Introduction to Muscle PDF

# Introduction to Muscle ## Introduction - **Question:** What is a muscle? - **Answer:** Tissue specialized to convert biochemical reactions into mechanical work. ## Muscle Function - Two main functions are to generate: - **Contraction** - **Expansion** - except when physically pulled by...

# Introduction to Muscle ## Introduction - **Question:** What is a muscle? - **Answer:** Tissue specialized to convert biochemical reactions into mechanical work. ## Muscle Function - Two main functions are to generate: - **Contraction** - **Expansion** - except when physically pulled by antagonistic muscle groups - Also generate heat & contribute to body temperature homeostasis. ## Muscle Types - Three types of muscle in the human body: - **Skeletal** - Attached to bones of the skeleton - Contract in response to a signal from a somatic motor neuron - Cannot initiate contraction on its own or be influenced by hormones - **Smooth** - Primary muscle of internal organs and tubes (e.g., stomach, blood vessels, urinary bladder) - Influences the movement of materials through the body. - **Cardiac** - Found only in the heart - Pump to move blood around the body - We will mainly focus on skeletal muscle in this unit. ## Gross Structure of Skeletal Muscle ### Characteristics - Responsible for positioning and movement of the skeleton - Skeletal muscles make up ~40% of body weight ### Structure - Outer connective tissue → **epimysium** - Contains bundles of muscle tissue called **fascicles** - Fascicles are covered by the **perimysium** → a connective tissue sheath - Nerves and blood vessels - Muscle fibers (muscle cells) are found within each fascicle - Covered by an innermost connective tissue sheath → **endomysium** - Within the muscle fibers are the functional units of skeletal muscle → **myofibrils** - Contain so many myofibrils that there is little room for other organelles - Cytosol contains many glycogen granules (energy storage) & mitochondria (ATP synthesis) ## Structure of a Muscle Fibre - Long, cylindrical cell - Several hundred nuclei on the surface of the fiber - Cell membrane is called the **sarcolemma** - Majority of the space is taken up by the **myofibrils** - Myofibrils are contractile and elastic protein bundles ## Organization of the Myofibril ### Characteristics - Myofibrils occupy most of the space in a muscle fiber - They are highly organized and consist of bundles of contractile elastic proteins. - **Contractile proteins** → actin & myosin - **Regulatory proteins** → troponin & tropomyosin - **Accessory proteins** → nebulin (aligns thin filament) & *titin* (elastic protein that returns stretched muscle to its relaxed state) ### Contractile Proteins - **Myosin** - A motor protein that consists of two coiled protein molecules (chains) that have two important parts → head & tail region - These two regions are joined by a flexible hinge - About 250 myosin molecules join - Arranged so that the heads are at the ends and the tails are together - **Actin** - Subunits G-actin (globular actin) - The G-actin subunits polymerize to form chains (F-actin) - Two F-actin chains twist together - this forms the basis of the thin filament - The coiled F-actin associates with regulatory proteins → *troponin* and *tropomyosin* - These proteins regulate muscle contraction. - Forms the completed thin filament. ## The Sarcomere - When observed under the light microscope, the myofibrils have stripes called striations (skeletal muscle = striated muscle) - The stripes were given names before anyone knew what they were for. - One repeated pattern of this unit forms a **sarcomere** - A sarcomere is made up of organized thick and thin filaments that results in the striations seen in skeletal muscle. - **Z-Line (disks)** → this is the site of attachment for thin filaments - One sarcomere is made of 2 Z discs & the filaments between them. - **I Band (isotropic)** → this is a region containing only thin filaments - A Z disc runs through the middle of an I band → thus each ½ of the I band is part of a different sarcomere. - **A Band (anisotropic)** → this is a region containing thick and thin filaments - The thick and thin filaments overlap at the outer edges of the A band. - **H Zone (part of the A band)** → this is a region containing only thick filaments. - The central region is lighter than the outer edges - **M Line** → this is a site of attachment for the thick filaments - The M line is the center of the sarcomere. ## Skeletal Muscle Contraction - Skeletal muscles only contract when stimulated by a signal from the nervous system. - Excitation-contraction coupling is the series of electrical and mechanical events in a muscle that leads to muscle contraction. - Occurs through an action potential in the muscle membrane → the EPPs are essentially always above threshold and cause muscle to contract. ### Contraction 1. **ACh** is released by the neuron into the synaptic cleft at the neuromuscular junction and binds to **nicotinic cholinergic receptors** on the motor end plate → the receptors are *Na+/K+ channels*. 2. The binding of **ACh** opens the channels → both *Na* & *K+* move across the membrane. - **ACh** is removed by *acetylcholinesterase*. - *Na+* influx exceeds *K+* efflux → local depolarization occurs at the synapse (called an *End Plate Potential - EPP*). 3. The **end plate potential** (action potential) then moves down the T-tubule system. - T-tubule membrane contains **dihydropyridine receptors (DHP receptors)** → L-type calcium channel - Depolarization changes the conformation of the DHP receptors. - DHP receptors are mechanically linked to the *Ca2+* channels of the *SR* known as *ryanodine receptors - RyR*. 4. **DHP receptor** then changes *RyR* conformation which results in the opening of *SR Ca2+* channels → *Ca2+* leaves the SR. - This increases cytosolic [Ca2+]. 5. *Ca2+* binds to **troponin** (a regulatory protein) on the thin filament → This shifts **tropomyosin** (another regulatory protein) into the "on" position revealing actin binding sites. 6. **Myosin** is now able to bind to actin and go through the cross bridge cycle. ## Crossbridge Cycle - **Myosin** is a motor protein capable of converting chemical energy (ATP) into movement. - Crossbridge cycling is easiest to understand when illustrated step-by-step: - **Step A:** Active site on *actin* is exposed when *Ca2+* binds to *troponin*. - **Step B:** Myosin head binds to *actin* at the actin binding site and forms a crossbridge. - **Step C:** The inorganic phosphate releases from *myosin* that causes the myosin head to pivot toward the center of the sarcomere (called the power stroke). - This pulls the thin filament towardsthe M line. - **Step D:** A new molecule of *ATP* attaches to the myosin head, causing the crossbridge to detach. - **Step E:** The myosin ATPase hydrolyses *ATP* → *ADP* + *P* which returns *myosin* to the cocked position. - Return to step B to continue the cycle if *Ca2+* is bound to *troponin*. - During contraction, the crossbridges do not all move simultaneously. - While some crossbridges are attached to *actin*, others are not. - At any one instance during contraction about 50% of the cross bridges are attached and produce contraction in the muscle. ## Skeletal Muscle Relaxation - Relaxation of skeletal muscle occurs when *Ca2+* is pumped back into the *SR* through the *Ca2+-ATPase*. - ↓ *Ca2+* in cytosol causes *troponin* & *Ca2+* to unbind. - The unbinding of *Ca2+* to *troponin* shifts *tropomyosin* into the "off" position and covers the binding sites on the actin subunits of the thin filament. - *Myosin* heads can no longer bind to the actin subunits. - Elastic elements pull filaments back to the relaxed position when *myosin* unbinds. ## ATP and Muscle Contraction ### Introduction - Muscles convert biochemical energy into mechanical work. - *Ca2+* controls muscle contractions → removed from the cytosol by *Ca2+-ATPase*. - *Na+* and *K+* ions are pumped back into/out of the cell using *ATP*. - *Myosin: Actin* interaction uses *ATP*. ### ATP - *ATP* is the main energy currency of the cell (*ATP* is a nucleotide triphosphate). - Muscle contraction requires a steady supply of *ATP*. - Energy is transferred from nutrients → - Occurs aerobically or anaerobically - *Glycolysis* occurs in the presence (aerobic) or absence (anaerobic) of oxygen. - Provides limited amount of *ATP*. - Generates unwanted metabolites in the absence of oxygen. - *Oxidative Metabolism* requires oxygen present (comes from the air you breathe) - Provides up to 15X more *ATP* per glucose molecule. - Does not produce toxic end products - Oxidative metabolism provides most of the energy required for muscle contraction when oxygen is available. ### Creatine Phosphate - Creatine phosphate is a high energy phosphate molecule. - Muscles have a high concentration of creatine phosphate. - Provides a rapid source of energy for the muscle. - Easily donates the inorganic phosphate to *ADP* to create *ATP* → provides a limited supply of *ATP*. - Used mainly to buffer [*ATP*] over very short time scales (i.e., seconds). - Reaction: creatine phosphate + *ADP* → *ATP* + creatine - Muscles contain large amounts of *CK*. - Resting muscles store energy in creatine phosphate ## Classification of Muscle Fibres - Two important terms relating to muscle contraction: - **Twitch** → single contraction-relaxation cycle - **Latent period** → short delay between the AP & the beginning of the muscle tension - Skeletal muscle is specialized for certain types of activity. - Functions are seen in morphological and biochemical adaptations in the muscle. - Three general types of muscle fibers: - **Slow-twitch fibers (type I)** - **Fast-twitch oxidative-glycolytic fibers (type IIA)** - **Fast-twitch glycolytic fibers (type IIX)** - Oxidative or glycolytic refer to the primary sources of ATP. - Oxidative fibers usually appear red due to the presence of *myoglobin*. - *Myoglobin* → an oxygen-carrying heme protein. - Oxidative fibers are smaller than glycolytic fibers, have numerous mitochondria & are better vascularized. - Fast or slow refers to the rate of *myosin* *ATPase* activity. - Fast fibers can split *ATP* more quickly and can contract/develop tension faster. - Result of the presence of different isoforms of *myosin*. - The length (duration) of contraction also varies between fibers. - Fast fibers have a shorter twitch (they can have more twitches per unit time) - Twitch duration is determined by the rate of removal of *Ca2+* from the cytosol. - Short twitch duration is useful for rapid, small muscle contractions (e.g. playing the piano, typing). - Long twitch duration good for long sustained movements (lifting heavy loads). - **Question:** Which type of muscle has the highest rate of *Ca2+* removal from the cytosol? ## Mechanics Of Muscle Contraction & Body Movement ### Muscle Contraction - **Resting Fibre Length** - The tension exerted by a muscle during a single twitch is influenced by: - The muscle type - Sarcomere length at the start of contraction - Sarcomere length is the degree of overlap between the thick and thin filaments. - **Too little overlap:** - Few crossbridges - Little force can be generated - **Too much overlap:** - Actin filaments start to interfere with each other. - Less force generated. - **Way too much overlap:** - Thick filaments collide with Z disk - Force rapidly decreases. ### Force Of Muscle Contraction - **NOTE:** A single twitch does not represent the maximum force the muscle fiber can develop. - Force of a muscle fiber can be increased increasing the rate of action potentials that stimulate the fiber. ### Summation - Increase in force generated by a muscle. - Due to repeated stimulation from action potentials that occur before the muscle has fully relaxed. ### Tetanus - Term for the state of a muscle when it reaches maximum force of contraction. - **Incomplete (unfused) tetanus:** - Slow stimulation rate → fiber relaxes slightly between stimuli - **Complete (fused) tetanus:** - Fast stimulation rate → fiber does not have time to relax ### Motor Unit - The motor unit is the basic unit of contraction in an intact skeletal muscle. - A muscle is made up of many different motor units. - A motor unit is composed of two components: - A group of muscle fibers - The somatic motor neuron that controls them. - All muscle fibers are of the same skeletal muscle fiber type. - An action potential in the somatic motor neuron → contraction of ALL muscle fibers in each motor unit. - Each motor unit contracts in an all-or-none fashion. ### Body Movement - Two main types of muscle contraction: - **Isotonic** - Creates force and moves a load. - The load is usually constant, and the muscle length changes. - **Isometric** - Creates force without movement. - Muscle length is constant. - The load is usually greater than the force that can be applied. - **Question:** How can an isometric contraction create force if there is no change in muscle length? - **Answer:** Even though sarcomeres shorten, muscle length stays constant because these elastic elements stretch to take up force until fully stretched. # Smooth Muscle ## Gross Structure of Smooth Muscle - **Where is Smooth Muscle Found in the Body?** - Walls of hollow organs & tubes → not attached to bones of skeleton - Some important smooth muscles → bladder sphincter, intestine, walls of blood vessels ## Arrangement of Smooth Muscle Cells - Smooth muscle can be arranged in two different ways: - **Single unit** → - Not necessary to electrically stimulate each individual fiber - Found on walls of internal organs → e.g., blood vessels - **Multi-unit** → - Each individual muscle fiber is separately innervated - e.g., iris of the eye, parts of reproductive organs ## Differences Between Smooth & Skeletal Muscle - **Whole muscle level:** - Contraction of smooth muscle changes muscle shape, not just length. - Smooth muscle develops tension (force) slowly. - Smooth muscle can maintain contraction longer without fatiguing → important because some are contracted most of the time (e.g.) - **Cellular level:** - Fibres much smaller in smooth muscle than skeletal muscle fibres. - About the same diameter as a single myofibril in a skeletal muscle fibre. - Actin & myosin are not arranged into sarcomeres - Thus, no banding pattern (no striations). - Actin & myosin arranged in long bundles diagonally around periphery of the cell. - Actin anchored at cell membrane structures called dense bodies - Not attached to the Z lines as in skeletal muscle - No T-tubules in sarcolemma, not much sarcoplasmic reticulum (SR) - Smooth muscle cells have special vesicles called *caveolae* that are invaginations of the sarcolemma that are specialized for cell signalling. - Force of contraction is related to the amount of *Ca2+* released. ## Smooth Muscle Contraction - **Role of *Ca2+* in Contraction** - Major difference between contraction of smooth muscle & cardiac muscle is the role of phosphorylation in regulating the smooth muscle contraction process. - Signal to initiate contraction is increase in cytosolic *Ca2+*. - *Ca2+* enters from extracellular fluid (ECF) through: - Voltage-gated channels → open when cell depolarizes - Stretch-activated channels → open when membrane stretched - Chemically-gated channels → open in response to hormones - *Ca2+* entry from the ECF results in the release of *SR Ca2+*. - *Ca2+* binds to *calmodulin (CaM)* in the cytosol. - *Ca2+/CaM* activates the enzyme *myosin light chain kinase (MLCK)* - *MLCK* activates *myosin* by phosphorylating the light chain of the *myosin* molecule in the head using energy and *P* from *ATP* → this *ATP* is used to activate the *myosin* through phosphorylation (not for crossbridge cycling) - When *myosin* is not phosphorylated, *ATPase* activity is blocked. - When *myosin* is phosphorylated, *ATPase* is active. - The phosphorylated myosin (active) can now interact with *actin* and go through crossbridge cycling and allow contraction to occur in the smooth muscle cell → remember, additional *ATP* is needed for each crossbridge cycle - *MLCK* uses the *P* from *ATP* to activate *myosin* (turn it "on"), but additional *ATP* is needed to go through crossbridge cycling for contraction to occur - *Note:* in smooth muscle the *myosin* is regulated → via phosphorylation of *myosin* - Whereas *actin* is regulated in skeletal muscle → via *troponin/tropomyosin* interaction with *actin*. - **Relaxation in Smooth Muscle** - Relaxation is a multi-step process: - *Ca2+* is removed from the cytosol - Pumped back into the SR using *ATP* to the extra-cellular environment through → - Decrease in *Ca2+* levels in the cytosol causes *Ca2+* to unbind from *calmodulin* → - *Myosin* light chains are dephosphorylated - Dephosphorylation of *myosin* does not automatically relax the muscle. - This allows the smooth muscle to enter the *latch state* → not fully understood. - Tension is maintained (*myosin* remains bound to *actin*) but with minimal *ATP* consumption. # Cardiac Muscle ## Gross Structure of Cardiac Muscle - Cardiac muscle cells are called **myocardial cells** → these are specialized muscle cells of the heart. - Shares features with both smooth and skeletal muscles - Most myocardial cells are typical striated muscle. - Contractile fibres organized into sarcomeres. - Cardiac muscle differs from skeletal muscle: - Cardiac muscle cells are much smaller with a single nucleus with about 1/3 of the cell volume is occupied by mitochondria. - T-tubules are much larger and branched and the sarcoplasmic reticulum (SR) is smaller. - Adjacent cells are joined by **intercalated discs** with desmosomes. - About 1% of cardiac muscle cells are *NOT* involved in contraction. - They are involved in the electrical excitation of the heart → known as the electrical conducting system of the heart. - They initiate heartbeat & allow the electrical excitation to spread rapidly throughout the heart. - They are connected to other cardiac cells via **gap junctions**. ## Cardiac Muscle Contraction - **Process of Contraction** - Like skeletal muscle contraction EXCEPT: - *Ca2+* enters through *Ca2+* channels on cell membrane as well as the SR. - *First: calcium enters through external* *Ca2+* *channels*. - *Next: calcium-induced calcium release* → *release of stored* *Ca2+* *from SR*. - *SR calcium provides about 90% of that* *needed* *for contraction*. - *Cardiac cells* have a *Na+/Ca2+* *antiport* (in addition to* *Ca2+-ATPase*). - *Removes* *Ca2+* *from cytosol and pumps it* *into the extracellular* *space*. - Exhibit **graded** (not all-or-none) contraction → the force generated x (is proportional to) number of active crossbridges. - Number of active crossbridges is proportional to cytosolic [*Ca2+*]. - Therefore, the force generated is proportional to cytosolic [Ca2+]. - **Factors Influencing Cardiac Muscle Contraction Force** - **Changes in [Ca2+]** - Regulated by epinephrine & norepinephrine → bind to & activate *ẞ1-adrenergic receptors*. - Binding then activates cAMP second messenger signaling pathway that leads to: - *a. Phosphorylation of voltage-gated* *Ca2+* *channels*: - *Increases the probability of the channel to open → increase* [*Ca2+* *] in the cytosol*. - *b. Phosphorylation of phospholamban*: - *Leads to increase* *SR Ca2+-ATPase* *activity → increase* *SR Ca2+*. - **Sarcomere length** - Tension generated length of muscle fibre. - Due to degree of overlap between actin and myosin→ - *Note that stretching a myocardial muscle cell may also allow more Ca2+to enter through cell membrane Ca2+channels → contributing to a more forceful next contraction*. ## Cardiac Muscle Contraction - Cardiac muscle is also an excitable tissue → can generate action potentials. - Major sequence of events: - **Phase 4:** Resting membrane potential. - **Phase 0:** Depolarization → The *AP* opens voltage-gated *Na* channels, causing a rapid increase in membrane *Na* permeability (close again). - **Phase 1:** Initial repolarization → open fast *K+* channels allow initial repolarization. - **Phase 2:** The plateau → initial depolarization triggered voltage-gated *Ca2+ channels* to slowly open, causing an increase in *Ca2+* permeability and the fast *K+* channels close. - **Phase 3:** Rapid repolarization → the *Ca2+* channels close and the slow voltage-gated *K+* channels open (initial depolarization was the trigger), and the resting stage ion permeability is restored (phase 4). - Result of sustained depolarization: - Typical action potential in neuron or skeletal muscle cell: 1-5 msec. - Typical action potential in cardiac muscle: >200msec - **Question:** Why is this important? - **Question:** Why don't cardiac muscle cells undergo summation and tetanus? - **Answer:** Because of the longer refractory period → means that the cell has finished contracting before the next action potential. ## Important Terms and Concepts ### Important Terms - Skeletal muscle - Cardiac muscle - Striated muscle - Muscle fibre - Myosin and actin - A band - I band - M line - Contraction - Sliding filament theory - Troponin - Excitation-contraction coupling - T-tubule - ACh - Acetylcholinesterase - Isotonic - Tetanus (complete and incomplete) - Fast twitch - Calcium-dependent calcium release - Smooth muscle - Tendon - Myofibril - Thick filament - Thin filament - Z line - H zone - Crossbridge - Relaxation - Myoglobin - Tropomyosin - Neuromuscular junction - EPP - Tension - SR - Load - Isometric - Summation - Phosphocreatine - Slow twitch - Dense bodies ### Important Concepts - Differences between skeletal muscle, smooth muscle, and cardiac muscle at whole muscle level, cellular level, and molecular level - Structure of the sarcomere - Sliding filament theory - Role of calcium - Excitation-contraction coupling - Role of *ATP* in muscle contraction. - Skeletal muscle fiber types - Tetanus, motor unit, and recruitment - Isotonic contraction and isometric contraction - Contraction mechanism in smooth muscle and cardiac muscle