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Regulation of Contraction I. How do nerves stimulate muscle contraction? II. Factors influencing muscle tension III. How does contraction of smooth/cardiac muscle differ from skeletal? Innervations of Muscle Fiber Sarcolemma SR Neuromuscular Junction • AP to synaptic terminal of motor neuron Neuromuscular Junction • AP to synaptic terminal of motor neuron • Release of Ach Neuromuscular Junction • AP to synaptic terminal of motor neuron • Release of Ach • AP spreads across sarcolemma Neuromuscular Junction • AP to synaptic terminal of motor neuron • Release of Ach • AP spreads across sarcolemma Skeletal Muscle Contraction Sarcolemma Skeletal Muscle Contraction Sarcolemma • AP passes down T tubules Skeletal Muscle Contraction • AP passes down T tubules • Release of Ca+ from the ____________ Skeletal Muscle Contraction • AP passes down T tubules • Release of Ca+ from the ____________ • Ca+ binds to Troponin Skeletal Muscle Contraction • AP passes down T tubules • Release of Ca+ from the ____________ • Ca+ binds to Troponin • Sliding Filament Animation Regulation of Contraction I. How do nerves stimulate muscle contraction? II. Factors influencing muscle tension III. How does contraction of smooth/cardiac muscle differ from skeletal? Factors Influencing Muscle Tension 1. Frequency of stimulation 2. Fiber length at onset of contraction 3. Number of muscle fibers contracting 4. Muscle fatigue Frequency of Muscle Fiber Stimulation Incomplete Frequency of Muscle Fiber Stimulation Incomplete Frequency of Muscle Fiber Stimulation Maximum Tension Incomplete Complete Tetanus • Sustained maximum muscle tension Tetanus Infection • Clostridium tetani • Muscles contract severely • Respiratory failure Factors Influencing Muscle Tension 1. Frequency of stimulation 2. Fiber length at onset of contraction 3. Number of muscle fibers contracting 4. Muscle fatigue Muscle Tone & L-T Relationship • Relaxed muscles are slightly contracted Muscle Tone & L-T Relationship • Relaxed muscles are slightly contracted • Tension during contraction depends on sarcomere length of optimum length Muscle Tone & L-T Relationship • Too much tone yields a weak contraction of optimum length Muscle Tone & L-T Relationship • Too much tone yields a weak contraction • Too little tone also yields a weak contraction of optimum length Tension maximum) of maximum) (% of Tension (% Length-Tension Relationship 100 50 60 80 100 120 140 % of resting length 160 Northern Leopard Frog Factors Influencing Muscle Tension 1. Frequency of stimulation 2. Fiber length at onset of contraction 3. Number of muscle fibers contracting • Each fiber can contract or not (all or none) • More fibers means a stronger contraction Factors Influencing Muscle Tension Factors Influencing Muscle Tension 4. Muscle fatigue • Muscle fibers cannot contract strongly until Ca+ is sequestered • Delay is “fatigue” Regulation of Contraction I. How do nerves stimulate a muscle contraction? II. Factors influencing muscle tension III. How does contraction of smooth/cardiac muscle differ from skeletal? Smooth & Cardiac Muscle Contraction • Contain actin & myosin • Use ATP to power cross-bridge cycling Smooth & Cardiac Muscle Contraction • Contain actin & myosin • Use ATP to power cross-bridge cycling • Differences in muscle excitation • Differences in muscle contraction Smooth Muscle Contraction • Myosin and Actin lattice • Slow, wavelike contraction Relaxed Contracted Smooth Muscle Contraction • Myosin and Actin lattice • Slow, wavelike contraction Relaxed • Influx of Ca+ from ECF • Self-excitable Contracted Smooth Muscle Contraction • Differs from skeletal and cardiac muscle: – Lacks troponin and tropomyosin – Ca+ binds to calmodulin (similar to troponin) Smooth Muscle Contraction • Differs from skeletal and cardiac muscle: – Ca+- calmodulin complex phosphorylates Myosin, increases its affinity for Actin – Ca+ removal causes Myosin-Actin dissociation Relaxed Contracted Cardiac Muscle Cardiac Muscle • Has characteristics of both skeletal and smooth Skeletal Similarity: • Troponin & tropomyosin Smooth Similarity: • Self-excitable Unique Feature: • Long AP’s AP in Cardiac Muscle • Long AP prevents tetanus Skeletal Muscle Cardiac Muscle Summary • Motor neurons stimulate muscle contraction. The AP then passes along the sarcolemma and down the T tubules, deep into the muscle fiber. This allows all myofilaments within the cell to be stimulated. • Release of Ca+ from the SR binds to troponin, causing tropomyosin to shift and expose the myosin head binding sites on actin. Sliding filament process then takes over. • Muscle fiber length, fatigue, and the number of fibers stimulated can all influence the strength of contraction. • Smooth/Cardiac contraction differs from Skeletal contraction.
