Nerve Impulses: Graded Potentials & Action Potentials

Graded Potentials

Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none. They occur due to ions moving across the neuronal membrane and are essential for initiating action potentials.

• Definition: Localized changes in membrane potential that decrease in magnitude with distance from the stimulus.

• Key Properties: Can be depolarizing or hyperpolarizing, summate with other graded potentials, and are not propagated over long distances.

• Example: Postsynaptic potentials in dendrites and cell bodies.

Action Potentials

Action potentials are rapid, all-or-none electrical signals that travel along neurons, allowing for long-distance communication within the nervous system.

• Definition: A brief reversal of membrane potential, typically from -70 mV to +30 mV.

• Key Properties: Do not decrease with distance, have an all-or-none response, and are self-propagating.

• Equation:

• Example: Nerve impulse transmission along axons.

Action Potential Properties/Details

Initiation Site

The initiation site of an action potential is typically at the axon hillock, due to the high density of voltage-gated sodium channels.

All-or-None Principle

• Definition: Once threshold is reached, an action potential will always occur with the same magnitude.

• Example: Neuronal firing in response to sufficient stimulus.

Phases of Action Potential

• Depolarization: Rapid influx of Na+ ions.

• Repolarization: Efflux of K+ ions restores resting potential.

• Hyperpolarization: Membrane potential temporarily becomes more negative than resting potential.

Self-Propagation

• Action potentials propagate along the axon as local currents depolarize adjacent regions, opening more sodium channels.

Conduction Velocity

• Definition: Speed at which an action potential travels along an axon.

• Factors: Myelination and axon diameter increase conduction velocity.

• Example: Myelinated axons conduct impulses faster than unmyelinated axons.

Refractory Periods

• Absolute Refractory Period: No second action potential can occur, due to inactivation of Na+ channels.

• Relative Refractory Period: A second action potential can occur if the stimulus is stronger than normal, during the repolarization phase.

Synaptic Transmission

Synapse

A synapse is the junction between two neurons where information is transmitted from one cell to another.

• Most synapses: Axon to dendrite, but can also be axon to cell body or axon terminal.

Chemical Synapse Steps

• Neurotransmitter release from presynaptic neuron

• Neurotransmitter diffuses across synaptic cleft

• Neurotransmitter binds to receptors on postsynaptic membrane

• Postsynaptic potential generated

Neurotransmitter Removal

• Away from synaptic cleft by diffusion

• By enzymes (degradation)

• By astrocytes or axon terminal reuptake

Example Neurotransmitters and Their Functions

• Acetylcholine: Activates skeletal muscles; involved in memory; decreased in Alzheimer's disease.

• Norepinephrine: Stress/adrenaline hormone involved in "fight-or-flight" response.

• Dopamine: Brain's reward mechanism; controls motor movement; decreased in Parkinson's disease.

• Serotonin: Effects mood; decreased in depression.

• GABA: Main inhibitory neurotransmitter; mimicked by sedative drugs.

• Glutamate: Main excitatory neurotransmitter; functions in learning and memory.

• Endorphins: Natural opiates that inhibit pain signaling.

• Substance P: Mediates pain signaling.

Types of Chemical Synapse Potentials

Excitatory Post-Synaptic Potential (EPSP)

• Cause: Increases the chance to produce an action potential in the postsynaptic membrane.

Inhibitory Post-Synaptic Potential (IPSP)

• Cause: Decreases the chance to produce an action potential in the postsynaptic membrane.

Synaptic Integration

Multiple synaptic inputs are integrated at the postsynaptic neuron. Summation of EPSPs can lead to action potential firing if threshold is reached.

• Presynaptic inhibition: Another neuron inhibits release of neurotransmitter from axon terminal.

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries inferred based on standard physiology knowledge.