Neuroscience: Synapses and Neurotransmission

Nervous System Physiology Essentials 3

• This chapter covers the fundamentals of synapses, specifically neuromuscular junctions, and different synapse types, including electrical and chemical synapses, along with various neurotransmitters.

Synapse

• A synapse is the connection between two neurons, a neuron and a muscle cell, or a neuron and a gland cell.
• In the CNS, both cells involved in a synapse are neurons.
• In the PNS, either a neuron, muscle, or gland cell can be the second cell.
• The synapse between a neuron and a muscle cell is called a neuromuscular junction (NMJ).

Presynaptic and Postsynaptic Cells

• The cell before the synapse is the presynaptic cell.
• The cell behind the synapse is the postsynaptic cell.

Muscle Neuromuscular Junction

• The presynaptic cell is the neuron.
• The postsynaptic cell is the muscle cell.

Synapse Types (Structural)

• Axodendritic: Neuron to dendrite
• Axosomatic: Neuron to cell body
• Axoaxonic: Neuron to axon

Synapse Types (Functional)

• Excitatory: Stimulate the postsynaptic cell.
• Inhibitory: Suppress the postsynaptic cell.

Electrical Synapse

• Direct connection between neurons.
• Ions flow freely through gap junctions between cells.
• Transmission is very fast and bidirectional.
• No synaptic delay.
• Found in smooth muscle and heart muscle.
• Gap junctions are relatively close (2-4 nm).

Chemical Synapse

• Indirect connection between neurons.
• Neurotransmitters are released from the presynaptic neuron to transmit signals across a synaptic cleft.
• Transmission is unidirectional (one way).
• Synaptic delay exists (time it takes for neurotransmitter release and receptor binding).
• Found in most synapses in the nervous system.
• Synaptic cleft is wider, 20-40 nm apart.

Neurotransmitters

• Chemical messengers that transmit signals between neurons.
• Stored in synaptic vesicles in the presynaptic neuron.
• Released via exocytosis when an action potential reaches the axon terminal of the presynaptic neuron.
• Bind to receptors on the postsynaptic cell, initiating a response.
• Examples: Glutamate, GABA, Acetylcholine.

Neurotransmitters and Their Functions in CNS

• Glutamate: Excitatory, crucial for learning and memory.
• GABA: Inhibitory, plays a role in reducing anxiety.
• Acetylcholine: Excitatory, important for muscle contractions, learning, and memory.

Conditions Affecting the Neuromuscular Junction

• Lambert-Eaton Syndrome: Autoimmune disease affecting voltage-gated Ca2+ channels, reducing acetylcholine release.
• Botulism: Blocks the release of acetylcholine, leading to muscle paralysis.
• Tetanus: Blocks the release of inhibitory neurotransmitters causing overactivity of motor neurons, leading to muscle contraction.
• Myasthenia Gravis: Autoantibodies attack acetylcholine receptors at the neuromuscular junction decreasing its function.
• Curare: Competes with acetylcholine for receptor sites, blocking muscle stimulation.

Botulinum toxin (Botox)

• Released by Clostridium botulinum.
• Blocks the release of acetylcholine.
• Used to treat wrinkles, muscle spasms, and other conditions.

Common Ways to Get Botulism Toxin Infection

• Poorly preserved/sterilized/sealed homemade canned foods.
• Commercial foods.
• Honey (for infants as they lack bacteria in stomach for proper digestion)

Tetanus

• Caused by Clostridium tetani.
• Destroys synaptobrevin in inhibitory interneurons.
• Blocks release of inhibitory neurotransmitters.
• Leads to muscle contraction and spastic paralysis.

Lambert-Eaton Syndrome (LES)

• Autoimmune disease attacking voltage-gated Ca2+ channels in the presynaptic terminal.
• Less ACh release by presynaptic neuron.
• Leads to muscle weakness.

Myasthenia Gravis (MG)

• Autoimmune disease attacking ACh receptors.
• Less ACh reception.
• Causes muscle weakness.

Curare

• Plant-derived compound that blocks acetylcholine receptors.
• Causes muscle weakness/paralysis.

Postsynaptic Potentials

• EPSP (Excitatory Postsynaptic Potential): Makes it more likely that the postsynaptic neuron will fire. Increases membrane potential.
• IPSP (Inhibitory Postsynaptic Potential): Makes it less likely that the postsynaptic neuron will fire. Decreases membrane potential.

Summation

• Temporal Summation: Successive rapid stimuli from one presynaptic neuron.
• Spatial Summation: Simultaneous stimulation from multiple presynaptic neurons.

Convergence and Divergence

• Convergence: Multiple presynaptic neurons converging onto a single postsynaptic neuron (information from multiple sources collected in one place).
• Divergence: Information from one neuron diverging to multiple postsynaptic neurons (information from one source going to several).
• Facilitation: Temporary increase in the neuron's excitability from repeated stimulation/activation.

###V-SNARE and T-SNARE

• Proteins that are essential for neurotransmitter release from synaptic vesicles into the synaptic cleft. Essential for V-SNARE proteins and T-SNARE proteins to properly fuse.