Physiology · MU Pleven

Synaptic Transmission

Introduces synapses as junctions for signalling between neurons and to muscles or glands, then describes presynaptic terminals, the transmitter vesicles and mitochondria, and how excitatory or inhibitory receptors determine the effect. It classifies synapses by target site (axodendritic, axosomatic, axoaxonic) and contrasts electrical synapses through gap junctions with chemical synapses. It closes with excitatory and inhibitory postsynaptic potentials and their durations, leaving summation to a picture.

Inside: 5 sections · 5 labelled pictures · 27 flashcards · 7 min

Topic 6 of 17 in the MU Pleven syllabus for Physiology (Medicine, Year 2). The free AceMed note covers it in 5 sections, about 982 words (7 min of reading), with 27 flashcards.

In this note

  1. TOPIC 6. SYNAPTIC TRANSMISSION. TYPES OF SYNAPSES. CHEMICAL SYNAPSES. POSTSYNAPTIC POTENTIALS. SUMMATION OF SYNAPTIC INPUTS – TEMPORAL AND SPATIAL.
  2. SYNAPTIC TRANSMISSION.
  3. TYPES OF SYNAPSES.
  4. POSTSYNAPTIC POTENTIALS
  5. SUMMATION OF SYNAPTIC INPUTS – TEMPORAL AND SPATIAL

SYNAPTIC TRANSMISSION.

SYNAPSES à specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands, and the junctions between cardiomyocytes and smooth muscle cells.

The figure shows a typical anterior motor neuron in the anterior horn of the spinal cord. It is composed of three major parts: the soma, which is the main body of the neuron; a single axon, which extends from the soma into a peripheral nerve that leaves the spinal cord; and the dendrites, which are great numbers of branching projections of the soma that extend as much as 1 mm into the surrounding areas of the cord.

  • As many as 10,000 to 200,000 minute synaptic knobs called presynaptic terminals lie on the surfaces of the dendrites and soma of the motor neuron.
  • About 80 to 95% of them on the dendrites and only 5 to 20% on the soma.

PRESYNAPTIC TERMINALS → ends of nerve fibrils that originate from many other neurons.

  • Have varied anatomical forms, but most resemble small round or oval knobs and, therefore, are sometimes called terminal knobs, boutons, end-feet, or synaptic knobs.
  • Separated from the postsynaptic neuronal soma by a synaptic cleft having a width usually of 200 to 300 angstroms.
  • The terminal has two internal structures important to the excitatory or inhibitory function of the synapse

Some secrete a transmitter substance that excites the postsynaptic neuron.

But other presynaptic terminals are inhibitory - they secrete a transmitter substance that inhibits the postsynaptic neuron.

Neurons in other parts of the cord and brain differ from the anterior motor neuron in

  1. 1. Size of the cell body
  2. 2. Length, size, and number of dendrites, ranging in length from almost zero to many centimeters
  3. 3. Length and size of the axon
  4. 4. Number of presynaptic terminals, which may range from only a few to as many as 200,000.

These differences make neurons in different parts of the nervous system react differently to incoming synaptic signals and, therefore, perform many different functions.

The transmitter vesicles contain the transmitter substance that, when released into the synaptic cleft, either excites or inhibits the postsynaptic neuron

→ Excites if the neuronal membrane contains excitatory receptors

→ Inhibits if the membrane contains inhibitory receptors

The mitochondria provide adenosine triphosphate (ATP), which in turn supplies the energy for synthesizing new transmitter substance.

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