Week 21 Flashcards

Question

Features of neurons: Soma (Cell body)?

Answer 1

Contains the normal cell organelles Main site of protein synthesis and degradation Has pronounced rough ER = ‘Nissl’ substance

Answer 2

Long, thin process Propagates nerve impulse to another neuron, muscle fibre or gland Often myelinated Terminates at axon terminals or synapses

Answer 3

Release of neurotransmitters to communicate with other cells (post synaptic)

Answer 4

Bipolar neurons – 1 main dendrite and 1 axon - Found: Retina of the eye, inner ear, olfactory area of brain Unipolar neurons - Just 1 process from the cell body, part way down the axon - Found: Always sensory neurons (pain, temperature, touch, pressure) Multipolar neurons - Many dendrites and 1 axon - Found: Most neurons in the CNS

Answer 5

Immune cells that survey the CNS and respond to signs of infection or damage Exist in a wide-range of morphologies depending on activation state : - Surveillant = smaller, with multiple processes - Activated = larger, with rounded cell body & shorter processes

Answer 6

Small, star-shaped cells that provide support for the development and homeostatic maintenance of the nervous system and cerebral blood vessels Heterogeneity (morphology, protein expression) across different brain regions Form a ‘glial scar’ after severe injury

Answer 7

Cells that form the lipid-rich sheath of myelin that wraps around some neurons to increase the speed at which information is transmitted by the neuron In the PNS, these cells are called Schwann cells

Answer 8

Areas of brain that contain myelin (bundles of axons) are called white matter, while regions of unmyelinated cell bodies are referred to as the grey matter

Answer 9

unmyelinated axon conduction velocities range from about 0.5 to 10 m/s myelinated axons can conduct at velocities up to 150 m/s

Answer 10

Blood vessels in the brain are made up of endothelial cells, astrocytes, pericytes (capillaries), smooth muscle cells (arteries) and neurons. This is referred to as the neurovascular unit

Answer 11

Endothelial cells of the brain express form tight junction proteins. These junctions allow the brain to create a physical barrier between the blood and the brain, called the blood-brain barrier (BBB) This gives the brain a high degree of selectively, thereby protecting it against entry by potentially harmful substances in the blood (may cause issues when trying to treat brain illness with drugs)

Answer 12

- contained within the ventricles and in the subarachnoid spaces of the brain and spinal cord - produced from filtered blood by the choroid plexus in the ventricles

Answer 13

- provides buoyancy for the brain and cushion it against injury, as well as waste removal - Because CSF is shared between the brain and spinal cord, samples of CSF collected from the spinal cord are used to provide a ‘snapshot’ of the brain environment

Answer 14

at dendrites (up to 100,000 synaptic inputs/neuron) and integrate in cell body

Answer 15

Information is transmitted along the axon in the form of electrochemical signals or nerve impulses (= action potentials) Action potentials are due to the flow of ions (Na+, K+) through specific protein channels in the membrane The lipid bilayer of the membrane is impermeable to these charged ions

Answer 16

Chemical force: - Differences in concentration: diffusion from a region of high concentration to a region of low concentration Electrical force: - Interior of cell is negatively charged so positively charged cations are retained and negative ions will be expelled Electrochemical driving force: - A combination of the chemical and electrical forces acting on any particular ion

Answer 17

Channels that are gated and require a stimulus to open - ligands, mechanical force or voltage - specific to particular ion(s) Channels that are always open and allow free movement of ions

Answer 18

Ligands gated = open when a ligan binds Mechanical gated = Mechanoreceptors – opens when a mechanism occurs such as temp, pressure Voltage gated Depends on voltage of neuron, positive or negative, to open – selective, only allow entry of certain ions. Open channels AKA leaky channels = selective as they're open all of the time such as only to Na+, electrochemical force only impact on these membranes.

Answer 19

Under resting conditions, the concentration of Na+ ions is ~ 10x higher outside the neuron compared to the concentration of Na+ ions inside At the same time, levels of K+ ions are ~ 15x higher inside the neuron compared to the extracellular environment

Answer 20

There is a constant flow of K+ ions down their concentration gradient, from the inside of the neuron to the outside This movement occurs via open (or leaky) K+ channels that are situated in the membrane of the neuron It will use a Na+/K+ ATPase pump to regulate this

Answer 21

The ion gradient is maintained by the continuous operation of the Na+/K+ ATPase pump It moves 3 Na+ ions from the inside of the neuron to the outside of the cell At the same time, 2 K+ ions are moved from outside the neuron to the inside of the cell

Answer 22

At each cycle of the Na+/K+ ATPase pump, the cell loses one positively charged ion from the intracellular environment

Answer 23

The ultimate result between the diffusion of K+ and the action of the Na+/K+ ATPase pump is a more positive charge outside the neuron compared to the inside of the neuron At rest, there is more positive charge outside the neuron compared to the inside of the neuron The difference in charge across the membrane of the neuron is referred to as polarisation

Answer 24

The difference in charge across the membrane of the neuron is referred to as polarisation

Answer 25

the resting membrane potential

Answer 26

When ion channels open, the chemical gradient drives ion movement from high concentration to low concentration In the absence of polarisation, diffusion would occur until chemical equilibrium was reached However, this does not occur because of electric forces

Answer 27

When Na+ channels open: chemical gradient drives ion movement into the cell electrical force pulls + ions into the cell both act in the same direction = Na+ will enter the cell

Answer 28

As Na+ moves into the neuron, the charge inside the cell starts to become positive and the electrical gradient decreases, along with the chemical gradient Eventually, the chemical and electrical forces will be exactly in balance and there will be no net flow through any open channels

Answer 29

the membrane potential required to exactly counteract the chemical forces acting to move one particular ion across the membrane. Specific to sodium!

Answer 30

When K+ channels open: chemical gradient drives ion movement out of the cell but electrical force pulls + ions into the cell two forces act in opposite directions chemical force > electrical force, so K+ moves out of the neuron

Answer 31

As K+ moves out of the neuron, the charge inside the cell starts to become even more negative, so the electrical gradient becomes stronger Eventually, the chemical force that drives K+ out of the cell = the electrical force driving K+ back into the cell At this point, there will be no net flow of K+ ions

Answer 32

Nernst Equation E = 61/z log Co / Ci Z = charge (valence) of ion Co = conc of ion outside neuron Ci = Conc of io inside neuron

Answer 33

Na+ = +60mV meaning no net movement of sodium at this point. K+ = -94mV meaning no net movement of potassium at this point

Answer 34

Incoming signals (from sensory stimulus or other neurons) can depolarise the cell membrane, causing the membrane potential to rise from its resting potential of -70 mV (e.g. open Na+ channels) If the membrane potential is depolarised beyond a certain critical level (threshold potential = -55mV) then an action potential is triggered in the neuron Other incoming signals can do the reverse and hyperpolarise the membrane (i.e. cause the membrane potential to decrease), so making an action potential less likely

Answer 35

-55mV (triggering an action potential)

Answer 36

Makes an action potential less likely to occur as it is caused by reverse incoming signals causing a decrease to membrane potential

Answer 37

Embedded in the plasma membrane of the neuron are ion channels that are sensitive to the voltage of the cell These channels open only when the voltage in the cell reaches a certain value These are called voltage-gated ion channels

Answer 38

Voltage-gated Na+ channels have both an activation gate and an inactivation gate. At rest, the activation gate is closed and the inactivation gate is open Voltage-gated K+ channels have one activation gate, which opens to allow the flow of K+ ions through the channel and closes to stop the flow of K+ ions

Answer 39

When the membrane potential is -70mV, voltage-gated Na+ channels are closed and the concentration of Na+ outside the cell is higher than inside the cell

Answer 40

When the neuron receives an excitatory signal or stimulus, ligand-gated ion Na+ channels open Small amounts of Na+ will move down their concentration gradient into the neuron and the resting potential will start to become more positive

Answer 41

Once the membrane potential reaches a critical threshold of -55 mV, voltage-gated activation gates in the Na+ channel open quickly, allowing Na+ to flood into the neuron As a result of the large influx of positively charged Na+, the neuron quickly loses its negative charge and undergoes depolarisation

Answer 42

When the inside of the neuron become highly positive, the pore of the voltage-gated Na+ channels is plugged by the inactivation gate and the flow of Na+ into the neuron stops

Answer 43

Eventually the intracellular environment of the neuron becomes sufficiently positive that voltage-gated K+ channels begin to open slowly Opening of these channels allows K+ to flow down its concentration gradient out of the cell This movement of K+ causes the inside of the neuron to quickly regain its negative charge in a process called repolarisation

Answer 44

1. Neuron at rest 2. Stimulation 3. Depolarisation 4. Inactivation of Na+ channels 5. Repolarisation 6. Hyperpolarisation 7. Refractory period

Answer 45

In response to the increasingly negative charge inside the neuron, the voltage-gated K+ channels close. Because this process is slow, some K+ ions continue to move outside the cell while the channel is closing This extra efflux of K+ causes the membrane potential to become more negative than the resting potential of -70 mV. This process is called hyperpolarisation

Answer 46

During the period of hyperpolarization, the neuron will not be able to fire another action potential. This is termed the refractory period Eventually, the action of the Na+/K+ ATPase pump will restore the resting membrane potential to -70mV and the neuron will be ready to fire another action potential

Answer 47

The process of depolarisation and repolarisation is referred to as an action potential A single action potential takes only milliseconds to complete, enabling the neuron to fire quickly in response to the hundreds of signals it receives every second

Answer 48

The process of depolarisation and repolarisation of a neurone cell due to a threshold being reached (-55mV)

Answer 49

At the base of the neuron in the region called the axon hillock

Answer 50

The action potential will be transmitted down the axon Small gaps in the myelin, called nodes of Ranvier, allow ion movement across the axon membrane at these sites This effectively permits the action potential to ‘jump’ from one node to another, thereby allowing the signal to be transmitted very quickly. This is type of transmission is called saltatory conduction

Answer 51

Unmyelinated = Each segment has to undergo the process of action potentials Myelinated = Only nodes of Ranvier have to undergo the process of action potentials, therefore the ions are propagated along the neuron quicker.

Answer 52

An action potential will not be triggered if the excitatory stimulus is not strong enough to raise the membrane potential to reach the threshold potential Action potentials are “all or none” – they either fire or they do not Information is coded by the frequency of the firing of action potentials (i.e. the number of spikes over a given period of time), rather than the size of the action potential, which is always the same

Answer 53

Neurons communicate with other cells via synapses Electrical synapses use gap junctions that directly connect the cytoplasm between 2 cells Chemical synapses involve the release of neurotransmitters from a pre-synaptic neuron that diffuse across the synaptic cleft & bind to post-synaptic neurons Chemical synapses are the most common type

Answer 54

- NT & Synaptic Vesicles = Neurotransmitters are stored in synaptic vesicles. Amount in one vesicle is called a quantum - Voltage-gated Ca2+ channel: Arrival of action potential causes influx of Cz2+ and fusion of vesicles with pre-synaptic membrane and release of transmitter into synaptic cleft - Receptor = Transmitter (crosses synaptic cleft and) Binds to receptor on post-synaptic membrane - Reuptake molecule/ enzyme = End of transmitter activity via i) End i) Catabolism (degradation) or ii) Uptake of transmitter into axon terminal or glial cells

Answer 55

Eventually, synaptic communication is terminated when the neurotransmitter is either taken back up into the presynaptic neuron (reuptake) or broken down in the synapse by enzymes.

Answer 56

When the depolarisation of the action potential reaches the presynaptic terminal, the voltage-gated Ca2+ ion channels open. In response to the increase in intracellular Ca2+, vesicles containing neurotransmitter fuse with the plasma membrane of the neuron. This causes the neurotransmitter to be released into the synapse. Neurotransmitter diffuses across the synapse and binds to receptors on the postsynaptic neuron. For excitatory neurotransmitters, this causes Na+ ion channels to open and entry of Na+ triggers an action potential in the postsynaptic neuron.

Answer 57

excitatory = if they raise the membrane potential towards the critical threshold (membrane potential less negative) inhibitory = if they lower the membrane potential away the critical threshold (membrane potential more negative) A single neuron communicates with hundreds of other neurons and therefore constantly receives both excitatory (EPSP = excitatory postsynaptic potential) and inhibitory (IPSP = inhibitory postsynaptic potential) signals

Answer 58

the process by which the neuron ‘sums up’ all the excitatory and inhibitory signals it receives over a period of time

Answer 59

1. Synthesised in the neuron 2. Present at presynaptic terminals, packaged within synaptic vesicles 3. Exogenous substance (drug) at reasonable concentration mimics exactly the action of endogenously released transmitter 4. Specific mechanism exists for removing transmitter from synaptic cleft

Answer 60

Excitatory postsynaptic potential Inhibitory postsynaptic potential

Answer 61

Acetylcholine - Excitatory at skeletal muscle, and both at other sites. Secretion = PNS & CNS, Neuromuscular junction Norepinephrine & Epinephrine - Both. Secretion = CNS & PNS Dopamine (Secretion = CNS + PNS) and serotonin (CNS) - Generally excitatory. Gamma aminobutyric acid - Inhibitory (CNS + Neuromuscular junction) ETC

Answer 62

1. Ionotropic receptors – transmitter binding = direct opening of ion channel - also termed ligand-gated ion channels - always stimulatory - fast – effect lasts a few milliseconds 2. Metabotropic receptors – transmitter binding = indirect activation of G-protein - also termed G-protein coupled receptors - can trigger opening or closing of a separate ion channel and downstream signalling cascade - slow – effect takes up to several hours

Answer 63

omposed of 4 or 5 subunits arranged around a central pore in the membrane Receptors can be made up of different combinations of subunits = increase diversity between different tissues

Answer 64

nicotinic acetylcholine (ACh) GABAA glycine 5-HT3 receptors

Answer 65

Composed of a single protein with 7 membrane-spanning regions (α-helices) seven transmembrane (7TM) receptor

Answer 66

muscarinic acetylcholine alpha and beta adrenergic receptor all 5-HT receptors except 5-HT3 rhodopsin olfactory receptors many others

Answer 67

At rest, the channel pore is closed and there is no movement of ions Binding of neurotransmitter to its receptor, causes the channel to open Ions will flow down their concentration gradient Channels will be permeable to anions (e.g. Na+, K+) or cations (Cl-)

Answer 68

Binding of neurotransmitter to its receptor causes activation of a G-protein The G-protein can act directly on an ion channel, causing the ion pore to open and/or the G-protein can activate a second messenger If a second messenger activated then Second messenger can bind to and open an ion channel or initiate a signalling cascade (enzymes, gene transcription, etc.)

Answer 69

Transmitter binds to receptor GTP exchanges for GDP on the G protein a subunit G protein dissociates from receptor – then ligand as well The 3 subunits (a, b, and g) of the G protein also dissociate The a subunit activates the ion channel The a subunit is inactivated by the hydrolysis of GTP to form GDP (GTPase activity is intrinsic to this subunit) The a subunit recombines with b and g subunits and attaches to the receptor, which can then bind another agonist

Answer 70

G proteins can stimulate (Gs) or inhibit (Gi) enzymes Most common enzyme targets: Cyclic AMP, cyclic GMP, inositol triphosphate & diacylglycerol. = All of which slow intracellular response, eg decrease heart rate

Answer 71

Agonists are drugs that mimic the actions of the neurotransmitter - Binding to the receptor = activation Antagonists are drugs that block the action of the neurotransmitter - Binding to the receptor = no activation

Answer 72

A neurodegenerative disease where neurones become demyelinated. This can be seen as lesions on in the brain and spinal cord. Therefore, this slows down action potential conduction causing problems with cognitive development and mobility