Most people experience mild, involuntary muscle twitches throughout their lifetime which are referred to as fasciculations. This page will review the research and theories about the neuromuscular structures involved with and causes of fasciculations.

A Motor Unit (1)

Functional Anatomy

Entire motor units are affected by fasciculations. A motor unit consists of an alpha motor neuron (lower motor neuron) and the corresponding muscle fibers the neuron innervates. Each motor neuron innervates multiple muscle fibers which are widely distributed within a single muscle.
The alpha motor neuron receives an electrical signal from an upper motor neuron or an interneuron within the spinal cord. If the signal is strong enough, the alpha motor neuron will generate its own action potential down the axon. The motor neuron begins in the anterior horn of the spinal cord, exits through the anterior root, and travels to the muscle fibers where it synapses at the neuromuscular junction. When the action potential reaches the neuromuscular junction, the electrical signal causes the muscle fiber to contract.2

Cause of Fasciculations

Fasciculations are involuntary muscle twitches which cause visible contractions of muscle fibers. For such a twitch to occur, an action potential must be produced down the axon of the alpha motor neuron. Fasciculations may occur regardless of age or gender. Much research has been done to identify the causes behind both benign and pathological fasciculations, yet benign origins remain undetermined even though the only possible locations for dysfunction are the muscle, the motor neuron or the neuromuscular junction.3

(Fasciculation from Neural Damage4)


  1. Benign fasciculation syndrome is the condition of having twitching occasionally or nearly continuously. The fasciculations are visible as twitches on the surface of the skin. The origin of the twitches is unknown.3
  2. Muscular exhaustion or overexertion can cause diffuse fasciculation without any atrophy. This is not clinically significant.5
  3. Metabolic:
    1. Deficiency of magnesium.6
    2. Deficiency of sodium chloride.7
  4. Fatigue, anxiety, and stress can increase the presence of fasciculations.8


  1. Amyotrophic lateral sclerosis (Lou Gehrig’s disease) is a disease characterized by upper and lower motor neuron degeneration. Symptoms: weakness and muscle atrophy are usually the first symptoms but may also include fasciculation, cramping, stiffness; weakness affecting an arm or leg; and difficult or nasal speaking.9
  2. Neuromyotonia (Isaac’s syndrome) is also called peripheral nerve hyperexcitability, cramp-fasciculation syndrome, myokymia and is suspected to be the result of an increased presence of voltage-gated potassium channel antibodies. Symptoms: muscle hyperactivity, stiffness, cramps, quivering muscles, excessive sweating, fasciculations, fatigue, exercise intolerance and other similar symptoms.10,11
  3. Organophosphate poisoning is a condition where organophosphates (found in some pesticides and herbicides) inhibit acetylcholinesterase so that acetylcholine accumulates in the body. Symptoms: muscle weakness, fatigue, muscle cramps, fasciculation and paralysis.12
  4. Spino muscular atrophy is a condition in which a genetic defect prohibits the production of a protein needed for the survival of motor neurons. Symptoms: areflexia, muscle weakness, fasciculations, arthrogryposis (multiple congenital contractures), and weight lower than normal.13
  5. Spinal and bulbar muscular atrophy (Kennedy disease) is a neurodegenerative disease originating from a recessive genetic defect. Muscular symptoms include fasciculations, cramps and muscle atrophy.13
  6. Metabolic: due to tetany (from calcium deficiency) or uremia (buildup of urea and nitrogenous waste in the blood stream which is normally eliminated by the kidneys).15


Many people, when discovering they have a fasciculation, may begin to search the latter list to determine if they have a serious disease. However, fasciculations are usually benign and should only be examined by a medical professional if other symptoms are present, especially muscle weakness. Muscle weakness is evidence that the neuron may be losing its control over the muscle and a more serious condition could exist. Benign fasciculations are more common and may merely be a sign of stress or the body's need of more nutrients.


-Areflexia: absence of reflexes.
-Atrophy: partial or complete wasting away of a part of the body.
-Clinical significance: being of a magnitude that conveys practical relevance.
-Contractures: a permanent shortening of a muscle or a joint.
-Fasciculation: involuntary muscle twitches which cause visible contractions of muscle fibers
-Myokymia: involuntary, spontaneous quivering of a group of muscles.
-Tetany: a muscle spasm, cramp or twitching.
-Urea: a material that aids the kidneys in eliminating nitrogenous waste.

Relevant Links

-Video of fasciculations in the bicep of a healthy 23-year-old man.

-Benign Fasciculation Syndrome

-Disorders of the Nervous System


1. True or False: Fasciculations are more prevalent in older adults.
2. Which of the following is not a part of a motor unit:
a. Muscle fiber
b. Alpha motor neuron
c. Gamma motor neuron
d. Neuromuscular junction

3. What are the electrical signals in neurons called?a. Voltagesb. Action potentialsc. Charges
4. True or False: magnesium deficiency causes tetany.
5. The alpha motor neuron originates in which part of the spinal cord?a. Anteriorb. Lateralc. Dorsald. Medial
Short Answer:6. Describe the path of the electrical signal from the upper motor neuron to the muscle.
7. What is thought to cause neuromyotonia?
8. If you see one of your leg muscles start to twitch, should you be worried? Why or why not?


  1. Saladin K. Anatomy & physiology : the unity of form and function. Dubuque: McGraw-Hill, 2010.
  2. Knierim J. (2012). Neuroscience Online: Motor Units and Muscle Receptors [Online]. University of Texas Health Science Center at Houston. ( [20 Dec. 2012].
  3. Greg D. (2011). Benign Fasciculation Syndrome (BFS) Constant Muscle Twitching [Online]. ( [20 Dec. 2012].
  4. Knierim J. (2012). Neuroscience Online: Disorders of the Motor System (Online). University of Texas Health Science Center at Houston. ( [20 Dec. 2012].
  5. Nielsen JM and Marvin SL. Syndrome of muscular fasciculation and atrophy. Bull. Los Angeles Neurol. Soc. 23: 51-64, 1958.
  6. Flink LM. (1957) Introduction to symposium on amyotrophic lateral sclerosis. Paper presented at the Pro. Staff Meeting of Mayo Clinic.
  7. Denny-Brown D, Pennybacker JB. Brain: Fibrillation and fasciculation in voluntary muscle. 61: 311-334. Print.
  8. Weisberg LA, Garcia C, Strub R. Essentials of Clinical Neurology: Abnormal Involuntary Movement Disorders (Dyskinesias) [Online]. Tulane University 20.
  9. Amyotrophic Lateral Sclerosis. (2012) [Online]. Wikipedia. ( [20 Dec. 2012].
  10. Hart IK, et al. (2002) Brain: Phenotypic Variants of Autoimmune Peripheral Nerve Hyperexcitability. 1887-95. Print.
  11. Neuromyotonia. (2012) [Online]. Wikipedia. ( [20 Dec. 2012].
  12. Organophosphate Poisoning. (2012) [Online]. Wikipedia. ( [20 Dec. 2012].
  13. Spinomuscular Atrophy. (2012) [Online]. Wikipedia. ( [20 Dec. 2012].
  14. Spinal and Bulbar Muscular Atrophy. (2012) [Online]. Wikipedia. ( [20 Dec. 2012].
  15. Eaton LM, Lambert EH. Electromyography and electric stimulation of nerves in disease of the motor unit. J.A.M.A. 163: 1117-1124.

Answers to Quiz:

1. False. Fasciculations can occur at any age.2. C. Gamma motor neurons are not a part of the motor unit.3. B. Action potentials are the name of the electrical signals that travel in a neuron.4. False. Calcium deficiency causes tetany.5. A. The alpha motor neuron originates in the anterior horn of the spinal cord.6. An action potential travels down the upper motor neuron via the spinal cord and synapses with the lower motor neuron. An action potential then travels down the axon of the alpha motor neuron (the lower motor neuron) and then reaches the muscle fiber at the neuromuscular junction causing the muscle to contract.7. Neuromyotonia is thought to be caused by an over-accumulation voltage-gated potassium channel antibodies.8. You should not be worried because it is common to have a leg twitch and is not necessarily indicative of the presence of a disease.