Muscular Tissue: Types and Organization

• Muscular and nervous tissues are two of the four main types of animal tissues.

Muscular Tissue

• Responsible for movement in the body

Types of Muscular Tissue

• Striated muscle: Includes skeletal and cardiac muscle
• Non-striated muscle: Includes smooth muscle
• Skeletal Muscle: Striated, voluntary and attached to the skeleton
• Cardiac Muscle: Striated, involuntary and found in the heart
• Smooth Muscle: Non-striated, involuntary and located in the walls of internal organs

Skeletal Muscle Organization

• Epimysium: A dense connective tissue layer that surrounds the entire skeletal muscle and is continuous with fascia and tendons; it binds muscle to bone
• Perimysium: A thin but dense connective tissue layer that wraps fascicles (bundles) of muscle fibers
• Endomysium: A delicate connective tissue layer surrounding individual muscle fibers within fascicles

Skeletal Muscles

• Composed of cylindrical, non-branched fibers
• Attached to the skeleton and accounts for 40% of body weight
• Muscle Fibers: Multinucleate because they form from fused embryonic cells
• Have obvious transverse striations and undergo voluntary contractions
• Sarcolemma: Plasma membrane of a muscle cell
• Sarcoplasm: Cytoplasm of a muscle cell
• Sarcoplasmic Reticulum: A muscle cell's smooth endoplasmic reticulum

Myofibrils

• Parallel bundles within muscle fibers containing thick and thin myofilaments
• Composed of a series of sarcomeres separated by Z lines (or discs)
• Mitochondria, glycogen granules, and SER cisternae are in the sarcoplasm between the myofibrils

Sarcomere

• The basic functional unit of muscle contraction
• Myofibrils consist of repeating sarcomeres
• Boundaries of sarcomeres are marked by two Z discs (or lines)

Sarcomere Composition

• Thick myofilaments: Made of myosin
• Thin myofilaments: Made of actin

Cardiac Muscle

• Myocardium: Bundles form this thick structure
• Cardiac muscle cells are single cells, and they branch and join at intercalated discs
• Cells have 1-2 nuclei in the center
• "Fiber" means a long row of joined cardiac muscle cells

Cardiac Muscle Characteristics

• Exists only in the heart wall
• Cells are striated and branched with involuntary contractions
• Function: to pump blood
• Cells attach via intercalated discs, are striated and have a single nucleus

Intercalated Discs

• Junctional specializations between cardiac muscle fibers
• Zonula adherens (A) and desmosomes (B) are in the transverse portion of the disc
• Gap junctions (C) are in the longitudinal portion

Smooth Muscle Characteristics

• Cells are spindle-shaped with one central nucleus, grouped into sheets, and have no striations (sarcomeres)
• Does not always require a nervous signal to contract; can be stimulated by stretching or hormones
• Contractions are involuntary, and they exist in the walls of hollow organs, such as the stomach, uterus, and blood vessels

Structure of the Nervous Tissue

• Central Nervous System (CNS): Consists of the brain and spinal cord
• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves
• PNS Subdivisions: Sensory (input) and motor (output) divisions

Central Nervous System Cell Types

• Nerve cells (neurons): Conduct electrical impulses
• Glial cells (neuroglia): Support and protect neurons

Peripheral Nervous System

• Nerve fibers, and only nerve cells, exist in ganglia
• Supported by connective tissue

Neuron

• The structural and functional unit of the nervous tissue
• Consists of the nerve cell body and its processes, dendrites and axons

Neuron Cell Body Nucleus

• Large
• Spherical
• Vesicular with a prominent nucleolus
• Typically central, but peripheral in autonomic neurons of the PNS

Neuron Cell Body

• Lacks centrioles, thus, cannot divide
• Nissl bodies/granules: rER + free ribosomes for protein and membrane production
• Present in: cell body and dendrites
• Absent in: axon hillock and axon

Neuron Cell Body Organelles

• Golgi apparatus synthesizes neurotransmitters and forms lysosomes
• Mitochondria are numerous
• Cytoplasmic matrix contains microtubules and neurofilaments in areas without Nissl granules; they are important for intracellular transport and part of the cytoskeleton

Synapse

• The site where information or signals are transmitted from one neuron to another or to other cell types (muscle or gland cells)
• Post Synaptic Contact Classifications: Axo-dendritic, Axo-somatic, Axo-axonic

Synapse Structure

• Axon terminal (Presynaptic membrane): Contains mitochondria and synaptic vesicles with neurotransmitters
• Synaptic cleft: The neurotransmitter passes through this small space between the pre- and post-synaptic membranes
• Post Synaptic Part: Axon, dendrite, or nerve cell body

Synapse Classification by Signal Transmission

• Chemical synapse
• Electrical synapse

Electrical Synapse

• Ions move from one neuron to another via gap junctions, transmitting the action potential of the presynaptic cell to the postsynaptic cell
• Less numerous than chemical synapses but faster