Structural Anatomy - Neurons Flashcards
Neurons are…
electrically conductive
3 parts of the neuron
- Axon - communicates with other cells, electrical charge moves down
- Dendrites - where cell receives input from other cells
- Cell body
Do all neurons look the same?
- NO!
- Neuron morphology - what neurons look like - is diverse and specialized
- Depends on function - such as location, how far it needs to communicate, etc.
What are processes?
- Processes (extensions) = axons and dendrites
- (Like siblings = brothers and sisters)
- The number of each type of process can dictate a cell’s name
- Unipolar
- Bipolar
- Multipolar
- Interneuron
Unipolar Process
- one process leaves cell body
- 1 additional axon coming off the cell body
Bipolar Process
- Two processes leave the body
- 2 big, additional processes coming off the cell body
Multipolar Process
- 3+ processes extend from cell body
- Lots of processes
Interneuron Process
- 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
How do driving forces in neurons come about?
- 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)
Molecules and ions cross into/out of neurons in 3 ways:
- Passive diffusion
- Facilitated diffusion through channels
- Active transport, requires energy, through pumps
Passive Diffusion
Molecules and ions cross into/out of neurons in 3 ways
- Pass through lipid-bilayer without needing a “special door” = can just move through (very left on image below)
Facilitated diffusion through CHANNELS
Molecules and ions cross into/out of neurons in 3 ways
- 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
Active transport, requires energy, through pumps
Molecules and ions cross into/out of neurons in 3 ways
- 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
A healthy neuron has a resting membrane potential (voltage) between…
- -60 mV and -80mV
- More electrically active/negative inside of the cell than the fluid that surrounds it
How can we measure resting membrane potential?
Using two electrodes/sensors:
- One measuring the charge of fluid on outside (extracellular)
- Other measuring the charge inside the cell (intracellular)
How can voltage in the membrane change (OVERALL)?
Na+ and K+ move in and out of the cell through channels
How is the resting membrane potential actively maintained?
- 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)
The neuron at rest:
- 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
At what voltage can a neuron fire an action potential?
When it exceeds -55 mV
Action potential stages:
- Rising phase
- Repolarization
- Hyperpolarization
Rising phase PT. 1: beyond -55mV
Action potential stages
- 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
Rising phase PT. 2: -30mV
Action potential stages
- 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
Start of Repolarization Phase: +50 mV
Action potential stages
- 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
Hyperpolarization: -70 mV
Action potential stages
- 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!
All or none principle - WHY?
because we need time to let those sodium/potassium pumps recharge the membrane back to normal, before starting again!
Does AP occur anywhere along the axon?
- 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
Why does charge across the membrane change to begin with?
- 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
How can charge across the membrane be affected by other neurons?
- 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
The synapse
small, active gap between two neurons
The synapse - sending…
= pre-synaptic
The synapse - receiving…
- 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)
Neurotransmitters can be (3 types)
- 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
When drugs act upon NT receptors, they can act as 2 things:
- agonists (turning on the receptor and activating its effects)
- antagonists (blocking the receptor from being turned on)
Neurotransmitter cleanup
- Diffuse away
- Broken down by enzymes
- Reuptake
○ By presynaptic neuron
○ By glia
Reuptake inhibitors
- Serotonin (5-HT) & norepinephrine (=noradrenaline) are neurotransmitters
- These drugs are generally used to treat depression, anxiety, and pain
Selective serotonin reuptake inhibitors (SSRIs) include…
…citalopram (Celexa), escitalopram (Cipralex), fluoxetine (Prozac), fluvoxamine (Luvox), sertraline (Zoloft)
Selective norepinephrine reuptake inhibitors
include desvenlafaxine (Pristiq), duloxetine (Cymbalta), levomilnacipran (Fetzima), venlafaxine (Effexor)
How to drugs work to keep neurotransmitters in the synapse for longer
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
Other cell in the nervous system
- Glia
- “glue,” “sticky”
- 10x more numerous than neurons
4 subtypes of glia
- Oligodendroglia
- Schwann cells
- Microglia
- Astroglia
Oligodendroglia
4 subtypes of glia
wrap around the axons of neurons in CNS, forming many myelin sheaths per cell
Schwann cells
4 subtypes of glia
wrap around the axons of neurons in PNS, forming one myelin sheath per cell
Microglia
4 subtypes of glia
- (smallest type of glial cell) respond to injury & disease, engulfing debris and triggering immune response
- Only in CNS
Astroglia
4 subtypes of glia
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
Blood-brain barrier (BBB)
- Protects brain
- Active transport for large molecules
What are glia like in relation to brain dysfunction?
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
