord

• Conus medullaris: Terminal end of the spinal cord at L1

• Terminal filum: Thin strand anchoring the spinal cord to the coccyx

• Spinal nerves: 31 pairs, named by region (C1–C8, T1–T12, L1–L5, S1–S5, 1 coccygeal)

The Meninges

Protective Coverings

The spinal cord and brain are protected by three layers of meninges:

• Dura mater: Outermost, tough and durable, anchored to vertebral cavity

• Arachnoid mater: Middle, thin, web-like, connects dura to pia mater

• Pia mater: Innermost, thin, rich in blood vessels, directly contacts CNS tissue

The subarachnoid space between arachnoid and pia mater contains cerebrospinal fluid (CSF), which cushions the CNS. CSF can be sampled via lumbar puncture for diagnostic purposes, such as detecting meningitis.

Cross-sectional Anatomy of the Spinal Cord

Gray and White Matter

Each spinal cord segment is associated with spinal nerves forming two nerve roots: posterior (dorsal, sensory) and anterior (ventral, motor). The cell bodies of sensory neurons are grouped in the posterior root ganglion.

• Gray matter: Butterfly-shaped, divided into horns (posterior, anterior, lateral)

• Posterior horn: Receives sensory input

• Anterior horn: Sends motor output

• Lateral horn: Present in thoracic, upper lumbar, and sacral regions; part of autonomic nervous system

• Gray commissure: Connects sides of gray matter, contains central canal

• White matter: Organized into columns (posterior, anterior, lateral)

• Ascending tracts: Carry sensory information to the brain

• Descending tracts: Carry motor commands from the brain

Nerve Plexuses

Organization and Function

Spinal nerves reorganize into nerve plexuses to innervate the periphery. Four major plexuses are found along the vertebral column:

• Cervical plexus: C1–C5, branches to neck, head, and phrenic nerve (diaphragm)

• Brachial plexus: C5–T1, nerves to arms (radial, median, ulnar, musculocutaneous, axillary)

• Lumbar plexus: L1–L4, nerves to pelvic region and anterior leg (femoral, obturator)

• Sacral plexus: L4–S4, nerves to posterior leg (sciatic, tibial, fibular)

Thoracic nerves (T2–T11) do not form plexuses but give rise to intercostal nerves for the muscles between ribs.

Table: Nerve Plexuses and Associated Nerves

Name	Associated Spinal Nerves	Major Associated Peripheral Nerves
Cervical	C1–C5	Phrenic
Brachial	C5–T1	Radial, median, ulnar, musculocutaneous, axillary
Lumbar	L1–L4	Femoral, obturator
Sacral	L4–S4	Sciatic

Key Terms and Concepts

• Spinal cord: CNS structure transmitting information between brain and body

• Meninges: Protective layers (dura, arachnoid, pia mater)

• Cauda equina: Bundle of nerves below spinal cord

• Gray matter: Neuronal cell bodies, divided into horns

• White matter: Myelinated axons, organized into columns

• Nerve plexus: Network of nerves reorganizing axons for peripheral innervation

Example: Sensory and Motor Pathways

For example, the radial nerve contains both sensory fibers (entering via posterior root) and motor fibers (exiting via anterior root) to the arm. The pathway for a sensory signal to excite a muscle via motor nerves involves:

1. Sensory nerve innervating the muscle

2. Posterior root

3. Posterior root ganglion

4. Posterior horn

5. Ascending tracts of white matter

6. Brain

7. Descending tracts of white matter

8. Anterior horn

9. Anterior root

10. Motor nerve innervating the muscle

Summary

The spinal cord is a complex structure essential for sensory and motor signaling, protected by meninges and organized into regions and nerve plexuses. Understanding its anatomy and function is fundamental for studies in anatomy and physiology.