Spinal Cord Structure

Spinal cord - extends from brainstem through the foramen magnum and down vertebral column

1. Deep anterior median fissure & less deep posterior median sulcus
2. Surrounded by 3 meninges: dura, arachnoid and pia maters
   1.   Attached to occipital periosteum and to the coccygeal ligament which attaches to the periosteum of the coccyx
3. Central gray matter comprised of soma, glial cells and unmyelinated axons
   1. Arranged as H-shape surrounding the central canal
      1. Posterior gray horns contain somatic and visceral sensory nuclei
      2. Anterior gray horns contain somatic motor nuclei
      3. Lateral gray horns located only in thoracic and lumbar segments contain visceral motor nuclei
      4. Gray commissure connects right and left halves of spinal cord, axon crossing
4. Surrounded by white matter comprised of myelinated and unmyelinated axons
   1. Divided into paired anterior, posterior and lateral white columns
      1. Each column contains tracts, either ascending sensory or descending motor
5. Enlarged areas have more gray matter
   1. Cervical enlargement & lumbar enlargement - innervate limbs
6. Spinal cord ends as the tapered conus medullaris at L1\2
   1. Filum terminale - strand of connective tissue extends from conus to S2 and merges into the coccygeal ligament, provides longitudinal support
7. 31 pairs of spinal nerves (mixed) exit through intervertebral foramen
   1. Group of spinal nerves L2-S5 emerging from conus form cauda equina
   2. Each spinal nerve is formed from a dorsal root (afferent or sensory) and a ventral root (efferent or motor)
      1. Dorsal root ganglion contain soma of sensory neurons
   3. Nerve plexus is an interwoven network of spinal nerve branches resulting from muscle fusion during development
      1. Cervical, brachial, lumbar, and sacral plexuses

Figure in class

Sensory and Motor Tracts

Major Ascending Sensory Pathways

From receptor to cerebral cortex or cerebellum

Posterior column pathway

• Carries fine touch, pressure, vibration and proprioception information
• First-order sensory neurons (s.c.) synapse in the nuclei gracilis and cuneatus (m.o.)
• Second-order neurons cross-over in the medulla oblongata go to the thalamus
• Third-order neurons (thalamus) projection to the primary sensory cortex

Spinothalamic pathway

two tracts located in anterior and lateral columns

• Carries crude touch, pressure, pain and temperature information
• First-order sensory neurons synapse with interneurons in spinal cord
• Second-order interneurons cross-over in the spinal cord and go to the thalamus
• Third-order neurons (thalamus) projection to the primary sensory cortex

Spinocerebellar pathway

two tracts located in each of the lateral columns

• Carries information about position of skeletal muscle, tendons and joints
• First-order sensory neurons synapse with interneurons in spinal cord
• Second-order interneurons traveling in the posterior tract cross-over in the spinal cord and go to the cerebellum
• Second-order interneurons traveling in the anterior tract do not cross-over, they go directly to the cerebellum
• There are no third-order neurons

Major Descending Motor Pathways

From cerebral cortex, brainstem, cerebellum to effector

Pyramidal system

conscious control of skeletal muscles

• Corticobulbar tracts - neurons descend through cranial nerves 3-7, 9, 11, 12
  • Control muscles in the eye, jaw, face and neck
• Corticospinal tracts - located in anterior and lateral columns
  • Control muscles below neck level
  • 85% of the neurons cross-over in the medulla oblongata, travel down the spinal cord and then synapse with a motor neuron
  • 15% of the neurons travel down the spinal cord and then cross-over before synapsing with the motor neuron

Medial pathway

subconscious control of trunk and proximal limb muscles

• Reticulospinal, vestibulospinal, and tectospinal tracts located in anterior columns
• Only motor neurons in tectospinal tracts show cross-over

Lateral pathway

subconscious control of distal part of upper limb muscles

• Rubrospinal tracts located in lateral columns
• Cross-over in mesencephalon