1: Neuronal structure and organelles

1: Neuronal Structure and Organelles

Neurons are the fundamental information-processing cells of the nervous system, uniquely specialized for rapid communication. Their structure is highly polarized and optimized for receiving, integrating, conducting, and transmitting electrochemical signals. Understanding neuronal anatomy is essential for grasping how the nervous system functions.

A typical neuron consists of four main regions:

1. Dendrites: These are highly branched, tree-like extensions radiating from the cell body. Their primary role is to receive incoming signals (chemical or electrical) from other neurons via synapses located on dendritic spines (small protrusions). The vast dendritic surface area provides numerous points for synaptic input, allowing integration of information from thousands of other neurons.
2. Cell Body (Soma): The soma is the metabolic and biosynthetic center of the neuron. It houses the nucleus, which contains the genetic material (DNA) and directs cellular activities. Surrounding the nucleus is the perikaryon (cytoplasm of the soma), packed with organelles critical for neuronal function:
   • Nucleus: Contains DNA and the nucleolus (site of ribosomal RNA synthesis).
   • Rough Endoplasmic Reticulum (RER) & Nissl Bodies: Stacks of RER (visible as Nissl bodies under light microscopy) are abundant and synthesize proteins, especially those needed for neurotransmission and membrane maintenance.
   • Smooth Endoplasmic Reticulum (SER): Involved in lipid synthesis, calcium ion storage, and detoxification.
   • Golgi Apparatus: Modifies, sorts, and packages proteins (like neurotransmitters and receptors) into vesicles for transport to their destinations (dendrites, axon terminals).
   • Mitochondria: Generate adenosine triphosphate (ATP), the primary energy currency, crucial for maintaining ion gradients, synaptic transmission, and axonal transport.
   • Lysosomes: Contain hydrolytic enzymes for breaking down waste materials and cellular debris.
3. Axon: A single, elongated, tubular process that conducts electrical impulses (action potentials) away from the cell body towards the axon terminals. It arises from a specialized region of the soma called the axon hillock, which has a high density of voltage-gated sodium channels and acts as the site for action potential initiation. The axon proper is often insulated by myelin (produced by glial cells) to speed conduction. The axon contains:
   • Axoplasm: The cytoplasm within the axon.
   • Cytoskeleton: Comprising microtubules (major highways for fast axonal transport), neurofilaments (intermediate filaments providing structural support and determining axon caliber), and microfilaments (actin filaments involved in growth cone movement and synaptic plasticity).
4. Axon Terminals (Terminal Boutons): The distal ends of the axon branch into numerous terminals. These contain synaptic vesicles loaded with neurotransmitters. When an action potential arrives, it triggers the release of neurotransmitters into the synaptic cleft, enabling communication with the next neuron (or effector cell like muscle).

This intricate structural organization – with dendrites for input, the soma for integration and biosynthesis, the axon for rapid conduction, and terminals for output – underpins the neuron's ability to transmit information precisely and efficiently throughout the nervous system.