McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 2 5 Muscle relaxants

Relaxed muscle fibre

Sensory neuron (axon)

Spindle (stretch sensor) stretches (in response to gravity)

Interneuron

Dorsal root

Dorsal horn

Cell body

Stimulates sensory nerve

Spinal cord cross section

Dorsal root ganglion

Spinal cord

Synapse

Ventral horn

Spinal nerve

Stimulates gamma nerve

Ventral root

Motor neuron (axon)

Muscle fibre contracts

Nerve ending in muscle fibres

No stretch

Sensory nerve not stimulated

FIGURE 25.1  Reflex arc showing the pathway of impulses. The spindle gamma loop reflex arc: The relaxing and contracting of muscle fibres causes muscle tone and ability to stand upright and promotes venous return.

Spindle (dendrite)

Muscle fibre

Gamma nerve not stimulated

Muscle fibre relaxes

NEUROMUSCULAR ABNORMALITIES All of the areas mentioned work together to allow for a free flow of impulses into and out of the CNS to coor- dinate posture, balance and movement. When injuries, diseases and toxins affect the normal flow of informa- tion 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. Muscle spasm Muscle spasms often result from injury to the musculo- skeletal 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 con- traction cuts off blood flow to the muscle fibres in the injured area, causing lactic acid to accumulate, 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 (see Figure 25.2). Muscle spasticity Muscle spasticity is the result of damage to neurons within the CNS rather than injury to peripheral

to help move the blood towards 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 centres to coor- dinate movement and position. 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 fibres control different types of move- ments. Those fibres 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 con- trolled muscle activity, and allows the body to make automatic adjustments in posture or position and balance. The extrapyramidal tract controls lower-level, or crude, movements.