Membrane and Action Potentials

Question 1

Nervous system cells:
2 cells types:

Answer 1

-Neuroglia: small cells that surround and wrap (glue) delicate neurons, outnumber neurons 50-1

-Neurons (nerve cells): excitable cells that conduct electrical signals

Question 2

Neuron special characteristics

Answer 2

-extreme longevity (last a persons lifetime)
-no mitosis
-high metabolic rate
–requires continuous supply of oxygen and glucose
-all have similar structure

Question 3

Neuron

Answer 3

-cell body:
aka soma
rough ER and ribosomes (Nissl bodies) -> synthesizes proteins
nucleus with nucleolus

-processes
dendrites:
-receptive (input) region or neuron
-convey incoming messages toward cell body
axons:
-nerve fiber of variable length
-conduct action potentials

Question 4

membrane potentials

Answer 4

-the voltage across the plasma membrane
-voltage (aka electrical potential) is the difference in electrical charge between two points
-this membrane potential exists in nearly all cells

Question 5

Generating membrane potential:

Answer 5

-the concentration of ions is different inside and outside of the cell
–inside of the cell has more K+ than outside
–outside of the cell has more Na+ than inside

-differences in concentration are maintained by the Na+-K+ pump
–3 Na+ out of cell, 2 K+ in per ATP

-leakage channels allow the flow Na+ and K+ through the membrane (down concentration gradient)

-membrane is 100x more permeable to K+ then Na+

-more K+ is flowing out of cell than Na+ is flowing in
–causes the inside of the cell to be more negative relative to the outside
(biggest contribution to the membrane potential)

-Na+-K+ pumps transport one more + ion out than in

-together this creates a membrane potential

Question 6

Neurons membrane potenital

Answer 6

-in a resting neuron, membrane potential ~-70–90 mV

-negative sign means that inside of the cell is negatively charge
-membrane is said to be polarized

Question 7

Ways to change membrane potentials:

Answer 7

1. anything that alters ion concentrations on both sides of the membrane
2. anything that changes membrane permeability to any ion

deopolarization: potential difference becomes smaller

hyperpolarization: potential difference becomes greater

Question 8

Graded potential

Answer 8

-short-lived, localized changes in membrane potential (depolarizations or hyper polarization)
-triggered by a change that opens gated ion channels
–chemical signals binding to receptors
–changes in charge across membrane

-spread as opposite charges attract each other
-magnitude of graded potentials declines with distance, current is lost due to leakage channels

Question 9

what does “graded” potential mean?

Answer 9

-graded means that magnitude varies with stimulus strength or frequency
–stronger stimulus opens more voltage gated Na+ channels, current travels further

-graded potentials can summate or add onto each other

Question 10

How are action potentials triggered?

Answer 10

-graded potentials are generated in the dendrites and cell body of a neuron
-they can trigger action potentials

Question 11

Action potential:

Answer 11

-rapid changes in membrane potential

-responsible for transmission of nerve signals
-only occur in neurons and muscle cells
-unlike graded potentials, they do not decay over distance

-during an action potential, the membrane potential briefly reverses before returning to normal

Question 12

4 Stages of an action potential:

Answer 12

1. Resting: all gated Na+ and K+ channels are closed
   2.Depolarization: Na+ channels open, allowing Na+ entry.
   3.Repolarization: Na+ channels are inactivating. K+ channels open, allowing K+ to exit
2. Hyperpolarization: Some K+ channels remain open, and Na+ channels reset

Question 13

what happens after an action potential?

Answer 13

-resting membrane potential has been restored
-resting ionic conditions are NOT restored

-Na+-K+ pump works to redistribute ions back to resting conditions

Question 14

Propagating action potentials:

Answer 14

Propagation- spreading
-AP is transmitted from origin down entire axon length
-Occurs in one direction

-Na+ influx through voltage gates in one membrane area cause opening of Na+ voltage gates in adjacent membrane areas
–leads to depolarization of that area, which in turn causes depolarization in next area

-since Na+ channels closer to AP origin are still inactivated, no new AP is generated here. AP only occurs in forward direction

Question 15

Refractory period + two types

Answer 15

time in which a neuron cannot trigger another AP
Two types:
1. absolute refractory period
2.relative refractory period

Question 16

Absolute refractory period:

Answer 16

-time from opening Na+ channels until resting of the channels
-purpose:
-ensure that AP is an all-or-none event
-enforces one-way transition of nerve impulses

Question 17

Relative refractory period:

Answer 17

-most Na+ channels in resting state, some K+ channels are still open
-repolarization occurring
-threshold for AP generation is elevated
–only exceptionally strong stimulus could stimulate AP

Question 18

What 2 factors does speed of AP conduction depend on?

Answer 18

1. Axon diameter:
   larger-diameter fibers= less resistance t current flow
   faster impulse conduction
2. degree of myelination:
   two types of conduction depending on presence or absence of myelin
   -1. continuous conduction (slower)
   -2. salutary conduction (faster)

Question 19

Myelin

Answer 19

-schwann cells wrap around axon in paper towel roll fashion
-the tight coil of wrapped plasma membrane is the myelin sheath
-nodes of ranvier: areas where schwann cells do not touch

Question 20

Continuous conduction:

Answer 20

NONMYELINATED AXONS
-slow b/c

-one locale stimulates adjacent part of membrane to produce AP
-AP regenerated at each gate along axon
-takes time for ions and gates to move

Question 21

Saltatory conduction:

Answer 21

MYELINATED AXONS
-30x faster than cantinas conduction

-myelin sheaths insulate and prevent leakage of charge
-voltage gated Na+ channels are located at nodes of ranvier
-APs generate only at gaps

Electrical signal appears to jump rapidly from gap to gap

Question 22

Threshold:

Answer 22

-graded potentials depolarize membrane
-if depolarization is enough to open the gated sodium channels, an action potential will occur
-this voltage needed to create an action potential is called threshold

All-or-none principle: once a stimulus reaches the threshold, action potential will occur

Question 23

Coding for intensity:

Answer 23

-all action potentials are alike in magnitude, regardless of stimulus intensity (unlike graded potentials.)

-CNS tells difference between a weak stimulus and a strong stimulus by frequency of impulses

-frequency is number of APs received per second
-higher frequency= stronger stimulus

Question 24

Reflexes

Answer 24

-quick, involuntary, stereotyped reactions of glands and muscles to stimulation
-occur the same way every time
-involve reflex arc

Question 25

5 components of a Reflex arc:

Answer 25

1. arrival of stimulus and activation of receptor
2. activation of sensory neuron
3. integration-connection between neurons
4. activation of motor neuron
5. response by effector

Question 26

reflex arc- receptors

Answer 26

-sensory receptor detects a stimulus
ex: thermoreceptors detect heat, when heat is detected, ion channels open up (creates graded potential -> action potential)

Question 27

reflex arc- sensory neuron

Answer 27

-transmits afferent action potential to the CNS

Question 28

reflex arc-integration center

Answer 28

-point of synaptic contact between neurons
-may include one more more interneurons
-synapse-> graded potential

Question 29

reflex arc-synapses

Answer 29

-connections between neurons
-incoming AP triggers release of neurotransmitter molecules that travel across synaptic cleft
-ion channels open in response to binding neurotransmitter

Question 30

reflex arc- motor neuron

Answer 30

-conducts efferent AP from the integration center to an effector organ

Question 31

reflex arc- effector

Answer 31

muscle or gland responds to efferent AP