Motor End Plate: Structure and Function

Tissue Types and Functions

• Motor End Plate:
  • Specialized region of contact between motor nerve fiber and muscle fiber
  • Axon terminal contains numerous mitochondria and synaptic vesicles with acetylcholine (neurotransmitter)
  • Synaptic cleft is the space between the axon terminal and muscle fiber
  • Junctional folds are deep invaginations in the sarcolemma under the synaptic cleft to increase the surface area
  • Muscle fibers depend on the nervous system for their integrity and function
  • Denervation atrophy occurs if the nerve supply to skeletal muscle is cut
• Cardiac Muscle:
  • Striated muscle of the heart
  • Cardiac muscle fibers are joined to each other by intercalated discs forming branched muscle fibers
  • Cardiac muscle has one or two nuclei centrally located and numerous mitochondria
  • No stellate cells, so unable to regenerate
• Smooth Muscle:
  • Non-striated and involuntary controlled by autonomic nervous system
  • Found in wall of viscera, each muscle fiber is enclosed by thin basal lamina and fine network of reticular fibers
  • Sarcolemma has numerous caveola, possibly having a role similar to that of striated muscle

Cytoplasm

• Gel-like fluid present between the plasma membrane and nucleus of the cell
• Three main components:
  • Cytoplasmic organelles: involved in metabolism (e.g., mitochondria and endoplasmic reticulum)
  • Cytoplasmic inclusions: small particles seen temporarily in the cytoplasm, may or may not be membrane-bound, metabolically inactive (e.g., lipid droplets, glycogen granules, pigment granules)
  • Cytoskeletons: provide the structural framework to the cell

Organelles

• Membranous organelles:
  • Rough endoplasmic reticulum
  • Smooth endoplasmic reticulum
  • Mitochondria
  • Golgi apparatus
  • Lysosome
  • Peroxisome
• Non-membranous organelles:
  • Ribosomes
  • Cytoskeleton
  • Microtubules
  • Microfilaments
  • Intermediate filaments

Muscle Types

• Muscular tissue:
  • Specialized tissue for contractility to produce movement in the organ
  • Divided into three types: skeletal muscle, smooth muscle, and cardiac muscle
• Skeletal muscle:
  • Attached to bone for movement, voluntary controlled by motor and sensory fibers
  • Formed of cylindrical muscle fibers with uniform thickness, doesn't branch
  • Striated with multiple peripheral nuclei
• Connective tissue coat of the muscle:
  • Epimysium: thick CT layer surrounds the whole muscle
  • Perimysium: thin CT layer surrounds each individual muscle
  • Endomysium: delicate CT surrounds each individual muscle fiber, consists of basal lamina synthesized by muscle fiber and reticular fibers and fibroblast

EM Structure of Skeletal Muscle

• Muscle filled with long striated myofibrils
• Myofibril is a bundle of myofilaments
• Two types of myofilaments: actin and myosin
• Muscle fibers are rich in mitochondria, SR (Sarcoplasmic reticulum), glycogen, myoglobin, Golgi complex

Sarcomere

• Structural and functional unit of a myofibril
• Formed by overlapping actin and myosin filaments
• Segment between two Z-lines, with an average length of 2.5µm
• Composed of a dark A band, which is intermediate between two light I bands
• A band is formed of myosin filaments with overlapping portions of actin filaments
• I band consists of actin filaments that do not overlap with myosin
• In the center of the A band, the H zone consists of only myosin
• H zone is bisected by the M line, the site of connection between myosin filaments

Mitochondria

• Ovoid-shaped structures, about 0.2-5µm in size, bound by a phospholipid bilayer membrane
• Outer membrane is smooth and encloses the entire organelle
• Inner membrane is folded to form cristae
• Matrix space is filled with mitochondrial matrix, containing matrix granules, rich in cations such as Ca and Mg
• Functions: cell respiration, absorption and secretion processes, protein synthesis, lipid metabolism, heat production, and calcium storage

Golgi Complex

• Present in nearly all cells, variable in position and size
• Consists of three components: flattened vesicles or cisternae, microvesicles or transfer vesicles, and macrovesicles or secretory vacuoles
• Functions: processing, modifying, and packaging of secretory products, addition of carbohydrate fractions to proteins, production of complex carbohydrates, mucoproteins, lipoproteins, and relatively pure proteins

Learn about the motor end plate, a specialized region of contact between motor nerve fibers and muscle fibers, and its role in muscle contraction.