Synaptic Plasticity

Synaptic plasticity serves integral functions in learning and memory.
- Emphasizes the importance of engaging in activities that improve synaptic plasticity when preparing for exams.

Neurons can produce multiple types of neurotransmitters and synaptic effects, referred to as co-transmitters.
- Example: Combination of small molecule neurotransmitters and neuropeptides can exist in a single neuron.
- A single neurotransmitter can produce various postsynaptic effects.

In the central nervous system (CNS), amino acids serve as major neurotransmitters:
- Excitatory Postsynaptic Potentials (EPSPs): Primarily produced by glutamate.
- Inhibitory Postsynaptic Potentials (IPSPs): Primarily produced by GABA and glycine.
- Volume transmission from neurotransmitters like acetylcholine, norepinephrine, dopamine, and serotonin, mainly occurring in lower concentrations in the brain.
- Different effects depending on the type of receptors in various tissues.

Ionotropic vs Metabotropic:
- Ionotropic receptors, specifically nicotinic receptors, cause direct ion channel gating.
- Example: Ligand-gated channels which open upon binding to ACh at neuromuscular junctions, resulting in muscle contraction.
- Metabotropic receptors (e.g., muscarinic receptors) are G-protein coupled and result in slower, modulatory responses, often involving second messengers like cAMP.

Nicotinic Receptors: Agonist effects by nicotine; antagonist effects by curare, leading to paralysis.
- In skeletal muscle, neglecting the contraction process leads to paralysis due to lack of stimulation.
Muscarinic Receptors: G-protein coupled receptors resulting in inhibition in cardiac muscles; effects of norepinephrine act to regulate heart rate effectively.

Neurotransmitter activation is transient (milliseconds); removal mechanisms include:
- Enzymatic digestion (e.g., ACh degradation by acetylcholinesterase).
- Reuptake mechanisms, used for norepinephrine without degradation.
- Synaptic activity regulation through different types of peptidases for neuropeptides.

Small molecule neurotransmitters synthesized at axon terminals.
- Release upon low-frequency stimulation through specific enzymes.
Neuropeptides synthesized in the nucleus, transported down axons, released under conditions requiring calcium presence.

Ionotropic receptors:
- Have five transmembrane segments; undergo conformational changes upon ligand binding.
Metabotropic receptors:
- Have seven transmembrane domains, activate secondary messenger pathways through G-proteins.

Synaptic plasticity mechanisms, associated with learning:
- Post-tetanic Potentiation: Increases in synapse strength with repeated stimulation.
- Found prominently in the hippocampus, tied to spatial learning.
- Repeated activity strengthens synaptic connections, promoting memory retention.

NMDA Receptors: Voltage-dependent, allowing calcium entry when magnesium blockage is removed.
- Enhanced calcium signaling activates kinases and transcription factors, increasing the number of dopamine receptors.

Calcium acts as a second messenger facilitating various intracellular processes such as phosphorylation of proteins and gene transcription.

Morris water maze experiments illustrate memory and learning impacts in knockout models lacking NMDA receptors, highlighting importance for synaptic plasticity.
- Mice lacking these receptors struggle to find the escape platform compared to normal mice.
- Experiments suggest a balance between learning improvements and potential negative outcomes (e.g., excitotoxicity or cancer) due to excessive receptor activity.

Synaptic plasticity is essential in learning, involving complex dynamics between neurotransmitter types, receptor actions, calcium signaling, and synaptic strength. Understanding these processes assists in elucidating foundational aspects of memory formation and retention.