UMN

Upper Motor Neurons (UMN)

  • Definition: Upper Motor Neurons (UMNs) are efferent pathways originating in the Central Nervous System (CNS) that synapse on Lower Motor Neurons (LMNs) to control muscles responsible for movements, posture, balance, and muscle tone.

  • Components of UMN:

    • UMNs consist of two primary systems:

    • Pyramidal System: Originates from the cortex.

    • Extrapyramidal System: Originates from the brainstem.

Pathways Involving UMNs

  • UMN Origin:

    • Cortex / Brainstem

    • Synapses on:

      • Cranial Nerve Nuclei: Control head, face, and neck movements.

      • Spinal Nerves / Ventral Horn: Control limb movements.

    • Specific Synapses:

    • Cranial Nerve Synapses: All cranial nerves except I (Olfactory), II (Optic), and VIII (Vestibulocochlear).

    • Regions controlled:

      • Head / Neck: C1-C8

      • Forelimb: C6-T2

      • Hindlimbs: L4-L6 (Patellar reflex) / L6-S1 (Withdrawal reflex).

Detailed Components of UMN

Pyramidal System

  • Composition:

    • Corticobulbar Tract: Synapses on cranial nerves; controls head, face, and neck movements.

    • Corticospinal Tract: Synapses on spinal cord; controls limb movements.

  • Function:

    • Responsible for voluntary movements.

Extrapyramidal System

  • Components:

    • Vestibulospinal Tract: Involved in posture stabilization.

    • Reticulospinal Tract: Influences involuntary movements and muscle tone.

  • Function:

  • Responsible for involuntary movements, related to posture, tone, and reflex responses.

Cerebral Motor Cortex

  • Regions and Function:

    1. Postcruciate Gyrus:

    • A. Controls pelvic limb movements.

    • B. Controls thoracic limb movements.

    1. Rostral Suprasylvian Gyrus:

    • C. Controls ear movement.

    • D. Controls eyelid movement.

    • E. Controls masseter and temporal muscles.

    • F. Controls lateral cervical muscles.

UMN Communication with LMN

  • Mechanisms:

    • UMNs communicate with LMNs using two types of proprioceptive receptors embedded in striated muscles:

    • Neuromuscular Spindles (NMS)

    • Golgi Tendon Organs (GTO)

Neuromuscular Spindles (NMS)

  • Structures:

    • Extrafusal Fibers: Main fibers controlling muscle contraction.

    • Intrafusal Fibers: Contained within the spindle, these provide sensory feedback on muscle stretch.

  • Types of Afferent and Efferent Fibers:

    • Afferent Fibers:

    • Type Ia: Respond to muscle stretch.

    • Type II: Respond to muscle position and movements.

    • Efferent Fibers:

    • Gamma (γ) motor neurons: Control intrafusal fibers of the muscle spindle.

    • Alpha (α) motor neurons: Control extrafusal fibers, generating movement.

  • Intrafusal Fibers:

    • Types:

    • Nuclear Bag Fiber: Sensitive to muscle length changes and speed.

    • Nuclear Chain Fiber: Sensitive only to muscle length.

Reflexes and Muscle Activation

Examples of Reflexes

  • Patellar Reflex:

    • Tapping on the patellar tendon stretches the muscle, activating Type Ia and II sensory neurons.

    • This reflex is monosynaptic as it synapses directly to an α motor neuron, causing muscle contraction, while the opposite side reacts via a polysynaptic pathway.

Golgi Tendon Organs (GTO)

  • Location: Found in tendons, detecting tension generated by muscle contractions.

  • Function:

    • Measure the degree of tension a muscle generates in 3D space, using Type Ib afferent fibers to convey information to the spinal cord.

  • Mechanism:

    • When muscle contracts, the tendon pulls, activating GTO and Type Ib fibers, sending signals to spinal cord. These fibers synapse on either excitatory or inhibitory interneurons, influencing α motor neurons to contract or relax the muscle accordingly.

Gait and Posture

Gait Generation

  • Phases of Gait:

    • Postural Phase: Involves extensors to maintain stability.

    • Protraction Phase: Flexors are activated to swing the limb forward.

Requirements for Gait

  • Postural Phase:

    • Controlled primarily by the reticulospinal and vestibulospinal tracts (extensors).

  • Protraction Phase:

    • Controlled by the rubrospinal tract and corticospinal tract (flexors) and the reticulospinal and vestibulospinal tracts (extensors).

UMN vs. LMN Signs

  • UMN Signs:

    • Spasticity

    • Hyperreflexia

    • Positive Babinski sign (normal plant of the foot flexing vs. an extensor response in humans).

  • LMN Signs:

    • Flaccid paralysis

    • Hypotonia

    • Hyporeflexia

    • Muscle atrophy

    • Fasciculations (twitching of muscles).

Common Cases of UMN and LMN Diseases

  • UMN Diseases:

    • Intervertebral Disk Disease

    • Wobblers Syndrome / Equine Protozoal Myeloencephalitis (EPM)

    • Symptoms: Can't walk but have reflexes present.

  • LMN Diseases:

    • Tick Paralysis

    • Botulism

    • Myasthenia Gravis

    • Equine Motor Neuron Disease

    • Symptoms: Can walk but lack reflexes.