Spinal Chord Pathways

Objectives

  • Describe the organization of the nervous system.

  • Understand the relationship between the spinal cord and vertebral canal.

  • Explain the spinal cord's function.

  • Describe the structure and function of somatosensory pathways in the spinal cord.

  • Differentiate between upper and lower motor neurons.

  • Contrast between flaccid and spastic paralysis.

  • Explain the central motor program and the process of converting environmental stimuli into motor activity.

  • Analyze the motor pathways in the spinal cord and the behaviors resulting from damage to these pathways.

Structure of the Central Nervous System

  • Major Components

    • Brain

      • Consists of:

        • Cerebrum

        • Brainstem (Midbrain, Pons, Medulla)

        • Cerebellum

    • Spinal Cord

      • Extends from base of the skull (medulla) to the L1-L2 vertebrae.

      • Contains cervical and lumbar enlargements.

      • Covered by meninges and has a central canal continuous with brain ventricles.

Spinal Cord Anatomy

  • Organization

    • Inner grey matter and outer white matter.

    • Grey matter includes:

      • Posterior/Dorsal horns (sensory functions)

      • Anterior/Ventral horns (motor functions)

  • Dorsal and ventral roots combine to form spinal nerves.

Spinal Nerves and Overview

  • Composition and Function

    • 31 pairs of spinal nerves:

      • 8 Cervical

      • 12 Thoracic

      • 5 Lumbar

      • 5 Sacral

      • 1 Coccygeal

    • Spinal cord ends at the L1-L2 level.

Sensory and Motor Information Flow

  • Continuous exchange of sensory (afferent) and motor (efferent) information between CNS and peripheral organs.

  • Millions of sensory neurons relay information to CNS, while motor neurons adjust organ system activities.

Somatic Sensory Pathways

  • Major Sensory Pathways

    1. Dorsal column/medial lemniscal pathway

      • Processes joint position and discriminative touch.

    2. Spinothalamic (anterolateral) pathway

      • Conveys pain and noxious temperature sensations.

    3. Spinocerebellar pathway

      • Involved in unconscious proprioception.

Dorsal Columns Pathway

  • Conveys:

    • Joint position (conscious proprioception)

    • Discriminative touch (3D recognition without vision).

  • Involves several neurons from skin or joint receptors to the sensory cortex via the medulla and thalamus.

Spinothalamic Pathway

  • Conveys pain and temperature from trunk and extremities.

  • Pathway Details:

    • First-order neuron from skin to dorsal horn.

    • Second-order neuron crosses over and ascends to thalamus.

    • Third-order neuron from thalamus to primary sensory cortex.

Spinocerebellar Pathway

  • Carries unconscious proprioception signals to the cerebellum, ensuring the body knows muscle and joint positions.

  • First-order neurons from receptors synapse in dorsal horn, with second-order neurons traveling to cerebellum.

Motor Systems

  • CNS issues motor commands through PNS to effectors.

  • Somatic motor pathways involve:

    • Upper Motor Neurons (UMN)

      • Cell body located in CNS; synapses onto Lower Motor Neurons (LMN).

    • Lower Motor Neurons (LMN)

      • Cell body in the ventral horn of the spinal cord; innervates skeletal muscles.

Motor Pathways Overview

  • Corticospinal (Pyramidal) Pathway

    • Extends from the cortex directly to spinal cord (LMNs).

  • Corticonuclear (Corticobulbar) Pathway

    • Connects cortex to cranial nerve nuclei in the brainstem for head and neck muscle regulation.

Voluntary Movements and Central Motor Program

  • Central Motor Program Steps:

    1. Identifying targets in space (posterior parietal cortex)

    2. Formulating movement plan (premotor cortex)

    3. Executing movement (primary motor cortex).