Spinal Cord Organization and Function: Structure, Pathways, and Clinical Relevance 5

Spinal Cord Organization and Function

Overview

The spinal cord forms the inferior part of the central nervous system (CNS) and serves as a critical conduit for information between the brain and the rest of the body. It is essential for both relaying and processing neural signals.

• Location: The spinal cord is a long, tubular organ located within the vertebral cavity of the dorsal body cavity.

• Connection to Brain: It passes through the foramen magnum and joins the brainstem, forming a continuous pathway for neural communication.

Spinal Cord Anatomy

Basic Features

• Extent: The spinal cord extends from the foramen magnum to the level between the first and second lumbar vertebrae (L1-L2).

• Length: Measures approximately 43–46 cm (17–18 inches).

• Diameter: Ranges from 0.65–1.25 cm (0.25–0.5 inch).

External Structure

• Posterior Median Sulcus: A narrow groove on the dorsal side.

• Anterior Median Fissure: A wider groove on the ventral side.

• Conus Medullaris: The tapered, lower end of the spinal cord.

• Filum Terminale: A fibrous extension of the pia mater anchoring the spinal cord to the coccyx.

• Enlargements: Cervical and lumbar enlargements correspond to the origins of nerves for the upper and lower limbs.

Functions of the Spinal Cord

Relay and Processing

• Relay Station: Acts as an intermediate between the body and the brain.

• Outgoing Information: Receives signals from the brain and sends them to the body.

• Incoming Information: Receives sensory input from the body and transmits it to the brain.

• Processing Station: Performs integration and processing of information, including spinal reflexes that can occur independently of the brain.

Spinal Cord Protection

Spinal Meninges

The spinal cord is protected by three connective tissue layers called meninges, which are continuous with those of the brain:

1. Dura Mater: The tough, outermost layer.

2. Arachnoid Mater: The middle, web-like layer.

3. Pia Mater: The delicate, innermost layer that adheres to the spinal cord surface.

• Special Features: The spinal dura mater lacks a periosteal layer. The pia mater anchors the spinal cord to the vertebral cavity via denticulate ligaments.

Spaces Associated with the Meninges

• Epidural Space: Located between the dura mater and the vertebral foramen; filled with veins and adipose tissue for cushioning and protection.

• Subdural Space: A potential space between the dura mater and arachnoid mater; typically minimal in healthy individuals.

• Subarachnoid Space: Between the arachnoid and pia mater; filled with cerebrospinal fluid (CSF) and is the site for lumbar punctures (spinal taps).

Internal Anatomy of the Spinal Cord

Gray and White Matter

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons. Arranged in a butterfly or H-shaped pattern.

• White Matter: Composed of myelinated axons organized into columns (funiculi) that carry information up (sensory) and down (motor) the spinal cord.

Gray Matter Regions

• Anterior (Ventral) Horn: Contains somatic motor neurons.

• Posterior (Dorsal) Horn: Contains interneurons that process somatic and visceral sensory information.

• Lateral Horn: Present from T1 to L2; contains cell bodies for autonomic (visceral motor) neurons.

• Gray Commissure: Connects the two sides of gray matter and surrounds the central canal, which contains CSF.

White Matter Regions

• Funiculi: The white matter is divided into anterior, lateral, and posterior columns (funiculi), each containing bundles of axons called tracts.

• Ascending Tracts: Carry sensory information to the brain (e.g., posterior columns, spinocerebellar tracts, anterolateral system).

• Descending Tracts: Carry motor commands from the brain to the body (e.g., corticospinal, reticulospinal, tectospinal, vestibulospinal tracts).

Spinal Nerves and Plexuses

Structure of Spinal Nerves

• Origin: Spinal nerves arise from the spinal cord and innervate regions below the head and neck.

• Roots: Each spinal nerve is formed by the union of a posterior (sensory) root and an anterior (motor) root.

• Posterior Root Ganglion: Contains cell bodies of sensory neurons.

• Connective Tissue Sheaths: Nerves are bundled by the epineurium (outer), perineurium (around fascicles), and endoneurium (around individual axons).

Cauda Equina

• Definition: A bundle of spinal nerve roots extending from the conus medullaris, resembling a horse's tail.

Nerve Plexuses

• Plexus: A network of intersecting nerves formed from the ventral rami of spinal nerves.

• Major Plexuses:

  • Cervical Plexus

  • Brachial Plexus

  • Lumbar Plexus

  • Sacral Plexus

• Brachial Plexus Example: Composed of the axillary, radial, musculocutaneous, median, and ulnar nerves, which innervate the upper limb.

Segmental Innervation and Dermatomes

Dermatomes

• Definition: A dermatome is an area of skin supplied by sensory fibers from a single spinal nerve root.

• Clinical Relevance: Dermatome maps are used to diagnose the level of spinal cord or nerve root injury.

• Note: All spinal nerves except the first cervical (C1) have associated dermatomes.

Decussation

Definition and Importance

• Decussation: The crossing over of nerve fibers from one side of the CNS to the other, often occurring in the spinal cord or brainstem.

• Functional Significance: Explains why each side of the brain controls the opposite side of the body.

Summary Table: Spinal Cord Meningeal Spaces

Space	Location	Contents	Clinical Significance
Epidural Space	Between dura mater and vertebral canal	Veins, adipose tissue	Site for epidural anesthesia
Subdural Space	Between dura mater and arachnoid mater	Potential space (minimal fluid)	Can fill with blood in subdural hematoma
Subarachnoid Space	Between arachnoid and pia mater	Cerebrospinal fluid (CSF)	Site for lumbar puncture (spinal tap)

Key Terms and Definitions

• Central Nervous System (CNS): Composed of the brain and spinal cord.

• Spinal Reflex: An automatic response mediated by the spinal cord, independent of brain input.

• Funiculus: A bundle of nerve fibers in the white matter of the spinal cord.

• Plexus: A network of intersecting nerves.

• Dermatome: Area of skin innervated by a single spinal nerve.

• Decussation: Crossing of nerve fibers from one side to the other in the CNS.