The Spinal Cord

Spine Protection

  • Vertebral Column

    • Composed of vertebrae and vertebral ligaments.

    • Provides structural support and encases the spinal cord.

  • Spinal Coverings

    • Epidural Space

      • Contains fat and connective tissue.

    • Dura Mater

      • Made up of dense, irregular connective tissue.

      • Extends from the level of the foramen magnum to the second sacral vertebra.

    • Subdural Space

      • Contains interstitial fluid.

Spinal Coverings Continued

  • Arachnoid Mater

    • Composed of connective tissue arranged in a spider web-like formation with collagen and elastic fibers.

    • Subarachnoid Space

      • Filled with cerebrospinal fluid (CSF).

  • Pia Mater

    • Loosely attached to the surface of the spinal cord and brain.

    • Contains collagen and elastic fibers along with numerous blood vessels providing nutrients and oxygen to the nervous tissue.

Peripheral Nervous System (PNS): Protection

  • Meningeal Coverage

    • All three spinal meninges (dura mater, arachnoid, pia mater) cover spinal nerves until they exit the vertebral column through the intervertebral foramina.

  • Denticulate Ligaments

    • Membranous extensions of the pia mater.

    • Suspend spinal cord in the dural sheath to offer protection against shock and sudden displacements.

External Anatomy of Spinal Cord

  • Location

    • Extends from the medulla oblongata to the level of the second lumbar vertebra in adults.

    • Conus Medullaris located between the first and second lumbar vertebrae.

  • Enlargements

    • Cervical Enlargement: C4 - T1

    • Lumbosacral Enlargement: L2 - S3

  • Cauda Equina

    • Roots of lumbar and sacral spinal nerves emerging from lower spinal cord.

  • Filum Terminale

    • Extension of pia mater anchoring the spinal cord interiorly to the coccyx.

Spinal Nerves and Segments

  • Spinal Nerve Pairs

    • Total of 31 pairs of spinal nerves:

      • 8 pairs of cervical spinal nerves (first pair between atlas and occipital bone).

      • 12 pairs of thoracic spinal nerves.

      • 5 pairs of lumbar spinal nerves.

      • 5 pairs of sacral spinal nerves.

      • 1 pair of coccygeal spinal nerves.

  • Grooves

    • Anterior Median Fissure: Deep, wide groove.

    • Posterior Median Sulcus: Shallow, narrow groove.

Core of Gray Matter

  • Gray Matter Components

    • Contains cell bodies of neurons and neuroglia.

    • Unmyelinated axons and dendrites of association and motor neurons.

    • Neural processing centers and origins for certain nerves.

    • Gray Commissure connects both sides of the spinal cord.

  • Subdivisions of Gray Matter

    • Anterior (Ventral) Gray Horn

    • Posterior (Dorsal) Gray Horns

    • Lateral Gray Horns (found in thoracic, upper lumbar, and sacral segments).

White Matter on Periphery

  • Central Canal

    • Extends throughout the entire length of the spinal cord and is continuous with the fourth ventricle of the brain.

  • White Matter

    • Composed of bundles of myelinated axons of motor and sensory neurons.

    • Anterior White Commissure connects the white matter of left and right sides.

  • Subdivisions of White Matter

    • Divided by anterior and posterior gray horns into regions termed columns (funiculi):

      • Anterior (Ventral) White Columns

      • Posterior (Dorsal) White Columns

      • Lateral White Columns

White Matter Columns

  • Nerve Fiber Bundles

    • Each column contains bundles of nerve fibers called tracts (fasciculi) which are continuous with tracts in the brain.

  • Tract Types

    • Ascending (Sensory) Tracts: transmit nerve impulses upward to the brain.

    • Descending (Motor) Tracts: transmit nerve impulses downward from the brain.

  • Variations in Size and Shape

    • Spinal cord segments vary in size, shape, amounts of gray and white matter, and distribution/shape of gray matter.

Sensory and Motor Tracts

  • Sensory Information

    • Spinothalamic Tracts:

      • Lateral: Pain and temperature sensations.

      • Anterior: Crude touch and deep pressure.

    • Fasciculus Gracilis and Fasciculus Cuneatus:

      • Responsible for proprioception, discriminative touch, two-point discrimination, pressure, and vibrations.

    • Spinocerebellar Tracts:

      • Involved in subconscious proprioception, posture, balance, and coordination of skilled movements.

  • Motor Output

    • Direct (Pyramidal) Pathways:

      • Lateral Corticospinal Tract, Anterior Corticospinal Tract, Corticobulbar Tracts (Cranial Nerves) - transmit impulse for precise, voluntary muscle movements.

    • Indirect (Extrapyramidal) Pathways:

      • Include Rubrospinal, Vestibulospinal, and Tectospinal Tracts - program automatic movements, coordinate body movements with visual stimuli, maintain muscle tone and posture, play major roles in equilibrium.

Reflexes

  • Gray Matter Function

    • Serves as the integrating center for spinal reflexes.

    • The brainstem serves as the integrating center for cranial reflexes.

  • Reflex Characteristics

    • Reflexes are fast, predictable, and automatic responses to environmental changes to help maintain homeostasis.

    • Somatic Reflexes involve contraction of skeletal muscles.

    • Visceral (Autonomic) Reflexes involve smooth muscle, cardiac muscle, or gland responses.

Spinal Nerve Components

  • Roots of Spinal Nerve

    • Each spinal nerve is attached to a spinal segment by two roots:

      • Posterior (Dorsal) Root:

        • Contains sensory nerve fibers transmitting nerve impulses from the periphery into the spinal cord.

        • Contains the Posterior (Dorsal) Root Ganglion with cell bodies of sensory neurons.

      • Anterior (Ventral) Root:

        • Contains motor neuron axons transmitting impulses from the spinal cord to the periphery.

        • Cell bodies of motor neurons located in the gray matter of the spinal cord.

  • Motor Neuron Locations:

    • Cell bodies of motor neurons for skeletal muscles: Anterior (Ventral) Gray Horns.

    • Cell bodies for smooth muscle, cardiac muscle, or glands: Lateral Gray Horns.

Reflex Pathways

  • Components of Reflex Circuits

    • Composed of five functional components:

      • Receptor: Dendrite or sensory structure that triggers nerve impulses upon stimulation.

      • Sensory Neuron: Transmits impulses from the receptor to gray matter of spinal cord or brainstem.

      • Integrating Center: Region within the CNS, may consist of:

        • Single synapse between sensory and motor neuron (monosynaptic reflex arc).

        • Multiple association neurons (polysynaptic reflex arc).

      • Motor Neuron: Transmits nerve impulses from the integrating center to the responder.

      • Effector: Muscle or gland responding to impulses (in somatic reflex: skeletal muscle, in autonomic reflex: smooth muscle, cardiac muscle, or gland).

Spinal Nerves and Plexuses

  • Plexus Formation

    • Anterior (ventral) rami of spinal nerves form networks (plexuses) on both sides of the body:

      • Cervical Plexuses (including Phrenic nerve).

      • Brachial Plexuses (containing Musculocutaneous, Median, Radial, Ulnar nerves).

      • Lumbar Plexuses (including Femoral and Obturator nerves).

      • Sacral Plexuses (including Sciatic and Pudendal nerves).

    • Intercostal (Thoracic) Nerves: Anterior rami of spinal nerves T2-T12 do not form plexuses and innervate directly to structures in intercostal spaces.

  • Dermatomes

    • Each spinal nerve supplies a specific area of the body (dermatome) providing sensory input to one pair of spinal nerves, overlapping slightly with adjacent dermatomes.

    • Somatic sensory neurons carry nerve impulses into the spinal cord and brainstem, while somatic motor neurons carry impulses out of the spinal cord to skeletal muscles.