Spinal Cord Anatomy

• Cross Section of Spinal Cord

Gray Matter and Horns

• Gray matter contains neuron cell bodies, dendrites, and axons.
• The gray matter is divided into horns.
• The dorsal horn contains cell bodies of sensory neurons.
• The ventral horn contains cell bodies of motor neurons that control skeletal muscles.
• The lateral horn, which is not always present, contains cell bodies of motor neurons controlling cardiac muscle, smooth muscle, and glands, part of the Autonomic motor system

Anterior Features

• The anterior median fissure is a wide groove on the anterior aspect of the spinal cord.
• The posterior median sulcus is a narrow groove on the posterior aspect.

Funiculus

• Funiculus relates to a column of white matter

Organization and Tracts of the Spinal Cord

• Gray matter is divided into laminae.
• White matter contains axons organized into tracts.
• White matter tracts transfer signals to the thalamus or to nuclei for synapse.
• White matter tracts are divided based on the direction of signal transmission: Ascending tracts carry sensory information, while descending tracts carry motor commands. Ascending tracts include sensory information via the posterior column
• The posterior column includes the gracile fasciculus and cuneate fasciculus.
• Ascending tracts carry sensory information from the spinal cord to the cerebellum or thalamus
• Descending tracts include the lateral corticospinal tract which runs from cortex to the spinal cord

Posterior White Column-Medial Lemniscal Pathway:

• Transmits discriminative touch, vibration, including conscious proprioception.
• Most receptors are involved, except free nerve endings.
• The primary somesthetic area (S I) in the cortex is involved in termination.

Neurons

• First-order neurons: pure sensory or motor dorsal root ganglion with peripheral process, cell body in (dorsal root ganglia), and central process that enters the spinal cord and ascends ipsilaterally in the posterior column, carrying fibers from upper and lower body parts
• Second-order neurons: dorsal column nuclei (nucleus gracilis and cuneatus) in the medulla, Internal arcuate fibers.
• Medial lemniscus: Fibers carry Big boss to the thalamus
• Third-order neurons: Thalamus (VPL), run through Internal Capsule and Corona Radiata, and terminates in Primary Somesthetic Area (S I) location in cortex

Modality & Gracile/Cuneate Nuclei

• Modality includes discriminative touch (high velocity/well advanced), conscious proprioception (sense of position/muscle joint sense), where sensors measure muscle or joint tension.
• Axons cross the midline in the medulla, leading to Lemniscal Decussation.
• Third-order neurons project from the VPL of the thalamus to the primary somatosensory cortex.
• Signals pass through the internal capsule.
• Fibers organize topographically on the cortex (homunculus).
• The fasciculus gracilis transmits information from inferior areas to T6.
• The fasciculus cuneatus transmits information from superior areas to T6.

Key Concepts

• The gray matter makes up more volume in the caudal/lower portion of the body
• Fibers from the lower body are medial in the spinal cord
• From the lower part of the body to the upper part of the body
• The fasciculus gracilis synapse is in the lower portion of the medulla
• After the 2nd order crossing the midline, information heads toward the VPL
• Homunculus indicates fine-tune vibration, pressure, and proprioception signals from the right side of the body
• Homunculus refers to area on the cortex