Synapses and Neurotransmitters

Synapses and Neurotransmitters

Action Potentials We have been talking about action potentials and how they allow an electrical impulse to travel from the dendrites to the end plates of a neuron. These action potentials do not just move down a single neuron and then stop.

Action Potentials Your brain is a network of millions of neurons that are, in essence, talking to one another. Action potentials are the message and the synapse is how the message is transferred from one neuron to another.

Synaptic Transmission The tiny spaces between neurons and effectors are called synapses or synaptic clefts. Synapses are formed between the end plates of one neuron and the dendrites, axon, or cell body of another neuron. They can also be formed between the endplates of one neuron and an effector (muscle)

Synaptic Transmission The terminal branches of a single neuron allow it to join with many different neurons allowing the message from one to be multiplied very quickly by sending it to many other neurons.

Synaptic Transmission Small vesicles in the end plates of neurons contain chemical messengers called neurotransmitters. As an impulse moves along a neuron, it causes the release of these neurotransmitters from the end plates. Neurotransmitters are released from the presynaptic neuron into the synaptic cleft.

Synaptic Transmission Once neurotransmitters are in the synapse, they diffuse across it until they attach to receptors on the dendrites, axon, or cell body of the postsynaptic neuron. This binding of neurotransmitters creates a depolarization of the postsynaptic neuron stimulating an action potential and allowing the message to move on.

Synaptic Transmission Stages: 1. Action potential moves toward end plates stimulating calcium channels to open stimulating movement of vesicles. 2. Vesicles with neurotransmitter move towards endplate of presynaptic neuron. 3. Neurotransmitters are released into synapse through exocytosis. 4. Neurotransmitters diffuse across synaptic cleft.

Synaptic Transmission Stages: 5. Neurotransmitters bind to receptors on postsynaptic neuron. 6. Bound neurotransmitter stimulates response. 7. Neurotransmitter fragments released after use. 8. Fragments move back to presynaptic neuron and re-enter cell through endocytosis for recycling.

Synaptic Transmission A synaptic cleft is extremely small (about 20nm wide). Even with this small space, diffusion is a slow process. So when neurotransmitters are diffusing across the synapse, the transmission of the message slows down a bit.

Synaptic Transmission Areas of the body with fewer synapses will result in quicker transmission of an impulse. What are examples of areas where we need to react quickly?

Synaptic Transmission Areas of the body with many synapses will result in slower transmission of an impulse. What are examples of areas where we react slower?

Neurotransmitters Neurotransmitters are used by neurons to change the membrane potential of postsynaptic neurons. They can either stimulate an action potential or inhibit one. Neurotransmitters that cause action potentials to occur are said to be excitatory while those that stop them from happening are called inhibitory.

Neurotransmitters Acetylcholine (a-see-tyl-kol-een) is an excitatory neurotransmitter found in the end plates of most neurons. When it attaches to the receptors on a postsynaptic neuron it causes sodium channels to open. Once this occurs, sodium ions rush in causing depolarization to occur in this neuron.

Neurotransmitters This stimulation of an action potential means that the message is being passed on, which is a good thing. But, remembering action potentials, we know that the cell needs to reach a repolarization stage which means that sodium channels need to close.

Neurotransmitters If acetylcholine remains attached, the postsynaptic neuron is stuck in the depolarization stage. An enzyme called cholinesterase (colon-esteraze) is released by the presynaptic neuron. This enzyme destroys acetylcholine allowing the postsynaptic neuron to begin the recovery stages of action potential.

Neurotransmitters Scientists have used this knowledge of acetylcholine and cholinesterase in the development of many insecticides. Cholinesterase is blocked which causes the heart of the insect (which is completely controlled by nerves) to contract but never relax therefore killing it.

Neurotransmitters In humans, low levels of acetylcholine has been related to deterioration of memory and mental capacity giving evidence to this depletion being a cause of Alzheimer s disease.

Neurotransmitters Although acetylcholine is considered an excitatory neurotransmitter, there are some cases where it can also be inhibitory. Inhibitory neurotransmitters cause the membrane of the postsynaptic neuron to become more permeable to potassium ions. This leads to a hyperpolarization of the membrane which means that an action potential cannot occur.

Neurotransmitters An example of an inhibitory neurotransmitter is serotonin. Action potentials are blocked to allow your brain to enter a state of rest and allows you to sleep. People with low levels of serotonin generally have a hard time falling asleep or staying asleep.

Neurotransmitters Another inhibitory neurotransmitter is gamma aminobutyric acid (GABA) GABA is the most abundant neurotransmitter in the brain and is used to calm action potentials in the brain. Having GABA in the brain allows you to prioritize information and to focus on many different things at once.

Neurotransmitters People with low levels of GABA neurotransmitters can suffer from certain anxiety disorders, panic disorders, and Parkinson s disease. Certain drugs, like caffeine, inhibits the release of GABA causing your brain to become more alert. AKA removing the inhibiting effect on action potentials.

Summation It needs to be understood that in many cases, the neurotransmitters released from a single neuron are not enough to reach the threshold level in the postsynaptic neuron which means an action potential will NOT occur. The effect produced by the accumulation of neurotransmitters released from two or more neurons is called summation.

Summation This graph shows that Neuron A and Neuron B are both excitatory because they are stimulating a change while Neuron C is inhibitory.

Summation It also shows that alone, Neurons A and B cannot reach the threshold level, but if you combine their reactions (summation), the threshold is met and an action potential will occur.

Questions Page 425 #3, 4, 5, 6