Chapter 10 Muscle Tissue Skeletal muscle, cardiac muscle and smooth muscle. Differ in their microscopic anatomy, location and how they are controlled by the endocrine and nervous system.
3 Types of Muscle Tissue 1. Skeletal muscle attaches to bone via tendon at the periosteum, skin or fascia striated with light & dark bands of actin & myosin voluntary control of contraction & relaxation controlled by hormones & release of neurotransmitters Cells are multinucleated
2. Cardiac muscle tissue muscle of the heart striated alternating light & dark bands involuntary control autorhythmic because of built in pacemaker cells do not need nerve stimulation to contract controlled by hormones and release of neurotransmitters of nerves branched cells with single nucleus
3. Smooth muscle tissue Located in hair follicles in skin wall of gastrointestinal (GI) tract walls of hollow organs lines most blood vessels nonstriated involuntary controlled by hormones & neurotransmitters spindle shaped cells with single nucleus
Functions of Muscle Tissue Producing body movements Stabilizing body positions Movement of substances within the body such as blood, lymph, urine, air, food and fluids, sperm Producing heat - approximately 85% of body temperature involuntary contractions of skeletal muscle (shivering)
Functional Characteristics of Muscle Tissue 1. Excitability Neurons are also excitable - respond to chemicals released from nerve cells or hormones & other chemicals to propagate an action potential 2. Contractility - ability to shorten and lengthen (relax) 3. Extensibility - ability to be stretched without damaging the tissue 4. Elasticity - ability to return to original shape after being stretched
Fusion of Myoblasts into Muscle Fibers In the embryo, myogenic stem cells produce: myoblasts satellite cells undifferentiated cells Myoblasts fuse together during fetal development and become multinucleated that can no longer divide = Muscle Fiber Vary in length from 1 meter (sartorius muscle in thigh) to few millimeters (muscles of inner ear) Muscle growth is a result of cellular enlargement & not cell division Satellite cells become active after muscle damage for repair but their activity is limited.
Satellite Cells Different from glial satellite cells in PNS 2 7% of all nuclei associated with a single fiber Very little cytoplasm Normally mitotically quiescent
Connective Tissue Components of Skeletal Muscle
Skeletal Muscle & Associated Connective Tissue Superficial fascia is loose connective tissue & fat underlying the skin which cushions, insulates, protects Deep fascia = dense irregular connective tissue around muscle to keep muscles separate / individual movement Connective tissue components of the muscle include 1. epimysium = surrounds the whole muscle 2. perimysium = surrounds bundles (fascicles) of 10-100 muscle cells 3. endomysium = separates individual muscle cells Epimysium extends to form tendon that attaches muscle to periosteal membrane of the bone
Muscle Fiber & Myofibers 1 Muscle Cell Skeletal Muscle cells are long, cylindrical & multinucleated Sarcolemma = muscle cell membrane Sarcoplasm filled with tiny threads called myofibrils & myoglobin (red-colored, oxygen-binding protein)
Transverse Tubules T (transverse) tubules are invaginations of the sarcolemma into the center of the cell filled with extracellular fluid carry muscle action potentials down into cell Mitochondria lie in rows throughout the cell near the muscle proteins that use ATP during contraction
Myofibrils & Myofilaments Myofibrils are the contractile elements/change position/not length Myofilaments (thick & thin filaments) are the contractile proteins of that make up the myofibrils Myosin threads are called thick filaments Actin threads are called thin filaments
Sarcoplasmic Reticulum (SR) System of tubular sacs Stores Ca 2+ in a relaxed muscle by active transport Release of Ca 2+ by diffusion triggers muscle contraction
Proteins of Muscle Contraction 3 Proteins involved in muscle contraction Contractile proteins 1. Thin filaments = Actin 2. Thick filaments = Myosin Regulatory proteins which turn contraction on & off 3. Troponin and Tropomyosin Structural Proteins keep actin and myosin in proper alignment, give myofibril elasticity & link the myofibrils to the sarcolemma
Filaments and the Sarcomere Thick (myosin) and thin (actin) filaments overlap each other in a pattern that creates striations (light I bands and dark A bands) The actin and myosin do not extend the entire length of the muscle cell They are arranged in compartments called sarcomeres separated by Z discs
Thick (myosin) & Thin (actin) Myofilaments Supporting proteins (M line, titin and Z disc help anchor the thick and thin filaments in place)
Myosin Thick filaments are composed of myosin each molecule resembles two golf clubs twisted together myosin heads (cross bridges) extend toward the actin filaments Held in place by the M line proteins.
Actin / Troponin / Tropomyosin Thin filaments are made of actin, troponin, & tropomyosin The regulatory protein (TT) covers and uncovers the myosin binding site on the actin molecule
Structures of the Neuromuscular Junction Synaptic end bulbs are swellings of axon terminals End bulbs contain synaptic vesicles filled with acetylcholine (ACh) Motor end plate - area on membrane contains 30 million ACh receptors.
Events at the Synapse 1. Arrival of nerve impulse at nerve terminal causes release of ACh from synaptic vesicles 2. ACh binds to receptors on muscle motor end plate opening the gated ion channels so that Na + can rush into the muscle cell 3. Inside of muscle cell becomes more positive, triggering a muscle action potential that travels over the cell and down the T-tubules 4. The release of Ca 2+ from the SR is triggered and the muscle cell will shorten & generate force 5. Acetylcholinesterase breaks down the ACh attached to the receptors on the motor end plate so the muscle action potential will cease and the muscle cell will relax.
Neuromuscular Junction (NMJ) or Synapse NMJ = neuromuscular junction end of axon nears the surface of a muscle fiber at its motor end plate region (remain separated by synaptic cleft or gap)
Steps in the Contraction Cycle
Sliding Filament Mechanism Of Contraction Myosin cross bridges pull on thin filaments Thin filaments slide inward Z Discs come toward each other Sarcomeres shorten.the muscle fiber shortens. The muscle shortens Notice :Thick & thin filaments do NOT change in length - Only Change Position
Contraction Vs. Relaxation
Relaxation 1. Acetylcholinesterase (AChE) breaks down ACh within the synaptic cleft 2. Muscle action potential ceases 3. Ca 2+ channels close 4. Active transport (using ATP) pumps Ca 2+ back into storage in the sarcoplasmic reticulum 5. Tropomyosin-troponin (TT) complex recovers binding site on the actin
STAGES OF A MUSCLE TWITCH: The latent period is a delay time after stimulus is applied. During this time, the action potential sweeps over the sarcolemma and Ca 2+ is released. During the contraction period, Ca 2+ binds to troponin, myosin binding sites are exposed and cross-bridges form. Peak tension occurs. During the relaxation period, Ca 2+ is transported back to SR, myosinbinding sites are covered by tropomyosin, myosin heads detach and tension decreases.
Refractory Period Period of time during which the muscle loses excitability Similar to refractory period in neuron Differs in skeletal and cardiac muscle
Overview: Contraction From Start to Finish
Atrophy Atrophy and Hypertrophy wasting away of muscles caused by disuse (disuse atrophy) or severing of the nerve supply (denervation atrophy) the transition to connective tissue can not be reversed Hypertrophy increase in the diameter (size) of muscle fibers resulting from very forceful, repetitive muscular activity and an increase in myofibrils, SR & mitochondria
Rigor Mortis Rigor mortis is a state of muscular rigidity that begins 3-4 hours after death and lasts about 24 hours After death, Ca 2+ ions leak out of the SR and allow myosin heads to bind to actin Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells.
Muscle Metabolism Production of ATP in Muscle Fibers Muscle uses ATP at a great rate when active Sarcoplasmic ATP only lasts for few seconds & has to be replenished 3 sources of ATP production within muscle 1. creatine phosphate 2. anaerobic cellular respiration / without O 2 3. aerobic cellular respiration / With O 2
Creatine Phosphate Excess ATP within resting muscle used to form creatine phosphate Creatine phosphate 3-6 times more plentiful than ATP within muscle Its quick breakdown provides energy for creation of ATP Sustains maximal contraction for 15 sec (used for 100 meter dash). Athletes tried creatine supplementation gain muscle mass but shut down bodies own synthesis (safety?)
Anaerobic Cellular Respiration ATP produced from glucose breakdown into pyruvic acid during glycolysis if no O 2 present pyruvic converted to lactic acid which diffuses into the blood& causes muscles to be sore Glycolysis can continue anaerobically to provide ATP for 30 to 40 seconds of maximal activity (200 meter race)
Aerobic Cellular Respiration ATP for any activity lasting over 30 seconds if sufficient oxygen is available, pyruvic acid enters the mitochondria to generate ATP, water and heat fatty acids and amino acids can also be used by the mitochondria Provides 90% of ATP energy if activity lasts more than 10 minutes
Muscle Fatigue Inability of muscle to maintain force of contraction after prolonged activity. Factor that contribute: Inadequate release of Ca 2+ from SR because of declining Ca 2+ in sarcoplasm Depletion of creatine Insufficient O 2 Depletion of glycogen and other nutrients Buildup of lactic acid Not enough release of acetylcholine
Muscle Cramps
Oxygen Debt The added oxygen, over and above resting oxygen consumption, that is taken into the body after exercise. Used to: Convert lactic acid back into glycogen stores in the liver Resynthesize creatine phosphate and ATP in muscle fibers Replace O 2 removed from myoglobin
Recovery Oxygen More than just repaying oxygen debt Increased temperature means more chemical reactions that require more ATP Heart and lungs working harder consumes more ATP Tissue repair at an increased pace requires more O 2
The Motor Unit Motor Unit: 1 somatic neuron and the muscle cell it innervates (stimulates) Afferent Neuron: sensory nerve Efferent Neuron: motor nerve The motor neuron leaves the spinal cord and synapses with the muscle cell (the effector)
The axon of the motor neuron branches and synapses to approximately 150 muscle cells Muscles for precision 1-2 muscle fibers per unit Large muscles 2000-3000 muscle fibers per unit
Motor Unit Recruitment Helps to resist fatigue Weakest units are recruited first and other are added if more force is needed Allows for smoother movement
Myoglobin Oxygen carrying protein in muscles Contains 1 heme (cofactor containing Fe 2+ ) group that can bind 1 molecule of O 2 More myoglobin = red meat
Exercise Type of fibers is genetically determined Total number of skeletal muscle fibers does not change with exercise, but aerobic training can cause a gradual change from FG to FOG fibers. FG fibers will increase in size and the number of mitochondria, blood supply and strength. Strength training increase the size and strength of FG fibers.
Anatomy of Cardiac Muscle Single centrally located nucleus / Skeletal is multinucleated Cells connected by intercalated discs with gap junctions Same arrangement of thick & thin filaments as skeletal
Physiology of Cardiac Muscle Autorhythmic cells contract without stimulation Contracts 75 times per min & needs lots O 2 Sustained contraction possible due to slow Ca 2+ delivery Ca 2+ channels to the extracellular fluid stay open
Smooth Muscle
Smooth Muscle Although they have thick and thin fibers, there are not arranged in orderly sarcomeres and therefore do not appear striated. They also lack transverse tubules and only have a small amount of SR for storage of Ca 2+ Ca 2+ ions stored in small pouch like invaginations of the plasma membrane call caveolae There is no troponin in smooth muscle fibers instead, calcium binds to calmodulin.
Smooth Muscle Contraction Dense bodies are similar to Z-lines in a sarcomere. Norepinephrine and acetylcholine can serve as neurotransmitters. Contraction can be affected by hormones (ex: childbirth) Smooth muscle is slower to contract & relax but can forcefully contract longer with the same amount of energy used.