Summary

Summary

The PNS consists of peripheral nerves, which ­connect the CNS to the rest of the body. It is divided into the SNS and ANS. Peripheral sensory receptors­ are specialized­ to respond to stimuli. They include ­chemoreceptors, mechanoreceptors, nociceptors, photoreceptors, and ­thermoreceptors. ­Sensory adaptation is the ­ability to ignore ­unimportant stimuli­. The general senses of touch, ­pressure, tempera-ture, and pain are spread throughout the body. Nonen-capsulated (free) nerve endings include tactile (Merkel) discs and hair follicle receptors. Encapsulated nerve endings include Meissner’s (tactile) corpuscles, Pacinian (lamellar) corpuscles, bulbous corpuscles (Ruffini end-ings), muscle spindles, tendon organs, and joint kines-thetic receptors.

Complex sense organs contain the receptors for the special senses, which include hearing, vision, equilibrium, smell, and taste. The somatosensory system serves the limbs and wall of the body. Sensory (afferent) nerves carry impulses toward the CNS, whereas motor (efferent) nerves carry impulses away from the CNS. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves. The main portions of the spinal nerves mostly combine to form complex networks called plexuses. Peripheral motor endings activate effectors via the release of neurotransmitters. The SNS stimulates skeletal muscles, whereas the ANS innervates glands, cardiac muscle, and smooth muscle. Reflexes are either inborn (intrinsic) or learned (acquired). A reflex arc is formed by five components: a receptor, a sensory neuron, an integration center, a motor neuron, and an effector.

Imbalances of the PNS affect either the sensory or motor component, but usually not both. Nerve conduction slows, and symptoms include buzzing or tingling sensations, motor weakness, and reduced reflexes. Regeneration of axons is possible, but cannot occur when the cell body dies. Aging of the PNS results in slower impulse conduction, decreased sensation, slower reflexes, and often, clumsiness. Conditions such as diabetes cause age-related functional changes to be more noticeable. Over time, the response of the PNS to injury becomes reduced. Older people are then more vulnerable to injury and disease.