A muscle relaxant is a drug that affects skeletal muscle function and decreases the muscle tone.

It may be used to alleviate symptoms such as muscle spasms, pain, and hyperreflexia. The term “muscle relaxant” is used to refer to two major therapeutic groups: neuromuscular blockers and spasmolytics. Neuromuscular blockers act by interfering with transmission at the neuromuscular end plate and have no central nervous system (CNS) activity.

They are often used during surgical procedures and in intensive care and emergency medicine to cause temporary paralysis. Spasmolytics, also known as “centrally acting” muscle relaxants, are used to alleviate musculoskeletal pain and spasms and to reduce spasticity in a variety of neurological conditions. While both neuromuscular blockers and spasmolytics are often grouped together as muscle relaxants, the term is commonly used to refer to spasmolytics only.

How Skeletal Muscle Contracts

A single α-motor neuron can innervate up to 200 muscle fibers, forming a complex called motor unit (Figure 1).¹⁰ With movement, an action potential originates from the UMN in the motor cortex.⁹ This action potential depolarizes the motor neuron terminal, resulting in the opening of voltage-gated calcium (Ca²⁺) channels and the subsequent release of the neurotransmitter acetylcholine (Ach) into the synaptic cleft. In the synaptic cleft, Ach binds to nicotinic cholinergic receptors on the muscle fiber membrane, leading to an influx of sodium (Na⁺) and a discharge of potassium (K) across the muscle fiber’s membrane, which results in depolarization of the muscle fiber.¹¹ This depolarization opens voltage-gated Ca²⁺ channels on the sarcoplasmic reticulum (via ryanodine and inositol triphosphate receptors), allowing for Ca²⁺ influx into the cytoplasm of striated muscle cells.¹²  The Ca²⁺ then binds to troponin C, which exposes myosin-binding sites on actin filaments. A cross-link forms between actin and myosin, leading to muscle contraction. The pumping of Ca²⁺ back into the sarcoplasmic reticulum, using adenosine triphosphate, leads to cessation of contraction.

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Further information

Muscle Spasms

A muscle spasm is a sudden involuntary contraction of a muscle group that involves jerking and twitching. Unlike spasticity, which is a disorder of the CNS, muscle spasms arise from a variety of peripheral musculoskeletal conditions, such as mechanical low back pain. Common skeletal muscle conditions that cause spasms include fibromyalgia, myofascial pain syndrome, and mechanical low back or neck pain.

Antispasm Agents

Most of the agents discussed here are FDA approved for adjunctive use to treat muscle spasms and pain associated with acute musculoskeletal conditions (Table 4). Health data from 2003 to 2004 revealed that cyclobenzaprine (Amrix, Fexmid, others), carisoprodol (Soma, others), and metaxalone (Metaxall, Skelaxin, others) accounted for more than 45% of medications prescribed for acute musculoskeletal pain.

Due to CNS depression, cyclobenzaprine, metaxalone, orphenadrine (Norflex, others), methocarbamol (Robaxin, others), carisoprodol, and chlorzoxazone (Lorzone, Parafon Forte DSC, others) are on the American Geriatrics Society’s Beers List of inappropriate drugs for elderly patients.

Despite this, approximately 300,000 annual prescriptions for skeletal muscle relaxants (15%) are issued to patients older than 65 years of age.

Skeletal muscle relaxants represent a diverse pharmacotherapeutic group of medications across several chemical classes that are structurally dissimilar. These agents are effective for spasticity, skeletal muscle spasms, or both.

Because of the breadth of pharmacologic mechanisms and variable pharmacokinetics, the drugs have a huge range of AEs and potential drug interactions.

Considering that these agents are most often used in the elderly and also as adjuvants for the treatment of chronic pain patients with multiple comorbidities who are likely receiving a polypharmaceutical regimen (including opioids), skeletal muscle drug selection for each patient requires careful attention to these factors.