Cytology · MU Sofia

67. Interneuronal synapses. Neurotransmitters.

Explains what a synapse is and its main components: the presynaptic terminal, the synaptic cleft and the postsynaptic membrane. Covers classification of synapses by transmission mechanism (chemical and electrical) and by morphological location, including the neuromuscular junction, then steps through the sequence of chemical neurotransmission. Finishes with the main neurotransmitter groups, such as acetylcholine, amino acids, monoamines, neuropeptides and others, and the ways neurotransmitters are inactivated.

Inside: 3 sections · 5 labelled pictures · 13 flashcards · 7 min

Topic 67 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 3 sections, about 981 words (7 min of reading), with 13 flashcards.

In this note

  1. Interneuronal Synapses. Neurotransmitters.
  2. I. Interneuronal Synapses
  3. II. Neurotransmitters

I. Interneuronal Synapses

A synapse is a specialised site where nerve impulses are transmitted from one neuron to another, or from neurons to other effector cells like muscles and glands. It is the functional unit for communication within the nervous system, allowing neurons to process and transmit information through electrical and chemical signals. The structure of a synapse ensures that this transmission is unidirectional.

A. Components of a Synapse

  1. 1. Presynaptic Axon Terminal (Terminal Bouton): This is the sending part of the neuron, from which neurotransmitters are released. It contains a large number of synaptic vesicles (containing neurotransmitters), numerous mitochondria, and smooth endoplasmic reticulum.
  2. 2. Synaptic Cleft: A narrow intercellular space, typically 20-30 nm wide, that separates the presynaptic and postsynaptic membranes. Neurotransmitters diffuse across this gap.
  3. 3. Postsynaptic Cell Membrane: This is the receiving part of the neuron or effector cell. It contains specific receptors for the neurotransmitters and ion channels or other mechanisms to initiate a new impulse or response.

B. Types of Synapses: Synapses can be broadly classified based on their mechanism of transmission and their morphological location.

  1. 1. Based on Mechanism of Transmission
  • Chemical Synapses: The most common type. Electrical activity in the presynaptic neuron is converted into a chemical signal (neurotransmitter release). This chemical binds to receptors on the postsynaptic cell, initiating a chemical or electrical response.
  • Electrical Synapses: Here, the membranes of adjacent cells are directly connected by gap junctions (or synaptic clefts capable of passing electric current). This allows for direct, rapid passage of electrical current and voltage changes from the presynaptic to the postsynaptic cell, enabling faster message transfer.
  1. 2. Based on Morphological Location
  • Axosomatic Synapse: An axon forms a synapse with a cell body (soma) of another neuron.
  • Axodendritic Synapse: An axon forms a synapse with a dendrite (or dendritic spine) of another neuron. These are common, and dendritic spines are key sites for synaptic signal processing and neural plasticity.
  • Axoaxonic Synapse: An axon terminal secretes into another axon. These synapses modulate the activity of axosomatic and axodendritic synapses.
  • Axosecretory Synapse: Axon terminals secrete directly into the bloodstream.
  • Axoextracellular Synapse: Axons secrete directly into the extracellular fluid.
  • Axosynaptic Synapse: An axon terminal ends on another synapse.
  • Neuromuscular Junction (NMJ): A specialized synapse between a motor neuron (effector nerve fiber) and a muscle fiber. The chemical transmitter here is acetylcholine.

C. Mechanism of Neurotransmission (at Chemical Synapses)

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