Skeletal Muscle Tissue

Skeletal Muscle Tissue Arrangement Myofibrils – contractile elements of muscle tissue

Skeletal Muscle Cont. Muscle fiber – Muscle cell; composed of several myofibrils

Skeletal Muscle Cont. Each muscle fiber is surrounded by a thin sheath of areolar connective tissue called endomysium

Muscle Tissue Cont. Fascicles – A bundle of muscle fibers. There are usually between 10 to 100 muscle fibers in a fascicle.

Muscle Tissue Cont. Each fascicle is surrounded by a layer of dense irregular connective tissue called perimysium

Muscle Tissue Cont. Whole muscle – made up of several fascicles

Muscle Tissue Cont. The whole muscle is surrounded by a dense irregular connective tissue called epimysium

Muscle Tissue All three connective tissues (endomysium, perimysium, epimysium) extend beyond the muscle fiber to form a tendon.

Muscle Tissue Tendon – Composed of dense regular connective tissue that attaches muscle to the periosteum of the bone

General Features of Skeletal Muscle Striated

General Features of Skeletal Muscle Voluntary

General Features of Skeletal Muscle Multinucleated

General Features of Skeletal Muscle Controlled by the somatic (voluntary) division of the nervous system

Microscopic Anatomy of Muscle Fibers Muscle Fiber = Muscle Cell

Microscopic Anatomy cont. Sarcolema – plasma membrane of muscle cells or muscle fibers

Microscopic Anatomy cont. The multiple nuclei of each muscle fiber is located beneath the sarcolema

Microscopic Anatomy cont. T (tranverse tubules) – Invagination of the sarcolema that tunnel in from the surface to the center of each muscle fiber

Microscopic Anatomy cont. Sarcoplasm – cytoplasm of a muscle fiber

Microscopic Anatomy cont. Sarcoplasmic reticulum – fluid filled system of membranous sacs. Calcium is stored here.

Microscopic Anatomy cont. Dilated ends of SR are called terminal cisterns

Microscopic Anatomy cont. Myofibrils are composed of functional units called sarcomeres responsible for the striations

Microscopic Anatomy cont. Each sarcomere is separated from the next by z discs

Microscopic Anatomy cont. Sarcomeres are composed of thick (myosin) and thin (actin) filaments

Microscopic anatomy cont. A band is the part of the sarcomere composed of thick (myosin) and thin (actin) filaments

Microscopic anatomy cont. The A band is the dark striation seen under the microscope

Microscopic Anatomy cont. I Band is the part of the sarcomere that contains only thin (actin) filaments

Microscopic Anatomy cont. I Band is the light striation seen underneath the microscope

Microscopic Anatomy The H zone is the part of the A band that contains only thick filaments (myosin)

Microscopic Anatomy M line is the middle of the sarcomere and is composed of supporting proteins that hold the thick filaments together

How does a nerve initiate contraction? Neuromuscular junction – the region of contact between a motor neuron and a skeletal muscle fiber

Initiation of Contraction Synaptic cleft – the region between the neuron and muscle fiber

Initiation of Contraction The tips of axon terminals are called synaptic end bulbs

Initiation of Contraction Synaptic vessicles – membrane – enclosed sacs that contain the neurotransmitter acetylcholine (Ach) located in the synaptic end bulb

Initiation of Contraction Motor end plate – the region of the sarcolema opposite of the synaptic end bulb

Initiation of Contraction Each motor end plate contains between 30 to 40 million Ach receptors

Initiation of Contraction / 4 Steps 1. Once the nerve impulse arrives at the synaptic end bulb, the synaptic vesicles release Ach via exocytosis.

Initiation of Contraction / 4 Steps 2. When two ACh molecules bind to the ACh receptors at the motor end plate it opens the cation channel and Na+ can flow across the membrane.

Initiation of Contraction / 4 Steps 3. Once the inside of the muscle fiber is more positively charged, a muscle action potential is triggered, which propogates along the sarcolema and into the T tubule system.

Initiation of Contraction / 4 Steps 4. ACh is broken down by acetylcholinesterase in the extracellular matrix of the synaptic cleft.

Calcium’s Role Once the action potential propagates along the sarcolema and into the T tubules Ca2+ release channels in the SR membrane open causing Ca2+ to flow out of the SR into the cytosol.

Calcium’s Role Calcium binds to troponin on the actin filaments causing the troponin- tropomyosin complexes to move away from the myosin binding sites on actin.

Contraction / 4 Steps 1. ATP hydrolysis – ATP is hydrolyzed into ADP and a phospate by ATPase on a myosin head

Contraction / 4 Steps 2. Attachment of myosin to actin to form crossbridges – myosin binds to actin on the myosin binding site and the phosphate is released.

Contraction / 4 Steps 3. Power stroke – The myosin pushes the thin filament past the thick filament toward the M line releasing ADP.

Contraction / 4 Steps 4. Detachment of myosin from actin – When ATP binds to the myosin head, the myosin head detaches from actin.

Contraction As the muscle contracts the I band and H zone decreases

Relaxtion Once nerve impulses stop; 1.Acetylcholinesterase breaks down the remaining acetylcholine 2.Muscle action potentials stop 3.Calcium levels in cytosol decreases 4.Contraction stops

How do calcium levels decrease? Ca2+ release channels close Ca2+ active transport pumps move Ca2+ back into the SR In the SR calsequestrin binds to Ca2+ enabling more Ca2+ to be sequestered within the SR

Rigor Mortis Calcium leaks out of the SR therefore allowing myosin heads to bind to actin. ATP production ceases so myosin cannot detach form actin. Muscles therefore become rigid (cannot contract or stretch)

Atrophy Muscle fibers decrease in size due to loss of myofibrils

Hypertrophy Muscle fibers increase in diameter due to the production of more myofibrils.

ATP and Muscle Muscle fibers need ATP for powering the contraction cycle and to pump Ca2+ into the SR.

ATP and Muscle ATP is made by; 1.Creatine phosphate 2.Anaerobic cellular respiration 3.Aerobic cellular respiration

Creatine Phosphate When the muscle is relaxed creatine kinase (CK) transfers a phosphate from ATP to creatine forming creatine phosphate and ADP.

Creatine Phosphate ATP + Creatine → ADP + Creatine Phosphate This reaction is catalyzed by creatine kinase

Creatine Phosphate When a muscle contracts CK tranfers a phosphate from creatine phosphate to ADP forming ATP and creatine.

Creatine Phosphate Creatine Phosphate + ADP → Creatine and ATP This reaction is catalyzed by CK

Anaerobic Cellular Respiration Does not require oxygen ATP is formed by a process called glycolysis A glucose is converted into two pyruvic acid molecules

Anaerobic Respiration Glycolysis uses two ATP but forms 4 ATP for a net gain of two Pyruvic acid is converted into lactic acid

Anerobic Respiration Muscle fibers attain their glucose via diffusion from the blood and glycogen stored within muscle fibers

Aerobic Respiration Requires oxygen Takes place in mitochondria The two molecules of pyruvic acid produced in glycolysis enter the kreb cycle. Aerobic respiration results in a net gain of 36 ATP.

Aerobic Respiration In aerobic respiration oxygen is attained via the diffusion of oxygen from blood and oxygen released by myoglobin

Aerobic Respiration Myoglobin is a protein found in muscle cells that binds oxygen

Motor Units There is only one neuromuscular junction per fiber.

Motor Units A somatic motor neuron branches out and forms neuromuscular junctions with many muscle fibers.

Motor Units A motor unit consists of a somatic motor neuron plus all the skeletal muscle fibers it stimulates

Motor Units All muscle fibers in a motor unit contract in unison

Motor Unit Muscles that produce precise movements are made up of small motor units.

Red Muscle Fibers Have a high myoglobin content

White Muscle Fibers Have a low myoglobin content

3 Main Types of Skeletal Muscle Fibers 1.Slow Oxidative Fibers 2.Fast Oxidative-Glycolytic Fibers 3.Fast Glycolytic Fibers

Slow Oxidative Fibers Smallest in diameter Contain large amounts of myoglobin Generate ATP by aerobic cellular respiration Large amounts of mitochondrial and blood capillaries ATPase in the myosin head hydrolyzes ATP slowly

Fast Oxidative-Glycolytic Fibers Intermediate in diameter High myoglobin content Generates ATP by aerobic and anaerobic respiration High content of mitochondria and blood capillaries ATPase hydrolyzes ATP quickly

Fast Glycolytic Fibers Largest in diameter Low myoglobin content Few blood capillaries and mitochondria Generate ATP by anaerobic respiration ATPase hydrolyzes ATP quickly

Motor Unit Muscle fibers of a single motor unit are of the same type

Origin and Insertion Most muscles cross at least one joint and are attached to the articulating bones that form the joint.

Origin and Insertion When a muscle contracts, it draws one articulating bone toward the other.

Origin and Insertion The attachment of the stationary bone is the origin.

Origin and Insertion The attachment of the movable bone is the insertion

Twitch contraction The contraction of all the muscle fibers in a motor unit in response to a single action potential

Myogram A record of a muscle contraction

Myogram of a Twitch Contraction 1.Latent period 2.Contraction period 3.Relaxation period

Myogram of a Twitch Contraction 1.Latent period – Lasts two milliseconds Calcium ions are released from SR

Myogram of a Twitch Contraction 2.Contraction period – 10 – 100 msec

Myogram of a Twitch Contraction 3.Relaxation Period – 10 – 100 msec Active transport of calcium into SR

Frequency of Stimulation Wave summation – When a second stimulus occurs before the muscle has relaxed, the second contraction is stronger than the first.

Frequency of Stimulation Unfused tetanus – When a skeletal muscle is stimulated at a rate of 20 to 30 times per second, it can only partially relax between stimuli resulting in a sustained but wavering contraction.

Frequency of Stimulation Fused tetanus – When a skeletal muscle is stimulated at a rate of 80 to 100 stimuli per second, a sustained contraction results in which individual twitches cannot be discerned.

Motor Unit Recruitment Not all motor units in a muscle are not stimulated at once to prevent fatigue.

Concenteric Isotonic Contraction A muscle shortens and pulls on a tendon, which produces movement and reduces the angle at a joint.

Eccenteric Isotonic Contraction The length of a muscle increases during contraction.

Isometeric Contractions The muscle doesn’t shorten because the force of the load equals muscle tension.