Describe the origin, course and function(s) of corticospinal tracts. Add a note on effect(s) of damage to these tracts and the resulting paralysis.

Origin of the Corticospinal Tracts

The corticospinal tracts originate from the motor cortex, specifically the precentral gyrus (Brodmann area 4). Neurons from this area give rise to the upper motor neurons that form the corticospinal tracts. Approximately 30-50% of these fibers originate from the leg area, 30% from the arm area, and the remaining 10-20% from the head and neck areas. These fibers initially descend through the internal capsule.

Course of the Corticospinal Tracts

The corticospinal tracts follow a distinct pathway:

• Internal Capsule: The fibers pass through the anterior limb of the internal capsule.
• Brainstem: As the tracts exit the internal capsule, they enter the brainstem. Here, a significant decussation (crossing over) occurs in the caudal medulla oblongata at the pyramidal decussation. Approximately 80-90% of the fibers cross to the contralateral side. The remaining 10-20% remain ipsilateral.
• Medulla Oblongata: After decussation, the tracts descend as the lateral corticospinal tract.
• Spinal Cord: The lateral corticospinal tract continues down the length of the spinal cord in the lateral funiculus. The anterior corticospinal tract (the uncrossed fibers) descends ipsilaterally in the anterior funiculus.
• Termination: The corticospinal tracts terminate on interneurons and lower motor neurons in the ventral horn of the spinal cord. These lower motor neurons then directly innervate skeletal muscles.

Types of Corticospinal Tracts

There are two main types of corticospinal tracts:

Tract	Decussation	Location in Spinal Cord	Function
Lateral Corticospinal Tract	Pyramidal Decussation (medulla)	Lateral Funiculus	Controls distal limb muscles for precise, skilled movements (e.g., hand, fingers)
Anterior Corticospinal Tract	May cross at spinal cord level or remain uncrossed	Anterior Funiculus	Controls axial muscles (trunk) and proximal limb muscles for gross movements (e.g., posture, gait)

Functions of the Corticospinal Tracts

The primary function of the corticospinal tracts is to mediate voluntary movement. Specifically, they are responsible for:

• Voluntary Movement: Initiating and controlling conscious movements.
• Fine Motor Control: Enabling precise and coordinated movements, particularly of the distal limbs.
• Posture and Balance: Contributing to the maintenance of posture and balance, especially through the anterior corticospinal tract.
• Reflex Modulation: Influencing spinal reflexes, allowing for voluntary control over reflexive actions.

Effects of Damage to the Corticospinal Tracts and Resulting Paralysis

Damage to the corticospinal tracts results in a characteristic pattern of weakness and paralysis known as upper motor neuron syndrome. The specific manifestations depend on the location and extent of the lesion:

• Complete Transection of the Spinal Cord: Results in complete paralysis and loss of sensation below the level of the lesion.
• Hemisection of the Spinal Cord (Brown-Sequard Syndrome): Causes ipsilateral weakness and spasticity, and contralateral loss of pain and temperature sensation below the level of the lesion.
• Cortical Lesions: Lead to contralateral hemiparesis (weakness on one side of the body) or hemiplegia (paralysis on one side of the body). The severity depends on the extent of the cortical damage.
• Internal Capsule Lesions: Produce contralateral hemiparesis/hemiplegia, often affecting the face, arm, and leg.
• Brainstem Lesions: Can cause a variety of motor deficits depending on the specific nuclei and tracts involved.

Key features of upper motor neuron lesions include spasticity (increased muscle tone), hyperreflexia (exaggerated reflexes), and clonus (rhythmic, involuntary muscle contractions). These are distinct from the flaccid paralysis, hypotonia, and areflexia seen in lower motor neuron lesions.