Neurons and the brain Flashcards
Structural info and neuronal transmission
How much of the brain do neurons make up?
10%
Other 90% are Glia cells
What is the difference between oligodendrocytes and schwann cells?
Oligo = support and insulate axons in CNS by creation/secretion of myelin sheath Schwann = axons in PNS - cell body actually wraps around the axon to create the myelin sheath
Where is myelin especially found?
Cells carrying motor signals
What are dendrites?
Branches that carry information FROM other neurons - the exact structure and number varies with type of neuron i.e. where it is and what it does
What is the cell body?
Contains nucleus and other organelles –> genetic code so involved in protein synthesis (neurotransmitters)
What is the cell membrane?
Barrier that determines what gets in and out of a neuron
Resting potential is -70mV (inside:outside) and only axons can produce action potentials because they have voltage gated ion channels in their membranes
What are axons?
Carries information TO other neurons in the form of action potentials - each neuron only ever has one but the exact structure and length varies with function (i.e. presence of any collaterals)
What is the myelin sheath and what happens in MS?
Fatty substance surrounding axon to speed up AP conduction - blocks normal sodium/potassium transfer along sections so AP jumps between nodes of ranvier
Myelin breaks down so neurotransmission in motor system is slower
How do impulses travel along an axon?
When AP reached one part of axon, it opens voltage-gated sodium channels in adjacent parts –> propagates AP
What is meant by “synaptic potential”?
Potential created within postsynaptic cells which is conducted passively through dendrites and soma towards axon hillock - passive transmission is short range as electrical signal is impeded by surrounding material and does not create change in electrical charge
What are neurotransmitters?
Chemical signals released from presynaptic neurons that affect properties/actions of postsynaptic neurons - imbalances can cause disorders
What is the process of PERCEPTION and what does it consist of?
Culmination of sensory neuron transmission and cognitive control/interpretation
SENSATION (bottom-up) - how sensory information is encoded by sensory organs and then transmitted to the brain
COGNITION (top-down) - how higher level systems affect how we perceive the world e.g. our past experiences and memories
Distinguish between neurons and nerves
A neuron is a the single entity consisting of axon, cell body etc
A nerve is a bundle of elongated axons belonging to hundreds of thousands of neurons
How can neurons be classified?
By neurotransmitter
By function e.g. sensory, motor
By structure e.g. unipolar, bipolar, pseudo-unipolar or multipolar (which can be motor neurons or “interneurons”
What are “inhibitory” synapses and what are the main inhibitory neurotransmitters?
Control spread of excitation i.e. keep activity “channelled” (reduce activity in post-synaptic cell)
Acetylcholine (generally excitatory but varies with receptor) - alzheimers (too little)
Dopamine (inhibitory and excitatory) - parkinsons (too little) and schizophrenia (too much)
GABA (inhibitory) - huntington’s (too little)
How is the strength of a signal entering a neuron determined?
each postsynaptic neuron sums together its synaptic potentials from different dendritic sites and if the excitatory signal from them is sufficiently strong at the axon hillock an AP will be generated
What are two properties of action potentials?
All-or-nothing i.e. amplitude always the same, and no decrease in size all the way along the axon –> transmission over long distance
No. propagated per second varies (spiking rate) - some neurons will have high spiking rate in some situations but not others
How does GABA exert its inhibitory effect on postsynaptic neurons?
Opens chloride channels so neuron more negative and harder to depolarise
What are the properties of the “resting state” of the action potential?
The resting potential inside relative to outside is -70mV (outside is arbitrarily considered 0mV)
If passive current across axon hillock is strong enough voltage gated sodium channels open
Potassium and negatively charged proteins inside cell
Sodium and potassium outside
What occurs during the depolarising phase?
The voltage-gated sodium channels open and sodium enters the axon
Inside charge becomes less negative, up to -50mV –> at the point the membrane becomes fully permeable and charge momentarily reverses up to +50mV
This sudden reversal is the action potential
What occurs during the repolarisation stage?
Potassium from inside flows outside through the voltage gated potassium channels, and makes the inside charge more negative again
The sodium channels close
What is meant by the “undershoot”?
The potassium channels continue to operate even when the sodium channels have closed so the membrane potential temporarily goes even more negative that resting state
Makes immediate re-depolarisation harder and also stops AP from being able to travel backwards - REFRACTORY PERIOD
What does the undershoot mean for increasing stimulus intensity?
Increasing stimulus intensity increases the rate of neuron firing but there is an upper limit to nerve impulses able to be conducted down axon per second (usually ~500-800 per second)
