ch 8 neurons and electrical signaling

Question 1

the nervous system

Answer 1

• the nervous system is set up in an input and output fasion
• CNS consists of the brain and spinal cord while the PNS is all of your nerves
• info gets sent TO the CNS through afferent fibers. it gets sent FROM the CNS through efferent fibers
• somatic nervous system controls voluntary movements
• autonomic kinds of sounds like automatic which is good bc autonomic controls involuntary movements

Question 2

the nervous system CNS and PNS

Answer 2

• CNS (integration)
• peripheral nervous system (input and outputs)
• efferent: signals AWAY from brain
• somatic (voluntary, motor neurons to skeletal muscle)
• autonomic (involuntary control): sympathetic (neurons to cardiac muscle, smooth muscle, glands), parasympathetic (enteric nervous system (neurons to GI tract)
• afferent: signals TO the brain: somatic senses (skin, muscle, joint-pain), special senses (hearing, vision, equilibrium, smell, taste), viseral senses (internal-stomach fullness, bld pressure, pH)
• afferents generally inform the brain about whats going on elsewhere in the body and it gets this info from your sensory organs
• efferents are signals that come from the brain and are basically orders to be carried out by other parts like glands or muscles

Question 3

neuronal anatomy

Answer 3

• dendrites carry information TO the cell body
• axons carry info FROM cell body
• Axon Away
• at a synapse, an axon from a neuron releases neurotransmitter onto another axon, a muscle, or a gland

Question 4

neuronal anatomy

Answer 4

• CNS: cell bodies often grouped in nuclei axons grouped in bundles or commissures
• PNS: cell bodies often grouped in ganglia axons grouped together in nerves

Question 5

glial cells

Answer 5

• non-neuronal nervous system cells
• provide support (structural, chemical) to neruons
• 90% of all cells in nervous system
• 5 types:
• astrocytes (development, maintenance of extracellular environment) and create road or pathway for other cells to grown on top or and reach their appropriate destination
• ependymal cells (lining of ventricles) and line the hollow spots between brain and skill and serves a natural cushion since its very difficult to compress a fluid so small blows to the head dont cause major damage bc this fluid spreads out and absorbs the impact
• the bld brain barrier is the result of very small capillaries that support bld to the brain. they are so narrow that REC have to pass through in a single file line. bc of this, things in the bld stream dont actually make it to the brian like WBC that make immue system which is why microglia is there
• microglia (immune response) immune support for brain
• olgiodendrocytes (myelin for CNS)
• schwann cells (myelin for PNS)

Question 6

how do neurons work?

Answer 6

• neurons are excitable cells
• they can change their membrane potentials in a regulated fashion
• all cells have a resting mem pot for most cells its -70mV
• neurons are unique bc they can change their mem pot in a regular fashion bc they have lots of gates ion channels

Question 7

membrane potential

Answer 7

• water is an excellent conductor of electricity
• dont get into a pool during a thunderstorm
• the intercellular and extracellular solutions are mostly water, so charge is conducted very easily in these fluids
• the mem is lipid, which is not a good conductor of electricity
• without transport protein like an ion channel, ions cant cross
• bc of this, there is a constant separation of charge between the inside and outside of the cell (mem pot)

Question 8

resting mem pot

Answer 8

• Na+ and K+ are permeable under resting conditions
• potassium much more permeable than sodium
• at rest Na+ will diffuse into the cells, and K+ will diffuse out of the cell through “leak” channels
• leak channels are channels that are always open
• there are only a few of these channels in a typical cell but there are more k+ leak channels than there are Na+ leak channels
• thats why there is more K+ leaving the cell at rest than there is Na+ going into the cell
• mem pot is always moving towards equilibrium pot for the most permeant ion
• the fact that resting mem pot is -70mV indicates that there is more K+ crossing the mem at rest than Na+
• at rest, there is a small amount of Na+ entering the cell but even more K+ exiting the cell

Question 9

Resting membrane potential

Answer 9

• both Na+ and K+ are permeable under “resting” conditions
• K+ is about 25x more permeable as Na+ under resting
• why? bc more K+ channels are open under resting conditions. K+ channels are leakier than Na+ channels at rest
• there are more K+ leak channels than Na+ leak channels

Question 10

ion current

Answer 10

• the actual movement of ions across the membrane
• current is defined as the flow of electrical charge
• current crossing the cell mem is carried by ions typically Na+, K+, Ca2+ or Cl-

Question 11

ionic current

Answer 11

• Na+ equilibrium pot is +55mV
• the closer the mem pot gets to the equilibrium pot for an ion, the smaller the current gets
• due to the presence of gated ion channels, channels that can be opened and closed in a regular fashion

Question 12

gated ion channels

Answer 12

• ligand-gated: chemical messenger, like paracrine, neurotransmitter, or hormone
• voltage-gated: open and close by changes in mem pot
• mechanically-gated: physically pulled open by another protein. common in muscle cells

Question 13

generic changes in Vm

Answer 13

• if ion channels open and ions cross the membrane, unless ions are moving equally in opposite directions, which only happens at equilibrium, the mem pot has to change
• polarized = extreme, bc at -70 there is a big difference between the charge on the inside and outside (-70 to be exact)
• depolarization is a positing change (-70 to -40)
• repolarization (returns to -70 again)
  -hyperpolarization is a negative change (-70 to -90)

Question 14

types of changes in mem pot

Answer 14

• graded pot: small electrical signals (sub-threshold), can be different sizes, gets smaller in magnitude with distance traveled
• action potentials: larger electrical signals (above threshold), all the exact same size, does not decrease with distance traveled

Question 15

types of changes in mem pot pt 2

Answer 15

• changes in mem pot can be classified based on their size
• small changes are graded bc graded means different size
• threshold for most cells is around -55mV
• once the mem pot reaches -55 the response will be the same every single time an action pot. this is a very large change in mem pot
• all action potentials are changes in mem pot that are smaller than -55 so the responses can be many different sizes

Question 16

graded potentials

Answer 16

• will either be in the form of an EPSP or IPSP
• EPSP: excitatory post synaptic pot is a depolarization that brings the post synaptic cell closer to threshold
• IPSP: inhibitory post synaptic potential is a hyper polarization that pushes the post synaptic cell further from threshold
• a depolarization is a positive change in mem pot which will bring the me pot closer to the threshold (-55) making action pot more likely
• a hyperpolarization is a neg change in mem pot which will bring the mem pot farther from threshold making an action pot less likley

Question 17

temporal summation

Answer 17

• graded pot are very small but they can add together to reach threshold and cause an action pot this is called summation
• temporal summation involves only one presynaptic input
• an input is a cell that is communicating with the purple cell this. this input is the presynaptic cell
• the the W cell stimulates the purple cell and allows the mem time to repolarize before it stimulates again, the response is a graded pot
• but if W stimulates before repolarization occurs, the graded pot will piggyback on top of each other and bring the purple cell to threshold
• so for temporal summation, there is one presynaptic input that stimulates the cell twice and one right after another and brings it to threshold

Question 18

spatial summation

Answer 18

• involves more than one presynaptic input
• both presynaptic cells stimulate the purple cell at the same time and an action pot is generated

Question 19

action potentials

Answer 19

• occurs when graded pot reaches threshold
• mem pot becomes briefly post
• only last a few milliseconds
• can propagte long distances without decreasing in amplitude
• based on changes in permeability to Na+ and K+ associated with selective opening and closing of voltage-gates ion channels

Question 20

action potential chart

Answer 20

• phase 1: rising phase (Na+ entering cell) and depolarization
• phase 2: falling phase (K+ leaving the cell) and repolarization
  phase 3: short hyperpolarization phase

Question 21

voltage gated Na+ channel gating

Answer 21

• the voltage gated Na+ channel begins the action potential
• it has 2 gates, an activation gate and an inactivation gate
• it has 3 conformations that it can be found in

Question 22

voltage-gates Na+ channel conformations

Answer 22

• closed but capable of opening: inactivation gate open, activation gate closed (like a door that is shut but not locked)
• open: both gates are open (open door)
• closed and incapable of opening: inactivation gate is closed and activation gate is open (like a door that is closed and locked)
• closed and inactivate do not mean the same thing
• a voltage gated Na+ channel can be closed but not inactivated this means the activation gate is closed but the inactivation is open
• think inactivation is the lock on the door. first conformation above is how the channel is at rest
• the threshold those channels are all open
• at the peak of the action potential, the inactivation gate closes. This means the channel is INACTIVATED. The activation gate is still open, but that doesn’t matter. The channels are INACTIVATED. This means they are not capable of being opened until the membrane potential returns to -70mV. They are completely out of play.