Clinical Neuroscience Week 6 – Ascending Pathways (Easy)

• Sensory information from the body travels to the cerebral cortex via ascending pathways in the spinal cord.
• These pathways relay touch, conscious proprioception, fast nociception, and temperature.
• Lesions in the spinal cord can cause specific sensory impairments depending on the location and side of the lesion.

Afferent/Sensory Input

• Afferent or sensory input includes touch, pressure, vibration, temperature, pain, vision, and hearing.
• Information from peripheral receptors in the upper/lower extremities and trunk enters the dorsal root ganglion.
• The information then enters the dorsal aspect of the spinal cord.

Conscious vs Unconscious Pathways

• Conscious level pathways include light touch, proprioception, vibration, pain, and temperature.
• Relevant conscious spinal cord pathways are the Dorsal Column-Medial Lemniscus (DCML) and Spinothalamic tracts.
• Unconscious level pathways include proprioception and kinesthesia.
• The relevant unconscious spinal cord pathway is the Spinocerebellar tract.

Ascending Tracts (Sensory)

• The Dorsal Column-Medial Lemniscus (DCML) tract is for deep touch, light touch, vibration, and conscious proprioception.
• The Lateral Spinothalamic Tract (LST) transmits pain and temperature.
• The Ventral Spinothalamic Tract (VST) also transmits pain and temperature.

DCML Pathway

• This is a 3-neuron pathway.
• First-order neurons: from receptors to the medulla; synapse and decussate in the medulla, then ascend contralaterally.
• Second-order neurons: from the medulla to the thalamus.
• Third-order neurons: project from the thalamus to the somatosensory cortex, located in the postcentral gyrus.
• Sensory information is processes contralaterally

Primary Somatosensory Cortex

• The lateral aspects of the cortex correspond to the head, neck, and upper extremity.
• Medial aspects correspond to the trunk and lower extremities.
• An MCA stroke can cause diminished sensation and strength.

Clinical Assessment Techniques

• Light touch is assessed using cotton wool or fabric on the skin.
• Vibration is assessed using a 128 Hz tuning fork.
• Proprioception is assessed by passively moving a joint (e.g., finger or toe) up and down, and asking the patient which way it is pointing.

Spinothalamic Pathway

• This pathway is primarily responsible for pain and temperature sensation.
• First-order neurons: Receptors to the dorsal horn and synapse at the level of the spinal cord.
• Second-order neurons: Cross the midline and ascend the spinal cord to the thalamus.
• Third-order neurons: Project to the thalamus and cerebral cortex.
• The Thalamus is responsible for all the sensory inputs from the DCML and the spinothalamic tract.
• The amygdala (responsible for emotions withing the brain) receives pain and temperature information.

Pain and Temperature Fibers

• A-beta fibers: Light touch, vibration, and dermis stretch; information arrives quickly at the spinal cord
• A-delta fibers: Cold, noxious mechanical stimuli.
• C fibers: Nociceptive and Thermal stimuli; generally poorly myelinated or unmyelinated, so information arrives slower.

Clinical Assessment - Pain

• Assessed with Pin prick, by having an individual identify if an object (bobby pins) placed on the skin is sharp, dull, or blunted.

Clinical Assessment - Temperature

• Assessed using heated/cooled probes placed on the skin, and having the individual identify if it is hot or cold.

Spinocerebellar Tracts

• Provide information about unconscious proprioception and kinesthesia awareness.
• There are two tracts: Anterior and Posterior Spinocerebellar tracts.
• These tracts are a two-order neuronal pathway.

Spinocerebellar Pathway

• Dorsal (posterior) tract: Uncrossed; synapses at the level of the spinal cord, and travels ipsilaterally to the cerebellum.
• Ventral (anterior) tract: Double crossed; synapses at the level of the spinal cord in the dorsal horn, crosses contralaterally, and recrosses to the ipsilateral side.

Spinotectal Tract

• Transmits sensory information related to pain and temperature.
• Involved in the body's response to sensory stimuli
• First-order neurons: Sensory neurons carrying pain and temperature information from the body.
• Second-order neurons: Located in the spinal cord, synapse with first-order neurons and transmit information upward to the tectum of the midbrain.
• Facilitates the perception of pain and temperature and plays a role in orienting the body’s response to painful or potentially harmful stimuli (head and eye movements toward the stimulus).

Spinoolivary Tracts

• A sensory pathway that transmits information from the spinal cord to the inferior olivary nuclei in the brainstem.
• First-order neurons: Carry sensory information from the body, primarily related to proprioception (position and movement of body parts).
• Second-order neurons: Located in the spinal cord, synapse with first-order neurons and ascend toward the brainstem.
• Involved in the integration of proprioceptive information for motor coordination and fine-tuning voluntary movements and maintaining balance.

Impaired Proprioception

• Presentation: excess movement, wiggling, not a lot of control (dysmetria).
• During cone tapping, the involved leg struggles to stay steady and controlled as the uninvolved leg lifts.

Spinal Cord Tracts - Impact

• Corticospinal tracts impact voluntary movement.
• Extrapyramidal tracts impact involuntary motor function.
• DCML impacts light touch, proprioception, and vibration.
• Spinocerebellar tracts impact unconscious proprioception and kinesthesia.
• Spinothalamic tracts impact pain and temperature.

Common Incomplete Spinal Cord Injury Types

• Brown-Sequard injury.
• Central cord syndrome.
• Anterior cord syndrome.
• Cauda equina/injury.

Brown-Sequard Syndrome

• Half of the spinal cord is lost.
• Ipsilateral loss of light touch, proprioception, and vibration (DCML).
• Contralateral loss of pain and temperature (spinothalamic tract).
• Sparing of the dominant hand is a predictor of increased function.
• Distal upper extremity sensation/motor are last to return.
• Rarely a pure injury, and 75% will ambulate.

Central Cord Syndrome

• Injury in the center of the spinal cord, sparing the outside perimeter.
• Often due to hyperextension injury (fall, whiplash).
• Greater motor function loss in upper extremities than lower extremities, and variable sensory deficits.
• Most common SCI syndrome, especially in individuals > 50 years, in cervical injuries due to a fall.

Anterior Cord Syndrome

• The anterior portion of the spinal cord is compromised, possibly due to an anterior spinal artery stroke or hyperflexion-based spinal injury.
• Structures in the anterior portion are damaged.
• Loss of motor function below the level of injury.
• Loss of pain and temperature sensation.
• DCML is spared, so sensory features are intact.

Cauda Equina

• Injury to the cord as it exits the spinal column.
• Impairments vary based on the nerves injured.
• Loss of bowel and bladder function may indicate nerve compromise.
• Areflexic bowel and bladder and is a Lower Motor Neuron Injury.
• Requires referral to a physician or ER.

Summary & Review - Tract Tracing

• Sensory information travels ipsilaterally (touch, proprioception, vibration) and enters the spinal cord.
• First-order neuron: Receptors to medulla.
• Synapses and decussates (crosses over).
• Second-order neuron: Medulla to thalamus.
• Synapses and travels to the primary sensory cortex (post-central gyrus).
• Third-order neuron: Projects to the thalamus and somatosensory cortex.
• Information is processed contralaterally.

Key Neural Structures Involved in Sensory Pathways

• Nociceptors.
• Gracile or cuneate fascicle.
• Medulla.
• Thalamus.
• Somatosensory cortex.

Sensory Tract Crossing Behavior

• Crosses at the DCML and Spinothalamic tracts.
• Double crosses at the anterior ventral spinocerebellar tracts.
• Does not cross at the dorsal spinocerebellar tracts.

Major Functions of Each Sensory Tract

• DCML: Deep touch, light touch, vibration, proprioception, conscious proprioception.
• Lateral spinothalamic tract (LST): Pain and Temperature
• Spinocerebellar Tracts: Posterior and Anterior Spinocerebellar Tract

Common Incomplete Spinal Cord Syndromes & Impairments

• Cauda equina: Loss of bowel and bladder function.
• Anterior Cord Syndrome: Loss of motor function, pain, and temperature.
• Central Cord Syndrome: Motor function loss in upper extremities and sensory deficits.
• Brown-Sequard Syndrome: Ipsilateral loss of DCML (light touch, proprioception, vibration) and contralateral loss of spinothalamic (pain and temperature) functions.

Assessing Sensory Deficits

• Light touch: Use cotton wool/fabric, and have the individual acknowledge where they felt the touch.
• Vibration: Use a 128 Hz tuning fork.
• Proprioception: Move a distal extremity (hand/foot) up/down, and have the individual identify the direction.

Test your knowledge of ascending sensory pathways. Questions cover the Dorsal Column-Medial Lemniscus (DCML) pathway, lateral spinothalamic tract, kinesthesia, proprioception, and Brown-Sequard Syndrome. Assess your understanding of sensory processing and related neurological conditions.