Neuroscience: Action Potentials and Receptors

Action Potentials

• Action potentials are nerve responses to stimuli.
• Sensory receptors detect environmental changes.
• These changes trigger a depolarization wave, called an action potential.
• This electrochemical message moves through nerve fibres.

Receptors

• Sensory neurons have receptors at their ends, often linked to collagen fibres and ion channels.
• Sensory receptors detect environmental changes.
• These changes trigger a depolarization wave (action potential).
• This is the electrochemical message transmitted through nerve fibres.

Receptors in Skin

• Free nerve endings detect pain and temperature.
• Merkel's discs detect touch.
• Meissner's corpuscles detect touch.
• Hair follicle receptors detect touch.
• Pacinian corpuscles (lamellar) detect vibration and pressure.
• Ruffini's endings detect stretch.

The Neuron

• Neurons transmit signals as electrochemical impulses in one direction.
• Depolarization waves travel along the axon's length.
• The wave is primarily the movement of Na+ and K+ ions across the axon's membrane.

The Neuron at Rest

• The outside of a neuron is more positive than the inside.
• Outside: higher Na+ and K+, Inside: lower Na+ and higher K+.
• A resting membrane potential of -70 mV exists.

All-or-None Response

• Data shows a correlation between the strength of stimuli (in mV) and the force of contraction (in N).
• The minimum level of stimulus (threshold level) must be reached for a neuron to fire an impulse.
• The strength of the response is uniform along the axon once the threshold is reached.
• A response is either complete or none.

The All-or-None Response

• Sensory neurons respond to chemical, light, heat, membrane disruption, or electrical stimuli.
• A stimulus must reach a threshold level for a neuron to fire.
• Response strength remains consistent regardless of stimulus above the threshold.
• The response is either total or absent; an all-or-nothing response.

How Can We Tell The Difference Between Warm and Hot?

• More intense stimuli (warmer vs. hotter) result in a varied frequency of impulses.
• Warmer stimuli result in a slower frequency of impulses.
• Hotter stimuli result in a higher frequency of impulses.
• Different neurons have different threshold levels; causing more neurons to fire with a greater intensity.

Depolarization

• When a nerve cell become excited, Na+ channels on the membrane open, causing Na+ to rush into the axon.
• K+ channels remain closed during this initial influx of Na+.
• The influx of Na+ causes a charge reversal resulting in the membrane potential reaching approximately +30 mV.
• Depolarisation in one part causes neighbouring Na+ channels to open.

Repolarization

• Na+ channels close after the split second of depolarization.
• K+ channels open, allowing K+ to move outside the axon.
• The membrane restores its resting potential (-70 mV).

Repolarization

• A sodium-potassium pump restores the resting membrane potential.
• The pump actively transports 3 Na+ ions out of the cell for every 2 K+ ions taken in.
• Nerves are unable to fire again until the membrane is repolarised.

Synaptic Transmission

• Tiny gaps between neurons are called synapses.
• Synapses span a distance of approximately 20 nm.
• Messages are carried from the pre-synaptic neuron across the synapse to post-synaptic neuron.

What are Neurotransmitters?

• Chemicals diffuse across the synaptic cleft and bind to receptors on the post-synaptic membrane.
• Neurotransmitters are contained within spherical vesicles.
• After binding, neurotransmitters can be degraded or reused.
• Approximately 100 different types, with many being amino acid derivatives or peptides.

What are Neurotransmitters? (Excitatory/Inhibitory)

• Neurotransmitters can either excite or inhibit post-synaptic neurons.
• Excitatory neurotransmitters open Na+ channels, causing depolarization.
• Inhibitory neurotransmitters open K+ channels, preventing depolarization.
• Some neurotransmitters can have both excitatory and inhibitory effects, depending on the receptor they bind to.

Excitatory and Inhibitory Neurotransmitters

• The interplay between excitatory and inhibitory neurotransmitters facilitates actions like throwing a ball.
• The triceps muscle receives excitatory signals to contract while the biceps muscle gets inhibitory signals to relax.

Acetylcholine

• Acetylcholine is an excitatory neurotransmitter.
• It is used in the neuromuscular junction to activate muscles.
• It is degraded by the enzyme acetylcholinesterase.
• Drugs interfering with cholinergic systems can cause convulsions and paralysis.

Making Connections (Homework)

• Research how pain relievers (like Advil) work.
• Research the interaction of psychoactive drugs (e.g., LSD, mescaline) with serotonin receptors.

Explore the fascinating world of action potentials and sensory receptors in this quiz. Understand how neurons transmit signals and how various receptors in the skin respond to different stimuli. Test your knowledge on these fundamental concepts of neuroscience.