Neuroscience: Transmission Across a Synapse

Transmission Across a Synapse

• Neurons are polarized, having a resting potential (-70mV), maintained by the Na-K exchange pump. This pump actively transports sodium ions out and potassium ions in.
• Action potentials occur when a stimulus reaches a neuron, triggering a depolarization. sodium (Na+) channels open, and Na+ ions rush into the neuron if the threshold is reached. This is an "all-or-none" response.
• The impulse travels down the axon, and myelinated axons transmit the impulse more quickly, jumping between nodes of Ranvier.
• Depolarization causes Na+ channels to close and K+ channels to open, initiating repolarization. Potassium (K+) ions move out of the neuron, restoring the resting potential.
• Hyperpolarization follows repolarization, the potential drops below resting potential briefly, as more K+ ions move out.
• The refractory period is the recovery time for a neuron to return to its resting potential before another stimulus can trigger an action potential.
• The Na-K exchange pump restores the neuron to its original resting potential.
• Nerve impulses transmit via a domino effect, with each neuron passing the signal to the next. This involves chemical events where the signal is picked up by the dendrites and transmitted through the axon to the next neuron.

Synapse

• Synapses (or synaptic clefts) are gaps between neurons or a neuron and another effector (like a muscle cell or gland).
• Impulses travel from one end of a neuron to the opposite end (synaptic terminal).
• Most synaptic terminals are not directly connected, but a gap (synaptic cleft) separates them.
• Neurotransmitters bridge this gap, carrying signals across to the next neuron or effector cell.

Neurotransmitters

• Neurotransmitters have an excitatory or inhibitory effect on the next neuron.
• Excitatory neurotransmitters stimulate the brain, triggering receptor proteins to open sodium channels, allowing sodium to flow into the postsynaptic neuron, slightly depolarizing it. If depolarization reaches the threshold (-70mV), then an action potential is produced.
• Inhibitory neurotransmitters trigger the flow of potassium out of the postsynaptic neuron, increasing its polarization (hyperpolarization).
• A neuron can receive multiple signals simultaneously (both excitatory and inhibitory).
• After producing their effects, neurotransmitters are broken down by enzymes in the cell or reabsorbed.

How Neurotransmitters function

• Neurotransmitters are released from the synaptic vesicles after an impulse reaches the axon terminal.
• Neurotransmitters bind to receptor proteins on the postsynaptic membrane, eliciting either an excitation or inhibition of the membrane.
• After their effect, enzymes break down the neurotransmitter, removing it from the receptors and the synaptic cleft.
• The ion channels then close returning the neuron to the resting potential.

Other Key Neurotransmitters

• Acetylcholine is an excitatory neurotransmitter critical for neuromuscular junctions. It crosses the synapse, exciting the muscle cell membrane which causes depolarization. For a muscle to relax, acetylcholinesterase breaks down acetylcholine. If acetylcholine remains in the synapse the muscle cell would not repolarize. The ion channel would remain open, maintaining muscle contraction.
• Norepinephrine: An excitatory hormone used by the brain and some autonomic neurons, working in conjunction with epinephrine to ready the body for stressful situations. Overproduction can lead to elevated blood pressure, anxiety and insomnia.
• Dopamine: Affects brain synapses for controlling body movements. It is linked to sensations of pleasure. Excessive production can lead to schizophrenia, and insufficient levels can cause Parkinson's disease.
• Serotonin: Regulates temperature and sensory perception and helps control mood. Inadequate amounts of serotonin are linked to depression.
• Endorphins: Natural pain relievers that affect areas of the brain related to emotions. A deficit can increase the risk of alcoholism.
• GABA (Gamma-aminobutyric acid): The most common inhibitory neurotransmitter in the central nervous system. It lessens the neuron's ability to send signals to other nerve cells and produces a calming effect.

How Drugs Affect the Brain

• Drugs can interact with neurotransmitters by mimicking or blocking their actions, or affecting the way they are broken down or reabsorbed. These effects greatly impacts the brain and can have various consequences on the body.

Specific Example: Congenital Myasthenia Gravis

• Congenital Myasthenia Gravis (MG) is a genetic disorder that causes a reduced number of acetylcholine receptors on muscle cells.
• Treating congenital MG involves inhibiting the action of acetylcholinesterase– preventing the breakdown of acetylcholine. This allows acetylcholine to remain active for longer, enhancing transmission at the neuromuscular junction.

Additional Points

• Summation is when multiple impulses add up to reach the threshold, triggering an action potential in a neuron.

Explore the intricacies of neuronal communication in this quiz about synaptic transmission. Delve into the processes of depolarization, repolarization, and the role of ion channels in action potentials. Test your knowledge of how neurons maintain their resting potential and the importance of the refractory period.