3-Neurons and electrical signaling

Question 1

What is the nervous system?

Answer 1

The nervous system includes: Central Nervous System which includes the brain and spinal cord.
Peripheral Nervous System which includes the efferents neuron and the afferent neurons.

Question 2

What are the efferent neurons?

Answer 2

Efferents neurons signal AWAY from the brain. They include Somatic, voluntary, motor neurons signaling to skeletal muscles, and
Autonomic involuntary motor neurons composed of the Sympathetic neurons which signal to cardiac muscle, smooth muscle, glands and Parasympathetic enteric Nervous System neurons signaling to GI tract.

Question 3

What are afferent neurons?

Answer 3

Afferents neurons signal TO the brain. They include the Somatic senses of the skin, muscles, and joints—pain.
The special senses of hearing, vision, equilibrium, smell, and taste.
The Visceral senses for internal stomach fullness, blood pressure, pH senses.

Question 4

What are the components of a neuron?

Answer 4

The neurons are composed of:
Synapses: the location at which one cell makes chemical or electrical connections with one another.
Dendrites: send info toward other cells.
Axon: Sends info away from cells.
Terminal: the tip of the axon forming synapses of dendrite to another cells.

Question 5

What are interneurons?

Answer 5

Interneurons are only located in the brain and spinal cord and relay messages down its tracks to afferent and efferent neurons.

Question 6

Describe how the Central Nervous system is organized?

Answer 6

The CNS cell bodies are grouped into nuclei. The axon are grouped in bundles or commissures.

Question 7

Describe how the Peripheral Nervous system is organized?

Answer 7

The PNS cell bodies are often grouped into ganglia and the axons are grouped together into nerves.

Question 8

What are glial cells?

Answer 8

Glial cells are non-neuronal nervous system cells. They provide support (structural and chemical) to neurons.
90% of all cells in nervous system are glial cells.
There are 5 types of glial cells:
Astrocytes responsible for development and maintenance of extracellular environment. Astrocytes provide a road map and a substrate for neurons to grow on.
Ependymal cells which are the lining of ventricles and help form cerebral fluid. Blurry CSP means menigitits. The CSP occupies the space between the brain and skull acting as a cushion allowing brain to move.
Microglia are immune response cells. Anything larger than RBC cannot get to the brain which is why it has its own immune system.
Oligodendrocytes which serve as myelin for CNS.
Schwann cells which are myelin for PNS.

Question 9

What is the blood brain barrier?

Answer 9

The blood brain barrier is a barrier of blood between the brain. It doesn’t allow everything to get to the brain including medicines. This means you have to go through the skull to treat the brain, but also protects the brain from disease.

Question 10

How do neurons work?

Answer 10

Neurons are “excitable” cells. Meaning that they can change their membrane potentials in a regulated fashion. All cell have a resting membrane potential. Neurons can change the membrane potential very quickly making electrical electrical signals. Neurons in brain and spinal cord are sending and receiving messages and are responsible for coordinating the activities you perform. Have more ions channels than most cells.

Question 11

What is a membrane potential?

Answer 11

A membrane potential is the total charge inside the cells compared to the outside. -70 means the inside the cell is -70mV more negative than the outside of the cell. The membrane itself is high resistance and low conductance because the membrane is non polar so anything polar are charged will not cross easily. Inside and outside of the cell have low resistance and high conductance meaning the charge moves around freely because they are mostly water . It has potential because it resist ions from crossing it. The separation of electrical charge is the potential (Voltage).

Question 12

What is the resting membrane potential?

Answer 12

The resting membrane potential is Determined by:
Concentration gradients of different ions across membrane (equilibrium potential—Nernst Equation)
Relative permeability of different ions through membrane (which ion channels present, open or closed).

Question 13

What does sodium and potassium have to do with resting membrane potential.

Answer 13

How much sodium is going in and how much potassium is going out which has to do with how many ion channels are there and if they are open or close describe the permeability. Because Na+ and K+ are permeable under resting conditions. There are more potassium channels than sodium channels open at rest. They are always open (leak channels), there are more K+ leak channels than Na+. Costant leakage of Na+ into the cell and K+ out of the cell would cause the ion gradin to reach equilibrium and membrane potential would be zero. The NA/K ATPase pumps 3 Na+ out and 2 K+ in to maintain concentration gradient.

Question 14

Can equilibrium potential be changed?

Answer 14

The equilibrium of Na+ and K+ are dependent on ion concentrations. Changing the ion concentrations changes the equilibrium potentials.

Question 15

What is the ionic current?

Answer 15

The ionic current is the actual movement of ions across the membrane. Current is the low of charge, i.e. the movement of charge.
I = current
g= conductance
Current = Conductance X driving force
Driving force = Membrane potential – equilibrium potential.
Conductance is how many channels of a particular ion are present and how many are open which equates to permeability. Further from equilibrium bigger the current…

Question 16

What are the types of gated ion channels?

Answer 16

Th gated ion channels are Ligand-gated, Voltage-gated, and Mechanically-gated. When these channels open, the membrane potential changes and the membrane can either go up or down .If we start at rest (-70mV) , if a channels opens it causes a positive change in the MP and the cell become polar. Depolarization can return the cells to its resting MP (repolarization).

Question 17

What are ligand gated ion channels?

Answer 17

Ligand gated ion channels is anything that binds to a receptor. Most of the time a neurotransmitter.

Question 18

What is a voltage gated ion channel?

Answer 18

Voltage gated ion channels open when the MP is within a certain range.

Question 19

What are mechanically gated ion channels?

Answer 19

Mechanical gated ion channels are physically pulled open by another protein or receptor.

Question 20

What are the types of changes in membrane potentials.

Answer 20

There are two types of changes in the membrane potential:

Graded Potentials which are small electrical signals (Sub-threshold). They can be different sizes and get smaller in magnitude with distance traveled (short distance travel). Can also hypopolarize.

Action Potentials (all or none responses) are larger electrical signals (above threshold). They are all the exact same size. Does not decrease with distance travel (long distance travel).

Threshold is -55. If the membrane potential get depollarized to get to -55, you will get an action potential. Hyperpolarizing brings the membrane further from threshold. Any change in threshold that isn’t enough to reach threshold is graded.

Question 21

Describe a graded potential?

Answer 21

Graded Potentials will either be in the form of an EPSP, or IPSP.

EPSP (Excitatory Post-Synaptic Potential): a depolarization (positive change) that brings the post-synaptic cell closer to threshold. Increases the likelihood of an AP and generally activates the cell.

IPSP (Inhibitory Post-Synaptic Potential): a hyperpolarization (negative change) that pushes the post-synaptic cell further from threshold. Decreases the likelihood of an AP and deactivates or inhibits the cell.

Graded potentials can sum together. If the summed potential exceeds threshhold, around -55 mV in most cells, an action potential is generated. This number is the point of no return for neurons.

Question 22

What is temporal summation?

Answer 22

In temporal summation one cell stimulates another cell twice before the first response has had a chance to die down. Two or more SUB-THRESHOLD stimuli add up to allow the postsynaptic cell to reach threshold. The stimuli builds on top of one another by stimulating one after the other.

Question 23

What is spatial summation?

Answer 23

In spatial summation two or more cells send simultaneous SUB-THRESHOLD stimuli to a cell that add up to get the postsynaptic cell above threshold. In spatial summation, to pre-synaptic cells/inputs stimuates the cell at the same time and they add up to the AP.

Question 24

What happens if there is a the temporal and spatial summation happening on the cell?

Answer 24

If A, B, and C all go at the same time, but A and B are EPSP and C is IPSP. You get no change in the MP because the EPSP and the IPSP cancel each other out.

Question 25

Describe in detail what a action potential is?

Answer 25

Action Potentials occur when graded potentials reach threshold. The membrane potential becomes briefly positive (shifts polarity), but only last a few milliseconds. Can propagate long distances without decreasing in amplitude. Based on changes in permeability to Na+ and K+ associated with selective opening and closing of voltage-gated ion channels. The membrane potential at any point will be approaching equilibrium of the most permeable ion.

Question 26

Describe the phases of action potentials.

Answer 26

Phase 1: The rising phase has sodium channels open because the MP is moving toward +55. There are more voltage gates (VG) for Na+ open.

Phase 2: The following phase now has potassium (K+) channels open because the membrane is slowly moving toward the equillibrium of K+. There are more VG K+ channels open.

Phase 3: There are all these VG channels. VG potassium channels are slow to open and even slower to close. The Sodium VG channels swing right open and snap shut. Potassium take its time. So in Phase 3, once the MP reaches threshold, on the way back down, the Potassium channels are told to close, but they are slow and take to long. Thus, to much K+ is leaving before it is corrected which is why phase 3 is called the undershoot phase.

At threshold both Na+ and K+ channels are told to open. Na+ ope fast which is why we have the rising phase, but K+ opens slowly which is why we have the following phase and the undershoot phase.

Question 27

Describe Voltage Gated Na+ channel gating.

Answer 27

Voltage gated channels have 2 separate gates – an activation gate and an inactivation gate. Activation gate is like a sliding door, the inactivation ate is like a ball and chain. Closed and inactivated don’t mean the same thing. If the inactivation gate is closed, nothing can cross, the channel is shut down.

Question 28

What are the voltage gated Na+ channels confirmations?

Answer 28

1. Closed but capable of opening: Inactivation gate open, activation gate closed. (Like a door that is shut, but not locked.)
2. Open: Both gates are open. (Like an open door)- during rising phase
3. Closed and incapable of opening: Inactivation gate is closed and activation gate is open. (Like a door that is closed and locked). -During phase 2

Question 29

Describe voltage gated K+ channels?

Answer 29

Voltage gated K+ channels activation gates also opened by depolarization. BUT, SLOWER activation gate (This is important for the rising phase of the action potential)
Much slower inactivation gate

Question 30

Describe action potential in more detail.

Answer 30

Phase 1: Na+ channel activation gates opening.

Phase 2: Na+ channel inactivation gates closing, K+ channel activation gates opening.

Phase 3: K+ channel activation gates closing. (Na+ channel inactivation gates opening, Na+ channel activation gates closed)

Question 31

Why are action potentials not graded?

Answer 31

Positive feedback mechanism of action potential rising phase means that maximum activation of voltage-gated Na+ channels is attained for each action potential.

Membrane potential cannot equal or exceed equilibrium of Na+ because of constant permeability to potassium. Most of these neurons are pretty uniform from one cell to the next, they all have about the same number o channels.

During the rising phase, because the sodium channels open over time- the cell reaches maximum permeability. So, if they all have the same number, they will all be the same.

Question 32

What are the absolute refractory periods?

Answer 32

Absolute refractory period are periods of time following action potential in which no stimulus of any strength can generate another action potential. All Na+ channels are either in the “open” conformation, during the rising phase, or in the “inactivated” conformation, during the falling phase. Cell must completely repolarized for Na+ inactivation gates to open again. No stimulus can generate another action potential because in order to do this, the Na+ channels ned to be open, but they were inactivated during rising phase.

Question 33

What relative refractory period?

Answer 33

Relative refractory period: period of time following action potential in which another action potential can be generated, but a much stronger stimulus must be used. Most Na+ channels are still inactivated, but some inactivation gates have opened. Also, some voltage-gated K+ channels are still open. Another action potential can be produced, but takes a larger stimulus because you have Na+ VGC still inactivated and some K+ VGC still open.

Question 34

What are refractory periods?

Answer 34

These are periods where the cell is ignoring any other AP until the AP finishes. Absolute always comes before relative. The absolute refractory period begins at threshold and ends when the cell returns to rest (this is a complete action potential). The relative refractory period happens in phase three - there are two things working against another acton potential.

1. These K+ channels that were told to close when the cell returns to rest, aren’t all closed. K+ are leaking out.
2. Some of these sodium channels are till inactivated during this time. It takes a short period of time for them to come back into play

Question 35

What is frequency coding?

Answer 35

Frequency coding describes the process of sending more action potential not stronger ones. The brain is constantly receiving action potentials and they are all the same. How can we determine between them. You code for intensity/size by producing more AP in a short period of time. The brain directs special attention to this new stimuli. An increase in frequency is an increase in magnitude.

Question 36

What is propagation of AP in un-myelinated axons?

Answer 36

Action potential do not travel down the axon in un-myelinated axon. Instead they get recreated at each site until it reaches the end of the axon.

Question 37

What are myelin?

Answer 37

Myelin are like rubber on electrical wires. The keep the action potentials propagating foward in each segment. Each segment is is refractory period directly after threshold. Entire membrane has to get to threshold which takes a lot of energy. In the CNS myelin is made of oligodendrocytes. In the PNS the myelin is made up of schwann cells. Have no ion channels in myelin. Charge cant enter or exit, only nodes need to be bought to threshold. Myelin makes AP propagation go faster. Multiple sclerosis destroys myelin and expose axons making them sensitive and painful.

Question 38

Describe the propagation of action potential in myelinated axons?

Answer 38

Myelinated action potential have saltatory conduction. Faster propagation than un-myelinated axons. Instead of having to bring every stretch of membrane to threshold in order for propagation to occur. Charge “jumps” over the myelinated regions of axons and enters and exits the axon at the Nodes of Ranvier.
Have to depolarize node, but only the nodes of Ranvier need be depolarized. Faster and more energy efficient.

Question 39

Describe propagation of action potentials.

Answer 39

Action potential propagate dow axon without decrement. APs in one patch activates potential in neighboring patch of membrane. No “back propagation” because of Absolute refractory periods. Larger diameter - faster AP propagation because of less internal resistance.