Muscle Tissue: Skeletal Muscle Fibers

Muscular Tissue

• Includes smooth muscles and striated muscles (skeletal and cardiac).

Skeletal Muscle

• Parallel muscle fibers arranged in bundles (fascicles).
• Bundles separated by loose connective tissue, known as perimysium.
• The entire muscle wrapped in dense connective tissue, i.e. epimysium, which is continuous with the tendon.
• Individual muscle fibers sheathed by endomysium.
• Blood vessels and nerve fibers run through the endomysium.

Skeletal Muscle Fibers (LM)

• Cylindrical, parallel fibers.
• Covered by a plasma membrane called the sarcolemma.
• Fibers are multinucleated with cigar-shaped peripheral nuclei.
• Sarcoplasm is acidophilic and shows transverse striations.

Skeletal Muscle Fibers (Types)

• Red: smaller in diameter, contain more myoglobin, highly populated with mitochondria that have packed cristae, slower contraction, resistant to fatigue, and have nerve endings differentiated from white fibers.

Skeletal Muscle Fibers (EM)

• Muscle fiber is mostly occupied by longitudinal myofibrils.
• Sarcoplasm contains mitochondria, Golgi apparatus, smooth endoplasmic reticulum (SER).
• Sarcoplasm includes glycogen granules and lipid droplets (energy sources), and myoglobin (pigment).

Sarcomere

• Myofibril demonstrates alternating dark and light bands.
• Myofibril is divided into repeating segments (sarcomeres), which are separated by dark Z-lines.
• Sarcomere measures about 2-3 um in length.
• Contains thick filaments (myosin) about 1.5 um in length forming the dark A-band in the center.
• Also includes thin filaments (actin) that extend from the Z-line for 1 um, forming the I-band, and extending into the A-band.
• H-band: area within the A-band not reached by thin filaments.
• M-line: at the center of thick filaments, a series of projections connect the thick filaments together in a transverse direction, forming a thin dark line in the middle of the H-band.

Sarcoplasmic Reticulum (SER) and Transverse Tubules

• SER forms terminal cisternae (TC).
• Sarcolemma extends a transverse tubule (T-tubule) between two terminal cisternae of the SER, together forming a triad.
• Gap junctions present between transverse tubules and terminal cisternae.
• Nerve impulses travel from the sarcolemma to the transverse tubules and then to the sarcoplasmic reticulum.

Cardiac Muscle

• Fibers create the bulk of the heart wall.
• Striated involuntary fibers.
• Cardiac fibers are cylindrical branching fibers, each cell possesses one or two central nuclei.
• Adjacent cardiac cells are associated via dark irregular lines known as intercalated discs.

Nervous Tissue

• Nervous system anatomically divided into:
  • Central Nervous System (CNS)- brain and spinal cord.
  • Peripheral Nervous System (PNS)- cranial nerves, spinal nerves, autonomic nerves, and associated ganglia.

Nervous Tissue Structure

• Composed of neurons and neuroglia.
• Neuron is the structural and functional unit.
• Neuron is composed of a cell body, axon, and dendrites.

Types of Neurons

• Based on processes:
• Unipolar: Rounded cells with one process that bifurcates (one dendrite and one axon); present in spinal ganglia and sometimes pseudounipolar.
• Bipolar: Fusiform cells with two processes (one dendrite and one axon); located in olfactory mucosa, retina, and spiral ganglion of the cochlea.
• Multipolar: Many dendrites and one axon.

Multipolar Neurons

• Have a variety of shapes
• Stellate neurons have star-shaped cell bodies, e.g., anterior horn cells and cells of autonomic ganglia.
• Purkinje cells have flask-shaped or fusiform cell bodies, located in the cerebellar cortex.
• Pyramidal cells have cell bodies in the form of pyramids, present in the cerebral cortex.
• Types based on function:
  • Motor neurons
  • Sensory neurons

Neuronal Cell Body (Histological Structure)

• Contains nucleus and cytoplasm.
• Nucleus is large, pale, spherical, with a defined nucleolus.
• Cytoplasm contains basophilic granules called Nissl granules.

Nerve Cell Body (EM Structure)

• Nissl granules are groups of free ribosomes and rough endoplasmic reticulum (RER).
• Golgi bodies located around nucleus, function in lysosome formation.
• Mitochondria are numerous.
• Microtubules and neurofilaments occupy cytoplasmic matrix as part of cytoskeleton and are important in intracellular transport.

Neuronal Cell Body (Inclusions)

• Lipofuscin pigment: represents the end product of lysosomal activity.
• Melanin pigment: present in the neurons of the midbrain.
• Lipid droplets: found occasionally.

Peripheral Nervous System (Organization)

• Cell bodies are present only in special groups called ganglia.
• Ganglia are present as stations in the course of nerves and classified into sensory and autonomic ganglia
• Axons or dendrites form bundles (fascicles) of nerve fibers.
• Nerves enclosed in a dense connective tissue sheath called the epineurium.
• Epineurium extends among fascicles to form interfascicular CT.
• Each fascicle is wrapped in a layer of CT called perineurium.
• Individual nerve fibers are enveloped in a thin CT layer called endoneurium.
• Axons are sheathed by Schwann cells (neurilemmal cells), forming a neurilemmal sheath.
• Sheath may or may not form myelin around axons.
• Fibers are myelinated or non-myelinated based on myelin sheath presence.
• Myelin: composed of successive layers of Schwann cell membrane wrapped spirally around the axon.
• Consists of lipid and protein.

Central Nervous System (Organization)

• Cell bodies aggregate in various forms (nuclei, columns, or layers).
• Fibers form groups or fascicles known as fiber tracts, traveling within the central nervous system.
• Oligodendroglia responsible for myelin formation.

Synapses

• Sites of contact between two neurons.
• Structure:
  • Presynaptic part: axon terminal, expanded, contains mitochondria and neurosecretory vesicles.
  • Synaptic cleft.
  • Postsynaptic part: dendrite, cell body, or axon of another neuron.
• Types:
• Axosomatic: End foot contacts a cell body.
• Axodendritic: End foot contacts a dendrite.
• Axoaxonic: End foot contacts an axon.

Impulse Transport

• Chemical synapse: relies on a chemical transmitter.
• Electrical synapse: impulse is transported through a gap junction between pre- and postsynaptic membranes.

Neuroglia

• Connective tissue of the nervous system binding neurons and blood vessels together.
• In the CNS:
  • Astrocytes: supportive and nutritive function.
  • Microglia: phagocytic cells.
  • Oligodendrocytes: myelin formation.
  • Ependymal cells: line brain ventricles and the central canal of the spinal cord.
• In the PNS:
  • Schwann cells: myelin formation.
  • Satellite cells: surround nerve cells of ganglia.