BIOL 3330 - Chapter 5 Notes

electrical vs chemical synapses

electrical synapse - the ionic current directly transfers from one cell to the next

• very fast

• postsynaptic potential (PSP)

• bidirectional

• synaptic integration - several PSPs needed to cause an action potential

chemical synapse - neurotransmitters are released to communicate from one cell to the next

• the synaptic cleft is 5-20 nm wide

• *image

synaptic arrangement in the CNS

• axodendritic - axon to dendrite

• axosomatic - axon to cell body (more control over what the cell does)

• axoaxonic - axon to axon (signal for the signal)

• axospinous - axon to dendritic spine

• dendrodendritic - dendrite to dendrite (coordinating their actions)

CNS synapses vary in size and shape

synaptic transmission

typically unidirectional - neuron (presynaptic) to target cell (postsynaptic)

Gap junction components

• channels that connect

• connexons on each membrane

• electrically coupled

• can be between neurons or neurons with glial cells

CNS synaptic membrane categories

Gray’s type I - asymmetrical, usually excitatory

Gray’s type II - symmetrical, usually inhibitory

*the receptor determines whether it’s excitatory or inhibitory; just noticed it’s usually

peripheral synapse

• type of synapse called varicosities

• releases simultaneously in all tissues

• part of the parasympathetic and sympathetic nervous systems

  • parasympathetic releases: acetylcholine (ACH)

  • sympathetic releases: norepinephrine (NE)

3 categories of neurotransmitters

amino acids

• small organic molecule

• stored in vesicles

• made in the nerve terminal

• glycine, glutamate, gamma-aminobutyric acid (GABA)

amines

• small organic molecule

• stored in vesicles

• made in the nerve terminal

• acetylcholine (ACH), dopamine (DA), epinephrine, histamine, norepinephrine (NE), serotonin (5-HT)

peptide

• short amino acid chain (proteins)

• stored in secretory granules

• made in endoplasmic reticulum + ribosomes, then cleaved in Golgi to form the product

• substance P, somatostatin, neuropeptide Y

fast-form synapses last from 10-100 milliseconds (msec)

slower forms can last from 100 msec to a minute

granules and vesicles are observed in the same terminals

molecular steps of neurotransmission

step 1: an action potential enters the terminal causing voltage-gated Ca 2+ channels to open

step 2: this causes the synaptic vesicle to fuse with the membrane, releasing the contents into the synaptic cleft

• if a vesicle isn’t already docked, Ca2+ influx will cause it to dock

step 3: the neurotransmitters bind to the receptors in the postsynaptic density

differences between transmitter-gated ion channels and G-protein-coupled receptors

transmitter-gated ion channels open when activated by their neurotransmitter

• the function is determined by the ion entering the cell (NOT THE NEUROTRANSMITTER)

• what the receptor does determines whether it is inhibitory or excitatory

G-coupled receptors (metabotropic receptors)

• gated-ion channel (shortcut pathway) or second messenger (cascade)

• they are slower, have longer lasting effects

EPSP

excitatory postsynaptic potential - transient postsynaptic membrane depolarization caused by presynaptic release of neurotransmitter

• ex: Ach and Glutamate

IPSP

inhibitory postsynaptic potential - transient hyperpolarization of postsynaptic membrane potential caused by a presynaptic release of neurotransmitter

• ex: GABA and Glycine

differences in neurotransmitter recovery and degradation

• diffusion of transmitter molecules away from the synapse

• reuptake: neurotransmitter re-enters the presynaptic axon terminal

• enzymatic destruction inside the terminal cytosol or in the synaptic cleft (can’t bind to receptor)

• desensitization: the continued presence of neurotransmitter causes the receptor to close for a while, even if new neurotransmitter is released (modified function of cell - received too much signal)

synaptic integration

• the process by which multiple synaptic potentials combine within one postsynaptic neuron

• the postsynaptic cell can have a few to several thousand neurotransmitter-gated channels

  • how many of these are activated depends on how much neurotransmitter is released

    • the smallest unit is one vesicle

    • each contains roughly the same amount of neurotransmitters (several thousand)

    • amount of neurotransmitter released is dependent on the number of vesicles released (the amount of calcium ions that enter the nerve terminal)

in the CNS, neurons require many EPSP to produce a postsynaptic depolarization

• spatial summation (different cells) - EPSP from different synapses add together to raise the threshold

• temporal summation (one cell w/ multiple) - rapid succession (1-15 msec) of EPSP at the same synapse

GABA in the nerve terminal

• converted from glutamate to GABA by Glutamic Acid Decarboxylase (GAD)

autoreceptors

• receptors commonly found in the membrane of the presynaptic axon terminal

• presynaptic receptors sensitive to the neurotransmitter released by the presynaptic terminal

• consequences of activing autoreceptors vary; the common effect is inhibiton of neurotransmitter release

• sends signal that the terminal is full of neurotransmitter to reduce or stop release (safety valve)

depolarization along a dendrite

• internal resistance - resistance to current flowing down the dendrite

• membrane resistance - resistance to current flowing across the membrane

modulation

synaptic transmission that modifies effectiveness of EPSPs generated by other synapses with transmitter-gated ion channels