Muscle Tissue and Structure

• Muscle tissue types include skeletal, cardiac, and smooth muscle.
• Skeletal muscle is striated and voluntary.
• Cardiac and smooth muscles are involuntary.
• Cardiac muscle is striated, whereas smooth muscle is nonstriated.

Muscle Cell Structure

• A muscle cell is also known as a muscle fiber.
• The cell membrane of a muscle cell is the sarcolemma.
• The cytoplasm of a muscle cell is the sarcoplasm.
• The endoplasmic reticulum is modified into the sarcoplasmic reticulum.
• Muscle cells contain nuclei, mitochondria, ribosomes, and myofilaments.

Connective Tissue Arrangement

• Connective tissue surrounds muscle components in a specific arrangement.
• The epimysium surrounds the entire muscle.
• The perimysium surrounds each fascicle.
• The endomysium surrounds each muscle fiber.

Skeletal Muscle Cell Characteristics

• Skeletal muscle cells are long, large-diameter fibers.
• Skeletal muscle cells are multi-nucleated, with nuclei located peripherally.
• Striations in skeletal muscle are due to dark A bands and light I bands.

Myofibrils

• Are rod-shaped and arranged parallel to each other.
• Myofibrils are densely packed, occupying about 80% of the cell volume.
• Striations within myofibrils are composed of dark A bands and light I bands.
• The H zone is the lighter region in the middle of the A band.
• The M line is dark and bisects the H zone.
• The Z disc is dark and bisects the I band.

Sarcomere

• The sarcomere represents the region between two adjacent Z discs.
• Each sarcomere includes an A band and half of an I band at each end.
• The sarcomere is the smallest contractile unit of muscle.

Myofibril Composition

• Myofibrils are composed of thick and thin filaments.
• The thick filaments are mainly composed of myosin and located in the central portion of the sarcomere.
• The thin filaments contain actin, tropomyosin, and troponin, and are located on the sides of the sarcomere.
• The A band consists of thick and thin filaments.
• The I band consists of thin filaments.
• The H zone consists of only thick filaments.
• The M line represents attachments between thick filaments.
• The Z line represents the attachment between thin filaments.

Sarcoplasmic Reticulum and T-Tubules

• The sarcoplasmic reticulum is arranged as an interconnecting system of tubules.
• Terminal cisternae are formed where A and I band junctions dilate.
• Sarcolemma penetrates into the cell interior as a tubule at A & I band junctions to form T-tubules.
• T-tubules run between the terminal cisternae and transmit electrical impulses from the sarcolemma to them.
• A triad consists of a terminal cistern, T-tubule, and another terminal cistern.

Neuromuscular Junction (NMJ)

• The pre-synaptic terminal represents the expanded terminal end of a motor nerve axon.
• It contains synaptic vesicles filled with acetylcholine.
• The post-synaptic membrane is the folded motor end plate of the muscle cell, containing nicotinic acetylcholine receptors (NAChR).
• The synaptic cleft represents a 30 nm gap between the pre-synaptic end and the post-synaptic membrane.

Muscle Contraction

• Relaxed muscle fibers feature thick and thin filaments that overlap slightly.
• In muscle contraction, thin filaments slide centrally, increasing the overlap.
• Sarcomere length is reduced during contraction.
• I bands shorten during contraction.
• The H zone disappears during contraction.
• Myofilament lengths remain the same during contraction.
• This process is described by the sliding filament theory.

Muscle Spindle

• Intrafusal fibers are contained within a connective tissue capsule filled with fluid.
• Two types of intrafusal fibers include long & spindle fibers.
• Two types of sensory nerve endings include spiral & sprinkled endings.
• Stretch receptors helps to maintain muscle tone.

Motor unit

• One motor neuron is connected to all the muscles fibers that it innervates
• There are many motor units in a muscle

Cardiac Muscle

• Cardiac muscle is striated and has a single, centrally located nucleus.
• Cardiac muscle cells branch and are connected by intercalated discs.
• Intercalated discs joins the cells together by desmosomes.
• Ionic echange is facilitated through gap junctions within the heart.
• Contains actin and myosin filaments
• Contraction occurs via the sliding filament hypothesis.
• The T tubules are found at the Z discs.
• One cistern of sarcoplasmic reticulum is in parallel to the transverse tubule forming diads

Smooth Muscle

• Smooth muscle is found in the walls of hollow viscera.
• The Tapering ends of smooth muscles have central bulges.
• Smooth muscles cells have single nuclei placed centrally.
• Actin and myosin filaments are arranged in a crisscross pattern.
• There is also an intermediate filaments between the thick and thin filaments
• Desmin is found in the intermediate filaments
• Thin and intermediate filaments are attached to dense bodies,
• Contraction of these filaments decreases the size of the cell and transmits the contractile force to adjacent muscle cells.

Muscle Tissue Repair

• Regeneration: Replacement of the tissue with the same type of tissue when damaged.
• Occurs by the division of the existing cells.
• Hypertrophy: Increasing the size of cells by increase demand.
• Hyperplasia: Increasing the number of cells due to increase demand.
• Occurs by the division of the existing cells.
• Atrophy: Loss of muscle mass due to non use.
• Occurs due to reduced size of existing cells