Module 6 (ET neurons)

Question 1

Neurons

Answer 1

Nerve cells that are the principal building blocks and instruments of communication of the central and peripheral nervous systems; they receive information, digest it and respond to it by transferring it to other cells

Question 2

Nervous system

Answer 2

PNS and CNS both form complex communication networks that allow an organism to interact in appropriate ways with its internal (contents of the body) and external (world outside the body) environments

Question 3

Neuroscience

Answer 3

A scientific concerned with the function and structure of the nervous system

Question 4

Communication

Answer 4

Electrical signals (dendrites cell body, axon) and chemical signals (synapses)

Question 5

Structure of a vertebrate neuron

Answer 5

Vary in size and shape; soma (cell body), two types of processes - dendrites and axon(s) with axon terminals

Question 6

RMP

Answer 6

Resting Membrane Potential; electrical potential difference across the cell membrane which results from separation of charge (carried by ions); can change during various stages of cell activity; more negative charges inside the cell compared to the EC fluid

Question 7

Types of ion channels which affect the membrane permeability

Answer 7

Non-gated channels (leak channels that are open at rest); gated channels (voltage or ligand gated that are usually closed at rest)

Question 8

RMP in neurons

Answer 8

Cytoplasm usually has a potential that is 50 to 70mV lower (more negative) than the potential of the extracellular space

Question 9

Excitable substances

Answer 9

Only muscle fibres and neurons can suddenly respond with a transient change of potential (i.e. with an action potential) in response to a stimulus; other cells have a negative RMP but it doesn’t change when various stimuli act on them

Question 10

What maintains the RMP

Answer 10

Unequal concentrations of Na+ and K+ outside the cell; unequal permeability of the cell membrane to these ions (the ratio of PK:PNa is about 40:1; due to concentration gradients, there is a steady diffusion of Na+ into the cell);
electrogenic action of the Na-K pump (Na/K ATPase); K+ and (to a smaller extent) Na+ non-gated channels

Question 11

How are intracellular potentials measured

Answer 11

Microelectrode recording technique (pentration of the cell membrane) or patch-clamp technique (no penetration of the cell membrane)

Question 12

Microelectrode recording technique

Answer 12

Penetration of cell membrane; glass capillary pulled so that its tip is very small; electrode is filled with electrolyte so it can conduct current and connected to an amplifier (voltmeter), second electrode is outside in EC space; allows measurement of PD between outside and inside

Question 13

Patch-clamp technique

Answer 13

No penetration of cell membrane; glass electrode with a larger top seals to membrane between lipid bilayer; bridge forms a contact between what is inside the cell which allows measurement of potentials and current flow; fill pipette with electrolyte which creates an artificial inside

Question 14

Concentrations of K+ and Na+ inside and outside neurons

Answer 14

K+: lower outside and higher inside (conc gradient out of cell)
Na+: higher outside and lower inside (conc graident in to cell)

Question 15

Na/K ATPase pump

Answer 15

Maintains concentration gradients and RMP; cell is slightly permeable to Na+ gradient, so will gradually diffuse into cell along it; membrane highly permeable to K+ so they will escape; pump removes more positive charges (Na+) than introduces (K+) to the cell

Question 16

Leak channels

Answer 16

Open at rest; in cell membrane of neurons, there are many leak K+ channels but very few leak Na+ channels (so they are much more permeable to K+ than Na+)

Question 17

Gated channels

Answer 17

Voltage-gated (pd change); ligand-gated (chemical/ligand); mechanically-gated (shape change)

Question 18

Equilibrium potential

Answer 18

Intracellular potential at which the net flow of ion is zero in spire of a concentration gradient and permeability; can be calculated for each ion by the Nernst equation

Question 19

Nernst equation

Answer 19

Used to calculate the equilibrium potential for each individual ion that contributes to the RMP; does not require knowledge of the permeability of the membrane for the ion;
EK+ = 61.5mV x log ([K+]outside/[K+]inside)
ENa+ = 61.5mV x log ([Na+]outside/[Na+]inside)

Question 20

When does the Nernst equation apply

Answer 20

In a situation when a cell membrane is permeable only to one ion (i.e. has leak channels only for one specific ion)

Question 21

Rule 1

Answer 21

The higher the permeability of the cell membrane to a particular ion, the greater the ability of this ion to shift the RMP to its equilibrium constant
In neurons, the RMP is closer to the equilibrium constant for K+ because the membrane permeability is much higher to K+ than to Na+

Question 22

Goldman equation

Answer 22

Method for calculating the value of the RMP by taking into account both the concentration gradients and the relative permeability of the resting cell membrane to K+ and Na+ ions;
Vm = 61.5mV x log (Pk[K+]outside + Pna[Na+]outside)/ Pk[K+]inside + Pna[Na+]inside)

Question 23

Hyperpolarisation

Answer 23

Potential inside neurons becomes more negative; potential moves closer to the EK+ and away from the ENa+

Question 24

Depolarisation

Answer 24

Potential inside neurons becomes less negative; potential moves away from the EK+ and closer to the ENa+

Question 25

Action potential

Answer 25

Very brief fluctuation (short-lasting change) in membrane potential caused by a transient opening of voltage-gated ion channels (mainly K+ and Na+) which spreads along axon; occurs after the membrane potential reaches certain voltage called the threshold (depolarised)

Question 26

Significance of an AP

Answer 26

Information is coded in the frequency of them; they can be regarded as a form of language by which neurons communicate; key element of the process of signal transmission along axons

Question 27

Stages of an AP

Answer 27

Slow and graded depolarisation evoked by a stimulus; fast depolarisation; repolarisation; after-hyperpolarisation

Question 28

Depolarisation to threshold AP

Answer 28

Stimulus causes a shift of membrane potential from the resting value to the threshold value; slow and graded depolarisation; strength of stimulus determines size of depolarisation

Question 29

Fast depolarisation AP

Answer 29

Once threshold is reached, potential inside quickly shifts to a positive value which does not last long (overshoot; reversal of polarisation); sudden activation of voltage-gated Na+ channels which open very fast (PNa+ increases and PK/PNa = 1:20)

Question 30

Repolarisation AP

Answer 30

Membrane potential goes back down to original RMP; Na+ channels inactivate and voltage-gated K+ channels open

Question 31

After-hyperpolarisation AP

Answer 31

AHP; membrane potential goes down more negative than the original RMP before returning to normal; votage-gated K+ channels remain open for a while (PK/PNa = 100:1) and then close

Question 32

Absolute refractory period

Answer 32

Fast depolarisation and repolarisation stages; time during AP when nerve cell is not excitable (after the stimulus evokes an AP, another stimulus would not evoke another AP within the same period - cell will not respond)

Question 33

Relative refractory period

Answer 33

Hyperpolarisation stage; if a strong stimulus was applied, it may evoke an AP, however it is harder for stimulus to reach threshold because it is lower that RMP

Question 34

Stimulus

Answer 34

Physical (electric current, light or mechanical stretch); chemical (drug or neurotransmitter)

Question 35

Voltage-gated Na+ channels in AP

Answer 35

When voltage threshold is reaches, Na+ channels open and the ions move into the cell along both the concentration and electrical gradient; residues detect small changes in potential and will change configuration to open activation gates

Question 36

Inactivation of voltage-gated Na+ channels

Answer 36

Slows down and then stops when the inside potential becomes positive (moves towards ENa+) and thus attracts Na+ ions less, and when Na+ channels inactivate; very important in nerve cells because they try avoid a large influx of Na+ ions which would depolarise the cell (losing the MP); this is when the inactivation gate changes configuration to block the channel

Question 37

All-or-none

Answer 37

Each AP is an all-or-none event which is in contrast to the graded small subthreshold depolarisations or hyperpolarisations; amplitude of APs is usually constant and does not depend on the stimulus intensity

Question 38

Evoking APs

Answer 38

Passing high current outside the nerve cell will evoke an AP under the cathode; the AP is not stationary and will move away from the point is is generated in both directions; the current will flow through the path with least resistance (outside the cell, which has no affect on excitability of membrane); across membrane and inside the axon only (can change RMP)

Question 39

Rule 2

Answer 39

When the current generated by an outside source flows through the cell membrane from outside to inside, hyperpolarisation occurs; when it flows from inside to outside, depolarisation occurs

Question 40

AP generation physiologically in CNS neurons

Answer 40

First generated in the axon initial segment (axon hillock) which has the lowest threshold (trigger zone); depolarisation to threshold evoked by EPSP which spread mainly passively from dendrites; once generated, APs are transmitted actively along the axon away from the cell body

Question 41

Axon initial segment

Answer 41

Axon hillock where the potential threshold for APs is lowest due to the higher density of voltage-gated Na+ channels (more excitable); serves as the trigger zone for APs

Question 42

EPSP

Answer 42

Cause a graident of potential which causes a flow of current that spreads along the dendrite and go through the membrane from inside to outside - depolarising the cell membrane; if it reaches threshold an AP will be generated and spread down the axon

Question 43

Unmyelinated axons

Answer 43

Small diameter (~1um); transmission of APs is slow and continuous (form point to point)

Question 44

Myelinated axons

Answer 44

Larger diameter (5-10um); transmission of APs is fast and saltatory (in large steps; discontinuous) and disrupted at nodes of Ranvier; oligodendrocytes in CNS and schwann cells in PNS (types of glia cells)

Question 45

Two stages of AP transmisison

Answer 45

Occurs in both types of axons; all about electricity (flow of ions) and therefore the flow of current 1. Passive spread (of current) 2. Generation of APs

Question 46

Passive spread of current

Answer 46

Subthreshold (has not reached the threshold) depolarisation at one region of the membrane; passive current flow (inside and outside the axon); depolarisation of adjacent parts of a membrane

Question 47

How far can current spread passively

Answer 47

Over a short distance; local (subthreshold; not activated voltage-gated channels) depolarisation induced by current injected into an axon by a glass microelectrode - AP gets smaller because some ions dissipate very quickly as it flows along axon

Question 48

AP transmission in unmyelinated axons

Answer 48

AP generated and a passive current flows (fast); depolarisation of adjacent parts of the membrane to threshold; voltage-gated Na+ channels in adjacent parts of the membrane open and a new full size AP is generated in these adjacent parts of the membrane - which takes time and so conduction velocity is very slow

Question 49

What does myelination do

Answer 49

Increases the AP conduction velocity by increasing the efficiency of passive spread of current; due to their insulating properties, there is less current dissipation as it flows along the axon, so they do not need to be regenerated at every part of the axonal membrane

Question 50

Saltatory conduction

Answer 50

Process of AP transmission in myelinated glia cells; APs generated at nodes of Ranvier and current flows passively between nodes

Question 51

Generation of AP in sensory neurons

Answer 51

Stimulus acting on receptors in sensory neurons does not immediately evoke APs; evokes a graded depolarisation in the sensory endings (receptor potential) which spreads passively to the nearby located trigger zone; APs spread along the axon toward the CNS

Question 52

AP only conducted in one direction

Answer 52

Under physiological conditions, only in one direction; the passive current flowing backwards is unable to reactivate the voltage-gated channels because they are in the absolute refractory period; by the time the ARP is over, the AP has moved down the axon

Question 53

Sensory neurons

Answer 53

PNS contains axons of sensory neurons; would be connected to sensory neurons in the skin and transmit information to the CNS; also have some in internal organs

Question 54

Strength of stimulus

Answer 54

Information encoded in the amplitude of the receptor potential and the frequency of APs

Question 55

Sensory neuron structure

Answer 55

Distal: trigger zone and connection to muscle fibre Proximal: synaptic terminal and sensory neuron cell body

Question 56

Synaptic transmission

Answer 56

Communication between neurons within the CNS in the brain or a neuron and muscle fibre; usually very fast process of transferring information between neurons or between neurons and muscle fibres; can occur through chemical or electrical synapses

Question 57

Neuromuscular junction

Answer 57

End plate; peripheral synapses

Question 58

Stages of synaptic transmission

Answer 58

1. Arrival of an AP from a cell body of a motor neuron is transmitted to the pre-synaptic terminal 2. voltage-gated Ca2+ channels open, influx of the ions from outside to inside (due to increased presynaptic Ca2+ permeability) 3. release of transmitter by exocytosis which is stored in synaptic vesicles, into the synaptic cleft 4. reaction of transmitter with postsynaptic receptors 5. Activation of ligand-gated ion channels (K+ and Na+) 6. Movement of ions leads to the depolarisation of the postsynaptic membrane; EPP and AP

Question 59

Main types of chemical synapses in the CNS

Answer 59

Excitatory: depolarisation of the postsynaptic membrane (EPSP) Inhibitory: hyperpolarisation of teh postsynaptic membrane (IPSP)

Question 60

Key features of a chemical synapse

Answer 60

Specificity: specific NTs have specific effects on the postsynaptic membrane Complexity: type, time course, strength, location, timing Plasiticity: changes in synaptic structure and function associated with development, agening and learning

Question 61

Neurotransmitters

Answer 61

Chemical messengers that open or close ion channels and lead to the depolarisation or hyperpolarisation of postsynaptic membrane; can bind to many different types of receptors, each of which produce a different affect on the neuron function

Question 62

EPSP

Answer 62

Excitatory postsynaptic synapse potential; neurotransmitters include glutamate, or ACh; open channels selective for K+ and Na+ (sometimes Ca2+), shifts the RMP towards threshold for generation of AP (depolarisation)

Question 63

IPSP

Answer 63

Inhibitory postsynaptic synapse potential; neurotransmitters include GABA or glycine; open K+ channels, hyperpolarises the membrane

Question 64

Direct gating

Answer 64

When the transmitter binds to the receptor/ion channel complex, causing the pore to open and ions to pass through, depolarising or hyperpolarising the cell membrane; very fast and short lasting

Question 65

Indirect gating

Answer 65

When the transmitter binds to receptors, activating a biochemical pathway involving a G-protein; these bind GTP when activated by a membrane receptor and leads to the production of secondary messengers (cAMP); secondary messengers activate activate protein kinases which phosphorylate specific ion channels leading to hyperpolarisation/depolarisation of membrane ; effects are slower and longer lasting

Question 66

Small molecule neurotransmitters

Answer 66

Classical NTs, usually fast action and act directly on postsynaptic receptors; amino acids, acetylcholine, amines

Question 67

Neuropeptides

Answer 67

Neuromodulators; large molecule chemicals that have an indirect action on postsynaptic receptors, or modulatory action on the effects of other NTs; slow and usually more diffuse action; neuropeptide Y (NPY), substance P, kisspeptin, enkephain

Question 68

Factors determining synaptic action

Answer 68

Type of NT; type of NT receptor/channel complex expressed in the postsynaptic membrane; amount of NT receptor present in the postsynaptic membrane

Question 69

Glutamate

Answer 69

Main excitatory NT in the CNS; binds to 3 main glutamate receptors which are directly gated ion channels (AMPA, NMDA, Kainate); each have different permeabilities to glutamate and different functions

Question 70

Excitotoxicity

Answer 70

Too much glutamate (activation of NMDA receptor) causes entry of Ca2+ and can damage/destroy the cell, leading to a stroke, epilepsy and traumatic brain injury

Question 71

NT inactivation and recovery after release

Answer 71

Diffusion (NTs removed from the synaptic cleft to some degree by diffusion) Enzymatic degradation (in the synaptic cleft by certain enzymes) Re-uptake (and re-cycling; act on postsynaptic receptors and then are transport back to where they were released)

Question 72

Integration of synaptic inputs of neurons

Answer 72

Each neuron recieves thousands of synapses (IP and EP); each of these, when activated, produce only very small postsynaptic potentials at the axon initial segment; in order to depolarise the initial segment to threshold, EPSPs need to be enhanced

Question 73

Temporal summation

Answer 73

Occurs when a high summation of APs in the presynaptic neuron elicits postsynaptic potentials that summate each other; can occur between individual EPSPs; can enhance the duration and amplitude of IPSPs

Question 74

Spatial summation

Answer 74

Effect of triggering an AP in a neuron from one or more presynaptic neurons; occurs when more than one EPSP originates simultaneously and at a different part of the neuron; can occur between individual EPSPs; can enhance the duration and amplitude of IPSPs