A. Division of peripheral from central nervous system is artificial, since they are really continuous: cell bodies of anterior horn neurons are located in spinal cord (central nervous system), while their axons are located in ventral roots and peripheral nerves (peripheral nervous system); dorsal root ganglion neuron cell bodies and their peripheral axons are considered part of peripheral nervous system, while their “central” axons (in spinal cord dorsal columns) are part of central nervous system; diseases often do not respect notions of “boundaries” between central nervous system and peripheral nervous system B. Nerve axons can be either myelinated or unmyelinated 1. Single oligodendrocytes produce myelin internode segments for multiple central nervous system axons 2. Each peripheral nervous system myelin segment (internodal segment) is formed by single Schwann cell cytoplasm; unmyelinated axons are also encased by cytoplasm of Schwann cells which do not form myelin; bundles of myelinated and unmyelinated fibers form fascicles enveloped by specialized connective tissue sheath (perineurium) which produces blood-nerve barrier (analogous to blood-brain barrier) C. Muscle is composed of multiple muscle fibers enclosed by fascial connectivec tissue sheath; muscle fibers are multinucleate cells containing thousands of cytoplasmic (sarcoplasmic) myofibrils composed of actin and myosin (contractile apparatus); each muscle fiber has single acetylcholine receptorrich endplate opposite motor nerve terminal; muscles are mosaics of two distinctive fiber types 1. Type 1 muscle fibers — slow twitch, red, aerobic, oxidative, and fatigue-resistant 2. Type 2 muscle fibers — fast twitch, white, anaerobic, glycolytic rapidly-fatiguing