# Muscle and Nervous Tissue Biology I · Cells and Tissues · https://tryals.app/learn/biology-i/muscle-and-nervous-tissue ## Tissues That Respond Two tissue types are **excitable**: they respond to stimulation by changing their membrane potential. One converts that into movement, the other into signalling. **Muscle** comes in three types: | Type | Control | Striated | Nuclei | Where | |---|---|---|---|---| | Skeletal | Voluntary | Yes | Many, peripheral | Attached to bone | | Cardiac | Involuntary | Yes | One, central | Heart only | | Smooth | Involuntary | No | One, central | Walls of hollow organs | Skeletal and cardiac muscle are **striated**, and the stripes are the sarcomeres, the repeating units of overlapping actin and myosin filaments. Contraction happens by the **sliding filament** mechanism: myosin heads bind actin, pivot, and pull the filaments past one another. The filaments themselves never shorten; only the overlap changes. This produces the **length-tension relationship**. Too stretched and there is too little overlap for many cross-bridges to form; too compressed and the filaments interfere. Maximum force comes at an intermediate length where overlap is optimal, which is why joint angle affects how much force you can exert. Cardiac muscle adds **intercalated discs** that couple cells electrically, so the heart contracts as a coordinated unit rather than fibre by fibre. **Nervous tissue** has two cell populations. **Neurons** carry signals; **glia** outnumber them and support, insulate and defend. A neuron has dendrites receiving input, a cell body integrating it, and an axon carrying the output. Conduction speed is the design problem, and there are two solutions. Increasing axon **diameter** works but scales poorly, the squid giant axon reaches 1 mm across to achieve about 25 m/s. **Myelination** works far better: a fatty sheath with gaps at the nodes of Ranvier lets the signal jump node to node, reaching 100 m/s or more in a fibre a fraction the diameter. Vertebrates chose insulation over bulk, and the loss of myelin in multiple sclerosis shows how completely conduction depends on it. > **Common pitfall:** thinking muscle filaments shorten during contraction. They do not, actin and myosin keep their length throughout, and the sarcomere shortens because the filaments *slide past* one another. The name of the mechanism is the correction. ## Practice questions 7 of this lesson's 11 practice questions, with answers. The full set is in the app. ### 1. A myelinated axon conducts at 100 m/s while an unmyelinated one of similar diameter conducts at 1 m/s. How many times faster is the myelinated fibre? **Answer:** 100 (within ±1) **Why:** **100 times** faster at similar diameter. Reaching that speed by thickening the axon instead would require an enormous fibre, which is exactly the route the squid took, and why its giant axon is a millimetre across. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/a-myelinated-axon-conducts-at-100-m-s-while-an-unmyelinated-one-of ### 2. Both muscle and nervous tissue are classified as excitable, yet they produce fundamentally different biological outcomes upon stimulation. What physiological event do they share, and why does their downstream function diverge? A. Electrical coupling synchronises tension in one and routes discrete signals in the other B. Membrane potential shifts trigger sliding filaments in one and axon conduction in the other C. Active cross-bridge cycling produces contraction in one and dendrite integration in the other D. Action potential generation shortens proteins in one and releases glial defence in the other **Answer:** B. Membrane potential shifts trigger sliding filaments in one and axon conduction in the other **Why:** Excitability refers strictly to the electrical response at the plasma membrane. Divergence occurs because muscle couples this electrical event to mechanical cross-bridge cycling via sliding filaments, whereas nervous tissue channels it into propagating signals along an axon. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/both-muscle-and-nervous-tissue-are-classified-as-excitable-yet-they ### 3. During muscle contraction the actin and myosin filaments themselves become shorter. **Answer:** False **Why:** False, the filaments keep their length throughout. The sarcomere shortens because actin and myosin *slide past* each other, increasing their overlap. The name of the mechanism states the correction directly. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/during-muscle-contraction-the-actin-and-myosin-filaments-themselves ### 4. Striation reflects the sarcomere arrangement, not the control system, which is why cardiac muscle is striated yet involuntary. Sort each feature by the muscle type it belongs to. **Answer:** - Skeletal muscle: Voluntary, many nuclei, Bone-bound, striated - Cardiac muscle: Intercalated discs, Striated, involuntary - Smooth muscle: Unstriated, organ walls **Why:** Striation tracks the sarcomere arrangement, not the control system, cardiac muscle is striated yet involuntary. Intercalated discs are the heart’s alone, and they are what make it contract as one coordinated unit. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/striation-reflects-the-sarcomere-arrangement-not-the-control-system ### 5. Match each conduction strategy to its consequence. **Answer:** - Increasing axon diameter → Faster, but scales very poorly with size - Myelination → Far faster in a much thinner fibre - Nodes of Ranvier → Gaps where the signal is regenerated - Loss of myelin → Conduction slows or fails, as in multiple sclerosis **Why:** Diameter and myelination are the two available routes to speed, and myelination is enormously more efficient. Multiple sclerosis demonstrates the dependence directly: the axon is intact, but without its sheath the signal cannot travel properly. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/match-each-conduction-strategy-to-its-consequence ### 6. A muscle is stretched to 1.6 times its optimal sarcomere length, well beyond the point of useful filament overlap. Set the relative force it can now generate, from 0 to 1. **Answer:** 0 (within ±0.12) **Why:** At 1.6 times optimal length the filaments barely overlap, so almost no cross-bridges can form and force approaches **0**. This is not a theoretical curiosity, it is why joints have ranges beyond which you can exert almost no force. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/a-muscle-is-stretched-to-1-6-times-its-optimal-sarcomere-length-well ### 7. Arrange these events of skeletal muscle contraction in order. **Answer:** 1. A nerve impulse arrives at the neuromuscular junction 2. Calcium is released from the sarcoplasmic reticulum 3. Binding sites on the actin filament are exposed 4. Myosin heads bind and pivot, pulling the filaments 5. The sarcomere shortens and the muscle contracts **Why:** The chain runs from nerve signal to calcium release to exposed binding sites to cross-bridge cycling. Calcium is the switch: without it the binding sites stay covered and no contraction is possible, which is why calcium handling is central to muscle disease. Page: https://tryals.app/practice/biology-i/muscle-and-nervous-tissue/arrange-these-events-of-skeletal-muscle-contraction-in-order