Unit Five Notes: The Neuron

Unit Five Notes: THE NEURON

Section 11.1: Overview of the Nervous System

  • Functionality: The nervous system receives, integrates, and responds to information.

    1. Direction of Information Flow
    • Sensory Input: Information is gathered by sensory receptors from both the internal and external environments.
    • Integration: Interpretation of sensory input occurs in the central nervous system (CNS).
    • Motor Output: Activation of effector organs (muscles, glands) produces a response.

  • Organization of the Nervous System
    A. Major Divisions

    1. Central Nervous System (CNS): Comprises the brain and spinal cord.
    2. Peripheral Nervous System (PNS): Comprises all neural structures outside the brain and spinal cord, including cranial and spinal nerves.
      B. Divisions of the PNS:
    3. Sensory Division (Afferent Division): Conveys impulses from the body to the CNS.
    4. Motor Division (Efferent Division): Transmits impulses from the CNS to effector organs, divided into:
      a. Voluntary: Somatic Nervous System (SNS) - conscious control of skeletal muscles.
      b. Involuntary: Autonomic Nervous System (ANS) - regulates smooth and cardiac muscles, and glands, further divided into:
      i. Sympathetic Division: Mobilizes body systems for activity.
      ii. Parasympathetic Division: Conserves energy.

Section 11.2: Neuroglia Support and Maintain Neurons

  • Types of Cells:
    1. Neurons: Excitable cells that can generate and conduct electrical signals.
    2. Neuroglial Cells (Glial Cells): Support neurons in various ways:
      a. Astrocytes: Star-shaped cells that anchor neurons and blood vessels in the CNS.
      b. Microglial Cells: Derived from white blood cells (WBC), these are phagocytic cells in the CNS.
      c. Ependymal Cells: Ciliated cells that produce and circulate cerebrospinal fluid, lining the hollow cavities of the CNS.
      d. Oligodendrocytes: Myelinate axons in the CNS.
      e. Schwann Cells: Myelinate axons in the PNS.

Section 11.3: Neurons - Structural Units of the Nervous System

  • Anatomy of a Neuron:

    • Dendrites: Extensions that receive signals.
    • Cell Body (Soma):
      • Nissl Bodies: Ribosome-rich areas indicating high protein synthesis.
      • Lack of Centrioles: Neurons do not undergo mitosis.
    • Axon:
      • Axon Initial Segment: Site for action potential generation.
      • Axonal Transport: Mechanisms facilitating movement of materials along the axon.
      • Telodendria: Ends of the axon that connect to target cells.
        • Axon Terminals: Regions containing synaptic vesicles that release neurotransmitters into the synaptic cleft.

  • Anatomy of Myelinated Axons:

    • Neurolemma: Outer membrane of Schwann cells surrounding an axon.
    • Internode: The segment of axon covered by myelin.
    • Node of Ranvier: Gaps between myelin segments, facilitating faster conduction.

  • Neuron Characteristics:

    • Longevity: Neurons can live for a person’s entire lifetime.
    • Amitotic: Neurons do not undergo cell division.
    • High Metabolic Rate: Neurons require continuous energy supply due to high activity.
    • Specialized Function: Primarily focused on generating and conducting electrical signals.

  • Classification of Neurons:
    A. Structural Classification: Based on the number of processes extending from the cell body:

    1. Multipolar Neurons: Numerous processes branching off.
    2. Bipolar Neurons: Two processes extending off the cell body.
    3. Unipolar Neurons (Pseudounipolar): One short process that divides into dendrites and axon.
      B. Functional Classification: Based on direction of information flow:
    4. Sensory Neurons (Afferent): Carry information into the CNS.
    5. Interneurons: Entirely contained within the CNS, facilitating communication.
    6. Motor Neurons (Efferent): Carry information out of the CNS to effector organs.

Section 11.4: Resting Membrane Potential and Ion Concentration

  • A Neuron at Rest: Neurons maintain a state of readiness and utilize energy to prepare for incoming messages, akin to a preheated oven.

    • Resting Membrane Potential (RMP): The electric charge inside the cell surface is distinct from outside, measured at approximately -70mV; this signifies the inside is 70mV more negative compared to the outside.
      • Polarization: Refers to the separation of charge across the neuron membrane, where both the extracellular fluid (ECF) and intracellular fluid (ICF) are electrically neutral, yet have differing ion concentrations.

  • Establishing and Maintaining RMP: Involves two types of channels:
    A. Passive Transport: Leakage (non-gated) channels that are always open, allowing ions to diffuse according to their concentration gradients, with a greater number of K+ channels than Na+ channels.
    B. Active Transport: Na+/K+ Pump utilizes ATP for counter-transport, ejecting 3Na⁺ from the neuron and importing 2K⁺, continuously working to maintain gradients and membrane potential.

  • Ion Concentration Gradient:

    • Na⁺ Concentration: High extracellularly, low intracellularly.
    • K⁺ Concentration: Low extracellularly, high intracellularly.
    • Leakage channels allow Na⁺ in and K⁺ out, yet the Na+/K+ pump restores balance continuously.

  • A Neuron at Work: Neurons have gated channels for conducting signals:

    • Dendrites and Cell Body: Contains chemically gated channels that generate graded potentials.
    • Axon: Contains voltage gated channels that generate action potentials.

Section 11.5: Graded Potentials - Short-Distance Signals

  • Definition of Graded Potential: A temporary change in voltage at the dendrites/soma, which diffuses towards the axon initial segment.
    • Mechanism: When a neurotransmitter (NT) binds to receptors on dendrites/soma, it opens channels specific to Na⁺ or K⁺ ions:
    1. If Na⁺ channels open, depolarization occurs.
    2. If K⁺ channels open, hyperpolarization occurs.
  • Characteristics:
    • Voltage change magnitude is proportional to the number of opened channels.
    • Graded potentials diminish over distance, similar to ripples in a pond.
      • Graded potentials are essential for potentially activating voltage gated channels in the axon if depolarization reaches threshold.

Section 11.6: Action Potentials - Long-Distance Signals

  • Definition of Action Potential: Indicates a nerve impulse, represented by an electric current traveling along an axon without diminishing.

  • Phases of Action Potential:
    A. Resting State: All voltage-gated Na⁺ and K⁺ channels are closed.
    B. Depolarization: When the axon reaches threshold (-55 mV), voltage gated Na⁺ channels open, resulting in Na⁺ influx and further depolarization to +30 mV.
    C. Repolarization: At +30 mV, Na⁺ channels inactivate and K⁺ channels open, allowing K⁺ efflux, reversing the charge.
    D. Hyperpolarization: The voltage gated K⁺ channels close more slowly, leading to a temporary state more negative than RMP.
    E. Return to Resting State: Voltage gated channels close, and the Na+/K+ pump restores the RMP.

  • Refractory Periods:

    • Absolute Refractory Period: Occurs when Na⁺ channels are open or inactive; no stimulus can stimulate an action potential.
    • Relative Refractory Period: During hyperpolarization, stronger stimuli can initiate another action potential.

  • Conduction Velocity Factors:
    A. Axon Diameter: Larger diameters conduct faster (Type A fibers: 300 mph; Type B fibers: 40 mph; Type C fibers: 2 mph).
    B. Myelin Sheath: Insulates axons, enhancing saltatory conduction, allowing action potentials to jump from node to node.

Section 11.7: Synapses - Transmission between Neurons

  • Chemical Synapse:

    • Action potentials cause neurotransmitter release via exocytosis into the synaptic cleft.
    • Voltage-gated Ca²⁺ channels open, inducing synaptic vesicles to fuse with the membrane.
    • Neurotransmitters bind to postsynaptic receptors, leading to ion channel opening.
    • Neurotransmitter removal methods include enzymatic degradation, reuptake, or diffusion.

  • Electrical Synapse:

    • Gap junctions allow ion flow directly, facilitating rapid electrical signal transmission, common in cardiac muscle tissues.

Section 11.8: Postsynaptic Potentials

  • Postsynaptic Potential (PSP): Refers to the graded potential caused by neurotransmitters; can either:

    • Excitatory Post Synaptic Potential (EPSP): Leads to depolarization, driving the membrane potential toward threshold.
    • Inhibitory Post Synaptic Potential (IPSP): Leads to hyperpolarization, moving the membrane potential farther from threshold.

  • Information Transfer Dynamics:

    • Synaptic Delay: Approximately 0.2-0.5 ms is required for neurotransmitter effect.
    • Synaptic Fatigue: Occurs when neurotransmitter supplies are depleted.
    • Summation: Neurons integrate multiple EPSPs and IPSPs:
    1. Temporal Summation: Rapid delivery of stimuli can accumulate to reach threshold.
    2. Spatial Summation: Different stimuli at various locations simultaneously contribute to summation.

### Videos:

  • Recommended viewing sequences on the resting membrane potential, generation of action potential, synaptic functions, and EPSP/Ipsp are suggested to reinforce learning and provide visual context for understanding these concepts.