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Overview of Neurons and Their Structures

  • Neurons are the basic unit of the nervous system, responsible for transmitting signals throughout the body.

Key Structures of Neurons

  • Cell Body (Soma)

    • Central part of the neuron.
    • Contains the nucleus and most organelles.

  • Processes

    • Neurons have two types of processes:
    • Dendrites
      • Smaller processes that receive incoming signals.
    • Axon
      • A long process that carries outgoing signals.
      • Each neuron has only one axon.
      • The signal travels in one direction: from dendrite to cell body to axon.

  • Axon Hillock

    • A triangular structure at the beginning of the axon where the axon begins.
    • Term 'hillock' means a small hill.

  • Nerves vs. Neurons

    • Nerve: A bundle of axons in the peripheral nervous system (PNS).
    • Neuron: An individual nerve cell with dendrites and axon.
    • In the central nervous system (CNS), a nerve bundle is referred to as a tract.

Microscopic Anatomy of a Neuron

  • Cell Membrane: Surrounds the neuron.

  • Axoplasm: The specialized cytoplasm of the axon; limited in components.

    • Managed by organelles such as mitochondria, Golgi apparatus (releases vesicles into the axon), and cytoskeletal elements (intermediate filaments and microtubules).

  • Myelin Sheath

    • An insulator around the axon designed to improve signal integrity.
    • Gaps in the sheath are called nodes of Ranvier, important for signal transmission and action potential resetting.

  • Terminal Ends (Synaptic End Bulb)

    • These branches connect with the dendrites of the next neuron to continue signal transmission.

Types of Neurons Based on Structure

  • Unipolar Neurons

    • One process that branches into two directions (not true unipolar in humans, called pseudo unipolar).
    • Primarily in invertebrates; used for sensory input.

  • Bipolar Neurons

    • Two processes: one axon and one dendrite.
    • Found in olfactory epithelium and retina—related to smell and vision.

  • Multipolar Neurons

    • Multiple processes (three or more).
    • Most common type of neuron in humans.

Classification by Shape and Function

  • Neurons can also be classified by shape, such as pyramidal neurons (found in the cerebral cortex) and Purkinje cells.

  • The olfactory neuron is another example based on function and location in the nervous system.

Axonal Transport

  • Axonal Transport: Mechanism of moving materials through the axon.
    • Fast Axonal Transport
    • Moves organelles at rates of up to 400 mm per day.
    • Antegrade: From cell body to axon terminal.
    • Retrograde: From axon terminal back to cell body.
    • Slow Axonal Transport
    • Moves materials through axoplasm at 0.2 to 2.5 mm per day.

Synaptic Transmission

  • Synapse: Junction where one neuron's axon terminal meets another neuron's dendrites.

    • Presynaptic Neuron: Sends the signal.
    • Postsynaptic Neuron: Receives the signal.
    • Synaptic Cleft: Gap between the two neurons.

  • Types of Synapses

    • Chemical Synapse: Most common; neurotransmitters are released to facilitate signal transmission.
    • Electrical Synapse: Less common; important for rapid communication, seen in embryonic tissues and some adult tissues (e.g., eyes).

  • Growth Cone: Bud from which the neuron grows during development.

    • Neurotrophic Factors: Encourage developing neurons to find their targets.

Glial Cells

  • Support neurons and perform specific functions depending on their location:
Central Nervous System Glial Cells
  1. Astrocytes
    • Star-shaped cells that form the blood-brain barrier and provide nutrition to neurons.
  2. Oligodendrocytes
    • Produce myelin sheaths around CNS axons.
  3. Microglia
    • Immune cells that protect neurons by removing debris.
  4. Ependymal Cells
    • Line brain ventricles and central canal of spinal cord.
Peripheral Nervous System Glial Cells
  • Satellite Cells: Help maintain an environment around neurons similar to astrocytes.
  • Schwann Cells: Produce myelin sheaths around PNS axons, similar to oligodendrocytes.

Myelin Formation

  • In the PNS, myelin is formed by Schwann cells, wrapping around axons in multiple layers to prevent signal loss.
  • Gaps known as nodes of Ranvier are crucial for the action potential resetting.

Neuronal Regeneration

  • Neuron regeneration mainly occurs in the PNS, allowing for axonal repair but not cell body regeneration.
  • The repair process involves:
    1. Axoplasm leaks out causing the damaged end to form a new growth cone.
    2. Schwann cells release chemicals to promote regrowth.
    3. Cellular debris is cleared by immune cells.

Conclusion

  • Neurons are complex cells crucial for nervous system function, with various structural forms and supportive glial cells ensuring proper signal transmission and repair mechanisms.