Synapses
Fall 2002

Ccommunication within the nervous system requires relaying messages:

a. Within a neuron--this is made possible by ions & action potentials
b. Between neurons--the topic of this lecture.
Historical background:
Greeks believed fluids (humors) flowed through body. The bicep bulges when te arm contracts because it fills with fluid.

Luigi Galvani (1791) did experiments with frog legs showing that electrical stimulation of the spinal cord would cause the frogs legs to move (called "Animal electricity"). Believed that electricity flowed through a continuous neural net.

Otto Loewi did an experiment (1921) showing that neurons used chemicals to communicate with targets in the body (i.e., it wasn't just electricity). Loewi put a live frog heart in a dish. He electrically stimulated the vagus nerve which is connected to the heart and heart rate slowed down. He then poured the fluid from this heart onto a second heart and found that the second heart also slowed down. His conclusion was that there must be a chemical substance that is released by the vagus nerve that controls heart rate. He called it vagusstoff. This chemical is now known to be a neurotransmitter called acetylcholine.
 

The billions of neurons in the nervous system communicate via connections called synapses. Neurons can synapse with: We are going to focus on neuron to neuron communication.
The axon of one neuron, in particular the terminal button, can contact another neuron at the: The synapse is the gap between the terminal button of one neuron and a target (Figure 5.2).
There are three parts to a synapse: a. The presynaptic element,
b. The synaptic cleft; &
c. The postsynaptic element Chemical messengers called neurotransmitters are located in vesicles inside the presynaptic element. When an AP enters the terminal button, calcium (Ca++) channels open and Ca++ flows in. Ca++ causes vesicles to fuse with the presynaptic membrane releasing the neurotransmitter (Figure 5.8).

The Neurotransmitter diffuses across the synaptic cleft and binds to receptors in the postsynaptic membrane. Receptors are proteins that have a special structure that allows only a particular neurotransmitter to bind to it (like a lock & key). The binding of the neurotransmitter to the receptor causes a change in the resting potential of the postsynaptic neuron.
 

The receptor changes the resting potential by causing ions to flow through the membrane.
This can happen in two different ways because of two different types of receptors:

1) Transmitter gated ion channel receptors: The neurotransmitter causes the receptor to change shape allowing ions to flow through the receptor (Figures 5.10, 5.11, & 5.12)

2) G-protein coupled receptors: The neurotransmitter causes the receptor to activate an intracellular G-protein which open ion channels or activate a second messenger which can alter many intracellular processes (Figure 5.13).
 

 As mentioned in an earlier lecture, the flow of ions can cause either a:
a. Depolarization (e.g., move Na+ or K+ in). A depolarization makes it more likely for the neuron to fire an action potential. Thus, this is called an Excitatory postsynaptic potential or EPSP (Figure 5.11).

b. Hyperpolization (move K+ out or Cl- in). A hyperpolarization makes it less likely for the neuron to fire an action potential. Thus, this is called an Inhibitory postsynaptic potential or IPSP (Figure 5.12)


The receptor decides if an EPSP or IPSP occurs by which ions fit through the receptor associated channel.
 

EPSPs & IPSPs are called local potentials because they don't spread very far. For example, local potentials may travel into the soma, but will not travel the length of an axon. Thus, we need to generate an AP or else the message will die (as mentioned in the previous lecture, APs travel the length of an axon because they are regenerated by Na+ channels).
 

Action potentials can only be generated by summing up the voltage changes of many local potentials. That is, the postsynaptic neuron sums many EPSPs & IPSPs. If the axon hillock in the postsynaptic neuron reaches a certain potential (e.g., -55 mV), then an action potential will be generated. This is called the All or none law (A neuron either reaches the threshold potential and fires an action potential or it doesn't).
 

Neurons sum local potentials across (Figure 5.15):

Space--called Spatial summation (Multiple inputs from different places), and
Time--called Temporal summation (Multiple inputs within a certain time). Both Temporal & spatial summation occur continuously (e.g., there are 10,000 inputs to each neuron). The neuron integrates all of the inputs and if the threshold potential is reached, then an AP is evoked. Inhibitory inputs can prevent APs despite excitatory input.
 

Return to Psych 372 Syllabus