Muscle Tissue Types and Properties

Muscle Tissue

• Derived from mesoderm
• Composed of cells optimized for contractility

Muscle Contractility

• Optimized by the sliding action of thick myosin filaments along thin actin filaments

Muscle Organelles

• Specialized organelles are referred to with the prefix sarco

Muscle Types

• Skeletal muscle consists of voluntary muscles mostly attached to bone
• Cardiac muscle consists of involuntary muscles found in the heart
• Smooth muscle consists of involuntary muscles found in visceral organs and soft tissues

Skeletal Muscle Properties

• Made of muscle fibers
• Long, cylindrical, and multinucleated contractile cells
• Possess elongated nuclei, located peripherally under the sarcolemma
• Organized by thin layers of collagenous supporting tissue

Skeletal Muscle Connective Tissue

• Endomysium surrounds the muscle fiber, consisting of loose (areolar) connective tissue
• Perimysium surrounds the muscle fascicle, consisting of dense irregular connective tissue
• Epimysium surrounds the whole muscle, consisting of dense irregular connective tissue
• Deep fascia overlies the epimysium and is continuous with joint connective tissue

Muscle Fiber Organization

• Skeletal muscle exhibits striations, alternating light and dark bands
• Dark bands are A bands
• Light bands are I bands
• The Z disc bisects each I band

Sarcomere Structure

• Sarcomere is the repetitive functional unit of the muscle contractile apparatus
• Sarcomeres extend from Z disc to Z disc
• They are arranged end-to-end from myofibrils
• A bands contain thick myosin filaments
• I bands contain thin actin filaments

Sarcoplasmic Reticulum

• It is a membranous smooth ER found in skeletal muscle fibers
• Sequesters Ca2+ during muscle contraction
• Terminal cisterns are expanded structures near T tubules

Transverse (T) Tubules

• Are invaginations of the cell sarcolemma
• It encircles each myofibril
• Located near the aligned A and I bands

Skeletal Muscle Fiber Types

• Slow oxidative fibers are for slow contractions over long periods
• They have many mitochondria and depend on oxidative metabolism
• They are dark red in color due to high myoglobin and surrounding capillaries
• Postural muscles of the back are an example
• Fast glycolytic fibers are for rapid, short-term contractions
• They have few mitochondria and depend on anaerobic metabolism
• They are white in color due to less myoglobin & surrounding capillaries
• Major muscles of the legs are an example
• Fast oxidative-glycolytic fibers have physiological and histological features that are intermediate between the other two types
• Extraocular muscles are an example

Cardiac Muscle

• Striated, similarly arranged to skeletal muscle tissue
• Fibers arranged in series are joined at interdigitating regions called intercalated discs
• Contains long, uninucleated, branching contractile cells
• Possess centrally-located nuclei
• Sparser, less well-organized myofibrils
• With fibroblast activity

Intercalated Discs

• Consist of many junctional complexes
• Transverse sections contain desmosomes and fascia adherens
• Longitudinal sections contain gap junctions

Smooth Muscle

• Possesses a rudimentary sarcoplasmic reticulum
• Lacks T tubules, contains caveolae
• Not arranged in sarcomeres (unstriated)
• Bundles of thick and thin myofilaments crisscross the sarcoplasm obliquely
• Actin filaments insert into dense bodies
• Lacks troponin and tropomyosin, uses calmodulin & MLCK
• Stimulation from the autonomic nervous system is propagated via gap junctions, allowing synchronized contraction

Skeletal Muscle Control

• It's under the influence of the autonomic nervous system and various hormones
• A component of visceral organs and blood vessels
• Specialized for slow, steady contractions
• Smooth muscle fibers are elongated, fusiform in shape, unstriated with a central nucleus
• Linked by numerous gap junctions
• Enclosed by an external lamina and a network of collagen fibers (Types I & III)
• Cells shorten and broaden on contraction

Nervous Tissue Derivation

• Derived from Ectoderm

Nervous Tissue Cells

• Parenchyma: Neurons
• Stroma: Glial cells

Nervous System Function

• Distributed throughout the body as an integrated communications network

Nervous System Divisions

• Central Nervous System includes the brain and spinal cord
• Peripheral Nervous System includes cranial, spinal, and peripheral nerves and ganglia

Neuron Definition

• Functional unit of both the CNS and PNS
• Responds to environmental stimuli by reversing the ionic gradient across the plasma membrane of the cell

Nerve Impulse

• Also known as the Action Potential/Depolarization Wave
• Consists of the reversal of the ionic gradient starting at the place where the stimulus is received
• Propagated across the cell's entire plasma membrane, traveling along the neuron's processes
• Signal is eventually transmitted to other neurons, muscles, and glands

Neuron Dendrites

• Numerous elongated processes extending from the cell body
• Receives stimuli from other neurons at unique sites called synapses

Neuron Cell Body

• Also known as Soma/Perikaryon
• Contains the nucleus and organelles
• Synthetic center for the entire neuron

Neuron Axon

• Single long process extending from the cell body
• Specialized to generate and conduct nerve impulses to other cells

Neuron Cytoplasm

• Contains Nissl bodies, regions with numerous ribosomes and highly developed RER
• Abundant cytoskeleton, including microtubules, actin filaments, and intermediate filaments

Neuron Classification

• Sensory/Afferent neurons receive stimuli from receptors
• Motor/Efferent neurons send impulses to effector organs such as muscle fibers and glands
  • Somatic motor neurons: voluntary; innervate skeletal muscle
  • Autonomic motor neurons: involuntary; innervate glands, cardiac muscle, and smooth muscle
• Interneurons establish relationships among other neurons, usually multipolar or anaxonic

Neuron Cell Body Components

• Contains the nucleus and cytoplasm
• Excludes the cell processes
• Nucleus is large and euchromatic

Dendrite Properties

• Highly branched, tapering processes emerging off the cell body
• Principal signal reception and processing site on neurons
• End in either specialized sensory receptors or form synapses with neighboring neurons
• Dendritic spines are intial processing sites for synaptic signals

Axon Features

• Usually singular
• Long cylindrical process with less branching
• Originates from the axon hillock which is a pyramid-shaped region of the perikaryon
• Ends in terminal boutons that terminate at other neurons or effector organs
• Contains a myelin sheath for insulation
• Axoplasm contains mostly mitochondria, microtubules, neurofilaments, and vesicles

Synapse Characteristics

• Sites where nerve impulses are transmitted from one neuron to the next cell, muscle, or gland
• Converts an electrical signal into a chemical signal
• Contains a presynaptic axon terminal (terminal bouton), synaptic cleft, and postsynaptic cell membrane

Glial Cells

• Support neuronal survival and fosters an optimal environment for neuronal activity
• Estimated to be 10x more abundant than neurons

CNS Glial Cells

• Oligodendrocytes form and maintain the myelin sheath
• Astrocytes provide physical and metabolic support
• Microglia are phagocytic cells
• Ependymal cells line the ventricles for the formation of CSF

PNS Glial Cells

• Satellite cells maintain the environment around the neuronal body in a ganglion
• Schwann cells form and maintain the myelin sheath

Cartilage

• Tough, durable form of connective tissue
• ECM characterized by large amounts of GAGs and proteoglycans

Cartilage Functions

• Cushioning/Shock absorption: able to bear stress without distortion due to firm but resilient ECM
• Lubrication: smooth surface allows sliding and facilitates bone movement
• Guides development of long bones

Cartilage Cells

• Parenchyma consists of Chondrocytes which synthesize & maintain ECM located in matrix cavities called lacunae

Cartilage Vessels

• Lacks vascular supply, receives nutrients by diffusion from capillaries in surrounding connective tissue

Perichondrium

• Sheath of dense CT that surrounds cartilage
• Harbors the blood supply and neural component
• Lacks nerve supply

Cartilage matrix

• Variation in composition dictates the type of cartilage
• Components include Type II Collagen fibrils, Hyaluronan, Proteoglycans, and Water

Hyaline Cartilage Properties

• Hyalos is Greek for glass (homogeneous, semitransparent in fresh state)
• Most common type of cartilage

Hyaline Cartilage Location

• Articular surface of movable joints
• Walls of larger respiratory passages
• Ventral ends of ribs (costal cartilages)
• Epiphyseal ribs of long bones for longitudinal bone growth
• Temporary skeleton in the embryo

Cartilage composition

• Collagen Type II is present
• Aggrecan binds to water
• Chondronectin enables adherence of chondrocytes to ECM

Perichondrium Structure

• Sheath covering hyaline cartilage made of dense irregular CT
• Outer fibrous layer contains type I collagen, fibroblasts, and blood vessels
• Inner cellular layer contains chondroblasts

Chondrocytes Properties

• Small, round nuclei appear shrunken
• Pale-staining cytoplasm
• Arranged in isogenous groups (clusters)

Types of cartilaginous matrix

• Territorial matrix--matrix that surrounds each chondrocyte or isogenous group
• Interterritorial matrix--matrix that fills the space between isogenous groups and chondrocytes

Elastic Cartilage features

• Similar to hyaline cartilage but abundant in elastic fibers & chondrocytes
• Yellowish in color
• Flexible support for tissues that need stretch
• Has a surrounding perichondrium
• Chondroblasts are responsible for oppositional growth of the matrix

Elastic cartilaginous matrix

• Contains elastic fibers to gives a rough appearance to the matrix
• Also contains Type II Collagen

Elastic Cartilage Locations

• Epiglottis
• Auditory canal / tube
• Larynx
• Pinna/auricle

Fibrocartilage characteristics

• Provides cushioning, firm support, and tensile strength
• Lacks perichondrium

Fibrocartilage locations

• Pubic symphysis
• Intervertebral disk
• Insertion of tendons/ligaments

Chondroyctes Properties

• Chondrocytes are smaller and fewer, arranged in lacunae in small groups in parallel rows or columns

ECM composition

• Type I collagen gives matrix a dense and coarse appearance
• Type II Collagen is also present

Bone definition

• Hard and strong connective tissue
• ECM is hard and calcified

Bone functions

• Protection & Support
• Calcium reserve
• Blood cell production
• Movement

Bone Structures

• Lined by endosteum (internal surface surrounding the marrow cavity)
• Periosteum is the external surface of the bone

Bone matrix properties

• 50% of dry weight is inorganic material
• Contains Calcium hydroxyapatite as the most abundant part of the matrix
• Also contains Bicarbonate, Citrate, Magnesium, Potassium, Sodium, and Calcium phosphate

Bone organic material

• Type I Collagen
• Proteoglycans and glycoproteins

Types of Bone Cells

• Osteocytes are small cells with cytoplasmic processes that extend into canaliculi
• They deposit matrix around themselves
• Occupies a space in the matrix called a lacuna

Osteoblasts

• Synthesize and secrete organic matrix components
• Can differentiate as osteocytes and become entrapped in lacunae or flatten to cover the matrix surface as bone lining cells – cuboidal or low columnar in shape

Osteoclasts

• Function in bone remodeling & resorption of calcified bone matrix
• They are large, multinucleated cells derived from monocytes

Periosteum vs Endosteum

• Periosteum: outer fibrous layer of dense connective tissue, Type I collagen, fibroblasts, blood vessels
• Endosteum: covers trabeculae of bony matrix

Types and Organization of Bone

• Compact bone: dense bone beneath the periosteum
• Spongy/Cancellous bone: deep area of the bone consisting of interconnecting cavities
• Woven bone: immature bone formed in osteogenesis or repair
• Lamellar bone: lamellae concentrically organized around small central canals

Osteogenesis

• Intramembranous ossification: flat bone formation; mesenchymal cells differentiate into osteoblasts and deposit osteoid - unmineralized prebone that later becomes calcified

• Endochondral ossification: long bone formation; hyaline cartilage serves a cartilage model; cartilage proliferates then calcifies, eventually replaced by bone

Joints Definition

• Regions where adjacent bones are capped and held together by CT

Types of Joints

• Synarthroses limited or no movement; can be fibrous or cartilaginous
• Synostoses essentially no movement – skull sutures
• Syndesmoses bones joined by dense CT – interosseous ligament of inferior tibiofibular joint, posterior sacroiliac joints
• Symphyses thick pad of fibrocartilage between thin articular cartilage – pubic symphysis, intervertebral disks
• Diarthroses free bone movement – elbow, knee; ligaments and capsule of dense CT maintain alignment

Specialized Joint Cells

• Type A cells / Macrophage-like synovial cells are derived from blood monocytes, remove wear and tear debris from synovial fluid, and regulate inflammatory events in diarthrotic joints
• Type B cells / Fibroblastic synovial cells produce abundant hyaluronan and smaller amounts of proteoglycans