Neural Communication: Action Potentials, Synapses, and Post-Synaptic Potentials

Neural Communication

How Do Neurons Communicate?

Neurons communicate through electrical and chemical signals, enabling the transmission of information throughout the nervous system. This process involves both action potentials (within a neuron) and synaptic transmission (between neurons).

• Action Potential: An electrical signal that travels along the axon of a neuron; a within-cell process.

• Synapse: The junction between two neurons where chemical communication occurs; a between-cell process.

• Synaptic Cleft: The small gap between the axon terminal of the presynaptic neuron and the dendrite of the postsynaptic neuron.

Resting Potential

Definition and Mechanism

The resting potential is the difference in electrical charge across the membrane of a neuron when it is not actively sending a signal. This potential is essential for the generation of action potentials.

• Membrane Potential: The voltage difference between the inside and outside of a cell.

• Typical Resting Potential: Approximately -70 mV (the inside of the neuron is more negative than the outside).

• Example Calculation:

  • Outside cell (extracellular): 110 mV

  • Inside cell (intracellular): 40 mV

  • Difference: $110\,\text{mV} - 40\,\text{mV} = 70\,\text{mV}$

Action Potential

Phases and Properties

An action potential is a rapid, temporary change in a cell's membrane potential, allowing the neuron to transmit a signal along its axon.

• Massive, momentary reversal of the membrane potential from -70 mV to +50 mV.

• All-or-None Response: The neuron either fires completely or not at all; there is no partial action potential.

• Depolarization: Sodium (Na+) ions enter the cell, making it less negative (less polar).

• Threshold: If the membrane potential reaches a certain level, the action potential is triggered.

• Rising Phase: Rapid influx of Na+ ions.

• Falling Phase: Potassium (K+) ions leave the cell, making the membrane potential more negative.

• Undershoot (Hyperpolarization): The membrane potential temporarily becomes more negative than the resting potential.

• Refractory Period: The neuron cannot fire another action potential immediately after one has occurred.

Axonal Transmission

Propagation of Action Potentials

Action potentials travel along the axon to transmit signals to other neurons or effectors. The speed and efficiency of this transmission depend on the axon's properties.

• Unmyelinated Axons: Require repeated generation of action potentials along the entire length.

• Myelinated Axons (Saltatory Conduction): Action potentials jump from node to node (Nodes of Ranvier), greatly increasing conduction speed.

• Signal Speed: Myelinated axons conduct signals much faster, similar to jumping rather than walking.

Synapse

Structure and Function

The synapse is the site where communication occurs between two neurons. It consists of the presynaptic neuron (sending the signal), the synaptic cleft, and the postsynaptic neuron (receiving the signal).

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Contains receptors that bind neurotransmitters, leading to changes in membrane potential.

Neurotransmission is the process of chemical communication between neurons via neurotransmitters.

Post-Synaptic Potentials (PSPs)

Types and Effects

When neurotransmitters bind to receptors on the postsynaptic neuron, they produce changes in the membrane potential called post-synaptic potentials (PSPs). These can be excitatory or inhibitory.

• EPSP (Excitatory Post-Synaptic Potential):

  • Depolarizes the postsynaptic membrane (e.g., from -70 mV to -67 mV).

  • Increases the likelihood that the neuron will fire an action potential.

• IPSP (Inhibitory Post-Synaptic Potential):

  • Hyperpolarizes the postsynaptic membrane (e.g., from -70 mV to -73 mV).

  • Decreases the likelihood that the neuron will fire an action potential.

Integration of Post-Synaptic Potentials

Summation and Action Potential Generation

Neurons integrate multiple EPSPs and IPSPs to determine whether to fire an action potential. This process is called summation and can be spatial (from different locations) or temporal (over time).

• Two simultaneous EPSPs can sum to produce a greater depolarization.

• Two simultaneous IPSPs can sum to produce a greater hyperpolarization.

• Simultaneous EPSPs and IPSPs can cancel each other out.

• If the combined effect of all PSPs reaches the threshold, an action potential is generated.

Type of PSP	Effect on Membrane Potential	Effect on Neuron Firing
EPSP	Depolarization (less negative)	Increases likelihood
IPSP	Hyperpolarization (more negative)	Decreases likelihood

Key Equations

• Resting Membrane Potential:

  • $V_{rest} = V_{outside} - V_{inside}$

• Threshold for Action Potential:

  • If $V_{membrane} \geq V_{threshold}$, then action potential is triggered.

Example

• If a neuron receives two EPSPs of +3 mV each at the same time, the total depolarization is +6 mV, which may be enough to reach the threshold and trigger an action potential.

• If a neuron receives an EPSP of +3 mV and an IPSP of -3 mV simultaneously, the effects cancel out, and the membrane potential does not change.

Additional info: The provided diagrams visually reinforce the concepts of synaptic structure and the summation of post-synaptic potentials, which are fundamental to understanding neural communication in psychology.