Nervpus And Synaptic System Flashcards
What are neurones?
Specialised cells that are adapted to carry nerve impulses from one part of the body to another
State the components of myelinated motor neurones, and their functions
Cell body- contains organelles, nucleus and large amounts of mitochondria and RER
Dendrites- carry nerve impulses to the cell body
Axon- a long fibre that carries nerve impulses away form the cell body
Schwann cells- rich in lipid, wrap around the axon many times for insulation. They make up the myelin sheath
Nodes of ranvier- the gaps between Schwann cells
Axon terminals- the end of the axon, opposite end to the cell body
Why are myelinated motor neurones advantageous?
They transmit nerve impulses faster than non-myelinated motor neurones do
What are nerve impulses?
The temporary reversals of the electrical potential differences across the axon membrane
The reversals are between the two states of resting and action potential
What is the resting potential?
When the electrical potential difference across the axon membrane is between -65 and -70 mV
Why is the resting potential difference negative?
Because the inside of the axon is more negative than the outside
How do sodium and potassium ions move across the axon membrane and why?
Active transport and facilitated diffusion, using specific transport proteins
They cannnot move by simple diffusion because they are charged molecules
What are the three types of intrinsic proteins found in the axon membrane that allow Na+ and K+ ions to move?
Gated channels- for either sodium or potassium ions
Leak channels- for either sodium or potassium ions
Sodium/ potassium pumps
Explain how the resting potential in the axon membrane is established?
Active transport of 3Na+ pout of the axon, and 2K+ into the axon by Na/K pumps
Higher conc. of K+ inside, and higher conc. of Na+ outside
The membrane is more permeable to K+ ions because there are more K+ channels than Na+ channels
Therefore the diffusion of k+ back out of the axon is much faster than the diffusion of Na+ back into the axon.
This creates a potential difference of -65/-70 mV inside compared to outside
The membrane becomes polarised
What is the action potential?
Where the axon membrane becomes depolarised due to a stimulus.
(Due to the reversal of charges across the axon membrane)
Describe the stages that lead to the action potential
Resting potential
A stimulus cases the gates Na channels to open, membrane is more permeable to Na+ so they rapidly move into the membrane by diffusion- Membrane is depolarised and potential difference is 40mV (Na+ channels close again)
Gated K channels open, K+ move back out of the axon membrane by diffusion- re polarisation, then hyperpolarisation and a potential difference of -95mV. (Large amounts of K+ move accross)
This also causes the refraction period top occur
The gated channels restore the concentrations of Na+ and K+, so the resting potential is reached again
What potential differences does the action potential fluctuate between?
40mV and -90mV
What is the all or nothing principe?
Where to generate the action potential, a specific threshold value must be reached by the stimulus. If not reached the voltage-gated sodium ions will not open and the axon membrane will not be depolarised
Once reached, all action potentials will be of a constant size and speed
How do you produce a larger action potential?
By increasing the frequency of action potentials
What is the refractory period?
A short period of time where the axon membrane is recovering from its own depolarisation.
Gated sodium ions cannot open during this period, so the action potentials cannot be started.
What are advantages of the refractory period?
Distinct and separate action potentials
Limits the number of action potentials
Ensures the action potentials only travel in one direction
Describe the passage of the action potential in a non-myelinated neurone
The sodium ions can travel across the membrane sideways.
This causes gated sodium channels in the next region to also open and allow diffusion of Na+
Polarisation moves along the axon in one direction, causing a wave of depolarisation
Describe the passage of the action potential in myelinated neurones
The action potential/ depolarisation only occurs in the nodes of ranvier- because the myelin sheath insulates the rest of the axon.
This causes saltatory conduction
The impulses therefore jump across the Schwann cells, from node to node
This increases the speed of the neurone
What is saltatory conduction?
Where impulses in myelinated neurones jump from node to node, with an increased speed
(Because the myelin sheath insulates the rest of the axon length)
What factors affect the conduction of impulses in the action potential?
Myelinated- faster conduction due to saltatory conduction (from node to node)
High temp- ions diffuse quicker and respiratory enzymes work quicker to produce more ATP
Wider axon diameter- less resistance to the flow of ions, so the potential difference is more stable
Why is transmission of impulses faster along myelinated axons than along non-myelinated axons?
Myelinated axons are insulated by the Schwann cells/ myelin sheath
This causes saltatory conduction, so depolarisation/ action impulses only occur on the nodes of ranvier
Impulses jump between nodes
However non-myelinated axons are not insulated, so depolarisation/ action impulses occur across the whole length of the axon
What are synapses?
The gaps between the axon of one neurone and the dendrites of another
Why are neurotransmitters required?
Action potentials cannot pass a cross synapses, so they are transmitted via neurotransmitters
What are the components of synapses? Describe how they link together
In the terminal axons:
Synaptic knob contains synaptic vesicles, which hold neurotransmitter substances
Synaptic vesicles fuse with the pre-synaptic membranes and release neurotransmitters into the synaptic cleft
Neurotransmitters bind to specific receptors on the post-synaptic membranes and diffuse into the dendrites
What is the synaptic cleft?
The area between the presynaptic membrane and the postsynaptic membrane
What is a neuromuscular junction?
The synapse between a motor neurone and a muscle. The postsynaptic membranes is the sarcolemma of the muscle cell
In cholinergic synapses, what is the neurotransmitter? And what is it made of?
Acetylcholine (ACh)
Made from ethanoic acid and choline
Describe the process of transmission across a cholinergic synapse
- the action potential arrives at the synaptic knob, and stimulates calcium ion channels to open. Calcium ions diffuse into the terminal axon
- This causes the synaptic vesicles to fuse with the presynaptic membranes and release ACh into the synaptic cleft by exocytosis
- ACh diffuses across the synaptic cleft and binds to receptors on sodium ion channels in the postsynaptic membrane
- this stimulates the sodium ion channels to open, sodium ions diffuse into the dendrites and cause depolarisation. If the threshold is reached an action potential is initiated
After transmission across a cholinergic synapse, how is the stimulation of action potentials stopped?
The continued presence of ACh causes an enzyme called acetylcholinesterase to hydrolyse the ACh. The products diffuse back across the synaptic cleft and into the presynaptic membrane where ACh can be reformed and packages into synaptic vesicles
Why does the presynaptic knob contain many mitochondria?
Cause lots of energy from ATP is required to generate the neurotransmitter
What are three features of synapses that are important to their functions?
Unidirectionality- synapses can only pass neurotransmitters in one direction
Summation- where the effect of different impulses can be combined to ensure the postsynaptic neurone is depolarised enough to exceed the threshold
Inhibition- some postsynaptic membranes have chloride ion channels that are opened by specific neurotransmitters. Rapid diffusion of Cl- into the neurone causes hyper polarisation, so depolarisation is less likely to occur and an action potential will not be generated
What are the two types of summation?
Spatial summation- many different synaptic knobs release neurotransmitters at the same time, so depolarise the postsynaptic membrane enough to reach the threshold.
Temporal summation- one presynaptic knob releases small amounts of neurotransmitters many times in a short period, so build up the depolarisation enough to reach the threshold.
What is the difference between inhibitory and excitatory synapses?
Inhibitory synapses use chloride ion channels to stop an action potential from being generated, when a specific neurotransmitter is detected
Excitatory synapses cause depolarisation and therefore an action potential
What are benefits of inhibitory synapses?
They prevent random impulses from being sent around the body
They allow only specific pathways to be stimulated in specific areas of the body
What synapses and transmitters are used at neuromuscular junctions?
Cholinergic synapses
Acetylcholine (ACh)
Give examples of how drugs can increase the action potentials
Similar shapes to neurotransmitters so bind to receptors on postsynaptic membrane- so can stimulate depolarisation
Reduce activity of enzymes that hydrolyse neurotransmitters (e.g acetylcholinesterase)
Causes the release of excess neurotransmitters
What are examples of how drugs can reduce action potentials?
Act as competitive inhibitors- block/ bind to receptors on postsynaptic membranes
Inhibitors the release of neurotransmitters
How is the effect of a drug dependent on the synapse it targets?
It can be an inhibitory synapse, or an excitatory synapse.
E.g blocking receptors on inhibitory synapses will increase action potentials (less chloride channels opened)
Blocking receptors on excitatory synapses will decrease action potentials
