Exam 2: Synapse & the Blood-Brain Barrier (BBB)

What is a synapse? Why do we say it is responsible for the chemical aspect of neurotransmission?

A synapse is a junction between 2 neurons into which neurotransmitters are released, causing a change in the resting membrane potential in the postsynaptic neuron; the point of communication between two cells.

• The neurotransmitter is released here (a chemical process) which changes the membrane potential of the postsynaptic cell. The presynaptic cell releases a neurotransmitter into the synaptic cleft (a tiny space between the two cells, as they don’t touch).

What is the difference between presynaptic and postsynaptic cells/membranes?

The presynaptic cell/membrane transmits a signal by releasing neurotransmitters, while the postsynaptic cell/membrane receives that signal through receptors.

Describe the stages leading to neurotransmitter binding to receptors on the postsynaptic cell.

1. An action potential of the presynaptic cell reaches the axon terminal (synaptic bouton)

2. Ca²⁺ channels open, and Ca²⁺ ions flow into the cell. It allows the synaptic vesicles to fuse with the cell membrane and release a particular neurotransmitter

3. The axon terminal releases a neurotransmitter which diffuses in the synaptic cleft and binds to receptors on the postsynaptic membrane

4. The neurotransmitter causes channels to open

What happens if an excitatory neurotransmitter (glutamate) is released in the synapse?

• Excitatory effects result in depolarization and cell firing.

• Glutamate is the principle excitatory neurotransmitter (lesser ones include acetylcholine, dopamine, and norepinephrine)

What happens to the neurotransmitters to terminate their effects?

Degradation by enzymes, reuptake into the presynaptic cell, and diffusion away from the synapse

What are neurotoxins?

Substances that damage or interfere with the function of the nervous system. In general neurotoxins cannot cross the blood-brain barrier, so they block neural transmission elsewhere in the body (ie the muscles).

How does curare affect the body?

Curare is a neurotoxin that is extracted from plants and has been used by hunters in South America to paralyze their prey (but it is safe to eat poisoned meat).

• It blocks the nicotine acetylcholine receptor at the neuromuscular junction (where a motor neuron transmits signals to the muscle fiber, causing contractions).

• It results in muscle paralysis (including limbs, but also lungs, etc), causing death by asphyxiation (insufficient oxygen).

What is the purpose of the capillaries?

The microcirculation in the brain is carried out by the capillaries (which are tiny, only 5-10 μm). They bring in oxygen, water, and nutrients and take away waste, etc.

What cells are essential for the blood-brain barrier (BBB)?

• Endothelial cells form the lining of the capillaries. These cells are mainly characterized by tight junctions between them. 

• The capillaries are surrounded by astrocytes (glial cells) that help transfer metabolites from the blood to the neurons, remove waste, take care of excess neurotransmitters, etc.

What are tight junctions?

They pass only small molecules through (such as water) or those that are lipid-soluble (such as alcohol, barbiturates, or drugs of abuse)

What is the basic purpose of the blood-brain barrier (BBB)?

The overall purpose of that barrier is to protect the brain from the debris in blood and to maintain a constant internal brain environment (because you don’t want the fine brain microenvironment disrupted by something released in your gut).

What can affect the blood-brain barrier (BBB)?

• The BBB can be broken down by hypertension, radiation, infection, trauma, or inflammation.

• There are certain areas of the brain where the BBB is weak: the area postrema (a vomiting center in the medulla oblongata) detects toxins in the blood and induces vomiting. This way, we can eliminate the toxic substance from our stomach before it harms the organism.

What is meant by the “brain” in BBB (neurons)?

Refers to neural tissue