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February 20, 2021
  

[Brain Image]    

PSY 340 Brain and Behavior

Class 07:  The Concept of the Synapse & Chemical Events at the Synapse (pt. 1)

   

The signals in the brain are transmitted using two different methods (the first of which we studied last week)
  • Along the axon the signal is transmitted electrically as an action potential.
  • At the synapse, the signal goes from the axon terminal button across to the post-synaptic membrane (on a dendrite, e.g.,) by chemical means.
[Types of Transmission]

So, now we turn to understand the synapse and how signals are sent chemically between one neuron and another.



The Puzzle of How Neurons Are Connected

[Are neurons connected at the synapse?] At the beginning of the 20th century, Santiago Ramón y Cajal proposed that there was some type of junction between two neurons. Neurons were distinct cells. However, what was the nature of that junction?

Sherrington   Sir Charles Scott Sherrington (1857-1952) & the Discovery of the Synapse


Charles Scott Sherrington was an English neurologist, neuroscientist, professor at Oxford University, and winner (along with Edgar D. Adrian) of the Nobel Prize for Physiology and Medicine in 1932 for the work reported here.
 
Reflexes
: automatic muscle responses to stimuli
[Reflex Arc
          Leg Flexion]Sensory neuron --> interneuron ("intrinsic") neuron --> motor neuron at level of the spinal cord = reflex arc

In experiments with dogs, Sherrington noted

From these observations, Sherrington concluded some of the most important qualities about synapses and transmission of messages within the nervous system.


[Gaps in neural signal]Speed of Reflex & Delayed Transmission at the Synapse

Summation: The effects of different kinds of stimulation on a neuron's tendency to fire

Two types: temporal and spatial

Temporal summation: repeated stimuli within a relatively short period of time can have a cumulative effect (see diagram below on left)

  [Eccles]

Sir John Eccles (1903-1997), a former student of Sherrington, showed temporal summation in single cells. Won the Nobel Prize in 1963 for his work on how inhibitory and excitatory processes occur at the synapse.  

Spatial summation: stimuli occurring at different locations can have a cumulative effect. (See diagram on the far right.)

 [Summation Diagram]


[EPSPs and IPSPs]
EPSPs and IPSPs: Effects at the Synapse

If one neuron stimulates another neuron, the first is called the presynaptic neuron and the second the postsynaptic neuron.

When the postsynaptic neuron receives a signal/message/firing across the synapse, various channels may open on the postsynaptic membrane and allow ions either to enter or to exit. This flow of ions causes a change in the electrical voltage on the inside of the postsynaptic neuron's membrane.

Excitatory Post-Synaptic Potential (EPSP)

Inhibitory Post-Synaptic Potential (IPSP)

Usually happens when K+ gates open up on the postsynaptic membrance and allow K+ ions to leave the neuron. Alternatively, channels for the chloride ion (Cl-) may open up and allow Cl- ions to enter the postsynaptic neuron. In either case, the inside of the postsynaptic membrane is, at least for a short while, more negative.
IPSPs make it less likely that the postsynaptic neuron will fire

Our text also notes that many neurons have a spontaneous firing rate without even being stimulated. For example a neuron may fire spontaneously 10  to 20 times per second. Recent research (Brembs, 2020)  argues that the ultimate foundation for this phenomenon is the nature of the brain itself as a  dynamic organ which is constantly active even in the absence of stimulation. Specific neurochemical factors which contribute to this spontaneous firing rate are quite complex (e.g., see Häusser et al., 2004, Mazzoni et al., 2007).



Chemical Events at the Synapse (pt. 1)

Otto Loewi (1873-1961) & Chemical Transmission at Synapses (1920)

In 1905, T. R. Elliott (Cambridge University, UK) had found that adrenaline (epinephrine) causes the heart to beat faster, relaxes the stomach muscles, dilates the pupils. These effects parallel stimulation of the sympathetic nervous system (SymNS) and, thus, led Elliott to wonder if the SymNS used a chemical like adrenaline to induce its effects.

Otto Loewi's
              Experiment on VagusstoffOne night in 1920, German physiologist, Otto Loewi, had an idea. He went to his laboratory and performed an experiment with a frog's heart. (see diagram). He used a frog's heart because it will continue beating on its own without stimulation if it is removed from the frog and placed in a liquid-filled beaker.

He stimulated the vagus nerve of the frog's heart repeatedly. The heart rate slowed down (this is an effect of the parasympathetic nervous system).

Then, he pumped the fluid from the beaker in which the stimulated heart was located into another beaker with a second frog's heart. That heart's beat also slowed down. He concluded that there was some chemical released by the heart after its vagus nerve had been stimulated which caused the second heart to slow down its beating. He did not know what that chemical was, but called it Vagusstoff (in English, "stuff from the vagus nerve"). Later on, the chemical was found to be acetylcholine, one of the neurotransmitters we will talk about below. He had stumbled upon a way in which, at the synapse, chemicals were used as transmission agents across the synaptic cleft.

Chemical Activity at the Synapse

Chemical
              Activity at the Synapse

References

Brembs, B. (2020). The brain as a dynamically active organ. Biochemical and Biophysical Research Communications. https://doi.org/10.1016/j.bbrc.2020.12.011

Häusser et al. (2004). The Beat Goes On: Spontaneous Firing in Mammalian Neuronal Microcircuits. Journal of Neuroscience, 24(42), 9215-9219.

Mazzoni A.,  et al. (2007) On the Dynamics of the Spontaneous Activity in Neuronal Networks. PLoS ONE, 2(5).

This page was first posted January 30, 2005