Ascending Pathways Study Notes

Terminology and Review
Overview of the sensory pathways:
- General summary
- Cortical representation
Posterior Column Medial Lemniscus Pathway:
- Description of pathway
- Pathway drawing
- Clinical significance
Anterolateral System:
- Overview of system
- Spinothalamic pathway

Definition:
- A bundle of nerve fibers
- May be:
- Ascending (sensory)
- Descending (motor)

Thalamus:
- Critical processing station for all sensory information (except olfactory) on its way to the cortex
- Plays key roles in processing motor information, integrating higher-order cognitive and emotional information
- Considered the “Gatekeeper” for the cortex

A fibers: (Fastest) Myelinated and divided into:
- Aα, Aβ (motor, proprioception)
- Aδ, Aγ
B fibers: Smaller, myelinated, typically preganglionic visceral motor (autonomic)
C fibers:
- Small diameter, unmyelinated
- Postganglionic visceral motor (autonomic) fibers and some sensory fibers

Exteroceptors:
- Superficially located sensory endings in the skin
- Respond to pain, temperature, touch, and pressure
Enteroceptors:
- Monitor events within the body (e.g., feeling movement through the gut)
Proprioceptors:
- Located in muscles, tendons, and joints
- Signal awareness of body position and movement

Sensory pathways bundle information according to modality
- Different modalities travel in different tracts
- All sensory information conveyed to the CNS via ascendant tracts through spinal cord and brainstem
- Some information goes to brainstem centers, some to cerebellum for the planning and refinement of movement
- Some information travels to cortex, mediating conscious awareness of stimuli

Posterior Column–Medial Lemniscus System
- Discriminative touch, vibration, pressure, proprioception
Anterolateral System (Spinothalamic Tracts)
- Pain, temperature, nondiscriminative touch
Spinocerebellar Tracts
- Proprioceptive information to the cerebellum

From the periphery, sensory information enters spinal cord through the posterior root of spinal nerves
Sensory neurons from receptors in the body have their cell bodies in the dorsal root ganglia
For the head and neck, sensory ganglia are associated with respective cranial nerves

Crossing of Sensory Information:
- All sensory information, except olfaction, crosses midline at some point to reach cortex of contralateral hemisphere
Primary Somatosensory Cortex:
- Located in the postcentral gyrus
Sensory Homunculus:
- Areas of the cortex correspond to specific areas of the body, referred to as somatotopic organization
- Results in a sensory map of the body on the cortex

Function: Detects discriminative touch, vibration, pressure, proprioception
Pathway Description:
- Starts in the dorsal/posterior column of the spinal cord
- Crosses in the medulla as the medial lemniscus
- Involves Aβ nerve fibers for fast conduction of information
First Order Neurons:
- Pseudounipolar, cell bodies in dorsal root ganglia
- Signal from the periphery travels to the ipsilateral posterior horn of the spinal cord
- Majority of fibers ascend in posterior columns and do not synapse until caudal medulla
Posterior Columns Composition:
- Fasciculus Gracilis: Carries info from lower limb and trunk up to T6
- Fasciculus Cuneatus: Carries info from trunk and upper limb above T6
- Some axon collaterals synapse with motor neurons in anterior horn, forming spinal reflex arcs
Medulla Synapse:
- Fibers synapse on respective second-order neurons located in the nucleus gracilis and nucleus cuneatus
- Axons from second-order neurons cross midline as internal arcuate fibers, ascend as the medial lemniscus
Thalamic Relay:
- The medial lemniscus fibers synapse in the contralateral ventral posterolateral (VPL) nucleus of thalamus
- Second-order neuron fibers synapse with third-order neurons
- Third-order neurons relay info through the internal capsule and corona radiata to primary somatosensory cortex for conscious perception of sensory info

Impact to the PCML system:
- Loss of ability to distinguish finer aspects of touch
- Distinction of Effects:
- Below medulla: Loss is peripheral
- Above medulla: Loss is central

Overview:
- Traverses the anterolateral region of spinal cord
- Primitive and slower than the PCML system
- Transmits pain, temperature, and nondiscriminative (crude) touch, itching, or tickle sensation
Constituents of Anterolateral System:
- Majority of fibers travel to thalamus via the Spinothalamic tract, mediating conscious perception of pain and temperature
- Other smaller tracts:
- Spinomesencephalic (to midbrain)
- Spinoreticular (to reticular formation)
- Spinobulbar (to select nuclei in brainstem)
- Spinohypothalamic (to hypothalamus)

Function:
- Mediates pain and temperature sensation
Neuronal body locations:
- Located in the dorsal root ganglia
Pathway Details:
- Fibers enter spinal cord through the posterior horn
- Ascend/descend 1-2 spinal cord levels in Lissauer’s tract before entering gray matter
- Fibers synapse in the posterior horn (gray matter) in Rexed Lamina II, known as Substantia Gelatinosa
Rexed’s Lamina:
- Ten distinct regions in the gray matter of spinal cord
Conduction Pathway:
- Axons from second-order neurons cross to contralateral side in the anterior white commissure
- Spinothalamic tract ascends to the thalamus
- From thalamus, fibers project through internal capsule and corona radiata to primary sensory cortex

Conscious Sensory Input:
- Spinothalamic tract as part of the anterolateral system mediates sensation of pain and temperature from the contralateral side of the body

Pathway drawings illustrate connections among the spinal cord, medulla, pons, midbrain, and cortex

Complex Regional Pain Syndrome:
- Pain triggered by an injury but more severe than the injury itself
- Occurs due to damage to Aδ and C fibers
- Miscommunication between brain and sensory nervous system leads to activation of sympathetic nervous system causing swelling, more pain, and skin changes.