Study Notes: The Spinal Cord and Spinal Nerves (Chapters 13-14)

The Spinal Cord

Overview and Location

The spinal cord is a major component of the central nervous system (CNS), housed within protective membranes called meninges and the vertebral column. It serves as a conduit for sensory and motor information between the brain and most other parts of the body, and gives rise to the spinal nerves.

• Protective membranes (meninges): Surround and protect the spinal cord.

• Vertebral column: Bony structure encasing the spinal cord.

• Function: Transmits sensory input to the brain and motor output to effectors.

Gross Anatomy of the Spinal Cord

The spinal cord extends from the brain to the level of vertebrae L1 and L2, and stops lengthening around age 4, although the vertebral column continues to grow.

• Regions: Cervical, thoracic, lumbar, sacral

• Bilateral symmetry: Left and right halves are mirror images.

• Segments: 31 segments, each giving rise to a pair of spinal nerves.

• Grooves: Divide the cord into left and right:

  • Posterior median sulcus (shallow groove)

  • Anterior median fissure (deeper groove)

• Central canal: Contains cerebrospinal fluid (CSF)

Key Structures and Features

• Cervical enlargement: Nerves of shoulders and upper limbs

• Lumbar enlargement: Nerves of pelvis and lower limbs

• Conus medullaris: Thin, conical spinal cord below lumbar enlargement

• Cauda equina: Nerve roots extending below conus medullaris

• Filum terminale: Thin strand of fibrous tissue at end of conus medullaris, attaches to coccygeal ligament

Additional info: More gray matter in regions dedicated to sensory and motor control.

Naming of Spinal Cord Segments

• Cervical nerves: Named based on the cervical vertebrae immediately inferior to it.

• Thoracic, lumbar, sacral nerves: Named based on the vertebrae immediately superior to it.

Spinal Nerves

Formation and Structure

Spinal nerves are formed by the union of posterior and anterior roots, with one pair emerging from each side at each vertebral level. Each spinal nerve contains both afferent (sensory) and efferent (motor) fibers, making them mixed nerves.

• White ramus communicans: Innervates glands and smooth muscle (myelinated axons)

• Gray ramus communicans: Innervates glands and smooth muscle (unmyelinated axons)

• Posterior ramus: Supplies skin/muscles of back

• Anterior ramus: Supplies most of body wall, skin, limbs

Cross-Sectional Anatomy

The spinal cord is organized into white matter and gray matter, with the following key features:

• Posterior root: Sensory input

• Anterior root: Motor output

• Spinal ganglion: Contains cell bodies of sensory neurons

• Central canal: Contains CSF

Spinal Meninges

Functions and Layers

The spinal meninges are specialized membranes that isolate the spinal cord from its surroundings, including the bony walls of the vertebral column. Their functions include protecting the spinal cord, providing physical stability and shock absorption, and supplying blood. The meninges are continuous with the cranial meninges.

• Meningitis: Viral or bacterial infection of the meninges

The Three Meningeal Layers

1. Dura mater: Outer layer, tough and fibrous collagen

   • Fuses with periosteum of occipital bone cranially

   • Joins filum terminale in coccygeal ligament caudally

   • Provides longitudinal stability

2. Arachnoid mater: Middle meningeal layer

   • Simple squamous epithelia

3. Pia mater: Innermost meningeal layer

   • Mesh of collagen and elastic fibers

   • Bound to underlying neural tissue

Spaces Associated with the Meninges

• Epidural space: Between spinal dura mater and vertebral canal; contains loose connective and adipose tissue; site for anesthetic injection

• Subdural space: Between arachnoid mater and dura mater

• Subarachnoid space: Between arachnoid mater and pia mater; contains collagen/elastic fiber network (arachnoid trabeculae); filled with cerebrospinal fluid (CSF); site for lumbar punctures

Cerebrospinal Fluid (CSF)

CSF carries dissolved gases, nutrients, and wastes. It acts as a shock absorber and facilitates exchange of materials. CSF is withdrawn via lumbar puncture or spinal tap, typically in the lumbar region to avoid damaging the spinal cord.

Additional Structures

• Denticulate ligaments: Extend from pia mater to dura mater; stabilize side-to-side movement

• Blood vessels: Along surface of spinal pia mater and within subarachnoid space; provide nutrients/oxygen and waste exchange

Gray Matter and White Matter

Gray Matter Organization

Masses of gray matter within the CNS are called nuclei and are organized into regions called horns:

• Posterior horns: Somatic and visceral sensory nuclei (incoming information from receptors)

• Anterior horns: Somatic motor nuclei (outgoing information to effectors)

• Lateral horns: Thoracic and lumbar segments; visceral motor nuclei

White Matter Organization

White matter is organized into three columns:

• Posterior white columns: Between posterior horns and posterior median sulcus

• Anterior white columns: Between anterior horns and anterior median fissure; includes anterior white commissure (axons cross from one side to the other)

• Lateral white columns: On each side of spinal cord, between anterior and posterior columns

Functional Organization of White Matter

• Tract: Bundle of axons in CNS relaying the same type of information in the same direction

• Ascending tracts: Sensory information up toward the brain

• Descending tracts: Motor commands down to the spinal cord

Spinal Nerves and Plexuses

Anatomy of Spinal Nerves

Each spinal cord segment is connected to a pair of spinal nerves, which are surrounded by three connective tissue layers that support structures and contain blood vessels.

Connective Tissue Layers of Spinal Nerves

Layer	Location	Function
Epineurium	Outer layer	Dense network of collagen fibers
Perineurium	Middle layer	Divides nerve into fascicles (axon bundles)
Endoneurium	Inner layer	Surrounds individual axons

Peripheral Nerves

• Form from branching and re-sorting of spinal nerves

• All are mixed nerves (sensory and motor)

• Same connective tissue layers as spinal nerves

Peripheral Distribution of Spinal Nerves

Motor Nerves

• Distribute motor commands from motor nuclei to thoracic/lumbar segments of spinal cord

• Rami Communicantes:

  • White ramus: Carries visceral motor fibers to sympathetic ganglion of autonomic nervous system

  • Gray ramus: Unmyelinated nerves from sympathetic ganglion

• Dorsal ramus: Contains somatic and visceral motor fibers; innervates the back

• Ventral ramus: Larger branch; innervates ventrolateral structures and limbs

Sensory Nerves

• Carry sensory information from periphery sensors to sensory nuclei in spinal cord (lumbar and thoracic)

• Dorsal, ventral, and white rami also carry sensory information

Dermatomes

• Specific bilateral region of skin

• Monitored by specific pair of spinal nerves

Summary Table: Spinal Cord Regions and Functions

Region	Main Function
Cervical	Innervates shoulders and upper limbs
Thoracic	Innervates trunk and some internal organs
Lumbar	Innervates pelvis and lower limbs
Sacral	Innervates pelvic organs and lower limbs

Key Terms and Definitions

• Spinal cord: Main pathway for information connecting the brain and peripheral nervous system

• Meninges: Three protective membranes covering the brain and spinal cord

• CSF (Cerebrospinal Fluid): Fluid that cushions the CNS and facilitates exchange of nutrients and waste

• Gray matter: Contains neuron cell bodies; involved in processing and integration

• White matter: Contains myelinated axons; involved in transmission of signals

• Spinal nerves: Mixed nerves carrying sensory and motor information

• Dermatome: Area of skin supplied by a single spinal nerve

Example Application

Damage to a specific spinal nerve can result in loss of sensation or motor function in the corresponding dermatome or muscle group. For example, injury to the lumbar region may affect lower limb movement and sensation.

Additional info: The lumbar region is preferred for lumbar punctures because the spinal cord ends above this level, reducing risk of injury.