spinal cord: ascending and descending

Overview of Nervous System Structures

  • Theme of Organization: In the nervous system, information is organized and segregated in distinct ways.

    • Example: Motor information originates from different regions of the cortex than sensory information.

    • The thalamus also shows high-level organization, segregating information by type.

    • Similar organization exists in the spinal cord for sensory and motor information.

Learning Objectives

  • Differentiate between ascending and descending tracts:

    • Ascending: Information traveling up the spinal cord to the brain.

    • Descending: Information traveling from the brain down to the spinal cord and ultimately to the periphery.

  • Trace the pathways of nerve signals in the spinal cord, focusing on the pathway for pain signals.

    • Refer to Saladin textbook, pages 480-485.

Cross-Section of the Spinal Cord

  • Orientation: Use landmarks to distinguish anterior (front) and posterior (back):

    • Anterior median fissure indicates anterior side.

    • Dorsal root ganglia help identify posterior side.

  • Structure Overview:

    • Central gray matter is surrounded by white matter.

    • Central canal contains cerebrospinal fluid.

  • Composition of Matters:

    • Gray Matter: Contains cell bodies and unmyelinated fibers.

    • White Matter: Comprising myelinated fibers.

Tracts in the Spinal Cord

  • Identification of sensory and motor tracts:

    • Sensory Tracts (Blue): Carry information towards the brain.

    • Motor Tracts (Red): Carry information away from the brain.

  • Important Note: Sensory and motor tracts are located on both sides of the spinal cord, and naming may indicate side orientation but does not reflect actual distribution in the body.

Names of Tracts and Their Significance

  • Name origin indicates location and function of tracts:

    • Posterior Columns: Located in the posterior region of the spinal cord.

    • Spinothalamic Tract: Carries information from the spinal cord to the thalamus.

    • Spinocerebellar Tract: Carries information from the spinal cord to the cerebellum.

    • Lateral Corticospinal Tract: Transmits information from the cortex to the spinal cord.

Flow of Information for Pain Signals

  • Overview of pain signal transmission from periphery to brain:

    • Step 1: Pain stimulus (e.g., stubbing toe) activates receptors, generating action potentials through sensory neurons.

    • Step 2: Sensory neurons transmit signals via the posterior root to the spinal cord, forming a synapse in the dorsal horn.

    • Step 3: The second-order neuron begins at the dorsal horn:

      • Axon crosses the spinal cord to the anterolateral or spinothalamic tract and ascends to the brain.

    • Step 4: Synapses in the thalamus activate the third-order neuron.

    • Step 5: Third-order neuron conveys information to the sensory cortex.

  • Neuronal Chain:

    • First-Order Neuron: Detects the painful stimulus.

    • Second-Order Neuron: Located in the central nervous system, relays information.

    • Third-Order Neuron: Conveys information to the sensory cortex for interpretation.

Motor Pathways

  • Motor pathways mirror sensory pathways in structure:

    • Chain of neurons transmits information from the cortex, through the midbrain, medulla, spinal cord, and finally out to the periphery.

  • Transition to discussing cutaneous innervation in the next section.