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.