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Neurons: An Overview

  • Neurons are specialized nerve cells essential for intercellular communication.

Learning Outcomes

  • Sketch and label a typical neuron's structure.
  • Describe the functions of each component of the neuron.
  • Classify neurons based on their structure and function.

Understanding Neuron Function

  • Knowledge of neuron function necessitates familiarity with its structural components.
  • Neurons serve as the basic functional unit of the nervous system.

Characteristics of Neurons

  • Neurons are long-lived and possess a high metabolic rate, similar to skeletal muscle cells.
  • Have an excitable plasma membrane capable of generating and propagating action potentials, which requires significant energy resources.
  • Energy expenditure is critical for synthesizing and secreting chemical compounds necessary for neurotransmission.
  • Mitochondria in neurons produce energy to meet these demands.
  • Most neurons lack centrioles, which prevents them from dividing; hence, they cannot be replaced if lost to injury or disease.
  • Exemptions include neural stem cells in the adult nervous system, like those in the nose and the hippocampus, which are involved in olfactory receptor regeneration and memory storage, respectively.
  • Research is ongoing to understand triggers for neural stem cell activity to prevent or reverse neuron loss.

Structure of Neurons

  • Neurons display various shapes and typically consist of four main regions:
    1. Cell Body (Soma)
    2. Dendrites (branching extensions)
    3. Axon (a long projection)
    4. Teledendria (terminal branches of the axon)
  • Axon Hillock: The region between the cell body and the axon.

Components of the Cell Body (Soma)

  • Contains a large nucleus with a prominent nucleolus.
  • The cytoplasm is referred to as the Perikaryon, which contains:
    • Neurofilaments and neurotubules for cytoskeleton support.
    • Organelles for energy production and neurotransmitter synthesis.
  • Mitochondria generate ATP for the neuron's activity.
  • Rough Endoplasmic Reticulum (RER) with ribosomes produces proteins and forms dark-stained regions known as Nissl bodies (named after Franz Nissl).
  • Nissl bodies contribute to the gray matter seen in the brain and spinal cord.

Dendrites, Axons, and Teledendria

  • Dendrites:
    • Highly branched, extending from the cell body.
    • Participate in intercellular communication with key functions in receiving information.
    • Dendritic spines (0.5 - 1 µm projections) engage in synapses and account for 80-90% of a neuron's surface area.
  • Axons:
    • Long cytoplasmic processes, potentially up to 1 meter in length, propagate action potentials.
    • Axoplasm contains neurofibrillas, neurotubules, vesicles, lysosomes, mitochondria, and enzymes.
    • Axolemma: The plasma membrane around the axoplasm.
    • Axons may branch into collaterals, allowing a single neuron to communicate with multiple other cells.
  • Teledendria:
    • Terminal branches of the axon that end in axon terminals or synaptic terminals involved in cell communication.

Axonal Transport

  • Movement of materials between the cell body and axon terminals is termed Axonal Axoplasmic Transport.
  • Transport types:
    • Slow Stream: A few millimeters per day (e.g., substances like proteins).
    • Fast Stream: Can reach speeds of up to 1,000 mm/day, average around 300 mm/day.
  • Anterograde Flow: Movement from the cell body to the axon terminal (via kinesin).
  • Retrograde Flow: Movement from axon terminal to cell body (via dynein).
  • Clinical Note: Rabies utilizes retrograde flow to enter the CNS after a bite from a rabid animal, showcasing how certain pathogens could bypass neural defenses through axonal transport.

Classification of Neurons

  • Neurons can be classified structurally or functionally.

Structural Classification

  • Neurons may be anaxonic, bipolar, unipolar, or multipolar based on dendrite and axon relationships:
    1. Anaxonic Neurons: Small; numerous dendrites without visible axons; found in the brain and special sense organs; functions are poorly understood.
    2. Bipolar Neurons: Have one dendritic and one axonal process; rare, occurring in special sense organs (like sight and smell); cell body is situated between the two processes; maximum length under 30 µm.
    3. Unipolar Neurons: One continuous process that merges dendrites and axon with the cell body off to the side; primarily sensory neurons in the PNS; can be very long.
    4. Multipolar Neurons: Most common in the CNS; feature multiple dendrites and one axon; all motor neurons controlling skeletal muscles are multipolar, transmitting motor commands from spinal cord to muscles.

Functional Classification

  • Neurons can also be classified into three main functional categories:
    1. Sensory Neurons (Afferent Neurons): Part of the PNS; their cell bodies are in peripheral sensory ganglia; deliver information from sensory receptors to the CNS; classified as unipolar neurons with processes known as afferent fibers.
    • Types of Sensory Receptors:
      • Interceptors: Monitor internal environment (cardiovascular, respiratory, etc.).
      • Exteroceptors: Monitor external environment (touch, temperature, taste, etc.).
      • Proprioceptors: Monitor muscle/joint position and movement.
    1. Motor Neurons (Efferent Neurons): Carry instructions from CNS to effector organs and tissues; axons known as efferent fibers; controlled systems include:
    • Somatic Nervous System (SNS): Consciously controlled motor neurons to skeletal muscles.
    • Autonomic Nervous System (ANS): Involuntarily innervates smooth muscle, cardiac muscle, and glands; includes preganglionic and postganglionic fibers.
    1. Interneurons (Association Neurons): Located between sensory and motor neurons; outnumber all other neuron types; mainly in the brain and spinal cord; responsible for integration; facilitate coordination of motor activity, memory, planning, and higher-level cognitive functions.

Summary and Review

  • Structural Components of a Neuron:
    • Cell body (soma), dendrites, axon, teledendria, Nissl bodies, neurofilaments, neurotubules, neurofibrils, axoplasm, axolemma, initial segment, axon hillock, collaterals.
  • Classification by Structure: Anaxonic, bipolar, unipolar, multipolar.
  • Classification by Function: Sensory neurons, motor neurons, interneurons.