Structural Anatomy - Neurons

Question 1

Neurons are…

Answer 1

electrically conductive

Question 2

3 parts of the neuron

Answer 2

• Axon - communicates with other cells, electrical charge moves down
• Dendrites - where cell receives input from other cells
• Cell body

Question 3

Do all neurons look the same?

Answer 3

• NO!
• Neuron morphology - what neurons look like - is diverse and specialized
• Depends on function - such as location, how far it needs to communicate, etc.

Question 4

What are processes?

Answer 4

• Processes (extensions) = axons and dendrites
• (Like siblings = brothers and sisters)
• The number of each type of process can dictate a cell’s name

1. Unipolar
2. Bipolar
3. Multipolar
4. Interneuron

Question 5

Unipolar Process

Answer 5

• one process leaves cell body
• 1 additional axon coming off the cell body

Question 6

Bipolar Process

Answer 6

• Two processes leave the body
• 2 big, additional processes coming off the cell body

Question 7

Multipolar Process

Answer 7

• 3+ processes extend from cell body
• Lots of processes

Question 8

Interneuron Process

Answer 8

• no axon or short axon (instead, lots of little stringy ends)
• Characteristic of brain regions that don’t receive sensory information nor send motor outputs - just connect brain areas to other brain areas

Question 9

How do driving forces in neurons come about?

Answer 9

• Inside vs. the outside of the neuron are chemically different
• Balance between inside and outside of neuron is KEY to the functioning of the neuron (otherwise, would be no driving forces)

Question 10

Molecules and ions cross into/out of neurons in 3 ways:

Answer 10

1. Passive diffusion
2. Facilitated diffusion through channels
3. Active transport, requires energy, through pumps

Question 11

Passive Diffusion

Molecules and ions cross into/out of neurons in 3 ways

Answer 11

• Pass through lipid-bilayer without needing a “special door” = can just move through (very left on image below)

Question 12

Facilitated diffusion through CHANNELS

Molecules and ions cross into/out of neurons in 3 ways

Answer 12

• Can go down their concentration gradients (middle on image)
• If there’s more of something on the outside than inside, then it’ll have the drive to equal these things out (by pulling more molecules from the outside)
• NOTE: still moving through without any added energy needed

Question 13

Active transport, requires energy, through pumps

Molecules and ions cross into/out of neurons in 3 ways

Answer 13

• Potentially more molecules on the inside rather than out - thus, not having that “drive” or concentration gradient to follow
• Inability to pass may also be due to charge
• However, can be brought into the cell using ADDED ENERGY
• Call these gates/channels PUMPS

Question 14

A healthy neuron has a resting membrane potential (voltage) between…

Answer 14

• -60 mV and -80mV
• More electrically active/negative inside of the cell than the fluid that surrounds it

Question 15

How can we measure resting membrane potential?

Answer 15

Using two electrodes/sensors:

• One measuring the charge of fluid on outside (extracellular)
• Other measuring the charge inside the cell (intracellular)

Question 16

How can voltage in the membrane change (OVERALL)?

Answer 16

Na+ and K+ move in and out of the cell through channels

Question 17

How is the resting membrane potential actively maintained?

Answer 17

• Maintained by sodium- potassium pumps (goal: to make inside of the cell more negative)
• 3 Na+ out: 2 K+ in
• Uses 2/3rds of brain’s total energy (ATP)

Question 18

The neuron at rest:

Answer 18

• More Na+ outside the cell: cant cross, but have the drive to
• More K+ inside the cell: cant cross, but have the drive to
• Each have dedicated channels (doors that only they fit through) that are closed at rest but open at predictable voltages
• Neither can cross the membrane when channels are closed

Question 19

At what voltage can a neuron fire an action potential?

Answer 19

When it exceeds -55 mV

Question 20

Action potential stages:

Answer 20

• Rising phase
• Repolarization
• Hyperpolarization

Question 21

Rising phase PT. 1: beyond -55mV

Action potential stages

Answer 21

• Voltage-gated Na+ channels open, Na+ outside the cell “falls down” its concentration gradient and enters the cell > cell becomes more +
• RESULT: charge inside the membrane becomes more positive

Question 22

Rising phase PT. 2: -30mV

Action potential stages

Answer 22

• Around -30 mV, K+ channels open, K+ “falls down” its concentration gradient and leaves the cell
• AT THIS POINT: K+ going out of the cell (more outside than inside), Na+ going into the cell
• Inside is still positive, but we now also have K+ simultaneously going out

Question 23

Start of Repolarization Phase: +50 mV

Action potential stages

Answer 23

• Once +50 mV is achieved, voltage-gated (sodium channels) channels close
• Once this is cut off, the inside of the cell is no longer becoming positive, and we still have potassium leaving - MAKING THE CELL MORE NEGATIVE

Question 24

Hyperpolarization: -70 mV

Action potential stages

Answer 24

• At around -70 mV, the potassium channels also close - so we no longer have potassium leaving the cell and making the cell more negative
• Lastly, sodium/potassium pumps work to reset the cell back to normal:
• Potassium that left, come back in!
• Sodium that came in, leave!

Question 25

All or none principle - WHY?

Answer 25

because we need time to let those sodium/potassium pumps recharge the membrane back to normal, before starting again!

Question 26

Does AP occur anywhere along the axon?

Answer 26

* Unmyelinated axons have Na+ channels (Nav) all along their surface * Myelinated axons not all covered by myelin; rather, have little bubbles and gaps between myelinated areas, leaving the axon exposed * Nav only at the nodes of Ranvier: where myelin is interrupted

Question 27

Why does charge across the membrane change to begin with?

Answer 27

* Could be caused directly by a stimulus in a sensory cell (E.g., light activates rods in the eye, stretch activates muscle sensory cells) * OR can be caused by inputs from other neurons

Question 28

How can charge across the membrane be affected by other neurons?

Answer 28

* At the end of each axon is a terminal button * Button has vesicles filled with molecules called neurotransmitters – molecules that allow neurons to communicate with one another * When an action potential reaches the terminal button, NTs are released into the synapse

Question 29

The synapse

Answer 29

small, active gap between two neurons

Question 30

The synapse - sending...

Answer 30

= pre-synaptic

Question 31

The synapse - receiving...

Answer 31

* postsynaptic neuron * Postsynaptic neurons have a variety of receptors that fit, like a lock and key, with specific neurotransmitters * When NTs bind to a corresponding receptor, they trigger changes that push the charge of the postsynaptic cell up (towards depolarizing) or down (towards hyperpolarizing)

Question 32

Neurotransmitters can be (3 types)

Answer 32

* Excitatory: make the receiving neuron more likely to fire an action potential * Inhibitory: make the receiving neuron less likely to fire an action potential * Modulatory: trigger other changes

Question 33

When drugs act upon NT receptors, they can act as 2 things:

Answer 33

* **agonists** (turning on the receptor and activating its effects) * **antagonists** (blocking the receptor from being turned on)

Question 34

Neurotransmitter cleanup

Answer 34

1. Diffuse away 2. Broken down by enzymes 3. Reuptake ○ By presynaptic neuron ○ By glia

Question 35

Reuptake inhibitors

Answer 35

* Serotonin (5-HT) & norepinephrine (=noradrenaline) are neurotransmitters * These drugs are generally used to treat depression, anxiety, and pain

Question 36

Selective serotonin reuptake inhibitors (SSRIs) include...

Answer 36

...citalopram (Celexa), escitalopram (Cipralex), fluoxetine (Prozac), fluvoxamine (Luvox), sertraline (Zoloft)

Question 37

Selective norepinephrine reuptake inhibitors

Answer 37

include desvenlafaxine (Pristiq), duloxetine (Cymbalta), levomilnacipran (Fetzima), venlafaxine (Effexor)

Question 38

How to drugs work to keep neurotransmitters in the synapse for longer

Answer 38

By blocking reabsorption of serotonin into the pre-synaptic cell, the drug lengthens the time serotonin is available in the synapse to act on the post-synaptic cell

Question 39

Other cell in the nervous system

Answer 39

* Glia * “glue,” “sticky” * 10x more numerous than neurons

Question 40

4 subtypes of glia

Answer 40

* Oligodendroglia * Schwann cells * Microglia * Astroglia

Question 41

Oligodendroglia ## Footnote 4 subtypes of glia

Answer 41

wrap around the axons of neurons in CNS, forming many myelin sheaths per cell

Question 42

Schwann cells ## Footnote 4 subtypes of glia

Answer 42

wrap around the axons of neurons in PNS, forming one myelin sheath per cell

Question 43

Microglia ## Footnote 4 subtypes of glia

Answer 43

* (smallest type of glial cell) respond to injury & disease, engulfing debris and triggering immune response * Only in CNS

Question 44

Astroglia ## Footnote 4 subtypes of glia

Answer 44

are the largest glial cells: * Support endothelial cells of the BBB * Provide nutrients to neurons * Maintain ion balance in CNS * Repair after injury * Communicate with neurons and glia * Control and maintain synapses Only in CNS

Question 45

Blood-brain barrier (BBB)

Answer 45

* Protects brain * Active transport for large molecules

Question 46

What are glia like in relation to brain dysfunction?

Answer 46

Glia likely play an underestimated role in many neurological conditions given their immune and “clean-up” roles Future drugs may target them Research is ongoing for topics including: * depression * stroke * spinal cord injury * multiple sclerosis * autism * Schizophrenia