Nervous system: action potentials, graded potentials, and synaptic transmission
steps of synaptic transmission
1. action potential arrives 2. VG Ca2+ open, increasing intracellular Ca2+ 3. Causes vesicles with neurotransmitter to exocytose, releasing neurotransmitters to synaptic cleft 4. neurotransmitters binds, leading to change in ion permeability in post-synaptic cell 5. degradation enzymes OR reuptake proteins on pre-synaptic cell diminish neurotransmitter concentration in synapse
steps of action potential
1. initial depolarization; if sufficient enough, VG Na+ will open 2. VG Na+ open, Na+ rushes in 3. VG K+ open but its effects are unseen since VG Na+ are still open 4. VG Na+ inactivate 5. VG K+ still open and can see its effects as K+ rushes out 6. VG K+ inactivate. Na/K pump causes return back to resting membrane potential
why is propagation unidirectional?
At each site of depolarization there is a refractory period such that when its neighboring ion channels open, it does not cause them to re-open again.
what if there a potential much larger than threshold?
actions retains same amplitude, same type of action potential
open state of VG channels
allows ions to pass through down electrochemical gradient
myelinating cells
assist with conductance of electrical messages, insulate axons with fatty myelin sheaths
anterograde transport
away from soma
where in the neuron does it decide whether to fire an action potential?
axon hillock; has highest amount of VG channels
relative refractory period
bigger depolarization is required to get to threshold; number of available VG Na+ channels still isn't 100%
central nervous system
brain and spinal cord, processes incoming information
afferents
carry information to CNS
connexins
collection of 6 connexons. forms basis of these gap junctions found at electrical synapses
importance of cytoskeleton in neuron
critical for maintaining physical features, helps to quickly move material from one soma to other parts of neuron
spatial summation
different synapses are active at the same time; could be all EPSPs, all IPSPs, or a mix of both
peripheral nervous system
everything that's not brain and spinal cord, carries information to CNS and deliver motor commands to the rest of the body
myelinated axon propagation (saltatory conduction)
faster, leak channels are covered by the myelin, electrical signals stay stronger over longer distances, less interactions with membrane
neuron
functional unit of the nervous system; use changes in membrane potential to conduct information both within same cell and between cells, communicate information, process incoming sensory information, initiate changes in physiology and body movement
propagation
how information spreads down length of axon, regeneration of action potentials
inhibitory post-synaptic potential (IPSP)
hyperpolarization/maintain resting membrane potential in post-synaptic cell (increases in K+ or Cl- permeability)
both action potentials and graded potentials
ion movement, changes in membrane potentials
synapse
junction between neuron and target cell. there's electrical and chemical synapses (latter is more common in body)
depolarization
less negative than rest, moving more positive from rest
action potential
long distance, actively regenerated, all or nothing response to depolarization at the axon, large and standard changes in membrane potential, require voltage-gated ion channels, systemic opening of Na+ VG channels
astrocytes
monitor nutrients/waste, maintaining proper ECF ion concenetrations
absolute refractory period
most of VG Na+ are inactive; no way for Na+ to move across membrane; exists as we climb action potential and persists until close to rest
repolarization
moves back to rest following depolarization
hyperpolarization
moves more negative from rest
oligodendrocytes
myelinating cells of the CNS
Schwann cells
myelinating cells of the PNS
interneurons
neurons that are completely within the CNS, involved in processing of information
why are membrane potentials important to a neuron?
neurons use changes away from resting membrane potential to transmit electrical information within and between cells; presence of various voltage-gated ion channels are embedded within membrane
what if there's not a large enough depolarization to open VG Na+?
no action potential; only sub-threshold potentials
inactive state of VG channels
no ion movement because it's closed and not allowed to open
closed state of VG channels
no ion movement but can still be opened with stimulus
excitatory post-synaptic potential (EPSP)
post-synaptic cell experiences depolarization (increases in Na+ permeability, could generate action potential)
gap junctions
protein tunnels that interact with each other; extend from each cell
post-synaptic density
proteins just beneath dendrites of post-synaptic cell that help stabilize receptors and anchor them
ionotropic receptors
receptor and ion channel are the same protein, quick
metabotropic receptrs
receptor and the ion channel it influences are two different proteins, utilize G proteins, takes longer, slower response to neurotransmitter binding, effects are longer lasting/acting
temporal summation
same synapse is active very close in time; either all EPSPs or all IPSPs because it's the same synapse
graded potential
short distances, not actively regenerated, decay pretty rapidly, most found in dendrites, caused by the opening of ion channels, magnitude depends on stimulus strength (smaller stimulus = smaller magnitude), confined to small region on membrane, decay quickly with space and time
unmyelinated axons propagation
slower, entire membrane works to regenerate actin potential because leak channels allow signal to dissipate quickly
microglia
specific to neural tissue; immune-like cells
glial cells
support neurons, 90% of the nervous system
summation
synaptic integration; process by which the axon hillock takes into account all incoming inputs
efferents
take information away from CNS
retrograde transport
toward soma
chemical synapse
translates electrical messages to chemical messages that diffuse to post-synaptic cell; information becomes electrical again in post-synaptic cell, slower, greater flexibility due to physical disconnection
electrical synapse
travels directly from cytosol of pre-synaptic cell to post-synaptic cell cytosol via gap junctions, fast, found where info transmission has to be quick, less flexible
axon hillock
where a neuron decides whether to send an action potential