Study Notes on Fasciculations and Motor Neuron Lesions

Fasciculations and Fibrillations in Motor Neuron Lesions

Definition of Fasciculations

  • Fasciculation is defined as an involuntary, pathological muscle contraction.
    • It can be likened to muscle twitches, which are generally benign.
    • Common areas where fasciculations can be observed include:
    • The eyelid
    • The tongue, which is served by the hypoglossal nerve.

Mechanism of Fasciculations

  • Fasciculations can only be seen with lower motor neuron lesions.
  • Lower Motor Neurons (LMN) are neurons that originate in the anterior gray horn of the spinal cord.
  • Key points regarding lower motor neurons:
    • Damage may occur to the cell body, axon, or axon terminal of the LMN.
    • This damage leads to a decrease in the release of acetylcholine (ACH).

Acetylcholine Action

  • Acetylcholine plays a crucial role in muscle contraction:
    • ACH binds to nicotinic receptors (ligand-gated ion channels) on the muscle cell membrane.
    • The binding of ACH opens the ion channels, allowing sodium ions (Na+Na^+) to flow into the cell, resulting in:
    • Increased positive charge within the cell
    • Generation of an action potential leading to muscle contraction.
Consequences of Decreased Acetylcholine
  • Damage to the LMN results in:
    • Decreased ACH levels.
    • Less interaction with acetylcholine receptors, leading to:
    • Decreased muscle stimulation.
    • This lack of stimulation triggers a compensatory response by the muscle to synthesize more acetylcholine receptors, a process termed upregulation.

Upregulation of Receptors

  • In response to decreased ACH:
    • The muscle cell increases the number of acetylcholine receptors to enhance sensitivity to any available ACH.
    • This causes the receptors to become mechanically sensitive, leading to:
    • Increased vulnerability to mechanical stimuli (e.g., light tapping), which can activate these channels.
    • This results in sodium flow into the cell, moving the resting membrane potential closer to the threshold potential, thus triggering more frequent action potentials, which causes:
      • Pathological muscle contractions called fasciculations.

Summary of Fasciculations

  • The pathological process can be summarized as follows:
    • Damage occurs to LMN --> Decreased ACH release --> Reduced receptor stimulation --> Upregulation of receptors --> Increased mechanical sensitivity --> Pathological contractions (fasciculations).

Distinction between Fasciculations and Fibrillations

  • Fibrillations:
    • Similar to fasciculations but are detected through electromyography (EMG).
    • Represented as fasciculations on the EMG output.
    • Key differences:
    • Fasciculations are visible muscular contractions, while fibrillations reflect electrical activity of the muscle that is not necessarily visible externally.

Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN) Lesions

  • Understanding the differences in clinical presentation between UMN and LMN lesions is vital:
    • Tone:
    • UMN lesions typically present with increased muscle tone (spasticity).
    • LMN lesions often result in decreased muscle tone (flaccidity).
    • Deep Tendon Reflexes:
    • Reflexes are exaggerated in UMN lesions (hyperreflexia).
    • Reflexes are diminished or absent in LMN lesions (hyporeflexia).
    • Strength:
    • UMN lesions may lead to weakness or paralysis but with sparing of certain muscle groups.
    • LMN lesions result in significant muscle weakness or paralysis affecting specific muscles innervated by the damaged neurons.