Week 19 Flashcards
1
Q
Central nervous system
A
consists of the brain and spinal cord
2
Q
Peripheral nervous system
A
the other aspects of the nervous system except the brain and spinal cord (e.g. nerves from sense organs to the CNS)
3
Q
Blood-brain barrier
A
- semi-permeable barrier between the blood and the brain
- produced by tightly packed cells in the capillary walls of the brain, with tiny gaps so small necessary molecules can permeate through (e.g. oxygen and nutrients required to fuel the brain - passive diffusion; glucose and amino acids - active transport)
- main protective mechanism for the brain to try to exclude bacteria, viruses and toxins so the brain is not exposed to harmful substances in the blood (important to remember that damaged neurons do not replace themselves as easily as other body cells so it is crucial to minimise the chance of the brain getting infected in the first place)
4
Q
Area postrema (in medulla)
A
- detects the presence of toxins in the bloodstream and initiates vomiting
- this is one of the only parts of the brain that requires access to the bloodstream so is outside the BBB
5
Q
The neuron doctrine
A
- used newly developed staining techniques to show that neurons are separable (i.e. small gap between neurons rather than them being connected)
- made it clear that the brain consists of millions of individual neurons
6
Q
Neurons (numbers)
A
- 70 billion in cerebellum
- 12-15 billion in cerebral cortex
- 1 billion in spinal cord
7
Q
Structure of neurons
A
- cells of the nervous system that are specialised in performing information-processing tasks
- all have roughly the same structure:
- soma (cell body, contains the nucleus and machinery of the neuron: mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus etc.)
- dendrites (branching fibres that get narrower at the end, receive information from other neurons)
- axon (thin fibre of a constant diameter that extends away from the soma to other neurons, most vertebrate axons are covered in myelin sheaths for insulation)
- also crucially they are separate from the outside world by a thin membrane (neuron’s edge): 8 nm thick and composed of lipid molecules and proteins - it is penetrable for small, uncharged molecules and some ions that can go through protein channels
8
Q
Major types of neurons
A
- motor neurons (efferent - carry signals away from the CNS to the muscles to provide movement)
- sensory neurons (afferent - carry information from the 5 senses to the brain, via the spinal cord)
- interneurons (connect other types of neurons together, e.g. Purkinje cells in cerebellum, pyramidal cells in the hippocampus)
9
Q
Neuroglial cells
A
- smaller than neurons and exceed neurons in number by around 1.2 times
- they are supportive cells in the nervous system, without these neurons would not be able to function
- during development, glial cells provide scaffolds for neurons to migrate to their final destinations (radial glia)
- oligodendrocytes (76%): insulate nerve cells with myelin sheaths in the CNS
- astrocytes (17%): provide structure, surround neurons and hold them in place; supply nutrients and oxygen to neurons; modulate neurotransmission by preventing toxic build up of neurotransmitters by clearing them out from synapses
- microglia (6%): remove dead neuronal tissue; immune defense of the CNS
10
Q
Resting potential of neurons
A
- at rest, there are more sodium and chloride ions outside than inside the cell + more potassium ions inside than outside
- resting potential is -70 mV (as there are more positive ions on the outside than the inside so more negative inside)
- at rest, the sodium-potassium pump moves 3 sodium ions out for every 2 potassium ions that are moved into the neuron to maintain this resting potential of an unstimulated neuron
11
Q
Concentration gradient
A
- force of diffusion, where ions move from an area of higher to lower concentration
- sodium ions have a gradient of moving into the cell
- potassium ions have a gradient of moving out of the cell
12
Q
Electrical gradient
A
- electrostatic pressure depends on the balance of positive and negatively charged ions
- both sodium and postassium ions want to move into the cell (due to negative internal resting membrane potential)
13
Q
Depolarisation
A
reduction of the neuron’s polarisation towards zero (i.e. making the neuron potential less negative), about +40 mV
14
Q
Hyperpolarisation
A
increased (even more negative charge) inside the neuron so that it is below its resting potential, about -90 mV
15
Q
Formation of an action potential
A
- if stimulation exceeds the threshold of excitation (around -55 mV) this produces a sudden massive depolarisation of the membrane
- at rest, protein channels have activation gates closed so ions are not moving in and out but when depolarised to threshold, activation gates open and ions begin moving in and out more freely (lots of sodium moves in to make inside more positive)
- leading to action potential (+40 mV)
- when max potential is reached, sodium channels are closed but potassium channels remain open so potassium can move out (resting potential can be re-established by sodium-potassium pump after all ion channels are closed again)
- depolarisation only lasts a couple of seconds, after this the neurons gets repolarised and potentially hyperpolarised into a refractory period
16
Q
All or nothing principle
A
- amplitude of an action potential is independent of the amount of current produced (i.e. larger currents do not create larger action potentials)
- if failed initiations do not exceed threshold level, the neuron goes back to its resting potential
17
Q
Saltatory conduction
A
- can only occur in the presence of a myelin sheath
- first action potential is generated in the axon hillock, then action potentials move down the axon by jumping between Nodes of Ranvier for faster propagation
- myelin is dielectric, meaning it insulates the axon and prevents any charge leakage and loss through the axon so action potential size does not change as it moves down the axon
18
Q
Multiple sclerosis
A
- a neurological condition that affects around 100,000 people in the UK
- common symptoms include vision problems, fatigue, difficulties walking, numbness in extremities etc.
- results from demyelination of axons in the CNS so there is lack of communication between different parts of the nervous system
- leads to formation of plaques in the brain due to the immune system mistaking myelin as a harmful substance and attacking it, leading to scarring on the brain
19
Q
What is the synapse?
A
- the junction at which the signal is passed from one neuron to the next
- no cytoplasm continuity between neurons (neuron doctrine) they are separated by the synaptic cleft which is 20-30 nm wide
- discovered as there was a delay in transmission at the synapses (i.e. no continuity between passage of information between neurons)
- chemical transmission here rather than electrical (either inhibitory or excitatory neurotransmitters released into synaptic cleft)
20
Q
Sequence of events at the synapse
A
- presynaptic neuron synthesises chemicals that serve as neurotransmitters
- neurotransmitters stored in vesicles in the axon terminal
- action potential arrives at the terminal of presynpatic neuron
- opens calcium ions channels leading to calcium influx
- neurotransmitter vesicles open
- neurotransmitters released into synaptic cleft
- attach to receptors on postsynaptic neurons (either ionotropic or metabotropic)
- induces either IPSP or EPSP (graded potentials - varying magnitudes rather than being all or nothing, i.e. more neurotransmitter = more ion channels open)
21
Q
Ionotropic receptors
A
- where neurotransmitter directly opens ion channels in the membrane
- effect is fast and short-lived
- useful for vision, hearing and muscle activity
22
Q
Metabotropic receptors
A
- where neurotransmitters open ion channels indirectly
- produce slower but longer lasting effects
- useful for behaviours such as hunger, thirst, fear etc.
23
Q
IPSP
A
- hyperpolarisation of postynaptic neuron, which decreases firing rate
- inhibitory neurotransmitters bind to receptors to allow negatively charged ions to enter into the cell and hyperpolarise it so it is more likely to be inhibited
24
Q
EPSP
A
- depolarisation of postsynaptic neuron, which increases firing rate
- excitatory neurotransmitters bind to receptors to allow positively charged ions in and depolarisation to occur so it is more likely to create action potentials
25
Q
Spontaneous firing
A
- at rest, postsynaptic neurons still randomly fire action potentials
- IPSPs decrease rate of action potentials in postsynaptic neuron relative to its spontaneous firing rate
- if above threshold, EPSPs increase rate of action potentials firing in the postsynaptic neuron
- IPSPs/EPSPs do not necessarily always result in a more excited/inhibited behaviour as an EPSP may activate a neuron which in turn inhibits many other neurons etc.
26
Q
Temporal summation
A
- IPSPs and EPSPs can accumulate over a short time in rapid sequence
- repeated sub-threshold stimulations can add together to make the neuron above the threshold overall to lead to action potential firing
27
Q
Spatial summation
A
- different subthreshold IPSP and EPSP inputs arrive simultaneously at different locations on dendrites and cell body
- these are combined into one to result in an action potential
28
Q
Combination of effects on neuron
A
- information integrator (via temporal and spatial summation)
- decision maker (combining excitatory and inhibitory inputs to determine whether to fire or not)
29
Q
Termination of action potential firing
A
- reuptake of neurotransmitters (e.g. serotonin, dopamine and norepinephrine detach from the receptor and are reabsorbed by presynaptic neuron for reuse)
- enzymatic degradation (e.g. acetylcholine is broken down by acetylcholinesterase into acetate and choline)
- reabsorption (glial cells can reabsorb neurotransmitters at some synapse and influence synaptic activity by granting or withholding such absorption, e.g. an astrocyte as part of the tripartite synapse where it reabsorbs glutamate from the synaptic cleft and recycles it into glutamine which can then be returned to the presynaptic terminal for reuse)
30
Q
Acetylcholine
A
- enables muscle action
- regulates attention, learning, memory, sleeping and dreaming
- in neurodegenerative diseases (e.g. Alzheimer’s), Ach-producing neurons deteriorate
