119. Ascending Pathways – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Ascending pathways carry sensory impulses from peripheral receptors to the thalamus and then to the primary somatosensory cortex for conscious perception. Most pathways consist of first-order, second-order, and third-order neurons, while higher cortical processing involves fourth-order neurons.
• First-order neurons have cell bodies in the dorsal root ganglia (DRG). Their peripheral processes arise from sensory receptors, and their central processes enter the spinal cord through the dorsal roots. Second-order neurons are located in the spinal cord or medulla and usually decussate (cross) before ascending to the thalamus. Third-order neurons arise from the thalamus and project to the sensory cortex.
• The two major ascending pathways are:
  • Dorsal column–medial lemniscal pathway.
  • Anterolateral system (spinothalamic pathways).
• The dorsal column pathway carries fine touch, vibration, conscious proprioception, tactile localization, tactile discrimination, and stereognosis.
  • Fibers ascend without crossing in the spinal cord.
  • Gracile fasciculus carries sensations from the lower limb and lower trunk.
  • Cuneate fasciculus carries sensations from the upper limb and upper trunk.
  • First-order neurons end in the gracile and cuneate nuclei of the medulla.
  • Second-order neurons cross as internal arcuate fibers, form the medial lemniscus, and terminate in the ventral posterolateral (VPL) nucleus of the thalamus.
  • Third-order neurons project to the primary somatosensory cortex.
• The anterolateral system is an older sensory pathway consisting of:
  • Anterior spinothalamic tract – carries crude touch.
  • Lateral spinothalamic tract – carries pain and temperature.
  • First-order neurons synapse in the dorsal horn (mainly laminae I, II, and V).
  • Second-order neurons cross through the anterior white commissure within one or two spinal segments and ascend on the opposite side to the VPL nucleus of the thalamus.
  • Third-order neurons project to the sensory cortex.
• Touch sensation:
  • Fine touch is transmitted through the dorsal column pathway.
  • Crude touch is transmitted through the anterior spinothalamic tract.
  • Touch receptors include Meissner corpuscles, Merkel discs, Pacinian corpuscles, Ruffini endings, and free nerve endings.
  • Aβ fibers are the principal afferent fibers for touch.
• Proprioception and kinesthesia arise from muscle spindles, Golgi tendon organs, joint receptors, and Pacinian corpuscles. Conscious proprioception travels through the dorsal column pathway, while collateral fibers also project to the cerebellum for coordination. Lesions produce sensory ataxia.
• Vibration sense (Pallesthesia) is detected mainly by Pacinian corpuscles and carried through the dorsal column pathway. Loss of vibration sense is an early feature of vitamin B12 deficiency, diabetic neuropathy, and dorsal column lesions.
• Two-point discrimination is the minimum distance at which two stimuli are perceived separately. It depends on intact dorsal columns, high receptor density, and a normal primary somatosensory cortex. Stereognosis is the ability to recognize a familiar object by touch with closed eyes and requires intact dorsal columns and cortical sensory areas.
• Important sensory abnormalities:
  • Anesthesia: Complete loss of sensation.
  • Hypesthesia: Partial loss of sensation.
  • Dissociated anesthesia: Loss of pain and temperature with preserved touch, classically seen in syringomyelia.
  • Hemianesthesia: Loss of sensation on one side of the body.
  • Hyperesthesia: Increased sensitivity.
  • Allesthesia: Stimulus perceived on the opposite side.
  • Paresthesia: Tingling or numbness without stimulation.
  • Agraphesthesia: Inability to recognize letters traced on the skin.
  • Astereognosis: Inability to identify a familiar object by touch.