Muscle Tissue

Pericytes

• Found wrapped around tiny blood vessels (capillaries and venules).

• Act like “support cells/stem cells” that help control blood flow, keep vessels strong, and help in healing.

• Found outside capillaries

Myofibroblast

• Special cells that appear when there’s injury or wound healing.

• They contract (pull the tissue together) to help close wounds.

• Think of them as “the body’s stitchers.”

Myoepithelial Cells

• Found in external surface of glands (like sweat, salivary, or mammary glands).

• They squeeze the glands to push out the fluid (like milk, sweat, saliva).

• Think of them as “tiny squeezers in the glands.”

Muscle Cells

• The actual cells that make up muscles (skeletal, smooth, cardiac).

• They contract to help us move, pump blood, or push food in the gut.

• Think of them as the “engines of movement.”

Muscle Tissue

• Demonstrates the greatest degree of contractility

  • Due to muscle cells (muscle fiber)

• Highly cellular tissue

• Supported and bound together by intercellular material

  • Consists of connective tissue

• Responsible for locomotion and movement

Muscle Fiber/Muscle Cells

• Elongated Cell

  • Called a “fiber” because it’s long, thin, and thread-like

• From Mesoderm EXCEPT iris (Ectoderm)

• Enveloped by Basal Lamina (Made of reticular tissue)

Cell membrane of Muscle Fiber/Tissue

Sarcolemma

Cytoplasm of Muscle Fiber/Tissue

Sarcoplasm

Smooth Endoplasmic Reticulum of Muscle Fiber/Tissue

Sarcoplasmic Reticulum

Mitochondria of Muscle Fiber/Tissue

Sarcosome

Actin & Myosin

Inside: Filled with special proteins _____ that allow contraction.

Myofibrils & Sarcomeres

Internal unit: Contains _____ → which are made up of repeating units called _____ (the contractile units).

Skeletal Muscle

• Striated (striped) due to Sarcomere → has light and dark bands under the microscope.

• Contraction → Quick & Forceful

• Forms Mouse Shaped organs → Muscles or Skeletal Muscles

• Voluntary → meaning we can control it consciously (EX: Limbs, Body wall, Face) (NOT AN EX: Pharynx and upper part of Esophagus)

It is typically attached at either end by a DENSE CONNECTIVE TISSUE called Tendon to a part of the SKELETAL SYSTEM called Bone or Cartilage. The attachment are referred to as Origin and Insertion.

Tendon

To what DENSE CONNECTIVE TISSUE is the Skeletal Muscle attached?

Bone or Cartilage

Skeletal muscle is attached to Tendon to a part of the SKELETAL SYSTEM called?

Muscle Fiber

• The basic structural unit of skeletal muscle.

• Long, cylindrical, multinucleated cells that contain myofibrils.

• Myofibrils are made up of sarcomeres (the contractile units with thin and thick filaments).

Fascicle

• A bundle of muscle fibers grouped together.

• Surrounded by connective tissue called perimysium.

• Fascicles give muscles their visible grain or striated appearance.

Muscle

• The whole skeletal muscle is formed by bundles of fascicles.

• Surrounded by a connective tissue covering called epimysium.

• Muscles work together with tendons to produce movement by contracting and pulling on bones

Endomysium

• Made of: Loose connective tissue (mainly reticular fibers and some collagen) that surround each individual muscle fiber.

• Contains capillaries and nerves that supply the muscle cells.

• Provides support and helps transmit force from one fiber to the next.

Perimysium

• Made of: Dense irregular connective tissue sheath that surrounds a bundle of muscle fibers, called a fascicle.

• Provides pathways for blood vessels and nerves to reach the muscle fibers inside.

• Also contributes to the strength of muscle by binding fibers together.

Epimysium

• Made of: Dense irregular connective tissue (rich in collagen fibers, mainly reticular fibers and some collagen) that surrounds the entire muscle.

• It merges into tendons, which attach muscles to bones.

• Protects the muscle from friction and helps distribute force generated during contraction.

Skeletal Muscle Fiber

• Instead, it is long, cylindrical, and multinucleated, meaning it has many nuclei scattered along its length.

• Composed of Myofibrils (made up of sarcomeres)

• Contract when stimulated by motor neurons at the neuromuscular junction (NMJ).

Myofibrils

Long protein structures inside the fiber, made of repeating sarcomeres (the contractile units).

Sarcoplasmic Reticulum (SR)

Store and release calcium ions (Ca²⁺) needed for muscle contraction.

T-tubules

Carry electrical signals deep inside the fiber.

Sarcomere

• Made up of Microfilaments.

Thin Filaments: Actin, Tropomyosin, and Troponin

• Contraction occurs through the sliding filament model, where actin and myosin filaments slide past each other, shortening the sarcomere. When sarcomeres shorten, the muscle contracts.

• It is the region of a myofibril between two Z-discs (Z-lines).

• The principal component of thin filaments is F-Actin which consist of 2 strands of Globular and Soluble Actin (G-Actin).

• One end of each thin filament is attached to a Z-Line while the other end is free

Thick Filament: Myosin

• Occupy the middle zone of a sarcomere

• Among the myofilaments, actin and myosin are the most abundant, accounting for about 60% of total muscle for protein.

1. Red Muscle Fibers

• Appearance: Dark red (due to high myoglobin and many mitochondria). Smaller and have richer blood supply than white muscle fibers

• Contraction: Slow but sustained; resist fatigue. “Slow-twitch” muscle fibers

• Sarcoplasm has more mitochondria, glycogen granules and myoglobulin

2. White Muscle Fibers

• Appearance: Pale/white (low myoglobin, fewer mitochondria).

• Contraction: Fast and forceful than Red, but fatigue quickly.

• “Fast-switch” muscle fibers

3. Intermediate Muscle Fibers

• Appearance: Pinkish (moderate myoglobin and mitochondria).

• Contraction: Faster than red fibers but more resistant to fatigue than white fibers.

• Morphology and characteristics are between Red and White

General Sensory Receptors

• Simple nerve endings

• Vater-Pacinian Corpuscles

• Ruffini’s Corpuscles

Proprioceptors

• Simple nerve endings

• Neuromuscular spindles

• Golgi tendon organs

Neuromuscular Spindle

• Encapsulated fusiform structure that has several striated muscle fibers (intrafusal fibers)

• Present in all skeletal muscles

• Embedded in Endomysium and Perimysium

• A stretch receptor that detects the degree and velocity stretch applied to a muscle (Prevents over stretching)

Example: When a doctor taps your knee, the muscle spindle detects the sudden stretch → sends a signal → causes the quadriceps to contract.

2 kinds of Intrafusal Fibers

1. Nuclear Bag Fibers

2. Nuclear Chain Fibers

Nuclear Bag Fibers

• Structure: The central region of the fiber is swollen and filled with nuclei that “bunch up” like a bag.

• Function:

  • Detect dynamic changes in muscle length (speed and rate of stretch).

  • Important for sensing when the muscle is being stretched quickly.

Nuclear Chain Fibers

• Structure: The nuclei are arranged in a straight chain/row along the length of the fiber.

• Function:

  • Detect static muscle length (steady or maintained stretch).

  • Important for sensing muscle position rather than speed.

Golgi Tendon Organ

• Location: In tendons, near where the muscle fibers connect to the tendon.

• Small, one mm-long structures that attach skeletal muscles to their insertions and origins

• Measures the tension that is generated by muscle contraction

Sensitive to muscle contraction than stretch

Cardiac Muscle

• Location: Found only in the walls of the heart (myocardium).

• Striations: Yes, like skeletal muscle, because of sarcomeres (thin and thick filaments).

• Control: Involuntary – you cannot consciously control it.

• Nucleus: Usually one nucleus per cell (sometimes two).

• Branching

• Cell junctions demonstrate intercalated disc

Cardiac Muscle Fiber

• Cylindrical and much shorter than skeletal muscle cells, branches at their ends unlike skeletal muscle

• Contains only 1 to 2 nuclei, centrally located

• Sarcoplasm is more abundant and mitochondnia more numerous and larger

• With myofibrils with cross striations that are not as prominent as that of skeletal muscle cell

Contraction of cardiac muscle

• Similar to the contraction of skeletal muscle, except that inside - sarcoplasmic reticulum

• Calcium ions also came from the outside of cell

• Cells contract without neural stimulation

• The impulse that initiates the contractions is generated by the sinoatrial node

Intercalated Disest

• Unique to cardiac muscle

• Appear as dark, transverse lines that occur at irregular intervals

Purkinje Fibers

Non-contractile cardiac muscle cells specialized to initiate and conduct electrical impulse that controls cardiac contraction

Smooth muscle

• Also known as visceral muscle

• Involuntary muscles

• Slower and less forceful than striated muscles

• No striations (unlike skeletal & cardiac muscles) because actin and myosin are arranged differently.

Smooth muscle cell

• Fusiform

• Single nucleus

• Fusiforn, broad at middle, and tapering at both ends

• Contains a single, oval nucleus that is located in the thick part of the cell.

• Acidophilic sarcoplasm, filled with myofilaments

• May be mistaken for collagen fibers

Organization of Smooth Muscle

• Fascicles are enveloped by endomysium

• Desmosomes and gap junctions attach the sarcolemma of adjacent cells

Skeletal musc

Repair and Regeneration: Skeletal Muscle Tissue

• Capable of regeneration despite its inability to undergo cell division.

  • Satellite cells (myeloblast - like stem cells) - source of new skeletal muscle cells

Repair and Regeneration: Smooth Muscle Tissue

Can regenerate from a type of stem cell called a “Pericyte”, which is found in some blood vessels. Pericytes allow smooth muscle cells to regenerate and repair much more readily than skeletal and cardiac muscle tissue.

Repair and Regeneration: Cardiac Muscle Cells

Have very limited to no regenerative capabilities

Smooth Muscle Contracyion

2 strands of Globular and Soluble Actin (G-Actin)

Coiled around each other much like the fibers of a rope

Neuromuscular junction (NMJ)

Where Skeletal muscle fibers contract when stimulated by motor neurons