The Muscular-Skeletal System

Textbook Chapter 35The Muscular-Skeletal System

Four Types of TissuesEpithelial Tissue: covers the outside of the body and lines the organs and cavities within the bodyConnective Tissue: binds, supports, and connects other tissues (ie. blood, tendons, ligaments, bones)Muscle Tissue: consists of long cells called muscle fibers, which contract in response to motor neurons; muscle fiber = one cellNervous Tissue: senses stimuli and transmits signals

Different Types of Muscle TissueSkeletal Muscle: Attached to the skeleton (voluntary)Contain multiple nucleiHave a striated pattern consisting of sarcomeres (myofibrils are parallel)Cardiac Muscle:Heart muscles that contract to pump blood (involuntary)Irregularly striatedHigh duration and continual useInitiates own contractions through the SA and AV nodesSmooth Muscle:Contract to move materials throughout the bodySlow and sustainedNot striatedLine hollow organsExample: digestive muscles move food through peristaltic contractions

Types of Muscle Tissue

Skeletal Muscle HierarchySkeletal muscle is characterized by a hierarchy of smaller and smaller unitsOne muscle, myofiber or myocyte is covered by epimysium (connective tissue)One muscle consists of hundreds or thousands of muscle fibers (one muscle fiber is one cell)Muscle fibers come in bundles called fasciles, which are surrounded by perimysium (connective tissue)Each muscle fiber is a long cell that runs parallel to the muscleThe functional contractile unit of a muscle fiber are the myofibrilsThe functional contractile unit of myofibrils are sarcomeres

Skeletal Muscle StructureEach muscle fiber contains many nuclei and is covered by a capillary-lined membrane, called the sarcolemmaSarcoplasm is the cytoplasm of muscle fibersThe T-tubules are in constant contact with the extracellular fluid, and conduct the action potentials into the fiber interiorThe sarcoplasmic reticulum is derived from the ER. The sarcoplasmic reticulum transports materials throughout the muscle fiber and releases Ca2+The sarcosomes are the mitochondrion of muscle fiber and produce ATP energy for the muscle fiberThe myofibrils are the functional unit of a muscle fiber. They are long bundles of the protein actin (thin filaments) and myosin (thick filaments) that contract Myofibrils are composed of subunits called sarcomeres, which give skeletal muscle is striation. Sarcomere units are separated by Z lines. Sarcomeres are composed of myosin suspended between actin. They also contains accessory proteins troponin and tropomyosin

Skeletal Muscle Structure

Sarcomere StructureSarcomeres are composed of myosin proteins suspended between actin proteinsEach individual sarcomere is separated by Z-lines, which are made of titin (connectin) and anchor actin proteinsI-band is the zone of actin not superimposed by myosinA-band is the whole length of a single actinH-band is the zone of myosin not superimposed by myosinA contraction is the result of myosin pulling actin filaments closer together

Skeletal Muscle Structure

Muscle Stimulation MechanismMuscle contraction of several fibers is stimulated by a motor neuronAcetylcholine is released from the synaptic bulb of the motor neuron into the synaptic cleftAcetylcholine binds to the sodium potassium pumps on the motor end plate of the muscle fiberThe sodium potassium pumps depolarize the environmentThis triggers a self-propogating action potential down the membrane of the muscle fiberAcetylcholinesterase on the membrane hydrolyzes the left-over acetylcholine to prevent over-stimulationThe action potential travels down the membrane and then into the T-tubulesIn the T-tubules, the action potential activates voltage-gated calcium channels that leads into the sarcoplasmic reticulumThe sarcoplasmic reticulum then releases calcium ions into the sarcoplasm

Sliding Filament MechanismEach actin filament has binding sites that the myosin head attaches toTropomyosin covers those binding sites to prevent the unnecessary binding of myosin and actinCalcium ions released by the sarcoplasmic reticulum binds to the troponin complex, which is also located on the actin filament. The troponin complex changes configuration and moves the tropomyosin aside, uncovering the binding sites on the actinMyosin heads in their high energy state bind to myosin binding sites on the actin, forming a crossbridge attachment After binding, the myosin head changes from its high energy configuration to its low energy configuration, which causes the myosin head to bend and pull the actin filaments towards the center of the sarcomereATP energy is used to unbind the two filaments and return myosin heads to its high energy straight configuration The binding, flexing, and detaching process pulls the actin filaments closer together, which shortens the length of the sarcomere, therefore contracting the muscleComplete Process Animation

Sliding Filament Mechanism

Complete Contraction Mechanism

Nervous Control of MuscleMuscle contraction is stimulated by acetylcholine released from motor neuronsA motor neuron stimulates multiple fibers, motor unitThe strength of muscle contraction depends on the number of fibers contracting and rates at which each fiber contractsThe strength of muscle contraction also depends on the number of motor neurons stimulated and the frequency of their action potentialsTeatanus is a state of smooth and sustained contraction produced when motor neurons deliver a volley of action potentials

Hydrostatic SkeletonA hydrostatic skeleton provides locomotion through the use of peristaltic movements and the use of fluidThe contraction of circular and longitudinal muscle builds up hydraulic pressure in the body cavityThe hydraulic pressure is used to propel the organism forwardFound in cnidarians, flatworms, nematodes, annelids

Hydrostatic Skeleton Movement

ExoskeletonExoskeletons are hardened and segmented encasements on an animals outer surface to which internal muscles are attached Exoskeletons are strong and flexibleExoskeletons are made of chitinFound in arthropods and mollusks

EndoskeletonThe endoskeleton consists of hard supporting elements, such as bone, located under soft tissue (muscle)Some bones are fused, others are connected at jointsFound in chordates

Functions of the Human Skeletal SystemSupport: vertebral column helps maintain posture)Protection: the thoracic cage protects internal organsMovement: antagonistic muscles on skeletons contractStorage: calcium, phosphorus, other minerals, fatsBlood cell formation: bone marrow produces red and white blood cells

Axial SkeletonThe central skeletonResponsible for mostly support and protectionSkull:Cranium: protects brainSinal Cavity: warms and moistens airVertebral column: protects spine and is segmented by intervertebral discsThoracic Cage: protects major organsMade of sternum, ribs, thoracic vertebrae

Appendicular SkeletonThe skeleton that forms the appendagesResponsible for mostly movement, storage, and blood cell formation, with the exception of the pelvisLimbsPelvisSupports the entire upper bodyProtects reproductive organs, bladder, and large intestine

Axial and Appendicular Skeleton

BoneResponsible for support, movement, storage, blood cell formation, and protectionOsteocytes are living and mature bone cellsCompact (cortical) bone is mainly responsible for strength, support, and storage of mineralsHas concentric rings, osteons, of hydroxyapititeHydroxyapitite is a salt made of calcium and phosphorus and gives the bone its strengthStores calcium and phosphorus through hydroxyapititeSpongy (cancellous) bone is responsible for most metabolic activity in bones and growthUsually found at the ends of long bonesContains SOME red bone marrow (MOST are in central cavity)Blood vessels and nerves run through the Haversian canal

BoneBone has a semi-hollow space in the middle called the central cavityThe central cavity is mostly responsible for blood cell production and fat storageContains most of the red and yellow bone marrowRed bone marrow produces red and white blood cellsYellow bone marrow stores fats and can be readily converted into red bone marrow when blood levels are dangerously low

Bone Structure

Bone Remodeling and GrowthBone growth and remodeling is mostly regulated by osteoblasts and osteoclastsOsteoblasts secrete collagen on top of boneCollagen hardens into hydroxyapititeOsteoblasts grow bone by increasing widthEventually die by apoptosis or change into osteocytesWork harder when there is stress on bonesOsteoclasts break down bone tissueOsteoclasts secrete enzymes, hormones, and acid that dissolve boneCalcium and phosphorus are releasedOsteoclasts work in response to PTHThe process of removing and building bone is called bone remodelingOsteoblasts grow bones in terms of width, but bones grow in terms of length through the epiphyseal plates (growth plates) found in the spongy bone

Bone Remodeling Video

JointsJoints cushion and hold connected bones togetherResponsible for movement by allowing for mobilityReduces friction of rubbing bonesJoint cells are called chondrocytesSecrete glycoproteins and collagen that reduces frictionFour main types of joints:Fused joints: do not move and connect bone (cranium)Pivot joints: allow for pivoting circular motion (vertebrae)Hinge joints: allow for 180 degree movement (elbow)Ball and socket joints: allow for 360 degree movement (femur)Joints can also be classified on their mobility:Synarthroses: immovable joints (skull)Amphiarthroses: slightly moveable joints (vertebrae)Diarthroposes: freely moveable joints (limbs)

Joint Types

Connective Tissue of Bone and MuscleResponsible for movementLigamentsAttaches bone to boneDense connective tissueTendonsAttaches muscle to boneCords of dense connective tissue