Please enable JavaScript.
Coggle requires JavaScript to display documents.
neuronal communication and homeostasis (communication and homeostasis…
neuronal communication and homeostasis
communication and homeostasis basics
responding to environment helps organisms survive
stimulus= any change in internal or external environment
they respond to changes in internal environment to ensure that conditions are at optimum for metabolism
receptors detect stimuli
receptors are specific to one particular stimulus e.g. light
different types of receptor detects different stimuli
some receptors are cells and some are proteins (phosphoproteins) on cell surface membrane
effectors= cells that bring about a response to the stimulus
communication
cell signalling is a method of communication which is needed to bring about a response
can be to adjacent cells (nervous system) cells secrete neurotransmitters which send signals to nearby cells e.g. muscle cells
can be to distant cells (hormonal communication) cells secrete hormones which travel in the blood and act as signals
homeostasis is the maintenance of a constant internal environment
changes in external environment can effect internal environment
homeostasis involves control systems that keep internal environment roughly constant
e.g. body temp
too high- enzymes will be denatured. enzymes bonds vibrate too much causing the hydrogen bonds to break which hold 3d shape. this changes shape of active site so it can no longer work as a catalyst so metabolic reactions are less efficient
too low- enzyme activity is reduced/ slowed, slowing rate of metabolic reactions
temperature has an optimum where enzyme activity is at highest
its also important to control blood glucose levels
homeostatic systems detect a change and respond by negative feedback
1) receptors detect when a level is too high/ too low and the information is communicated via the hormonal or the nervous system to effectors
2) effectors repsond to counteract change
3) mechanism restores level to normal
positive feedback mechanisms amplify a change from a normal level
the effectors respond to increase levels away from normal
e.g. blood clotting, contractions during labour
receptors and neurones
the nervous system send s informations as nerve impulses
sensory neurones= transmit nerve impulses from receptors to CNS (brain/ spinal chord)
motor neurones= transmit nerve impulses from CNS to effectors
relay neurones= transmit nerve impulses between sensory and motor neurones
stimulus--> receptor--> CNS--> effectors--> response
sensory receptors convert stimulus energy into nerve impulses
receptors convert stimulus energy into electrical energy (they are transducers)
1) when a nervous system receptor is at resting state there's a charge difference between inside and outside of cell this is generated by ion channels and pumps (voltage difference)
2) when stimulus is detected the cell membrane becomes excited and ore permeable allowing ions to move in and out of cell changing the potential difference. change in pd= generator potential
3) a bigger stimulus excites membrane more causing a bigger movement of ions so producing bigger generator potential
4) generator potential triggers action potential (nerve impulse) along a neurone, it is only triggered if generator potential exceeds threshold
pacinian corpuscles
detect pressure and vibrations and found on surface of skin and are attached to sensory nerve ending
when it becomes stimulated the lamellae are deformed and press on sensory nerve ending. this causes deformation of stretch mediated sodium channels. sodium channels open and they diffuse into cell creating generator potential which triggers action potential
neurone cell membranes are polarised at rest
in resting state the outside of membrane is more positively charged than inside (sodium potassium pumps- move 3 Na+ out for every K+ in)
membrane becomes polarised. the voltage across membrane is the resting potential (-60mv) which is maintained by sodium and potassium pumps which pump the ions out/in via active transport
the membrane isnt permeable to sodium (no channels open) so it cannot diffuse back into once actively transported out which creates an electrochemical gradient
membrane is permeable to potassium ions (some channels open) so they diffuse out via channels
this makes outside more positive than inside
structure of neurones
many are long to transmit action potential over a longer distance
cell membranes contain gated ion channels e.g. Ca2+, K+, Na+
ion pumps use atp to actively transport ions in/ out of cell
neurones maintains pd across the membrane
cell body contains: nucleus, ribosomes and many mitochondria
dendrites connect neurones together and carries impulse towards body
axon carries impulse away from body
neurones are surrounded by a myelin sheath made if Schwann cells
differences
motor neurones have cell body in CNS and have long axon
sensory neurones have long dendron and a short axon
relay have short dendrites and short axon
action potentials
neurone cell membrane becomes depolarised when stimulated
1) stimulus- this excites the neurone cell membrane causing sodium ion channels to open. membrane becomes more permeable to Na+ so they diffuse into cell down the sodium ion electrochemical gradient making inside less negative
2) depolarisation- if the potential difference reaches threshold (around -50mv) voltage gated sodium ion channels open so more sodium ions diffuse into cell (positive feedback) making inside more positive
3) repolarisation- at a pd of +40mv the Na+ channels close and voltage gated K+ channels open. the membrane is more permeable to K+ so they diffuse out of the neurone down the conc gradient this causes neurone to return to resting potential
4- hyperpolarisation- K+ channels are slow to close so there is an overshoot where too many K+ diffuse out of neurone. the pd may become slightly more negative than resting potential (refractory period- ensures action potentials are transmitted in 1 direction)
5) resting potential- sodium/ potassium pumps return membrane to resting potential
1) the membrane has a resting potential of -60mv and is polarised. there is a higher conc of Na+ outside than inside, there is a higher conc of K+ inside than outside
a bigger stimulus causes more frequent impulses
once the threshold has been reached the action potential will always fire with the same voltage no matter how big stimulus is
if threshold isnt reached no action potential will be created (all or nothing)
a bigger stimulus won't cause a bigger action potential but will cause them to fire more frequently
action potentials go faster in myelinated neurones
myelin sheath is an electrical insulator
in peripheral nervous system myelin sheath is made of schwann cells
gaps in myelin sheath are the nodes of ranvier (sodium ion channels are concentrated at these nodes)
in a myelinated neurone depolarisation only occurs in nodes of ranvier where sodium ions can get through membrane
the neurones cytoplasm conducts enough electrical charge to depolarise the next node so the impulse jumps from node to node (saltatory conduction- speeds up transmission)
in a non myelinated neurone the impulse travels as a wave along the whole axon membrane which is slower
myelination enables a more rapid response to stimulus
non myelinated neurones are shorter and carry action potential over shorter distances than myelinated
local currents
1) when action potential occurs sodium ion channels open at one point in membrane
2) sodium ions diffuse down conc gradient into neurone. the conc of Na+ rises at the point where the sodium channels are open
3) sodium ions diffuse sideways along the neurone away from the area of high conc. this movement of charged particles= local current
4) local current causes slight depolarisation further along neurone which effects the voltage gated sodium ion channels causing them to open, the open channels allow rapid influx of Na+ causing depolarisation further along the neurone causing the action potential to move along the neurone
synapses
a synapse is a junction between a neurone and the next cell
synaptic cleft= gap between cells at a synapse
the presynaptic neurone has a swelling called a synaptic knob which contains vesicles with neurone transmitters
when an action potential reaches the end of a neurone it causes neurotransmitters to be released into the synaptic cleft which diffuse across to the post synaptic membrane and bind to specific receptors
when neurotransmitters bind to receptors they trigger an action potential which stimulates a response
neurotransmitters are removed from the cleft to stop the response continuing e.g. they are taken back to the presynaptic membrane or broken down by enzymes
neurotranmitters e.g. acetylcholine and noradrenaline
the process
1) action potential triggers calcium influx:
action potential arrives at synaptic nob of presynaptic membrane
action potential stimulates voltage gated calcium ion channels to open
calcium ions diffuse into synaptic knob
2) calcium influx causes neurotransmitter release:
influx of calcium ions into the synaptic knob causes synaptic vesicles to move into presynaptic membrane and fuse with it
the vesicles release the neurotransmitter into synaptic cleft via exocytosis
3) the neurotransmitter triggers an action potential in the postsynaptic neurone:
the neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane
this causes sodium ion channels to open causing sodium ions to diffuse in causing depolarisation . an action potential is then generated if the threshold is reached
the neurotransmitter is then removed to prevent response continuing
actetylcholinerase
an enzyme found in synaptic cleft
hydrolyses acetylcholine into choline and ethanoic acid
stops transmission of signals so no more action potentials are produced
ethanoic acid and choline enter pre synaptic bulb by diffusion and are recombined using ATP to acetylcholine and stored in vesicles
synapses play vital roles in nervous system
synaptic divergence- when one neurone connects many neurones to different parts of body
synaptic convergence- when many neurones connect to one neurone information can be amplified
spatial summation- when neurones converge the small amount of neurotransmitter released from each could be enough to reach threshold and generate action potential . multiple stimuli can be coordinated into a single response
temporal summation- when two or more nerve impulses arrive at quick succession from the same presynaptic neurone which makes an action potential more likely
synapses make sure impulses are only transmitted one way
a small amount of acetylcholine diffusing across the membrane produces an excitatory post synaptic potential (EPSP) which isnt large enough to generate an action potential alone
summation= when the effects of several EPSP's combine to increase the membrane generation to threshold
some presynaptic neurones can produce inhibitory post synaptic potentials (IPSPs) which reduce effect of summation and prevent action potential being generated- lots of EPSPs can be stopped by 1 IPSP
low level action potentials can be amplified by EPSPs in summation
after repeat stimulation of a synapse it may run out of vesicles containing neurotransmitter so it will no longer respond to stimulus
creating reaction pathways in nervous system where action potential is more likely to be fired is done by increasing receptors
structure
pre synaptic bulb
contains lost of mitochondria- ATP is required for active transport
large amount of SER- packages neurotransmitter into vesicles
large number of vesicles- contains acetylcholine the neurotransmitter which diffuses across synaptic cleft
voltage gated calcium ion channels on cell surface membrane
post synaptic membrane
sodium ion channels that respond to neurotransmitter which consist of 5 polypeptide chains
2 of the polypeptide chains have a receptor site which is specific to acetylcholine and has a complementary shape
when acetylcholine is present it binds to the receptor sites causing channels to open