Spinal Anatomy Tracts

The mature spinal cord has an H-shaped central gray area which contains the cell bodies of the spinal neurons. This is surrounded by an area of white matter which contains the axons that ascend the cord to the brain, or that descend the cord from the brain.

The spinal cord gray matter is functionally divided into three major areas: the dorsal horn, the ventral horn, an the intermediate zone.

• contains ascending neurons and interneurons
• relays incoming sensory information to higher centers

• contains descending neurons and interneurons
• controls motor functions of the muscles of the trunk and limbs

• contains autonomic pre-ganglionic neurons which control motor functioning of the viscera.
• also contains neurons (Clarke's column) which send sensory information to the cerebellum.

The spinal cord white matter is functionally divided into three pairs of funiculi: the posterior funiculi, the lateral funiculi, and the anterior funiculi.

contains ascending axons which relay sensory information to the medulla oblongata

contains sensory, motor, and autonomic axons

contains axons descending from the brain which relay motor information to the muscles

He proposed a scheme for subdividing the gray matter of the spinal cord in 1952, into 10 layers or laminae. His system was based on neuronal cytoarchitecture. His system has endured because later it was found that the neurons in the different laminae are functionally distinct with different patterns of projections.

in the dorsal horn

approximately equivalent to the intermediate zone

comprise the ventral horn

consists of the gray matter surrounding the central canal

• the marginal zone
• the most superficial region of the dorsal horn
• important sensory relay for pain and temperature

• the substantia gelatinosa
• receives afferent information from nonmyelinated fibers and integrates it with that from thinly myelinated afferent fibers that project to Lamina I

• contain the nucleus proprius
• this integrates sensory input with information that descends from the brain and the region of the base of the dorsal horn where many neurons which project to the brain stem are located.

• Contains Clarke's nucleus or cell column only in the thoracic and upper lumbar segments.
• This relays information about limb position and movement to the cerebellum.
• The intermediolateral nucleus or cell column also located in the thoracic and upper lumbar segments contains autonomic preganglionic neurons

Contains interneurons that are important in regulating skeletal muscle contraction.

• The motor nuclei of the ventral horn.
• Contains motor neurons which innervate skeletal muscles

Receives afferent input similar to Laminae I and II

Contains central branches of small-diameter fibers

Contains axons of propriospinal neurons that interconnect different regions of the spinal cord

• Located in the posterior funiculi.
• Ascend the cord ipsilaterally until the medulla oblongata, where they decussate, at the cervico-medullary junction.

• In the spinal cord, at the level of T-6, the columns divide and form two bundles or fascicles of axons: the gracile fascicle and the cuneate fascicle.
• The gracile fascicle ascends medially, and contains fibers from the ipsilateral sacral, lumbar, and lower thoracic segments. The cuneate fascicle ascends laterally and contains fibers from the upper thoracic and cervical segments.

After the fibers decussate at the cervico-medullary junction, they continue travelling by way of the medial lemniscus to the VPL (Ventral Posterior Lateral) nucleus of the thalamus. The sensory information they carried to the thalamus then goes from the thalamus to the post central gyrus of the parietal lobe, to the somatic sensory cortex (AKA somesthetic cortex) for localization.

Relay the following modalities: discriminative touch, vibration, and joint position sense. Testing in the office is done by testing joint position sense, swaying (Romberg's Test), vibratory sense, touch.

• Located in the antero-lateral column.
• Contains fibers from ascending pathways which originate in different laminae of the dorsal horn of the spinal cord.
• Decussates segmentally and ascends the cord contralaterally.

• The fibers from this tract terminate in the brain stem, hypothalamus, and thalamus.
• Tracts involved in this system include the spinothalamic (lateral and ventral), spinoreticular, and spinomesencephalic tracts.

• The spinothalamic tract is the most prominant ascending nociceptive pathway in the spinal cord.
• It carries information from neurons in laminae I, and V through VII.
• Terminates in the thalamus at the VPL nucleus.
• Information then goes from the thalamus to the somatic sensory cortex in the post central gyrus of the parietal lobe for localization.

• The spinoreticular tract contains nociceptive information from the neurons in laminae VII and VIII.
• These axons terminate on neurons in the reticular formation of the medulla and pons.
• This information is then relayed onto the thalamus and other structures in the dicephalon.

The spinothalamic and spinoreticular tracts mediate noxious and thermal sensations which are relayed to them from A-delta and C fibers.

• The spinomesencephalic (AKA spinotectal) tract contains information nociceptive neurons in laminae I and V.
• This tract terminates in the tectum (roof) of the midbrain, in the superior colliculus.
• It also projects to the mesencephalic periaqueductal gray region, which surrounds the cerebral aqueduct. This area is important because it contains neurons that are a part of a descending pathway the regulates pain transmission.

• The second major ascending system for mediation of somatic sensation.
• Relays the following modalities: pain, temperature, crude touch.
• Testing is done in the office by pinwheel, temperature of water in test tubes, randomly touching the patient.

• Located in the lateral funiculi.
• Information from the lower extremities ascends the cord in the ventral spinocerebellar tracts.
• Lower extremity information enters the posterior aspect of the spinal cord and decussates immediatley.
• The tract travels with the anterolateral system to the brain stem.
• At the brain stem, the ventral spinocerebellar tract decussates back to the side of its origin.
• It enters the superior cerebellar peduncle (AKA bracium conjunctivum).
• From here, the information goes to the floccular-nodular lobe of the cerebellum.
• Ultimatley, the information reaches the somesthetic cortex of the parietal lobe; the rest of the cortex including the motor area; and the vestibular nuclei.

• Information from the upper extremities ascends the cord via the dorsal spinocerebellar tracts.
• These DO NOT decussate in the cord; their neurons are in Clarke's column.
• The dorsal spinocerebellar tracts enter the inferior cerebellar peduncle (AKA restiform body).
• From here, the information travels to the floccular-nodular lobe of the cerebellum.
• This information ultimately reaches the same areas as that of the ventral spinocerebellar tracts.

These tracts relay information to the cerebellum regarding body position, position of body parts relative to each other. Therefore, they convey unconscious proprioception.

• Originate in the motor cortex of the brain.
• The majority of the tracts decussate at the level of the medullary pyramids (hence the name pyramidal tracts) and continue contralaterally down the spine as the lateral corticospinal tracts.

Those that DO NOT decussate at the level of the pyramids continue down the spine ipsilaterally as the ventral corticospinal tracts. These may then decussate at the segmental cord level, or remain ipsilateral.

This is the direct pathway for voluntary movement. Function in voluntary, fine movement; innervate the fine flexors of the fingers and toes.

• Originate primarily in the reticular formation of the mesencephalon, pons and medulla oblongata.
• Have both excitatory and inhibitory connections with spinal interneurons and motor neurons.

• The mesencephalic reticulospinal tracts descend to the cord both:
• Ipsilaterally as the homologue of the IML and
• Contralaterally to terminate in the voluntary extensor muscles of the posterior compartments of the limbs and the back.

The medullary reticulospinal tracts inhibit the action of the ipsilateral mesencephalic reticulospinal tracts.

Function in conscious postural maintenance and pain control.

Originate at the vestibular nuclei; there are 4 of these.

The tract from the lateral vestibular nuclei (AKA Dieter's Nucleus) descends the cord ipsilaterally as the lateral vestibulospinal tract. This tract innervates all involuntary extensor and anti-gravity muscles of the back.

The tracts from the superior, inferior, and medial nuclei also descend the cord ipsilaterally and contralaterally. These also ascend as the medial longitudinal fasiculus to end on cranial nerves III and IV (mesencephalon) and VI (pons) to drive the eyes.

Function in posture/extensor muscles.

• Originates in the magnocellular protion of the red nucleus in the midbrain.
• This tract decussates immediately, and descends through the medulla to the dorsal part of the lateral column of the spinal cord.

Innervates the voluntary flexor musculature of the limbs above T-6 (some books say in the cervical spine only).