Chapter 13: The Spinal Cord and Spinal Nerves

Chapter 13: The Spinal Cord

Spinal Cord Anatomy

The spinal cord is a vital part of the central nervous system, protected by the vertebral column and meninges. It serves as a major pathway for information traveling between the brain and the rest of the body.

• Protective Structures: Vertebral column and meninges provide physical stability and protection.

• Meninges: Three layers:

  • Dura mater (outermost, tough)

  • Arachnoid mater (middle, vascular)

  • Pia mater (innermost, delicate)

• Spaces associated with meninges:

  • Epidural space (fat)

  • Subdural space (interstitial fluid)

  • Subarachnoid space (cerebrospinal fluid, CSF)

External Anatomy of the Spinal Cord

The spinal cord has distinct external features and regions that correspond to the origins of spinal nerves.

• Enlargements: Cervical and lumbar regions where nerves to limbs arise.

• Conus medullaris: Tapered end of the spinal cord.

• Cauda equina: Bundle of nerve roots below the conus medullaris.

• Filum terminale: Extension of pia mater anchoring the cord.

• Posterior root ganglion: Contains cell bodies of sensory neurons.

• Spinal nerves: Emerge from the cord and pass through intervertebral foramina.

Internal Anatomy of the Spinal Cord

Internally, the spinal cord is organized into gray and white matter, each with specific functions and structures.

• Gray matter: Shaped like an H or butterfly, contains neuron cell bodies.

• White matter: Surrounds gray matter, contains myelinated axons.

• Central canal: Contains circulating CSF.

• Anterior, posterior, and lateral horns: Regions of gray matter for sensory and motor processing.

• Columns (funiculi): Bundles of white matter tracts.

Sensory and Motor Tracts

Tracts are bundles of axons in the white matter that carry sensory and motor information.

• Ascending tracts: Carry sensory impulses to the brain.

• Descending tracts: Carry motor impulses from the brain to effectors.

• Tract naming: Indicates origin and destination (e.g., spinothalamic tract).

Spinal Nerves

Spinal nerves are mixed nerves that connect the spinal cord to the body, facilitating communication.

• 31 pairs: Cervical (C1–C8), thoracic (T1–T12), lumbar (L1–L5), sacral (S1–S5), coccygeal (Co1).

• Connective tissue coverings:

  • Epineurium: Surrounds entire nerve

  • Perineurium: Surrounds fascicles

  • Endoneurium: Surrounds individual axons

• Fascicle: Bundle of fibers/axons

Distribution of Spinal Nerves

Spinal nerves branch and form networks to supply different body regions.

• Posterior (dorsal) ramus: Supplies skin and muscles of the back.

• Anterior (ventral) ramus: Supplies limbs and anterior trunk.

• Plexuses: Networks of axons (cervical, brachial, lumbar, sacral, coccygeal).

• Rami communicantes: Connect spinal nerves to sympathetic chain ganglia.

Plexuses of Spinal Nerves

Plexuses are networks formed by anterior rami of spinal nerves, supplying specific regions.

• Cervical plexus: Supplies skin and muscles of head, neck, diaphragm, and upper shoulders.

• Brachial plexus: Supplies shoulders and upper limbs.

• Lumbar plexus: Supplies abdominal wall, external genitalia, and part of lower limbs.

• Sacral plexus: Supplies buttocks, perineum, lower limbs; includes the sciatic nerve (largest in body).

• Coccygeal plexus: Supplies small area of skin in coccygeal region.

Reflexes and Reflex Arcs

Reflexes are rapid, involuntary responses to stimuli, often mediated by the spinal cord.

• Reflex: Automatic, fast, involuntary response to a stimulus.

• Spinal reflex: Integration occurs in the spinal cord.

• Reflex arc: Pathway followed by nerve impulses producing a reflex. Components:

  1. Sensory receptor

  2. Sensory neuron

  3. Integration center

  4. Motor neuron

  5. Effector

Types of Reflexes

Several types of spinal reflexes help maintain posture, protect from injury, and regulate muscle tension.

• Stretch reflex: Causes contraction of a skeletal muscle in response to stretching; maintains muscle tone and coordination.

• Tendon reflex: Controls muscle tension by causing relaxation when tension is excessive; prevents muscle damage.

• Flexor (withdrawal) reflex: Moves a limb away from painful stimulus.

• Crossed extensor reflex: Extends joints in the opposite limb to maintain balance during withdrawal reflex.

Summary Table: Spinal Cord Meninges and Spaces

Layer/Space	Location	Contents
Epidural space	Between vertebrae and dura mater	Fat, blood vessels
Dura mater	Outermost meningeal layer	Tough connective tissue
Subdural space	Between dura mater and arachnoid mater	Interstitial fluid
Arachnoid mater	Middle meningeal layer	Vascular connective tissue
Subarachnoid space	Between arachnoid mater and pia mater	Cerebrospinal fluid (CSF)
Pia mater	Innermost meningeal layer	Delicate connective tissue

Additional info:

• The sciatic nerve is the largest nerve in the body, originating from the sacral plexus.

• Funiculi are columns of white matter in the spinal cord, containing ascending and descending tracts.

• Cerebrospinal fluid (CSF) provides cushioning and nutrient transport within the central canal and subarachnoid space.