Chapter 2 Flashcards
synapse
a specialized gap as a point of communication between two neurons anatomy of chemical events
An impulse travelling through a synapse in the spinal cord is slower than one travelling a similar distance along an uninterrupted axon.
The speed of conduction along an axon is about 40 m/s.
The speed of conduction through a reflex arc is slower and more variable, sometimes 15 m/s or less. Presumably, the delay occurs at the synapse.
graded potentials
Local potentials
Small voltage fluctuation on a part of the cell membrane
It is not spontaneous; the axon must be stimulated
hyperpolarization
increase in electrical charge across a membrane (more negative)
Inward flow of Cl- ions or outward flow of K+ ions
Tetraethylammonium (TEA) blocks K+ channels and blocks hyperpolarization
“Inhibitory Postsynaptic Potentials”= IPSP
depolarization
decrease in electrical charge across a membrane (more positive)
Usually due to the inward flow of Na+
Tetrodotoxin (TTX) blocks Na+ channels and blocks depolarization
“Excitatory postsynaptic potentials” = EPSP
Neurotransmission
A neurotransmitter is a chemical released by a neuron at the presynaptic terminal onto a target.
Has an excitatory or inhibitory effect, depending on the receptors of the post-synaptic cell
Outside the CNS, many of these chemicals circulate in the bloodstream as hormones.
amino acids
glutamate, GABA, glycine, aspartate, and maybe others
A modified amino acid: acetylcholine
monoamines (mod. from amino acids)
glutamate, GABA, glycine, aspartate, and maybe others
A modified amino acid: acetylcholine
neuropeptides
(chains of A acids): endorphins, substance P, neuropeptide Y, and many others
purines:
ATP, Adenosine
gases:
NO (nitric oxide)
synaptic vesicle
organelle consisting of a membrane structure that encloses a quantum of neurotransmitter
synaptic cleft
gap that separates the presynaptic membrane from the postsynaptic membrane
chemical synapse
junction where messenger molecules are released when stimulated by an action potential.
presynaptic membrane
a neuron that releases neurotransmitter (output side of synapse)
postsynaptic membrane
a neuron that receives neurotransmitter (input side of synapse)
types of synapses
Synapses in the mammalian nervous system are either chemical or electrical (i.e., the gap junction)
Electrical synapse: fused presynaptic and postsynaptic membrane that allows an action potential to pass directly from one neuron to the next
Depolarization in one affects the other very quickly as if the 2 neurons were 1
Electrical synapses are faster than chemical synapses
chemical neurotransmission in 6 steps
Synthesis: NT is synthesized (axon terminal or cell body) and stored in the axon terminal and stored in the vesicle along the axon. NT that leaks from vesicle is broken down by enzymes
Action Potential: propagates along the axon and causes vesicle to bind with presynaptic membrane due to the opening of voltage-sensitive Ca++ channels
Exocytosis: NT is brought to the presynaptic membrane and released into the synaptic cleft and (via diffusion) reaches the postsynaptic membrane to the receptors.
Separation: once released in the synaptic cleft, NT is able to activate the receptors on the postsynaptic membrane, and then they separate from the receptor.
Deactivation: the NT must be turned off (diffusion, deactivating enzyme, or reuptake by glia or presynaptic cell), or else it will continue to work indefinitely.
Retrograde transmitters: the postsynaptic cell may send messages to control (increase/decrease) the release of NT by the presynaptic cell.
neurotransmitters are synthesized in two general ways:
axon terminal (smaller neurotransmitters)
building blocks from food (via bloody supply) are pumped into cells via transporters
–transporters are protein molecules embedded within the cell membrane
MAO (monoamine oxidase)
breaks down excess neurotransmitters
autoreceptors
receptors that defect the amount of transmitter released and inhibit further synthesis and release
postsynaptic neurons
respond to stimulation by releasing chemicals that travel back to the presynaptic terminal where they inhibit further release
whats a neurotransmitter?
Must be synthesized or present in the neuron
Must produce a response in a target cell
Must have the same response when placed on the target
There must be a way to remove
ionotropic receptors
- direct
- binding sites, channel
- Occurs when a neurotransmitter attaches to receptors and immediately opens ion channels
Glutamate (+) opens sodium channels, GABA (-) opens chloride channel
metabotropic receptors
- indirect
- binding site, no channel
- proteins and enzymes play a key role
- G-protein activation: coupled to guanosine triphosphate (GTP), an energy-storing molecule
Increases the concentration of a “second messenger”
2nd messenger communicates to areas within the cell
May open or close ion channels, alter the production of activating proteins, or activate chromosomes
varieties of synapses
Different sorts of neural connections…
Dendrodendritic
Axoaxonic
Axodendritic
Axosomatic
Axosynaptic
Axoextracellular
Axosecretory
Type 1 Synapse
Excitatory
Dendrites
Large active zone
Wide cleft
Round vesicles
Dense material on membranes
Type II Synapse
Inhibitory
Cell body
Small active zone
Narrow cleft
Flat vesicles
Sparse material on membranes
hormones
Chemicals produced by endocrine glands (e.g., pineal gland, pituitary gland, etc) and secreted by glands (or other tissue)
Have an impact at more than one location in the body
It can trigger long-lasting changes in many parts of the body
