nutritive and supportive connections between neurons and capillaries in the CNS
function of microglial cells
phagocytosis of damaged cells; the macrophages of the CNS
differences between oligodendrocytes and Schwann cells
oligodendrocytes in CNS and Schwann cells in PNS; oligodendrocytes myelinate many cells while Schwann cells myelinate only one cell
white matter
one of two components of the CNS; consists mainly of myelinated cells; appears white because myelin is made up mainly of lipids
blood-brain barrier
separates circulating blood from cerebrospinal fluid; composed mainly of tight junctions in capillary endothelial cells; filters blood plasma; permits entry of ions but not proteins into CSF
functions of blood-brain barrier
prevention of entry of pathogens into CNS; maintenance of homeostasis in CSF
choroid plexus
specialized tissue that secretes cerebrospinal fluid into the ventricles; found in multiple locations in the ventricles of the brain; filters blood and actively transports ions to produce CSF
arachnoid villi
small protrusions of arachnoid mater in the brain that uptake circulating cerebrospinal fluid and allow it to re-enter the bloodstream
Wallerian degeneration
"the process of axonal degeneration following nerve damage to the axon & separation of the axonal process from the cell body; occurs in both the CNS and the PNS; ""clean-up"" assisted by microglia"
chromatolysis
lysis of neuronal chromatin; occurs in response to cellular damage; involves swelling of the cell and dispersal of the rER
steps in electrical signaling
input; summation; conduction; output
axon hillock
the most electrically sensitive part of the axon; has the lowest threshold for an action potential; initiation site for action potential; involved in summation
resting membrane potential
the difference in voltage between the interior and exterior of a quiescent cell; primarily determined by potassium in neurons
Nernst equation
used to calculate the membrane potential based on the internal and external concentrations of a single ion; assumes that only that ion is permeable
relative ion concentrations
sodium low inside & high outside; potassium high inside & low outside; chlorine low inside & high outside; calcium very low inside & low outside
Goldman-Hodgkin-Katz equation
used to calculate the membrane potential based on the internal and external concentrations of several monovalent ions
mechanism of unidirectional action potential propagation
sodium influx provokes depolarization of membrane; depolarization provokes opening of neighboring sodium channels; opening of sodium channels leads to further sodium influx; channels are temporarily inactivated after opening to prevent reversal of direction
voltage clamp
used by electrophysiologists to measure ion flow while holding membrane potential steady; used by Hodgkin and Huxley to elucidate ionic causes of action potentials
command potential
the electric potential at which a cell membrane is held in experiments involving voltage clamps
space constant
represented by lambda; describes the distance that passive voltage will spread along an axon or dendrite; depends on the resistance across and inside the membrane; depends on the number of open ion channels and the diameter of the cell process
time constant
describes the speed at which a cell membrane potential changes; represented by tau
sodium-potassium pump
ATP-driven enzyme that pumps 3 sodium ions out of the cell for every 2 potassium ions it pumps into the cell
evidence for existance of a sodium-potassium pump in neurons
removal of external K+ decreases rate of Na+ transfer; blockage of ATP synthesis decreases rate of Na+ transfer
mechanism of sodium-potassium pump action
sodium binding on inner cell membrane; phosphorylation of membrane receptor; conformational change of membrane receptor; sodium release; potassium binding; dephosphorylation of receptor; potassium release
synapse
a junction for communication between two cells
areas in which synaptic transmission occurs
between two neurons; between a pre-synaptic neuron and a post-synaptic non-neuronal target; between a pre-synaptic sensory cell and a post-synaptic neuron
types of synapses
electrical and chemical
differences between electrical and chemical synapses
electrical synapses are much smaller; electrical synapses have cytoplasmic continuity while chemical synapses do not; electrical synapses have no synaptic delay while chemical synapses have variable-length synaptic delays; electrical synapses are theoretically bidirectional while chemical synapses are unidirectional
gap junction plaque
occurs at electrical synapses; made up of connexon units that form channels; allow passage of small molecules such as cyclic AMP
synaptic densities
visible as thick black lines on EM of a synapse; composed of exocytotic machinery on the presynaptic side and neurotransmitter-binding receptors on the postsynaptic side
active zone
the region of the presynaptic plasma membrane at which synaptic vesicles dock
synaptic folds
found in neuromuscular junctions; postsynaptic membrane displays folding
steps in chemical signaling at the neuromuscular junction
depolarization of nerve terminal; opening of voltage-gated calcium channels; calcium influx; exocytosis of ACh; activation of nAChRs; opening of sodium and potassium channels; depolarization of post-synaptic cell membrane; conformational change of calcium channels on sarcoplasmic reticulum; calcium release; muscle contraction
neurotransmitter families
small molecues; peptides and polypeptides
examples of NTs: small molecules
amines - dopamine and norepinephrine; amino acids - glutamate and GABA and glycine; nucleosides and nucleotides; lipids
examples of NTs: peptides and polypeptides
hypothalamic peptides - oxytocin and vasopressin; gut peptides - somatostatin and vip; endorphins
differences between small molecule and peptide/polypeptide neurotransmitters
small molecules are synthesized locally while peptides are synthesized at the cell body and transported; small molecules are packaged in small clear vesicles while peptides are packaged in large dense core vesicles; small molecule vesicles are recycled while peptide vesicles are not; small molecules require low Ca2+ concentration for release while peptides require high Ca2+ concentration for release
acetylcholine
a small molecule neurotransmitter syntheized in the cytosol from acetate and choline; occurs in neuromuscular junctions
two processes by which neurotransmitter is exocytosed
"""kiss and run"" or complete fusion/conventional vesicle exocytosis"
"""kiss and run"" neurotransmitter release"
a process by which vesicles release neurotransmitter into the synaptic cleft; involves incomplete fusion with the cell membrane and formation of a fusion pore; involves vesicle recycling; occurs when large amounts of neurotransmitter are being released
conventional vesicle exocytosis
a process by which vesicles release neurotransmitter into the synaptic cleft; involves complete fusion with the cell membrane; involves recovery of the vesicle membrane via clathrin-mediated endocytosis
SNARE hypothesis
describes the mechanism by which vesicle fusion occurs; involves recognition of vesicle SNARES by target SNARES on the cell membrane
t-SNARES
target SNARES; protein complexes on the cell membrane that interact with v-SNARES to promote vesicle docking
v-SNARES
vesicle SNARES; protein complexes on the vesicle that interact with t-SNARES to promote vesicle docking
role of coiled-coil interactions in exocytosis
the coiled-coil protein domains of v-SNARES and t-SNARES interact to pull the vesicle into a docked position on the membrane for exocytosis
synaptotagmin
a protein involved in exocytosis; two C2 domains bind calcium; acts as a calcium sensor
tetanus toxin
a toxin that blocks inhibitory neuronal transmission via proteolysis of the SNARE family VAMP-2; causes rigidity and spasms
botulism toxin
a toxin that blocks excitatory neuronal transmission via proteolysis of the SNARE family VAMP-1 at the NMJ; causes flaccid paralysis
local synaptic potentials
small graded electric responses provoked at the postsynaptic membrane; can be excitatory or inhibitory
factors upon which EPSP amplitude depends
quantity of neurotransmitter released; quantity of neurotransmitter that reaches receptors; number of receptors activated
miniature EPSPs
spontaneous EPSPs that result in the absence of presynaptic stimulation; result from the release of one vesicle's contents
quantal gradation of EPSPs
EPSP amplitude is quantal because it depends on the number of presynaptic vesicles released
post-synaptic excitation
excitatory post-synaptic potentials increase the probability that an action potential will occur post-synaptically; usually involve depolarization
post-synaptic inhibition
inhibitory post-synaptic potentials decrease the probability that an action potential will occur post-synaptically; usually involve hyperpolarization
ionotropic receptors
are linked directly to post-synaptic ion channels; mediate fast synaptic transmission; decay is due to diffusion or hydrolysis of neurotransmitter
metabotropic receptors
movement of ions through post-synaptic channels depends on intermediary molecules; mediate slow synaptic transmission; decay depends on lifetime of second messengers
examples of ionotropic receptors
nAChRs; GABAARs; NMDA-Rs
examples of metabotropic receptors
G-protein couple receptors such as beta-adrenergic receptors
beta-adrenergic receptors
a family of G-protein coupled receptors; stimulate heart contraction; stimulation leads to a cascade that eventually results in phosphorylation of an ion channel; compete with mAChRs (inhibitory)
neurotransmitter inactivation
NTs can be inactivated by diffusion or reuptake or enzymatic degradation
steps in chemical synapse transmission
presynaptic events; neurotransmitter diffusion across cleft; binding of NT to post-synaptic receptors; activation of ionotropic or metabotropic receptors; postsynaptic potentials; transmitter inactivation
myasthenia gravis
an autoimmune disease that affects nAChRs; antibodies decrease nAChRs at the NMJ by causing them to cluster and be internalized and degraded; causes muscle fatigue
electromyogram; records summation of action potentials in muscle fibers
bungarotoxin
a toxin that blocks nicotinic acetylcholine receptors
factors that affect the strength of synaptic transmission
quantity of neurotransmitter per vesicle; number of vesicles available; number of vesicles released; number of post-synaptic receptors; sensitivity of receptors to transmitter; transmitter inactivation