Neuronal Communication Flashcards
What is a nerve
Bundle of neurones
State 4 features of a neurone
Cell body
Dendron
Dendrites
Axon
Describe cell body
Has nucleus surrounded by cytoplasm
Large numbers of ER + mitochondria = involved in production of neurotransmitters
Describe dendron
Short extension from cell body
Transmit electrical impulses TOWARDS cell body
Describe dendrite
Smaller branches extended from dendron
Transmit electrical impulses TOWARDS cell body
Describe axon
Singular elongated nerve fibres
Transmit impulses away from cell body
Can be very long
Fibre is cylindrical + has narrow region of cytoplasm surrounded by plasma membrane
Describe sensory neuron
Transmit impulses form sensory receptor cells to relay neurone or motor neurone or brain
1 dendron – carries impulse to cell body
1 axon – carries away from cell body
Relay neurone
Transmit between neurones
Many short axons + dendrons
describe a motor neurone
Transmit from relay neurones or sensory neurone to effector
1 long axon
Many short dendrites
draw a sensory relay + motor neurone
label
- dendron
- dendrites
- axon
- cell body
- myelin sheath
what is myelin sheath
protective covering that surrounds and insulates axons in the nervous system.
how is myelin sheath produced
Schwann cells
produce these layers of membrane by growing around the axon many times
Each time they grow around – a double layer of phospholipid bilayer is laid
what is the purpose of myelin sheath
enhance the speed and efficiency of nerve signal transmission.
as an electrical insulator - preventing the loss of electrical signals + allowing the nerve impulses to travel more rapidly along the axon.
what are nodes of ranvier and what is its function
small gaps or nodes that exist between adjacent myelin sheath segments
essential for the saltatory conduction of nerve impulses
whats the size of nodes of ranvier
2-3 micrometres
what do sensory actors act as
transducer – convert energy
convert stimulus into nerve impulse called generator potential
examples of sensory receptors
photoreceptors - light
chemoreceptors - chemicals
mechanoreceptors - mechanical strain
baroreceptors - blood pressures
osmoreceptors - body fluids
what do olfactory cells detect
Presence of volatile chemicals
what do taste buds detect
Presence of soluble chemicals
what does the cochlea detect
Vibrations in air / sound waves
what does the pacinian receptor detect
mechanical pressure
where are pacinian receptors found
feet / fingers / external genitalia / joints
what does a pacinian corpuscle consist of
consists of a single sensory neurone, surrounded by layers of connective tissue which are each separated by a gel = lamella
gel between the layers contains Na+
what channels are in the membrane of pacinian corpuscle
stretch-mediated sodium ion channels
describe how an action potential is formed in the pacinian corpuscle
Resting state – stretch mediated ion sodium channels too narrow for Na+ to pass through
at resting potential
When pressure applied = corpuscle changes shape + membrane stretches
Sodium ion channels widen + Na+ diffuse in
Influx of positive sodium ions depolarises cell – generator potential
Creates action potential
describe how an action potential is created in a photoreceptor cell
In dark – membrane of rod cells depolarised at their resting state
This is because – rod cells actively transport sodium ions out of cell which flow back in
This depolarisation – triggers release of neurotransmitters = inhibit bipolar neuron = no action potential sent to brain
In light – light causes rhodopsin to break into retinal and opsin
Causes sodium ion channels to close
Still sodium actively transported out but no come back in
Charge difference across membrane
Inside is more negative – cell is hyperpolarised
Stops releasing neurotransmitters
No more inhibition of bipolar neuron
Now can send impulse
what does the term resting potential mean
in a resting axon inside of the axon always has a more negative electrical potential compared to the outside of the axon
what is the normal resting potential
-70 mV
what are the two causes of the resting potential
active transport of sodium ions and potassium ions
Differential membrane permeability
how is a resting potential established
Sodium-potassium pumps in membranes – carrier proteins
Use ATP to pump 3 sodium ions out for every 2 potassium ions in
Larger conc of positive charge on outside
Establishes electrochemical gradient
Now more Na+ on outside and more K+ on inside
K+ channels open and Na+ channels closed
K+ diffuse out of cell
More +ve on outside
Channels are less soluble to Na than K
what is a nerve impulse
action potential that starts at one end of a neurone + is propagated along axon to other end of neurone
state the 5 stages of how an action potential is generated
stimulus
depolarisation
repolarisation
hyperpolarisation
restoration
describe how an action potential is generated
Stimulus triggers sodium ion channels in membrane to open
allows sodium ions to diffuse down electrochemical gradient into cell
Voltage-gated sodium channels in the axon membrane open
(Negative protein attracted to positive inner cell)
Sodium ions come into axon down electrochemical gradient - sudden influx
(Positively charged sodium attracted to negative inside axon + conc gradient)
More sodium ions in = inside less negative= depolarisation
Change in charge causes more channels to open = more sodium
Positive feedback
Reach +40mV
Positive protein repelled by positive inside cell and blocks channel for sodium channel
No more sodium coming in
Negative protein on potassium channel attracted to positive inside cell =opens channel
Potassium ions diffuse out – down conc gradient
Returns potential difference back to normal = -70mv
Negative feedback
Potassium ion channels = slow to close and as a result
too many potassium ions diffuse out of the neurone
short period of hyperpolarisation
briefly more negative
once closed – sodium-potassium pumps restore resting potential
describe the stimulus stage in generating an action potential
Stimulus triggers sodium ion channels in membrane to open
allowing sodium ions to diffuse down electrochemical gradient into cell
describe the depolarisation stage in generating an action potential
Voltage-gated sodium channels in the axon membrane open
(Negative protein attracted to positive inner cell)
Sodium ions come into axon down electrochemical gradient // sudden influx
(Greater conc of sodium on outside)
(Positively charged sodium attracted to negative inside axon)
More sodium ions in = inside less negative= depolarisation
Change in charge causes more channels to open = more sodium
Reach +40mV
describe the repolarisation stage in generating an action potential
Now +40 mV
Positive protein repelled by positive inside cell and blocks channel for sodium channel
No more sodium coming in
Negative protein on potassium channel attracted to positive inside cell =opens channel
Potassium ions diffuse out – down conc gradient
Returns potential difference back to normal = -70mv
describe the hyperpolorisation stage in generating an action potential
Potassium ion channels = slow to close and as a result
too many potassium ions diffuse out of the neurone = short period of hyperpolarisation
briefly more negative
describe the restoration stage in generating an action potential
once closed – sodium-potassium pumps restore resting potential
what voltage is the cell depolarised
+40mV
what type of process is depolorisation
positive feedback
what type of process is repolorisation
negative feedback
draw out an action potential graph + label all the stages
how are action potentials transmitted along an axon
when action potential happens in one = stimulate action potential in adjacent part of neurone
happens because sodium ions that diffuse into the neurone diffuse sideways causes voltage-gated ion channels in the next portion to open
sodium ion channels open there too = more positive inside
the wave moves away from part of neurone that just fired action potential
that neurone will be in refractory period / cannot be stimulated because sodium ion channels remain closed
what is the refractory period
period of time when both sodium ion channels are closed ( repolarisation)
and potassium ion channels are closed
( hyperpolarisation).
whats the significance of a refractory period
Ensure discrete event – stopping them from merging together
Ensure new action potentials are generated ahead – only in one direction
state the factors that affect speed of conduction
Myelination
The diameter of the axon
Temperature
how does myelin sheath affect speed of conductioj
increases speed as it allows for saltatory conduction
what is saltatory conduction
In myelinated segments depolarisation cant occur = myelin sheath stops diffusion of Na + / K+
Action potentials can only occur – nodes of Ranvier (small uninsulated sections of axon)
nerve impulses jump from one section to the next – saltatory conduction
how does axon diameter effect speed of conduction
Impulse will be quicker along thicker axons
Thicker axons – axon membrane with greater surface area – greater rate of diffusion
Less resistance to flow of ions in cytoplasm
how does temperature effect speed of conduction
Higher temp = faster nerve impulse
Ions diffuse faster but only up to 40 degrees = proteins denature
what is the all or nothing principle
If stimulus is strong enough to increase cell potential above threshold potential – stimulate action potential
if not - no action potential
what do difference strengths of stimulus result in
different frequencies that action potentials are being fired
the bigger the stimulus = more often an action potential will occur along the neurone.
what is a synaptic cleft
gap between axon of one neurone and dendrite of next
how large is a synaptic cleft
20-30nm across
what is the presynaptic neurone
neurone which impulse has travelled along + has vesicles containing neurotransmitters
what is the postsynaptic neurone
neurone that receives neurotransmitter
what is a synaptic knob
swollen end of presynaptic neurone
– has mitochondria + a lot of ER to make neurotransmitters
what are synaptic vesicles
vesicles that contain neurotransmitters in
fuse with presynaptic membrane + release into cleft
where are neurotransmitter receptors found
on postsynaptic membrane
2 types of neurotransmitter
excitatory
inhibitory
function of excitatory neurotransmitter + example
Result in depolarisation of the postsynaptic neurone
Opens sodium ion channels = action potential
acetylcholine
function of inhibitory neurotransmitter + example
Result in hyperpolarisation of postsynaptic neurone
Opens potassium ion channels – no action potential
GABA
what is a cholinergic synapse
Uses Ach – acetylcholine neurotransmitter
where are cholinergic synapses found
Common in CNS of vertebras + at neuromuscular junctions = where motor neurone + muscle cell meets
describe how an action potential is transmitted across a cholinergic synapse
Action potential at the presynaptic membrane causes depolarisation of membrane
Stimulates voltage-gated calcium channels to open
Ca 2+ diffuse down electrochemical gradient
(from tissue fluid surrounding the neurones into the synaptic knob)
Stimulates Ach containing vesicles to fuse with presynaptic membrane (snare proteins shorten)
releases Ach molecules into synaptic cleft by exocytosis
Diffuse across + bind with choline receptors in post synaptic membrane
Sodium ion channels to open
Diffuse into the cytoplasm of postsynaptic neuron
Causes depolarisation – re starting electrical impulse once threshold is reached
Ach broken down + recycled
ATP released by mitochondria recombines choline + ethanoic acid
what enzyme catalyses the break down of Ach
acetylcholinesterase
what does acetylcholinesterase do
hydrolyses Ach into choline and ethanoic acid (acetyl)
whats the significance of breaking down Ach
Prevents sodium channels staying open
Stops permanent depolarisation of post synaptic membrane
why are synapses important
Unidirectionality
Allow impulse from one neurone to be transmitted to a number of neurones at multiple synapses = single stimulus creating a lot of responses
Number of neurones may feed into same synapses – stimuli from different receptors
what ensures unidirectionality across a synapse
Neurotransmitter from one side + receptors from another
how can a single impulse be insufficient to generate an action potential
Only small amount of acetylcholine is released into the synaptic cleft
small number of the gated ion channels are opened in the axon membrane
insufficient number of sodium ions pass through the membranes
threshold potential is not reached
small amount of acetylcholine attached to receptors is broken down rapidly by acetylcholinesterase
what is summation
process by which postsynaptic potentials are combined to determine whether an action potential is generated in the postsynaptic neurone
whats the benefits of summation
avoids nervous system being overwhelmed
synapses act as barrier = slows down rate = as only allowed to pass on if has input from other neurones
what is temporal summation
multiple impulses arrive within quick succession
impulses can be ‘added’ together to generate an action potential
describe how temporal summation work
many action potentials arrive in quick succession
effects cumulate together
large amount of acetylcholine is released into the synaptic cleft
large number of the gated sodium ion channels open
sufficient number of sodium ions pass through the membrane
what is spatial summation
Multiple impulses arriving simultaneously at different synaptic knobs stimulating the same cell body
