Neuroscience Action Potentials

Action Potentials

• Nerve response to stimuli
• Sensory neurons have receptors on their ends, often linked to collagen fibers and ion channels.
• These sensory receptors detect environmental changes, triggering a depolarization wave called an action potential.
• Action potentials are electrochemical messages transmitted through nerve fibers.
• Action potentials travel in one direction along the axon.
• The axon sends a wave of depolarization along its length.
• The wave of depolarization is primarily the movement of Na+ and K+ ions across the axon's membrane.

Receptors in Skin

• Free nerve endings (pain, temperature), Merkel's disc (touch), Meissner's corpuscle (touch), hair follicle receptor (touch), Pacinian (or lamellated) corpuscle (vibration and pressure), Ruffini's ending (stretch) are types of receptors.

Synaptic Transmission

• Tiny gaps between neurons are called synapses; they span 20 nm.
• Messages are carried from the presynaptic neuron to the postsynaptic neuron across the synapse.

Neurotransmitters

• Chemicals released by nerve endings that diffuse across synaptic clefts.
• Neurotransmitters bind to receptors on post-synaptic membranes.
• Neurotransmitters are contained in spherical, membrane-bound vesicles.
• Approximately 100 types; many are amino acid derivatives or peptides (e.g., glutamate, endorphins).
• Neurotransmitters can be excitatory or inhibitory.
• Excitatory neurotransmitters cause Na⁺ channels to open.
• Inhibitory neurotransmitters prevent action potentials by opening K⁺ channels.
• Some neurotransmitters can both excite or inhibit, depending on the receptor they bind to.

Acetylcholine

• An excitatory neurotransmitter used at neuromuscular junctions
• Motor neurons release acetylcholine to activate muscles.
• Degraded by acetylcholinesterase.
• Drugs that interfere with cholinergic systems can cause dangerous effects (e.g., convulsions, paralysis)

Depolarization

• When a nerve cell is excited (by a stimulus), Na⁺ channels open and Na⁺ rushes into the axon.
• K⁺ channels remain closed (during depolarization), resulting in charge reversal.
• Membrane potential becomes approximately +30 mV.
• This process is called depolarization.
• Depolarization of one part of the axon causes neighboring Na⁺ channels to open, continuing along the axon.

Repolarization

• Immediately after depolarization, Na⁺ channels close.
• K⁺ channels open, allowing K⁺ to move outside the axon.
• This process restores the membrane to its resting potential.
• A sodium-potassium pump actively transports 3Na⁺ out of the axon for every 2K⁺ in.
• The time between impulses when the membrane is repolarizing is the refractory period.

All-or-None Response

• Sensory neurons can be stimulated by chemicals, light, heat, membrane disruption or electricity.
• A minimum stimulus level (threshold level) must be reached before a neuron fires an impulse.
• Once the threshold level is reached, the strength of the response is uniform along the axon.
• This strong response is called the all-or-none response; there are no mild or strong responses.

How to distinguish warm from hot

• The more intense a stimulus, the greater the frequency of impulses.
• Warm stimuli have slower frequencies, while hot stimuli have higher frequencies.
• Each neuron has a threshold level, and more neurons fire when the stimulus is more intense, leading to a greater intensity of the response from the brain.

The Neuron at Rest

• The outside of the neuron is more positively charged than the inside.
• (Outside: ↑Na⁺, K⁺; Inside: ↓Na⁺, ↑K⁺).
• A resting membrane potential of -70 mV exists.
• The uneven concentrations of Na⁺ and K⁺ on either side of the neuron membrane maintain an external positive charge.

Homework Questions

• Read section 9.2 and answer questions #1-8 and #10 on page 426.

Other Topics

• Making connections: Research how pain relievers (like Advil) work.
• Action of psychoactive drugs, and how LSD and mescaline, both hallucinogenic drugs, interact with serotonin receptors.

Explore the mechanisms of action potentials and synaptic transmission in this quiz. Understand how sensory receptors detect stimuli and the role of ion movement in depolarization waves. Test your knowledge of nerve response and receptor types in the skin.