3. Nervous System 1 (pt. 1)

an electrical impulse (signal) may be generated and propagated along the axon (called a nerve impulse)

- differences in the concentration of ions and molecules between a cell's intracellular and extracellular fluids

- signaling by some cells (especially nerve and muscle cells).

- gives cells their electrical properties

- ionic concentration differences across membrane (gradients)

- permeability of cell membrane to ions

o   K⁺, Na⁺, Cl^-, Ca⁺⁺

o   large negatively charged organic ions (org^-) – are non-diffusable proteins

- [Na⁺] is high in the ECF (low in ICF)

- [K⁺] is low in the ECF of the cell and high in the ICF

- maintained by the activity of the Na⁺/K⁺- ATPase (pump) in the cell membrane

- Cl^- is repelled by org^-  (large, negatively charged organic ions) so has a higher concentration in the ECF than the ICF

- [Ca²⁺] is higher in the ECF due to various transporters in the cell membrane and the endoplasmic reticulum membranes (i.e. the smooth endoplasmic reticulum is a storage area for calcium ions, that keeps Ca++ out of the cytosol)

- they stay inside the cell (non-diffusible proteins)

- ion channels: ions diffuse through them and down concentration gradients

non-gated channels (leakage channels) and gated channels

- they are always open

- more K⁺ than Na⁺ non-gated channels, therefore the cell membrane is more permeable to K⁺ at rest (no stimulus)

- these channels (especially K⁺ channels) are important in establishing the resting membrane potential (RMP)

- not involved at rest, open in response to stimuli

- types: voltage gates, chemical/ligand gates, thermal gates, and mechanical gates

Closed K+ leakage channel

Open ligand/chemical gated channel

Voltage-gated channel

- the difference in electrical charge between the inside and outside of a cell

- created by the movement of ions, which are charged particles, across the cell membrane

- measured in millivolts (mV)

- essential for processes like nerve signaling, muscle contraction, and maintaining homeostasis

- the charge difference (potential difference) just across the cell membrane in of a resting (not stimulated) cell

- approximately -70 mV (inside of cell is more negative)

- Na⁺/K⁺-ATPase (Na⁺/K⁺ pump) - not a channel

- org^- (large negatively charged organic ions) inside cell – can’t cross membrane

- K⁺ is the major determinant of RMP because there are more non-gated K⁺ channels than Na⁺ (so more permeable to K⁺ than Na⁺ at rest)

- contributes a little (a few mV) to RMP (pumping more positive ions out than in)

- maintains concentration gradients of Na⁺ and K⁺

- breaks down 1 ATP and uses energy to pump 3 Na⁺ out and 2 K⁺ in

- active transport as both ions are pumped against their concentration gradients

- RMP is - 70mV

- net movement is 0 as Na+ (pos charge) moving in is equal to the K+ (pos charge) moving out

- K⁺ diffusion slows as inside becomes increasingly negative

- Na⁺ diffusion into cell increases due to increasing attraction to negatively charged cell interior

- until -70 mV is reached, the amount of K⁺ moving out of the cell is greater than the amount of  Na⁺ moving in (greater K⁺ permeability)

- once at -70mV, the amount of K⁺ moving out equals the amount of Na⁺ moving in

- electrical gradient increases the rate of Na+ entry into the cell, and slows down the K+ exiting cell (creating RMP, 0 net movement of charges)

- only muscle and nerve cells

- capable of producing departures from RMP in response to stimuli (= changes in the external or internal environment)

- gated ion channels open

- MP changes, producing a graded potential.

- If the threshold potential is reached (from GP) it triggers an action potential

a small change in RMP caused by a stimulus, usually on dendrite or cell body by opening gated channels (changing membrane permeability)

- more positive than RMP = depolarization e.g. -70 mV to -65 mV (closer to zero)

- more negative than RMP = hyperpolarization e.g. -70 mV to -75 mV

- short distance signals - die away quickly (short lived)

- charges that enter the cell move towards adjacent areas of opposite charge and causes depolarization or hyperpolarization on adjacent membrane

- Ion movement = current flow

- varies directly with the strength of the stimulus

- a larger stimulus leads to a larger graded potential that travels further

o   repolarization = return to RMP after depolarization or hyperpolarization

- A large enough GP (caused by stimuli) or a sum of GPs, must be depolarization (to -55mV)

- steps: stimuli, GP reach threshold, AP