Neuronal communication Flashcards
What are neurones?
Specialised cells that carry electrical signals in the nervous system
What are common features of neurones?
Cell body, dendrites, axon, synapses
How do sensory neurones work?
Transmit info from sensory receptors to CNS. Dendron carries action potential from receptor to cell body. Axon carries signal to CNS.
What is the function of relay neurones?
Carry electrical signals between sensory and motor neurones. Normally have short, highly branched dendrites and axons.
What is the function of motor neurones?
Receive signals and transmit them to effectors, which carry out the response.
What are sensory receptors and how do they work?
Specialised cells that detect physical stimuli. They convert stimulus energy into electrical energy = generator potential.
What is the generator potential?
The depolarisation of the receptor cell membrane due to a stimulus.
What are Pancinian corpuscles?
A type of sensory receptor
What is the resting potential?
The potential difference across the membrane while the neurone is at rest.
What is happening while the neurone is at rest?
There are more cations inside the membrane than outside - it is polarised.
How is the resting potential maintained and created?
Potassium and sodium ion channels and the sodium-potassium pump, which keeps more cations outside that inside the cell.
How does the sodium-potassium pump work?
It uses ATP to pump 3 Na out and 2 K in, creating a negative resting potential. At rest, the membrane is permeable to K but not Na so K travels out down the gradient.
What is an action potential?
A rapid change in potential difference across a membrane. This occurs when a neurone is firing a nerve impulse. To be triggered, Na ions need to enter the neurone.
How is the electrochemical gradient created?
The concentration of Na inside is lower than outside.
What happens when enough depolarisation occurs due to the generator potential?
The voltage-gated sodium channels open, so Na enters (more depolarisation). More channels open. So many enter the cell it becomes positive (40 mv) compared to the outside.
When does repolarisation occur?
The K ions move out the cell, decreasing the membrane potential. The membrane potential overshoots and is more negative than the resting potential.
What voltage does the membrane potential reach during depolarisation?
+30 mv
What is the refractory period?
The period in an action potential where the axon can’t be depolarised to start a new action potential.
Why does the refractory period occur?
It is impossible to initiate another action potential as the high Na and low K concentrations stop it. Must be restored to normal first.
What is the resting membrane potential in mv?
-60 mV
What is the threshold value of depolarisation?
-50 mV
What is the value of further depolarisation?
40 mV
Why is action potential an all or nothing principle?
Each action potential depolarises the axon to the same voltage, and the threshold potential must be reached.
Are all nerve impulses identical in strength?
Yes, as the strength of the stimulus is not related to the size of the action potential.
How do action potentials travel along the whole neuron?
As a wave of depolarisation. When depolarisation occurs, voltage gated sodium channels open further down, and earlier parts have begun repolarising.
How do action potentials only occur in 1 direction?
Behind the travelling wave, the axon is in the refractory period.
What is the structure of the myelin sheath?
It is made up of Schwann cell membranes, stopping ions moving across the membrane.
How is a local current created?
When an action potential occurs, Na ions diffuse into the cell through channels.
What is saltatory conduction?
The action potential jumps from 1 node of Ranvier to the next in myelinated neurones.
What is the main function of the myelin sheath?
Prevents Na ions being attracted to the negative charge outside the membrane.
Why does myelination increase transmission speed?
Via saltatory conduction, as the action potential moves down the axon in bigger jumps.
How does saltatory conduction work?
The movement of ions across the membrane to create an AP only occurs where there is no myelin. Na diffuse along between nodes of Ranvier. The membrane is depolarised at these sections, increasing the length of the local current.
What is the function of the synapse?
It links 2+ neurones and allows the AP to be communicated between them.
How does the AP travel across the synapse?
It causes the release of neurotransmitters, which diffuse from the presynaptic to the postsynaptic, initiating an AP.
Why are many mitochondria present at synapses?
Produce ATP for active processes.
What features does the pre-synaptic neurone have?
- Vesicles with neurotransmitters
- extensive SER (to make vesicles)
- many mitochondria
- voltage gated Ca ion channels
What does the post synaptic neurone have on the membrane?
Neurotransmitter receptors - specialised Na ion channels that open/close when neurotransmitter binds to specific receptor site.
What are the channels on post synaptic neurones made of?
5 polypeptide molecules (2 make up specialised receptor site)
How is acetylcholine released into the synaptic cleft?
Via exocytosis to diffuse across the synaptic cleft. It binds to receptors on the Na channels.
What are cholinergic synapses?
Synapses that use acetylcholine as their neurotransmitter.
How is the neurotransmitter acetylcholine inactivated?
By the enzyme acetylcholinesterase. Hydrolyses into choline and acetic acid to be recycled (with ATP).
What is an excitatory synapse (EPSP)?
When a little acetylcholine is released, it binds to the Na receptors. causing Na ions diffuse into the neurone. A small amount of depolarisation occurs - not enough for AP.
What is an inhibitory synapse?
Stimulation hyperpolarises the post-synaptic neurone in response to neurotransmitter binding. Decreases the resting membrane potential - much harder to initiate AP.
What is Summation?
The effects of several EPSPs added together.
When does summation occur?
When there is enough build up of neurotransmitter in the synapse to produce a membrane depolarisation. This reaches the threshold potential to trigger AP.
What are the 2 ways summation occurs?
Spatial - 1 neurone receives signals from multiple pre-synaptics. Produce specific response.
Temporal - low level signals amplified. Persistent low level stimulus generates multiple AP in little time.
When are low level signals filtered out?
When summation does not occur. If it creates an AP, unlikely to be transmitted as not enough acetylcholine released to reach threshold potential.
Why does unidirectionality occur?
Synaptic vesicles containing acetylcholine only on the presynaptic. Receptors only on postsynaptic.
What is habituation?
The reduction in neural signalling due to repeated stimulation and leads to reduced levels of neurotransmitter being released.
It is why we become used to stimuli and ignore them eg. smells.
It avoids overstimulation of an effector - can be damaging.
What happens when a neurone is repeatedly stimulated?
Can run out of synaptic vesicles containing neurotransmitters. It is fatigued. An action potential can no longer be stimulated.
How are synapses adaptable?
The vesicles of neurotransmitter have a great effect, so the chance of a successful action potential is higher and the synapse is more sensitive.
eg. synaptic strength amplified by increasing the number of receptors.
What are memory and conscious thought based on?
Connectivity between specific neurones, which can be increased or decreased by changing the strength of certain synapses to improve memory or thought.
What is the role of synapses?
- Neurones can communicate
- Memory and conscious thought
- Filter out low level signals.
What does ‘all or nothing’ mean?
Threshold potential must be met in order for an action potential to be generated.
Why does the deformation of the plasma membrane case it to become more permeable to Na+?
Na+ channels open so temporary gaps appear in the bilayer.
How is the strength and intensity of the stimulus communicated to the brain?
The frequency of the action potential - higher frequency is a stronger stimulus.
