Cell Biology Quiz: Vesicles and Mitochondria

Cell Structures Related to Neurons

• Mitochondria: Spherical, double-membraned organelles with cristae vital for chemical energy production; scattered throughout the cell.
• Neurofibrils: Linear fibrils (10 nm) made of microfilament bundles that give shape to neurons; run parallel from dendrites through the cell body to the axon.
• Microfilaments: Thin filaments (3–5 nm) forming a dense cytoskeletal network beneath the plasma membrane; involved in cell retraction and transport.
• Microtubules: Larger linear tubes (25 nm) that facilitate cell transport; extend from dendrites through the cell body to the axon.
• Lysosomes: Small vesicles (8 nm) acting as cell scavengers, containing primary, secondary, and residual forms; involved in waste processing.
• Centrioles: Paired hollow cylinders of microtubules, localized to the cell body cytoplasm; play a role in cell division and microtubule maintenance.
• Lipofuscin: Yellowish-brown granules scattered in the cytoplasm; they represent metabolic by-products derived from lysosomal activity.
• Melanin: Yellowish-brown granules found in the substantia nigra of the midbrain; associated with dopamine formation.
• Plasma Membrane: Fluid-like membrane facilitating the initiation and conduction of nerve impulses; characterized by rapid and slow axonal transports.

Impulse Conduction and Action Potentials

• Depolarization: Occurs when Na+ ions diffuse into the cell cytoplasm due to neuronal excitation.
• Action Potential: A brief occurrence lasting about 5 msec; spreads along the plasma membrane as a self-propagated nerve impulse.
• Refractory Period: Nonexcitable state where a new action potential cannot be generated; helps regulate impulse frequency.
• Sodium and Potassium Channels: Integral proteins that participate in the depolarization process; essential for maintaining ionic balance during nerve impulses.

Axon Structure and Function

• Axon: Longest process extending from the cell body; conducts nerve impulses away from the neuron.
• Axon Hillock: Conical elevation where the action potential is initiated.
• Axon Terminals: Distal ends of axon branches where communication occurs with other neurons.
• Varicosities: Swellings of autonomic axons resembling beads; involved in neurotransmitter release.
• Axolemma and Axoplasm: Plasma membrane and cytoplasm of the axon, respectively; axoplasm lacks Nissl granules and Golgi complex.
• Initial Segment: Most excitable part of the axon critical for action potential generation.

Synaptic Transmission

• Synapse: Junction where neurons communicate; can be synapses between axon and dendrite (axodendritic), axon and soma (axosomatic), or axon to axon (axoaxonic).
• Chemical Synapses: Involve neurotransmitter release, which diffuses across the synaptic cleft and binds to postsynaptic receptors; includes neurotransmitters like acetylcholine, dopamine, serotonin, and GABA.

Mechanism of Neurotransmitter Action

• Neurotransmitters are released upon nerve impulse arrival, leading to calcium influx, synaptic vesicle fusion, and release into the extracellular fluid.
• The effect on the postsynaptic neuron varies based on overall summation of excitatory and inhibitory signals; depolarization can trigger action potentials and propagate nerve impulses.

Transport Mechanisms

• Rapid Transport: Achievable speeds of 100 to 400 mm per day using motor proteins kinesin (anterograde) and dynein (retrograde).
• Slow Transport: Slower movement (0.1 to 3.0 mm per day) involving cytoplasm transport, primarily anterograde, likely mediated by a kinesin family motor.