Lower Motor Neurons: Flaccid Paralysis Pt. 1

Lower Motor Neurons

Overview of the Motor System

• Definition: The motor system is an integrated action of neurons and pathways that allow normal movements to occur.

• Subgroups of the Motor System:

  • Lower Motor Neurons (LMN)

  • Pyramidal System

  • Basal Ganglia

  • Cerebellar System

  • Brainstem Motor Centers

How Do We Move?

1. Initiation of Movement:

   • Voluntary Process: Motor movement is considered a voluntary act.

   • Origin of Idea/Desire: This desire to move originates in the association area of the cerebral cortex.

2. Processing of Movement:

   • Basal Ganglia: Responsible for the initiation of movement and postural adjustments.

   • Cerebellum: Controls the programming for the coordination of movement.

3. Pathway of Impulses:

   • The impulses are sent from the Basal Ganglia and Cerebellum to the motor and premotor areas of the cerebral cortex.

   • The impulses are then carried by the Pyramidal System to the Lower Motor Neurons (LMN).

4. Execution of Movement:

   • Lower Motor Neurons carry the motor commands to the muscles, resulting in movement.

Detailed Structure of Motor System Interconnections

• Association Areas: Desire to move initiated here.

• Cerebral Cortex:

  • Premotor Area: Involved in programming for movement;

  • Motor Area: Sends commands for movement.

• Basal Ganglia: Involved in initiation and postural adjustments.

• Cerebellum:

  • Posterior Lobe: Plays a role in coordination.

  • Anterior Lobe: Involved in equilibrium via vestibular receptors.

  • Flocculonodular Lobe: Affects balance and eye movements.

• Brainstem Centers: Responsible for supraspinal reflex activity.

• Lower Motor Neurons: Execute the commands.

• Intrafusal Stretch Receptors: Provide feedback about muscle stretch.

• Extrafusal Contractile Fibers: The fibers directly responsible for muscle contraction.

Feedback Mechanism During Movement

• Muscle Feedback: Muscles send feedback information to the cerebellum.

• Feedback System: This system fine-tunes movements based on current performance (e.g., did we step far enough?).

• Efficiency in Movement: The feedback allows for adjustments that improve efficiency during current and future motor planning.

Motor Units

Definition and Components

• Lower Motor Neurons (LMNs):

  • Also known as Alpha Motor Neurons.

  • Serve as the direct connection between the Central Nervous System (CNS) and skeletal muscle.

  • Feature large myelinated axons.

  • Axons synapse at motor end plates (the neuromuscular junction).

• Acetylcholine: The primary neurotransmitter at the neuromuscular junction.

• Motor Unit Consists of:

  1. Alpha motor neuron

  2. Its axon

  3. The extrafusal muscle fiber it innervates.

Functions of Motor Units

• Fiber Innervation:

  • The number of muscle fibers a motor unit innervates correlates with functionality.

  • For fine motor actions (e.g., eye movements), a motor unit could consist of 1 axon with fewer than 12 muscle fibers.

  • For gross motor actions (e.g., lifting heavy objects), a motor unit could consist of 1 axon innervating thousands of muscle fibers.

Alpha Motor Unit

• Components: Includes the motor end plate, cell body of the lower or alpha motor neuron, and axon.

• Visual Reference: Figure 5-2 illustrates the components of the alpha motor unit, including the interruption indicating the myelinated length of the axon.

Gamma Motor Neurons

• Innervation: Directly innervate intrafusal fibers of muscle spindles.

• Muscle Spindles: Sensory organs responsive to muscle stretching.

• Function: Activating the Gamma Motor Neurons increases tension on muscle spindle receptors, which play a crucial role in muscular tone.

Brainstem Lower Motor Neurons

• Cranial Nerves: Most cranial nerves contain axons of LMNs, with cell bodies grouped in paired nuclei at various brainstem levels.

• Exceptions: Olfactory, Optic, and Vestibulocochlear nerves do not follow this pattern.

Cranial Nerves with Lower Motor Neurons

• Cranial Nerves Involved: CN III, IV, V, VI, VII, IX, X, XI, XII

Oculomotor Nucleus and CN III

• Location: The Oculomotor Nucleus is the “V” shaped ventral part of the periaqueductal gray of the midbrain, found at the level of the superior colliculus.

• Function: Innervates five eye muscles:

  1. Superior rectus

  2. Medial rectus

  3. Inferior rectus

  4. Inferior oblique

  5. Superior levator (raises the eyelid).

Lesions of CN III

• Symptoms Presented in Patients:

  • Ipsilateral ophthalmoplegia (eye turns down and out).

  • Ipsilateral ptosis (sagging/drooping eyelid).

  • Ipsilateral mydriasis (dilated pupil).

  • Loss of accommodation for near vision.

Trochlear Nucleus and CN IV

• Location: Ventral border of the periaqueductal gray of the midbrain at the level of the inferior colliculus.

• Function: Innervates the superior oblique muscle of the eye.

• Actions:

  • Primarily: Intorsion (internal rotation)

  • Secondary: Depression

  • Tertiary: Lateral rotation of the eye.

• Mnemonic: “SO4” = Superior oblique (CN 4).

Lesions of CN IV

• Patient Presentation:

  • Extorsion (outward rotation of the eye).

  • Impaired depression of the eye.

  • The patient compensates by tilting the head slightly downward and to the opposite shoulder.

  • Symptoms vary based on the location of the lesion:

  • Trochlear nuclei damage: contralateral symptoms.

  • Trochlear nerve damage: ipsilateral symptoms due to the decussation.

Motor Trigeminal Nucleus and Motor CN V

• Location: Dorsolateral part of the tegmentum at the midpontine level.

• Function: Innervates muscles of mastication (chewing).

  • Muscles involved:

  1. Masseter

  2. Temporalis

  3. Medial pterygoid

  4. Lateral pterygoid

Lesions of CN V

• Symptoms:

  • Paralysis and muscular wasting of ipsilateral muscles of mastication.

  • Upon attempting to open their mouth, the jaw will deviate towards the ipsilateral side.

Abducens Nucleus and CN VI

• Location: Beneath the facial colliculus in the floor of the 4th ventricle, located in the caudal pons.

• Function: Innervates the lateral rectus muscle of the eye which is responsible for lateral eye movement.

Lesions of CN VI

• Symptoms:

  • Medial deviation of the eye (esotropia) due to the paralysis of abduction of the ipsilateral eye.

Facial Nucleus and Motor Root of CN VII

• Location: Lateral part of tegmentum within the caudal pons.

• Function: Innervates muscles of facial expression and the stapedius muscle of the inner ear.

Lesions of CN VII

• Symptoms:

  • Ipsilateral facial musculature paralysis affecting both upper and lower facial muscles.

  • Most common cause: Bell's Palsy (inflammatory reaction to CN VII).

  • Recovery typically within a few months.

  • Symptoms of Bell's Palsy include:

  • Unilateral facial muscle weakness.

  • Inability to close the eye.

  • Sagging or drooping of the affected side of the face.

Nucleus Ambiguus and Motor Roots of CN IX, X, XI

• Location: Elongated column of alpha motor neurons found in the ventrolateral reticular formation of the medulla.

• Function: Innervates muscles of the palate, pharynx, larynx, and upper esophagus; involved in swallowing and speech production.

Lesions of the Ambiguus Nucleus

• Consequences:

  • Lesion in the rostral part affects the glossopharyngeal nerve (CN IX), resulting in dysphagia (difficulty swallowing).

  • Lesion affects the vagus nerve (CN X) leading to:

  • Paralysis of vocal cords (hoarseness and vocal weakness).

  • Paralysis of palatal muscles, causing sagging of the ipsilateral palatal arch.

  • Deviation of the uvula towards the contralateral side.

  • Bilateral lesions can result in airway obstruction due to complete closure.

Hypoglossal Nucleus and CN XII

• Location: Elongated motor nucleus located in the floor of the 4th ventricle near the midline of the medulla.

• Function: Innervates ipsilateral muscles of the tongue.

Lesions of CN XII

• Patient Presentation:

  • Paralysis and atrophy of the ipsilateral muscle of the tongue.

  • Upon requesting to “stick out the tongue,” it will deviate towards the side of the lesion.