Spinal Cord Anatomy

Overview

The spinal cord is a complex structure that serves as the main pathway for information connecting the brain and peripheral nervous system. It is organized into distinct regions of gray and white matter, each with specialized functions.

• Gray matter is located centrally, surrounding the central canal, and is H- or butterfly-shaped.

• White matter is found on the outside of the gray matter, opposite to the arrangement in the brain.

Gray Matter: Horns

The gray matter of the spinal cord is divided into regions called horns, each with specific functions:

• Anterior (Ventral) Horn: Contains somatic motor neuron cell bodies. Axons exit via ventral roots to innervate skeletal muscles.

• Lateral Horn: Prominent in thoracic and upper lumbar regions. Contains autonomic motor neuron cell bodies. Axons exit via ventral roots to visceral organs.

• Posterior (Dorsal) Horn: Receives afferent (sensory) fibers from peripheral receptors. Cell bodies are located in the dorsal root ganglion (often pseudounipolar neurons). These fibers may ascend in tracts or synapse with association neurons.

White Matter: Columns and Tracts

Organization

White matter consists of myelinated and unmyelinated fibers organized into columns (funiculi):

• Anterior (ventral) funiculus

• Lateral funiculus

• Posterior (dorsal) funiculus

Fibers may be ascending (sensory), descending (motor), or commissural (crossing sides).

Ascending (Sensory) Tracts

Ascending tracts carry sensory information to the brain. They are also called afferent tracts.

• Dorsal White Column: Composed of fasciculus cuneatus and fasciculus gracilis.

  • Fasciculus cuneatus: Carries fine touch, pressure, and proprioception from upper limbs, trunk, and neck.

  • Fasciculus gracilis: Carries similar information from lower trunk and limbs.

• Spinocerebellar Tract: Posterior (dorsal) spinocerebellar tract carries subconscious proprioception to the ipsilateral cerebellum (does not cross over).

• Lateral Spinothalamic Tract: Carries pain and temperature. Crosses over immediately upon entering the spinal cord, then ascends contralaterally to thalamus and sensory cortex.

• Anterior Spinothalamic Tract: Carries crude touch and pressure. Also crosses over before ascending.

Proprioception

Proprioception is the conscious awareness of body position, such as knowing if your knee is bent or straight without looking.

Clinical Relevance: Sensory Loss Patterns

• Damage to one side of the spinal cord affects sensation differently:

  • Fine touch (fasciculus gracilis/cuneatus): Loss on the same side as the damage.

  • Pain and temperature (spinothalamic tracts): Loss on the opposite side, since these fibers cross over before ascending.

Descending (Motor) Tracts

Pyramidal Tracts

The pyramidal tracts are major descending pathways that carry voluntary motor commands from the brain to skeletal muscles.

• Lateral Corticospinal Tract: Originates in the primary motor cortex, crosses over (decussates) in the medulla pyramids, then descends to the spinal cord and synapses with somatic motor neurons.

• Anterior Corticospinal Tract: Descends ipsilaterally, then crosses over at the level of the synapse with the second neuron before exiting to the muscle.

Extrapyramidal Tracts

• Tectospinal Tract: From superior colliculi to contralateral eye muscles; important for visual tracking.

• Vestibulospinal Tract: From medulla to ipsilateral muscles; maintains balance.

• Rubrospinal Pathway: From red nucleus in brainstem to contralateral muscles; maintains muscle tone.

• Reticulospinal Tracts: From reticular formation to both ipsilateral and contralateral muscles; helps with muscle tone, visceral muscle movements, and unskilled movements.

Summary Table: Major Spinal Cord Tracts

Key Definitions

• Afferent: Carrying information toward the central nervous system (sensory).

• Efferent: Carrying information away from the central nervous system (motor).

• Decussation: Crossing over of nerve fibers from one side to the other.

• Proprioception: Awareness of body position and movement.

Relevant Equations

While spinal cord anatomy is not heavily reliant on equations, the following formula is relevant for nerve conduction velocity:

Additional info: Academic context was added to clarify tract functions, clinical relevance, and definitions for completeness.