Axial Musculature
Axial Musculature

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- skeletal muscle cell specializations: - excitability: excitable, responding to nerve stimulation - elasticity: stretch (which is limited by interstitial fibroconnective tissue) - contractility: forcefully shorten when stimulated - skeletal muscle functions: - locomotion, posture (antigravity), manipulation - inspiration and, when necessary, forced expiration - eye movements - swallowing, speech, facial expressions - control of urination and defecation - skeletal muscle cell (myofibril) is innervated at the neuromuscular junction (synapsis) - alpha motor neuron in ventral horn of spinal cord central gray matter emits an axon which splays out in the skeletal muscle to synapse on one or more muscle cells - fineness of control depends on how few muscle cells innervated per axon - at the synapse, motor axon releases acetylcholine neurotransmitter into synaptic cleft, initiating muscle cell membrane (sarcolemma) depolarization, which passes inside on transverse (‘T’) tubu les, causing calcium channels in sarcoplasmic reticulum (modified smooth ER) to release calcium ions into muscle cell cytoplasm, initiating contraction - skeletal muscle origin and physiology - myogenesis: skeletal muscl es are of mesodermal origin, from paraxial mesoderm - small ‘satellite cells’ are stem cells which can divide to replace damaged myofibrils - myoblasts fuse to form long cells (myofibrils) with multiple nuclei, located peripherall y - muscle cell cytoplasm: parallel arrays of contractile proteins, actin and myosin - fiber types based on glycogen/myoglobin ratio and velocity of contraction - glycogen: glucose stored as polymer; myogl obin: oxygen storage protein - ‘dark’ muscles: more myoglobin and mitochondria for oxidative respiration - type I (high oxidative, slow twitch) = slow contractions, resist fatigue - i.e. postural muscles and soleus muscle - type IIa (high oxidative, fast twitch) = fast contractions, resist fatigue - i.e. extraocular muscles and gastrocnemius muscle - ‘light’ muscles: more glycogen for anaerobic respiration - type IIb (low oxidative, fast twitch) = fast contractions, fatigue easily - i.e. quadriceps femoris (IIb is m ost abundant of fast twitch fiber types) - determined by innervation: muscle cells in a motor group are the same fiber type - most skeletal muscles are mixed, but with a preponderance of one type skeletal myology = study of muscle, specifically of skeletal muscle 656 skeletal muscles in the human body = about 40% of body weight - names based on: - shape (quadratus, teres) - size (maximus, minimus, longus, brevis) - direction (rectus, obliqu e, transverse) - location (sternocleidomastoid, coracobrachialis, intercostal) - action (flexor digitorum, supinator, tensor fascia lata) - number of heads (biceps, triceps, quadriceps) cross - sectio n of skeletal muscle cells with myofilaments, and peripheral nuclei ©