Pharmacology Chapter 25 Muscle Relaxants

Neuromuscular Abnormalities

All of the areas mentioned work together to allow for a free flow of impulses into and out of the CNS to coordinate posture, balance, and movement. When injuries, diseases, and toxins affect the normal flow of information into and out of the CNS motor pathways, many clinical signs and symptoms may develop, ranging from simple muscle spasms to spasticity—or sustained muscle spasm—and paralysis.

Nerves and Movement

Calcium in MusclesClick to Show Muscle Contraction animationClick to Show Posture, balance, and movement are the result of a constantly fluctuating sequence of muscle contraction and relaxation. The nerves that regulate these actions are the spinal motor neurons. These neurons are influenced by higher level brain activity in the lower areas of the brain: The cerebellum (associated with conscious muscle movements) and basal ganglia (associated with unconscious muscle movements). This brain activity provides coordination of contractions, and the cerebral cortex allows conscious thought to regulate movement.

Centrally Acting Skeletal Muscle Relaxants

Centrally acting skeletal muscle relaxants (Table 25.1) include baclofen (Lioresal), carisoprodol (Soma), chlorzoxazone (Parafon), cyclobenzaprine (Amrix), metaxalone (Skelaxin), methocarbamol (Robaxin), orphenadrine (Banflex, Flexon), and tizanidine (Zanaflex). Diazepam (Valium), a drug widely used as an anxiety agent (see Chapter 20), also has been shown to be an effective centrally acting skeletal muscle relaxant. It may be advantageous in situations in which anxiety may be precipitating the muscle spasm. Other measures in addition to these drugs should be used to alleviate muscle spasm and pain. Such modalities as rest of the affected muscle, heat applications to increase blood flow to the area to remove the pain-causing chemicals, physical therapy to return the muscle to normal tone and activity, and anti-inflammatory agents—including nonsteroidal anti-inflammatory drugs (NSAIDs)—if the underlying problem is related to injury or inflammation may help.

Brain Control

Many areas within the brain influence the spinal motor nerves. Areas of the brainstem, the basal ganglia, and the cerebellum modulate spinal motor nerve activity and help to coordinate activity among various muscle groups, thereby allowing coordinated movement and control of body muscle motions. Nerve areas within the cerebral cortex allow conscious, or intentional, movement. Nerves within the cortex send signals down the spinal cord, where they cross to the opposite side of the spinal cord before sending out nerve impulses to cause muscle contraction. In this way, each side of the cortex controls muscle movement on the opposite side of the body. Different fibers control different types of movements. Those fibers that control precise, intentional movement make up the pyramidal tract within the CNS. The extrapyramidal tract is composed of cells from the cerebral cortex, as well as those from several subcortical areas, including the basal ganglia and the cerebellum. This tract modulates or coordinates unconsciously controlled muscle activity, and it allows the body to make automatic adjustments in posture or position and balance. The extrapyramidal tract controls lower level, or crude, movements. Many are now not using the terms pyramidal and extrapyramidal tracts to describe movement since so many movements can't be clearly classified into one or the other tract.

Muscle Spasm

Muscle spasms often result from injury to the musculoskeletal system—for example, overstretching a muscle, wrenching a joint, or tearing a tendon or ligament. These injuries can cause violent and painful involuntary muscle contractions. It is thought that these spasms are caused by the flood of sensory impulses coming to the spinal cord from the injured area. These impulses can be passed through interneurons to spinal motor nerves, which stimulate an intense muscle contraction. The contraction cuts off blood flow to the muscle fibers in the injured area, causing lactic acid to accumulate and resulting in pain. The new flood of sensory impulses caused by the pain may lead to further muscle contraction, and a vicious cycle may develop (Figure 25.2).

Muscle Spasticity

Muscle spasticity is the result of damage to neurons within the CNS rather than injury to peripheral structures. Because the spasticity is caused by nerve damage in the CNS, it is a permanent condition. Spasticity may result from an increase in excitatory influences or a decrease in inhibitory influences within the CNS. The interruption in the balance among all of these higher influences within the CNS may lead to excessive stimulation of muscles, or hypertonia, in opposing muscle groups at the same time, a condition that may cause contractures and permanent structural changes. This control imbalance also results in a loss of coordinated muscle activity. For example, the signs and symptoms of cerebral palsy and paraplegia are related to the disruption in the nervous control of the muscles. The exact presentation of any chronic neurological disorder depends on the specific nerve centers and tracts that are damaged and how the control imbalance is manifested.

Spinal Reflexes

The spinal reflexes are the simplest nerve pathways that monitor movement and posture (Figure 25.1). Spinal reflexes can be simple, involving an incoming sensory neuron and an outgoing motor neuron, or more complex, involving interneurons that communicate with the related centers in the brain. Simple reflex arcs involve sensory receptors in the periphery and spinal motor nerves. Such reflex arcs make up what is known as the spindle gamma loop system; they respond to stretch receptors or spindles on muscle fibers to cause a muscle fiber contraction that relieves the stretch. In this system, nerves from stretch receptors form a synapse with gamma nerves in the spinal cord, which send an impulse to the stretched muscle fibers to stimulate their contraction. These reflexes are responsible for maintaining muscle tone and keeping an upright position against the pull of gravity and are important in helping venous return when the contracting muscle fibers massage veins to help move the blood toward the heart. Other spinal reflexes may involve synapses with interneurons within the spinal cord, which adjust movement and response based on information from higher brain centers to coordinate movement and position.