What are some ways communication takes place in our cells? Why are most of them not as productive? (3)
What is an ideal way to communicate?
- Connections between cells through Gap Junctions (in animals) or Plasmodermeta (in Plants)
- Hormones - centrally secreted, active locally
- Secondary Messenger - Signal Transduction within the cell
The methods listed above work via diffusion, making them slow
ELECTRICITY is the best (fastest) way to communicate.
What is the most effective way to communicate within our bodies and between cells?
Communication via electrical signals through nerve cells known as NEURONS
This uses electricity which is much faster.
What are the components of a Neuron?
- Cell Body - Includes the Nucleus. Mitochondria, Golgi Bodies, Ribosomes, ER, etc.
It is connected to the dendrites and axon terminal. - Dendrites - Detect Stimulus and Recieve a signal. They covert the chemical signals into electrical signals (Increase SA)
- Axon - Conducts electrical signals and stores neurotransmitters
What are connections between neurons called?
Connection between axon-dendrites is called a SYNAPSE
The chemical signal moves between axon terminal and dendrite
What are electrical and membrane potentials?
A difference in charge between 2 points creates an ELECTRICAL POTENTIAL or a voltage.
When an electrical potential exists across a plasma membrane, the separation of charges is called a MEMBRANE POTENTIAL.
What is Resting Potential?
Describe what is happening to the charges during this state. (K+)
When a neuron is not communicating with other cells, the difference in charge across the membrane is called the Resting Potential.
In Resting potential, the membrane is selectively permeable to K+.
K+ slowly leaves the cell along its concentration gradient, leaving the inside negatively charged relative to the outside. (-65 mV)
In this state the neuron is POLARIZED
How does the Sodium-Potassium Pump work?
(AKA as the Na+/K+ - ATPase).
The sodium-potassium pump is a form of active transport (needs ATP) that brings
TWO K+ ions in and sends THREE Na+ ions out.
(High conc of K+ inside, Low conc of Na+ inside)
Attached is a photo that shows the process in intricate (big words) detail 🦧.
What are the K+ leak channels?
At rest, the membrane of the neuron is relatively impermeable to most cations, however, the neuron membrane has a high number of potassium channels that allow K+ ions to leak across the membrane.
They are known as K+ Leak Channels
What is the Equilibrium Potential for K+?
(4)
- The K+ gradient established by the Na+/K+ ATPase (high K+ inside the cell, low K+ outside) favours the net diffusion of K+ out of the cells via leak channels
- Results in a more and more negatively charged inside relative to the outside.
The buildup of a negative inside begins to attract K+ ions which are positively charged. - There is now a CONCENTRATION gradient that favours K+ OUT and an ELECTRICAL gradient that favours K+ IN
- Results in the Equilibrium Potential for K+. There is no longer a net movement of K+ 🦧
There are other channels on the membrane such as the Sodium channel. The sodium channel is usually closed and tightly controlled. What would happen in the Sodium channel was opened? (5)
- The Sodium-Potassium pump is actively pumping 3 Na+ ions out of the cell. This results in a higher concentration of Na+ outside of the cell
- If the Sodium Channel were to open, Na+ ions would follow their concentration gradient and enter the cell where there are fewer Na+ ions.
- Furthermore, since the inside of the cell is already negatively charged (-65 mV) this attracts even more of the positively charged sodium ions inside.
- Now the OUTSIDE of the cell is LESS POSITIVE and the INSIDE of the cell is LESS NEGATIVE.
The net result is that the charge inside the cell goes from -65 mV to +40 mV - The neuron is now DEPOLARIZED
How and When does Polarity change in the Neuron?
(4) (PDRH)
- At the resting potential, inside the cell, there are high K+ ions and low Na+ ions. The inside is -65 mV relative to the outside.
The neuron is POLARIZED.
- When the sodium channels open at the beginning of an impulse, Na+ ions flow into the cell. The inside is now +40 mV relative to the outside
The neuron is DEPOLARIZED.
- Once the Na+ channels close again, the Na+/K+ pump and K+ leak channels will continue working and
The neuron is REPOLARIZED
- After the action potential, there is a refractory period where the membrane potential is lower than when at rest.
The neuron is HYPERPOLARIZED
What is an Action Potential?
An action potential is a rapid, temporary change in membrane potential. It may qualify as the most important type of electrical signal in cells.
Allows neurons to communicate with other neurons, muscles or glands.
Occurs when the neuron switches from polarization to depolarization.
Graph what is happening during an action potential.
What do the different slopes represent?
- Starts at rest (-65 mV) sodium gates closed (polarized)
- Sodium Gates open (+40 mV) (depolarized)
- Sodium Gates close again (repolarization)
- Refractory Period where membrane potential is slightly lower than resting. (< -60 mV) (Hyperpolarization)
What is the “All or None” Principle in the Neuron?
There are no big or small action potentials in one nerve cell - all action potentials are the same size.
Therefore, the neuron either does not reach the threshold or a full action potential is fired
What causes the opening of the Na+ channel?
Change in membrane potential known as Threshold Potential (somewhere above -60 mV)
Why do Action Potentials only travel in one direction? Why not backwards?
Because the Hyperpolization stage leads to a refractory period that makes it harder to reach the threshold potential.
What affects the speed at which the action potential moves down the axon?
Depends on the Diameter of the Axon:
the larger the axon, the faster charges will move
invertebrates have large axons (“giant axons”)
but fewer neurons
* squid
* locust 😫
* earthworm
Neurologists prefer to work with these species when studying the nervous system!
We (Vertebrates) need far more neurons than invertebrates. Therefore our neurons need to be smaller and work faster. How do we achieve this?
Schwann cells wrap around the axon and help insulate it with Myelin Sheaths
What are Schwann Cells? What are their functions?
Schwann cells wrap around the axon and help insulate it with Myelin Sheaths.
Leads to complete isolation and does not allow anything to pass through
What are the Nodes of Ranvier?
The Nodes of Ranvier are the gaps between the myelin insulation of Schwann cells which insulate the axon of neurons.
Their function is to speed up propagation of action potentials along the axon
How do the Nodes of Ranvier, Schwann cells and Myelin Sheath work together?
I have no clue, look at the photo 👹
How long are axons?
Axons can be very long or short
The longest-known axon is in the giraffe.
The motor neuron of the giraffe originates from the spinal cord and ends up in the foot. (3m-4m)
What are Tetrodotoxins?
A neurotoxin from the pufferfish that blocks sodium channels: action potential cannot be generated. Neurons don’t work
What are the areas before and after the synapse called?
The Presynaptic Neuron (on the axon side)
and
The Postsynaptic Neuron (on the dendrite side)
