176 Anatomy & Physiology Essentials Copyright Goodheart-Willcox Co., Inc. a parallel fiber arrangement. However, the pennate arrangement makes it possible to pack more fibers into the muscle. This means that the muscle can generate more force. SELF CHECK 1. What is the difference between fast-twitch and slow-twitch fibers? 2. Which fiber type helps a sprinter get out of the blocks quickly? 3. Why can pennate-arranged fibers generate more force than parallel-arranged fibers? Pennate Fiber Architecture In a pennate (featherlike) fiber architecture, each fiber attaches obliquely to a central tendon, and in some cases, to more than one tendon. As Figure 6.9b shows: • Fibers that are aligned in one direction to a central tendon are unipennate. • Fibers that attach to a central tendon are bipennate. • Fibers that attach to a central tendon in more than two directions are multipennate. Certain muscles of the hand are unipennate, the rectus femoris (a member of the quadriceps group in the thigh) is bipennate, and the deltoid is multipennate. A muscle with a pennate fiber arrangement does not shorten as much upon contraction as a muscle with Adaptations of the Neuromuscular Junction The neuromuscular junction (NMJ) is a specialized synapse between a motor neuron and a skeletal muscle fiber. Full functionality of the NMJ is essential for muscle contraction and movement. NMJs on fast- and slow-twitch muscle fibers differ in structure and function. Presynaptically, compared to slow-twitch motor neuron endings, fast-twitch motor neuron endings have a rougher surface that enables the discharge of more acetylcholine at a time. When stimulated, the fast-twitch endings release more acetylcholine, but have fewer acetylcholine vesicles, so they fatigue more quickly with continued stimulation. Postsynaptically, compared to slow-twitch fibers, the fast-twitch fibers have more and deeper acetylcholine receptor sites. This enables them to respond to given amounts of acetylcholine with larger postsynaptic electrical currents. Adaptations with Aging After peak strength is achieved in young adulthood, aging is associated with a progressive decline in neuromuscular function, characterized by loss of muscle mass and strength. Like many structures in the human body, mature NMJs undergo structural and functional changes, or remodeling, throughout adult life. Whether these changes cause or result from the loss of muscle mass and strength is a topic of current research. Research has shown that changes in NMJ remodeling do precede the loss of fast-twitch motor units with aging. Structurally in the NMJ, both the nerve terminal area and the number of fiber acetylcholine receptors progressively diminish with aging. As a result, motor nerve conduction velocity becomes slower and the amplitude of the muscle action potential is reduced. Adaptations with Exercise A logical question is whether exercise can slow or postpone the aging-related remodeling of the NMJ. Studies were conducted in which groups of young and healthy elderly participants performed a single bout of resistance exercise. The exercise was done with one leg, with the other (non-exercised) leg serving as a control for each person. Biopsies from muscles in both exercised and control legs were collected daily for a week, beginning immediately post-exercise. The results of these studies showed the expected, age-related loss of functional muscle fibers in the elderly participants. Chemical and microscopic analyses of the biopsies revealed improved NMJ transmission of acetylcholine in the exercised leg compared to the control leg for both young and old participants. Together, the data support the role of exercise in stimulating the stability of the NMJ, but also demonstrate age-related changes, even in healthy elderly individuals. Research Notes Tonhom1009/Shutterstock.com A muscle biopsy is a small plug of muscle tissue collected by inserting a biopsy needle into a muscle.