Neuron Structure, Function, and Transport Systems

  • Sensory Neuron (Afferent Neuron)

    • Receives information from the periphery and transmits it to the central nervous system (CNS).
    • Typically unipolar, with two exceptions mentioned but not detailed in the lecture (referenced in PowerPoint).
    • Olfactory Neurons: A specific exception that goes directly into the olfactory bulb and branches to the brain without spinal cord delay. Most other sensory neurons pass through cranial or spinal nerves.
    • Types of Sensory Endings:
    • Encapsulated endings.
    • Free nerve endings.
    • Endings with a receptor that activates, sends a signal to the dendrite, and then to the CNS.
    • Direction: Defined as sending information from the peripheral to the nervous system.

  • Interneuron (Association Neuron)

    • Located exclusively within the central nervous system.
    • Three Distinctive Types:
    • A type with a cell body and dendrites but no axon (sometimes referred to as dendrite or anacite).
    • Other types with axons are multipolar.
    • Function: Relates information from the peripheral nervous system to the brain for integration (especially between cortical regions) and then sends processed information back to the periphery.
    • Acts as a "middleman" and can magnify signals.

  • Motor Neuron (Efferent Neuron)

    • Transmits reactions or responses from the CNS to effector organs (muscles or glands).
    • Typically multipolar neurons.
    • Longest Motor Neuron: Extends from the sacral region all the way to the toes and lower leg (calf muscle).
    • Clinical Correlation: This long pathway makes the muscles it innervates (like the calf) highly susceptible to muscle cramps due to the distance the nerve signal must travel.
    • Function: Responsible for muscle contraction, glandular secretion, and other efferent functions.
    • Direction: Defined as sending information from the nervous system to the effector.

  • Typical Neuron Structure and Function

    • Multipolar Neuron: Often characterized by a long axon.
    • Cell Body (Soma)
    • Contains the nucleus, which is the production center for proteins, organelles, and all essential materials needed for maintaining the neuron's function and integrity.
    • Acts as an integrating center for all information received by the dendrites.
    • Excitable Cell Property: Neurons are "excitable cells," meaning their membrane permeability can change, allowing ions to flow in (e.g., Na+Na^+ influx), leading to depolarization or hyperpolarization (excitation or inhibition). This involves numerous gated ion channels.
    • Occupies roughly 1/101/10 of the neuron's volume.
    • Dendrites
    • Primary structures for receiving information from neighboring cells or receptors.
    • Multipolar neurons have highly branched dendrites.
    • Signal transmission normally occurs from dendrite to cell body and then to the axon (anterograde transport).
    • Dendritic Spines:
      • Protrusions from the main shaft of dendrites, resembling a "baseball glove."
      • Function: Increase the surface area available to receive much more information and provide feedback to communicating structures.
      • Dynamic Nature: Dendritic spines are dynamic, changing their shape, configuration, and size over time. This plasticity is believed to be crucial for learning, memory, and enhancing communication within the nervous system. Studying and learning, for example, can lead to changes in these spines.
    • Axon
    • Each neuron typically possesses a single axon.
    • Originates from the cell body at a specialized region called the Axon Hillock, also known as the Trigger Zone.
    • Trigger Zone Function: This area determines whether the integrated information received from the dendrites and cell body is sufficient to generate an action potential. The signal must exceed a certain threshold to initiate an action potential.
    • Action Potential vs. Graded Potential: Signals at the dendritic and cell body level are called graded potentials; only once they reach and surpass the threshold at the trigger zone do they become action potentials.
    • Myelin Sheath:
      • A fatty, lipid-rich insulating layer that wraps around many axons.
      • Appears white when stained, forming the "white matter" in the CNS.
      • Crucial Function: Myelination ensures rapid, faithful (accurate), and high-speed transmission of action potentials over long distances. It prevents signal decay.
      • Cell bodies and dendrites do not have myelinated coverings.
      • Not all axons are myelinated, but most are, especially those requiring fast transmission like motor neurons (e.g., to the toe).
      • Mechanism: Relies on voltage-gated sodium channels which, when opened, cause a rapid influx of Na+Na^+ ions, making the inside of the membrane more positive and propagating the action potential.

  • Axonal Transport Systems

    • A sophisticated system to transport essential materials (such as organelles, vesicles containing neurotransmitters, etc.) produced in the cell body over long distances along the axon.
    • Anterograde Transport:
    • Forward movement, from the cell body towards the axon terminal.
    • Mediated by kinesin proteins, which act as "motor proteins" literally "walking" along microtubules within the axon, carrying cargo.
    • Critical for delivering components needed at the synapse and maintaining the axon's structure.
    • Retrograde Transport:
    • Backward movement, from the axon terminal towards the cell body.
    • Mediated by dynein proteins, which also walk along microtubules.
    • Important for conveying waste products for recycling, neurotrophic factors, and signaling molecules back to the soma.
    • Clinical Example: Shingles Virus:
    • The Varicella-Zoster virus (which causes chickenpox) can, after initial infection, enter the presynaptic terminals of neurons.
    • It then uses retrograde transport to travel along microtubules to the neuron's cell body, where it remains dormant.
    • Under conditions of stress, the dormant virus reactivates and uses anterograde transport to travel back down the axon to the periphery, causing the painful blisters and rashes associated with shingles.

  • Synaptic Communication

    • Involves the release of neurotransmitters from the presynaptic axon terminal into the synaptic cleft.
    • These neurotransmitters then bind to receptors on the postsynaptic dendrite (or other postsynaptic structures like muscle cells or glands).
    • This allows for communication between neurons (neuro-neuro communication) or between neurons and effector cells (e.g., motor neuron to muscle).

  • Other Concepts Mentioned

    • Syncytial Contraction: A description of how the entire heart contracts as a single unit or functional syncytium, a concept that earned a Nobel Prize. (A brief tangential mention).
    • Cytosolic Space: Mentioned in context of the environment where transport proteins operate, referring to the intracellular fluid.