Neuron Communication and Synapses

• Neurons communicate and adapt by sending and receiving messages, utilizing neurotransmitters, and adapting synapses.
• Neurons communicate through electrical and chemical signals.
• The communication occurs in two steps: electrical signals (action potential) traveling along the axon to the synapse, and chemical signals via neurotransmitters, which are released at the synapse to excite or inhibit the next neuron.
• A neurotransmitter is a chemical messenger that influences the postsynaptic neuron.
• Neurotransmitters function in three ways: carrying a message forward (anterograde transmission), sending messages backward (retrograde transmission) to regulate presynaptic release, and changing the structure of a synapse (synaptic plasticity).
• A chemical synapse involves neurons transferring messages using neurotransmitters.

Key Parts of a Synapse

• Presynaptic Terminal (Axon Terminal) contains synaptic vesicles filled with neurotransmitters.
• Microtubules transport substances within the neuron.
• Mitochondria provide energy for neurotransmitter release.
• Synaptic vesicles store neurotransmitters.
• The synaptic cleft (gap) is the space in which neurotransmitters travel.
• The postsynaptic membrane contains receptors that receive neurotransmitters.
• Postsynaptic receptors bind neurotransmitters and trigger a response.

Steps of Anterograde Synaptic Transmission

• Action potential reaches the axon terminal, opening voltage-sensitive calcium (Ca2+) channels.
• Calcium binds to calmodulin, forming a complex.
• Vesicles release neurotransmitters into the synapse (exocytosis).
• Neurotransmitters bind to receptors on the postsynaptic neuron.
• Neurotransmitters are removed or broken down (inactivation).
• Exocytosis is an active process where neurotransmitters are packaged into vesicles, moved to the cell membrane, and released.

Neurotransmitter Inactivation

• After neurotransmitters complete their job they must be removed from the synapse via:
• Diffusion, where neurotransmitters drift away.
• Degradation, where enzymes break them down.
• Reuptake, where transporters return neurotransmitters to the presynaptic neuron.
• Astrocyte uptake, where glial cells absorb excess neurotransmitters.
• Neurotransmitters are chemical messengers that can be excitatory (increase activity) or inhibitory (decrease activity).

Small-Molecule Neurotransmitters

• Acetylcholine (ACh) facilitates muscle movement, learning, and memory.
• Epinephrine (EP) and Norepinephrine (NE) contribute to alertness and stress response.
• Dopamine (DA) is involved in reward, pleasure, and movement.
• Serotonin (5-HT) affects mood, sleep, and appetite.
• GABA is the main inhibitory neurotransmitter and reduces brain activity.
• Glutamate is the main excitatory neurotransmitter and increases activity.

Neurotransmitter Versatility

• Neurotransmitters can be excitatory at one location and inhibitory at another.
• Two neurotransmitters can work together (synergistic effect) to strengthen signals.
• Neurotransmitters can act outside the brain as hormones, traveling through the bloodstream.
• Neurotransmitters bind to specific receptors to send signals.
• Two main categories of neurotransmitter receptors: ionotropic and metabotropic.

Ionotropic Receptors

• Have an ion channel where ions (Na+, K+, Cl-, Ca2+) can enter.
• They cause immediate effects by changing membrane voltage.
• An example is nicotinic ACh receptors in muscles.

Metabotropic Receptors

• They do not have an ion channel; instead, they activate a G-protein.
• They trigger slower, longer-lasting effects through second messengers.
• Examples include dopamine and serotonin receptors.