6.5 Neurones and synapses Flashcards
What do neurons transmit?
Electrical impulses
What are the two systems of the body that are used for internal communication?
Endocrine system and the nervous system
What does the endocrine system do?
Consists of glands that release hormones
What does the nervous system consist of?
Consists of nerve cells called neurons
How do neurons help with internal communication?
By transmitting nerve impulses
What is a nerve impulse?
An electrical signal
What do neurons look like?
- cell body with cytoplasm and nucleus
- narror outgrowths called nerve fibres along which nerve impulses travel
Along what do nerve impulses travel?
Narrow outgrowths called nerve fibres
* Dendrites and axons
What are two types of nerve fibres?
Dendrites and axons
What is the difference between axon and dendrites?
Dendrites: short branched nerve fibres (those used to transmit impulses between neurons in one part of the brain or spinal cord)
Axons: very elongated nerve fibres (those that transmit impulses from the tips of the toes or the fingers to the spinal cord.
What is the basic structure of a nerve fibre?
- the fibre is cylindrical in shape
- plasma membrane encosing a narrow region of cytoplasm
- diameter in most cases is about 1 micrometer
What are some nerve fibres coated with?
Some nerve fibres are coated along most of their length by a material called myelin
What does myelin consist of?
Many layers of phospholipid bilayers
How are myelin deposited?
Schwann cells deposit the myelin by growing round and round the nerve fibre.
* each time they grow around the nerve fibre a double layer of phospholipid bilayer is deposited
* there may be 20 or more layers when the schwann cell stops growing
What is the gap between the myelin?
Node of Ranvier
What can nerve impulses do in myelinated nerve fibres?
Saltatory conduction
* They can jump from one node of Ranvier to the next
What is saltatory conduction?
When nerve impulses jump from one node of Ranvier to the next
How is saltatory conduction beneficial?
It is much quicker than continuous transmission along a nerve fibre so myelinated nerve fibres transmit nerve impulses much more rapidly than unmyelinated enrve fibres
How is saltatory conduction beneficial?
It is much quicker than continuous transmission along a nerve fibre so myelinated nerve fibres transmit nerve impulses much more rapidly than unmyelinated enrve fibres
What is the resting membrane potentional of a neuron?
-70mV
When a neuron is not transmitting a singal because of the potential difference or voltage across its membrane
What causes the potential difference or voltage across a membrane of a neuron?
This potential is due to an imbalance of positive and negative charges across the membrane
What do sodium-potassium pumps do?
They transfer sodium (Na+) and potassium (K+) ions across the membrane.
Na+ ions are pumped out
K+ ions are pumped in
How is a concentration gradient for both ions created by the sodium-potassium pumps?
For every 3 Na+ ions are pumped out, only 2 K+ ions are pumped in
Why is the concentration gradient across the membrane steeper for Na+ than K+?
The membrane is about 50x more permeable to K+ ions than Na+ ions, so K+ ions leak back acorss the membrane faster than Na+ ions
How is a charge imbalance created?
There is a steeper concentration gradient for Na+ than the K+ gradient
What increases the charge imbalance in addition to the K+ and Na+ ions?
The proteins inside the nerve fibres are negatively charged (organic anions)
What is the resting membrane potential value of a neuron?
-70mV
What is an action potential?
is a rapid change in membrane potential, consisting of two phases
* depolarization and repolarization
What are the two phases of action potential?
Depolarization and repolarization
What is the difference between depolarization and repolarization?
Depolarization - a change from negative to positive
Repolarization - a change back from positive to negative
What causes depolarization?
negative to positive
* due to the opening of sodium channels in the membrane
* Allowing Na+ ions to diffuse into the neuron down the concentration gradient
* the entry of Na+ ions reverses the charge imbalance across the membrane, so the inside is positive relative to the outside.
What causes the reversal of the charge imbalance across the membrane?
The entry of Na+ ions, so the inside is positive relative to the outside
What is the membrane potential value after depolarization?
+30mV
When does repolarization occur?
Repolarization happens rapidly after depolarization due to the closing of the sodium channels and opening of potassium channels
What happens during repolarization?
- Sodium channels close, potassium channels open in the membrane
- K+ ions diffuse out of the neuron, down their concentration gradient
- which makes the inside of the cell negative again relative to the outside
- K+ ions remain open until the membrane has fallen to a potential close to -70mV
- the diffusion of potassium repolarizes the neuron, but does not restore the resting potential
How is the inside of the cell negative again after repolarization?
K+ ions diffuse out, down their concentration gradient
Until when does the potassium channels remain open?
Until the membrane has fallen to a potential close to -70mV
Why doesnt the diffusion of potassium restore the resting potential?
The diffusion repolarizes the neuron but does not restore the resting potential as the concentration gradients of Na+ and K+ ions have not yet been re-established
How long does the repolarization process take?
take a few miliseconds and the enruon can then transmit another nerve impulse
What is a nerve impulse?
Is an action potential that starts at one end of a neuron and is ropagated aong the axon to the other end of the neuron
Why does the propagation of the action potential happen?
because the ion movements that depolarize one part of the neuron trigger depolarization in the neighbouring part of the neuron
How do nerve impulses move along neurons?
In one direction in along neurons in humans and other vertebrates
Why can nerve impulses only move in one direction?
Because an impulse can only be initiated at on e terminal of a neuron and can only be passed on to other neurons or different cell types at the other terminal
What prevents propagation of an action potential backwards along an axon?
There is a refractive period after a depolarization that prevents porpagation of an action potential backwards along an axon
Which ion causes the propagation of an action potential along an axon?
due to the movements of Na+ ions
During depolarization, what happens to the sodium ions concentations?
Sodium channels open -> reduces the conc. of Na+ outside the axon and increases it inside
Does depolarization happen at the same time along the axon? What does that mean?
No, it happens in sections and that causes a different sodium ion concentration to the neighbouring part of the axon that has not yet depolarized
-> Na+ ions diffuse between these regions both inside and outside the axon
In what case do sodium ions diffuse between both inside and outside the axon?
When a neighbouring part of the axon that has not yet depolarized is depolarized
What are local currents?
The movement when the sodium ions diffuse along inside the axon to the neighbouring part that is still polarized.
Outside the axon the concentration gradient is in the opposite direction so sodium ions diffuse from the polarized part back to the part back to the part that has just depolarized
What do local currents reduce?
the concentration gradient in the part of the neuron that has not yet depolarized
Due to local currents, what happens to the membrane potential value?
This makes the membrane potential rise from the resting potential of -70mV to about -50mV.
What happens when the membrane potential of -50mV is reached by local currents?
Sodium channels in the axon membrane are voltage-gated and will open when a membrane potential of -50mV is reached
How do sodium channels in the axon membrane open?
They are voltage-gated and open when a membrane potential of -50mV is reached
What is the threshold potential?
-50mV
The opening of the sodium channels and causes depolarization
What do local currents cause?
A wave of depolarization and then repolarization to be propagated along the axon at a rate of between one and a hundred (or more) metres per second
What are synapses?
junctions between cells in the nervous system
Where are the synapses in sense organs?
Between sensory receptor cells and neurons
Where are the synapses in the brain and spinal cord?
There are immense numbers of synapes between neurons
Where are the synapses in muscles and glands?
between neurons and muscle fibres or secretory cells
What are mucles and glands sometime called?
Effetors
Why are muscles and glands sometimes called effectors?
Because they effecct (carry out) a response to a stimulus
What chemicals are used to send signals across synapses?
Neurotransmitters
Where are neurotransmitters used?
THe systen of using neurotransmitters to send signals across synapses is used at all synapses
What separaes the pre-synaptic and post-synaptic cells?
A fluid-filled gap called the synaptic cleft
What is the purpose of the synaptic cleft?
so electrical impulses cannot pass across pre-synaptic and post-synaptic cells
When are neurotransmitters released in to the synapse?
When the pre-synaptic neurons are depolarized
Synaptic transmission occurs very rapidly as a result of what events?
What does depolarization of the pre-synaptic membrane cause?
cause calcium ions (Ca2+) to diffuse through channels in the membrane into the neuron
What does the influx of calcium cause?
Cause vesicles containing neurotransmitter to move to the pre-synaptic membrane and fuse with it
How are neurotransmitters released?
The calcium cause vesicles containing neurotransmitters to fuse with the pre-synaptic membrane and is released into the synpatic cleft by exocytosis
Where does the neurotransmitter go after being released into the synpatic cleft?
It diffuses across the synaptic cleft and binds to receptors on the post-synpatic membrane
What does the binding of the neurotransmitter to the receptors cause?
cause adjacent sodium ion channels to open
As the sodium channels open due to the neurotransmitters binding to the receptors, what happens to the sodium ions?
They diffuse down their concentration gradient into the post-synaptic neuron, causing the post-synaptic membrane to reach the threshold potential
What is triggered when the post-synaptic membrane reaches the threshold potential?
an action potential is triggers in the post-synaptic membrane and is propagated on along the neuron
What happens to the neurotransmitter after opening the sodium channels?
They are rapidly broken down and removed from the synaptic cleft
What is acetylcholine used as and in which synapse?
- it is used as a neurotransmitter in many synapses including synapses between neurons and muscle fibres
How is acetylcholine produced?
Produced in the pre-synaptic neuron by combining choline (diet), with an acetyle group (aerobic respiration)
What do acetylcholine bind to?
receptors specific to acetylcholinei n the post-synaptic membrane
How long does acetylcholine remain obund to the receptor?
a short time, during which only one action potential is initiated in the post-synaptic neuron
How many action potential is initiated when acetylcholine binds to the receptor?
Only one
Why does acetylcholine only remain on the receptor for a short time and only initiates on action potential?
Because the enzyme acetylcholinesterase is present in the synaptic cleft and rapidly breaks acetylcholine down into choline and acetate
What enzymes breaks down acetylcholine?
Acetylchoinesterase
What products is produced when acetylcholine breaks down?
choline and acetate
What happens to the choline after acetylcholine breaks down?
It is reabsorbed into the pre-synaptic neuron, where it is converted back into active neurotransmitter by recombining it with an acetyl group
What are neonicotinoids?
Synthetic compounds similar to nicotine
What do neonicotinoids bind to?
acetylcholine receptors in cholinergic synapses in the central nerous system of insects
Why is the binding of neonicotinoids and acetylcholine receptors irreversible?
the enzyme acetylcholinesterase does not break down neonicotinoids
What happens when the receptors of acetylcholine is blocked?
acetylcholine is unable to bind and synaptic transmission is prevented. The consequence in insects is paralysis and death
What is an advantage of neonicotinoids as pesticides?
They are not highly toxic to humans and other mammals
Why is neonicotinoids pesticides not toxic to humans and other mammals?
- a much greater portion of synapses in the central nervous system are cholinergic in insects thant in mammals
- neonicotinoids bind much less strongly to acetylcholine receptors in mamals than insects
Under what condition will an action potential be initiated?
if the threshold potential is reaches because only at this potential do voltage-gated sodium channels start to open, causing depolarisation
What is the positive feedback effect invovled in action potentials?
The opening of some sodium channels and the inward diffusion of sodium ions increases the membrane potential causing more sodium channels to open
If the threshold potntial is reached there will therefore always be a full depolarization
What happens when the amount of neurotransmitter decreted is not enough to cause the threshold potential to be reached in the post-synaptic membrane?
The post-synaptic membrane does not then depolarize.
The Na+ ions that have entered the post-synaptic neuron are pumped outby sodium-potassium pumps and the post-synaptic membrane returns to the resting potential
What may be needed for threshold potential to be reached and a nerve impulse to be initiated?
It may be necssary for several of pre-synaptic neurons to release neurotransmitter at the same time for the threshold potential to be reached and a nerve impulse to be initiated in the post-synaptic neuron
