The Spinal Cord & Spinal Nerves

Spinal Cord Location and Function

• Location: Found in the vertebral canal, extending from above the foramen magnum to L1/L2.

• Functions:

  • Center for reflexes (quick response to stimuli).

  • Integrates nerve impulses (summation of excitatory and inhibitory).

  • Contains tracts for sensory (upward) and motor (downward) information.

Protection of Spinal Cord

• Bony Protection: Enclosed by vertebrae.

• Meninges (three connective tissue membranes):

  • Dura mater (outer)

  • Arachnoid mater (middle)

  • Pia mater (inner, adheres to spinal cord).

Meningeal Spaces

• Epidural Space: Fat-filled; surrounds dura mater.

• Subdural Space: Minimal space filled with interstitial fluid.

• Subarachnoid Space: Contains cerebrospinal fluid (CSF).

External Anatomy of Spinal Cord

• Cervical and Lumbar Enlargements: Nerves to upper/lower extremities.

• Conus Medullaris: Cone-shaped end of spinal cord.

• Filum Terminale: Stabilizes the spinal cord; extensions anchor it to the coccyx.

Internal Structure of Spinal Cord

• Gray Matter: H-shaped or butterfly-shaped; includes:

  • Posterior Horn: Sensory input.

  • Anterior Horn: Motor neuron output.

  • Lateral Horn: Interneurons.

• White Matter: Surrounds gray matter; divided into:

  • Ascending Tracts: Carry impulses to the brain.

  • Descending Tracts: Carry impulses from the brain.

Spinal Nerves

• There are 31 pairs of spinal nerves:

  • C1-C8 (Cervical)

  • T1-T12 (Thoracic)

  • L1-L5 (Lumbar)

  • S1-S5 (Sacral)

  • Co1 (Coccygeal)

Structure of Spinal Nerves

• Comprised of sensory & motor fibers plus connective tissue:

  • Endoneurium: Wraps around individual fibers.

  • Perineurium: Surrounds fascicles.

  • Epineurium: Covers entire nerve.

Branches of Spinal Nerves

• Each spinal nerve has four main branches:

  • Posterior Ramus: Supplies back skin/muscles.

  • Anterior Ramus: Supplies extremities/trunk.

  • Meningeal Ramus: Supplies meninges and vertebrae.

  • Rami Communicantes: Autonomic supplies to internal organs.

Plexus Formation

• Cervical Plexus: C1-C5, supplies neck & diaphragm.

• Brachial Plexus: C5-T1, supplies upper extremities.

• No Thoracic Plexus: Forms intercostal nerves.

• Lumbar Plexus: L1-L4, supplies lower abdominal wall.

• Sacral Plexus: L4-S4, supplies buttocks/lower extremities.

Reflexes

• Definition: Rapid, involuntary muscle/gland responses to stimuli.

• Reflex Arc Composition:

  1. Receptor

  2. Sensory Neuron

  3. Integration Center (CNS)

  4. Motor Neuron

  5. Effector

Types of Reflexes

• Innate vs. Acquired: Innate reflexes are genetically determined; acquired are learned.

• Visceral vs. Somatic: Visceral controls smooth/cardiac muscles; somatic controls skeletal muscles.

• Spinal vs. Cranial: Spinal occurs in the spinal cord; cranial occurs in the brain.

• Monosynaptic vs. Polysynaptic: Monosynaptic has one synapse; polysynaptic has multiple synapses.