Inhibitory Neurotransmitters Hyperpolarize Neurons to Prevent Action Potentials
- Neural communication relies on the integration of signals from multiple presynaptic neurons to determine whether a postsynaptic neuron will generate an action potential.
- This integration occurs through a process known as summation, where the combined effects of excitatory neurotransmitters (which depolarize the membrane) and inhibitory neurotransmitters (which hyperpolarize the membrane) determine the postsynaptic response.
- If the threshold potential is reached, the postsynaptic neuron will fire an action potential, illustrating the all-or-nothing principle.
Definition
Summation
Summation refers to the process by which multiple signals (neurotransmitters) from presynaptic neurons are combined at the postsynaptic neuron to determine if it will fire an action potential.
How Summation Works
1. Spatial Summation
- Spatial summation occurs when multiple presynaptic neurons send simultaneous signals to a single postsynaptic neuron.
- The combined effect of these signals is evaluated by the postsynaptic neuron at the same time.
- The threshold potential is typically around $−50 mV$.
- If the combined effects of excitatory and inhibitory signals do not reach this level, no action potential will occur.
How does it work?
- Signals can come from multiple synapses located at different places on the postsynaptic neuron's dendrites or cell body.
- If enough excitatory signals reach the postsynaptic neuron at the same time, the membrane potential will depolarize and, if it reaches the threshold potential, will trigger an action potential.
- Inhibitory signals can counteract the excitatory signals. If there are enough inhibitory signals, they may hyperpolarize the postsynaptic neuron, preventing it from firing.
- Imagine that three presynaptic neurons are firing simultaneously at different locations on a postsynaptic neuron.
- If each one releases an excitatory neurotransmitter, their effects will add up (summate), and if they reach the threshold potential, an action potential will be generated.
2. Temporal Summation
- Temporal summation occurs when a single presynaptic neuron fires action potentials rapidly in a short period.
- The effects of the signals build up over time, as the postsynaptic neuron doesn’t have time to fully return to its resting potential before the next signal arrives.
