14. Ascending Tracts

Ascending Sensory Tracts Overview

  • Course: 2024-2025 MED 203

  • Instructor: Dr. Badegül Sarikaya

Learning Objectives

  • Explain the ascending tracts pathway.

Overview of Sensory Information

  • Sensory Receptors: Detect stimuli from within and outside the body.

    • Exteroceptors: Sense external stimuli (e.g., pain, temperature, touch, vibration).

    • Proprioceptors: Monitor stimuli from within the body (e.g., muscles, tendons, joints); signal body position and movement.

    • Enteroceptors: Monitor events within internal organs (e.g., viscera and gut).

Ascending Sensory Pathways

Pathway to the CNS

  • Sensory information is sent to the central nervous system via ascending tracts through the spinal cord and brainstem.

  • Pathways diverge, sending signals to:

    • Brainstem centers

    • Cerebellum for movement planning and refinement

    • Cortex for conscious stimuli awareness

Sensory Neuron Anatomy

  • Cell bodies located in spinal ganglia or cranial nerve sensory ganglia.

  • Sensory signals enter spinal cord via posterior root.

Conscious vs Nonconscious Information

  • Conscious Sensation: Perception of stimuli.

  • Nonconscious Processing: Adjusts body function without conscious awareness.

Modality Specificity

  • Sensory information is bundled by modality; travels in different tracts:

    • Posterior Column-Medial Lemniscus: Discriminative touch, pressure, vibration, proprioception.

    • Anterolateral System: Pain, temperature, non-discriminative touch.

Cerebellar Information Processing

  • Proprioceptive Information: A copy sent to the cerebellum for fine-tuning and predicting sensory consequences of movement through spinocerebellar tracts.

Medial Longitudinal Fasciculus

  • Connects ascending and descending pathways, involving coordination of eye movements and vestibular function.

  • Most sensory information, except olfaction, crosses the midline on its pathway to the cortex.

Posterior Column-Medial Lemniscus Pathway

  • Function: Carries general somatic afferent information (e.g., touch, vibration).

    • High conduction velocity due to large-diameter fibers (Aβ).

    • Limited synaptic relays for quicker processing.

  • Anatomy: Primary afferent fibers associated with peripheral receptors.

    • Cell bodies in spinal ganglia; signals travel to the posterior horn of the spinal cord.

Spinal Reflex Arc Integration

  • In the spinal cord, collaterals synapse with motor neurons for reflex actions.

  • Majority of fibers ascend in the posterior columns without synapsing until the caudal medulla.

Medial Lemniscus Pathway

  • Carries information from the lower (fasciculus gracilis) and upper (fasciculus cuneatus) body to the thalamus.

  • Somatotopic organization continues in the medial lemniscus through the brainstem to the primary somatosensory cortex.

Anterolateral System

Structure and Function

  • Encodes pain, temperature, and nondiscriminative touch.

  • Majority travel to thalamus via the spinothalamic tract for pain and temperature perception.

  • Smaller tracts modulate sensations in the brainstem (spinomesencephalic, spinoreticular).

Fiber Types and Pathways

  • Aδ Fibers: Rapidly conducted sharp pain (20 m/s), thermoreceptors for heat and cold.

  • C Fibers: Slower conduction (2 m/s), responsible for dull pain, itch, and burning sensations.

Neuronal Anatomy in Anterolateral System

  • Cell bodies in spinal ganglia; central processes enter spinal cord.

  • Synapse in superficial laminae (I, II) and nucleus proprius.

  • Fibers cross midline in anterior white commissure to ascend as the anterolateral system.

Lesion Effects

  • Spinothalamic Tract Lesion: Loss of pain and temperature sensation contralaterally, a few levels below the injury.

  • Brown-Séquard Syndrome: Loss of touch and proprioception ipsilaterally; loss of pain and temperature sensation contralaterally.

Spinocerebellar Tracts Overview

  • Relay limb position and movement information to the cerebellum.

  • Pathways: posterior spinocerebellar (lower body), cuneocerebellar (upper body), anterior spinocerebellar, rostral spinocerebellar.

Function of Posterior Spinocerebellar Tract

  • Integrates proprioceptive and tactile information from lower limbs.

  • Fibers synapse in Clarke’s nucleus and travel through inferior cerebellar peduncle.

Function of Cuneocerebellar Tract

  • Analogous to posterior spinocerebellar tract but for upper limb.

  • Fibers synapse in the accessory cuneate nucleus before reaching the cerebellum.

Anterior Spinocerebellar Tract

  • Ascends from neurons in anterior horn, integrating proprioceptive information with descending signals.

Clinical Significance

  • Lesions lead to ataxias due to loss of proprioceptive input.

  • Ataxia may be masked in conditions involving other tract lesions (e.g., hemisection).

Summary of Spinocerebellar Information Flow

  • Overall integration is crucial for coordination and stability of movements.

  • Recent findings: Clarke nucleus processing is critical for motor planning, not solely cerebellum.

Conclusion

  • Understanding of ascending sensory pathways is vital for recognizing the complexity of sensory processing and the implications of lesions in these tracts.