Skeletal System Lecture Notes PDF

**UNIT 2** **The Skeletal System** The skeletal system is composed of: - Bones - Cartilage - Precursor for most bones - Covers many joint surfaces - Ligaments - Hold bones together at joints - Bone marrow ALL joined together to form flexible framework of the body!...

**UNIT 2** **The Skeletal System** The skeletal system is composed of: - Bones - Cartilage - Precursor for most bones - Covers many joint surfaces - Ligaments - Hold bones together at joints - Bone marrow ALL joined together to form flexible framework of the body! **Functions of the Skeletal System** - Support - Framework of our body - Protection - Encloses vital organs like the brain, spinal cord, heart, etc. - Movement - Movement is the result of the action of muscles on bones - Blood formation - Red bone marrow is the major producer of blood cells - Storage - Main reservoir of calcium & phosphorus **Osseous Tissue** Connective tissue w/ a hard calcified matrix. Exist in 2 Forms: 1. Compact (dense) bone - Has a solid matrix 2. Spongy bone - Is porous w/ spaces Contains: - Blood supply - Cartilage - Adipose tissue - Fibrous connective tissue **Bone Cells** - Osteogenic cells - Osteoblasts - Osteocytes - Osteoclasts **Osteogenic Cells** - Bone cells - Stem cells that give rise to osteoblast - Occur on bone surfaces beneath fibrous connective tissue that covers bone - The only bone cell capable of dividing & making more bone cells **Osteoblasts** - Bone cells - Bone forming cells - Synthesize the organic matter of the bone & promote mineralization - Lie in a single layer on the bone surface **Osteocytes** - Bone cells - most abundant bone cell - develop from osteoblasts that have become trapped in the matrix they developed - Reside in cavities called: lacunae - Lacunae are connected by small channels called canaliculi - Cytoplasmic process connects to neighboring cells via canaliculi - Functions: - Orchestrate bone remodeling by directing bone reabsorption & deposition - Play a role in maintaining bone density - Sense mechanical stress & secrete signals that adjust bone shape & density to adapt - This is why weight bearing exercises help preserve bone integrity & strength **Osteoclasts** Crater= osteoclasts - Bone cells - Bone dissolving cells - Develop from separate stem cell line - Have ruffled border on side facing bone - This surface secrete hydrochloric acid & enzymes to dissolve osseous tissue **Bone Matrix** - Matrix is a hard matters that surrounds osteocytes & lacunae - 1/3 is organic matter: mainly collagen & other proteins - Collagen gives bone its flexibility & strength - Can bend slightly before snapping - 2/3 are inorganic: mainly calcium phosphate & minerals - Minerals prevent bones from bending under weight **Spongy Bone** Consist of: - Porous lattice of slender rods & plates called traveculae - Spaces are filled w/ bone marrow & small blood vessels **Compact Bone** - Forms outer shell of bone - Prevents bone marrow from leaking out - Provides solid attachment surfaces for muscles, tendons, & ligaments - Organized in elongated cylinders called osteons - Look like tree rings w/ concentric layers called lamellae - Arranged around a central canal containing small blood vessels & nerves - Osteocytes occupy lacunae between the lamellae **Bone Development- Ossification** Ossification= the formation of bone 2 methods: 1. Intramembranous 2. Endochondrial **Intramembranous Ossification** - Produces the flat bones of the skull - Begins w/ a sheet of embryonic connective tissue - Embryonic cells deposit collage in this membrane - Mineral salts are then deposited to harden tissue - Creates thin calcified plates (trabeculae) - Inner & outer surfaces are calcified - Sandwiches spongy bone between compact bone **Endochondrial Ossification** - Produce most of the bone of the body - Begins w/ hyaline cartilage "model" in approximate shape of bone it will become - Begins around 6^th^ week of feetal development & continues into early 20's= happens intrauterine- while fetus is still inside mothers uterus. **Endochondrial Ossification: Step 1** Future bone begins as hyaline cartilage **Endochondrial Ossification: Step 2** - Chondrocytes begin to enlarge & die in primary ossification center - Near middle of hyaline cartilage - Thin walls between them begin to calcify - Osteoblast begin to deposit a thin layer of bone around cartilage model - Forms a collar that reinforces shaft of future bone - Fibrous sheath around collar become periosteum **Endochondrial Ossification: Step 3** - Blood vessels pierce periosteum at primary ossification center - Stem cells arrive via blood - Stem cells differentiate into osteoclasts & digest calcified tissue - Hollows out shaft & creates primary marrow cavity - Other stem cells differentiate into osteoblasts - Deposit layers of bone, thickening the shaft - Secondary ossification center forms at one end of the bone - Similar to the primary ossification center **Endochondrial Ossification: Step 4** - By the time of birth: - Secondary ossification center has hollowed out secondary marrow cavity - Another secondary ossification center forms at the other end of bone **Endochondrial Ossification: Step 5** - Throughout childhood & adolescence: - Primary & secondary marrow cavities are separated by the epiphyseal plate - Consist of a middle layer of hyaline cartilage bordered on each side by an area where cartilage transitions to bone - Plate function as a growth zone, allowing individuals to grow in limb length & height **Endochondrial Ossification: Step 6** - By late teens- early 20's: - Cartilage in the epiphyseal plate is depleted - The primary & secondary marrow cavities unite into a single medullary cavity - Bones can no longer grow - One attains max adult height **Growth & Remodeling** - Bones grow & remodel themselves throughout life - Change size & shape to accommodate the changing forces applied to the skeleton - Tension on skeleton stimulates increase in bone mass - People who exercise regularly or have manual labor jobs have greater bone mass than sedentary people - Surface osteoblasts deposit osteoid tissue, calcify it, & become trapped in it as osteocytes **Mineral Homeostasis** Bone is a metabolically active organ - Exchanges minerals w/ extracellular fluid - Skeleton is primary reservoir for calcium & phosphate - Phosphate used as a component of DNA, RNA, ATP, phospholipids, etc. - Calcium is used for muscle contraction, blood clotting, exocytosis, etc - Homeostasis maintained via - Deposition - Resorption Mineral deposition - Osteoblasts extract calcium, phosphate, & ions from blood & deposit them is osseous tissue Mineral Resorption - Osteoclasts dissolve bone, releasing minerals into the blood - Making them available for other uses  **Muscular System** **Types of Muscle:** - Skeletal - Cardiac - Smooth \*The muscular system only refers to skeletal muscle\* **Functions of Muscles** - Movement - Stability - Control of body opening & passages - Heat generation - Glycemic control **Movement of Muscles** Includes external visible movements Ex: lifting your arm Also includes internal movements - Breathing, moving contents within digestive tract, pumping blood, etc. **Stability** - Prevent unwanted movement - Maintaining posture - Ability to move one bone, while keeping another one still  **Control of Body Openings & Passages** - Ring shaped [sphincter muscles] - Can regulate the movement of content from one area to the next - Ex: digestive tract **Heat Generation** The skeletal muscle produces up to 30% of our body heat at rest & up to 40x as much during exercise **Glycemic Control** - Regulation of blood glucose - Skeletal muscle plays a significant role in stabilizing blood sugar level by absorbing a large share of it **Skeletal Muscle Fibers** - Skeletal muscle is primarily voluntary - Subject to concious control - Skeletal muscle is also strated - Has striations which are: - Alternating light & dark bands - Overlapping arrangement of internal proteins that enable it to contract **Structure of Muscle Fibers**  - Skeletal muscle cells are called: muscle fibers - Named this based on long slender shape - Muscle fibers have multiple nuclei - Bundles of contractile proteins within muscle cells called myofibrils - Range in \# from 7- 1,000+ - Numerous mitochondria, network of smooth ER, glycogen, & red oxygen- binding pigment, myoglobin packed between bundles **Plasma Membrane of Muscle Fibers** - Called the sarcolemma - Sarcolemma has tunnel like infoldings called transvers (T) tubules - Penetrate through fiber & emerge on other side - Function: carry an electrical current from surface of the cell to the interior when the cell is stimulated Smooth ER of the Muscle Fiber Smooth ER of muscle fiber is called sarcoplasmic reticulum Forms web around each myofibril & T tubules - Have dialated sacs called terminal cisterns SR is a reservoir for calcium ions - Calcium releases a flood of calcium into cytosol to activate contraction process **Myofilaments & Striations** Myofibrils are packed contractile proteins called myofilaments 2 main kinds: - Thick filaments - Thin filaments **Thick Filaments** - Made of several hundred proteins called myosin - Myosin head is shaped like a gold club - 2 polypeptides intertwined to form a shaft like tail - Double globular head projecting at an angle - Think of thick filaments as a bundle of 200-500 "gold clubs" w/ head pointed outward ina helical array **Thin Filaments** - About half as wide as thick filaments - Composed mainly of intertwined strands of a protein called actin - Looks like a strong of globular subunits - Also has 2 proteins called tropomyosin & troponin - Act as a molecular "switch" that either allow or inhibit muscle contraction **Myofilaments & Striations** - Patterns of myofilaments overlap gives muscle fibers striations Dark bands are called A bands - Where thick & thin filaments overlap Light bands are called I bands - Consists of only thin filaments - Intersected by Z discs - Made of proteins that anchor the thin filaments Segment of a myofibril from one Z disc to next is called a sarcomere - Muscle shortens when contracting b/c sarcomeres shorten & pull Z discs closer to each other **The Nerve Muscle Relationship** Skeletal muscle must be stimulated by nerve to contract - Nerve cells that stimulate skeletal muscles are called s neurons - Located in brain & spinal cord - Their axons, part that transmits electrical signal led to muscles - Each motor neuron stimulate all the muscle fibers of a group of fibers, such as in a muscle, to contract at once - One motor unit + all muscle fibers supplied by it are called a motor unit **The Nerve-Muscle Relationship** Where axon meets another cell is called a synapse Where axon meets a skeletal muscle fiber, synapse is called a neuromuscular junction Axons ends look bulbous & are called axon terminal - Nestled in a depression on the muscle fiber Two cells don't actually touch, separated by a gap called the synaptic cleft Axon terminal contains membrane bound sacs called synaptic vesicles which contain a signaling molecule called acetylcholine (Ach) - Ach is a neurotransmitters - Chemical signal sent by NS to a cell When neuron releases Ach, diffuses across synaptic cleft, bind to Ach receptor on surface of muscle fiber - This binding stimulates muscle fiber to contract Along w/ Ach, muscle fiber also has acetylcholinesterase (AChE) which breaks down Ach - Stops stimulation of the muscle causing it to relax - AChE also found in the synaptic cleft **Muscle Excitation** Excitation: is the process of converting electrical nerve signal to an electrical signal in the muscle fiber. **[Muscle excitation occurs in 3 step process:]** **Step 1:** - Nerve signals arrives at synapse - Stimulates synaptic vesicles to release acetylcholine (Ach) into synaptic cleft **Step 2:** - Ach bind to Ach receptors in the sarcolemma - Each receptor is a gated channel that opens in response to Ach - Sodium ions rush into muscle fiber & potassium exits - Driven by concentration & electrical gradients **Step 3:** - Ion movement electrically excite sarcolemma & initiate a wave of electrical changes called an action potential - Action potential spread in all directions away from neuromuscular junction - Like ripples spreading out in a pond - Muscle fiber is now "excited" - ***3 events must happen before contraction*** **3 event that must happen before contraction: Preparing for Contraction** - Rapid cyclic interactions between myosin & actin of thin & thick filaments drive contraction - In relaxed muscle, regulatory proteins block actin & myosin interaction - Excitation initiates a chain of events that allows myosin & actin to interact 3. **Step Process for Preparing for Contraction** 1. Excitation of T tubules open calcium channels in sarcoplasmic reticulum - Calcium floods cytosol of muscle fiber 2. Calcium binds to troponin molecules attached to thin filaments 3. Causes the associated tropomyosin to shift position, exposing the myosin binding active sites on the actin **Contraction** - Contraction - Muscle fiber develops tension & may shorten - Due to sliding filament model - Thick & thin filaments slide across each other, shortening the muscle fiber - **Occurs in 4 steps** **Contraction (occurs in these 4 steps)** Step 1 of contraction: - Each myosin head binds an ATP molecule & hydrolyzes it into ADP & a phosphate group - Causes head to move from flexed position to extended (high energy position) Step 2 of contraction: - Each myosin head bind to an active site on the thin filament - This link between a myosin head & actin filament is called a cross-bridge. Step 3 of contraction: - Myosin then releases the ADP & phosphate - Release moves myosin head back to original low-energy flexed position - Pulls thin filament along with it - This is called the power stroke - Head remains bound to actin until a new ATP binds Step 4 of contraction: - When new ATP binds, myosin head releases actin - Head is now ready to repeat the same process - Myosin hydrolyzes ATP to ADP and phosphate - Moves to high energy extended position - This is called the recovery stroke - Attaches to a new site farther down the thin filament & produces another power stroke This 4 step cycle repeats about every 5 strokes per second - Each stroke consumes one ATP A thin filament is pulled along the thick filament, it pulls a Z disc along with it - Thin filaments anchored to Z discs Meaning: sarcomeres shorten **Relaxation** - When nerve stops stimulating it, a muscle fiber relaxes - Returns to resting length - Relaxation occurs in 4 steps: 1. Never signal stops: axon terminal stops releasing Ach 2. AChE breaks down Ach- stimulation to muscle fiber stops 3. Sarcoplasmic reticulum reabsorbs calcium using active- transport pumps - ATP is required for muscle contraction and relaxation 4. Without calcium, troponin-tropomyosin complex shifts back and blocks myosin from binding to actin Muscles usually DON'T fully relax Usually in a state of partial contraction called: muscle tone - Keeping muscles firm at rest helps stabilize joints **Whole- Muscle Contraction** - Minimum contraction exhibited by a muscle cell is called a muscle twitch - Single cycle of contraction & relaxation - Twitch is very brief, 7 milliseconds to 0.1 seconds - Too brief & weak to do muscular work such as move a joint - Work requires summation of multiple twitches that occur when multiple nervouse stimuli occur in rapid succession - Rapid enough nervous stimuli allows muscle to relax only partially between twitches - Results in smooth contraction called incomplete tetanus - Motor units work in shifts to achieve this **Isometric vs Isotonic Contraction** - Contraction does NOT always mean shortening - Imagine lifting something heavy - Can feel tension build as the weight resists movement - This is called: isometric contraction - Tension rises enough to move heavy object & muscle shortens while maintaining constant tension - This is called isotonic contraction **Isotonic: Concentric vs Eccentric** Concentric: - Muscle shortens as it maintains tension - Ex: biceps brachii contracts & flexes the elbow during a bicep curl Eccentric - Muscle maintains tension as it lengthens - Ex: as you lower weight in becep curl - Maintain control Injuries during weight lifting are usually during the eccentric phase - Sarcomeres & connective tissue of muscle are pulling in one direction while weight is pulling muscle in opposite direction **Muscle Metabolism** All muscles contraction requires ATP - 2 ways of generating ATP: 1. Anaerobic fermentation 2. Aerobic respiration **Muscle Metabolism: Anaerobic Fermentation** - Pathway in which glucose is converted to lactate - For each glucose molecule, 2 ATP are produced - Does NOT use oxygen - Advantage - A way for muscle to produce ATP when demand is high but oxygen cannot be delivered fast enough to meet needs of aerobic respiration - Short bursts of exercise - Disadvantage - ATP yield is low **Muscle Metabolism: Aerobic Respiration** - Glucose is metabolized to pyruvate which is oxidized by mitochondria to CO~2~ and H~2~O - This produces 30 ATP per one glucose - Advantage - Far more efficient than anaerobic fermentation - Disadvantage - Requires oxygen - Cannot keep up w/ demand during intense exercise Muscle Metabolism: Example of shift during intense brief exercise - Uses aerobic respiration to start - Muscle fiber can convert some ADP back to ATP using phosphate storage molecule phosphate creatine - This is called the phosphagen system - Supplies enough ATP for about 6 second of sprinting or about 1 minute of brisk walking - When ATP sources are exhausted, shifts to anaerobin fermentation - Eventually faster heart rate & breathing provides enough oxygen to match demand & shift back to aerobic **Fatigue & Endurance** Muscle fatigue: - Progressive weakness that results from prolonged muscle use There are numerous cause of fatigue: - Depletion of glycogen & blood glucose - Leakage of calcium from sarcoplasmic reticulum - Accumulation of K+ in ECF, reduces excitability of the muscle fiber Endurance also depends on several factors - Muscles supply of myoglobin & glycogen - Density of blood capillaries - Number of mitochondria - Ones maximal rate of oxygen uptake - Will power **Types of Muscle Fibers** 2 primary types: 1. Slow twitch 2. Fast twitch Slow twitch: - Respond slowly but are relatively resistant to fatigue - Well adapted for aerobic respiration - Many mitochondria,, myoglobin, and blood capillaries Fast twitch: - Respond quickly but also fatigue more quickly - Well adapted for anaerobic respiration - Rich in enzymes for anaerobic fermentation, SR releases and reabsorbs CA++ quickly Muscular Strength & Conditioning - Resistance training: such as weight lifting - Muscle cells do not undergo mitosis: do NOT increase in number - Preexisting muscle cells instead increases in size - Does NOT significantly improve fatigue resistance Endurance (aerobic) exercise: such as running - Improves fatigue resistance - Increases glycogen stores, mitochondria numbers, and density of capillaries - Promote more efficient ATP production - Does NOT significantly increase strength - Optimal performance comes from cross- training **Nervous System** Coordinates cellular functions in 3 basic steps:  1. Senses information & sends messages to the central nervous system (CNS) - Changes within the body or changes in external environment 2. CNS receives & processes the information & determines appropriate response 3. CNS issues commands to skeletal muscles & other organs to carry out a response **Nervous System Anatomical Subdivisions: CNS** - Central nervous system (CNS) - Composed of brain & spinal cord - Enclosed in skull & vertebral column - Carries out processing (integrating) functions of nervous system **Nervous System Anatomical Subdivisions: PNS** - Peripheral nervous system (PNS)  - Composed of nerves leading to and from the CNS - Provides pathways of sensory input & motor output to structures that carry out its commands Functually divided into 2 divisions: 1. Sensory 2. Motor PNS: Sensory Division - Carries signals from various receptors to the central nervous system - Receptors in sense organs or sensory nerve ending - Provides information to central nervous system **Motor Division: Somatic Motor Division** - Carries signals to skeletal muscles - Produces voluntary movements - Movements under conscious control Motor Division: ANS - Carries signals to cardiac muscle, smooth muscle, and glands - Produces involuntary response - Not under conscious control - Ex: don't have to tell your body to breathe - ANS is subdivided into - Sympathetic division - Prepares the body for action - Think "fight or flight" - Activated by fear or when we need extra energy for exercise - Parasympathetic division - Involved in calming the body - Think "rest and digest" **Neurons** - Carry out the main function of the nervous system: communication - About 1 trillion neurons in the nervous system - Usually consists of a sort of globular cell body where the nucleus is located & 2 or more extensions that reach out to other cells **Neurons Basic Structure: Neurosoma** - Highly variable in shape - Neurosoma (soma or cell body) - Contains the nucleus= control center of the neuron - Contains organelles as well - Mitochondria - Lysosomes - Golgi complex - Rough endoplasmic reticulum - NO centrioles!!! - Neurons do NOT undergo mitosis - Neurons that die and NOT replacable **Neurons Basic Structure: Dendrites** - Thick arms arising from neurosoma & branching further - Look like branches on a leafless tree - "receiving end" of a neuron receives input from neighboring neurons - Anywhere from 1 to thousands of dendrites - More dendrites = more information it can receive to incorporate into its response - Dendritic spines - Tiny protusions form dendrites that increase connectivity between neurons - Dynamic processes which change their density & structure in response to stimuli - This "plasticity" is related to learning, memory, and cognitive function **Neuron Basic Structure: Axon** - Axon hillock on one side of neurosoma give rise to an axon - Output pathway for signals that it sends to other cells - **"**sending end" - Neurons have no more than one - Cylindrical structure- branches at ends into bulbs called exon terminals - Junction between axon terminal & cell its sending signal to = synapse **Structural & Functional Classes of Neurons** - Again, highly variable - Vary in shape depending on function & location - Can be classified both structurally & functionally Structurally: based on number of processes that arise from the neurosome - Multipolar - Bipolar - Unipolar Functionally: - Sensory (afferent) neurons - Interneurons - Motor (efferent) neurons **Structural Classes: multipolar** - One axon - Multiple dendrites - Most common type **Structural Classes: Bipolar** - One axon - One dendrite - Includes neurons associated w/ sense organs - Neurons in nose for olfacation (smell) - Neurons in retina for vision - Sensory neurons of the ear **Structural Classes: Unipolar** - Only process leading away from the neurosoma - Branches into a T - One are of T receives input via dendrites - Other arm is axon - Dendrites receive signals from sources such as skin and joints - Involved in senses of touch and pain - Axon leads to spinal cord **Functional Classes: Sensory (afferent) neurons** - Specialized to detect stimuli and send information about it to the CNS - Orginate in structures such as the: - Eyes - Ears - Skin - Joints - Carry information about sound, light, touch, pain, etc to the central nervous system - Afferent refer to signal traveling toward the central nervous system - Think "A" for Arrival - Usually unipolar or bipolar **Functional Classes: interneurons** - Perform integrative functions - Process, store, and retrieve information and "make decisions" about how the body should respond to a stimuli - Only found in the central nervous system - Most abundant of all the neurons - Usually multipolar Functional Classes: Motor (efferent) neurons - Specialized to carry outgoing signals form the central nervous system to the cells and organs that carry out commands - Begin in central nervous system and extend to muscle fibers and gland cells - Efferent refers to carrying signal away from central nervous system - Usually: Multipolar **Neuroglia** - Outnumber neurons at least 10 to 1 - We know less about neuroglia than neurons - Complete range of functions are still actively being researched - Support neurons and play central in healthy functioning of the nervous system - 4 kinds of glial cells in the central nervous system - Oligondendrocytes - Ependymal cells - Microglia - Astrocytes - 2 kinds of glial cells in the parasympathetic nervous system - Satellite cells - Schwann cells **Neuroglial Cells of the Central Nervous System** **Oligondendrocytes** - Resemble an octopus- multiple arm like processes - Arms surround nearby axons & form a layer of insulation called the myelin sheath Ependymal Cells - Line the internal fluid filled cavities of the brain and spinal cord - Produce cerebrospinal fluid (CSF) - Have cilia on their surface which help circulate cerebrospinal fluid Microglia - Small phagocytic cells that wander the central nervous system and destroy pathogens, debris, or foreign matter - Become concentrated in areas damaged by infection, trauma or stroke Astrocytes - Most abundant glial cells - Constitute over 90% of brain tissue in some areas - Roles: - Form structural frame work of nervous tissue - Extensions called perivascular feet contact blood vessels and maintain the blood-brain barrier (separates central nervous system from rest of the body) - Supports energetic demands of neurons by adjusting local blood flow - Secrete growth factors that promote neuron growth and synapse formation - Maintain extracellular environment (ex: regulate K+ levels) - Convert glucose to lactate to supply energy to neurons - Form scar tissue in damaged regions of the central nervous system -