PSYB65H3 Chapter Notes - Chapter 5: Synaptic Vesicle, Electron Microscope, Axon Terminal

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Published on 11 Nov 2015
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Brain & Behaviour
Ch. 5 How do Neurons use Electrochemical Signals to Communicate & Adapt?
5.1 A chemical Message
Lowewis successful heartbeat experiment, marked the beginning of research into how chemicals
carry information from one neuron to another. Loewi was the first to isolate a chemical
messenger.
We now know the chemical as acetylcholine (ACh), the same transmitter that activates skeletal
muscles. ACh acts to inhibit heartbeat, to slow it down
ACh activates skeletal muscles in the somatic nervous system and may either
excite/inhibit internal organs in the automatic system.
Ach is the chemical massager that slows the heart in diving braycardia (‘slow heart’ – it is
a survival strategy; conserves the bodys oxygen when your not breathing i.e., when
swimming).
Loewi stimulated another nerve to the heart, the accelerator nerve, and obtained a speeded-up
heart rate. As before, the fluid that bathed the accelerator heart increased the rate of beating of a
second heart that was not electrically stimulated.
Loewi identified the chemical that carries the message to speed up heart rate in frogs as
epinephrine (EP), also known as adrenaline: chemical messenger that acts as a hormone to
mobilize the body for fight or flight during times of stress and as a neurotransmitter in the CNS.
Adrenaline (Latin) & Epinephrine (Greek) are the same substance, produced by the
adrenal glands located atop the kidneys.
Adrenaline is a more common name b/c a drug company used it as a trade name, but EP
is common phase in the neuroscience world.
Norepinephrine (NE, also noradrenaline), a chemical closely related to EP. The results of Loewis
complementary experiments showed that Ach from the vagus nerve inhibits heartbeat, and EP
from the accelerator nerve excites it.
Messenger chemicals released by neuron onto a target to cause an excitatory or inhibitory effect
of the same chemicals, EP among them, circulate in the blood stream as hormones.
Under control of the hypothalamus, the pituitary gland directs hormones to excite or inhibit
targets such as the organs and glands in the ANS.
In part b/c hormones travel through the bloodstream to distance targets, their action is slower than
that of CNS neurotransmitters pushed by the lighting-quick nerve impulse.
Loewis discoveries led to the search for more neurotransmitters and their functions. Whether a
chemical is accepted as neurotransmitter depends on the extent to which it meets certain criteria.
Clinical Focus 5.2
Structure of Synapses
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Loewi’s discovery about the regulation of heart rate by chemical messengers was the first of two
important findings that from the foundation for current understanding of how neurons
communicate. The second is the intervention of the electron microscope, which enabled scientists
to see the structure of a synapse.
The election microscope uses some of the principles of both an oscilloscope & a light
microscope.
The electron microscope works by projecting a beam of electrons through a very thin
slice of tissue.
The varying structure of the tissue scatters the beam onto a reflective surface where it
leaves an image, or shadow, of the time.
The resolution of an electron microscope is much higher than that of a light microscope b/c
electron waves are smaller than light waves, so there is much less scatter when the beam strikes
the tissue.
If the tissue is stained with substances that reflect electrons, very fine structural details can be
observed.
Chemical Synapses
The first good micrographs, made in the 1950s, revealed the structure of a synapse for the first
time. In the center of the micrograph, the upper part of the synapse is the axon terminal, or “end
foot”; the lower part is the receiving dendrite; there are the round granular substances in the
terminal. They are the synaptic vesicles continuing the neurotransmitter.
Synaptic vesicle: organelle consisting of a membrane structure that encloses a quantum of
neurotransmitter.
The dark patches on the dendrite consists mainly of protein receptor molecules that receive
chemical messages.
Dark patches on the axon terminal membrane are protein molecules that serve largely as ion
channels and pumps to release the transmitter or to recapture it after its release.
The terminal and the dendrite are separated by a small space, the synaptic cleft: gap that
separates the presynaptic membrane from the postsynaptic membrane.
The synaptic cleft is central to synapse function b/c neurotransmitter chemicals must
bridge this gap to carry a message from one neuron to the next.
Surrounding the centrally located synapse that you can see in an electron micrograph are glial
cells, other axons, dendritic processes, and other synapses.
The surrounding glia contribute to chemical neurotransmission in a number of ways – by
supplying the building blocks for the synthesis of neurotransmitters or by cleaning up excess
neurotransmitter molecules, for example.
Chemical synapses: junction at which messenger molecules are released when stimulated by an
action potential.
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Presynaptic membrane: membrane on the transmitter-output side of a synapse (axon terminal);
forms the axon terminal.
Postsynaptic membrane: membrane on the transmitter-input side of a synapses (dendritic spine);
it forms the dendritic spine, and the space between the two is the synaptic cleft.
Within the axon terminal are specialized structures, including mitochondria, the organelles that
supply the cell’s energy needs; storage granules, large compartments that hold several synaptic
vesicles; and microtubules that transport substances, including the neurotransmitters, to the
terminal.
Storage granule: membranous compartment that holds several vesicles containing a
neurotransmitter.
Microtubule: Transport structure that carries substances to the axon terminal.
Synaptic vesicle: Round granule that contains neurotransmitter.
Storage granule: Large compartment that holds synaptic vesicles.
Postsynaptic receptor: Site to which a neurotransmitter molecule binds.
Postsynaptic membrane: Contains receptor molecules that receive chemical messages.
Presynaptic membrane: Encloses molecules that transmit chemical messages.
Synaptic cleft: Small space separating presynaptic terminal and postsynaptic dendritic spine.
Electrical Synapses
Chemical synapses are the rule in mammalian nervous system, but they aren’t the only kind of
synapse. Some neurons influence each other each other electrically through a gap junction, or
electrical synapse”, where the “prejunction” and “postjunction” cell membrane are fused.
Gap junction: fused prejunction and postjunction cell membrane in which connected
ions channels form a pore that allows ions to pass directly from one neuron to the next.
Ion channels in one cell membrane connect to ion channel in the other membrane, forming a pore
that allows ions to pass directly from one neuron to the next.
This fusion eliminates the brief delay in information flow – about 5 milliseconds per synapse – of
chemical transmission.
Gap junctions are found in the mammalian brain, where in some regions they allow groups of
interneurons to synchronize their firing rhythmically.
Gap junctions also allow glial cells & neurons to exchange substances.
Why, if chemical synapses transmit messages more slowly, do mammals rely on them more than
on gap junctions? The answer is that chemical synapses are flexible in controlling whether a
message is passed from one neuron to the next, they can amplify or diminish a signal sent from
one neuron to the next, and they can change with experience to alter their signals and so mediate
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