Action Potentials

Graded Potentials vs. Action Potentials

Neurons communicate using electrical signals. Graded potentials are local changes in membrane potential, but they are not sufficient to transmit signals over long distances. Action potentials are large, rapid changes in membrane potential that can travel along the entire length of the axon.

• Graded potentials are not large enough to communicate between the cell body and axon terminal.

• Action potentials are required for long-distance communication in neurons.

• Action potentials affect the entire excitable membrane of the neuron.

Generation of Action Potentials

An action potential is triggered when a graded potential brings the membrane potential to a critical threshold, typically around -55 mV.

• Depolarization to threshold: Voltage-gated sodium (Na+) channels open when the membrane potential reaches threshold.

• Rapid depolarization: Na+ rushes into the cell, causing the membrane potential to rise to about +30 mV.

• Repolarization: Na+ channels close, potassium (K+) channels open, and K+ exits the cell, returning the membrane potential toward resting levels.

• Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential (can reach -90 mV) before stabilizing at the resting value (-70 mV).

• Restoration: The Na+/K+ pump restores the original ion distribution.

All-or-none principle: Once threshold is reached, an action potential will always occur with the same amplitude; it does not vary in size.

Refractory Periods

• Absolute refractory period: No new action potential can be initiated, regardless of stimulus strength.

• Relative refractory period: A stronger-than-normal stimulus is required to initiate another action potential.

Propagation of Action Potentials

Continuous Propagation

Occurs in unmyelinated axons. The action potential moves along the axon in a stepwise fashion.

• Each segment of the axon depolarizes and then repolarizes sequentially.

• The action potential cannot move backward due to the refractory period.

• Conduction speed is relatively slow (about 1 m/s).

Saltatory Propagation

Occurs in myelinated axons. The action potential "jumps" from one node of Ranvier to the next, greatly increasing conduction speed.

• Myelin sheath prevents ion flow across the membrane except at the nodes.

• Impulse conduction is much faster than in unmyelinated axons.

• Larger diameter axons conduct impulses faster due to lower resistance.

Synaptic Transmission

Chemical Synapses

Most synapses in the nervous system are chemical synapses, where neurotransmitters mediate communication between neurons.

• Depolarization of the axon terminal opens voltage-gated Ca2+ channels.

• Influx of Ca2+ triggers exocytosis of neurotransmitter vesicles.

• Neurotransmitters bind to receptors on the postsynaptic cell, eliciting a response.

Postsynaptic Potentials

Types of Postsynaptic Potentials

Postsynaptic potentials are graded potentials that develop in the postsynaptic cell membrane in response to neurotransmitter binding.

• EPSP (Excitatory Postsynaptic Potential): Brings the postsynaptic membrane closer to threshold (depolarization).

• IPSP (Inhibitory Postsynaptic Potential): Moves the postsynaptic membrane further from threshold (hyperpolarization).

Summation of EPSPs and IPSPs determines whether the postsynaptic neuron will fire an action potential.

• Multiple EPSPs may be required to reach threshold.

• IPSPs can counteract EPSPs.

Neuronal Pools and Neural Circuits

Neuronal Pool

A neuronal pool is a functional group of interconnected neurons that process specific types of information. These neurons do not have to be located in the same anatomical area.

Neural Circuits

Neural circuits are patterns of synaptic connections among neurons in a neuronal pool, often described as the "wiring diagram" of the nervous system.

• Divergence: One neuron sends information to several others, allowing broad distribution of a specific input (e.g., sensory neurons sending information to multiple areas of the CNS).

• Convergence: Several neurons synapse on a single postsynaptic neuron, allowing that neuron to be activated in multiple ways.