2.4: the dendrites

Refractory Period

  • Definition: The refractory period is a time during which a neuron cannot fire another action potential after one has just occurred.
  • Types of Refractory Periods:
    • Absolute Refractory Period:
    • Occurs shortly after an action potential.
    • Both activation gates are open and inactivation gates are closed on voltage-gated sodium channels, preventing reactivation.
    • Lasts about 2 milliseconds.
    • Relative Refractory Period:
    • Follows the absolute refractory period.
    • Some sodium channels reset, allowing for a possible second action potential, but a stronger stimulus is required.
    • Membrane may remain hyperpolarized, increasing the threshold for firing an action potential.

Action Potentials and Unidirectional Propagation

  • Unidirectional Propagation: Action potentials propagate in one direction, from the cell body to the axon terminal. This occurs due to the refractory period:
    • The area just activated is in its refractory period, making it less capable of firing again.
    • The influx of sodium during depolarization influences adjacent voltage-gated channels, leading to activation downstream.

Experimental Scenarios

  • Stimulating Middle of Axon:
    • If stimulated in the middle, action potentials may travel in both directions due to simultaneous activation of regions above and below the stimulated point.
    • Sodium ions diffuse down the concentration gradient but cause action potentials to stop in the refractory region.

Speed of Action Potentials

  • Factors Affecting Speed:
    • Axon diameter: Larger diameters reduce leakage and allow faster propagation.
    • Myelination: Insulating Schwann cells prevent loss of ions and facilitate faster action potential transmission.

Myelination and Nodes of Ranvier

  • Myelination: Schwann cells wrap around axons, creating nodes of Ranvier.
    • Action potentials jump from node to node (saltatory conduction), speeding up transmission due to reduced time for regeneration of action potentials.

Graded Potentials vs. Action Potentials

  • Graded Potentials:
    • Local changes in the membrane potential that vary in size and can add together (summation).
    • Occur in dendrites and are often triggered by neurotransmitter binding to receptors.
  • Action Potentials:
    • Are all-or-nothing events that occur when the membrane potential reaches a certain threshold (-55 mV) at the axon hillock.

Dendrites and Signal Integration

  • Role of Dendrites:
    • Dendrites receive inputs from other neurons and sensory receptors, integrating signals to determine whether to generate action potentials.
    • They lack voltage-gated channels and generate only graded potentials.

Synaptic Transmission

  • Chemical Synapses:
    • Neurotransmitters are released from presynaptic neurons and bind to receptors on postsynaptic neurons.
    • Excitatory Synapses: Lead to depolarization by allowing sodium influx.
    • Inhibitory Synapses: Often involve chloride influx and lead to hyperpolarization, making action potentials less likely.

Decision-Making in Neurons

  • Neurons integrate multiple inputs at dendrites to decide whether to transmit a signal via action potentials.
  • The axon hillock is the critical point where the decision is made, influenced by graded potentials received from dendrites.

Long-Term Changes in Neuronal Activity

  • Second Messenger Systems:
    • Neurons can undergo long-term adaptations to regulate their sensitivity and responsiveness to stimuli, influencing action potential generation over longer periods.
  • G-Protein Coupled Receptors (GPCRs):
    • Play a role in long-term changes in membrane potential and neuron integration, adjusting excitability.