Dendrites and Local Potentials

  • Dendrites are structures of neurons receiving information.
    • Local potentials can be either excitatory or inhibitory.

Types of Local Potentials

  • Excitatory Local Potential:

    • Involves sodium ions (Na+) entering the cell.
    • This influx leads to depolarization of the membrane.
    • Example molecules: odorant molecules facilitating sodium entry.

  • Inhibitory Local Potential:

    • Involves negatively charged ions, like fluoride ( ext{F}^-), entering the cell or positive ions, such as potassium ( ext{K}^+), leaving the cell.
    • Both actions make the interior of the neuron more negative than the resting potential.

Office Hours and Clarification

  • The instructor encourages students to utilize office hours for doubts, specifically regarding local potentials.

Action Potentials

  • Defined as a rapid, transient change in membrane potential as a neuron transmits information.

Characteristics of Action Potentials

  • Excitatory Message: The goal of action potentials is to depolarize the membrane to trigger action potentials.

Mechanism of Action Potentials

  1. Resting Membrane Potential:

    • Resting potential is approximately -70 mV.

  2. Threshold for Action Potential Activation:

    • Local potential must depolarize the membrane to -55 mV (threshold).

  3. Depolarization Phase:

    • Sodium channels open, sodium enters the neuron due to concentration gradient and charge attraction; membrane voltage rapidly rises to approximately +35 mV.
    • Na+ current leads to increased membrane potential (from -70 mV to +35 mV).

  4. Repolarization Phase:

    • Once +35 mV is reached, sodium channels close, and potassium channels open allowing K+ to exit the cell, causing the voltage to drop.

  5. Hyperpolarization Phase:

    • Potassium voltage-gated channels remain open too long, leading to a temporary drop below the original resting membrane potential (more negative than -70 mV).
    • This is referred to as hyperpolarization.

  6. Restoration of Resting Potential:

    • The sodium-potassium pump works to reestablish resting potential by moving Na+ out and K+ back into the cell.
    • Astrocytes assist by absorbing excess ions to stabilize the environment.

Comparison of Potentials

  • Local Potentials:

    • Graded: Strength varies based on stimulus (can be weak or strong).
    • Detrimental: Can fade away and are not strong over distance.
    • Reversible: Can be undone by different ion movements.
    • Can be excitatory or inhibitory depending on ions involved.

  • Action Potentials:

    • All-or-nothing: Once the threshold is reached, the action potential will occur at full magnitude every time.
    • Nondecremental: Strength remains constant as it propagates along the axon.
    • Irreversible: Once triggered, cannot be undone.
    • Only excitatory; they do not carry inhibitory actions.

Refractory Periods

  • Absolute Refractory Period:

    • No new action potentials can occur during this phase as the neuron is completely engaged in the current action potential.

  • Relative Refractory Period:

    • A stronger-than-normal stimulus is required to initiate a new action potential due to hyperpolarization.
    • The neuron is merging back from a state where it could potentially activate again, but must overcome a more negative starting point.

Signal Conduction in Axons

  • Each action potential along an axon can be likened to a series of falling dominoes, where one action potential triggers the next.

Types of Axonal Conduction

  • Continuous Conduction:

    • Common in unmyelinated fibers.
    • Action potentials travel segment by segment, making the transmission slower.

  • Saltatory Conduction:

    • Occurs in myelinated fibers where action potentials leap from node of Ranvier to node of Ranvier.
    • Faster transmission due to gaps in myelin sheath where channels concentrate.

Factors Affecting Conduction Speed

  1. Fiber Diameter: Larger fibers conduct signals faster due to increased surface area and lower resistance.
  2. Myelination: Myelinated fibers are significantly faster than unmyelinated fibers.
  3. Example Speeds:
    • Small unmyelinated fibers: 0.5 to 2 meters/second.
    • Small myelinated fibers: 3 to 15 meters/second.
    • Large myelinated fibers: up to 120 meters/second.

Neurotransmitters and Synaptic Communication

  • Communication occurs via synapses, which allow signals to transfer between neurons.

Types of Synapses

  1. Axo-dendritic: Most common type, connecting axon to dendrite.
  2. Axo-somatic: Connects axon to the cell body.
  3. Axo-axonic: Connects one axon to another axon, affecting neurotransmitter release.

Neurotransmitter Release Mechanisms

  • Vesicular Release: Presynaptic neurons release vesicles containing neurotransmitters into the synaptic cleft.
  • Presynaptic and Postsynaptic Cells: Presynaptic releases transmitters; postsynaptic receives them.

Discovery of Neurotransmitters

  • Acetylcholine: First neurotransmitter discovered by Otto Loewy, known for its role in muscle contraction and heart rate regulation.
    • Influences autonomic nervous system action.

Major Neurotransmitter Classes

  1. Acetylcholine:

    • Excitatory for skeletal muscles, inhibitory in heart; involved in memory.

  2. Amino Acids:

    • GABA: Main inhibitory neurotransmitter in the brain.
    • Glycine: Main inhibitory neurotransmitter in the spinal cord.
    • Glutamate: Major excitatory neurotransmitter aiding in learning and memory formation.

  3. Monoamines:

    • Catecholamines:
      • Dopamine: Involved in mood regulation and can impact motivation and motor control.
      • Norepinephrine and Epinephrine: Control fight or flight responses, affect alertness, and are related to anxiety.
    • Serotonin and Histamine: Involved in mood, sleep, and blood flow regulation.