Chapter 9 Muscle and muscle tissue

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth, each serving distinct functions throughout the body.

• Skeletal muscle: Responsible for voluntary movements and is attached to bones.

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

• Smooth muscle: Involuntary muscle found in walls of hollow organs, aiding in processes like digestion and blood flow.

• Skeletal muscle characteristics: Striated appearance, multi-nucleated, and controlled consciously by the nervous system.

• Cardiac muscle characteristics: Striated like skeletal muscle but consists of single cells that are connected at intercalated discs, allowing for coordinated contractions; it is also regulated by the autonomic nervous system.

• Smooth muscle characteristics: Non-striated appearance, spindle-shaped cells that are also controlled involuntarily, allowing for slow and sustained contractions essential for functions such as peristalsis and digestive tract.

• Differences between muscle types:

  • Smooth muscle is primarily found in internal organs;

  • Skeletal muscle is attached to bones for voluntary movements;

  • Cardiac muscle is exclusive to the heart, facilitating rhythmic contractions crucial for blood circulation.

  Characteristics of Skeletal Muscle

• Excitability (responsiveness) : Refers to the ability of a cell to receive and respond to a stimulus by changing its membrane potential (which a stimulus is a chemical)

• Contractility: This property allows muscle fibers to shorten and generate force, enabling movement and stability of the skeleton.

• Extensibility: This characteristic refers to the ability of muscle fibers to be stretched while still being able to return to their original shape after the stretching force is removed.

• Elasticity: What allows them to recoil and resume its resting length after strteched

Muscle Functions:

• Produce movement: Responsible for locomotion and manipulation

• Maintain posture: helps stabilize joints and ensures the body maintains an upright position.

• Stabilize joints:plays a crucial role in supporting the skeleton and preventing injuries during physical activities.

• Generate heat: Muscles generate heat as they contract, which plays a role in maintaining body tepm.

Connective Tissue Sheaths:

• Epimysium: Dense Irregular connective tissue surrounding the whole muscle

• Perimysium: Fibrous connective tissue that surrounds individual fascicles within the muscle.

• Endomysium: A fine sheath of connective tissue surrounding each muscle fiber, providing structural support and facilitating the exchange of nutrients. Consisting of areolar connective tissue

  The sheaths of the muscle provide entry and exit for blood vessels and nerve fibers that serve the muscle.

Attachements:

• Insertion : The point where a muscle attaches to the bone that moves during contraction, allowing for the action of that muscle to effectively change the position of the body part.

• Origin: The point where a muscle attaches to the stationary bone, serving as the anchor point during contraction and playing a crucial role in stabilizing the movement.

• Indirect attachments: These are connections where the muscle does not directly attach to the bone; instead, the muscle's connective tissue forms a tendon (aponeurosis that connects to the bone, providing flexibility during movement while maintaining strength.

Skeletal muscle fibers:

• Sarcolemma: The specialized plasma membrane surrounding each muscle fiber.

• Sacroplasma: the cytoplasma of the muscle cell that contain gyclosomes (granules of stored gylcogen that provide glucose during ATP) and Myoglobin, red pigment that stores oxygen

• Myofibrils: Long, thread-like structures within the muscle fiber that contain the contractile units called sarcomeres, which are essential for muscle contraction.

• Striations : repeated dark and light bands along the length of each myofibrils.

• A Bands are dark and I bands are lights, giving the cell the appearance.

• A band has a lighter region called “H” Zone. Each H zone is bisected by a dark line called M line which is formed by molecules

• Each I band has a midline interruption called Z disc

• Sarcomere: The smallest functional unit of a muscle fiber, comprised of thick (myosin) and thin (actin) filaments, arranged in a specific pattern to facilitate contraction.

• Myoilaments: Smalled structures within the sarcomere. Which contain two type of contractile myofiliaments.

• Thick filaments: containing myosin that extend to the length of the A band and connected to the middle of the sarcomere

• Thin filaments: Contain actin across the I band and partway into the A band. The Z disc, a protein sheet, anchors the thin filaments. We describe the third type of myofilament,

• The H zone of the A band appears Jess dense because the

  thin fila,nents do not extend into this region.

• The M line in the center of the H zone is slightly darker because of the fine protein strands there that hold adjacent thick filaments together.

• The myofilaments are held in alignment at the Z discs and the M lines, and are anchored to the sarcolemma at the Z discs.

Molecular Composition of Myofilaments
- Muscle contraction depends on Actin and Myosin.

• Each myosin consist of six polypeptied chains. Two heavy chain and four light chains. Each heavy chain ends in a gobular head.

• Gobular head: They link the thick and thin filaments forming a cross bridge.

• Each thick filament contains 300 myosin molecule.

• Thin filaments are composed of protein actin.

• Actin contains a polypeptied subunits called G-actin (globular actin), which polymerize to form long chains known as F-actin (filamentous actin) which act as the back bone of the thin filaments.

• Polypetide strands of Tropomysion (protein) help stiff and stabilize the thin filaments. Tropomysion block mysion sits on actin so mysion gobular heads cannot bind to the filament.

• Troponin (another protein) is a gobular protein with three polypeptied subunits. One subunit attaches troponin to actin and another subnit binds tropomysion and helps position actin, another bind Ca2+.

• Elastic filaments: composed of protien titian, which extends to Z disc to thick filament to thin and attach to M line, Titian helps muscle cell sping back into shape after stretching.

• Dystrophin, another important protein that links thin filaments to the integral protein of the sarcolemma

Sacroplasmic Reticulum (SR) ansT tubules.

• The SR and T Tubules are two sets of intracellular tubules that help muscle contraction.

• Sarcoplasmic Reticulum plays a critical role in regulating calcium ion concentration. It stores calcium and releases it on demand when contracted.

• SR Tubules are longitudinally aling the myofibril communicating with each other at the “H” zone.

• T-Tubules: These tubular extensions of the sarcolemma penetrate into the muscle fiber and facilitate the rapid transmission of action potentials. The impulse down the t-tubles release trigger response to the release of Calcium (Ca2+) from Terminal Cisterns.

• Triads form the three membranuous structure: T-tubles and Two terminal Cisterns.

The role of the Traid:

• The protruding interegral protein of the T tubule act as voltage sensors and The Integral protein of the SR form gated channled throughout which the terminal cisterns release Ca2+

Sliding fillaement Contraction

• Contraction refers to the activation of mysion’s cross bridge.

• Sliding filament of contraction states that during contraction the thin filaments slide part the thich filaments, so myosin and actin overlab , but neither change length.

• Cross bridge attachments form and break every single time during.

• I bands shorten, Z diszcs shortent. Z disc attach and are pulled toward M line, H zone disappear and the contigous A band moves closer but length stays the same.

How Muscle contraction work:

• Neurons and Muscles respond to external stimuli by changing resting potential. The changes in a resting potential are known as electrical signals known as Action Potentials (AP) which is a change in the membrane that spread over the cell.

• Chemical signal diffuse across small gaps between cells (Neurotransmitter)

• Acetylocholine (ACh) is the primary neurotransmitter that transmits signals from neurons to muscle cells, initiating the process of muscle contraction.

Ion Channels

• Changing the membrane potential in neuron require opening and closing of membrane channels. Which allow for the movement of ions throught these channels and changes in the membrane voltage.

• Chemically gated ion channels open chemically gated ion channels are opend by neurotransmitter, which creates a local changes. ACh receptor that is single protein in the plasma membrane that is both receptor and ion channel. These channels is the initial change in the membrane and cause a local depolarization that triggers the voltage gated ion channels to create a AP.

• Voltage gated ion channels open or close to the response in the membrane potential. These channels underlie all AP in the skeletal muscle.

Anatomy of Motor neurons and the Neuromuscular Junction:

• Motor neurons reside in the spinal cord , containing a threadlike extension known as Axon, that extend from the cell body in the spinal cord to serve the muscle fibers. Axons exit spinal cord and pass throughout body as nerves. When it reaches muscle fibers axon divides and form oval neuromuscular juntion or motor neuron plane.

• Each muscle fiber has only one neuromuscular juntiion.

• Each axon has has an end called the Axon terminal which are close to the muscle fibers but are seperared by a synaptic cleft which is filles with extracellular fluid (glycoproteins and collagen fibers)

• At the axon terminal are synaptic vesicles that contain the neurotransmitter Acetylcholine (ACh), which is released into the synaptic cleft to initiate muscle contraction.

• The neuromuscular junction is the region where motor neuron contacts skeletal muscle and it consist of multiple axon terminal and underly junctional folds.

Sequence of events that lead to muscle contraction :

1. Events at the neuromuscular junction:

   • A motor neruon fires an AP down the axon

   • Motor neuron axon terminal release ACh into the synaptic cleft

   • ACh binds to the receptors in the junctional folds

   • ACh binding causes a local depolarization called end plate potential (EPP)

2. Muscle fiber excitation:

   • The local depolarization triggers an AP in the adjacent sarcolemma.

3. Excitation contaction coupling

   • Ap in sarcolemma travels down T-tubules.

   • Sarcoplasmic reticulum release CA2+

   • Ca2+ binds to troponin which shifts tropomyosin to uncover the myosin binding sites on actin. Myosin heads bind to actin.

4. Cross bridge formation:

   • Contraction occurs via cross bridge cycling.

Events of the neuromuscular Junction:

1. Action potential arrives at axon terminal of Motor neuron

2. Voltage gated Ca2+ channels open. Ca2+ enters the axon, moving down electrochemical gradient.

3. Ca2+ entry cause ACh to be released by exocytosis

4. ACh diffuses across synaptic cleft and binds to ACh receptors on the sarcolemma

5. ACh binding opens chemically gated ion that allow sodium ions to enter the muscle fibers and K+ out of the muscle fibers. (More NA+ ions to enter and K+ ions to exit, which produces local change in the membrane potential called EPP)

6. ACh effects are terminated by its breakdown in the synaptic clift and diffued away from the junction.

EPP:

1. An end plate potential is generated at the neuromuscular junction:

   EPP causes a wave of depolarization that spreads to the adjacent sarcolemma

2. Depolarization of the sarcolemma opens voltage gated sodium channnels. Na+ enters, following electrochemical gradient. AP is generated and spread in adjacent areas of the sarcolemma and opens voltage gated Na+ channels, propagating

3. Repolarization: Restoring the sarcolemma to its initial polarized state. (Negative. inside and positive outside) Voltage gated Na+ chnnels close and Voltage gated channels open. The potassium ion is higher inside the cell than inthe extracellular fluid, leading to K+ leaving the muscle fiber. This restores the negatively charge conditions inside which are the characteristics of a sarcolemma at rest.

Excitation - Contraction coupling:

• Sequence of events when a trasnmission of AP along sarcolemma cause mypofilaments to slide.

Steps in E-C coupling:

1. AP propagates along the sarcolemma and down T tubules

2. Calcium ions are released: AP transmission along T-tubles cause the release of Ca2+ from the terminal cisterns of the sarcoplasmic reticulum allowinf Ca2+ into cytosol.

3. Calcium binds to troponin and removes blocking tropomyosin

4. Contraction begins: Myosin binding to actin forms cross bridge and contraction beings (E-C Coupling is over)

Generation of and Action Potential across sarcolemma:

• Followed by action potential depolarization and repolarization.

• During repolarization a muscle fiber is at a refactory period where it cannot respond to further stimulation, ensuring that the muscle can only contract in an orderly and controlled manner. The ATP - dependent Na+ and K+ pump restores the ionic conditions of the resting states.

Cross Bridge cycling:

• Cross bridge binding formation requires Ca2+

• When Ca2+ is low is low muscle is relaxed. When Ca2+ is high, muscle contraction occurs as the myosin heads attach to actin. Two Ca2+ must bind to troponin causing change shape and roll tropomyosin.

Graded Skeletal Muscle contraction:

• Muscle tension: Force exerted by contracting muscle on an object.

Motor Unit :

• A motor unit consists of a single motor neuron and all the skeletal muscle fibers it innervates.

• The number of motor unites per motor fiber differ depending of the type of muscle control. In general, smaller motor units are found in muscles that need fine control, such as those in the eye, while larger motor units are present in muscles responsible for generating greater force, such as those in the legs.

Muscle Twitch: A muscle twitch is a brief contraction of a muscle fiber in response to a single stimulus, characterized by its three distinct phases: the latent period, contraction phase, and relaxation phase. Muscle contraction is measured by a muscle being attached to an apparatus that produces a myogram and records the contractile activity.

• Lantent period is the first few miliseconds. Durinf this period cross bridge begin to cycle but muscle tension is not measurable to

• Period of contraction cross bridge is active and onset to the peak of tension development.

• Period of relaxation lasting 10-100 ms due to pumping of Ca2+ back to the SR.

Grading muscle contraction:

• Muscle twitch can be a result to a neuromuscular problem.

• General Muscle contraction is graded in two ways

  1. Increase in the frequency f stimulation causes temporal summation ( the higher the frequency the greater the strength of contraction of a given motor unit)

  2. An increase strength of stimulation cause a recruitment ( the stronger the stimulation the more motor unit are activated the stronger the contraction)

• The nervous system can achieve greater muscular force by increasing the firing rate of motor neurons.

• Seconfd contraction is greater than the first because the muscle is already partially contracted and because even more calcium is in the cytosol.

If the muscle is stimulated at and increasingly faster rate:

• The relaxation time between twitches becomes shorter and shorter

• The concentration of Ca2+ in the cytosol rise higher and higher

• The degree of wave summation increases, processing to a sustained but quivering contraction referred to and Unfused of incomplete tetanus.

• When the muscle relaxation disappears and the contraction fuse into smooth sustained contraction it is known as a fused or complete tetnus.

Multiple motor unit summitation: controls the force of contraction more percicely.

• Subthreschold stimuli - stimuli that produce no observable contractions

• Threshold stimulus is the first observable contractions

• Maximal stimulus : strongest stimulus that increase contractile force

Muscle tone: when muscles are slightly contracted.

Isotonic contraction: when the muscle tension developed overcomes the load and muscle shortens. Consisting of two contractions:

1. Concentric contraction : when muscles shortens and does work. Example: picking up a book

2. Eccentric contraction: occurs when a muscle actively contracts while it is being stretched, resulting in the muscle lengthening while still producing tension, such as downward phase of a bicep curl. When you straighten your arm to the side after a bicep curl, it exemplifies an eccentric contraction as the bicep elongates while still maintaining tension against the weight.

Isometric Contractions: when muscle tension develops but the load does not move. The muscle neither shortens or lengthens due to the load being greater than the force.

Providing Energy for Contraction:

• As muscle contracts it is using ATP supplies the energy to move and detach the cross bridge it operated the Ca2+ pump in SR and operate Na+ - K+ pump in the plasma membrane.

• Three pathways regenerate ATP within a fraction of a second: A: Direct phosphorylation of ADP by creatine phosphate B: Anarobic glycolysis which converts glucose to lactic acid C: Areobic recperation.

Direct Phosphorylation:

• ATP stored in working muscles is consumed within a few twitches. CP (creatine phosphate, a unique energy molecule stored in muscles) is tapped to regenerate ATP.

Creatine phosphate + ADP —— (Creatine kinase) —- Creatine + ATP.

• Muscle cells store more CP than ATP. Together CP and ATP provide a maximum muscle power for 15 seconds.

Anaerobic Pathway:

• As ATP and CP are exhausted, more ATP is generated by breaking down (catabolizing) glucose from the blood or glycologyen stored in muscles.

• Glycolysis is the initial phase of glucose breaking down. Due to it being not present with oxygen it is called an “Anaerobic pathway." Oxygen delivery is impaired because bulging muscles compress the blood vessels within them In this process, glucose is converted to pyruvate acid molecules, which can then lead to the production of energy to form ATP.

• When both the oxygen and blood flow are impaired during muscle contraction it is called a lactic acid. This accumulation of lactic acid can lead to muscle fatigue , due to it being the end product of glucose.

• When enough oxygen is present the pyruvic acid produced during glycolysis enters the mitochondriand and still produce more ATP in the oxygen pathway called aerobic respiration.

Aerobic Respiration:

• 95% of the ATP used for muscle activity comes from aerobic resperation. Requiring oxygen and mitochondria.

• It begins witth glycolysis and breaks down to glucose to water and carbob dioxide and generates large amounts of ATP.

• Aerobic Endurance; The ammount of time a muscle can continue to contract.

Muscle Fatigue:

• Is the state of physiological inability to contract. Involving chemical changes such as:

• Ionic imbalance

• Increased inorganic phosphate

• Decreased ATP and Increased magnesium ions can also contribute to impaired muscle function, leading to fatigue and decreased muscle performance.

• Decreased Glycologen

Muscle Contraction : depends on four factors

1. Frequency of stimulation

2. Number of muscle fiber recruited

3. Size of muscle fibers

4. degree of muscle stretch

Velocity and Duration of Contraction influenced by muscle fibers , load, and recruitment.

• Speed of contraction slow and fast fibers

• Major pathways for formi ng ATP. The cells that rely mostly on the oxygen-using aerobic pathways for ATP generation are oxidative fibers. Those that rely more on anaerobic glycolysis and creatine phosphate are glycolytic fibers.

Different Type of smooth muscle :

Unitary Smooth Muscle (Visceral Muscles) : due to it being on the walls of the hallow organs except the heart muscle.

• Smooth muscle are innervated by varicosities of autonomic nerve fibers

• Electrically coupled by gap junctions

• responf to various chemical stimulie

Multi Unit Smooth muscle : Smooth muscle that is in large airways , arrector pili, and internal eye muscles

• Gap Junctions and Depolarization is absent

• richly supplied with nerve endings

• Respond to neural stimulation with graded contractions that involve recruitment.