Chapter 2: Synapse Flashcards
Synapse:
specialized gap between neurons.
Sherrington
studied reflexes (automatic muscular responses to stimuli) and introduced the term synapse
Reflex arc:
the circuit from sensory neuron to muscle response
- Temporal summation:
Repeated stimuli within a brief time having a cumulative effect.
Presynaptic neuron:
The neuron that delivers the synaptic transmission.
Postsynaptic neuron:
The neuron that receives the message.
Graded potential:
Either depolarization (excitatory) or hyperpolarization (inhibitory) of the postsynaptic neuron.
excitatory postsynaptic potential (EPSP)
A graded depolarisation which occurs when Na+ ions enter the postsynaptic neuron.
- EPSPs are not action potentials: The EPSP’s magnitude decreases as it moves along the membrane.
- A quick sequence of EPSPs combine to exceed the threshold and produce an action potential
Spatial summation:
Several synaptic inputs originating from separate locations exerting a cumulative effect on a postsynaptic neuron.
a. For example, pinching one point does not produce a reflex but pinching two points at once does.
b. This happens because the two points activated separate sensory neurons, whose axons converged onto a neuron in the spinal cord.
c. Spatial summation critical to brain functioning
Inhibitory postsynaptic potential (IPSP):
A temporary hyperpolarization of a postsynaptic cell (this occurs when K+ leaves the cell or Cl- enters the cell after it is stimulated).
Spontaneous firing rate:
The ability to produce action potentials without synaptic input (EPSPs and IPSPs increase or decrease the likelihood of firing action potentials).
The Discovery of Chemical Transmission at Synapses
- T.R. Elliott (1905):
- Otto Loewi (1960):
- T.R. Elliott (1905): Suggested that the sympathetic nerves stimulate muscle by releasing adrenaline or a similar chemical. However, the evidence was not convincing and most scientists continued to belief that synapses communicated electrically.
- Otto Loewi (1960): Fluid from a stimulated frog heart was transferred to another heart. The fluid caused the new heart to react as if stimulated.
- Lead to the conclusion that synaptic transmission depends on chemical rather than electrical stimulation.
- This discovery revolutionized our understanding and led to research developing psychiatric drugs.
neurotransmitters:
a. Chemicals that are released by one neuron at the synapse and affect another neuron
Categories of neurotransmitters include:
Amino acids Monoamines Acetylcholine Neuropeptides (chains of amino acids) Purines Gases (such as nitric oxide, which dilate nearby blood vessels, increasing blood flow to nearby brain areas)
Catecholamines (dopamine, epinephrine, and norepinephrine):
Three closely related compounds containing a catechol and an amine group.
Synthesis of Transmitters
Neurons synthesize neurotransmitters (from amino acids) from precursors derived originally from food.
Vesicles:
Tiny nearly spherical packets that store high concentrations of neurotransmitters near the presynaptic terminal
- Nitric oxide is an exception to this rule, as neurons do not store nitric oxide for future use.
MAO (monoamine oxidase):
break down transmitters into inactive chemicals because it is possible for a neuron to accumulate excess levels of a neurotransmitter
- The first antidepressant drugs that psychiatrists discovered were MAO inhibitors, by blocking MAO they increased the brain’s supply of serotonin, dopamine and norepinephrine
exocytosis:
When an action potential reaches the axon terminal, the depolarization causes voltage-dependent calcium gates to open. As calcium flows into the terminal, the neuron releases a neurotransmitter into the synaptic cleft within 1-2 milliseconds
A neurotransmitter can have two types of effects when it attaches to the active site of the receptor:
ionotropic (faster but shorter effect) or metabotropic (slower but longer effects) effects.
Ionotropic effects:
- Sodium and potassium channels along an axon are transmitter-gated or ligand-gates channels. When the neurotransmitter attaches, it opens a channel.
- Well suited to conveying visual and auditory information along with anything else that needs to be updated ASAP
Metabotropic Effects:
when the neurotransmitter attaches to the receptor, it alters the configuration of the rest of the receptor protein, enabling a portion of the protein inside the neuron to react with other molecules. Activation of the receptor by the neurotransmitter leads to activation of G-proteins, which are attached to the receptor.
- These synapses are better for more enduring effects such as taste, smell and pain
G-proteins:
A protein coupled to the energy-storing molecule guanosine triphosphate (GTP)
Second messenger:
Chemicals that carry a message to different areas within a postsynaptic cell; the activation of a G-protein inside a cell increases the amount of the second messenger.
- A metabotropic synapse, by way of its second messenger, influences activity in much or all of the cell and over longer time