descending

DESCENDING TRACTS

• Presented by: Dr. Omer Tagelsir Abdalla

General Concepts

• Motor Paths:
    - Descend from the brain to spinal cord.
    - Control voluntary movements and are at conscious control.
      - This is referred to as the pyramidal tract.
    - Descend from the cerebellum or brainstem to spinal cord.
    - Control involuntary movements and are at unconscious control.
      - This is also referred to as pyramidal tract.

Descending Tracts

• Definition:
    - Descending tracts consist of axons of cells that transmit efferent impulses.
    - Originates from:
      - Cortex
      - Brainstem
      - Spinal cord
      - Spinal nerve

• Types of Motor Neurons:
    - Upper Motor Neurons (UMNs)

Upper Motor Neurons

• Definition:
    - Nerve fibers that descend in the white matter from various supraspinal nerve centers.
    - They are segregated into nerve bundles, also called descending tracts.

• Characteristics:
    - Entirely located within the Central Nervous System (CNS).
    - Start in:
      - Cerebral Cortex
      - Cerebellum
    - End at spinal cord.

Lower Motor Neurons

• Definition:
    - Begin in the spinal cord, specifically from the anterior horns.
    - Composed of alpha motor neurons (A-α).
    - Form spinal and cranial nerves.
    - End at the junction with muscles.

Motor Pathway Overview

• Motor Cortex:
    - Upper Motor Neuron of the Brain ⟶
    - Lower Motor Neuron ⟶
    - Muscle

Pyramidal System

• Definition:
    - The pyramidal tracts derive their name from the medullary pyramids of the medulla oblongata through which they pass.
    - These pathways are responsible for the voluntary control of musculature.

• Functional Subdivision of Pyramidal Tracts:
    - Corticospinal Tracts:
      - Supply musculature of the body.
    - Corticobulbar Tracts:
      - Supply musculature of the head and neck.

• Pathway Distribution:
    - Corticospinal Tract:
      - Brain to spinal cord, supplying muscles of limbs and trunks through spinal nerves.
    - Corticobulbar Tract:
      - From brain to brainstem, supplying muscles of head and neck through cranial nerves.

Corticospinal and Corticobulbar Tracts Functions

• Corticospinal Tract Functions:
    - Controls primary motor activity for the somatic motor system from the neck to the feet.

• Corticobulbar Tract Functions:
    - Controls primary motor activity for the somatic motor system from the head.

Order of Neurons in Motor Pathways

• First Order Neuron:
    - From the cortex to anterior horn cells.

• Second Order Neuron:
    - Interneurons.

• Third Order Neuron:
    - From anterior horn cells to skeletal muscles.

Lesions in Corticospinal Tracts

• Characteristics of Lesions:
    - Reduced muscle tone
    - Weakness
    - Not complete paralysis (complete paralysis occurs when both pyramidal and extrapyramidal systems are involved).

Upper Motor Neuron Lesions (UMNL) vs. Lower Motor Neuron Lesions (LMNL)

Extrapyramidal System

• Definition:
    - Extrapyramidal tracts originate in the cerebellum and brainstem, carrying motor fibers to the spinal cord.

• Components:**
    - Cerebral Peduncle
    - Internal Capsule
    - Midbrain
    - Cerebellum
    - Pons
    - Red Nucleus and Rubrospinal Tract
    - Medulla Oblongata

• Function:
    - Responsible for involuntary control of musculature such as muscle tone, balance, and posture.

• Pyramidal vs. Extrapyramidal Tracts:
    - Pyramidal Tracts:
      - Lateral corticospinal tracts cross in pyramids of the medulla; voluntary control to limb muscles.
      - Ventral (anterior) corticospinal tracts cross at spinal cord; voluntary control to axial muscles.
    - Example of Extrapyramidal Tracts:
      - Rubrospinal Tract, which is an extrapyramidal tract.

Specific Extrapyramidal Tracts

• Rubrospinal Tract:
    - Origin: Red nucleus in the brainstem.

• Vestibulospinal Tract:
    - Originates in vestibular nuclei, primarily involved in maintenance of upright posture.

• Tectospinal Tract:
    - Originates from superior colliculus, controlling reflex movements of the head and neck.

• Reticulospinal Tract:
    - Involved in various autonomic functions and modulating motor activities.

Function and Dysfunction in Extrapyramidal System

• Normal Function:
    - Regulation and integration of voluntary motor activities.

• Dysfunction:
    - Involuntary motor activities, such as tremors.
    - Extrapyramidal system maintains muscle tone and regulates associated movements.
    - Symptoms of dysfunction:
      - Hypertonia and Rigidity
      - Bradykinesia
      - Mask-like facies
      - Infrequent blinking
      - Loss of swinging during walking

Clinical Correlation

• Damage to Corticospinal Tracts:
    - Commonly occurs due to cerebrovascular accidents (CVA) in the internal capsule.
    - Characteristics: Symptoms appear on the contralateral side of the body if there is a unilateral lesion.

• Cardinal Signs of Upper Motor Neuron Lesions:
    - Hypertonia (increased muscle tone)
    - Hyperreflexia (increased muscle reflexes)
    - Clonus (involuntary rhythmic contractions)
    - Babinski sign (extension of hallux in response to stimulation of foot sole)
    - Muscle weakness

• Damage to Extrapyramidal Tracts:
    - Commonly seen in degenerative diseases, encephalitis, and tumors.
    - Results in various types of dyskinesias, specifically incoordinate movements.