Chapter 2 Synapses

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.

2.1 The Concept of the Synapse Neurons communicate by transmitting chemicals at junctions, called “synapses” The term was coined by Charles Scott Sherrington in 1906 to describe the specialized gap that existed between neurons Sherrington’s discovery was a major feat of scientific reasoning

The Properties of Synapses Sherrington Investigated how neurons communicate with each other by studying reflexes (automatic muscular responses to stimuli) in a process known as a reflex arc Example Leg flexion reflex: a sensory neuron excites a second neuron, which excites a motor neuron, which excites a 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.

Presynaptic neuron: neuron that delivers the synaptic transmission Postsynaptic neuron: neuron that receives the message

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: Action potential moves toward end plates stimulating calcium channels to open stimulating movement of vesicles. Vesicles with neurotransmitter move towards endplate of presynaptic neuron. Neurotransmitters are released into synapse through exocytosis. Neurotransmitters diffuse across synaptic cleft. Neurotransmitters bind to receptors on postsynaptic neuron. Bound neurotransmitter stimulates response. Neurotransmitter fragments released after use. 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.

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 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.

An example of an inhibitory neurotransmitter is serotonin 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.

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. 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.

Spatial Summation, Part 1 Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not Thus, idea of spatial summation Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse

Recordings From a Postsynaptic Neuron During Synaptic Activation Figure 2.3 Recordings from a postsynaptic neuron during synaptic activation. © Cengage Learning

Spatial Summation, Part 2 Spatial summation is critical to brain functioning Each neuron receives many incoming axons that frequently produce synchronized responses Temporal summation and spatial summation ordinarily occur together The order of a series of axons influences the results

Temporal Summation Sherrington observed that repeated stimuli over a short period of time produced a stronger response Thus, the idea of temporal summation Repeated stimuli can have a cumulative effect and can produce a nerve impulse when a single stimuli is too weak

Three Important Points About Reflexes Sherrington’s observations Reflexes are slower than conduction along an axon Several weak stimuli present at slightly different times or slightly different locations produce a stronger reflex than a single stimulus As one set of muscles becomes excited, another set relaxes

Spatial Summation, Part 1 Sherrington also noticed that several small stimuli in a similar location produced a reflex when a single stimuli did not Thus, idea of spatial summation Synaptic input from several locations can have a cumulative effect and trigger a nerve impulse

Recordings From a Postsynaptic Neuron During Synaptic Activation Figure 2.3 Recordings from a postsynaptic neuron during synaptic activation. © Cengage Learning

Spatial Summation, Part 2 Spatial summation is critical to brain functioning Each neuron receives many incoming axons that frequently produce synchronized responses Temporal summation and spatial summation ordinarily occur together The order of a series of axons influences the results

Temporal and Spatial Summation Figure 2.4 Temporal and spatial summation © Cengage Learning

Spontaneous Firing Rate The periodic production of action potentials despite synaptic input EPSPs increase the number of action potentials above the spontaneous firing rate IPSPs decrease the number of action potentials below the spontaneous firing rate

Activating Receptors of the Postsynaptic Cell The effect of a neurotransmitter depends on its receptor on the postsynaptic cell

https://www.youtube.com/watch?v=Tqwo9dmIXAQ

Increasing and decreasing the effect of NTMs Agonist: A drug (or poison) increases activity of a NTM. How? Mimics shape Prevents reuptake by pre-synaptic neuron Blocks enzymes that break down NTM in synapse Antagonist: A drug (or poison) that reduces NTM activity Blocks release of NTM from its terminal button Blocks receptor on post synaptic dendrite

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