Chapter 8 - Neurons Flashcards
Two types of disrupting events
1) Graded potentials (unexcitable)
2) Action potentials (excitable)
Graded Potential (def and characteristics)
-a change in membrane
potential that varies in size
• found in unexcitable membranes
• unexcitable: has ligand-gated and
mechanosensitive channels, found in
soma and dendrites
a) magnitude of response is directly
proportional to magnitude of the
stimulus
b) does not transmit over long distance
(decremental conduction)
c) effects can be summated (summation)
Action Potential (def and characteristics)
-change in a membrane potential of excitable membranes
• found in neurons and some non-
neuronal tissue (i.e. muscle)
• excitable: contains voltage-gated
channels, found in axon and axon
terminals
a) action potential are “all-or-nothing”,
magnitude is “fixed” after threshold
b) capable of transmitting over distances
(>1m)
c) do not summate
Threshold (def)
the minimum stimulus necessary to elicit a response (action potential)
An above-threshold stimulus applied to the membrane can cause
a change in voltage resulting in the opening of voltage-gated channels (e.g. during the rising phase of an action potential Na can increase 600X over resting values)
Membranes also possess voltage-gated K channels
- which open in response to changes in Vm, but are approximately 10x slower than Na channels
- increased K conductance results in K efflux and repolarization
- K channels don’t inactivate line Na channels only close when Vm returns to ???get notes
How AP permit long-distance transmission?
- disproportionate ion flux, Na in and K out, creates depolarization
- Na influx must be greater than K efflux
Refractory period (def and characteristics)
- time period after an action potential in which Na gates may be inactive and the K channels may be open so that a second AP is impossible or more be open so that a second AP is impossible or more difficult to achieve (requires a larger than normal stimulus)
- creates unidirectional action potentials (axon hillock to axon terminals)
Local current flow
movement of AP from region to neighboring region
Rate of transmission is dependent on
axon diameter, wider diameter has lower resistance
Neuroglial cells (def and types)
-“support” the neuron, prevent charge leakage
• Schwann: myelin forming cells of the
PNS, help to prevent leakage of charge
•Oligodendrocytes: myelin forming cells of
the CNS, help prevent leakage of charge
• Astrocytes: star-shaped that surround
unmyelinated cells that give structural
support
• Microglia: small scavengers, phagocytic to damaged neurons
Nodes of Ranvier (def)
voltage-gated ion channels concentrated; AP “jumps” to each node
Saltatory Conduction (def)
AP propagation by “jumping” form node to node in myelinated fibers (neuron)
Synapse (def)
Anatomically specialized junction between two cells where the electrical activity of one cell influences the excitability of another
Two types of synapses
1) electrical: gap junctions, heart, later chapter
2) chemical: neurotransmitter flow between a neuron and an adjacent cell
Synapse mechanism
1) AP travels down axon to terminal bouton
2) depolarization open voltage-gated calcium channels leading to calcium influx
3) Ca2+ influences cytoskeleton and causes fusion of neurotransmitter filled vesicles with plasma membrane
4) Neurotransmitter released into synaptic cleft and diffuses across to postsynaptic membrane
5) NT binds to receptors on postsynaptic membrane leading to graded potential
Types of fast post synaptic graded potentials
- excitatory post-synaptic potential (EPSP)
- inhibitory post-synaptic potential (IPSP)
- grand post-synaptic potentials (GPSP)
EPSP (def)
Depolarize graded potential in postsynaptic neuron
IPSP (def)
Inhibits depolarization in postsynaptic neuron
- hyperpolarization from K+ efflux
- stabilization by Cl- channels opening
GPSP (def)
The cumulative change in membrane potential, all EPSP and IPSP
Two methods to enhance a GPSP to initiate an AP
1) temporal summation: summation of two stimuli from the same neuron that follow one another in time
2) spatial summation: summation of graded potentials from several sources
Common Neurotransmitters
1) Acetylcholine
2) Biogenic Amines- amino acid derivatives
- primarily catecholamines
- Epi, NE, DA
3) Amino acids
4) Miscellaneous
All postsynaptic receptors work by either
- ligand gated ion channels
- G-protein coupled receptor
Postsynaptic Receptors
-Cholinergic: respond to Ach
1) nicotinic: receptors act as Na channel
-found in skeletal muscle
-found in autonomic PNS and CNS
2) muscarinic: utilizes G-protein and 2nd
messenger system
- multiple subtypes of receptor
- found in autonomic PNS and CNS
-Adrenergic: respond to Epi/NE
1) alpha 1,2: utilize 2nd messenger system
2) beta 1,2: utilize cAMP 2nd messenger
system
-found in autonomic PNS
Enzymes that degrade NTs
-Acetylcholinesterase (AchE): degrades Ach
-Monoamino oxidase (MAO)
-catechol-o-methyltransferase (CoMT):
degrade Epi/NE
