Biology 12 - Crossing the Synapse

Neurotransmitters

These are chemical messengers of the nervous system which pass an impulse from one neuron to the next. 

This could be a sensory neuron communicating with an interneuron in the spinal cord or an interneuron in the spinal cord communicating with a motor neuron.   

They are released from the terminal button of the axon end brush.

Synapse

The tiny fluid-filled space between two neurons.

Synaptic cleft

The space between the presynaptic and postsynaptic membranes.

Information in the form of electrical impulses traveling down the  axon is converted into a chemical message that crosses the

Information in the form of electrical impulses traveling down the  axon is converted into a chemical message that crosses this.

When the neuron is at rest, the concentration of calcium ions is greater outside the membrane than inside. These calcium ions are in high concentration in the fluid filled space. 

At the end brush are membrane proteins which act as calcium gates.

What’s in the Synapse?

First step in crossing the synapse

Action Potential reaches the axon end brush.

Second step in crossing the synapse

Voltage gated calcium gates open allowing Ca²⁺ to rush in.

Third step in crossing the synapse

The increase in Ca²⁺ stimulates vesicles   containing neurotransmitters to move to the membrane.

Forth step in crossing the synapse

Vesicles fuse with the membrane and expel the neurotransmitters by exocytosis. The neurotransmitters then diffuse across the synapse.

Fifth step in crossing the synapse

The neurotransmitter binds to ligand gated channels (protein receptors) on the postsynaptic dendrite.

Sixth step in crossing the synapse

The effect of the neurotransmitter on the neighboring dendrite depends on the number and type of ligand gated channels that are activated by the neurotransmitters. 

Excitatory Neurotransmitters

Binding of excitatory neurotransmitters (such as acetylcholine) to its receptors on the postsynaptic membrane causes ligand gated sodium channels to open.

This causes an influx of Na⁺ ions thereby reverses the polarity of the neuron causing depolarization of the postsynaptic membrane.

Providing the threshold level is reached an action potential is generated in the postsynaptic cell.

Inhibitory Neurotransmitters

As inhibitory neurotransmitters bind to receptor sites on the postsynaptic membrane ligand gated potassium channels open allowing more K+ ions to diffuse out of the cell.

The diffusion of K+ ions increases resulting in a hyperpolarized condition

It is also possible that the inhibitory neurotransmitter binds to ligand gated chloride channels causing them to open.  Cl- ions to enter the cell also resulting in hyperpolarization. 

In either case, the membrane becomes more negative and prevents the continuation of the impulse

Principle of Summation

Generally one single synapse is below the threshold level and is therefore unable to initiate an action potential on its own. The postsynaptic potential (PSP) of the membrane is then determined by the sum total of several synapses.   

Excitatory Postsynaptic Potential (EPSP) 

Inhibitory Postsynaptic Potential (IPSP) 

The effect on the postsynaptic membrane will be determined by the overall total  neurotransmitters released which then bind to the postsynaptic membrane. This will be influenced  by both the EPSP and the IPSP at each synapse.