Muscular Tissue Functions of Muscular Tissue Muscle makes up a large percentage of the body s weight (40-50%) Their main functions are to: Create motion muscles work with nerves, bones, and joints to produce body movements Stabilize body positions and maintain posture Store substances within the body using sphincters and move substances by peristaltic contractions Generate heat through thermogenesis Types of Muscular Tissue (a) Skeletal muscle (b) Cardiac muscle (c) Visceral smooth muscle Properties 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) Skeletal Muscular Tissue 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
Connective Tissue Components 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) Organization of a single muscle belly Epimysium Perimysium Organization of a Fasciculus Organization of a Muscle Fiber 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 A muscle, a fasciculus, and a fiber all visualized
Many large muscle groups are encased in both a superficial and a deep fascia An aponeurosis is essentially a thick fascia that connects two muscle bellies This epicranial aponeurosis connects the muscle bellies of the occipitalis and the frontalis to form one muscle: occipitofrontalis Veins, arteries, and nerves are located in the deep fascia between muscles of the thigh Real Anatomy, John Wiley and Sons Nerve & Blood Supply Skeletal muscles are well supplied with nerves and blood vessels Generally, an artery and one or two veins accompany each nerve that penetrates a skeletal muscle Somatic motor neurons provide the nerve impulses that stimulate skeletal muscle to contract Blood capillaries bring in oxygen and nutrients and remove heat and waste products of muscle metabolism Microscopic Anatomy of a 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 The terminal processes of a motor neuron in close proximity to the sarcolemma of a skeletal muscle fiber Motor neuron Sarcolemma 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
Increasing the level of magnification, the myofibrils are seen to be composed of filaments Thick filaments Thin filaments 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 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. 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 The thin filaments are comprised mostly of the structural protein actin, and the thick filaments are comprised mostly of the structural protein myosin However, in both types of filaments, there are also other structural and regulatory 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
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 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 Ca 2+ Ca 2+ binds to troponin which changes the shape of the troponin-tropomyosin complex and uncovers the myosin binding sites on actin 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 Contraction & Relaxation of Skeletal Muscle Fibers: The Sliding Filament Mechanism With exposure of the myosin binding sites on actin (the thin filaments) in the presence of Ca 2+ and ATP the thick and thin filaments slide on one another and the sarcomere is shortened The sliding of actin on myosin (thick filaments on thin filaments) can be broken down into a 4 step process
Step 1: ATP hydrolysis Step 3: Power Stroke Step 2: Attachment Step 4: Detachment Sarcomere shortening produces tension within a muscle Compressed thick filaments Limited contact between actin and myosin The Neuromuscular Junction Excitation-Contraction coupling (EC coupling) involves events at the junction between a motor neuron and a skeletal muscle fiber 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
Conscious thought (to move a muscle) results in activation of a motor neuron, and release of the neurotransmitter acetylcholine (AcCh) at the NM junction The enzyme acetylcholinesterase breaks down AcCh after a short period of time 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 AcCh. Another way to say this: The receptors for AcCh are on the ligand-gated sodium channels on the motor end plate The muscle AP is propagated over the surface of the muscle cell membrane (sarcolemma) via voltage (electrical)-gated Na + and K + channels 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) The chemical events at the NMJ transmit the electrical events of a neuronal action potential into the electrical events of a 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
The flow of ions through cell a membrane looks a lot like a "piece" of electricity flowing through a wire (but not as fast) Generating an AP on the muscle membrane involves the transfer of information from an electrical signal (down the neuron), to a chemical signal (at the NMJ), back to an electrical signal (depolarization of the sarcolemma) This added complexity (changing from electrical to chemical back to electrical signals) provides necessary control of the process 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 Ca 2+ from the sarcoplasmic reticulum Sliding of thick on thin filaments in sarcomeres Generation of muscle tension (work) Role Players in E-C coupling The brain The motor neuron Acetylcholine (ACh) Acetylcholinesterase enzyme Ach receptors on the motor endplate Na + -K + channels on the sarcolemma Na + flow K + flow Regenerate AP The T-tubules The SR Ca 2+ release Troponin/Tropomyosin ATP Myosin binding Filaments slide Muscles contract