Nervous System

Action Potential

Fundamentals:

Environment and Problems

1)Extracellular fluid (blood plasma) contains water and nutrients (notably ions and glucose)
2)A cell needs three things
- 1)energy (ATP)
- 2)building blocks (to create biomolecules and maintain self)
- 3)nutrients (ions for cellular processes)

BUT...

PROBLEM1: cell membrane (lipid bilayer) is impermeable to ions and polar molecules which is needed
PROBLEM2: extracellular water "has a salt concentration equivalent to sea water" - BCH Shrt Course
Meaning a cell is in an environment that SHOULD cause it to die via dehydration (shriveling up) and malnutrition (lack of nutrients) and structural instability (lack of building materials to maintain shape)

Solutions and Proteins

Membrane proteins specifically the GLUT bring in glucose Wikipedia
- Brain needs it - to think
- Adipose tissue needs it - normal metabolism
- Muscle Tissue needs it - to move
  - So regardless of problem1 it was able take in Glucose to solve the need1 and this ATP will be able to furnish the other needs
    - Remember that Cell Metabolism take the Glucose and eventually makes ATP for energy
ATP can then be used to manipulate the permeability of the cell membrane

HOW?... Solutions and ATP

ATP is used with the Sodium Pump/NaK-ATPase Pump to pump out 3 sodium ions and 2 potassium ions in against the gradient

WAIT WHY WOULD IT WANT TO PUMP OUT SODIUM?...

Because it allows the Cotransport of Sodium and Glucose (against the gradient!) since there is a little predicament of Glucose concentrations:
- Glucose concentration intracellular is HIGH
- Glucose concentration extracellular is LOW
  - Left to its own devices, glucose would want to leave the cell
    - So this recycles sodium concentration so that the highly permeable water is tricked to stay in the cell while fulfilling the needs of ATP and Glucose, furthermore at the same time there is a gradient established between the cell membrane betweeen Na and K

Results of The Cell Fundamentals: Establishment of Resting Potential

1)Differing concentrations of ions BUT there is an electroneutrality (meaning there is a net 0 difference in cations vs anions intracellularly and extracellularly):
- Intracellular
  - K+ - HIGH
  - Na+ - LOW
  - Cl - LOW
  - Bicarb - LOWtoEVEN
  - Proteins - add to the net negative charge
- Extracellular
  - K - LOW
  - Na - HIGH
  - Cl - HIGH
  - Bicarb - EVEN
2)Permeability of cell membrane
- K will diffuse out of the cell
- Na will diffuse into the cell

○ There are nongated channels in the membrane that permit the passage of some Na+ ions back into the neuron, and K+ ions out of the neuron (again, using diffusion to achieve a concentration equilibrium), however, the membrane is not very permeable to Na+ ions. Hence many more K+ ions leave the cell than Na+ ions enter. This causes an excess of negative charge in the cell. in terms of the relative size (atomic radius) K is bigger than Na so why is the plasma membrane not more permeable to the smaller atom
--could it be due to the fact that there are more electrons? ○ The K+ ions continue to leak out until there is an equilibrium reached between the concentration gradient and the electric potential (i.e., the attraction of K+ positive ions back to the negatively charged intracellular fluid) ○ The voltage differential, again, is –70 mV on average

Functions:

Relay information form the central and peripheral nervous systems
Action potentials cause the release of neurotransmitter into the synaptic cleft

Components:

General Parts to AP

Stimulus
Receptors
AP Initation

Resting membrane potential - potential difference btwn the inside of the neuron and the extracellular space (-70 mV)
Neurons use selective permeability to ions and the Na/K ATPase to maintain a negative internal environment
Fairly impermeable to charged species
Iions are unlikely to cross the nonpolar barrier because it is energetically unfavorable
[K] is high and [Na] is low
Outside the neuron [Na] is high whereas [K] is low
Negative resting potential is generated by both negatively charged proteins within the cell and the relatively greater permeability of the membrane to K compared with Na
K is positively charged so its movement out of the cell results in a cell interior that is negative
Na/K ATPase is important for restoring this gradient after AP have been fired
Three Na out for every two K into the cel at the expense of one ATP
Inside is more negative

A/MP Initiation

All or nothing response
Neurons can receive excitatory and inhibitory input
Type of this input makes them ore likely to fire an AP whereas the latter makes them less likely
Depolarizaiton - excitatory, makes the cell less NEG

    INITIATEST THE AP
Hyperpolarizaiton - inhibitory, makes the cell more NEG
Axon hillock is depolarized to the threshold value (-55 to -40 mV) an AP will be triggered
Ion channels in the membrane open in response to the depolarizaiotn
Voltage gated ion channels there are two types
    Na voltage gated channels
    K voltage gated channels
They are both rpesent in each of the places but their activation is different
Na/K ATPase isn't only present in only neurons
Na wants to go INTO cell - more negatvie inside
    because there is less Na inside

Na channels respond to depolarizaiotn
Strong electric and chemical gradient for sodium to move into the cell
The cell potential becomes positve as a result of the sodium ion influx
Sodium channels rapidly close when the memPOT reaches about +35 mV
After the sodium depolarization, the potassium channels will cause repolarization
Speed depends on the length and cross sectional area of the axon
    Longer the axon the higher the resistance = slower the conduction
    Greater diameter of the axon lower the resistance = faster the conduction
        Greater effect is diameter and conduction
Mebrane is only permeable to ion movement at the nodes of Raniver
    Saltatory Conduction: Signal hops from node to node

Concepts:

Application:

Caffeine - stimulant that increases the rate of transmission across the synapse.

Van Castillo

Pages

Monday, August 12, 2013