Cytological Features of the Neuron
Nerve cells are distinguished by a high metabolic rate. The neuronal nucleus is typically pale, dominated by euchromatin (decondensed chromatin), which indicates active transcription processes. The synthetic apparatus is represented by a well-developed rough endoplasmic reticulum (rER), a prominent Golgi apparatus, and numerous mitochondria and lysosomes. This robust machinery is essential for the continuous synthesis of RNA and proteins to maintain function and support the regeneration of long processes.
Specific structures are identified in the cytoplasm (perikaryon) of the neuron:
- Chromophilic substance (Nissl bodies, tigroid substance). These are aggregations of rER cisternae. They stain intensely with basic dyes due to the abundance of RNA in ribosomes. Nissl substance provides for the synthesis of export, membrane, and lysosomal proteins. It is present in the soma and dendrites, but completely absent from the axon and axon hillock.
- Neurofibrils. Cytoskeletal elements (bundles of neurofilaments and neurotubules). Upon silver impregnation, they form a dense three-dimensional network in the cell body and run in parallel bundles within the processes. Their appearance during embryogenesis is the primary sign of neuroblast differentiation.
- Lipofuscin. A brown pigment that accumulates as the cell ages (can occupy up to 50% of the cytoplasmic volume in advanced age).
- Neurosecretory granules. Characteristic of hypothalamic neurosecretory neurons. These are membrane-bound vesicles containing peptides transported along the axon to blood vessels.
Processes: Axons and Dendrites
The primary classification criterion for processes in classical histology is the direction of impulse conduction, rather than their length or myelination.
Dendrites Conduct nerve impulses afferently, i.e., toward the neuronal soma. There are usually multiple dendrites, which branch close to the cell body. The surface of dendrites features specialized outgrowths called dendritic spines, which are necessary for synapse formation. Smooth endoplasmic reticulum cisternae (calcium ion stores) are located inside the spines. The higher the cellular activity, the more spines are present on its dendrites.
Axon (Neurite) Conducts impulses efferently—away from the neuronal soma. There is always only one axon. It typically does not branch along its course, giving off collaterals only in its terminal portion. It originates from a specialized region of the perikaryon called the axon hillock, where the nerve impulse is generated.
Morphological Classification of Neurons
Based on the number of processes extending from the cell body:
- Unipolar. Possess only a single axon. In humans, they are found only during embryogenesis (neuroblasts).
- Pseudounipolar. The points of origin of the dendrite and axon are maximally fused: it appears as a single trunk emerging from the cell body, which then T-branches. These are sensory neurons; their cell bodies always lie outside the central nervous system—in sensory ganglia (e.g., dorsal root ganglia). A long dendrite extends peripherally to receptors, while a short axon enters the spinal cord.
- Bipolar. Possess two processes (a dendrite and an axon) extending from opposite poles. A rare type, found in special sense organs (retina, olfactory epithelium).
- Multipolar. The most common type. They have one long axon and multiple branching dendrites. Functionally, they are associative or effector neurons.
Functional Classification and Impulse Conduction
A neuron processes information in 4 stages: reception (signal perception), state alteration (excitation/inhibition), conduction, and transmission. The excitation mechanism is based on plasma membrane transport systems. At rest, the membrane is polarized. Upon depolarization, sodium channels open, sodium rushes into the cell, and the membrane potential decreases (becomes less negative). Inhibition (hyperpolarization) is mediated by the opening of anion channels.
Functionally, neurons are divided into:
- Sensory (afferent): perceive impulses from receptors. Cell bodies lie exclusively in ganglia.
- Associative (interneurons): form connections between cells within the CNS or autonomic ganglia.
- Effector (motor/secretory): transmit commands to effector organs (muscles, glands).
Signal transmission to other cells occurs primarily via synapses using chemical neurotransmitters, or via the bloodstream (neuroendocrine transmission through axovascular contacts).