9 Muscles - Muscles and Muscle Muscles and Muscle Tissue...

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Unformatted text preview: Muscles and Muscle Muscles and Muscle Tissue Functional characteristics of muscle tissue Functional characteristics of muscle tissue Excitability – they can respond to stimuli by generating action potentials. Contractility – the ability to shorten forcibly when stimulated Extensibility – can be stretched Elasticity – are somewhat elastic and may return to their resting length, but not in the way you may think of a rubber band What other type of cell in the body can generate an action potential? Muscle Functions Muscle Functions Produce movement (e.g., movement of the body, pumping blood, movement of food) Maintain posture and body position Stabilize joints Generate heat Maintain blood pressure Types of Muscle Tissues Types of Muscle Tissues Skeletal Muscle – under voluntary (conscious control) Controls movement of the body Contracts rapidly but tires easily Has obvious stripes called striations Focus of this chapter Cardiac Muscle – makes up the heart Involuntary and does not need nervous system stimulation to contract (Intrinsic Pacemaker) Cells are connected by intercalated discs Striated Types of Muscle Tissues cont. Types of Muscle Tissues cont. Smooth Muscle – is associated with the digestive tract, urinary bladder, uterus ect. It is not striated and is involuntary Forces food and other substances through internal body channels Visceral smooth muscle ­ most common type Multiunit smooth muscle ­found in large airways of the respiratory system, large arteries, arrector pili muscles, and internal eye muscles. Cells connected by gap junctions, and contract as a single unit. May contract spontaneously (without neural stimulation). Muscle cells of this tissue are structurally independent and they are innervated to form motor units (motor units will be described later) Muscle Basics Muscle Basics Sarcolemma – muscle plasma membrane Sarcoplasm – cytoplasm of a muscle cell Prefixes – myo, mys, and sarco all refer to muscle Muscle Contraction of all three types of muscle depends on the interactions of actin and myosin myofilaments Gross Anatomy of a Skeletal Muscle Gross Anatomy of a Skeletal Muscle • A muscle is made up of several types of tissue and muscle cells which are referred to as muscle fibers •The fibers are wrapped in a series of connective tissue membranes: Epimysium – an overcoat of dense regular CT that surrounds the entire muscle Perimysium – fibrous CT that surrounds groups of muscle fibers called fascicles Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber Muscle Attatchment Muscle Attatchment When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin Muscles attach: Directly – epimysium of the muscle is fused to the periosteum of a bone Indirectly – CT wrappings extend beyond the muscle as a tendon or aponeurosis Microscopic anatomy of a skeletal muscle fiber Microscopic anatomy of a skeletal muscle fiber •Each muscle fiber consists of a large number of rodlike myofibrils •These myofibrils contain the contractile elements of the muscle fiber •The striations = repeating series of dark A bands and light I bands along the length of the myofibril Myofilaments Myofilaments 2 types of myofilaments (thick & thin) make up myofibrils. Thick myofilaments are made the protein myosin A single myosin protein resembles 2 golf clubs whose shafts have been twisted about one another About 300 of these myosin molecules are joined together to form a single thick filament Each thin filament is made up of 3 different types of protein: actin, tropomyosin, and troponin. Each thin filament consists of a long helical double strand. This strand is a polymer that resembles a string of beads. Each “bead” is the globular protein actin. On each actin subunit, there is a myosin binding site. Loosely wrapped around the actin helix and covering the myosin binding site is the filamentous protein, tropomyosin. Bound to both the actin and the tropomyosin is a trio of proteins collectively known as troponin. Sarcomere – the smallest contractile unit of muscle contraction and is the region of myofibrils between two successive Z discs ­The actin is held in place in the myofibril by Z­discs. ­M lines hold the mysoin in place ­Myosin filaments have structures (crossbridges) that grab onto the actin filaments and the Z­discs are pulled together, causing the muscle to shorten. When a muscle contracts, the I bands When a muscle contracts, the I bands get narrow and the A bands stay the same length. Why? Here are 2 electron micrographs of the same sarcomere. Here are 2 electron micrographs of the same sarcomere. Do you see the Z discs, A band, H zone, M line, and I bands? How do the 2 pictures differ? What happened? Sarcoplasmic Reticulum(SR) and T Tubules Sarcoplasmic Reticulum(SR) and T Tubules •SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril •Functions in the regulation of intracellular calcium levels •Elongated tubes called T tubules penetrate deep into the cell’s interior at each A band–I band junction •T tubules associate with the SR to form triads Sliding filament model of contraction Sliding filament model of contraction In the 1950's two British scientists, both named Huxley but working independently, developed the "sliding filament theory" of muscle contraction. The electron microscope enabled them to determine that the crossbridges within the cell change orientation when the muscle contracts so that the thin filaments slide over the thick filaments They also showed that the thick and thin filaments do not change in length during contraction What must happen first in a muscle contraction? Because skeletal muscle is voluntary A nerve must send an electrical impulse telling the muscle to contract Physiology of Muscle Physiology of Muscle Contraction Chpt 11 Nervous System Intro. Chpt 11 Nervous System Intro. All cells have a voltage difference across their plasma membrane. This is the result of several things: 1. The outside of the cell (ECF) is high in Na+ . The inside of the cell is high in K+ 2. The Na+/K+ pump is constantly pumping 3 Na+ ions out and 2 K+ ions in for every ATP used. Thus more positive charge is leaving than entering. 3. There are protein anions (i.e., negatively charged proteins) within the ICF that cannot travel through the PM. This causes cells to be polarized with a more negative charge on the inside of the cell compared to the outside. The PM is impermeable to Na+ but slightly permeable to K+. As a result, K+ is constantly leaking out of the cell. In other words, positive charge is constantly leaking out of the cell. Chpt 11 Nervous System Intro. Chpt11 Nervous System Intro. Action Potential Step 1: Resting polarized cell = Avg. ­70mv Voltage gated Na+ channels open (Steps of an action potential­How electrical signals are sent in the body) Step 2: Na Channels Close + Step 3: K Channels Open + Na+ K+ Na+ K+ + ­ KNa+ ­ Na+ ­ Na+ K+ K+ ­ ­Na+ Na+Cell Na+ ­ K+ ­ K+ Na+ K+ ­ ­ Na+ Na+ ­ ­ ­ ­ K+ ­ ­ ­ Na+ ­­ K+ K+ Na+ ­­­ ­ ­ ­ ­­ K+ Na+ Na+ Na+ K+ K+ Na+ K+ Na+ K+ Na+ Neuromuscular Junction Neuromuscular Junction The site of interaction between a neuron and any other cell is known as a synapse. The synapse between a neuron and a muscle is called the neuromuscular junction. 1. When an action potential traveling down an axon reaches the end, Voltage regulated Calcium channels open, allowing Calcium to enter the axon. 2. Synaptic vessels containing acetycholine fuse to the membrane and release acetycholine by exocytosis 1. Acetycholine diffuses across the cleft. 2. Acetycholine binds to receptors on the sarcolemma. 1. Opening of chemically­regulated sodium channels. What happens when sodium channels open? What happens in the neuromuscular What happens in the neuromuscular junction? Manipulation of the Neuromuscular Junction Manipulation of the Neuromuscular Junction Curare – arrowhead poison Binds to acetycholine and prevents acetycholine from binding to receptors. What happens? Japanese Puffer Fish Toxin ­ Destruction of Acetylcholine Destruction of Acetylcholine ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase (AChE) AChE activity prevents continued muscle fiber contraction in the absence of additional stimuli (sequence of events that couple the action potential moving along the sarcolemma to the sliding of the myofilaments) Excitation­Contraction Coupling­ Excitation­Contraction Coupling Once generated, the action potential: Ca2+ binds to troponin and causes: Is propagated along the sarcolemma Travels down the T tubules Triggers Ca2+ release from terminal cisternae of the SR The blocking action of tropomyosin to cease Actin active binding sites to be exposed http://msjensen.cehd.umn.edu/1135/Links/Animations/Flash/0010­swf_action_potenti.swf Contraction Step 1. Myosin will attach to actin when binding sites on actin are exposed. Myosin has just hydrolyzed ATP to ADP and Pi from the step before and these products are still bound to myosin. Step 2. Myosin releases ADP and Pi and this causes the myosin conformation change and pivoting of the head = Step of filament sliding Step 3. New ATP attaches to myosin and myosin lets go of actin tep 4. Myosin hydrolyzes ATP into ADP and Pi and is ready to repeat the cycle as long as calcium bound to troponin Review­ Regulation of Contraction Review­ Regulation of Contraction In order to contract, a skeletal muscle must: Be stimulated by a nerve ending Propagate an electrical current, or action potential, along its sarcolemma Have a rise in intracellular Ca2+ levels, the final trigger for contraction ??? What happens to the Ca2+ that is released from the SR after muscle contraction? A muscle twitch is the response of a muscle to a single brief threshold stimulus The three phases are: Latent period – first few milliseconds after stimulation when excitation­contraction coupling is taking place Contraction – cross bridges actively form and the muscle shortens Relaxation – Ca2+ is reabsorbed into the SR, and muscle tension goes to http://lessons.harveyproject.org/development/muscle/twitch~1.htm zero Muscle Twitch Muscle Twitch Muscle Response to Varying Stimuli Muscle Response to Varying Stimuli A single stimulus results in a single contractile response – a muscle twitch Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force ­Wave summation If stimuli are given quickly enough, complete tetanus results What is complete tetanus? Figure 9.13 Muscle Response: Stimulation Strength Muscle Response: Stimulation Strength Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs Beyond threshold, muscle contracts more vigorously as stimulus strength is increased Multiple motor unit summation or recruitment = activation of more muscle fibers from an increase in voltage stimulation ­ “Recruiting more motor units” Important Definition: Motor Unit = Treppe – increased contraction in response to multiple stimuli of the same strength Contractions increase because: There is increasing availability of Ca2+ in the sarcoplasm Muscle enzyme systems become more efficient because heat is increased as muscle contracts Treppe Treppe Figure 9.14 What are two ways to increase the What are two ways to increase the force of muscle contraction? Isotonic Contractions Isotonic Contractions •Muscle length changes and moves the load Figure 9.15a Isometric Contractions Isometric Contractions •Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens •Occurs if the load is greater than the tension the muscle is able to develop (No change in length of muscle) Examples: Muscles that maintain posture, lifting something heavy you cannot pickup Figure 9.15b Muscle Fiber Type –mainly grouped according Muscle Fiber Type to the speed of contraction and means of ATP production Slow oxidative muscle fibers ­ have a relatively slow myosin and rely primarily on aerobic ATP production. Fast oxidative muscle fibers ­have fast myosin, which cycles ATP very rapidly Typically have many mitochondria for the efficient production of ATP. Fatigue­resistant and good for long­term use. Have a great quantity of the protein myoglobin, with holds a reserve supply of oxygen within the muscle cell. Also have a large number of mitochondria. Contract quite rapidly, and are fairly fatigue­resistant. They have few mitochondria Fatigue rapidly. Good for short bursts of work Fast glycolytic muscle fibers ­ have fast myosin and rely primarily on anaerobic glycolysis for ATP production ...
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