Graded: Local: Receptor Potentials
Graded Potentials
Definition & Terminology
Graded potentials, also known as local potentials or receptor potentials, are changes in membrane potential that vary in magnitude based on the strength of the stimulus.
Characteristics of Graded Potentials
Graded:
A stronger stimulus results in a greater change in membrane potential (either depolarization or hyperpolarization) compared to a resting membrane potential.
The change in potential can be visualized with a graph where:
Y-axis shows membrane potential in millivolts (mV).
X-axis shows time in milliseconds.
Common resting membrane potentials for neurons: around mV, varying based on cell type: hence range could be from mV (e.g., skeletal muscle) to mV (important for action potentials).
Local:
Graded potentials are localized changes occurring near the point of stimulation, primarily in the dendrites of neurons. They do not propagate over long distances.
Decay with Distance:
These potentials diminish in amplitude as they travel away from the site of origin due to charge dissipation. This is one of the key characteristics that define graded potentials.
Mechanism of Action
Stimulus Application:
When a stimulus (such as a mechanical stimulus, chemical reaction, or light energy) affects a neuron:
Weak stimulation leads to a minimal depolarization, akin to a ripple from a small pebble thrown into a pond.
Stronger stimulation results in larger depolarizations, like a larger stone creating a wider ripple effect.
Ion Movement:
Involves the opening of ion channels (especially sodium channels), leading to an influx of positive ions ( ext{Na}^+) following the concentration gradient.
Depolarization occurs when sodium enters the cell, making the inside of the neuron less negative.
Hyperpolarization may occur if negative ions (like chloride, ext{Cl}^-) enter the cell or if positive ions leave the cell (like potassium, ext{K}^+).
Function and Importance of Graded Potentials
Role in Neuronal Communication
Graded potentials integrate signals received by the neuron, especially at its axon hillock, the decision-point for initiating action potentials.
They summate, or add together, influencing whether the axon hillock reaches threshold potential of approximately mV, triggering an action potential if reached.
Receptor Potentials
Graded potentials often occur in specialized cells called receptors which can detect specific forms of energy (e.g., light, temperature).
Types of Receptors and Their Response:
Chemically gated channels (activated by neurotransmitters).
Mechanically gated (activated by physical forces).
Thermoreceptors (activated by temperature changes).
Photoreceptors (activated by light photons).
Receptor Potentials and Specific Examples
Examples of Receptors
Nicotinic Receptor: A type of chemically gated receptor where the binding of acetylcholine (ACh) allows ext{Na}^+ channels to open:
When ACh binds, sodium ions flow into the cell, leading to depolarization (further action can include neurotransmitter release).
GABA Receptors: Can lead to inhibitory postsynaptic potentials (IPSPs) where either potassium ($ ext{K}^+$) leaves or chloride ($ ext{Cl}^-$) enters the cell resulting in hyperpolarization, moving the membrane potential further away from threshold.
Mechanics of Sensation
Example of Light Energy:
Light striking photoreceptor cells alters retinal configuration, leading to changes in their permeability, affecting what signals are sent to the brain.
Example of Mechanical Energy:
Pressure applied to skin off thresholds channels (e.g., in cochlear hair cells) causing sensory transduction related to hearing and touch.
High-frequency sound waves impact mechanical receptors to open ion channels and create receptor potentials, also leading to varying potential intensities.
Summary
Key Features of Graded Potentials
Graded: Represent changes relative to stimulus strength; the magnitudes vary.
Local: Function over short distances, mainly affecting the neuron where they originated.
Decay: The strength of graded potentials diminishes with distance traveled from the initial site of stimulation.
Integration: Several graded potentials can sum at the axon hillock, determining whether an action potential is triggered or inhibited, thus regulating neuronal communication.