Nerve impulse & Synaptic transmission Flashcards
(24 cards)
How is resting potential maintained in neurone’s membrane
Na-K pump uses A.T to move 3 Na+ out of neurone for every 2 K+ moved in. Required ATP. K+ channel allows F.D of K+ out of neurone down their conc gradient
What is action potential
Action potential is when a neurone’s voltage increases beyond a set point from the resting potential, generating a nerve impulse
What is depolarisation
An increase voltage above resting potential, created my axon membrane becoming more permeable to Na+
What is resting potential
When the neurone is not being stimulated (-70mv)
Outline the process during an action potential
Stimulus excites neurone membrane causing Na+ ions to open, membrane becomes more permeable so Na+ diffuses into neurone down electrochemical gradient. If potential difference reaches -55mv more Na+ channels open, more Na+ diffuses into neurone. At +40mv Na+ channels close and K+ channels open. Membrane more permeable to K+ so more diffuse out down conc gradient. K+ channels slow to close so slight overshoot so potential difference is <-70mv. Ion channels reset and Na-K pump returns membrane to resting potential
After action potential why cant membranes be excited again
-Ion channels recovering and cant be made to open
-Na+ channels closed during repolarisation and K+ closed during hyperpolarisation. Called refractory period
How is does action potential move along the neurone
Na+ which enters neurone diffuse sideways, causes Na+ channels in next region of neurone to open and Na+ diffuses into that part. Causes wave of depolarisation travelling along neurone. Wave moves away from parts of membrane in refractory period
What are the benefits of the refractory period
-Action potential don’t overlap, but pass along as discrete impulses
-Limits the frequency at which the nerve impulses can be transmitted
-Action potentials are unidirectional
What is the All or Nothing principle
Once threshold is reached action potential will always fire with same voltage no matter the stimulus. If threshold isn’t reached, action potential wont fire.
Will a bigger stimulus cause a bigger action potential
No, it will cause them to fire more frequently
What 3 factors affect speed of conduction of Action potential
Myelination
Axon diameter
Temperature
How does myelination affect action potential
Myelin sheaths are electrical insulators, made of Schwann cells in the PNS. Between Schwann cells are node of Ranvier. Na+ channels concentrated here. Depolarisation only occurs here for myelinated neurones. Cytoplasm conducts enough electrical charge to depolarise next node. This is called saltatory conduction
How does axon diameter affect action potential
Action potentials are conducted quicker along axons with larger diameters as there is less resistance to the flow of ions. Depolarises reaches other cell membranes quicker
How does temperature affect action potential
Speed of conduction increases with temperature as ions diffuse faster. After 40°C proteins denature
How are impulses unidirectional
Receptors are only on postsynaptic membrane
Outline structure of synapse
-Gap between cells at synapse is called synaptic clef
-Swelling at presynaptic neurone is called a synaptic knob, contains vesicles filled with neurotransmitters
-When action potential reaches neurone, causes neurotransmitters to release into clef and diffuse across postsynaptic membrane and binds to complementary receptors
-This binding may trigger action potential
-Neurotransmitters removed from clef so response does not keep happening or broken down by enzyme
outline transmission across Cholinergic Synapse
-Action potential arrives at synaptic knob
-Action potential stimulates Voltage gated Ca2+ channels to open
-Ca2+ diffuses in synaptic knob
-Influx of Ca2+ causes synaptic vesicles to move the presynaptic membrane, fuse with it.
-Vesicles release Acetylcholine (ACh) in synaptic clef via exocytosis
-ACh diffuses across clef and binds to specific cholinergic receptors on postsynaptic membrane (P.M)
-Causes Na+ channels to open in postsynaptic neurone
Influx of Na+ causes depolarisation. A.P on P.M generated and threshold reached
-ACh removed from cleft so response doesn’t keep happening. Broken down by enzyme AChE and products reabsorbed by Presynaptic neurone, used to make more ACh
What are excitatory neurotransmitters
Depolarise the postsynaptic membrane making it fire an action potential if threshold is reached. E.g. ACh is excitatory neurotransmitter at cholinergic synapses in CNS
What are inhibitory neurotransmitters
Hyperpolarise the postsynaptic membrane preventing it from firing an action potential. E.g. ACh is an inhibitory neurotransmitter at cholinergic synapses in heart
What is spatial summation
-Many neurones connect to one neurone
-Small amount of neurotransmitter released from each neurones can be enough altogether to reach threshold in Postsynaptic neurone and trigger A.P
-If some neurones release inhibitory neurotransmitter then it may result in no action potential
What is temporal summation
Two or more impulses arrive in quick succession from the same presynaptic neurone. Make A.P more likely as more neurotransmitter released into clef
What is a neuromuscular junction
Synapse between motor neurone and a muscle cell
Describe a Neuromuscular junction
-Use ACh which binds to cholinergic receptors called nicotinic cholinergic receptors
-Differences between cholinergic synapse are:
-Postsynaptic membrane has lots of folds that form clefs, which store enzyme AChE
-Postsynaptic membrane has more receptors than other synapses
-ACh is always excitatory at Neuromuscular junction. When Motor neurone fires A.P it doesn’t always trigger response
