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AN3.1-3 | General features of Muscle — Part 1
CLINICAL SCENARIO
A 35-year-old construction worker in Chennai is brought to casualty after a fall from scaffolding. The orthopaedic resident notices his heel is abnormally flat and he cannot stand on tiptoe. An ultrasound confirms: the Achilles tendon is ruptured.
Why did a tendon — not the muscle itself — fail? What is so different about the structure of a tendon versus the fleshy muscle belly? And why do orthopaedic surgeons also worry about the "compartment" around muscles when there is swelling?
Understanding the general features of muscle — how they are classified, how they are structured, and how they attach to bones — answers these questions directly.
WHY THIS MATTERS
Muscles make up roughly 40% of body weight in a healthy adult male. Every clinical action — walking, breathing, swallowing, pumping blood — depends on muscle.
As a future doctor you will:
• Diagnose injuries by knowing which tendon or muscle belly is involved
• Order imaging (USS/MRI) based on anatomical knowledge of muscle layers
• Perform injections into specific muscles (vaccines, botulinum toxin) without damaging nerves
• Understand shunt vs spurt to counsel physiotherapists on rehabilitation
This is not memorisation for exams — it is the anatomical language of clinical practice.
RECALL
Before we begin, let's connect to what you already know:
- From Cell Biology — muscle cells are called myocytes. They are specialised for contraction.
- From Basic Histology — you have seen that skeletal muscle looks striped (striated) under the microscope.
- From NCERT Biology (Class 11) — you know there are three types of muscle tissue: skeletal, cardiac, smooth.
In Physiology right now, you are studying how muscles contract (neuromuscular junction, action potential). In Biochemistry, you are learning about collagen — the protein that forms tendons. Today's Anatomy content ties these three threads together.
How Muscles Are Classified
Muscles are classified by four criteria — you can remember them as SSRA: Structure, Size, Shape, Region, Action.
1. Classification by Structure (Histology)
This is the most fundamental classification:
- Skeletal muscle (also called striated voluntary muscle)
- Long, cylindrical fibres with alternating light and dark bands (striations)
- Multiple nuclei pushed to the periphery (edge) of the cell — this is the key histological marker
- Under voluntary (conscious) control
- Found attached to bones — produces body movement
- Example: biceps brachii, quadriceps femoris
- Cardiac muscle (also called striated involuntary muscle)
- Short, branched fibres with striations
- Single central nucleus — distinguishes it from skeletal on histology
- Connected by intercalated discs — specialised junctions allowing coordinated contraction
- Involuntary — you cannot consciously stop your heart
- Found only in the wall of the heart (myocardium)
- Smooth muscle (also called non-striated involuntary muscle)
- Spindle-shaped cells with no visible striations
- Single central nucleus — similar to cardiac histologically, but smooth has no intercalated discs
- Involuntary — controls hollow organs (gut, blood vessels, bladder, uterus)
- Examples: wall of intestine (peristalsis), uterus (labour contractions)
2. Classification by Control
• Voluntary: skeletal muscle (under somatic nervous system)
• Involuntary: cardiac + smooth muscle (under autonomic nervous system)
3. Classification by Location
• Somatic: muscles of the body wall and limbs (all skeletal)
• Visceral: muscles of internal organs — mostly smooth, heart is cardiac
Memory aid: "Skeletal = Striated + Voluntary + Peripheral nuclei" — the S-V-P rule.
Naming Muscles: Size, Shape, Region, and Action
Muscle names are descriptive — they tell you exactly what the muscle looks like or does. Understanding the Latin/Greek roots saves you from pure memorisation.
By Shape
• Deltoid (Greek delta = triangular letter Δ) — the shoulder muscle is triangular
• Rhomboid — rhombus-shaped muscle of the back
• Trapezius — trapezoid-shaped muscle of the upper back
• Serratus anterior — saw-toothed (serratus = saw) edges along its origin
• Orbicularis oculi / Orbicularis oris — circular (orbis) muscles around eye/mouth
By Size
• Gluteus maximus / medius / minimus — large, medium, small buttock muscles
• Pectoralis major / minor — large, small chest muscles
• Adductor longus / brevis / magnus — long, short, large thigh adductors
By Region
• Brachialis (brachium = arm) — in the arm
• Tibialis anterior — at the front of the tibia (shin bone)
• Femoris (femur = thigh bone) — associated with the femur
• Brachioradialis — spans from brachium (arm) to radius (forearm)
By Action
• Flexor carpi radialis / ulnaris — flexes the wrist at the radial / ulnar side
• Extensor digitorum — extends the fingers (digiti)
• Supinator — supinates (rotates palm upward) the forearm
• Levator scapulae — elevates (levare = to lift) the scapula
By Number of Heads
• Biceps (2 heads) — biceps brachii, biceps femoris
• Triceps (3 heads) — triceps brachii (all 3 heads meet at one tendon → olecranon)
• Quadriceps (4 heads) — quadriceps femoris (all unite via patellar tendon → tibial tuberosity)
SELF-CHECK — Classification Check
A muscle biopsy shows elongated cells with peripheral nuclei, visible striations, and no intercalated discs. What type of muscle is this?
A. Cardiac muscle
B. Smooth muscle
C. Skeletal muscle
D. Mixed striated and smooth
Reveal Answer
Answer: C. Skeletal muscle
The muscle named "Flexor carpi ulnaris" was named by which criterion?
A. Shape
B. Number of heads
C. Action and region
D. Size
Reveal Answer
Answer: C. Action and region
The Anatomy of a Skeletal Muscle — From Outside to Inside
Think of a skeletal muscle like a cable of wires inside a protective sheath — each level has its own wrapper.
The Connective Tissue Hierarchy
| Level | Structure | Wraps |
|---|---|---|
| Outermost | Epimysium | Entire muscle |
| Middle | Perimysium | Each fascicle |
| Innermost | Endomysium | Each muscle fibre |
- Epimysium (epi- = upon; mys = muscle) — dense connective tissue coat around the whole muscle. Continuous with the deep fascia of the region. Like the outer sheath of a cable.
- Perimysium (peri- = around) — surrounds groups of fibres called fascicles (Latin fasciculus = small bundle). Blood vessels and nerves travel within the perimysium to reach individual fibres. Like the insulation around each wire bundle.
- Endomysium (endo- = within) — delicate reticular connective tissue surrounding each individual muscle fibre (myocyte). Capillaries and nerve terminals make direct contact here. Like the thin insulation coat on each individual wire.
- Fascicle: a bundle of 10–100 muscle fibres, enclosed by perimysium. The visible "grain" when you cut meat.
The Muscle Fibre (Myocyte)
• Long, cylindrical cell — up to 30 cm length (e.g., sartorius) and 100 µm diameter
• Multinucleated: hundreds of nuclei, all at the periphery (under sarcolemma)
• Sarcolemma — the plasma membrane of the muscle fibre
• Sarcoplasm — the cytoplasm of the muscle fibre (sarco- = flesh)
• Myofibrils — thin thread-like contractile structures running the length of the fibre; each contains thousands of sarcomeres
The Sarcomere: Functional Unit
• The smallest contractile unit of muscle
• Bounded by Z-discs (Z-lines)
• Contains actin (thin filament) and myosin (thick filament)
• When muscle contracts → sarcomeres shorten → fibres shorten → whole muscle shortens
(You will study the sliding filament mechanism in detail in Physiology — today, just know the structural hierarchy.)
You are currently learning about Extracellular Matrix in Biochemistry — the collagen fibres and ground substance that make up the epimysium, perimysium, and endomysium come directly from fibroblasts using the ECM components you are studying right now.