Ch 12 pt 2 – Membrane Potentials, Action Potentials & Synapses

Action Potential (AP)

Brief reversal of membrane potential used to transmit nerve impulses along neurons.

Phases of action potential

1. Resting state

2. Depolarization

3. Repolarization

4. Hyperpolarization

Resting phase of action potential

Depolarization phase of action potential

Repolarization phase of action potential

Hyperpolarization of action potential

Role of sodium-potassium pump in action potential

Treshold

propagation of action potential

All-or-None Principle

Action potentials either occur fully or not at all once threshold is reached.

Absolute refractory period

Relative refractory period

Saltatory conduction

Graded potential

How do you get “pros” and “cons”

How do you get various strength input?

Summation

EPSP

IPSP

Temporal summation

Spatial summation

Synaptic transmission

Presynaptic neuron

Postsynaptic neuron

Electrical synapses

Chemical synapses

Axonal terminal

Receptor region

Synaptic cleft

Reuptake of neurotransmitters

Termination of neurotransmitter effects

Neurotransmitter direct/ion channels

Neurotransmitters indirect/G proteins

Threshold Potential

The minimum depolarization level (≈ -55 mV) needed to trigger an action potential.

Resting State of an Axon

Na⁺ and K⁺ channels closed; maintains -70 mV potential.

Depolarization Phase

Na⁺ channels open; sodium enters the cell, making the interior positive.

Repolarization Phase

Na⁺ channels close; K⁺ channels open, allowing potassium to exit and restore negativity.

Hyperpolarization Phase

K⁺ channels remain open too long; inside becomes more negative than resting level.

Role of Sodium-Potassium Pump

Restores ionic balance after action potential by actively moving Na⁺ out and K⁺ in.

Propagation of Action Potential

Movement of the AP along the axon as local depolarization triggers adjacent regions.

Absolute Refractory Period

Time when a neuron cannot fire another AP (Na⁺ channels open/inactivated).

Relative Refractory Period

Period when a stronger-than-normal stimulus is needed to initiate an AP.

Conduction Velocity

Speed of impulse transmission; faster with larger axon diameter and myelin sheath.

Saltatory Conduction

AP jumps from node to node in myelinated axons; increases speed dramatically.

Continuous Conduction

Slower AP propagation along unmyelinated axons.

Graded Potentials

Small, local changes in membrane potential that decrease in strength over distance.

Characteristics of Graded Potentials

Short-lived, variable in size, and may depolarize or hyperpolarize the membrane.

Summation (Definition)

The additive effect of multiple graded potentials to reach threshold.

Temporal Summation

One presynaptic neuron releases neurotransmitters in rapid succession.

Spatial Summation

Multiple presynaptic neurons release neurotransmitters simultaneously.

Excitatory Postsynaptic Potential (EPSP)

Depolarization of the postsynaptic membrane; increases likelihood of firing.

Inhibitory Postsynaptic Potential (IPSP)

Hyperpolarization of the postsynaptic membrane; decreases likelihood of firing.

Synapse (Definition)

Junction that allows communication between neurons or between neuron and effector.

Presynaptic Neuron Function

Sends impulses and releases neurotransmitters into the synaptic cleft.

Postsynaptic Neuron Function

Receives neurotransmitters and generates graded potentials.

Electrical Synapse

Direct connection via gap junctions; allows rapid ion flow between cells.

Chemical Synapse

Uses neurotransmitters to transmit signals across a synaptic cleft.

Steps of Chemical Synaptic Transmission (1/2)

1️⃣ AP arrives at axon terminal. 2️⃣ Voltage-gated Ca²⁺ channels open and Ca²⁺ enters.

Steps of Chemical Synaptic Transmission (2/2)

3️⃣ Ca²⁺ causes vesicles to release neurotransmitter. 4️⃣ Neurotransmitter binds to postsynaptic receptors and triggers a response.

Neurotransmitter Removal Methods

Degraded by enzymes, reabsorbed by presynaptic cell or astrocytes, or diffuse away.

Reuptake (Definition)

Process of neurotransmitters being reabsorbed by the presynaptic terminal.

Enzymatic Degradation

Neurotransmitter broken down by enzymes (e.g., acetylcholinesterase for ACh).

Neurotransmitter Diffusion

Neurotransmitters drift away from the synaptic cleft into extracellular fluid.

Excitatory Neurotransmitters

Cause depolarization (e.g., glutamate).

Inhibitory Neurotransmitters

Cause hyperpolarization (e.g., GABA, glycine).

Neurotransmitters with Dual Roles

Act as excitatory or inhibitory depending on receptor (e.g., acetylcholine).

Neuronal Pool

Functional group of neurons that process and integrate incoming information.

Diverging Circuit

One neuron stimulates many others; amplifies the signal (e.g., motor pathways).

Converging Circuit

Many inputs funnel into one neuron; allows integration of different signals.