Synaptic Transmission 1.4

Architectural Components of a Neuron: * Dendrites: These are the primary receiving regions of the neuron where stimuli are typically first detected. * Cell Body (Soma): The central part of the neuron containing the nucleus; it integrates incoming signals. * Axon: The long projection from the cell body that carries electrical impulses (action potentials) away from the cell body toward target cells. * Axolemma: The specialized plasma membrane that surrounds the axon. * Collateral Branch: A side branch of an axon that allows a single neuron to communicate with multiple target cells. * Telodendria: Fine terminal extensions at the end of the axon and its collaterals. * Synaptic Terminals: The specialized endings of telodendria where the neuron communicates with another cell.
The Synaptic Relationship: * Presynaptic Cell: The neuron that sends the signal; the message originates here and travels down the axon to the terminals. * Postsynaptic Cell: The target cell that receives the signal; this can be another neuron or a specialized effector cell.

Neuron-to-Neuron Synapses: * A neuron can form a synapse with another neuron to continue the transmission of a signal through the nervous system.
Neuromuscular Junctions: * The specific synapse formed between a neuron (motor neuron) and a skeletal muscle fiber. * Functional outcome: Coordination of muscle contraction.
Neuroglandular Synapses (Neuroglandular Junctions): * The synapse between a neuron and a gland cell or endocrine cell. * Functional outcome: Coordination of glandular secretions.

General Principle: Neurons must communicate with various target cells to coordinate bodily functions. This communication requires the transfer of signals across a synapse (the gap between cells).
Electrical Synapse: * The electrical signal (ionic flow) is passed directly between cells via specialized protein channels called gap junctions. * This allows for rapid, direct communication without the need for chemical intermediates.
Chemical Synapse: * The neuron converts an electrical signal (action potential) into a chemical signal (neurotransmitter). * Vast Majority: Chemical synapses represent the vast majority of communication within the human nervous system. * They are necessary because action potentials cannot "jump" physically across the gap from one cell to another; they require chemical messengers to bridge the divide.

Neurotransmitter Release Stimulus: Release is triggered by the arrival of an action potential at the axon terminal.
Step-by-Step Sequence of Events: 1. Arrival of Action Potential: The action potential reaches the presynaptic axon terminal. 2. Calcium Influx: The change in membrane potential causes voltage-gated $Ca^{2+}$ channels to open. $Ca^{2+}$ ions flow INTO the presynaptic neuron from the extracellular fluid. 3. Vesicle Docking and Exocytosis: The presence of $Ca^{2+}$ acts as a signal for synaptic vesicles containing neurotransmitters to move to and dock at the presynaptic membrane. The vesicles then release their chemical contents into the synaptic cleft via the process of exocytosis. 4. Diffusion and Binding: The neurotransmitter molecules diffuse across the synapse and bind to specific receptors located on the postsynaptic neuron membrane. This binding causes ligand-gated channels to either open or close. 5. Generation of Response: The change in channel state generates a response in the postsynaptic cell, known as a post-synaptic potential.

Definition and Basic Properties: * Neurotransmitters are chemical messengers that enable neuronal communication. * They are synthesized and stored specifically within the presynaptic neuron. * They are released only in response to stimulation (the action potential and subsequent calcium influx).
Diversity and Specificity: * There are more than $60$ different neurotransmitters identified in the human body. * Different neurons within the brain are specialized to release different types of neurotransmitters.
Classification Schemes: * Structural Classification: * Amino Acids. * Amines. * Neuropeptides. * Other categories. * Functional Classification: * Excitatory. * Inhibitory. * Some can perform both roles depending on the context and target.

Dopamine: Involved in the regulation of movement, cognition, and the reward system.
Serotonin: Primarily regulates mood and sleep cycles.
Glutamate: Essential for memory, learning, and general cognitive function.
GABA ( $\gamma$ -Aminobutyric acid): Crucial for brain function and sleep; acts as a major inhibitory signal.
Epinephrine / Norepinephrine: Responsible for the "Fight or Flight" response.
Acetylcholine (ACh): Involved in various functions throughout the nervous system and at the neuromuscular junction.

Nature of the Response: * When neurotransmitters bind to receptors, they open or close channels, resulting in a graded potential. * A graded potential is defined as a local change in the membrane potential of the postsynaptic cell. * Important Note: Unlike the action potential, the post-synaptic response is NOT an "all or nothing" event. It is variable and depends on the number, timing, and nature of the signals received.
Factors Determining Action Potential Generation: 1. Type/Nature of the Signal: Whether the input is excitatory or inhibitory. 2. Strength of the Signal: Determined by the number of signals and their timing.
Excitatory Postsynaptic Potential (EPSP): * Results in a depolarizing effect (making the membrane potential more positive). * Occurs when ligand-gated $Na^+$ and/or $Ca^{2+}$ channels open, allowing positive ions to move into the cell. * Example: Glutamate binding often leads to $Na^+$ influx and depolarization (shifting the potential toward $-30\,mV$ or higher).
Inhibitory Postsynaptic Potential (IPSP): * Results in a hyperpolarizing effect (making the membrane potential more negative). * Occurs when negative ions move in, typically through the opening of ligand-gated $Cl^-$ channels. * Example: GABA binding often leads to $Cl^-$ influx and hyperpolarization (shifting the potential further below the resting state, e.g., toward $-80\,mV$ ).

Small Local Changes: All graded potentials cause small, local changes in the membrane (either depolarization or hyperpolarization).
Reaching Threshold: A neuron will only fire an action potential if these local changes add together (summate) to reach the required threshold.
Types of Summation: * Spatial Summation: The integration and addition of signals coming from multiple different neurons arriving at the postsynaptic cell simultaneously. * Temporal Summation: The integration of consecutive signals coming from a single presynaptic neuron in quick succession.
The Final Decision (Net Outcome): * Net Inhibition: If inhibitory signals outweigh excitatory signals, the cell remains below threshold. * Net No Change: If excitatory and inhibitory signals cancel each other out. * Net Excitation: If excitatory signals dominate and reach the threshold, an Action Potential (AP) is successfully generated.