Muscle Tissue Overview

Types of Muscle Tissue

Muscle tissue is one of the four primary tissue types in the human body, alongside epithelial, connective, and nervous tissues. There are three main types of muscle tissue, each with distinct structure and function:

• Skeletal Muscle Tissue: Voluntary muscles attached to bones, responsible for body movement.

• Cardiac Muscle Tissue: Involuntary muscle found only in the heart, responsible for pumping blood.

• Smooth Muscle Tissue: Involuntary muscle found in walls of hollow organs (e.g., intestines, blood vessels), controlling internal movements.

Properties of Muscle Tissue

• Excitability (Responsiveness): Ability to respond to stimuli.

• Contractility: Ability to shorten forcefully.

• Extensibility: Ability to be stretched without damage.

• Elasticity: Ability to return to original length after stretching.

Functions of Skeletal Muscle Tissue

Major Functions

Skeletal muscle tissue performs several essential functions in the body:

• Produce Skeletal Movement: Primary function; muscles pull on bones to create movement.

• Maintain Posture and Body Position: Continuous muscle contractions stabilize body positions.

• Support Soft Tissues: Muscles support and protect internal organs.

• Guard Entrances and Exits: Sphincter muscles control openings of digestive and urinary tracts.

• Maintain Body Temperature: Muscle contractions generate heat.

• Store Nutrient Reserves: Proteins in muscle fibers can be broken down for energy.

Organization of Skeletal Muscles

Components of a Skeletal Muscle

• Muscle Tissue (muscle fibers or cells)

• Connective Tissues

• Nerves

• Blood Vessels

Connective Tissue Organization

• Epimysium: Outermost layer of collagen fibers surrounding the entire muscle; separates muscle from surrounding tissues and connects to deep fascia.

• Perimysium: Surrounds bundles of muscle fibers called fascicles; contains blood vessels and nerves.

• Endomysium: Surrounds individual muscle fibers; contains capillaries, nerve fibers, and myosatellite cells (stem cells for repair).

Muscle Attachments

• Tendon: Bundle of connective tissue fibers attaching muscle to bone.

• Aponeurosis: Sheet-like connective tissue attachment.

Vascular and Nervous Supply

• Muscles have an extensive blood supply for oxygen, nutrients, and waste removal.

• Skeletal muscles are controlled by the somatic nervous system (voluntary), with exceptions for somatic reflexes and subconscious diaphragm control.

Neuromuscular Junction (NMJ) and Muscle Fiber Structure

Key Structures

• Neuromuscular Junction (NMJ): Site where a motor neuron communicates with a muscle fiber.

• Sarcolemma: Plasma membrane of a muscle fiber; excitable like a neuron's membrane.

• Sarcoplasm: Cytoplasm of a muscle fiber.

• Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases calcium ions (Ca2+).

• Terminal Cisternae: Enlarged ends of the SR containing voltage-gated Ca2+ channels.

• T-tubule (Transverse Tubule): Invaginations of the sarcolemma that transmit action potentials deep into the muscle fiber.

• Triad: Structure formed by a T-tubule and two adjacent terminal cisternae.

Muscle Fiber Anatomy

• Myofibrils: Cylindrical structures within muscle fibers, composed of repeating units called sarcomeres.

• Sarcomere: The functional contractile unit of a muscle fiber, extending from one Z disc to the next.

Sarcomere Structure and Myofilaments

Myofilaments

• Thin Filament: Composed mainly of actin (specifically, globular actin or G-actin). Two strands of G-actin form a dimer, and each G-actin has an active site for myosin binding.

• Thick Filament: Composed of myosin molecules, each with a head (binds actin and hydrolyzes ATP) and a tail.

Zones and Bands of the Sarcomere

• Z disc: Boundary of each sarcomere.

• A band: Dark region containing overlapping thick (myosin) and thin (actin) filaments.

• I band: Light region containing only thin (actin) filaments.

• M-line: Center of the sarcomere, supporting thick filaments.

• H-zone: Central region of the A band with only thick filaments (no actin).

Regulatory Proteins

• Troponin-Tropomyosin Complex: Regulates access to actin's active sites.

• Troponin: Binds Ca2+; when bound, it moves tropomyosin away from actin's active sites.

• Tropomyosin: Blocks actin's active sites when troponin is not bound to Ca2+.

Mechanism of Muscle Contraction

Sliding Filament Theory

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere and thus the muscle fiber. This process is powered by ATP and regulated by Ca2+ availability.

1. Ca2+ is released from the SR; ATP is required for contraction.

2. Cross bridge cycle occurs (myosin binds actin, pulls, detaches, and re-cocks).

3. Actin slides toward the M-line, shortening the sarcomere.

Events at the Neuromuscular Junction (NMJ)

1. Action potential arrives at the axon terminal; voltage-gated Ca2+ channels open.

2. Ca2+ influx triggers release of acetylcholine (ACh) into the synaptic cleft.

3. ACh binds to receptors on the motor end plate, causing Na+ influx and depolarization of the sarcolemma.

4. Depolarization triggers opening of voltage-gated Ca2+ channels in the terminal cisternae.

5. SR releases Ca2+ into the sarcoplasm, initiating contraction.

Cross Bridge Cycle

1. Cross Bridge Formation: Myosin head binds to actin; inorganic phosphate (Pi) is released.

2. Power Stroke: ADP is released; myosin head pivots, pulling actin toward the M-line.

3. Detachment: ATP binds to myosin head, causing it to release from actin.

4. Reactivation: ATP is hydrolyzed to ADP + Pi; myosin head returns to cocked position.

Ending Muscle Contraction

1. No more action potentials; ACh release stops.

2. Acetylcholinesterase (AChE) breaks down remaining ACh.

3. Ca2+ is pumped back into the SR; troponin releases Ca2+.

4. Tropomyosin covers actin's active sites; cross bridges cannot form.

Rigor Mortis

• After death, ATP production ceases; myosin heads cannot detach from actin.

• Muscles become rigid due to persistent cross bridges.

Pharmacological and Pathological Effects

Curare

• Acts as a competitive antagonist at ACh receptors at the NMJ.

• Prevents action potential propagation; causes paralysis and potentially fatal asphyxiation.

Sarin Gas

• Potent inhibitor of acetylcholinesterase (AChE).

• Causes accumulation of ACh in the synaptic cleft, leading to continuous muscle contraction and death by respiratory paralysis.

Effects of Repeated Stimuli on Muscle Contraction

Key Terms and Definitions

• Excitability: Ability of muscle tissue to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil to resting length after stretching.

• Myosatellite Cells: Stem cells in muscle tissue involved in repair.

• Acetylcholine (ACh): Neurotransmitter that stimulates muscle contraction.

• Acetylcholinesterase (AChE): Enzyme that breaks down ACh in the synaptic cleft.

Summary Table: Sarcomere Bands and Zones

Key Equations

• ATP Hydrolysis by Myosin:

• Calcium Active Transport (Sequestration):

Example: Sequence of Skeletal Muscle Contraction

1. Action potential arrives at NMJ; ACh is released.

2. ACh binds to receptors, causing depolarization of sarcolemma.

3. Depolarization spreads via T-tubules to SR, releasing Ca2+.

4. Ca2+ binds troponin, moving tropomyosin and exposing actin sites.

5. Myosin heads bind actin, initiating cross bridge cycling and contraction.

6. Contraction ends when Ca2+ is pumped back into SR and ACh is degraded.

Additional info: The above notes expand on the original lecture outline by providing definitions, context, and examples for each key concept, as well as summarizing the physiological and pharmacological relevance of muscle contraction mechanisms.