5.3 Neuronal communication Flashcards
stimulus definition
change in energy levels in environment
receptors definition
specialised cells that detect stimulus
transducers
why receptors are transducers
converts one type of energy into another
different receptors and energy
rods and cones (in eyes): light -> electrical
specialised hairs in ears: kinetic -> electrical
chemoreceptors in taste buds: chemical -> electrical
olfactory chemoreceptors in nose: chemical -> electrical
how touch receptors work
Pacinian corpuscles pressure on skin causes connective tissue deforming it sodium ion channels distort and open sodium ions diffuse into axon produced a.p.
sensory neurone function
carries action potential from sensory receptor
sensory neurone structure
long dendron
short axon
relay neurone structure
short dendrites
no dendron
short axon
relay neurone function
connects sensory and motor neurones
motor neurone function
carried action potentials from CNS to effector
motor neurone structure
short dendron
long axon
neurone general structure
can be long (transmit a.p. long distances)
plasma membrane gated ion channels (uses ATP to pump ions in and out to maintain potential difference across plasma membrane)
cell body (contains nucleus, lots of mitochondria, ribosomes)
dendrites and dendron (connect to other neurones, carry a.p. towards cell body)
axon (carries a.p. away from cell body)
myelin sheath around axon and dendron (series of Schwann cells)
myelinated neurones features
myelin sheath
series of Schwann cells associated and wrapped tightly around neurone
nodes of Ranvier (2-3um gaps every 1-3mm along neurones
wider neurone
why myelination speeds up transmission of a.p.
myelin sheath wrapped tightly around neurone
prevents movement of ions across neurone plasma membrane
ion can only move across at nodes of Ranvier
impulse jumps from one node to the next (saltatory conduction)
conduction is more rapid
non-myelinated neurone features and why they are slower at transmission
Schwann cells associated
several neurone enshrouded in one loosely wrapped Schwann cell
a.p. travels along neurone in wave instead of jumping from node to node (local currents)
narrower
advantages of myelination
myelinated neurones able to carry a.p. over long distances more quickly
enables faster response to stimulus
where non-myelinated neurones tend to be used
shorter distances
occurs in neuronal body cells and dendrites (grey matter)
coordination body functions e.g. breathing, digestive system where speed of transmission not so important
normal resting state of axon
resting potential
when neurone is at rest (no stimulus)
p.d. = -70mv
membrane is polarised
potential difference definition
difference in electrical charge
how resting potential is established
sodium-potassium ion pumps 3 Na+ out, 2K+ in
K+ leaks out through open potassium ion channels
anions are also inside of axon
membrane polarised
depolarisation method
axon is stimulated
Na+ channels open (becomes permeable to Na+)
influx of Na+ diffuses into axon
causes inside of axon to become more positive (depolarises)
potential difference reaches -55mV (threshold is met)
potential difference reaches +35 mV (with the help of positive feedback causing nearby voltage-gated Na+ channels to open)
action potential moves along axon
Na+/K+ pump keeps operating
repolarisation method
0.5 ms after depolarisation
voltage-gated Na+ channels close
voltage-gated K+ channels open
membrane impermeable to Na+ and permeable to K+
K+ flood out of axon
inside of axon becomes negative again compared to outside
hyperpolarisation method
K+ channels remain open
inside temporarily becomes too negative until resting potential is reached
refractory period
follows a.p. along neurone
in absolute refractory period, no impulse can be generated
in relative refractory period, impulse can only be generated if stimulus more intense than normal threshold level
voltage-gated Na+ channels close (stops another impulse from being generated) so resting potential can be restored
ensures impulses are separated, only pass in one direction along axon
all-or-nothing response
if threshold value met, action potential is generated
if not, generator potential is generated
threshold value
-55mV
why hyperpolarisation occurs
K+ channels close too slowly
too much K+ diffuse out
generator potential
generated when threshold value is not met (-55mV)
how action potentials are transmitted (local currents)
stimulus causes opening of Na+ channels
Na+ diffuses into neurone
a.p. generated, disrupts resting potential ion balance
higher conc of Na+ where they enter neurone, lower conc of Na+ to the side
Na+ ions diffuse sideways towards negative region
movement of ion = local current
continues along neurone
how voltage-gated sodium ion channels work
Na+ diffuse along membrane
reduces p.d. across membrane
causes voltage-gated sodium ion channels to open
causes more Na+ to enter membrane (example of positive feedback)
saltatory conduction definition
when impulses jump from one Node of Ranvier to another
excitatory neurotransmitter features
result in depolarisation of post synaptic neurone
if threshold reached in postsynaptic neurone, a.p. is triggered
e.g acetylcholine
inhibitory neurotransmitter features
results in hyperpolarisation of post synaptic membrane
prevents action potential from being triggered
e.g. GABA
transmission of impulses across synapse method
a.p. arrived at pre-synaptic neurone
causes Ca+ channels to open, Ca+ diffuse into pre-synaptic knob
causes synaptic vesicles to move to pre-synaptic membrane
vesicles fuse with membrane, releases acetylcholine into synaptic cleft via exocytosis
acetylcholine diffuses across synaptic cleft
binds to receptors on post-synaptic membrane
Na+ channels open on post-synaptic membrane, Na+ diffuses into post-synaptic membrane
causes it to be depolarised , a.p. generated at post-synaptic neurone
acetylcholinesterase definition
enzyme that hydrolyses acetylcholine into acetic acid and choline in synaptic cleft
why acetycholine in hydrolysed
stops continuous production of action potential in post-synaptic neurone
enables repolarisation of post synaptic membrane by unblocking receptors (stops Na+ channels from staying open)
recycles acetylcholine
what happens after hydrolysis of acetylcholine
acetic acid and choline diffuse back into pre-synaptic knob
combined back into acetylcholine using ATP
stored into vesicles
where myelinated neurone used and why
voluntary muscles
occurs along long axons within nervous system (white matter)
role of synapse
transmit information between neurones
ensure one way transmission of impulses (vesicles with Ach only in presynaptic knob, receptors for Ach only on post-synaptic membrane)
acclimatisation (fatigue and stop responding to stimulus as it runs out of neurotransmitters, helps avoid overstimulation of effectors that may cause damage)
divergence of nervous pathways
why 1 action potential in pre-synaptic neurone may not result in action potential in post-synaptic neurone
low level stimulus may generate a.p.
may not cause release of enough ach vesicles to cause a.p. in post synaptic neurone
just causes generator potential (excitatory post-synaptic potential = EPSP)
won’t reach threshold value
helps avoid overstimulation
how a.p. is generated in post-synaptic neurone when only low level stimulus present
several a.p. generated in short time
causes more vesicles of ach to be released
in post-synaptic neurone, several EPSPs combine to produce a.p.
temporal summation definition
several generator potentials come from same pre-synaptic neurone to create action potential in postsynaptic neurone
spatial summation definition
many a.ps arrive from converting pre-synaptic neurones causes few vesicles each to be released into same synapse, causing a.p. in postsynaptic neurone
