Synaptic Transmission Quiz

Synaptic Transmission

• Synapses are junction points between neurons where impulses are transmitted.
• Information travels in the central nervous system as nerve action potentials (impulses) through a series of neurons, one after another.
• Synaptic transmission is crucial for communication between neurons.

Intended Learning Outcomes

• Recognize the mode of synaptic transmission.
• Differentiate different types of postsynaptic potentials.
• Distinguish between presynaptic and postsynaptic inhibition.
• Describe the features of synaptic transmission.
• Understand neurotransmitter types and their functions.

Synapses

• Synapses are the connection points between neurons.
• Impulse transmission occurs from one neuron to another across a synapse.
• The structure comprises presynaptic and postsynaptic neurons, separated by a synaptic cleft.

Synapses Function

• Impulses are transmitted through a succession of neurons.
• Impulses can be blocked in the transmission, transformed into repetitive impulses, or combined into complex patterns during transmission.
• Synapses serve as targets for many drugs' actions.

Synapses, Types

• Chemical synapses are the most common type in the CNS.
• Chemical synapses involve neurotransmitter release from a presynaptic neuron across a synaptic cleft to bind to a receptor on a postsynaptic neuron.
• Electrical synapses transmit impulses directly between cells through gap junctions.
• Conjoint synapses combine chemical and electrical processes.

Chemical Synapses

• At a chemical synapse, one neuron's axon terminal connects to another neuron's dendrites, soma, or axon.
• The axon of a presynaptic neuron ends on the dendrites, soma, or axon of a postsynaptic neuron.

Chemical Synapses, Types

• Axo-dendritic connections are the least excitable.
• Axo-somatic synapses are slightly more easily stimulated.
• Axon-axon synapses are highly excitable, have low threshold stimulation, and possess high sodium channel density.
• There's a synaptic cleft separating the presynaptic and postsynaptic membranes, ranging from 20-40 nanometers in width.

Chemical Synapses; Properties

• Presynaptic terminals contain mitochondria and membrane-bound vesicles.
• Neurotransmitters are synthesized in the cell body and transported to the presynaptic terminals.
• Stimulation of presynaptic neurons releases neurotransmitters via exocytosis into the synaptic cleft.
• Neurotransmitters bind to specific receptor sites on the postsynaptic neuron.

Neurotransmitters

• More than 50 neurotransmitters are recognized.
• Some well-known neurotransmitters are acetylcholine, norepinephrine, epinephrine, histamine, GABA, glycine, serotonin, and glutamate.
• Neurotransmitters' effect depends on the receptor type on the postsynaptic neuron and the enzymatic processes in the postsynaptic neuron.

Electrical Synapses (gap junctions)

• Electrical synapses transmit impulses directly using gap junctions that connect the cytoplasm of adjacent cells.
• These synapses offer rapid impulse transmission.

Conjoint Synapses

• Conjoint synapses utilize both chemical and electrical processes to transmit impulses.
• Conjoint synapses are rare.

Synaptic Transmission; Properties

• The transmission process isn't instantaneous.
• A synaptic delay takes around 0.5 milliseconds.
• Neurotransmission can be determined by measuring synaptic delay in polysynaptic pathways.

Synaptic Fatigue

• Synaptic transmission declines after repeated stimulation.
• Causes include neurotransmitter depletion and postsynaptic receptor downregulation.

Synaptic Transmission; Properties; Chemical Changes

• Synaptic transmission is influenced by chemical changes such as variations in blood H+ concentration and blood O2 levels.
• Elevated H+ (acidosis) decreases synaptic transmission, while decreased H+ (alkalosis) increases it.
• Decreased O2 (hypoxia) reduces synaptic transmission.

Synaptic Transmission; Properties; Drugs

• Certain drugs affect synaptic transmission by increasing or decreasing it.
• For example, some anesthetic and anxiolytic drugs decrease transmission by affecting GABA receptors.
• Other drugs, like theophylline and caffeine, can increase transmission.

Botulism and Tetanus Toxins (Clinical Note)

• Clostridia bacteria produce potent toxins, including botulinum and tetanus toxins.
• Tetanus toxin blocks the release of inhibitory neurotransmitters, leading to spastic paralysis.
• Botulinum toxin blocks acetylcholine release at neuromuscular junctions leading to flaccid paralysis.

Postsynaptic Potentials; Types

• A postsynaptic potential (PSP) is a temporary change in the postsynaptic membrane potential.
• Excitatory postsynaptic potentials (EPSPs) cause depolarisation, making the neuron more likely to fire an action potential.
• Mechanism involves opening sodium channels, increasing sodium influx.
• Inhibitory postsynaptic potentials (IPSPs) cause hyperpolarization, making the neuron less likely to fire an action potential.
• Mechanism includes opening chloride channels, thus promoting chloride influx.

Postsynaptic Potentials; Summation

• Spatial summation occurs when multiple presynaptic neurons stimulate a postsynaptic neuron simultaneously.
• Temporal summation occurs when a single presynaptic neuron stimulates a postsynaptic neuron repeatedly in rapid succession.

Synaptic Transmission; Properties: Additional Details

• One primary direction of transmission: from the presynaptic to postsynaptic neuron.
• Summation processes, which have been discussed in previous sections, are also key features.
• Synaptic delay is the duration required for transmission from a presynaptic to a postsynaptic neuron (typically around 0.5 milliseconds), including transmitter release, receptor binding, and generation of the postsynaptic potential

Synaptic Transmission: Contral inhibition

• Central excitation occurs when excitatory postsynaptic potentials (EPSPs) are predominant.
• Central inhibition occurs when inhibitory postsynaptic potentials (IPSPs) are predominant.