Spinal cord II: ascending and descending pathways

Spinal Cord II: Descending Fiber Pathways

Introduction

• Lecturer: Prof. S.O. Chan

• School: School of Biomedical Sciences

• Sources:

  • Clinical Neuroanatomy and Neuroscience by FitzGerald, Gruener, and Mtui, 6th edition, Elsevier/Saunders

  • Barr’s The Human Nervous System by Kiernan, 7th edition, Lippincott-Raven

  • Clinical Neuroanatomy by Waxman, 27th edition, Lange

Major Functions of the Brain

• Convey Sensory Signals: Collects/input different sensory signals from the periphery.

• Analyze and Integrate: Analyzes and integrates new information with stored information in the brain.

• Perform Motor Responses: Executes corresponding motor responses through the descending fiber tracts.

Anterior/Ventral Gray Horn

• Structure: Each column of motor neurons supplies a group of muscles with similar functions.

• Layers:

  • Layer VIII and IX consist of flexors and extensors organized in a specific manner based on their target structures and functional roles.

  • Specific Muscle Groups:

  • Flexors and Extensors:

    • Flexors of hand/foot, forearm/leg, arm/thigh, trunk

    • Extensors of hand/foot, forearm/leg, arm/thigh, trunk

• Somatotopic Organization: Illustrated in Figure 13.1, shows how different muscles are mapped to specific columns in the anterior gray horn.

Descending Spinal Tracts

• Key Descending Tracts:

  • Corticospinal (pyramidal) tract

  • Extrapyramidal System

  • Reticulospinal tract

  • Vestibulospinal tracts

  • Tectospinal tract

  • Raphespinal tract; aminergic

  • Autonomic system: necessary for general movements but not delicate movements.

Corticospinal Tract - Origin

• Composition:

  • About 40% of the fibers originate from the primary motor cortex.

  • Contributions also come from supplementary and premotor areas.

  • Additional input from the sensory cortex and superior parietal lobule to dorsal column nuclei and spinal cord dorsal horn, influencing pain transmission.

• Functional Integration: Link between the frontal lobe and limbic system suggesting emotional implications; limiting excitation can prevent constant stress in patients.

Corticospinal/Pyramidal Tract Overview

• Functions: Controls voluntary motor functions.

• Termination:

  • Terminates in cranial nerve motor nuclei in the brainstem via corticonuclear or corticobulbar tracts.

  • Also terminates in motor neurons in the ventral horn of spinal cord.

• Decussation:

  • Approximately 80% of fibers decussate at the medulla-spinal cord junction (known as pyramidal decussation), descending to form the lateral corticospinal tract.

• Neurotransmitter:

  • Utilizes glutaminergic, excitatory action as its neurotransmitter.

Reflex and Motor Control Mechanisms

• Reflex Arc:

  • Involves muscle spindles, Ia afferents sending sensory signals to motor neurons, influencing muscle movement.

• Feedback Loop:

  • Inhibitory control maintains muscle readiness when resisted, aiding muscle tone; can inhibit reflexive muscle contractions.

Upper and Lower Motor Neurons

• Definitions:

  • Upper Motor Neurons: Corticospinal or corticonuclear neurons and their fibers.

  • Lower Motor Neurons: Motor neurons in brainstem and spinal cord ventral horn, along with their axons.

• Descendence: Contains fibers extending from upper motor neurons that innervate spinal interneurons to excite/inhibit motoneurons.

• Reflex Responses: Disruption of upper motor neuron function can lead to abnormal reflex inputs, resulting in increased reflexes and muscle contractions.

Upper Motor Neuron Lesions

• Clinical Findings:

  • Presentation includes spasticity in left hand and wrist, spastic gait in stroke patients.

  • All upper limb muscles contract.

Effects of Upper Motor Neuron Lesions

• Symptoms:

  • Clonus, hyper-reflexia, and positive Babinski's reflex.

Extrapyramidal System

Reticulospinal Tract

• Function: Involved in locomotion and posture control.

• Origin: Arises from reticular formation in pons and medulla; has bilateral effects.

• Mechanism: Acts via interneurons upon motor neurons supplying trunk and proximal limb muscles, facilitating routine locomotion controls such as walking/running.

Raphespinal Tract

• Function: Modulates pain transmission in the spinal cord's dorsal horn, along with influences from higher centers via serotonergic synapses.

Vestibulospinal Tract

• Origin: Arises from vestibular nuclei in the brainstem.

• Function: Keeps the center of gravity between the feet by innervating motor neurons that control antigravity muscles.

Additional Descending Tracts

Tectospinal Tract

• Origin: Arises from the tectum of the midbrain; crosses over and descends to the ventral gray horn of cervical and upper thoracic levels.

• Function: Helps orient the head toward visual or auditory stimuli.

Central Autonomic Pathways

• Origin: Arises from the hypothalamus and brainstem.

• Termination: Reaches preganglionic sympathetic neurons in thoracic segments and parasympathetic neurons at sacral levels.

Summary of Descending Pathways

• Fiber Crossing:

  • Various pathways exhibit differing degrees of crossing at cervical and upper thoracic levels; most important are noted.

• Unique Pathways: Includes raphespinal, lateral corticospinal, medullary reticulospinal, pontine reticulospinal, vestibulospinal, tectospinal, anterior corticospinal pathways, highlighting their distinctive roles.

Spinal Cord Lesions

• Types of Lesions:

  • Small central lesions

  • Large central lesions

  • Dorsal column lesions

  • Irregular lesions

  • Complete hemisections

  • Dorsal root tumors

  • Compression by extramedullary mass

Brown-Sequard Syndrome

• Characteristics:

  • Ipsilateral lower motor neuron paralysis at the lesion segment.

  • Ipsilateral upper motor neuron paralysis below the lesion level.

  • Ipsilateral cutaneous anesthesia at the lesion segment.

  • Ipsilateral loss of proprioceptive, vibratory, and two-point discrimination senses below the lesion.

  • Contralateral loss of pain and temperature senses below the lesion.