Makindo Medical Notes

Overview of the Corticospinal Tracts

The corticospinal tracts are the major pathways responsible for voluntary motor control. These tracts originate in the cerebral cortex and project to the spinal cord, facilitating movement of the limbs and trunk.

Divisions of the Corticospinal Tracts

• Lateral Corticospinal Tract: The lateral corticospinal tract is responsible for controlling voluntary movements of the limbs. It decussates (crosses) at the pyramids of the medulla oblongata, which allows one side of the brain to control the opposite side of the body.
• Anterior Corticospinal Tract: This tract controls voluntary movements of the axial muscles, such as those in the neck and trunk. Unlike the lateral corticospinal tract, it does not decussate at the medulla but instead crosses over at the level of the spinal cord.

Pathway of the Corticospinal Tracts

• Origin: The corticospinal tracts begin in the motor cortex, specifically from the precentral gyrus (primary motor cortex).
• Internal Capsule: The fibres pass through the posterior limb of the internal capsule, which acts as a conduit between the cortex and lower motor neurons.
• Brainstem (Pyramidal Tract): As the fibres descend through the midbrain and pons, they form the pyramids in the medulla.
• Decussation: In the medulla oblongata, about 90% of the corticospinal fibres decussate to form the lateral corticospinal tract, while 10% remain uncrossed, forming the anterior corticospinal tract.
• Spinal Cord: The tracts continue to descend through the spinal cord, where they synapse on lower motor neurons in the anterior horn of the spinal cord.

Functions of the Corticospinal Tracts

• Lateral Corticospinal Tract: Controls fine, skilled movements of the distal limbs, such as hand and finger movements.
• Anterior Corticospinal Tract: Regulates movements of the axial and proximal muscles, especially in the trunk and shoulders.

Motor

Sensory

Overview of Ascending Sensory Pathways

Ascending sensory pathways are responsible for transmitting sensory information from the periphery (such as skin, muscles, and joints) to the brain. These pathways carry information related to touch, proprioception, pain, and temperature.

Major Ascending Sensory Pathways

The primary ascending sensory pathways in the spinal cord and brain are:

• Dorsal Column-Medial Lemniscus Pathway (DCML)
• Function: The DCML pathway transmits information related to fine touch, vibration, and proprioception.
• Pathway:
  • First-order neurons: Sensory information is received from peripheral receptors and enters the spinal cord via the dorsal root ganglion.
  • Dorsal columns: The first-order neurons ascend ipsilaterally in the dorsal columns, either through the fasciculus gracilis (lower body) or fasciculus cuneatus (upper body).
  • Medulla: First-order neurons synapse in the nucleus gracilis or nucleus cuneatus. Second-order neurons decussate (cross over) in the medulla and ascend as the medial lemniscus.
  • Thalamus: The second-order neurons synapse in the ventral posterolateral nucleus (VPL) of the thalamus. Third-order neurons then project to the primary somatosensory cortex in the postcentral gyrus.
• Spinothalamic Tract (Anterolateral System)
• Function: The spinothalamic tract carries sensory information related to pain, temperature, and crude touch.
• Pathway:
  • First-order neurons: Sensory input is received by peripheral receptors and enters the spinal cord via the dorsal root ganglion.
  • Spinal cord: First-order neurons synapse on second-order neurons in the dorsal horn. These second-order neurons decussate (cross) at the level of the spinal cord and ascend contralaterally in the anterolateral columns.
  • Brainstem and Thalamus: The second-order neurons ascend through the brainstem and synapse in the VPL nucleus of the thalamus. Third-order neurons project to the primary somatosensory cortex.
• Spinocerebellar Tracts
• Function: The spinocerebellar tracts convey proprioceptive information from muscles and joints to the cerebellum for coordination of movement.
• Pathway:
  • First-order neurons: Proprioceptive information is received by peripheral receptors and enters the spinal cord via the dorsal root ganglion.
  • Spinal cord: First-order neurons synapse on second-order neurons in the dorsal horn.
  • Two Main Pathways:
    • Dorsal Spinocerebellar Tract: This tract carries proprioceptive information from the lower body. It ascends ipsilaterally in the spinal cord and projects to the cerebellum via the inferior cerebellar peduncle.
    • Ventral Spinocerebellar Tract: This tract carries proprioceptive information from the lower limbs. It decussates twice (once in the spinal cord and once in the cerebellum), ultimately entering the cerebellum via the superior cerebellar peduncle.