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BIOLOGY 251 Human Anatomy & Physiology Chapter 10 Muscle Tissue Lecture Notes Chapter 10 Muscular Tissue Lecture slides prepared by Curtis DeFriez, Weber State University Copyright © John Wiley and Sons, Inc. All rights reserved. Properties of Muscle Tissue • Extensibility – ability to be stretched without damaging the tissue • Elasticity – ability to return to original shape after being stretched • Excitability – respond to chemicals released from nerve cells • Conductivity – ability to propagate electrical signals over membrane • Contractility – ability to shorten and generate force Functions of Muscular Tissue Like nervous tissue, muscles are excitable or "irritable” they have the ability to respond to a stimulus Unlike nerves, however, muscles are also: Contractible (they can shorten in length) Extensible (they can extend or stretch) Elastic (they can return to their original shape) Copyright © John Wiley and Sons, Inc. All rights reserved. Three Types of Muscular Tissue (a) Skeletal muscle (b) Cardiac muscle (c) Visceral smooth muscle Copyright © John Wiley and Sons, Inc. All rights reserved. Three Types of Muscular Tissue Location Skeletal skeleton Cardiac heart Visceral (smooth muscle) G.I. tract, uterus, eye, blood vessels Function movement, heat, posture Appearance Control striated, multinucleated, fibers voluntary parallel pump blood striated, one central continuously nucleus involuntary Peristalsis, blood pressure, no striations, one pupil size, central nucleus involuntary erects hairs Copyright © John Wiley and Sons, Inc. All rights reserved. Skeletal Muscle Location Skeletal skeleton Cardiac heart Visceral (smooth muscle) G.I. tract, uterus, eye, blood vessels Function movement, heat, posture Appearance Control striated, multinucleated, fibers voluntary parallel pump blood striated, one central continuously nucleus involuntary Peristalsis, blood pressure, no striations, one pupil size, central nucleus involuntary erects hairs Copyright © John Wiley and Sons, Inc. All rights reserved. Skeletal Muscle Copyright © John Wiley and Sons, Inc. All rights reserved. Skeletal Muscle Skeletal muscle fibers are very long “cells” - next to neurons (which can be over a meter long), perhaps the longest in the body The Sartorious muscle contains single fibers that are at least 30 cm long A single skeletal muscle fiber Copyright © John Wiley and Sons, Inc. All rights reserved. Organization of Muscle Tissue The epimysium, perimysium, and endomysium all are continuous with the connective tissues that form tendons and ligaments (attach skeletal muscle to bone) and muscle fascia (connect muscles to other muscles to form groups of muscles) Copyright © John Wiley and Sons, Inc. All rights reserved. Organization of Muscle Tissue Organization of a fasciculus Copyright © John Wiley and Sons, Inc. All rights reserved. Organization of Muscle Tissue Organization of a muscle fiber Copyright © John Wiley and Sons, Inc. All rights reserved. Organization of Muscle Tissue In groups of muscles the epimysium continues to become thicker, forming fascia which covers many muscles This graphic shows the fascia lata enveloping the entire group of quadriceps and hamstring muscles in the thing Copyright © John Wiley and Sons, Inc. All rights reserved. Organization of Muscle Tissue Many large muscle groups are encased in both a superficial and a deep fascia Real Anatomy, John Wiley and Sons Copyright © John Wiley and Sons, Inc. All rights reserved. Organization of Muscle Tissue Veins, arteries, and nerves are located in the deep fascia between muscles of the thigh. Copyright © John Wiley and Sons, Inc. All rights reserved. The Skeletal Muscle Fiber Beneath the connective tissue endomysium is found the plasma membrane (called the sarcolemma) of an individual skeletal muscle fiber The cytoplasm (sarcoplasm) of skeletal muscle fibers is chocked full of contractile proteins arranged in myofibrils Copyright © John Wiley and Sons, Inc. All rights reserved. The Skeletal Muscle Fiber You should learn the names of the internal structures of the muscle fiber Sarcolemma Sarcoplasm Myofibril T-tubules Triad (with terminal cisterns Sarcoplasmic reticulum Sarcomere Copyright © John Wiley and Sons, Inc. All rights reserved. The Skeletal Muscle Fiber Increasing the level of magnification, the myofibrils are seen to be composed of filaments Thick filaments Thin filaments Copyright © John Wiley and Sons, Inc. All rights reserved. The Skeletal Muscle Fiber The basic functional unit of skeletal muscle fibers is the sarcomere: An arrangement of thick and thin filaments sandwiched between two Z discs A scanning electron micrograph of a sarcomere Copyright © John Wiley and Sons, Inc. All rights reserved. The Skeletal Muscle Fiber Muscle contraction occurs in the sarcomeres The “Z line” is really a Z disc when considered in 3 dimensions. A sarcomere extends from Z disc to Z disc. Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins Myofibrils are built from three groups of proteins Contractile proteins generate force during contraction Regulatory proteins help switch the contraction process on and off Structural proteins keep the thick and thin filaments in proper alignment and link the myofibrils to the sarcolemma and extracellular matrix Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins The thin filaments are comprised mostly of the contractile protein actin, and the thick filaments are comprised mostly of the contractile protein myosin However, in both types of filaments, there are also other structural and regulatory proteins Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins In the thin filaments actin proteins are strung together like a bead of pearls In the thick filaments myosin proteins look like golf clubs bound together Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins In this first graphic, the myosin binding sites on the actin proteins are readily visible. The regulatory proteins troponin and tropomyosin have been added to the bottom graphic: The myosin binding sites have been covered Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins In this graphic the troponin-tropomyosin complex has slid down into the “gutters” of the actin molecule unblocking the myosin binding site Myosin binding site exposed The troponin-tropomyosin complex can slide back and forth depending on the presence of Ca2+ Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins Ca2+ binds to troponin which changes the shape of the troponin-tropomyosin complex and uncovers the myosin binding sites on actin Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins • Besides contractile and regulatory proteins, muscle contains about a dozen structural proteins which contribute to the alignment, stability, elasticity, and extensibility of myofibrils • Titan is the third most plentiful protein in muscle, after actin and myosin - it extends from the Z disc and accounts for much of the elasticity of myofibrils • Dystrophin is discussed later as it relates to the disease of muscular dystrophy Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Proteins Structural proteins cont’d. • Myomesin is the protein that forms the M line. The M line proteins bind to Titan and connect the adjacent thick filaments to one another and hold they in alignment at the M line. • Nebulin is a long non-elastic protein wrapped around the entire length of each thin filament. It helps anchor the thin filaments to the Z disc. Copyright © John Wiley and Sons, Inc. All rights reserved. The Sliding-Filament Mechanism With exposure of the myosin binding sites on actin (the thin filaments)—in the presence of Ca2+ and ATP—the thick and thin filaments “slide” on one another and the sarcomere is shortened Copyright © John Wiley and Sons, Inc. All rights reserved. The Sliding-Filament Mechanism The “sliding” of actin on myosin (thick filaments on thin filaments) can be broken down into a 4 step process Copyright © John Wiley and Sons, Inc. All rights reserved. The Sliding-Filament Mechanism Copyright © John Wiley and Sons, Inc. All rights reserved. Step 1: ATP hydrolysis Step 2: Attachment Copyright © John Wiley and Sons, Inc. All rights reserved. Step 3: Power Stroke Step 4: Detachment Copyright © John Wiley and Sons, Inc. All rights reserved. The Sliding-Filament Mechanism Copyright © John Wiley and Sons, Inc. All rights reserved. Length-Tension Relationship Sarcomere shortening produces tension within a muscle Compressed thick filaments Limited contact between actin and myosin Copyright © John Wiley and Sons, Inc. All rights reserved. Sources of Muscle Energy Stored ATP 3 seconds Energy transferred from stored creatine phosphate 12 seconds Anaerobic glucose use 30-40 seconds Aerobic ATP production Minutes - hours Copyright © John Wiley and Sons, Inc. All rights reserved. Sources of Muscle Energy Copyright © John Wiley and Sons, Inc. All rights reserved. Sources of Muscle Energy Copyright © John Wiley and Sons, Inc. All rights reserved. Sources of Muscle Energy Copyright © John Wiley and Sons, Inc. All rights reserved. Neuromuscular Junction Excitation-Contraction coupling (EC coupling) involves events at the junction between a motor neuron and a skeletal muscle fiber Copyright © John Wiley and Sons, Inc. All rights reserved. Neuromuscular Junction An enlarged view of the neuromuscular junction The presynaptic membrane is on the neuron while the postsynaptic membrane is the motor end plate on the muscle cell. The two membranes are separated by a space, or “cleft” Copyright © John Wiley and Sons, Inc. All rights reserved. Neuromuscular Junction Conscious thought (to move a muscle) results in activation of a motor neuron, and release of the neurotransmitter acetylcholine (ACh) at the NM junction The enzyme acetylcholinesterase breaks down ACh after a short period of time Copyright © John Wiley and Sons, Inc. All rights reserved. Neuromuscular Junction The plasma membrane on the “far side” of the NMJ belongs to the muscle cell and is called the motor end plate The motor end plate is rich in chemical (ligand) - gated sodium channels that respond to ACh. Another way to say this: The receptors for ACh are on the ligand-gated sodium channels on the motor end plate Copyright © John Wiley and Sons, Inc. All rights reserved. Neuromuscular Junction The chemical events at the NMJ transmit the electrical events of a neuronal action potential into the electrical events of a muscle action potential Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Action Potential The muscle AP is propagated over the surface of the muscle cell membrane (sarcolemma) via voltage (electrical)-gated Na+ and K+ channels Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Action Potential By placing a micropipette inside a muscle cell, and then measuring the electrical potential across the cell membrane, the phases of an action potential (AP) can be graphed (as in this figure) Copyright © John Wiley and Sons, Inc. All rights reserved. Muscle Action Potential The behavior of the Na+ and K+ channels, at various points in the AP, are seen in this graphic Na+ gates open during the depolarization phase K+ gates open during the repolarization phase Copyright © John Wiley and Sons, Inc. All rights reserved. Excitation-Contraction Coupling Copyright © John Wiley and Sons, Inc. All rights reserved. Excitation-Contraction Coupling EC coupling involves putting it all together The thought process going on in the brain The AP arriving at the neuromuscular junction The regeneration of an AP on the muscle membrane Release of Ca2+ from the sarcoplasmic reticulum Sliding of thick on thin filaments in sarcomeres Generation of muscle tension (work) Copyright © John Wiley and Sons, Inc. All rights reserved. Excitation-Contraction Coupling Copyright © John Wiley and Sons, Inc. All rights reserved. Excitation-Contraction Coupling Role Players in E-C coupling The brain Regenerate AP The motor neuron The T-tubules Acetylcholine (ACh) The SR Acetylcholinesterase enzyme Ca2+ release Ach receptors on the Troponin/Tropomyosin motor endplate ATP Na+-K+ channels on the Myosin binding sarcolemma Filaments slide Na+ flow Muscles contract K+ flow Copyright © John Wiley and Sons, Inc. All rights reserved. The Motor Unit Motor Unit is composed of a motor neuron plus all of the muscle cells it innervates High precision • Fewer muscle fibers per neuron • Laryngeal and extraocular muscles (2-20) Low precision • Many muscle fibers per neuron • Thigh muscles (2,000-3,000) Copyright © John Wiley and Sons, Inc. All rights reserved. The Motor Unit Florescent dye is used to show the terminal processes of a single neuron which terminate on a few muscle fibers Copyright © John Wiley and Sons, Inc. All rights reserved. The Motor Unit Activities requiring extreme precision (like the subtle and rapid movements of the eye) involve muscles with very small motor units (1-4 muscle fibers/neuron) Copyright © John Wiley and Sons, Inc. All rights reserved. The Motor Unit All-or-none principle of muscle contraction When an individual muscle fiber is stimulated to depolarization, and an action potential is propagated along its sarcolemma, it must contract to it’s full force— it can’t partially contract Also, when a single motor unit is recruited to contract, all the muscle fibers in that motor unit must all contract at the same time Copyright © John Wiley and Sons, Inc. All rights reserved. Tension in a Muscle There is a brief delay called the latent period as the AP sweeps over the sarcolemma and Ca2+ ions are released from the sarcoplasmic reticulum (SR) During the next phase the fiber is actively contracting This is followed by relaxation as the Ca2+ ions are re- sequestered into the SR and myosin binding sites are covered by tropomyosin Temporary loss of excitability is call the refractory period – All muscle fibers in a motor unit will not respond to a stimulus during this short time Copyright © John Wiley and Sons, Inc. All rights reserved. Tension in a Muscle A twitch is recorded when a stimulus that results in contraction (force) of a single muscle fiber is measured over a very brief millisecond time frame Copyright © John Wiley and Sons, Inc. All rights reserved. Tension in a Muscle Applying increased numbers of action potentials to a muscle fiber (or a fascicle, a muscle, or a muscle group) results in fusion of contractions (tetanus) and the performance of useful work Copyright © John Wiley and Sons, Inc. All rights reserved. Tension in a Muscle Two motor units, one in green, the other in purple, demonstrate the concept of progressive activation of a muscle known as recruitment Recruitment allows a muscle to accomplish increasing gradations of contractile strength Copyright © John Wiley and Sons, Inc. All rights reserved. Figure 2.4 Copyright © John Wiley and Sons, Inc. All rights reserved.