11-2: Neurons at Work

Question 1

Functions of neuron

Answer 1

• receive signals
• integrate received information and produce output signal
• conduct signal
• transmit signal to other nerve cells

Question 2

structure of neuron

Answer 2

• cell body contains nucleus and other organelles
• insulated by myelin sheath, serves for conduction. causes depolarization (signal conduction) to spread farther along axon making the distant lengths of axon reach threshold sooner

Question 3

axon hillock

Answer 3

region where signals travel down the axon

Question 4

synapse

Answer 4

end of axon where signals are transmitted to other cells

Question 5

reflex

Answer 5

-direct response to signal without involving brain
-When pain receptors are
stimulated, sensory neurons activate interneurons in the spinal cord that directly activate motor neurons serving muscles in the arm

Question 6

sensory input

Answer 6

-Sensors detect external stimuli, such as light, sound, smell, etc., and also detect internal
conditions, such as blood pressure or muscle tension. Sensory neurons (nerve cells) transmit information from sensors to the central nervous system (CNS).

Question 7

Intergration

Answer 7

In the CNS, interneurons integrate (analyze and interpret) received information.

Question 8

motor output

Answer 8

• Motor neurons leave the CNS with information communicated to the effector cells.

Question 9

CNS

Answer 9

• central nervous system

- included brain and spinal cord

Question 10

PNS

Answer 10

• peripheral nervous system

- includes neurons, sensory and nervous

Question 11

ionic gradient

Answer 11

-contributes to membrane potential
-sodium potassium pump and maintains the ionic gradients
of Na+ and K+ ions
-large number of open potassium channels(no gates present), allows for significant leak of K+ ions out of cell (LEAK CHANNELS)
-Because the membrane is just weakly permeable to chloride and other ions, it is this outflow of potassium that results in a net negative charge inside the cell.

Question 12

membrane potential

Answer 12

-voltage at any living cell’s plasma membrane
- (-70)mV
-maintained by ion channels
-messages are transmitted as changes in membrane potential
-The membrane potential of a
neuron not sending signals is
called resting potential.

Question 13

sodium potassium pump

Answer 13

• removes 3 Na+ out of cell and brings in 2 K+ into cell
• uses ATP
• creates uneven distribution of charges

Question 14

Action potential and its characteristics

Answer 14

• rapid, temporary change in a
  membrane potential, as a response to stimuli that can open or close ion channels in the neuron plasma membrane.
  -action potentials are:
  • All-or-none phenomenon
  • They travel in one direction
  • They are fast
  -Action-potentials are known to be:
  • They regenerate themselves as they travel
  • They leap from one node of Ranvier to another
  (this is known as saltatory conduction)

Question 15

steps of action potential

Answer 15

• depolarization phase
• repolarization phase
• undershoot

Question 16

Depolarization phase

Answer 16

-has to shift from its resting potential at -70 mV to about -55 mV, which is termed threshold potential.
• The membrane becomes less polarized than before (hence the membrane potential becomes more positive).
• If the membrane potential reaches the threshold (-55 mV)
some Na+ channels in the axon’s membrane will open, and Na+ ions will rush into the axon
• If a stimulus changes the distribution of charge across the membrane
sufficiently, the gated sodium channels open.
-Both Na+ and K+ voltage- gated channels are involved in the production of an action potential. Both channels are opened by depolarizing the membrane,
but they respond independently and sequentially.
- Na+ channels open before K+ channels, K+ still closed
-• If depolarization reaches the threshold, the action potential will be triggered.
The Na+ influx causes further depolarization, which opens even more
Na+ gates, allowing even more Na+ to enter the cell, and so on.
• If depolarization reaches the threshold, the action potential will be
triggered.

Question 17

Repolarization phase

Answer 17

• The membrane potential reaches close to +40 mV.
- change is triggered by the closing of Na+ ion channels and the opening of K+ ion channels in the membrane
• At that time the membrane experiences a rapid
repolarization (membrane potential becomes more and
more negative), because K+ ions flow out of the axon.
• After equal numbers of K+ ions have moved out for the number of Na+ ions that had moved in, the charge difference
across the membrane returns to the resting level (positive on the outside and negative on the inside).
• However, repolarization event tends to result in hyperpolarization and the membrane becoming more negative than the resting potential.
• Most Na+ channels close
• Most K+ channels open
• The inside of the cell is
negative again

Question 18

undershoot

Answer 18

Undershoot:
• The membrane becomes hyperpolarized (membrane
potential is more negative) than at the beginning.
• It happens when more K+ ions have left the cell than Na+
ions had entered.
-This occurs because the action potential moves in just one direction through the axon, and Na+ channels are refractory
Na+ channels are closed
• Some K+ channels are open
• K+ channels will eventually close
• The membrane potential will come to the resting phase again once Na+ channels open again.

Question 19

Voltage-gated channels

Answer 19

• Have gates; open or close due to changes in the electrical
membrane potential near them
Involved in generating the action potentials, as they
respond to changes in the membrane potential.
• In the neuron, there are two types of voltage-gated channels: sodium and potassium. These channels aid in propagating electrical signals in one direction only.

Question 20

resting potential

Answer 20

• both gated Na+ and K+ channels are closed. Non-gated channels (not shown) maintain the resting potential

Question 21

summary of changes

Answer 21

• Maintaining membrane potential: K+ leak out of the cell, and Na+/K+ pump keeps pumping 3 Na+ out for 2 K+ in.
• Resting potential: ­Na+ outside is high, ­K+ inside is high; membrane is positive outside and negative inside.
• Depolarization: ¯Na+ outside is low, high ­K+ inside (and Na+ too);membrane is negative outside and positive inside.
• Repolarization: ­high K+ outside ­high Na+ inside; membrane is
positive outside and negative inside.

Question 22

Neural activity transmission

Answer 22

• First neuron’s axon -> the synaptic cleft -> another neuron’s dendrite
• The information is what is transmitted.
• The junction between the neuron and the other cell is called a synapse.
• It can be electrical (in the cardiac muscle) and chemical

Question 23

synaptic cleft

Answer 23

The narrow gap between
the sending neuron and
the receiving neuron

Question 24

synapse process

Answer 24

• Excitatory postsynaptic potentials affect channels that allow extracellular Na+ to flow into the postsynaptic neuron, causing local depolarization and bringing the neuron closer to the threshold for the initiation of an action potential.
-• As such, Na+ ions flow into the postsynaptic neuron, causing further
depolarization and bringing the axon closer to the threshold for
initiation of the excitatory postsynaptic potential.
• At some synapses, the ligand-gated ion channel is permeable to both
K+ and Na+. When such channel opens, the membrane potential
depolarizes toward a value closer to the threshold.

Question 25

IPSPs

Answer 25

-inhibitory post synaptic potentials
• At some other synapses, the ligand-gated ion channel is selectively
permeable to either K+ or Cl- ions.
• When such a channel opens, the postsynaptic membrane
hyperpolarizes. This produces an inhibitory postsynaptic potential,
because it moves the membrane potential further away from
threshold.
• As such, K+ ions flow out, or Cl- ions flow into the neuron, increasing
the polarization within the neuron, making it less likely that an
action potential will be triggered.

Question 26

What happens in the postsynaptic neuron

then?

Answer 26

Action potential
• Involves voltage-gated ion
channels
• All-or-none (independent of the strength of the triggering
stimulus)
• Regenerate themselves

Postsynaptic potential
• Involves ligand-gated channels
• Graded (the amount of
neurotransmitters)
• Do not regenerate themselves

-Once the postsynaptic membrane reaches the threshold (if so), the postsynaptic potential becomes an action potential.

Question 27

The effect of multiple synapses

Answer 27

-EPSPs make action potentials more likely in post synaptic cells
-IPSPs make action potetnials less liekly in post synaptic cells
-simultaneous EPSPs and IPSPS cancel each other out
- single excitatory postsynaptic potential cannot trigger an action potential in a postsynaptic neuron, because it is not strong enough.
• However, if 2 EPSPs (excitatory postsynaptic potentials) occur in rapid succession at a single synapse, then the second EPSP can trigger an action potential before the first one reaches the refractory phase. This is known as a temporal summation.
• Spatial summation occurs when a different synapse on the same receiving neuron receives another EPSP