Neuronal Communiation Flashcards
What are neurones and nerves?
Specialised cells of nervous system which carry electrical impulses around body
Bundle of neurones = NERVE
Features fund in all types of neurone?
Axon
Cell body - contains nucleus/other cellular structures
Axon terminal - end of axon , contain many nerve endings
These nerve endings allow neurones to connect to many other neurones /receive impulses from them , forming network for easy communication
What is meant when some neurones are myelinated? How is it beneficial?
Axon is INSULATED by a myelin sheath , with small uninsulated sections along its length —> NODES OF RANVIER
Myelin sheath made of specialised cells —> Schwann cells
So electrical impulses don’t travel down whole axon- jump from one node to next , reducing time wasted transferring impulse from 1 cell to another
Why does impulse travel more slowly non myelinated neurones ?
Axon is uninsulated so impulse travels along entire length of axon
- travels more slowly
- whole axon must be depolarised
3 types of neurone and function?
Sensory : carry impulses from RECEPTORS —> CNS (brain or spinal chord)
Relay : found within CNS and connect SENSORY —> MOTOR NEURONES
Motor: carry impulses from CNS —> EFFECTORS
Structure of 3 types of neurone?
Motor : large cell body at one end that lies in SPINAL CHORD/BRAIN
- nucleus in cell body
- highly branched dendrites extending from cell body —> large SA for axon terminals of other neurones when communicating
Relay : short, high branched axons and dendrites
Sensory: cell body that branches off middle of cell
- single long dendron carries impulses TO cell body /axon that carries impulse AWAY from cell body
What is a stimulus and a receptor cell?
Stimulus: Detectable change in internal/external environment of an organism
Receptor cells respond to a stimulus
- they are transducers - convert energy from one form into energy in an electrical impulse within sensory neurone
What are pacinian corpuscles ?
Found at ends of sensory neurone axons - made of layers of membrane around end of neurone separated by a gel
- gel between layers contain sodium ions (na+)
Section of axon surrounded by layers of membrane contain stretch-mediated sodium ion channels - OPEN when SUFFICIENT PRESSURE APPLIED
- allows Na+ to flow into neurone - electrical potential difference across axon membrane established - GENERATOR POTENTIAL
What is resting potential?
In a resting axon (not transmitting impulses) , INSIDE of axon always has NEGATIVE electrical potential compared to OUTSIDE
- this potential difference is usually -70mV inside axon
Sodium potassium pumps* - actively transport USING ATP , 3Na+ ions OUT of axon and 2 K+ ions INTO axon
Differential membrane permeability - membrane not as permeable to Na+ ions - channels mainly closed- so cant diffuse back in —> CREATES SODIUM ION ELECTROCHEMCIAL GRADIENT
K+ ION CHANNELS are mainly open so membrane permeable to K+ ions - diffuse back OUT axon down electrochemical grad (HIGH —> LOW)
- More NEGATIVE inside neurone - increase ELECTROCHEMICAL GRAD
2 factors contributing to establishing/maintaining resting potential?
- Active transport of sodium ions and potassium ions
- Differential membrane permeability
What are action potentials caused by?
Rapid movement of sodium and potassium ions across the membrane of axon
What are voltage gated channel proteins?
Channel proteins in axon membrane that changes shape to allow sodium and potassium ions to pass through
- open and close depending on voltage (electrical potential) across axon membrane
Closed when *axon membrane at resting potential
Stage 1 of an action potential?
STIMULUS (electrical impulse/chemical change to membrane of neurone)
- triggers Na+ ion channels in membrane to open —> Na+ ions diffuse into neurone down ELECTROCHEMICAL GRADIENT
Makes AXON MORE POSITIVE - generator potential established
When large enough stimulus detected by neurone, RESTING POTENTIAL —> ACTION POTENTIAL (when generator potential reaches threshold potential (-55mV))
Stage 2 of an action potential?
DEPOLARISATION
Threshold of -55mV is reached , an action potential is stimulated and so:
More Voltage-gated sodium ion channels in the axon membrane open
Sodium ions pass into the axon down the electrochemical gradient (high conc -> low conc)
- inside of the axon is - , attracting the + charged sodium ions
The movement of sodium ions reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – depolarisation
Depolarisation triggers more channels to open, allowing more sodium ions to enter and causing more depolarisation - positive feedback
The action potential will reach a potential of around +30mV
Stage 3 of an action potential?
REPOLARISATION
Once pd reached 30mV , all Na+ voltage gated channel proteins CLOSE in this section , stopping further diffusion of Na+ into axon
K+ ion voltage gated channel proteins in this section OPEN —> diffusion of K+ OUT of axon (DOWN CONC GRAD)
- returns pd to Normal (-70mV)- REPOLARISATION /example of negative feedback
Stage 4 of an action potential?
HYPERPOLARISATION
K+ ion channels are slow to close so too many K+ ions diffuse out of neurone - hyperpolarisation
- pd across this section of axon membrane is more negative than normal resting potential
Stage 5 of action potential?
Once the K+ ion voltage gated channel proteins are closed , the SODIUM-POTASSUM PUMP restores resting potential
- sodium ion channel proteins in this section of membrane become responsive to depolarisation again
How are action potentials transmitted along an axon?
An action potential triggered —> depolarisation* of that section of the axon - sodium ions channels open at that point
Na+ ions can diffuse ACROSS membrane into neurone
Na+ ions diffuse sideways along neurone, away from area of high conc —> movement of charged particles is LOCAL CURRENT
Local current causes slight depolarisation further along neurone, causing voltage gated Na+ channels to OPEN - cause FULL DEPOLARISATION along neurone (bc more Na+ ions flood in)
The previous section of the axon is in the repolarisation stage (the Na+ channels are closed and K+ channels are open) and is unresponsive
- makes the action potentials discrete events/ the impulse can only travel in one direction
What is the all or nothing principle?
An impulse is only transmitted from receptor to sensory neurone , if the initial stimulus is sufficient to increase membrane potential ABOVE threshold potential
Stronger stimulus = increased frequency of impulses
Without reaching threshold potential - AP NOT INITIATED
What is the refractory period?
When both Na+ and K+ channels are CLOSED
- when this occurs, this section of the axon membrane is in a period of recovery and is unresponsive
Importance of refractory period?
- ensures action potentials are discrete events —> stopping from merging into one another
- ensures that ‘new’ action potentials are generated ahead (further along axon) as region behind is recovering from the action potential that has just occurred
- impulse can only travel in one direction - for the successful /efficient transmission of nerve impulses
- minimum time between action potentials
- determining max frequency at which impulses are transmitted along neurones
What is the speed of conduction and what is it determined by?
How quickly the impulse is transmitted along a neurone
Determined by:
Myelination
Diameter of the axon
Temperature
How does myelination affect speed of conduction?
Unmyelinated neurones —> speed is very SLOW
- Bc depolarisation must occur along whole membrane of axon
When myelin is present —> increases SPEED
- action potentials can only occur at the nodes of ranvier (uninsulated section of axon)
Presence of Schwann cells mean action potentials ‘jump’ from one node to next —> SALTATORY CONDUCTION - allow impulse to travel much faster
How does diameter affect speed of conduction ?
Thicker axons - impulse conducted at higher speed
- as axon membrane will have larger SA for diffusion of ions , increasing rate of diffusion through channels - INCREASE RATE of DEPOLARISATION/ACTION POTENTIALS
Axons with larger diameter also have greater volume of cytoplasm (contains ions) - reduce electrical resistance so action potential can push into next section faster
How does temperature affect the speed of conduction?
Colder conditions can slow down conduction
- less KE available for facilitated diffusion of K+ and Na+ ions during an action potential - reduced rate of depolarisation, therefore reduced rate of transmission
At too high temps, channel proteins can denature - slower transmission due to membrane damage
What is a synapse and what do they consist of?
Junction where 2 neurones meet
Consists of :
Presynaptic Knob
Synaptic cleft - small gap between 2 neurones
Postsynaptic membrane
What are cholinergic synapses?
Synapses that use acetylcholine (ACh) as a neurotransmitter
What is a neurotransmitter?
Chemical used as a signalling molecule between 2 neurones in a synapse
How are electrical impulses transmitted across a synapse?
- Action potential arrives at synaptic bulb
- Voltage gated Ca+ ion channels OPEN /ions diffuse into synaptic bulb
- Ca+ ions cause synaptic vesicles to move to/fuse with PRE SYNAPTIC MEMBRANE
- Acetylcholine released by EXOCYTOSIS /diffuses across the cleft
- ACh bind to receptor sites on Na+ channels in POST SYNAPTIC MEMBRANE
- Na+ ion channels OPEN /ions diffuse across post synaptic membrane INTO post synaptic NEURONE —> DEPOLARISED so generator potential created
- If sufficient generator potentials COMBINE the potential will reach threshold potential / new action potential created in POST SYNAPTIC NEURONE
- ACh molecules broken down /recycled —> stops Na+ channels staying open/stops permanent depolarisation of post synaptic membrane
- ACETYLCHOLINESTERASE catalyses hydrolysis of ACh into ethanoic (acetic) acid + Choline - Choline absorbed back into presysnaptic membrane / reacts with acetyl coenzyme A to form ACh ( USES ATP FROM RESPIRATION )- then packed into vesicles to repeat process
Features of synapses ?
Unidirectionality: ensure 1 way transmission of impulses - neurotransmitters released on one side/its receptors on the other
Summation : when effects of multiple impulses are added together
- allows for effect of stimulus to be MAGNIFIED
- combination of different stimuli can trigger RESPONSE
- avoids nervous system from being overwhelmed by impulses - synapses are barrier/slow rate of transmission
Why is single impulse sometimes insufficient to generate an AP in post synaptic neurone ?
Only small amount of ACh released into synaptic cleft
Only small no. Na+ ion channels are opened so insufficient no. Na+ ion pass through membrane
So threshold potential is NOT REACHED + ACh attached to receptors is broken down rapidly by acetylcholinesterase
What is temporal and spatial summation?
Temporal: multiple impulses from same PRE-SYNAPTIC NEURONE* occur in succession + their effect is added together to generate an action potential
Spatial : action potentials arrive from several different pre-synaptic neurones - multiple pre synaptic neurones connect to same POST synaptic neurone (combo of both excitatory and inhibitory neurones are “added tgt” to determine if AP will be generated)
Both result in large amount of ACh released
- so large no. Na+ ions channels open + large no. Na+ ions pass through membrane
Therefore reaches threshold potential —> new AP GENERATED
What do excitatory and inhibitory neurotransmitters do?
EXCITATORY: stimulate generation of an AP in POST SYNAPTIC NEURONE - by opening Na+ ion channels in post synaptic membrane causing DEPOLARISATION if threshold is reached
INHIBITORY: prevent generation of an AP in post synaptic neurone - open the K+ ion channels in post synaptic membrane - cause HYPERPOLARISATION of membrane
What happens when neurones are subject to both excitatory and inhibitory synapses ?
Na+ ions enter cell body following stimulation by excitatory synapse
Stimulation of inhibitory synapses causes K+ ions to diffuse out of cell body
Cancels out effect of Na+ ions entering
Threshold potential is not reached so no AP generated
