Pyramidal & Descending Tracts

Overview: The motor system is divided into two main categories based on control and execution of voluntary movements. - Pyramidal Tract: Considered the CEO, it issues direct executive commands for voluntary movement (e.g., "flex index finger"). It leads to movements that can be jerky if executed in isolation. - Extrapyramidal System: Functions like a project manager, handling logistics without issuing commands. Its role is to smooth out motions and support coordinated movements.

The longest and largest descending fiber tract in the human central nervous system (CNS).
Involved in voluntary, discrete, skilled movements, controlling muscles for speech.
Named for its fibers that pass through the medullary pyramids.

The pyramidal tract fibers arise from different regions of the brain: - Primary Motor Cortex (Brodmann Area 4): Approximately 1/3 of axons arise here. - Primary Sensory Cortex (Brodmann Areas 3, 1, 2): Accounts for another 1/3. - Secondary Motor Cortex (Brodmann Area 6): This area anterior to the precentral gyrus contributes 1/3 as well. - Frontal Eye Fields (Brodmann Area 8): These areas also contribute axons to the pyramidal tract related to eye movements.
Functional blood supply: - Medial surface: Anterior Cerebral Artery (ACA). - Lateral surface: Middle Cerebral Artery (MCA).

A somatotopic representation of the body on the Primary Motor and Premotor Cortex (Brodmann Areas 4 & 6). - The medial surface represents lower extremities; the lateral surface represents face/lips. - Notably larger representations for body areas involved in skilled movements due to higher neuron density.

Area surrounding the central sulcus representing the lower limbs and comprising various Brodmann Areas (BA 4, 3, 1, 2, 5).
Clinical Correlation: - MCA vs. ACA Occlusion: - MCA: Weakness in face/upper extremity over lower extremity. - ACA: Weakness in lower extremity over upper extremity.

Located in the medial portion of BA 6, anterior to the paracentral lobule.
Integrates movements performed simultaneously on both sides of the body (e.g., bilateral tasks).

1st Order Neuron: Starts in the cortex; long axons descend to the spinal cord.
2nd Order Neuron (Internuncial Neuron): Located in the anterior gray column of the spinal cord, integrates commands.
3rd Order Neuron (Lower Motor Neuron, LMN): Innervates skeletal muscles through the anterior root of spinal nerves.

A reflex is an involuntary response to a stimulus important for maintaining muscle tone and posture without requiring higher center processing.
Components include: - Receptor Organ - Afferent Neuron - Efferent Neuron - Effector Organ
Example: Knee Jerk/Patellar Tendon Reflex.

The corticospinal tract pathway begins in the cerebral cortex where 1st order neurons originate.
Pathway details: - Ascends through the corona radiata and internal capsule, then descends through the brainstem. - Fibers form the pyramids in the upper medulla, crossing over at the pyramidal decussation. - Forms lateral and anterior corticospinal tracts based on whether they decussate.

Major pathways through which motor commands flow: - Lateral Corticospinal Tract: ~85-90% of fibers; controls distal limbs. - Anterior/Corticospinal Tract: ~10-15% of fibers; controls proximal muscles.
Key Observations: Damage to corticospinal tracts leads to inability to perform complex finger movements while basic walking ability may remain intact.

The activation tends to initiate impulses to skeletal muscles for motor control.
UMN lesions lead to specific clinical signs: - Increased muscle tone and hyperreflexia. - LMN lesions show flaccidity and reduced or absent reflexes.

Responsible for voluntary control of face, jaw, and speech functions.
Cranial nerves innervated include CN III, IV, V, VII, IX, X, XI, and XII, with bilateral projections from cortex to facial nucleus, except for CN VII and XII, responsible for controlling specific facial movements.

Include tracts like the tectospinal tract which mediates visual and auditory reflexes and the rubrospinal tract that influences flexor muscle tone.

Tracks involved in maintaining posture and balance by influencing extensor muscles and reflexive actions.

Results in ipsilateral loss of motor function and contralateral pain sensation, based on decussation points of various sensory pathways.

Results in specific cranial nerve deficits with motor deficits contralateral to the lesion.

Produces dense contralateral motor weakness across all extremities and facial regions.

Leads to abnormal mental status exams with norms intact for cranial nerves unless the lesion is particularly large.

Review questions cover key attributes of the pyramidal tract, locations and impacts of lesions, and anatomical functions related to motor control.

A thorough list of literature used for lecture material and studies relevant to the anatomy and functions of the pyramidal and descending motor tracts.