Muscles
On this page 9 sections
Overview
| Feature | Smooth Muscle | Skeletal Muscle | Cardiac Muscle |
| Striations | Absent | Present | Present |
| Cell Shape | Spindle-shaped | Cylindrical | Branched |
| Nuclei per Cell | Single (Centrally located) | Multiple (Peripherally located) | Single or double (Centrally located) |
| Control | Involuntary | Voluntary | Involuntary |
| Key Junctions | Gap junctions (in single-unit) | None | Intercalated discs (Gap junctions + Desmosomes) |
Skeletal Muscle
In skeletal muscle, these proteins are broadly divided into thick filaments and thin filaments.
1. Myosin
- Filament Type: Thick filament.
- Structure: Shaped like a tiny golf club with a long tail and a double globular head. Hundreds of these molecules bundle together to form a single thick filament, with their heads protruding outward.
- Function: It is a motor protein. During muscle contraction, the myosin heads bind to actin (forming "cross-bridges") and use energy from ATP to bend, pulling the thin filaments toward the center of the muscle unit (sarcomere) to shorten the muscle.
2. Actin
- Filament Type: Thin filament.
- Structure: Individual, spherical proteins (G-actin) link together like a strand of beads to form a twisted, helical structure (F-actin).
- Function: It serves as the structural backbone of the thin filament. Each actin bead contains a specific binding site where the myosin head hooks on during a contraction.
3. Tropomyosin
- Filament Type: Thin filament (Regulatory).
- Structure: A long, thread-like, fibrous protein that wraps around the actin helix.
- Function: In a resting (relaxed) muscle, tropomyosin physically blocks the binding sites on actin so that myosin cannot attach to it. It acts like a safety guard preventing unwanted muscle contraction.
4. Troponin
- Filament Type: Thin filament (Regulatory).
- Structure: A small, three-protein complex attached at regular intervals along the tropomyosin strand.
- Function: It acts as the switch. When a nerve signal tells a muscle to contract, calcium ions Ca2+ are released and bind to troponin. This causes troponin to change its shape, physically tugging the attached tropomyosin molecule out of the way. Once tropomyosin moves, the binding sites on actin are exposed, allowing myosin to bind and initiate contraction.
Receptors
Nicotinic Receptors
(Ligand-Gated Ion Channels)
Highly rapid, ionotropic receptors that cause direct cell depolarization upon binding.
| Subtype | Primary Locations | Major Physiological Function |
| N_M | Neuromuscular junction (skeletal muscle) | Excitation and contraction of skeletal muscle. |
| N_N | Autonomic ganglia (Sympathetic & Parasympathetic), Adrenal medulla, CNS | Postganglionic neuronal excitation; release of catecholamines (epinephrine/norepinephrine). |
Muscarinic Receptors
(G-Protein Coupled Receptors)
Slower, metabotropic receptors split into odd numbers (Gq-coupled / excitatory) and even numbers (Gi-coupled / inhibitory).
| Subtype | Signaling Pathway | Primary Locations | Key Effects |
|---|---|---|---|
| M1 | Gq(Stimulatory) | CNS, Gastric parietal cells | CNS excitation, memory processing, gastric acid secretion. |
| M2 | Gi(Inhibitory) | Heart (SA node, AV node, Atria) | Decreases heart rate (negative chronotropy) and decreases conduction velocity. |
| M3 | Gq(Stimulatory) | Smooth muscle & Exocrine glands(e.g., salivary, sweat, lacrimal glands, bronchioles, GI tract, bladder, pupils) | Increases secretions (salivation, sweating); contracts smooth muscle (bronchoconstriction, micturition, GI motility, pupillary constriction/miosis). |
| M4 & M5 | Mixed (Gi/Gq) | Central Nervous System (CNS) | Modulates locomotion, dopamine release, and complex behaviors. |