Muscle and Nervous Tissue

Tissue Types

• Epithelial Tissue: Covers body surfaces and lines body cavities.
• Connective Tissue: Supports and protects organs.
• Muscle Tissue: Generates force to allow movement.
• Nervous Tissue: Uses electrical signals for communication.
• Epithelial cells are aggregated polyhedral cells with small amounts of extracellular matrix, and line surfaces/cavities to provide glandular secretion.
• Connective tissue has fixed and wandering cells, abundant extracellular matrix and supports plus protects tissues/organs.
• Muscle tissue has elongated contractile cells, a moderate amount of extracellular matrix, and provides strong contraction for body movements.
• Nervous tissue has elongated cells with fine processes and a very small amount of extracellular matrix, and transmits nerve impulses.

Lecture Outline

• Muscle tissue and nervous tissue will be discussed.
• Muscle tissue topics include types, skeletal muscle histology, cardiac muscle histology, and smooth muscle histology.
• Nervous tissue topics include neurons, glial cells, the central nervous system, and the peripheral nervous system.

Types of Muscle Tissue Definitions

• Muscle Fiber: Muscle cell.
• Sarcoplasm: Cytoplasm of muscle cells.
• Sarcosomes: Mitochondria.
• Sarcoplasmic Reticulum: The smooth ER.
• Sarcolemma: muscle cell membrane and its external lamina.

Skeletal Muscle Histology

• Skeletal muscle is categorized by bundles of multinucleated cells, cross-striations, quick/forceful contraction, and voluntary contraction.
• Skeletal Muscle organization: skeletal muscle → fascicles → muscle fibers → myofibrils → myofilaments
• Connective tissue layers surround and organize the contractile fibers.
• Epimysium surrounds an entire muscle.
• Perimysium surrounds a fascicle.
• Endomysium surrounds individual muscle fibres.
• Collagen fibers transmit mechanical forces generated by contracting muscle cells/fibres; individual fibres seldom extend from one end to other.
• The endomysium, perimysium, epimysium, and deep fascia are continuous to the connective tissue of the tendon at myotendinous junctions.
• The connective tissues join muscles to bone, skin, or another muscle.
• I bands are light and each bisected by a Z disc (dark).
• The Z disc marks the ends of sarcomeres which are contractile apparatuses.
• The sarcomere extends from Z disc to Z disc.
• I bands are formed by thin (F-actin) myofilaments.
• Titin connects thick myofilaments to Z disc.
• A bands are dark and located at the centre of sarcomeres.
• A bands are formed by thick (myosin) myofilaments.
• H Zone is found in the centre of an A band.
• The central part of thick filaments are not overlapped by thin filaments.
• M line bisects the H zone.
• Myosin molecules are polarized.
• Globular heads contain ATPase and facilitate binding to actin to move the head and produce a power stroke.
• Antiparallel association of myosin molecules forms thick filaments.
• In half of the thick filament, myosin heads are oriented in one direction and those are in the other half are facing the opposite direction.
• Tails of the myosin molecules overlap and yield a bare central shaft
• Each thin filament runs between the thick filaments.
• Thin Filaments: Consist of a double helix of filamentous actin, where F-actin are linear polymers of globular G-actin subunits.
• Each G-actin monomer contains a myosin binding site.
• Tropomyosin and troponin are proteins associated with actin.
• Tropomyosin and troponin respond to varying calcium ion concentrations.
• They enable/disable the interaction and formation of cross bridges between actin/myosin heads.
• Myosin heads bind to actin and draw the thin filament a short distance along the thick filament, which slides the myofilaments of each sarcomere past each other.
• Thin filaments then overlap thick filaments of A band, pulling the Z discs closer together.
• During contraction the sarcomere shortens, the width of the A band remains constant, I bands start do disappear, and H zone disappears.
• Titin contains elastic elements that act as molecular springs and contribute to the passive elasticity of muscle

Striations

• Striations are identified by a fibroblast nucleus, muscule nuclei, A band, and I band.
• Skeletal muscle is composed of multinucleated cells, with peripherally located nuclei.
• Single motor nerve branches off to innervate skeletal muscle fibres and forms a motor end plate.
• Motor end plates occur at the point where axons (nerve fibres) terminate on muscle fibers.
• Neurotransmitter is released from neurons to excite muscle fibre.
• There is one motor end plate per muscle fibre.
• The motor unit consists of a single neuron and muscle fibres.
• Regeneration occurs through reserve muscle satellite cells, not mitosis.
• Myoblasts that do not fuse remain on the external surface of muscle fibres inside the developing external lamina

Cardiac Muscle Histology

• Elongated, branched cells bound to one another at intercalated discs, being unique to cardiac muscles.
• Vigorous, rhythmic, and involuntary contractions characterize cardiac muscles.
• Myocytes (myocardial cells) have striated appearance from organized myosin and actin filaments.
• Cardiac muscle has a rich vasculature vs. skeletal and smooth muscle.
• Abundance of capillaries to supply adequate oxygen/nutrients and meet metabolic needs of cardiac cells.
• Cardiac Muscle Cells: eosinophilic sarcoplasm containing mitochondria (sarcosomes), with one (or two) centrally-located nuclei.
• Cardiac muscles are branched, joined end-to-end at intercalated discs.
• Intercalated discs: Specialized junctions that connect individual cells and are perpendicular to the direction of muscle fibres.
• Intercalated discs contain fascia adherens which connect to actin filaments to transmit contraction and link up the myofibrils of adjacent cells in series.
• Cardiac muscle cells contain macula adherens (desmosomes) for cell adhesion and gap junctions which mediate passage of electrical signals.
• Action potential spreads from one cell to adjacent cells.
• Gap junctions are also characterized by Low electrical resistance.
• Some residue of lysosomal digestion accumulates as yellow-brown granules near the nucleus of some cardiac cells; this is lipofuscin.
• Purkinje fibers conduct electrical impulses, are modified cardiac muscle cells, and are part of the cardiac conduction system.
• Purkinje fibres are modified cardiac muscle cells specialized.
• These are larger, thicker than ordinary cardiac muscle cells and have scattered myofibrils around the cell periphery.
• Purkinje Fibres: Presence of glycogen in the sarcoplasm around the centrally placed nucleus
• Cardiac muscle lacks satellite cells.
• Cardiac muscle has low capacity for regeneration, where defects/damage from infarcts are replaced by proliferating fibroblasts and dense connective tissue.
• Defects in cardiac muscle lead to formation of myocardial scars.

Smooth Muscle Histology

• Composed of fusiform cells (spindle-like shape that is wide middle and tapers at both ends).
• Smooth muscle has no striations, slow contraction at low force, and is involuntary and located in blood vessels and walls of internal organs.
• Characterized by mononucleated cells, a nucleus found in the widest part of the cell, a centrally located nucleus, and eosinophilic cytoplasm.
• All cells within a smooth muscle mass contract together without any motor subunits.
• Relaxed smooth muscle features nuclei elongated with rounded ends where contracted smooth muscles have spiral, kinked, or twisted nuclei.
• Thick (myosin) and thin (actin) filaments are scattered throughout the sarcoplasm..
• The filament is not arranged into myofibrils.
• Smooth Muscle: Appears smooth.
• Multiunit smooth muscle contracts independently, is innervated individually, and has more precise muscle control
• Eye cilia/iris/hair erector muscles.

Single Unit Smooth Muscle

• Cells are electrically connected by gap junctions and contract together as a single unit, such as the walls of internal organs and blood vessels (visceral smooth muscle).
• Smooth muscle is regulated by the autonomic nervous system (such as the visceral nervous system in intestines).
• Neurotransmitters like Norepinephrine and Acetylcholine regulate smooth muscles.
• Hormones like estrogen/oxytocin and tissue hormones like prostaglandins/histamine all regulate smooth muscle.
• This results in involuntary contraction.
• Smooth muscles undergo rapid regeneration.
• Regenerating cells/fibres are small and relatively less differentiated.
• Injury Renews Mitotic Activity: Replace the damaged tissue
• Vascular smooth muscle layer can be restored by activation of mesenchymal stem cells, muscle cells, progenitor cells and multipotent vascular stem cells.

Muscle Tissue Summary

• Skeletal muscle fibers are single multinucleated cells vs aligned cells of cardiac and small fusiform cells of smooth muscle.
• Skeletal muscle has a cylindrical fiber shape and cells can be many cm long, whereas cardiac cells are 50-100 micrometers in length and smooth muscle cells are 50-200 um long.
• Striations: Present in skeletal and cardiac, absent in Smooth.
• The nucleus is peripheral adjacent to the sarcolemma for skeletal muscle, central in cardiac, and central at the widest point of cell for smooth muscle.
• T tubules found at A-I junctions for skeletal, dyads at Z discs for cardiac, and absent in smooth muscle.
• Well-developed sarcoplasmic reticulum is found in skeletal muscle with triads at T tubules, less well in cardiac muscle, and are irregular without good organization in smooth cells..
• Skeletal and cardiac muscle structures are organized with sarcomeres, and smooth muscle has gap junctions, calveolae, and dense bodies.
• Efferent Innervation: Motor in skeletal, autonomic in cardiac/smooth muscle
• Control of Contraction is troponin C for skeletal, similar to skeletal for cardiac, and calmodulin for smooth.
• Locations: Skeletal= skeletal, cardiac= heart, smooth=blood vessels and digestive tracts etc.
• Cell response to increased load: Hypertrophy in skeletal and cardiac muscles, hypertrophy plus hyperplasia in smooth muscle.
• Regeneration: Limited to satellite cells in skeletal muscles, very poor in cardiac, and good with mitotic cells in smooth.

Nervous System

• Cells found in the nervous system are neurons (nerve cells) and glial cells (supporting).
• The central nervous system (brain and spinal cord) contains relay neurons (interneurons).
• The peripheral nervous system includes cranial and spinal nerves, and contains sensory/motor neurons,.
• Nerves are a whitish fiber of neuron cells which carry impulses to the central nervous system and from the central nervous system to the effector organs and only found in the peripheral nervous system.
• Neurons are specialized cells involved in transmitting nerve impulses.
• Impulses are found in both peripheral and central nervous systems and composed of an axon, cell body, and dendrites.
• Nerves act as a conducting zone to transport signals while neurons generate chemical and electronic signals from sensory/motor inputs.
• These signals may also come from interneurons.

Nervous Tissue: Neurons

• Neurons Four Regions: dendrites, cell body, axon, and synaptic terminal.
• Dendrites are the receptive portions which receive synaptic afferent inputs and carry signals to cell bodies. Cell Body: Integrates signals and contains organelles and nucleus surrounded by cytoplasm.
• Cell bodies contain Nissl substance (rough endoplasmic reticulum for protein synthesis).
• Axons extend from the soma to conduct signals away from the direction in one direction towards terminal branches.
• Synaptic Terminals: Found at the end of axons where high concentration of neurotransmitter vesicles elicit effector signals to excite/inhibit next neuron.
• Multipolar neurons have >2 dendrites (and one axon) and are most common.
• Bipolar neurons have one dendrite and one axon, in the retina, olfactory epithelium, and inner ear.
• Unipolar Neurons: a single short process that bifurcates close to perikaryon with branches towards the peripheral (longer) and central (shorter) nervous system
• Anaxonic Neurons: many dendrites, no true axon; regulate electrical changes of adjacent CNS neurons, but do not produce action potentials.
• Neurons are excitable and move action potential along the axon to either excite another neuron or other effector cell.
• Synapses transmit the impulse by neurotransmitters that bind receptors on the postsynaptic cell to initiate a new action potential.

Nervous Tissue: Glial Cells

• Glial Cells: Supporting cells; Ependymal (CNS), Oligodendrocytes (CNS), Astrocytes (CNS), Microglia (CNS), Satellite (PNS), and Schwann (PNS) cells.
• Ependymal cells are epithelial-like and line fluid-filled cerebral ventricles and central canal of the spinal cord to produce cerebrospinal fluid (supportive, protective roles in the CNS).
• Oligodendrocytes support myelin sheaths to insulate large diameter axons in the CNS (facilitate nerve impulses), have a smaller/darker nucleus than astrocytes and a fried-egg appearance.
• Astrocytes bind neurons to blood vessels to transfer molecules and are part of the blood-brain barrier and are also star shaped CNS glia and the most numerous glia in the CNS.
• Blood-brain barrier prevents the passage of toxins/germs from the blood to the CNS.
• Microglia: Phagocytes that originate from blood monocytes.
• Microglia encircle degenerating neurons and brain tissue, microglia can also transform into brain macrophages.
• Microglia Mediate Immune Defence: mediate immune defence within CNS neurons.
• Satellite cells are found at the periphery of nerve cell bodies, enclosing perikaryons to regulate microenvironments.
• Schwann cells are large, oval and enclose all axons in PNS, producing myelin sheaths of 80% lipid and 20% protein large diameter axons with Nodes of Ranvier augmenting conductivity.

Overview of Central and Peripheral Nervous Systems

• The brain has gray matter on the outside and white matter on the inside.
• The spinal cord has white matter on the outside and gray matter on the inside.
• The CNS is enclosed by connective tissue called meninges (dura, arachnoid with CSF, and pia mater) with big blood vessels.
• The delicate pia mater layer directly contacts neural tissue.
• The choroid plexus contains water with villi specialized with epithelial ependyma that transfer into ventricles CSF.
• In Central Nervous System Plasticity and Regenerative Ability - Differentiating into neurons, astrocytes, oligodendrocytes to replace lost neural cells by secreting solute factors and protecting neural cells.

Peripheral Nervous System

• The peripheral nervous system consists of 12 pairs of cranial nerves and spinal cord
• Nerves consist of bundles of nerve fibres called fascicles which consist of connective tissue layers surrounding and organising the nerves.
• These nerves conduct information to the central nervous system.
• Within these, the Epineurium surrounds the entire nerve, the Perineurium surrounds the axon fascicle, and the Endoneurium surrounds the individual axons.
• Axons are typically myelinated with sheaths that are damaged and removed over 2 weeks.
• After 3 weeks, the damaged fibres suffer denervation atrophy and the Scwhann cells will proliferate to stimulate axonal regeneration to the new muscle.
• The nerve fibre regeneration is successful within the 3 months following injury, as connections to the muscle now exist.