Clinical Neuroscience Week 6 – Ascending Pathways (Medium)

Ascending Sensory Pathways: An Overview

• Afferent/sensory input includes touch, pressure, vibration, temperature, pain, vision, and hearing.
• Sensory information from the periphery travels to the dorsal root ganglion, then enters the dorsal aspect of the spinal cord.
• Conscious level pathways include those for light touch, proprioception, vibration, pain, and temperature.
• Relevant spinal cord pathways at the conscious level are the Dorsal Column-Medial Lemniscus (DCML) and Spinothalamic tracts.
• Unconscious level pathways involve proprioception and kinesthesia.
• A relevant spinal cord pathway at the unconscious level is the Spinocerebellar tract.

Dorsal Column-Medial Lemniscus Tract

• Ascending sensory tract.
• Dorsal columns carry information about deep touch, light touch, vibration, and conscious proprioception.
• Conscious proprioception is the ability to detect where a joint or body part is in space.

Lateral and Ventral Spinothalamic Tracts

• The Lateral Spinothalamic Tract (LST) is responsible for pain and temperature sensation.
• The Ventral Spinothalamic Tract (VST) is also involved in sensory pathways.

DCML Pathway Details

• This is a 3-neuron pathway.
• Sensory information (touch, proprioception, vibration) enters ipsilaterally into the spinal cord.
• The 1st order neuron goes from receptors to the medulla, where it synapses and decussates (crosses over).
• 2nd order neuron travels from the medulla to the thalamus.
• 3rd order neuron projects from the thalamus to the somatosensory cortex, located in the postcentral gyrus.
• Sensory information is processed contralaterally.
• The DCML divides into two components: the gracile fascicle (lower extremities) and the cuneate fascicle (upper extremities).

Somatosensory Cortex

• The sensory cortex is organized similarly to the motor homunculus.
• Lateral aspects of the cortex are responsible for the head, neck, and upper extremities.
• Medial aspects perceive sensory information from the trunk and lower extremities.
• An MCA stroke can diminish both strength and sensation in the upper extremity.

Clinical Assessment Techniques

• Light touch is assessed using cotton wool or fabric on the skin.
• Vibration is assessed using a tuning fork, typically 128 Hz.
• Proprioception is assessed by moving a distal extremity (hand or foot) and having the patient identify the direction of movement.

Spinothalamic Pathway Specifics

• First-order neurons go from receptors to the dorsal horn of the spinal cord, synapsing at that level.
• Information decussates immediately to the contralateral side before ascending.
• Second-order neurons cross the midline and ascend to the thalamus.
• Third-order neurons project to the thalamus and cerebral cortex.
• The thalamus processes sensory inputs from both the DCML and spinothalamic tracts.
• Pain and temperature information projects primarily to the thalamus, with some input to the amygdala (for emotional response).

Pain and Temperature Fibers

• A-beta fibers: Light touch, vibration, dermis stretch with high myelination and quick signal transmission.
• A-delta fibers: Cold, noxious mechanical stimuli (like a hammer hitting your toe), poorly myelinated.
• C fibers: Nociceptive and thermal stimuli, unmyelinated, and slow signal transmission.

Clinical Assessments for Pain and Temperature

• Pain: Assessed via pinprick, with the individual identifying sharp vs. dull sensations.
• Temperature: Assessed using heated or cooled probes on the skin, with the individual identifying hot or cold.

Spinocerebellar Tracts

• Provide information about unconscious proprioception and kinesthesia.
• Two tracts: anterior and posterior spinocerebellar tracts.
• These are two-order neuronal pathways.

Spinocerebellar Pathway Details

• Posterior tract: uncrossed, synapses in the spinal cord, and travels ipsilaterally to the cerebellum.
• Anterior tract: double crossed.
• Information enters, synapses, and crosses in the spinal cord, ascends contralaterally, and then recrosses to the ipsilateral side.

Spinotectal Tract

• Part of the spinal cord's sensory pathways.
• 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, these neurons synapse with first-order neurons and transmit information upward.
• Pathway: Second-order neurons ascend to the tectum of the midbrain. Primarily reach the superior colliculus, which is involved in motor functions like eye and head movements.
• Function: Facilitates the perception of pain and temperature. Plays a role in orienting the body’s response to painful or potentially harmful stimuli (head and eye movements toward the stimulus).
• Clinical Relevance: Important for understanding how the brain processes and responds to sensory input, particularly related to pain and temperature. Damage to the spinotectal tract can impair the ability to react to pain through proper motor coordination (e.g., eye and head movements).

Spinoolivary Tract

• A sensory pathway that transmits information from the spinal cord to the inferior olivary nuclei in the brainstem.
• Structure: 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.
• Pathway: The second-order neurons project to the inferior olivary nuclei, located in the medulla oblongata. The olivary nuclei then relay this information to the cerebellum, which is involved in motor control and coordination.
• Function: Involved in the integration of proprioceptive information for motor coordination. Plays a role in fine-tuning voluntary movements and maintaining balance.
• Clinical Relevance: Damage to the spinoolivary tract can impair proprioception and motor coordination. Disruptions may affect balance and movement precision. 

Impaired Proprioception

• Leads to excess movement and lack of control, called dysmetria.
• Individuals struggle to maintain steady posture when the uninvolved leg is in motion.

Spinal Cord Tracts and Their 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: Responsible for pain and temperature.

Common Incomplete Spinal Cord Injury Syndromes

• Brown-Sequard syndrome, central cord syndrome, anterior cord syndrome, and cauda equina injury.

Brown-Sequard Syndrome

• Definition: Loss of one half of the spinal cord.
• Ipsilateral loss of light touch, proprioception, and vibration due to damage to the DCML.
• Contralateral loss of pain and temperature sensation due to damage to the spinothalamic tract.
• Predictors of increased function include sparing of the dominant hand.
• Distal upper extremity sensation/motor function is typically the last to return.
• Rarely a pure injury. 75% of patients will ambulate.

Central Cord Syndrome

• Injury to the center of the spinal cord, often due to hyperextension injuries like falls or whiplash.
• Motor function loss is more pronounced in the upper extremities than in the legs.
• Sensory deficits may be present depending on the extent of damage.
• Most common SCI syndrome in individuals over 50.

Anterior Cord Syndrome

• Compromise to the anterior portion of the spinal cord, often due to anterior spinal artery stroke or hyperflexion injuries.
• Loss of motor function below the level of injury due to damage to fibers exiting the ventral root.
• Loss of pain and temperature sensation.
• Sensory features (light touch, proprioception) are typically intact due to sparing of the posterior aspect of the spinal cord (DCML).

Cauda Equina Syndrome

• Injury to the nerve roots as they exit the spinal column.
• Impairments vary based on the specific nerves injured.
• Loss of bowel and bladder function is a key concerning sign.
• Areflexic bowel and bladder.
• Considered a Lower Motor Neuron Injury.

Overview of ascending sensory pathways, including conscious and unconscious pathways. Focus on the Dorsal Column-Medial Lemniscus (DCML) and Spinothalamic tracts, which are important for relaying sensory information to the brain. Covers light touch, proprioception, vibration, pain, and temperature.