Lower Motor Neurons: Flaccid Paralysis Pt. 2

Lower Motor Neurons

Overview of Lower Motor Neurons

  • Flaccid Paralysis: A condition caused by damage to lower motor neurons that results in weakness or inability to move specific muscles.

Spinal Cord Lower Motor Neuron Organization

  • Location: The anterior/ventral horn of the spinal cord contains organized motor nuclei.

  • Proximal vs Distal Innervation:

    • Medial Nuclei: Innervate proximal muscles.

    • Lateral Nuclei: Innervate distal muscles.

  • Main Areas of Focus:

    • Cervical Enlargement

    • Lumbar Enlargement

Cervical Region

  • Muscle Innervation:

    • Medial Placement: Neurons that innervate shoulder muscles are located more medially.

    • Lateral Placement: Neurons that innervate hand muscles are found laterally.

  • Lesion Effects: Any neuron lesion in this area results in paralysis of the respective innervated muscle groups.

Lumbar Region

  • Comparison to Cervical Region:

    • Follows the same organizational concept (medial = proximal, lateral = distal).

    • Difference: Size of the neurons may vary.

  • Lesion Effects: Similar to the cervical region, a lesion results in paralysis of the innervated muscle groups.

Clinical Connections to Lower Motor Neurons

  • Spinal Accessory Nerve (CN XI):

    • Innervation: Supplies Sternocleidomastoid and trapezius muscles.

    • Lesion Consequences:

    • Weakness when turning the head to the opposite direction.

    • Weakness in shoulder shrugging on the same side (ipsilateral).

  • Phrenic Nerve:

    • Origin: Arises from spinal roots C3, C4, and C5.

    • Innervation: Supplies the diaphragm.

    • Effects of Lesion:

    • Ipsilateral Lesion: Paralysis of half the diaphragm.

    • Bilateral Lesion: Can lead to respiratory failure.

    • Mnemonic: “C3, C4, C5 keep the diaphragm alive.”

Myotomes Definition

  • Myotomes:

    • Defined as the muscles innervated by a single spinal cord segment.

    • Used clinically to assess the integrity of spinal nerve levels.

Myotome Chart

  • Nerve Level and Actions:

    • C4: Trapezius - Shoulder elevation.

    • C5: Deltoid - Shoulder abduction.

    • C6: Biceps - Elbow flexion.

    • C7: Triceps - Elbow extension.

    • C8: Flexor digitorum - Finger flexion.

    • L1/L2: Iliopsoas - Hip flexion.

    • L3/L4: Quadriceps - Knee extension.

    • L5: Extensor Hallucis - Great toe extension.

    • S1: Gastrocnemius - Ankle plantar flexion.

    • S3/S4: Rectal Sphincter - Bladder and rectum motor control.

Injury to Lower Motor Neurons

  • Consequences of Injury:

    • Results in flaccid paralysis as signals cannot reach muscles leading to muscle contraction.

    • Hypotonia: Low muscular tone may occur.

    • Decreased or Absent Reflexes: Reduced or lack of deep tendon reflexes (DTR).

    • Muscle Atrophy: If the injury persists, muscle wasting occurs.

  • Causes of LMN Syndrome:

    • May arise from lesions in either the Central Nervous System (CNS) or the Peripheral Nervous System (PNS).

Pathophysiology of the Motor Unit

  • Disorders Classification:

    • Myopathic Disorders: Characterized by muscle dysfunction.

    • Symptoms: Proximal limb weakness.

    • Neuropathic Disorders: Involve dysfunction of motor neurons or axons.

    • Symptoms: Distal limb weakness.

  • Motor Neuron Disease:

    • Symptoms include weakness, muscle atrophy, and fasciculations (involuntary muscle contractions occurring synchronously).

  • Demyelinating Neuropathies:

    • Examples: Guillain-Barré Syndrome (GBS) and Multiple Sclerosis (MS).

    • Reference: Review Chapter 1 for additional details.

Reflex Activities of Spinal Motor Neurons

  • Types of Reflexes:

    1. Myotatic Reflex

    2. Inverse Myotatic Reflex

    3. Gamma Loop Reflex

Myotatic Reflex (Stretch Reflex)

  • Mechanism:

    • Involves the annulospiral receptors of muscle spindles.

    • Defined as the contraction of a muscle in response to stretching.

    • Testing Method: Typically assessed with a reflex hammer at the tendon.

Commonly Tested Myotatic Reflexes

  • Muscle/Tendon, Nerve, and Spinal Segment Connection:

    • Biceps Tendon: Musculocutaneous nerve - Spinal Segment C6.

    • Triceps Tendon: Radial nerve - Spinal Segment C7.

    • Patellar Tendon: Femoral nerve - Spinal Segment L4.

    • Achilles Tendon: Tibial and Sciatic nerves - Spinal Segment S1.

Inverse Myotatic Reflex (Autogenic Inhibition Reflex)

  • Mechanism:

    • Involves Golgi Tendon Organs (GTO).

    • GTO detects changes in muscle tension and inhibits lower motor neurons, resulting in relaxation of the muscle.

    • Purpose: Protects tendons from injury (e.g., tearing).

Gamma Loop Reflex

  • Function:

    • Regulates the stretch and tautness of the annulospiral receptors located in muscle spindles.

  • Components of the Gamma Loop:

    • Gamma Motor Neuron: Exits the CNS, synapsing on gamma motor neurons.

    • Alpha Efferent Axon: Innervates extrafusal fibers, controlling muscle contraction.

    • Tendon: Connects muscle to bone.

    • Intrafusal Muscle Fiber: Part of the muscle spindle that senses stretch.

    • Annulospiral Receptor: Responsible for detecting stretch in the muscle.

    • Action: Excitation of gamma motor neurons causes contraction of intrafusal fibers, which results in stretch of the annulospiral receptors, thus regulating muscle spindle excitability.