Spinal Cord Anatomy and Function

• The lecture discusses the spinal cord and its anatomy

Objectives

• Master external features (location, regions, segments, limits, enlargements, relationship to vertebrae)
• Understand the organization of gray and white matter
• Know major ascending and descending pathways (dorsal column-medial lemniscus, anterolateral system, corticospinal tract)
• Understand how spinal cord lesions affect the body
• Understand: Intermediomedial/intermediolateral nuclei, substantia gelatinosa, nucleus proprius, dorsal nuclei, and spinal reflex

Location

• Lies in the vertebral canal
• Runs from the foramen magnum to the L1 vertebra in adults
• It runs to the L3 vertebra in newborns
• The vertebral column is around 70cm in males and 60cm in females
• Spinal cord length is around 42-45cm

External Features

• A segment constitutes where a portion of the spinal cord provides attachment to pairs of spinal nerves
• There are 31 spinal cord segments:
  • 8 Cervical
  • 12 Thoracic
  • 5 Lumbar
  • 5 Sacral
  • 1 Coccygeal
• There are two enlargements
  • Cervical enlargement (C4-T1)
  • Lumbosacral enlargement (L2-S3)
• Spinal cord cross-sections vary at different levels (C5, T2, T8, L1, L2, S2, S3)
• The cervical enlargement which starts at C4 and ends at T1, has nerves, emerging from this region, and serve the upper extremities
• Nerves from the lumbar enlargement serve the lower extremities

Inferior End

• Features of the inferior end of the spinal cord include:
  • Conus medullaris, the tapering terminal portion
  • Filum terminale, a fibrous strand of pia mater
  • Cauda equina, resembles a horse's tail

External Features, Membranes, Fissures and Sulci

• Dura mater, arachnoid mater, and pia mater are external features of the spinal cord
• Denticulate ligaments are paired, lateral triangular extensions of the spinal pia mater that attach to the dura mater.
• Anterior median fissure, posterior median sulcus, anterolateral sulcus, posterolateral sulcus, and posterior intermediate sulcus are all fissures and sulci located on the spinal cord
• The posterior intermediate sulcus can be found from C1-T4

Internal Features

• Gray matter is centrally located with ventral horns/gray columns, dorsal horns, and lateral horns (T1-L3)
• Central canal, dorsal and ventral gray commissures are also present
• White matter is peripherally located with posterior, anterior, and lateral funiculi, and a white commissure

Gray Matter Components, Rexed Laminae

• The posterior horns contain interneurons
• Anterior horns contain interneurons and motor neuron cell bodies
  • Their axons project through the ventral roots to skeletal muscles.
• Lateral horn neurons are sympathetic motor neurons that serve visceral organs.
  • Their axons also exit via the ventral root.
• Afferent sensory fibers form the dorsal roots of the spinal cord.
  • The somas of these sensory fibers are located in the dorsal root ganglion.

Rexed Laminae

• Rexed laminae divide the gray matter into regions.
• Key nuclei within these laminae include:
  • Posteromarginal nucleus (I) which receives input from dorsolateral tract
  • Substantia gelatinosa (II) which integrates sensory stimuli for pain and temperature
  • Nucleus proprius (III, IV, V) receives sensory information, including light touch, pain and temperature
  • Nucleus thoracicus (VII) relays posterior spinocerebellar tract information (C8-L3)
  • Intermediolateral nucleus (VII) which relays sympathetic preganglionic neurons (T1-L3)
  • Intermediomedial nucleus (VII) which is responsible for the sensation of viscera
  • Sacral parasympathetic nucleus (VII) which lies in S2-S4 and contain parasympathetic preganglionic neurons
  • Motor neurons (VIII, IX)

Autonomic Nervous System

• Lower centers are located in four pairs of parasympathetic nuclei in the brainstem
• Sacral parasympathetic nucleus are also lower centers found in cord segments S2-S4
• Located in the lateral gray horn of spinal cord segments T1-L3

White Matter

• Descending tracts originate from the cerebral cortex or brainstem
• Ascending pathways consist of dorsal columns, spinocerebellar tracts, spinothalamic, and spinoreticular tracts

Ascending Sensory Tracts

• Posterior column (dorsal column-medial lemniscus) includes fasciculus gracilis and fasciculus cuneatus.
• Anterolateral (spinothalamic) tract.
• Spinocerebellar tract.
• Posterior Column pathway provides deep sensations
  • The fasciculus gracilis arises from the spinal ganglion below T5 and terminates upon the nucleus gracilis.
  • The fasciculus cuneatus arises from the spinal ganglion above T4 and terminates upon the nucleus cuneatus.
  • This pathway mediates vibration, position sense, and fine touch.
• Two-point discrimination is tested with sharp points during a neurological examination and reflects how finely innervated an area of skin is

Anterolateral Pathway

• The spinothalamic tract's origin is laminae I, IV-VII in spinal cord and termination is the dorsal thalamus.
• Its function consist of sensing pain and temperature (lateral), and crude touch (anterior).

Spinocerebellar pathway

• Consist of ascending tracts
  • Anterior spinocerebellar tract from laminae V-VII terminating at the cerebellum
  • Posterior spinocerebellar tract from nucleus thoracicus to the cerebellum
• Function in nonconscious proprioceptive sensation.

Descending Motor Tracts

• Lateral corticospinal tract
• Rubrospinal tract
• Reticulospinal tract
• Vestibulospinal tract
• Medial longitudinal fasciculus
• Anterior corticospinal tract
• Tectospinal tract
• The lateral corticospinal tract is 75-90% of descending motor tracts and deals with limb muscles
• The anterior is important for axial muscles

Reflexes

• Reflexes have components: receptor, sensory neuron, integrating center, motor neuron, and effector.
• Pathological reflex indications: areflexia, hyporeflexia, or hyperreflexia.

Spinal Cord Lesions

• Spinal cord hemisection results in Brown-Sequard syndrome
  • Causes paralysis, vibration, and position sense loss below the level of the lesion on the same side
  • Also results in pain and temperature sense loss below the level of the lesion on the opposite side.