Muscular Movement — Revision Notes

⚡ 30-Second Revision

Muscle Types: — Skeletal (voluntary, striated), Smooth (involuntary, non-striated), Cardiac (involuntary, striated, intercalated discs).
Sarcomere: — Functional unit. Z-line to Z-line.
Filaments: — Actin (thin), Myosin (thick).
Bands: — A-band (myosin length, constant), I-band (actin only, shortens), H-zone (myosin only, shortens).
Sliding Filament Theory: — Actin slides over myosin; filaments don't shorten.
Key Ions: — $Ca^{2+}$ (binds to troponin, exposes actin sites).
Key Molecule: — ATP (detachment of myosin, re-cocking of myosin head).
Regulatory Proteins: — Troponin, Tropomyosin.
Neuromuscular Junction: — ACh released, binds to sarcolemma.
Sarcoplasmic Reticulum (SR): — Stores and releases $Ca^{2+}$.
Energy Sources: — Creatine Phosphate, Anaerobic Glycolysis, Aerobic Respiration.

2-Minute Revision

Muscular movement is powered by the contraction of specialized muscle cells. There are three types: skeletal (voluntary, striated, attached to bones), smooth (involuntary, non-striated, in organs), and cardiac (involuntary, striated, in heart, with intercalated discs).

The fundamental unit of contraction in striated muscles is the sarcomere, composed of actin (thin) and myosin (thick) filaments. Contraction occurs via the sliding filament theory, where actin slides past myosin, shortening the sarcomere without changing filament length.

This process is initiated by a nerve impulse at the neuromuscular junction, releasing acetylcholine. An action potential then propagates along the sarcolemma and T-tubules, triggering the release of calcium ions ($Ca^{2+}$) from the sarcoplasmic reticulum.

$Ca^{2+}$ binds to troponin, which moves tropomyosin away from actin's myosin-binding sites. Myosin heads then bind to actin, perform a power stroke (pulling actin), and detach upon binding a new ATP molecule.

ATP hydrolysis re-cocks the myosin head. Relaxation occurs when $Ca^{2+}$ is pumped back into the SR. ATP for contraction comes from creatine phosphate, glycolysis, and aerobic respiration.

5-Minute Revision

Muscular movement is essential for all bodily functions and relies on the contractile properties of muscle cells. We distinguish three types: Skeletal muscle for voluntary movements, characterized by striations and multiple peripheral nuclei.

Smooth muscle for involuntary actions in internal organs, lacking striations and having a single central nucleus. Cardiac muscle, unique to the heart, is involuntary, striated, and features branched cells with intercalated discs for synchronized contraction.

The core mechanism of contraction, particularly in skeletal and cardiac muscle, is the Sliding Filament Theory. Within a muscle fiber, myofibrils are composed of repeating units called sarcomeres, which are delimited by Z-lines.

Sarcomeres contain thin (actin) and thick (myosin) filaments. During contraction, actin filaments slide past myosin filaments, causing the sarcomere to shorten. The A-band (myosin length) remains constant, while the I-band (actin only) and H-zone (myosin only) shorten.

Excitation-Contraction Coupling is the sequence of events: A motor neuron releases acetylcholine (ACh) at the neuromuscular junction. ACh binds to receptors on the sarcolemma, generating an action potential. This action potential travels down T-tubules, triggering the release of **calcium ions ($Ca^{2+}$) from the sarcoplasmic reticulum (SR)**. $Ca^{2+}$ then binds to troponin, causing it to shift tropomyosin away from the myosin-binding sites on actin.

With sites exposed, myosin heads, energized by ATP hydrolysis (ATP $\rightarrow$ ADP + Pi), bind to actin, forming cross-bridges. The release of ADP and Pi causes the power stroke, pulling actin towards the sarcomere center.

A new ATP molecule binds to myosin, causing detachment. ATP hydrolysis then re-cocks the myosin head. This cycle continues as long as $Ca^{2+}$ and ATP are available. Relaxation occurs when $Ca^{2+}$ is actively pumped back into the SR, and tropomyosin re-blocks the actin sites.

Energy for contraction is primarily ATP, generated rapidly by creatine phosphate, then by anaerobic glycolysis (producing lactic acid), and for sustained activity, by aerobic respiration in mitochondria. Common disorders include Myasthenia gravis (autoimmune, ACh receptor attack) and muscular dystrophy (genetic, muscle degeneration).

Prelims Revision Notes

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Muscle Types:

* Skeletal: Voluntary, striated, multinucleated, peripheral nuclei, fast contraction, fatigues. Attached to bones. * Smooth: Involuntary, non-striated, single central nucleus, spindle-shaped, slow sustained contraction, fatigue resistant. Walls of internal organs. * Cardiac: Involuntary, striated, branched, single/binucleated, central nucleus, intercalated discs, rhythmic contraction, highly fatigue resistant. Heart wall.

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Skeletal Muscle Structure:

* Muscle fiber (cell): Multinucleated, contains myofibrils. * Myofibril: Composed of repeating sarcomeres. * Sarcomere: Functional unit, Z-line to Z-line. * Thin filaments: Actin, Troponin, Tropomyosin.

Anchored to Z-lines. * Thick filaments: Myosin. Located in the center. * Bands/Zones: * A-band: Dark, full length of myosin. Constant length during contraction. * I-band: Light, actin only.

Shortens during contraction. * H-zone: Lighter region within A-band, myosin only. Shortens/disappears during contraction. * Z-line: Bisects I-band, anchors actin. * M-line: Center of A-band, anchors myosin.

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Sliding Filament Theory: — Actin filaments slide over myosin filaments. Filaments themselves do NOT shorten.
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Excitation-Contraction Coupling (Sequence):

* Motor neuron releases Acetylcholine (ACh) at Neuromuscular Junction (NMJ). * ACh binds to receptors on Sarcolemma (muscle cell membrane), causing depolarization and Action Potential (AP).

* AP propagates along sarcolemma and into T-tubules. * AP in T-tubules triggers release of **Calcium ions ($Ca^{2+}$) from Sarcoplasmic Reticulum (SR)**. * $Ca^{2+}$ binds to Troponin (on actin filament).

* Troponin changes shape, pulling Tropomyosin away from myosin-binding sites on actin. * Myosin heads (already energized by ATP hydrolysis) bind to exposed actin sites, forming cross-bridges.

* Power Stroke: Release of ADP + Pi causes myosin head to pivot, pulling actin towards M-line. * New ATP binds to myosin head, causing detachment from actin. * ATP hydrolysis (ATP $\rightarrow$ ADP + Pi) re-energizes and re-cocks myosin head.

* Cycle repeats as long as $Ca^{2+}$ and ATP are present.

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Muscle Relaxation: — ACh broken down, AP stops, $Ca^{2+}$ actively pumped back into SR. Tropomyosin re-blocks actin sites.
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Energy for Contraction:

* ATP: Direct source. * Creatine Phosphate: Rapid ATP regeneration for initial seconds. * Anaerobic Glycolysis: Glucose $\rightarrow$ Lactic acid + ATP (fast, less efficient, no $O_2$). * Aerobic Respiration: Glucose/Fatty acids $\rightarrow$ $CO_2$ + $H_2O$ + ATP (slow, efficient, requires $O_2$).

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Muscle Disorders:

* Myasthenia Gravis: Autoimmune, antibodies attack ACh receptors at NMJ, leading to muscle weakness. * Muscular Dystrophy: Genetic, progressive degeneration of skeletal muscles. * Tetanus: Bacterial toxin (Clostridium tetani) causes sustained muscle contraction (spastic paralysis) by interfering with inhibitory neurotransmitters. * Rigor Mortis: Post-mortem muscle stiffness due to ATP depletion, preventing myosin detachment.

Vyyuha Quick Recall

To remember the sequence of muscle contraction initiation: All Cats Try Crunching Tasty Mice.

Acetylcholine release
Calcium release (from SR)
Troponin binds Calcium
Conformational change (in tropomyosin)
Thick filament (myosin) binds to actin
Muscle contracts