Comprehensive Study Notes on Synapses and Neurotransmission

General Definition and Characterization of Synapses

• Functional Unit: A synapse is the functional unit established between two excitable cells.

• Primary Purpose: Synapses are specialized for the transmission of information between cells.

• Classifications: There are two primary types of synapses identified based on their mechanism of transmission:

  • Electrical synapses.

  • Chemical synapses.

Electrical Synapses

• Mechanism of Transmission: Information is transmitted through electrical impulses.

• Directionality: These synapses characterized by two-way transmission, allowing signals to flow in both directions between cells.

• Structural Features:

  • Small Intercellular Distance: The gap between the membranes of the two cells is extremely narrow, measuring approximately $4\,nm$.

  • Low Membrane Resistance: The membranes have low electrical resistance, facilitating the easy flow of current.

  • Gap Junctions: Communication is mediated through gap junctions, which are composed of protein structures known as connexons (or connexin hemichannels).

• Functional Syncytium:

  • Definition: This is a functional unit of cells that are interconnected by electrical synapses, allowing them to react as a single, coordinated whole to stimuli.

  • Principle: They operate on the "all or none" principle, meaning if the stimulus reaches a certain threshold, the entire unit responds.

  • Biological Locations: Functional syncytia are found in the myocardium (heart muscle) and visceral smooth muscles.

Chemical Synapses

• Mechanism of Transmission: Information is transmitted through chemical substances known as mediators, neurotransmitters, or ligands.

• Directionality: Chemical synapses allow for one-way transmission only, from the presynaptic to the postsynaptic cell.

• Biological Locations: These are found in neuromuscular synapses (between nerve and muscle) and between nerve cells.

• Structural Sub-types Based on Junction Location:

  • Axo-dendritic: Between an axon and a dendrite.

  • Axo-somatic: Between an axon and the cell body (soma).

  • Axo-axonic: Between two axons.

Structural Components of a Chemical Synapse

• Presynaptic Membrane:

  • Belongs to the sending neuron.

  • Characterized as potential-dependent (voltage-dependent).

  • Contains synaptic vesicles filled with mediators (neurotransmitters).

• Synaptic Cleft:

  • The extracellular space between the presynaptic and postsynaptic membranes.

  • The width is approximately $20\,nm$.

• Postsynaptic Membrane (PSM):

  • Belongs to the receiving neuron or cell.

  • Characterized as ligand-dependent.

  • Contains membrane receptors, which are protein molecules on the cell surface possessing a high affinity for specific ligands (neurotransmitters).

Physiological Processes in a Chemical Synapse

1. Presynaptic Events: Depolarization of the presynaptic membrane occurs, leading to a calcium ($Ca^{2+}$) influx. This influx provokes the exocytosis of the neurotransmitter from the vesicles into the synaptic cleft.

2. Diffusion: The neurotransmitter travels across the synaptic cleft to reach the postsynaptic membrane (PSM).

3. Binding: The neurotransmitter binds specifically to the membrane receptors located on the PSM.

4. Generation of Postsynaptic Potential (PSP): The receptor-ligand binding provokes the opening or closing of ion channels. This changes the membrane permeability and generates a postsynaptic potential (PSP).

5. Termination: The neurotransmitter is eventually inactivated (via degradation or reuptake), causing the ion channels to return to their previous state (closing or opening again).

Postsynaptic Potentials (PSP)

• Excitatory Post-Synaptic Potential (EPSP): A depolarization that brings the membrane potential closer to the firing level (threshold).

• Inhibitory Post-Synaptic Potential (IPSP): A hyperpolarization or stabilization that moves the membrane potential away from the firing level, inhibiting the generation of an action potential.

Mediators and Neurotransmitters

• Synaptic Specificity: In any given synapse, only one type of mediator is utilized. Consequently, a specific synapse can be classified as strictly inhibitory or strictly excitatory.

• Classification by Physiological Effect:

  • Excitatory Effect: Glutamate is a primary excitatory neurotransmitter.

  • Inhibitory Effect: Glycine and $\gamma$-amino butyric acid (GABA) are primary inhibitory neurotransmitters.

  • Dual Effect: Some mediators, such as catecholamines, can have either excitatory or inhibitory effects depending entirely on the type of membrane receptor they bind to.

Detailed Classification of Mediators

Low-Molecular Weight Mediators

• 1. Acetylcholine: Operates via two types of choline receptors:

  • N (nicotinic) Choline Receptors: These are the receptors found in the neuromuscular synapse; they can be blocked by the drug curare.

  • M (muscarinic) Choline Receptors: These can be blocked by the drug atropine.

• 2. Biogenic Amines:

  • Adrenalin (Epinephrine): Acts on $\alpha1, \alpha2, \beta1$, and $\beta2$ receptors.

  • Norepinephrine: Acts on $\alpha1, \alpha2$, and $\beta1$ receptors.

  • Dopamine.

  • Serotonin.

• 3. Amino Acids:

  • Glutamate: Responsible for producing EPSP.

  • y-amino butyric acid (GABA): Responsible for producing IPSP.

  • Glycine: Responsible for producing IPSP; its action can be blocked by the toxin strychnine.

Neuropeptides

• Endogenous Opiates: Includes endorphins and enkephalines.

• Receptors: These peptides act upon specific opioid receptors.

Other Mediators

• Nitric Oxide (NO): A gaseous signaling molecule.

• Histamine.

• Adenosine Triphosphate (ATP).

The transmission of excitation (or inhibition) from one excitable cell to another primarily occurs through synapses, which are the functional units between these cells. There are two main types of synapses: electrical and chemical.

Electrical Synapses allow direct communication between cells via electrical impulses, facilitating two-way transmission. They are characterized by exceptionally small intercellular distances of approximately $4 ext{ nm}$ and low membrane resistance, permitting a seamless flow of current. The communication occurs through gap junctions, which consist of connexons that form a functional syncytium. This interconnected unit reacts to stimuli as one, adhering to the "all or none" principle, typical in the myocardium and smooth muscle tissues.

Chemical Synapses, on the other hand, utilize chemical substances known as mediators or neurotransmitters for information transfer. Transmission occurs in a unidirectional manner, from the presynaptic neuron to the postsynaptic neuron. Chemical synapses are further categorized based on junction locations into axo-dendritic, axo-somatic, and axo-axonic connections. The presynaptic membrane contains synaptic vesicles filled with neurotransmitters, while the postsynaptic membrane is lined with receptors specific to these mediators.

The physiological processes in a chemical synapse can be broken down into sequential events. First, there is a depolarization of the presynaptic membrane, allowing $Ca^{2+}$ influx, which triggers neurotransmitter exocytosis into the synaptic cleft. The released neurotransmitter then diffuses across the cleft to bind with receptors on the postsynaptic membrane, provoking changes in ion channel permeability. This process results in postsynaptic potentials (PSP). There are two types of PSP:

1. Excitatory Post-Synaptic Potential (EPSP) which brings the membrane potential closer to the firing threshold, and

2. Inhibitory Post-Synaptic Potential (IPSP) which moves the potential away from firing levels and inhibits action potential generation.

The effectiveness of neural transmission can also be influenced by the summation of postsynaptic potentials. Individual EPSPs can sum together (temporal or spatial summation) to reach the threshold for action potential generation or be overridden by inhibitory potentials (IPSPs), leading to a more complex interaction of excitatory and inhibitory signals.

In conclusion, synapses play a crucial role in neuronal communication, facilitating the transmission of excitation and inhibition through intricate mechanisms involving electrical impulses in electrical synapses and chemical signals in chemical synapses. The modulation of postsynaptic potentials, whether excitatory or inhibitory, fundamentally shapes the neural pathways governing diverse physiological functions.