Graded Potentials: Videos & Practice Problems

Graded potentials are crucial changes in membrane potential that occur in the soma and dendrites of neurons, particularly in the context of chemical synapses, which are the most common type of synapse in the human body. When a presynaptic neuron communicates with a postsynaptic neuron, it generates postsynaptic potentials that alter the voltage of the receiving neuron. These changes can be classified into two main types: excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).

EPSPs are depolarizing events that make the membrane potential more positive, effectively "exciting" the neuron and increasing the likelihood of firing an action potential. This can be visualized as the neuron urging to "fire" and send a message. Conversely, IPSPs are hyperpolarizing events that make the membrane potential more negative, inhibiting the neuron and reducing the chances of action potential generation, akin to a neuron advising against firing.

When an EPSP occurs, the process begins with the release of neurotransmitters that bind to receptors on the postsynaptic neuron, leading to the opening of gated sodium channels. As sodium ions rush into the cell, the membrane potential shifts from its resting state of approximately -70 mV towards a more positive value, such as -60 mV, indicating depolarization. This local depolarization is strongest at the site of stimulation and diminishes as it spreads along the membrane due to the presence of leak channels that allow some sodium to escape, resulting in a weaker signal as it travels.

For example, if the initial depolarization reaches a peak of -60 mV, it may drop to -65 mV or -68 mV as it propagates away from the stimulation site, eventually returning to the resting potential. This attenuation of the signal raises an important question: how can multiple EPSPs combine to reach the threshold needed for an action potential? This concept will be explored further in subsequent discussions, emphasizing the cumulative effect of multiple graded potentials in neuronal signaling.

In the context of neuronal communication, the concepts of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) play a crucial role in determining whether a neuron will fire an action potential. Each individual EPSP or IPSP is relatively small and may not significantly influence the membrane potential on its own. However, neurons are interconnected through synapses, allowing for the cumulative effect of multiple postsynaptic potentials, a process known as summation.

Summation occurs at the initial segment of the neuron, where the changes in membrane potential from various synapses are combined. If the cumulative effect reaches a threshold of approximately -55 millivolts, an action potential is generated. If the threshold is not met, the neuron remains inactive.

There are two primary types of summation: temporal summation and spatial summation. Temporal summation involves the rapid firing of a single neuron at one synapse, where successive EPSPs overlap in time. This rapid succession allows the potentials to add together, leading to a greater depolarization of the membrane. For instance, if a neuron sends multiple EPSPs in quick succession, the cumulative effect can push the membrane potential closer to the threshold.

On the other hand, spatial summation refers to the simultaneous activation of multiple synapses located in close proximity on the neuron's membrane. This can involve a combination of EPSPs and IPSPs from different neurons. The close proximity allows the effects of these potentials to blend together, resulting in a net change in membrane potential. Depending on the strength and nature of the incoming signals, the neuron may experience either depolarization or hyperpolarization.

Ultimately, whether a neuron reaches the threshold for an action potential depends on the balance of these summed potentials. If the cumulative effect of EPSPs and IPSPs results in sufficient depolarization at the initial segment, the neuron will fire, leading to the propagation of the action potential along the axon.

In this scenario, we analyze the synaptic activity of 15 neurons that connect to a single postsynaptic neuron. Among these, 13 neurons generate excitatory postsynaptic potentials (EPSPs) of 2 millivolts each, while 2 neurons produce inhibitory postsynaptic potentials (IPSPs) of 3 millivolts each. The resting potential of the postsynaptic neuron is -70 millivolts, and the threshold for triggering an action potential is -55 millivolts.

To determine whether an action potential will occur, we first calculate the total contribution of the EPSPs and IPSPs. The 13 EPSPs contribute:

13 \times 2 \text{ mV} = 26 \text{ mV}

Conversely, the 2 IPSPs contribute:

2 \times (-3 \text{ mV}) = -6 \text{ mV}

Next, we combine these values to find the net change in membrane potential:

26 \text{ mV} - 6 \text{ mV} = 20 \text{ mV}

Starting from the resting potential of -70 mV, we add the net change:

-70 \text{ mV} + 20 \text{ mV} = -50 \text{ mV}

Since -50 mV is more positive than the threshold of -55 mV, the postsynaptic neuron has indeed surpassed the threshold. Therefore, an action potential will be generated as a result of this synaptic activity.