Graded Membrane Potential Changes in Neurons Graded Membrane Potential Changes in Neurons Overview of Neurons

Neuron Structure
- Soma: The cell body of the neuron, often depicted in red.
- Axon: Conveys electrical impulses away from the cell body, depicted in green.
- Dendrites: Branching extensions of the neuron where synaptic inputs are received, depicted in blue.

Charge Separation
- Stable charge separation exists across the neuron's membrane.
- Cations: Positively charged ions located on the outside of the neuron membrane (e.g., Na⁺).
- Anions: Negatively charged ions located on the inside of the membrane (e.g., K⁺).
Resting Membrane Potential
- Reference point: The outside of the neuron is defined as 0 mV.
- Common value of resting membrane potential: Approximately $-60 ext{ mV}$ .
Graphical Representation
- X-axis: Time.
- Y-axis: Membrane potential in millivolts.
- Resting potential is indicated at $-60 ext{ mV}$ where it remains stable when no inputs are affecting the neuron.

Definition
- Graded potentials are transient changes in membrane potential that occur in response to inputs (stimuli).
Location of Occurrence
- Graded potentials typically occur in the dendrites and soma of the neuron.
Influences on Graded Potentials
- Size and duration of the graded potentials depend on the size and duration of the inputs, which can be either:
- Excitatory Inputs: Increase membrane polarity towards depolarization.
- Inhibitory Inputs: Decrease membrane polarity towards hyperpolarization.
Graded Potential Characteristics
- Graded potentials do not propagate into the axon of neurons.
- Action potentials, a different type of membrane potential change, are generated in the axon, particularly at the trigger zone (initial segment of the axon).

Threshold Potential
- Defined as the membrane potential that must be reached to trigger an action potential (~ $-50 ext{ mV}$ common).
Action Potential Generation
- When the trigger zone's membrane potential reaches or exceeds the threshold value due to the summation of graded potentials, an action potential is generated and propagates along the axon.

Summation
- The process by which multiple graded potentials combine to reach threshold potential.
Types of Summation
- Temporal Summation:
- Occurs when multiple graded potentials happen in succession (within a short time frame).
- Graded potentials can add together if they occur close enough in time, potentially doubling the depolarization effect.
- Spatial Summation:
- Occurs when multiple graded potentials activate simultaneously from different locations on the neuron.
- For instance, two inputs close to each other can lead to a significantly larger depolarization compared to potentials originating farther apart.

Post-Synaptic Potentials (PSPs)
- Graded potentials that occur as a result of neurotransmitter release at synapses.
- Excitatory Post-Synaptic Potential (EPSP):
- A depolarization resulting in the membrane potential moving closer to 0 (less negative).
- Inhibitory Post-Synaptic Potential (IPSP):
- A hyperpolarization resulting in the membrane potential moving further from 0 (more negative).
Receptor Potentials:
- Graded potentials generated by sensory stimuli (e.g., light, odorants).

Decay of Graded Potentials
- Graded potentials decay over time and distance.
- Effectiveness diminishes if the input occurs after the previous potential has decayed.
Location Dependence
- The closer the synaptic potential starts to the trigger zone, the greater its influence on whether an action potential occurs since it decays less over distance than if it started farther away (e.g., a dendrite located far from the trigger zone).
Small Size of Synaptic Potentials
- Typically, synaptic potentials are less than 1 mV.
- Multiple synaptic potentials are required (via temporal and spatial summation) to reach the threshold for firing an action potential, necessitating potentially a total change of approximately 10 mV from resting to threshold.

Graded potentials play a crucial role in the complex processing of neuronal information.
These potentials help neurons integrate multiple synaptic inputs from various sources within their network, influencing the likelihood of generating action potentials effectively.