B15 - Neurones and nervous coordination Flashcards
What are dendrons?
Dendrons - extension which divide into dendrites, carry impulses towards the cell body
What is the axon?
Axon - single long extension that carries the impulse away from the cell body. It can be metres at length and less than a micrometer in diameter
What are Schwann cells?
Schwann cells - surround the axon with their myelin-rich membranes. They protect and insulate
What are the nodes of ranvier?
Nodes of Ranvier - 2-3 μm long gaps between adjacent Schwann cells about every 1-3 mm in humans
What is the resting potential in a neurone, and why is it negative?
-65mV
It is negative as there is an unequal distribution of ions. Sodium-potassium pumps actively transport 3 sodium ions out of the cell and 2 potassium ions into the cell. Potassium ions freely diffuse out of the cell through permanently open potassium channel proteins
What happens in depolarisation of the axon?
A stimulus occurs and the energy causes some of the voltage-gated Na+ channels to open rapidly, Na+ ions diffuse into the axon. The change in potential difference causes more voltage-gated Na+ channels to open and a greater influx of Na+ ions
What happens in repolarisation of the axon?
The Na+ ion channels close and the voltage-gated K+ channels open, K+ ions diffuse out of the axon. The more K+ ions diffuse out, the more voltage-gated K+ channels open
What happens in hyperpolarisation of the axon?
K+ channels close slowly, which causes a temporary overshoot of outward K+ ion diffusion, the potential difference become more negative than usual
How is resting potential re-established after hyperpolarisation?
After hyperpolarisation, the closable K+ channels close and the Na+/K+ pump begins
How is an action potential passed along an unmyelinated axon?
A section is depolarised, which triggers the next section to become depolarised, while the previous section is being repolarised. This wave of depolarisation ensures the action potential is unidirectional
How is an action potential passed along a myelinated axon?
In a myelinated neurone, the myelin sheath acts as an insulator, limited local current flow to the nodes of Ranvier. The localised circuits occur at these nodes, effectively making the action potential to jump from node to node - saltatory conduction
What is the all or nothing principle?
Below threshold value there is no action potential
Any stimulus above the threshold will result in an action potential
How is a larger stimulus detected?
By a higher frequency of impulses
What is the refractory period?
After an action potential the inward movement of Na+ ions is prevented as the voltage-gated sodium channels are closed. This is called the refractory period and during this time no further action potential can be generated. This limits the number of action potentials, and thus limits the strength of stimulus that can be detected (brain is not overloaded)
What is an excitatory synapse?
An excitatory synapses produces new action potential in the postsynaptic neurone
What does an inhibitory synapse do?
They do not create a new action potential
What are neurotransmitters?
Chemicals neurones use to communicate
What is the synaptic knob?
Synaptic knob - swollen end of axon, contains a lot of mitochondria and SER. The synaptic vesicles store neurotransmitters. Neurotransmitters leave presynaptic neurone by exocytosis and diffuse across the synaptic cleft to the postsynaptic neurone
What are the steps of synaptic transmission?
- Action potential causes Ca2+ channels to open, influx of Ca2+ ions
- Ca2+ ions cause synaptic vesicles to migrate to and fuse with the presynaptic membrane
- Acetylcholine molecules diffuse across synaptic cleft (Na+ channels are closed)
- When the acetylcholine molecules bind to the receptor site on the Na+ channels, the channels open and Na+ ions diffuse into the postsynaptic cell
- Influx of Na+ ions generate a new action potential in the membrane of the postsynaptic neurone
- Acetylcholinesterase hydrolyses acetylcholine into ethanoic acid and choline which diffuse back into the presynaptic neurone (recycling)
- ATP from mitochondria is used to recombine choline and ethanoic acid into acetylcholine.
How do inhibitory synapses work?
Neurotransmitter binds to Cl- channels. When Cl- channels open, Cl- ions move into the postsynaptic neurone. This causes K+ channel to open and K+ ions to diffuse out of the postsynaptic neurone. Axon membrane is hyperpolarised (-80mV), it is more difficult to excite it
What is spatial summation?
Spatial summation - more than one presynaptic neurone to one postsynaptic neurone, so there is a high enough concentration of neurotransmitters and therefore an action potential will be started at the postsynaptic neurone
What is temporal summation?
Temporal summation is only one presynaptic neurone to a post synaptic neurone but many impulses. Low-frequency AP (action potentials) - neurotransmitters broken down rapidly, whereas in high-frequency AP - many releases over short period of time, concentration of neurotransmitter is above threshold
What is the neuromuscular junction?
The point where a motor neurone meets a skeletal muscle fibre.
There are many junctions along the muscle to allow rapid and coordinated muscle contraction
What is similar and different between neuromuscular junctions and a normal synapse?
Similar to normal synapse as neurotransmitters are transported by diffusion, neurotransmitters binding to its receptor causes rapid influx of Na+ ions. Na+/K+ pump repolarises axon. Enzymes used to break down neurotransmitter
But they cannot be inhibitory and the action potential ends here
