NERVOUS SYSTEM Flashcards
DORSAL ROOT GANGLION CONTAINS CELL BODIES OF WHAT?
SENSORY NEURONS
A BUNDLE OF FIBRES LOCATED WITHIN THE CNS IS A WHAT?
TRACT
ACTION POTENTIAL VS GRADED POTENTIAL DISTANCES
AP - short + long distances
GP - short distances only
WHAT DO LIGAND-GATED CHANNELS RESPOND TO?
chemical stimuli (ligand binds to receptor)
WHAT DO MECHANICALLY-GATED CHANNELS RESPOND TO?
mechanical vibration or pressure stimuli
WHAT DO VOLTAGE-GATED CHANNELS RESPOND TO?
direct changes in membrane potential
WHAT DO LEAK CHANNELS RESPOND TO?
they randomly open + close
RESTING MEMBRANE POTENTIAL
-> membrane of non-conducting neurons are positive OUTSIDE and negative INSIDE:
- unequal distribution of ions across plasma membrane + selective permeability to Na+ and K+
- most anions (-ve) cannot leave cell
- Na+/ K+ pumps
POTENTIAL ENERGY DIFFERENCE AT REST
-70mV
WHAT HAPPENS DURING AN AP?
neuron undergoes a rapid depolarisation of a large fixed size and then repolarises again
AP arises at trigger zone + propagates down the axon + each spike is followed by a refractory period
AP is an all or none phenomenon
magnitude of response given by spike rate (not spike size)
WHAT HAPPENS DURING A GRADED POTENTIAL?
neuron undergoes depolarisation (excitatory) or hyperpolarisation (inhibitory) of variable size
amplitude (size) of potential indicates response magnitude
GPs arise usually in dendrites, don’t propagate or have refractory periods
if enough GPs occur within an area of membrane, an AP may be generated
THRESHOLD OF MEMBRANE POTENTIAL
-55mV (initiates an AP)
ACTION POTENTIAL GENERATION
once membrane potential hits threshold:
voltage-gated Na+ activation gates open; Na+ rushes in + depolarises neuron until membrane potential reaches +30mV
voltage-gated K+ channels open; outflow of K+; Na+ channels inactivating + RMP reached
outflow of K+ continues; Na+ channels in resting state; K+ gates closing + RMP reached
REFRACTORY PERIOD
occurs after AP generation, neuron needs to rest ~0.4-4ms before next spike can occur
WHERE DOES AP ARISE?
at trigger zone (axon hillock) - propagates down the axon
ABSOLUTE REFRACTORY PERIOD
impossible to evoke another AP - Na+ channels are inactivated
RELATIVE REFRACTORY PERIOD
later, a stronger than usual stimulus is required to evoke an AP (part of Na+ channels recovered)
WHAT CHANNELS ARE ONLY PRESENT AT THE NODES?
Na+ channels
SALTATORY VS CONTINUOUS CONDUCTION
saltatory conduction occurs in myelinated axons only
continuous conduction occurs in unmyelinated axons only
THE LARGER THE AXON, WHAT HAPPENS TO IMPULSE PROPAGATION?
larger the axon, the faster the impulse propagation
TEMPORAL SUMMATION
many stimuli in a close span of time
repeated stimuli can have a cumulative effect + can produce a nerve impulse when a single stimuli is too weak
SPATIAL SUMMATION
many neurons firing simultaneously in the same location
synaptic input from several locations can have a cumulative effect + trigger a nerve impulse
combining of EPSPs + IPSPs across dendrite from simultaneous arrival of APs at various synapses
SYNAPTIC INTEGRATION
combining of excitatory + inhibitory signals acting on adjacent membrane regions of a neuron
for an AP to occur, sum of both excitatory + inhibitory postsynaptic potentials must be greater than a threshold value
ELECTRICAL SYNAPSE
electric current from one neurone is passed directly through a GAP JUNCTION (bidirectional)
sends simple depolarising signals
cell membranes aligned parallel
between large presynaptic neuron + small postsynaptic neuron (a lot of current to depolarise a cell)
CHEMICAL SYNAPSE
neurotransmitter released from presynaptic neuron
diffuses across the synaptic cleft
binds with a receptor on the postsynaptic membrane
fast - transmitter gated ion channels (uses amino acids)
slow - G protein coupled ion channels
NEUROTRANSMITTERS
- must be present in the presynaptic neuron
- must be released in response to presynaptic depolarisation + release must be calcium dependent
- specific receptors for it must be present on postsynaptic cell
SYNAPTIC TRANSMISSON
** transfer of info from end of axon of one neuron to the next
AP depolarises presynaptic membrane of synaptic terminal - Ca++ influx through voltage-gated channels
calcium activates proteins (sterine + neurine) attached to vesicles (containing a neurotransmitter) - pulling vesicles to membrane - opening vesicles + dumping their neurotransmitter contents into synaptic cleft (exocytosis - active transport)
neurotransmitter molecules diffuse across synaptic cleft + bind to receptors on subsynaptic membrane initiating response
WHAT ARE THE TYPES OF SYNAPTIC CONNECTIONS BETWEEN NEURONS?
AXODENDRITIC - axon terminal ends on a dendrite
AXOSOMATIC - axon terminal ends on a cell body
AXOAXONIC - axon terminal ends on another axon
DENDRODENDRITIC SYNAPSE - dendrite makes synapse with another dendrite
IONOTROPIC RECEPTORS
channel opens in response to ion binding
contains 4-5 subunits
fast speed of action
ligand-gated ion channels
METABOTROPIC RECEPTORS
channel opens in response to second messengers
contains 1 subunit
slow speed of action
G-protein coupled receptors
RHEOBASE
measure of membrane excitability
CHRONAXIE
minimum time required for an electric current double the strength of the rheobase to stimulate a muscle or a neuron
GRADED POTENTIAL
change in membrane potential that varies in size
EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP)
promotes excitation of postsynaptic membrane
GP that decays over time + space
cumulative effect of EPSPs are the basis for temporal + spatial summation
if Na+ ions are the carrier, MP of postsynaptic cell is depolarised
INHIBITORY POSTSYNAPTIC POTENTIAL (IPSP)
temporary hyperpolarisation of a membrane - prevents APs
if Cl- or K+ ions are carrier, MP of postsynaptic cell is hyperpolarised
SYNAPSE COMPOSITION
- presynaptic terminal
- postsynaptic cell
- zone of apposition
GAP JUNCTION CHANNELS
connect communicating cells at an electrical synapse
consist of a pit of cylinders (connexons)
always open
serve to synchronise the activity of a set of neurons
LENGTH CONSTANT
measure of how effective a given synapse is in contributing to spatial summation
gives distance that it takes an EPSP or IPSP to decrease 37% of its original value at synapse
directly proportional to membrane resistance
+ inversely proportional to longitudinal resistance of cytoplasm:
- leakier the membrane, shorter the length constant
- narrower the dendrite, shorter the length constant
WHY CHEMICAL TRANSMISSION?
more flexible; produces more complex behaviours
plasticity
can amplify neuronal signals
directly gated are comparatively fast
- mediate behaviour
indirectly are slower
- memory, learning
