Spinal Cord Anatomy: Sensory Pathways

Spinal Cord Anatomy Part II

• Presented by Kevin McCarthy, CNIM (Director of Education and Training) and Kent Rice, REPT, CNIM, DABNM (Sr. Manager Education and Training).

Afferent Spinal Cord Tracts

• The presentation includes the spinal cord anatomy agenda.
• Including topics like sensory pathways, motor pathways, and functional physiology

Dorsal Column - Medial Lemniscus Pathway

• Transmits discriminative touch, vibrotactile sensation, and proprioception (position sense).
• Key receptors are located in the skin, joints and muscles.
• Primary sensory fibers enter the dorsal root and ascend the ipsilateral fasciculus cuneatus tract.
• Termination occurs in the nucleus cuneatus.
• Synapse and decussation occur via internal arcuate fibers in the medulla
• Secondary fibers ascend the medial lemniscus up to the ventral posterolateral (VPL) nucleus of the thalamus.
• Thalamocortical fibers project to the hand region of the contralateral post-central gyrus.

Central DC-ML Pathway

• The primary somatosensory receiving area is in the post central gyrus, contralateral to the stimulated sensory fibers.
• Information crosses the midline (decussates) in the medulla.

SSEP Pathway

• Pathway transmits afferent signals for proprioception, pressure, and vibration forming the dorsal column.
• Contains the largest number of sensory fibers, accounting for the majority of SSEP recordings.

Dorsal Column Pathway Input

• Processes multiple types of sensory receptors and their input travels in the dorsal column path.
• Pacinian corpuscles, unencapsulated receptors, Golgi-Mazzoni corpuscles, and Meisner's corpuscles are all receptor inputs.
• 1st order neuron: Dorsal root ganglion
• 2nd order neuron: Nucleus gracilis and cuneatus
• 3rd order neuron: VPL of the thalamus
• Cortex: Postcentral gyrus

Spinothalamic Tracts

• Transmits sensory information about pain, temperature, and crude touch.
• Sensory pathway on the anterolateral aspect of the spinal cord.
• Note that decussation occurs in the spinal cord

Output Pathways

• Descending motor pathways originate from both the forebrain and the brainstem.
• Forebrain pathways: corticospinal and corticobulbar tracts
• Brainstem pathways: rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts
• Output is into the pre-motor network: supplementary motor cortex, primary motor cortex, and somato motor pathways

Descending Pathways Information

• Motor pathways are descending, specific white matter bundles (tracts)
• Motor pathways convey motor information down the spinal cord from the brain

Pyramidal Pathway Information

• Lateral Corticospinal Tract controls fine movement of the limbs
• Anterior Corticospinal Tract controls the central axial and girdle muscles.
• Decussation of the Pyramids: Approximately 80% of the corticospinal fibers cross.

Pyramidal Pathway Tracts

• Corticospinal tracts: upper motor neuron axons terminating in the anterior horn of the spinal cord.
• Corticobulbar tracts: upper motor neurons terminating in the cranial nerve nuclei of the brainstem.

Extra-Pyramidal Tracts

• Reticulospinal Tract: motor to flexors
• Vestibulospinal Tract: involved with balance and innervates motor of extensor and flexors
• Medial Reticulospinal Tract: motor to extensors
• Tectospinal Tract: motor of the neck and trunk

Distribution of Lower Motor Neurons (LMN)

• Distributed in the ventral horn gray matter, organized into somatotopic map.
• Proximal muscles represented medially.
• Distal muscles are represented laterally

Motor Unit Review

• Each motor unit includes one neuron and multiple muscle fibers.
• Multiple motor units recruited according to force and load.
• Smaller motor units recruited before larger to increase muscle loads.

Reflex Arc

• Collateral branches of sensory fibers involved in a reflex arc synapse in the spinal cord.
• Allows reflexes to activate spinal motor neurons without the delay of signals throughout the brain.
• Brain receives that sensory input while the reflex action occurs.

Reflex Arc and Muscle Spindle

• Group Ia afferents synapse directly on alpha motor neurons
• Results in contraction of homonymous muscle groups from a sensory stimulus.
• Group Ia afferents also synapse on local circuit neurons with Ia inhibitory interneurons, inhibiting antagonist muscles.