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.