Muscle Tissue Overview

Overview of Muscle Tissue

• Muscle cells optimize contractility, originating from the mesoderm.
• Differentiation involves gradual cell lengthening and increased synthesis of myofibrillar proteins like actin and myosin.

Three Types of Muscle Tissue

• Skeletal muscle features long, multinucleated cells with cross-striations, enabling quick, forceful, and voluntary contractions.
• Cardiac muscle also has cross-striations, made of branched cells connected by intercalated discs, promoting involuntary, vigorous, and rhythmic contractions.
• Smooth muscle is composed of fusiform cells without striations, resulting in slow, involuntary contractions.

Muscle Contraction

• Contraction in all muscle types occurs via the sliding of thick myosin filaments along thin actin filaments.
• Forces for sliding are generated by proteins that affect weak interactions in the bridges between actin and myosin.

Special Terminology for Muscle Cell Organelles

• Cytoplasm is referred to as sarcoplasm.
• Smooth ER is the sarcoplasmic reticulum.
• The cell membrane and its external lamina form the sarcolemma.

Medical Application: Muscle Fiber Variation

• The variation in muscle fiber diameter depends on muscle, age, gender, nutrition, and physical training.
• Exercise leads to the enlargement of skeletal muscles by stimulating myofibril formation and increasing the diameter of individual muscle fibers.
• This process is called hypertrophy, in which cell volume increases.
• Hyperplasia, tissue growth by increasing the number of cells, readily occurs in smooth muscle due to its retained mitotic capacity.

Muscle Contraction Mechanism

• Sliding interaction of thick myosin filaments along thin actin filaments causes contraction.

Structure of Skeletal Muscle

• Skeletal muscle is striated and consists of long, cylindrical, multinucleated fibers.
• Nuclei are elongated and located peripherally, just under the sarcolemma.
• Satellite cells are reserve progenitor cells adjacent to differentiated muscle fibers.

Endomysium, Perimysium, and Epimysium

• The endomysium, a delicate layer of reticular fibers and fibroblasts, surrounds individual muscle fibers inside the external lamina.
• The perimysium is a thin connective tissue layer that immediately surrounds bundles of muscle fibers, known as a fascicle.
• The epimysium, an external sheath of dense connective tissue, surrounds the entire muscle, and septa carry larger nerves, blood vessels, and lymphatics.

Oxygen Supply to Muscle Fibers

• Capillaries form a rich network in the endomysium, delivering O2 to fibers.

Organization of Skeletal Muscle Fibers

• Longitudinal sections of skeletal muscle fibers show cross-striations of alternating light and dark bands.

Banding Patterns

• A bands are dark and anisotropic, or birefringent, in polarized light microscopy.
• I bands are light and isotropic, located between A bands, bisected by a dark transverse Z disc in the TEM.

A Band Structure

• Close observation of the A band reveals a lighter H zone in the center, which contains only rodlike portions of the myosin molecule and no thin filaments.
• Bisecting the H zone is the M line, which contains myomesin, holding the thick filaments in place, and creatine kinase.

I Band Components

• I bands, each bisected by a Z disc, consist of the portions of the thin filaments that do not overlap the thick filaments.

Myofilaments

• Thick and thin filaments compose the contractile protein arrays inside myofibrils.

Thick Filament Composition

• A myosin protein is the major protein comprising the thick filaments.

Thin Filament Composition

• F-actin is a main protein, that comprises thin filaments.

Regulatory Proteins

• Thin filaments tightly associate with regulatory proteins like tropomyosin and troponin.

Regulatory Protein Functions

• Tropomyosin is a long coil of two polypeptide chains located in the groove between the two twisted actin strands.
• Troponin is a complex of three subunits: TnT attaches to tropomyosin, TnC binds Ca2+, and TnI regulates the actin-myosin interaction.

Accessory Proteins

• Titin, an important accessory protein in I bands, has scaffolding and elastic properties, supporting the thick myofilaments and connecting them to the Z disc.
• Nebulin binds each thin myofilament laterally, helps anchor them to α-actinin, and specifies the length of the actin polymers during myogenesis.

Sarcomere State in Relaxed Muscle

• In the relaxed state the sarcomere, I band, and H zone are at their expanded length.
• Titin molecules, spanning the I band, helps pull thin and thick filaments past one another in relaxed muscle.

Sarcomere Contraction

• During muscle contraction the Z discs at sarcomere boundaries are drawn closer together, moving toward the ends of thick filaments in the A band, and titin molecules are compressed.

Sarcoplasmic Reticulum (SR)

• The SR is a membranous smooth ER that contains pumps and proteins for Ca2+ sequestration, surrounding myofibrils.

Transverse Tubules (T-tubules)

• These are long fingerlike invaginations of the sarcolemma.
• The T-tubules trigger Ca2+ release from SR throughout the fiber simultaneously, and cause uniform contraction of myofibrils.
• They permit rapid transmission of the action potential into the cell.

Terminal Cisternae and Triads

• Adjacent to every T tubule are expanded terminal cisternae of the SR.
• The complex of a T tubule with two closely associated small cisterns of SR is known as a triad.
• Complex allows sarcolemma depolarization in a T-tubule to affect the sarcoplasmic reticulum and trigger the release of calcium ions into the cytoplasm surrounding the thick and thin filaments

Rigor Mortis

• In the absence of ATP, actin-myosin bridges become stable after death, causing muscle rigidity (rigor mortis).

Motor Nerve Innervation

• Myelinated motor nerves branch within the perimysium, with terminal twigs passing through the endomysium to form synapses with muscle fibers.
• Schwann cells enclose axon branches and their contact points with the muscle cells, with the Schwann cell's external lamina fusing with the sarcolemma.

Motor End Plate

• Each axonal branch forms a dilated termination situated within a trough on the muscle cell surface, thus forming a synaptic structure.

Acetylcholine

• Acetylcholine is a neurotransmitter for muscle contraction found in the axon terminals within numerous synaptic clefts and associated mitochondria.

Synaptic vs Junctional Clefts

• Synaptic cleft is a space between the axon and muscle.
• Adjacent to the synaptic cleft, the sarcolemma is thrown into deep junctional folds, increasing the postsynaptic surface area for acetylcholine receptors.

Acetylcholine Elimination

• Main neurotransmitter is broken down by acetylcholinesterase, preventing prolonged contact with its receptors.

Motor Unit

• A single axon and all the muscle fibers in contact with its branches.

Muscle Fiber Type Classification

• Classification is based on the maximal rate of contraction (fast or slow fibers) and major pathway for ATP synthesis (oxidative phosphorylation or glycolysis).
• Eye and eyelid muscles need to contract rapidly, while bodily posture muscles need to maintain tension for longer.

Muscle Fiber Characteristics

• Muscles often active with slow contractions and for long periods tend to have more mitochondria for oxidative phosphorylation and ATP production and to have a higher density of surrounding capillaries.

Myoglobin

• This a is cytoplasmic protein containing iron and stories oxygen molecules (gives fibers a red colour).

Slow oxidative muscle fibers

• Adapted for slow contractions occurung over long periods without fatigue.
• They are characterized by lots of mitochondria, surrounding capillaries, and myoglobin, giving the fibers a dark or red color.

Fast glycolytic fibers

• Are specialized for rapid, short term contraction.
• The have Limited mitochondria or capillaries.
• They primarily use anaerobic metabolism of glucose derived from stored glycogen, making the fibers appear white.
• Rapid contractions leads to rapid fatigue as lactic acid produced by glycolysis accumulates.

Fast oxidative-glycolytic fibers

• Have intermediate physiological and histological features to those of the other two fiber types.

Mesenchymal vs Cardiac Development

• Mesenchymal cells around the primitive heart tube align into chainlike arrays (rather than fusing as in skeletal muscle).
• Cardiac muscle cells form complex junctions between interdigitating processes.

Cardiac Muscle Organization

• The fibers consists of separate cells joined at interdigitating regions called intercalated discs.
• The transverse regions of these discs have abundant desmosomes and other adherent junctions for firm adhesion.
• The longitudinal regions of the discs having gap junctions.

Cardiac Contraction

• Intrinsic and happens spontaneously. is intrinsic and spontaneous.
• Heartbeat impulses are initiated, regulated, and coordinated by nodes of unique myocardial fibers, are Intiated, regulated, and coordinated locally, and that are specialized for impulse generation and conduction.

Smooth Muscle

• Visceral, specializes for slow, steady contraction, and controlled variably, which is involuntary.
• The are major component of blood vessels and respiratory, urinary, and reproductive tracts.
• Elongated, tapering, and unstriated with a network (Type 1 and 3) collagen fibers (comprising the endomysium).
• Is not under voluntary motor control and its fibers typically lack well-defined neuromuscular junctions.
• Most commonly stimulated by autonomic nerves.

Nervous tissue overview

• The nervous system regulates internal conditions and maintains behavioral patterns through collecting, analyzing, and integrating information.
• Major divisions: the central nervous system (brain & spinal cord) and the peripheral nervous system (cranial, spinal, and peripheral nerves & ganglia).
• Neurons are responsive to environmental stimuli, creating long processes to signals to other neurons, muscles, and glands.
• The cells are the nerves (numerous long processes) and glial (short processes), and supporting.
• Both, stabilize intrinsic conditions of the body within normal ranges.

Nerve tissue development

• Develops from the ectoderm.
• Axial structure, thickens the neural plate.
• Migrating extensively and differentiating, its migrate extensively.

Neurons characteristics

• Multipolar (one axon and two or more dendrites).
• Bipolar (one dendrite and one axon – retina and olfactory).
• Unipolar (a single process, bifurcates close to the perikaryon).
• Anaxonic (many dendrites with no axon – does not produce action potentials, but regulate electrical changes).
• Has 3 parts: cell body (the nucleus) and dendrites long processes).

Nissl Bodies

• Are the RER (indicates active production cytoskeletal proteins).
• Intermediate Filaments neurofilaments.

PNS, CNS, and Grey Matter

• Perikarya occur in the grey, with axons that are concentrated in the CNS’ white matter.
• Is bundle from the nerves in ganglia.

Dendrites Structure

• Short process Subdivision, that is a signal reception and processes.

###Axons structure

• Is specialize to transport signals.