Chapter 6

Chapter 6 – The Muscular System Use the terminology associated with the musculature system Learn about the following: Different types of muscle cells Muscle tissue development Gross and fine muscle structure Gross muscle function Muscle cell physiology Muscle types and actions Muscle development and growth Understand the aging and pathology of the musculature

Muscle cells change their shape by shortening along one or more planes; this is also called contraction. Over half the body’s mass is composed of muscle tissue, and over 90% of this muscle tissue is involved in skeletal movement. Overview Chapter 6 – The Muscular System

Functions of the Muscular System Moves the skeletal system Passes food through the digestive system Helps in dilation and constriction of blood vessels Helps in movement of air in and out of lungs Helps with movement of urine out of the bladder. Pumping blood throughout the body

Muscle Tissue Muscle is composed of contractile cells. Contractile cells can change their shape. Over half of the body’s mass is composed of muscle tissue. 90% of this muscle tissue is involved in skeletal movement. The rest would be used in cardiac tissue (the heart) and smooth muscle tissue (the digestive organs and circulatory system)

Muscle Tissue Contractile cells have high energy needs. So they need a lot of blood supply. Blood supplies muscles (contractile cells) with –Glucose –Oxygen –Electrolytes-ions essential for muscle contractions Blood removes large amounts of metabolic wastes. Muscles along with nervous tissue consume almost 70% of the food energy taken into the body every day. Like the skeletal system muscle consumes a lot of calcium Body Mass Index (BMI) is an indirct measure of body density.

Three types of muscle are found in the human body: Smooth muscle Cardiac muscle Skeletal muscle Muscle Chapter 6 – The Muscular System

Skeletal Muscle Long cylindrical cells Many nuclei per cell because several myoblasts are fused together Striated Voluntary Rapid contractions

Cardiac Muscle Branching cells One or two nuclei per cell Striated Involuntary Medium speed contractions

Smooth Muscle One nucleus per cell Nonstriated Involuntary Slow, wave-like Contractions Location: lining of blood vessels, digestive organs, urinary system

Skeletal Muscle Structure  1st level- Most basic -the muscle fiber or cell  Each muscle fiber is covered with a connective layer called the endomysium.  2nd level- Bundles of muscle cells are called fascicles (fasciculi)  Perimysium-a thin connective tissue covering that surrounds each fascicle.  3rd and highest organ level of skeletal muscle structure Epimysium-a fibrous connective tissue that covers the gross muscle and also the tendons that attach muscle to bone and skin.

Development of Muscle tissue Myogenesis- process of muscle tissue developing from mesoderm cells. Myoblasts- stem cells that form muscle tissue. Growth factors-are chemicals that act as signals to initiate cell division and differentiation of muscle tissue. Over a dozen genes involved in muscle cell development.

Muscle Cell Structure Skeletal muscle cells are long, cylindrical cells covered with an excitable membrane and filled with a specialized cytoskeleton. Sarcolemma-membrane of muscle cells Sarcoplasm- cytoplasm of muscle cells Cytoskeleton is located in the sarcoplasm -- Cytoskeleton is composed of bands of proteins called myofilaments. Three types of myofilaments: –Thick- composed of myosin –Thin- composed of actin (majority), wrapped around a length of tropomyosin, and speckled on the coils of the actin are small proteins called troponin. –Vertical- composed of the protein titin, it is considered an elastic myofilament.

Structure of the muscle fiber (muscle cell) Myofibrils-long cords of myofilaments (bands of proteins making up the cytoskeleton) that form parallel bundles that comprise most of a muscle cell’s interior. Sarcomere-the contractile unit of a muscle cell. (These chains of sarcomeres form myofibrils). Muscle cell (fiber)-is made up of many bundled myofibrils that run parallel to one another for the length of the cell. Thick and thin myofilaments arrange to form an overlapping pattern within a sarcomere. The thin myofilaments (actin) are attached to a protein structure called the Z-line. The thick myofilaments (myosin) seem to be floating between the rungs of the thin myofilaments, but they are held in place by invisible titin filament that attaches them to Z-line.

Microanatomy of Skeletal Muscle

Z line

Muscle Cell Structure Z-line Function To keep the thick and thin filaments aligned. To help control the stretch and recoil limits in a muscle. Serves a role in muscle contractions. It anchors the sarcomeres to the sarcolemma. Any movement of the Z-line changes the length of the muscle. Sarcoplasmic reticulum Function A system of inner membrane tubes that store and transport large amounts of calcium needed for muscle contraction.

H Band

Sarcomere Relaxed

Sarcomere Partially Contracted

Sarcomere Completely Contracted

Muscle cell contraction Simultaneous shortening of all the sarcomeres within a cell. Three stages: Neural stimulation Muscle cell contraction Muscle cell relaxation Neural stimulation Takes place at neuromuscular junction Contraction is initiated when end of nerve cell releases neurotransmitter Neurotransmitter is acetylcholine which binds to receptors in sarcolemma This causes changes in sarcolemma and allows transport of ions across the membrane Sodium ions flow into the muscle cell and potassium flows out of the cell Causes sarcoplsmic reticulum to release calcium The flow of calcium initiates the muscle contraction phase.

Muscle cell contraction Calcium binds to troponin on actin myofilamentts This causes binding site to open up so that myosin can bind to actin Also activates the attachment of ATP to myosin ATP provides energy for myosin head to swivel and hook on to the binding site on actin The swivel movement brings the two Z lines closer together This shortens the sarcomere The complete contraction of a muscle cell requires several cycle of neural stimulation and contraction phases. Muscle cell relaxation This begins when there is no more neural stimulations The sodium and potassium levels are back like they were originally The sarcoplasmic reticulum has recovered most of the calcium This causes a release of the myosin heads from actin There is no mechanism within the muscle cell for lengthening the sarcomere The muscle cell remains contracted The muscle is fully recovered when a body movement causes the sarcomere to stretch.

Rigor mortis Causes when calcium leaks out of the SR into the sarcomere. This is common after death Eventually, muscle cell structures begin to decay Causing muscles to become soft and loose Other factors that ensure adequate muscle contractions Having creatine phosphate present It is a molecule that stores energy in muscle cells It collects energy from ATP and can store energy for long periods of time. It then transfers energy back to ATP when muscle contractions require energy Having Glycogen present it is a stored form of glucose, important source of energy reserve for muscle action Having Myoglobin present It is a red colored chemical that stores oxygen for certain muscle cells. Having oxygen in muscle cells permits them to provide large amounts of ATP during continuous or heavy work

Neuromuscular Junction

1.Axon of motor junction3. Muscle fiber 2. Neuromuscular junction4. Myofibril

1.Presynaptic terminal3. Synaptic vesicles 2. Postsynaptic terminal 4. Synaptic cleft (sarcolemma)5. Mitochondria Neuromuscular Junction

Acetylcholine Opens Na + Channel

Muscle Contraction Summary Nerve impulse reaches myoneural junction Acetylcholine is released from motor neuron Ach binds with receptors in the muscle membrane to allow sodium to enter Sodium influx will generate an action potential in the sarcolemma

ATP

Creatine Molecule capable of storing ATP energy Creatine + ATPCreatine phosphate + ADP

Creatine Phosphate Molecule with stored ATP energy Creatine + ATPCreatine phosphate + ADP

Human Skeletal Muscles