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A Principles of Physiology > Nervous System > Flashcards

Nervous System Flashcards

1
Q

Diagram of axon

A
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2
Q

Function of dendrites

A

Reach out and receive information from other cells

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3
Q

Function of axon initial segment (also called axon hillock)

A

Generates an electrical impulse/action potential

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4
Q

What is at the end of a neuron (not dendrite end)

A
  • Synaptic connections with target cells
  • At this synapse, it releases a signalling molecule
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5
Q

Cell biology of a neuron

A
  • Proteins are synthesized in the cell body, but the distant tips of the dendrites could be a few millimiters away from the cell body (this is unusual and not usually seen in other cells).
  • To get proteins to the tips of dendrites that are 2 millimiters away is a challenge. To get proteins to the end of a very long axon that may be 3 meters long in some animals is an even bigger challenge.
  • If something goes wrong with the cell, the distant parts of the cell that are furthest of the body will be the most likely to show signs of damage.
  • Therefore, long-projecting neurons are often the first to show signs of disease (neurological symptoms appear in hands and feet because the cell bodies of those neurons are in the spinal cord).
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6
Q

What occurs in the cell body?

A

Transcription and translation

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7
Q

Information flow through a neural network

A
  • Afferent information comes into the central nervous system (sensory information)
  • There are neurons within the central nervous system that process that information and make a ‘decision’ - what’s the appropriate motor output?
  • Efferent motor neurons carrying commands out of the CNS, affecting target tissue e.g. a muscle, gland, or another neuron
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8
Q

Voluntary vs. reflexive movements

A
  • Voluntary is initiated by the brain
  • Reflexive is initiated automatically
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9
Q

What is the difference between cardiac muscle and skeletal muscle?

A

Cardiac muscle is innervated by autonomic neurons, so it’s part of the autonomic/visceral system

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10
Q

Smooth muscle

A
  • Visceral motor
  • E.g. lines digestive tract
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11
Q

Skeletal muscle

A
  • Somatic muscle
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12
Q

Steps of information flow through a neuron

A
  • Dendrites receive inputs (not shown on the diagram that there could be thousands of other cells converging onto this one neuron)
  • Cell body ‘integrates’ all input
  • Axon hillock ‘decides’ on whether an action potential is fired. Decided by voltage-gated sodium channels in the axon hillock. This part of the cell has a high density of sodium channels
  • Once the decision is made to fire an action potential, the axon conducts the AP to the very end of the axon
  • If there is a 100 mV depolarization & repolarization at the beginning of the axon, you want to have the same 100 mV depolarization & repolarization at the end of the axon
  • At the presynaptic terminal, there is release of a chemical messenger- neurotransmitter. It goes a very short distance across the synaptic cleft and binds to the receptor that recognizes that signaling molecule, and that receptor does something e.g. opens/closes an ion channel. In the end, it changes the state of the following cell.
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13
Q

What happens at the presynaptic terminal?

A
  • There is release of a chemical messenger- neurotransmitter.
  • It goes a very short distance across the synaptic cleft and binds to the receptor that recognizes that signaling molecule, and that receptor does something e.g. opens/closes an ion channel. In the end, it changes the state of the following cell.
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14
Q

What happens at the postsynaptic membrane

A

It has receptors that, when bound, cause a change in cell function

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15
Q

Synaptic cleft

A

The small space between the presynaptic terminal and the postsynaptic terminal

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16
Q

Are neurons good at conducting electricity?

A

No, the only way you can get an action potential generated at the cell body to travel more than a mm is if you regenerate that action potential along the way.

  • This happens at the break in the Myelin. Another group of voltage-gated sodium channels gets depolarized, opens up and lets a fresh influx of sodium occur to generate a full action potential again. This is propagation.
  • At every node, the action potential is regenerated so that it gets to the end at the same size that it was when it began.
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17
Q

What is the effect of local anaesthetic on action potentials?

A
  • The action potential will fail to propagate because the nodes of Ranvier (the sodium channels) will be blocked
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18
Q

Blood-brain barrier

A
  • Neurons are ‘high-maintenance’ cells
  • They’re very sensitive to changes in/low levels of oxygen, pH, glucose, temperature, ion concentration etc.
  • The neurons within the CNS are even more fragile
  • The blood-brain barrier protects neurons from sudden changes in these factors
  • Enough blood has to get to the brain and it also has to be selective about what comes out of the blood plasma and what goes into the extracellular fluid surrounding neurons
  • The capillaries are surrounded by support cells in the brain - astrocytes (one of the glial cells)
  • Not fully depicted in the diagram- these cells cover the capillary like a blanket and regulate movement of substances
  • So the permeability of a capillary in your brain is less than in the muscle cells, liver, etc.
  • This is part of the reason why so much blood goes to the brain every minute- it’s more challenging to get nutrients out and waste back in
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19
Q

Membrane potential overview (Vm)

A
  • All cells in the body demonstrate an electrical potential across their membrane (inside negative compared to outside, outside positive compared to inside)
  • This potential energy is used to do work- move things across the membrane
  • Excitable cells- nerves and muscle- use the change in membrane potential to carry information
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20
Q

What is a typical resting membrane potential?

A

-65mV

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21
Q

How do cells change the membrane potential?

A
  • Cells use energy (ATP) to pump ions out of equilibrium to establish a gradient (K+ high inside; Na+ high outside).
  • Make cell membrane (K+ channels) permeable to K+ and Vm becomes negative
  • Make cell membrane (Na+ channels) permeable to Na+ and Vm becomes positive
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22
Q

Ionic concentrations inside and outside the cell

A
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23
Q
A
24
Q

What is the chemical gradient?

A

Same as diffusion gradient- concentration of solutes on either side of a membrane

25
Q

If have Na+ and Cl- on either side of a membrane (with channels that let them pass through), and you put a battery in, making one side of the membrane positive and the other negative, what will happen to the ions?

A

The Na+ will be repelled from the positive side and will move into the negative side, and the Cl- will do the opposite.

26
Q

Explain the electrochemical equilibrium

A
  • If you have sodium and chloride on either side of a membrane that has a sodium channel but no chloride channel, and there is more sodium and chloride on the left, sodium will diffuse to the right.
  • But because the chloride doesn’t move across the membrane, and it is negative (Cl-), there will be an excess negative charge on the left side.
  • With more sodium diffusing to the right side, there will be an excess more positive charge on the right.
  • The sodium ion will be sitting in an electric field, and there’s an electrical gradient that would tend to hold that sodium in on the more negative side.
  • At a certain point, there will be sufficient charge on the membrane to prevent sodium from going down its chemical gradient.
  • This is the electrochemical equilibrium. The sodium ion is just as likely to go left or right because the two forces are equal and opposite.
27
Q

What does a membrane potential of -65mV mean in terms of the distribution of ions

A
  • For every 100,000 equally-balanced pairs of negative and positive ions on either side of the membrane, there’s one extra negative in and one extra positive out.
  • Relatively few sodium ions have to move before you get a change in the membrane potential.
28
Q

Nernst Equation

A
  • Allows you to calculate the equilibrium potential (membrane potential at which there is an equilibrium) for an ion
  • [ion]out / [ion]in is the concentration gradient
  • Z is the charge of the ion: +1 for sodium and potassium
29
Q

3 ways to think of the Nernst equation

A
  1. Restates the concentration gradient in electrical terms.
  2. Determines the Vm (membrane potential) at which there is no net flux of this ion across the membrane
  3. Determines where the membrane potential will be drawn to if the membrane is made more permeable to this ion.
30
Q

If the membrane potential is at the equilibrium potential, what will be the net movement of ions?

A

None

31
Q

What is the inverse of resistance?

A

Conductance

32
Q

Ohm’s Law

A
  • Current (coming through an ion channel) is equal to the conductance (number of channels open) times the driving force
  • When the membrane potential is at the equilibrium potential (Vm - Eion) = 0, the driving force is 0, so there is 0 current
33
Q

What are the two ways to have zero current?

A
  • Membrane potential is at equilibrium potential –> driving force is 0
  • 0 ion channels open
34
Q

What is the driving force on ions?

A
  • The difference between the membrane potential and the equilibrium potential
  • It changes because the membrane changes
35
Q

When the membrane potential is at the equilibrium potential, ___

A

The forces acting upon the ion are equal and opposite, so there is no net movement of the ion through the channel

36
Q

What if only one ion is permeable?

A

Watch lecture at 5:25

37
Q

What if more than one ion is permeable?

A

Watch lecture at 10:26

38
Q

GHK equation (constant field equation)

A
  • Actually used to calculate membrane potential
  • Weighted average of the Nernst Potentials of all permeable ions with permeability as the weighting factor.
39
Q

What would happen to the GHK equation if the permeability to sodium were 0?

A

The equation would say: the membrane potential (Vm) equals the Nernst potential for potassium

40
Q

What would happen to the GHK equation if the permeability to potassium were 0?

A

The equation would say: the membrane potential (Vm) equals the Nernst potential for sodium

41
Q

If the permeability to potassium is 0, and the membrane is permeable to sodium, what will happen to sodium?

A
  • Sodium will go in until the membrane potential reaches +61 Mv (the equilibrium potential for sodium)
42
Q

When both sodium and potassium are permeable, what will dictate where the membrane potential goes?

A
  • The relative permeability to potassium and sodium
  • If potassium is more permeable, it will be closer to the equilibrium potential for potassium (-80 mV)
  • If sodium is more permeable, it will be closer to the equilibrium potential for sodium (+61 mV)
43
Q

How does the membrane potential change?

A
  • Assuming that the concentrations are maintained, the sodium and potassium and pumps and other ions are regulated properly, its the permeability of the membrane to the ions through the opening and closing of ion channels that will change the membrane potential
  • As you open sodium channels, concentrations don’t change (with the one exception of calcium)
44
Q

What is permeability due to?

A

The number of open ion channels

45
Q

Membrane potential in a motor neuron axon (most simple action potential)

A

Watch lecture at 26:00

46
Q

Voltage-gated sodium channels

A
  • The channel forms a pore in the membrane
  • The membrane-spanning region is an electric field. There are amino acids in the protein that have charged groups on them that are sitting in this electric field.
  • The magnitude of the electric field changes the movement of the amino acid groups, which changes the 3D shape of the protein.
47
Q

Why do we describe voltage-gated sodium channels opening as a positive feedback loop?

A
  • As the membrane potential becomes more positive on the inside, it changes the shape of the voltage-gated channel and increases the probability that the channel will open.
  • The consequence of the opening (depolarization opens the channel) is more depolarization, because more sodium comes in.
  • This, in turn, causes more depolarization
48
Q

When does an ion channel become inactivated?

A
  • When the membrane potential becomes the height of an action potential
  • The gate on the intracellular side ‘slams shut’, clogs the pore, and prevents sodium from getting through
49
Q

The more negative the membrane potential, the more likely it is that sodium channels will be ___

A

Closed, ready to open

50
Q

Once a voltage-gated sodium channel is inactivated, can it go back to an open state?

A
  • No, it must first be deinactivated back to the closed state, after which it can open
  • This explains the refractory period
51
Q

What explains the refractory period?

A
  • Once a voltage-gated sodium channel is inactivated, it cannot go back to an open state, and must first be deactivated back to the closed state, which requires a negative membrane potential
  • So, once an action potential fires, the membrane must repolarize down to a negative value before you can hae another action potential
  • Therefore, each action potential is a discrete event
52
Q

What is hyperpolarization caused by?

A
  • The opening of additional potassium channels
  • They are voltage-gated, so when the membrane depolarizes, it opens these delayed potassium channels, causing potassium to leave and the inside to be hyperpolarized again, causing them to close (negative feedback loop)
  • This is also combined with the potassium leak channels
53
Q

When are leak potassium channels open?

A

Always

54
Q

Diagram of action potential

A
55
Q

Another diagram of action potential with graph of relative membrane permeability

A
56
Q
A
57
Q
A

A Principles of Physiology (25 decks)

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