Module 5-Neurones Flashcards
(37 cards)
Describe the resting potential and depolarisation stage of nervous transmission
-the neurone has a resting potential-it isn’t transmitting any impulse. Some potassium ion channels are open but all sodium ion channels are closed. Resting potential is at -70mV
-the energy of the stimulus triggers some sodium voltage gated ion channels to open, making the membrane more permeable to sodium ions. Sodium ion channels diffuse into the axon down their electrochemical gradient. This makes the inside of the neurone less negative
-this change causes more sodium ion channels to open allowing more sodium ions to diffuse into the axon
-once the threshold of -55mV is reached the membrane will become depolarised you will then always reach the action potential of +40mV
-when the potential difference reaches approx +40mV the voltage gated sodium ion channels close and voltage gated potassium ion channels open
Describe the repolarisation stage in neurotransmission
-potassium diffuses out of the axon down the concentration gradient and the potential difference becomes more negative
Describe the hyperpolarisation stage in neurotransmission
-as potassium ion channels don’t close immediately, inside the axon becomes more negative than its normal resting potential which prevents the neurone from being restimulated instantly
-the voltage gated potassium channels now close and the axon returns to its resting potential
What is the all or nothing principle?
depolarisation is an all or nothing response so if the potential difference reaches the threshold, depolarisation will always take place and the change in potential difference will always be the same
What will happen if the stimulus is stronger in neurotransmission?
action potentials will be produced more frequently
What is the refractory period and why is it needed?
-ion channels are recovering so an action potential can’t be stimulated instantly to make sure each is separate
-Ensure waves of depolarisation travel in one direction and not spreading out into 2 directions which would prevent a response from happening
-Limits number of impulse transmission to prevent over reaction to stimulus
Define synapse
A synapse is a junction between two neurones (e.g. a relay neurone and a motor neurone) or between a neurone and an effector (e.g a motor neurone and a muscle cell).
Define synaptic cleft
the gap that separates the axon of one neurone from the dendrite of the next.Is approx 20-30 nm across. When an action potential reaches a synapse, it must be transmitted across the synaptic cleft.
Define presynaptic neurone
the neurone before the synapse along which the impulse has arrived. When an action potential reaches the end of the neurone, it is transmitted across the presynaptic membrane to the postsynaptic membrane or to an effector cell.
Define synaptic knob
The end of the axon of the presynaptic neurone is called the synaptic knob. It is a swelling which contains synaptic vesicles. It is the location where the nerve impulse is transmitted across the synaptic cleft. There are also lots of mitochondria in the synaptic knob because lots of energy is needed to synthesise neurotransmitters.
Define synaptic vesicles
are vesicles located in the synaptic knob which contain neurotransmitters. When the vesicles fuse with the presynaptic membrane, neurotransmitters are released into the synaptic cleft.
Define excitatory neurotransmitters and give an eg
When they bind to the receptors on the postsynaptic membrane, the membrane is depolarised which generates an action potential if the threshold is reached at the postsynaptic membrane. E.g. When acetylcholine binds to receptors on the postsynaptic membrane in the central nervous system (CNS) an action potential is established.
Define inhibitory neurotransmitters
When the neurotransmitters bind to the receptors on the postsynaptic membrane, the membrane is hyperpolarised which prevents an action potential from being generated in the postsynaptic cell E.g. When acetylcholine binds to receptors on the postsynaptic membrane in the heart, potassium ion channels are opened in the membrane. This prevents an action potential from being established.
Define neurotransmitters
the chemicals that allow an action potential to be transferred across a synapse. When neurotransmitters are released from the synaptic vesicles into the synaptic cleft, they bind to specific receptors on the postsynaptic membrane.
Define post synaptic membrane
the membrane of the postsynaptic neurone or effector cells. Receptors on the postsynaptic membrane have a complementary shape to the neurotransmitters released from the synaptic knob. When neurotransmitters bind to their receptors, the action potential continues. There are only receptors on the postsynaptic membrane which ensures the nerve impulse only moves in one direction.
Define summation
process where neurotransmitters from multiple neurones are summed together to produce a response. There are two types of summation: spatial and temporal.
Define spatial summation
takes place when multiple presynaptic neurones form a junction with a single neurone. Each presynaptic neurone releases neurotransmitters so overall there are many neurotransmitters that bind to the receptors on one postsynaptic membrane. Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.
Define temporal summation
takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time. More neurotransmitters are released into the synaptic cleft, so more neurotransmitters are available to bind to receptors on the postsynaptic membrane. Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.
Define neuromuscular junction
is a synapse between a motor neurone and a muscle cell. An action potential is transmitted across the synapse using the neurotransmitter acetylcholine.
Define temporal summation
takes place when multiple nerve impulses arrive at the same synaptic knob within a short period of time. More neurotransmitters are released into the synaptic cleft, so more neurotransmitters are available to bind to receptors on the postsynaptic membrane. Together the neurotransmitters can establish a generator potential that reaches the threshold value and an action potential is generated.
Describe transmission of impulse across the synapse
1)An action potential arrives at the synaptic knob at the end of the motor neurone. The action potential depolarises the membrane of the synaptic knob. This causes voltage-gated calcium ion channels to open. Ca2+ ions diffuse into the synaptic knob.
2)The Ca2+ ion concentration inside the synaptic knob begins to increase. This causes the synaptic vesicles to move and fuse with the presynaptic membrane. The neurotransmitter inside the vesicles, is released into the synaptic cleft. This process is called exocytosis.
3)the NT diffuses down the concentration gradient across the synaptic cleft where it binds to specific receptors on the postsynaptic membrane which opens sodium ion channels in the postsynaptic muscle cell. As Na+ ions diffuse into the cell, the membrane becomes depolarised. If the potential difference reaches the threshold value, an action potential is generated and flows along the motor cell.
4)An enzyme breaks down the NT in the synaptic cleft. The products of the breakdown are reabsorbed by the presynaptic neurone and reused to synthesise more NT. It is important that the NT is removed from the receptors. This stops action potentials from being continuously generated in the postsynaptic cell.
What are cholinergic synapses?
Synapses that use acetylcholine as a neurotransmitter
Describe transmission across cholinergic synapses
-the arrival of an ap at the end of the presynaptic neurone causes calcium ion channels to open and calcium ions enter the synaptic knob.
-The influx of calcium ions into the presynaptic neurone causes synaptic vesicles to fuse with the presynaptic membrane, so releasing acetylcholine into the synaptic cleft.
-Acetylcholine molecules fuse with receptor sites on the sodium ion channel in the membrane of the postsynaptic neurone.
This causes the sodium ion channels to open, allowing sodium ions to diffuse in rapidly along a concentration gradient.
-The influx of sodium ions generates a new ap in the postsynaptic neurone.
-Acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl), which diffuse back across the synaptic cleft into the presynaptic neurone (= recycling). In addition to recycling the choline and ethanoic acid, the breakdown of acetylcholine also prevents it from continuously generating a new action potential in the postsynaptic neurone.
-ATP released by mitochondria is used to recombine choline and ethanoic acid into acetycholine. This is stored in synaptic vesicles for future use. Sodium ion channels close in the absence of acetylcholine in the receptor sites.
What’s the differences between neuromuscular and cholinergic synapses?
-Cholinergic synapses are between two neurones. Neuromuscular junctions are between a motor neurone and a muscle cell.
-There are less receptors in the postsynaptic membrane at a cholinergic synapse than at a neuromuscular junction.
-A cholinergic synapse can trigger an inhibitory or excitatory response in the postsynaptic membrane. An action potential at a neuromuscular junction always triggers an excitatory response in the muscle cell.
-In a cholinergic synapse, depolarisation of the postsynaptic membrane results in an action potential. At a neuromuscular junction, depolarisation of the postsynaptic membrane results in muscle contraction.
-Acetylcholinesterase is the enzyme that breaks down acetylcholine after it has bound to the receptors on the postsynaptic membrane. In cholinergic synapses the enzyme is located in the synaptic cleft but at a neuromuscular junction the enzyme is stored in clefts in the postsynaptic membrane.
