ascending

ASCENDING PATHWAYS

Dr. Omer Tagelsir Abdalla

Internal Features of the Spinal Cord

  • The spinal cord is composed of:
      - Inner core of gray matter
      - Outer covering of white matter

  • Gray Matter Composition:
      - Looks like an H-shaped pillar
      - Contains anterior and posterior horns

Composition of Gray and White Matter

  • Gray Matter:
      - Composed of cell bodies

  • White Matter:
      - Composed of axons of the nerves

Horns of the Spinal Cord

  • Anterior Horn:
      - Contains motor neurons

  • Posterior Horn:
      - Contains sensory neurons

Specific Structures of Gray Matter

  • Dorsal Horn

  • Intermediate Gray (between dorsal and ventral horns)

  • Ventral Horn:
      - Contains motor neurons for flexors and extensors

  • Nuclei:
      - Posteromarginal Nucleus
      - Substantia Gelatinosa
      - Nucleus Proprius
      - Nucleus Dorsalis (Clarke's Column)
      - Intermediolateral Cell Group
      - Intermedial Cell Group
      - Medial Group
      - Lateral Group

White Matter Organization

  • Divided into anterior, lateral, and posterior white columns (or funiculi)

  • Nerve fibers gather together to form bundles known as tracts

Ascending Tracts of the Spinal Cord

  • Function of Ascending Tracts:
      - Sensory nerve fibers travel from the periphery to the spinal cord and ascend to higher centers in the brain.
      - Bundles of ascending fibers are referred to as ascending tracts.

Types of Sensory Information

  • Exteroceptive Information (from outside the body):
      - Includes sensations of pain, temperature, and touch.

  • Proprioceptive Information (from inside the body):
      - Related to information from muscles and joints.

Conscious vs. Unconscious Sensation

  • Conscious Sensation:
      - Ends in the cerebrum
      - Includes sensations of pain, touch, and temperature
      - Characterized as ascending sensory pathways

  • Unconscious Sensation:
      - Ends in sub-cortical areas (does not reach the brain)
      - Involves muscle information and joint position
      - Also classified under ascending sensory pathways

Muscle Spindles and Reflexes

  • Mechanism:
      - Sudden changes in weight can cause muscles to extend.
      - This action stretches muscle spindles, triggering a reflex to maintain tension in the muscle.

Three Order Neurons in Ascending Pathways

Classes of Neurons:

  1. First Order Neuron:
       - Pathway: From skin or joint to the spinal cord or medulla

  2. Second Order Neuron:
       - Pathway: From spinal cord or medulla to the thalamus (thalamus is known as the secretory of the brain)

  3. Third Order Neuron:
       - Pathway: From thalamus to the brain (specifically to the cortex, parietal lobe = postcentral gyrus)

Pathway and Decussation

  • Second Order Neuron:
      - Crosses to the opposite side
      - Representation: The right side of the body is represented in the left side of the brain.
      - As this neuron ascends towards the brain, its nerve fibers construct the tract.

Ascending Pathways Summary

  • Three-neuron pathways:
      - Primary Sensory Neurons: Relay from external receptors
      - Secondary Neurons: Form tracts in the spinal cord and brainstem
      - Tertiary Neurons: Connect from thalamus to the primary sensory cortex

Types of Ascending Pathways Based on Consciousness

  • For Conscious Perception:
      - Ends in the brain, includes:
        - Spinothalamic System
        - Medial Lemniscal System

  • For Unconscious Perception:
      - Does not reach the brain, includes:
        - Spinocerebellar
        - Spino-olivary
        - Spinotectal
        - Spinoreticular

Spinothalamic Tract Characteristics

  • Carries sensations of pain, touch, and temperature

  • Receptors involved are free nerve endings

  • Secondary axons:
      - Decussate through anterior gray and white commissures
      - Form the spinothalamic tract, traveling in the white matter of the spinal cord
      - Project to the thalamus
      - Tertiary fibers end at the parietal lobe

Pathway of the Spinothalamic Tract

  • In details, the sensory impulses travel from the skin through:
      - Nociceptors (pain receptors) and thermoreceptors (temperature receptors)
      - First-order neuron (afferent)

  • Structure Overview:
      - Cross reference includes sections from medulla oblongata, pons, and cerebrum

Medial Lemniscus Tract

  • Also known as the Posterior Column Tract

  • Function:
      - Carries sensations related to joint position consciously, meaning it ends in the brain.

  • First Neurons:
      - Ascend along the entire length of the spinal cord, synapsing with secondary neurons in the medulla.

  • Secondary Neurons:
      - Travel from the medulla oblongata to the thalamus.

  • Third Neurons:
      - Extend from the thalamus to the brain.

  • Decussation:
      - Secondary fibers cross over to ascend to the thalamus and continue to the primary sensory cortex.

Medial Lemniscus System Details

  • Includes the pathway known as the Posterior Column-Medial Lemniscus Pathway
      - Connects to various parts of the primary somatosensory area of the cerebral cortex
      - Specific nuclei involved:
        - Nucleus gracilis
        - Nucleus cuneatus

  • Sensation specifics include:
        - 2-point discrimination
        - Graphaesthesia
        - Stereognosis

Summary of Ascending Pathways Function

  • First Neuron:
      - Spinothalamic tract: Pain, touch, and temperature
      - Dorsal column: Vibration and joint position

  • Order of Pathways:
      - Spinothalamic ends in the spinal cord
      - Dorsal column ends in the medulla oblongata

  • Final Pathway to Cortex:
      - From spinal cord to thalamus, and from thalamus to the cortex (
      - Located in parietal lobe = postcentral gyrus)

Muscle and Joint Sense

  • Categories:
      - Conscious Sense: Processed in the cortex
      - Unconscious Sense: Processed in the cerebellum
        - Relays include the medial lemniscus and spinocerebellar pathways.

Spinocerebellar Tracts

  • Types include:
      - Posterior Spinocerebellar Tract
      - Anterior Spinocerebellar Tract

  • Functions in unconscious muscle and joint sense, aiding in coordination and posture.

  • Information from muscle spindles, tendon organs, and joint receptors is utilized.

  • Cerebellum's Role: Facilitates coordination of limb movements and posture maintenance.

Specific Tract Pathways and Composition

  • Unconscious proprioception connected pathways:
      - Dorsal Spinocerebellar Tract: Involves axons of second-order neurons from nuclei in the spinal cord.

  • Cerebellum structure:
      - Involves superior and inferior cerebellar peduncles linking cerebellar processes.

Third Neuron Pathway Connection

  • Third Neuron Locations:
      - Thalamus
      - Medial Lemniscus

  • Sensory Pathways for:
      - Position sense, conscious proprioception, vibration, touch
      - Pressure, pain, and temperature processes.

Conclusion of Ascending Pathways Mechanism

  • Overall, the ascending pathways play a critical role in processing sensory information, whether conscious or unconscious, and connect various forms of sensory input to appropriate processing centers in the brain.