Lecture 7-Neurotransmitters and Receptors

• Neurotransmitters are released from the presynaptic cell.
• Neurotransmitters bind to receptors on the postsynaptic cell membrane.
• Receptors have one or more binding sites for neurotransmitters.

Ionotropic Receptors

• Contain a neurotransmitter binding site and an ion channel within the same protein.
• This is a ligand-gated channel.
• Excitatory neurotransmitters open cation channels.
• Inhibitory neurotransmitters open anion (Cl-) channels.
• Ion channels are selective based on size and charge, allowing only certain ions to pass, like Na+ and K+.

Shape of AP

• The shape of an action potential depends on the extracellular concentration of Na+.

Metabotropic Receptors

• Have a neurotransmitter binding site but no ion channel directly connected.
• They connect indirectly to ion channels through G protein mechanisms, either activating ion channesl directly or employing a second messenger system.
• Inhibitory neurotransmitters using metabotropic receptors often open K+ or Cl- channels and have a signal amplification.
• G protein activation may influence a signal amplification.

Postsynaptic Effects

• The same neurotransmitter can have excitatory or inhibitory effects depending on receptor structure and cell type.
• A single signal molecule can induce different responses in different target cells.
• Acetylcholine (ACh) can be either excitatory or inhibitory.
• Acetylcholine binds to ionotropic receptors and opens cation channels, leading to EPSP.
• Acetylcholine binds to metabotropic receptors, opens K+ channels, leading to IPSP.

Removal of Neurotransmitters

• By enzyme degradation, like acetylcholinesterase breaking down acetylcholine.
• Through diffusion out of the synaptic cleft.
• By uptake into cells via neurotransmitter transporters.

Spatial and Temporal Summation

• An average neuron has 1,000 to 10,000 synapses creating numerous points of communication.
• Integration occurs through the summation of postsynaptic potentials.

Spatial Summation

• Involves the integration of signals from multiple neurons.
• The number of neurons and membrane coverage affect spatial summation.

Temporal Summation

• The integration of signals over time, with EPSPs lasting around 15 milliseconds.
• Postsynaptic cells receive many signals that can result in various outcomes.
• EPSP equals IPSP results in no net change with 1 Na+ and 1 Cl-.
• If excitatory effects are greater than inhibitory, EPSP occurs, but remains below threshold.
• If inhibitory effects are greater than excitatory, IPSP and hyperpolarization occur.
• A nerve impulse is generated if EPSP surpasses the threshold.

Stimulus Encoding

• Relies on the frequency of action potentials as all impulses are of the same strength and size.
• Frequency is high at the trigger zone.
• Frequency is initiated by both graded potentials, and also a stronger stimulus.
• The number of neurons responding contributes to the intensity of the signal.
• Stronger signals have more neurons stimulated.

Neural Pools

• These are large groups of neurons associated with a particular function, like heart or breathing rate.
• Discharge zone: neurons in this zone have more synapses and elicit spatial summation.
• Facilitated zone: neurons require input neuron assistance for AP to be generated ("has a vote").

Neural Circuits: Neural Pathways

• Diverging circuit: One neuron influences many downstream neurons, such as motor output.
• Converging circuit: Many neurons influence fewer neurons, integrating various signals.
• Reverberating circuit: The signal is sent to some earlier neurons through repetitive loop, and the circuit continues for 2 secs.
• Parallel after-discharge circuit: Signals from one neuron propagate through parallel series of other output neurons each with different numbers of neurons and signal propagation times, stops the feedback loop to create a longer signal

Neurotransmitters (NTs)

• Acetylcholine (ACh) functions at neuromuscular junctions.
• Amino acids, such as inhibitory GABA and excitatory Glutamate.
• Biogenic amines, which lack a carboxyl group, include emotional and mental health related substances, some of which are targeted by drugs.
• Purines, like Adenosine, are inhibitors in the brain, and are blocked by caffeine, using ATP.
• Gases, such as nitric oxide (NO) and carbon monoxide (CO).
• Neuropeptides, consisting of 2-40 amino acids such as substance P and endorphins, are used in pain pathways.

Neuromodulators

• Do not directly cause EPSPs or IPSPs, but hang around synapse.
• Affect the transmission strength presynaptically via synthesis, release, degradation, and reuptake or post synaptically by changing the of membrane sensitivity.

Drugs of Abuse - Effects

• Cocaine blocks dopamine reuptake proteins and causes overstimulation of dopamine effects.
• MDMA (ecstasy) targets serotonin-releasing neurons, may cause brain damage can cause problems sleeping and memory loss.

This lesson covers the basics of neurotransmitters and their receptors, including ionotropic and metabotropic receptors. It explains how neurotransmitters released from the presynaptic cell bind to receptors on the postsynaptic cell membrane. The lesson also details how ionotropic receptors contain a neurotransmitter binding site and an ion channel within the same protein.