Neuron Structure, Classification, and Function Notes

Core Characteristics and Structure of Neurons

• Primary Role: Structural units of the nervous system that conduct messages via action potentials.

• Key Characteristics:

  • Extreme Longevity: Capable of functioning optimally for an entire lifetime.

  • Amitotic Nature: Inability to divide once mature (exceptions include olfactory epithelium and hippocampal regions).

  • High Metabolic Rate: Requires an uninterrupted supply of oxygen and glucose; cannot survive more than a few minutes without oxygen.

• Cell Body (Soma):

  • Spherical nucleus and cytoplasm ranging from $5\,\mu\text{m}$ to $140\,\mu\text{m}$ in diameter.

  • Biosynthetic and metabolic hub housing rough endoplasmic reticulum (chromatophilic substance or Nissl bodies), free ribosomes, Golgi apparatus, microtubules, and neurofibrils.

  • Contains pigments including melanin, a red iron-containing pigment, and lipofuscin.

  • Clusters of cell bodies are designated as nuclei in the CNS and ganglia in the PNS.

• Processes:

  • Dendrites: Short, branching extensions with dendritic spines that serve as receptive regions, conveying graded potentials toward the soma.

  • Axon: Single long process originating from the axon hillock; conducts nerve impulses along the axolemma away from the cell body to axon terminals, which release neurotransmitters.

  • Axons lack rough endoplasmic reticulum and Golgi apparatus, making them entirely dependent on the soma for protein maintenance.

Axonal Transport and Clinical Correlates

• Mechanisms:

  • Anterograde Transport: Moves mitochondria, cytoskeletal elements, membranes, and enzymes away from the cell body toward terminals via the motor protein kinesin along microtubules.

  • Retrograde Transport: Moves organelles and signal molecules back toward the cell body for recycling or degradation via the motor protein dynein.

• Clinical Relevance:

  • Pathogens exploit retrograde transport to reach the soma (e.g., polio, rabies, herpes simplex viruses, tetanus toxin).

  • Retrograde pathways provide a therapeutic avenue to deliver corrective genes or microRNA into neurons.

Myelination in the PNS and CNS

• Myelin Sheath Function: Whitish, fatty, segmented covering on axons that insulates fibers, prevents signal loss, and significantly speeds action potential propagation. Dendrites are always nonmyelinated.

• PNS Myelination:

  • Formed by Schwann cells wrapping around an axon in concentric layers, squeezing cytoplasm to the periphery.

  • Characterized by myelin sheath gaps (nodes of Ranvier) spaced approximately $\sim 1\,\text{mm}$ apart.

• CNS Myelination:

  • Formed by oligodendrocytes, each extending processes to wrap up to $60$ axons simultaneously.

  • Lacks the outer collar of perinuclear cytoplasm found in Schwann cells.

Structural and Functional Neuron Classifications

• Structural Classification:

  • Multipolar: Three or more processes (one axon, multiple dendrites); accounts for over $99\%$ of neurons; includes Purkinje cells of the cerebellum and pyramidal cells.

  • Bipolar: Two processes (one axon, one dendrite); rare; found in special sense organs (retinal cells, olfactory cells).

  • Unipolar (Pseudounipolar): Single short process dividing into peripheral and central branches; characteristic of sensory neurons in PNS ganglia (e.g., dorsal root ganglion cells).

• Functional Classification:

  • Sensory (Afferent): Transmit impulses from sensory receptors toward the CNS; primarily unipolar with cell bodies in sensory ganglia.

  • Motor (Efferent): Transmit impulses away from the CNS to effector organs (muscles, glands); multipolar with somas inside the CNS.

  • Interneurons (Association Neurons): Shuttle signals through CNS pathways; mostly multipolar; constitute over $99\%$ of all neurons.

Questions & Discussion

• Brain Nucleus vs. Neuron Nucleus:

  • Question: What is the difference between a nucleus in the brain and a nucleus in a neuron?

  • Answer: A nucleus in the brain is an anatomical cluster of neuron cell bodies within the CNS, whereas a nucleus in a neuron is the intracellular organelle housing genetic material.

• CNS Myelin Formation and Role:

  • Question: How is the myelin sheath formed in the CNS, and what is its primary function?

  • Answer: Oligodendrocytes wrap around multiple axons to form myelin, electrically insulating the axons to accelerate electrical impulse propagation.

• Reflex Pathway Activation Sequence:

  • Question: Which neurons are activated first and last during a withdrawal reflex from a finger burn?

  • Answer: A unipolar, afferent (sensory) neuron is activated first to carry impulses toward the CNS; a multipolar, efferent (motor) neuron is activated last to stimulate muscle contraction.

• Nerve Fibers vs. Muscle and Connective Fibers:

  • Question: How do nerve fibers differ from connective tissue and muscle fibers?

  • Answer: A nerve fiber is a neuronal axon extension; connective tissue fibers are non-cellular extracellular proteins; muscle fibers are elongated, multi-nucleated or single cells capable of contraction.