anatomy part one

what is membrane potential maintained by

ion concentration differences, ion channels, and the NaK, pump

ion channels

the gatekeepers

allow Na, K, Ca to move changing membrane potential.

voltage gated channels

open with electrical charges

ligand-gated channels

open with chemical messegers

what is an action potential

rapid shifts in membrane potential caused by ion movement

depolarization

making the inside of a cell more positive

repolarization

returns cell to negative resting state

what are the two types of cells in the heart

contractile (non-pacemaker) cells

pacemaker cells

contractile cells (non-pacemaker)

artia and ventricles

they need a trigger to have a plataeu phase that allows for prolonged contraction and prevents tetanus.

pacemaker cells

- SA and AV nodes, fire spontaneously and do not have plataeu phase; responsible for initiating and regulating heartbeats.

why do heart cells matter

explains heart rhythm , ECG pattern, and how drugs or disease effect heart rate and contraction

what is membrane potential

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

normal charge for the inside of a cell

negative

what are the 3 detirmining factors of MP

• concentration of ions on both sides of the cell membrane (Na,K,Ca)

• the membrane's permeability to ions which is controlled by ion channels

• activity of ion pumps which actively move ions in or out of the cell to maintain proper balance

what is phase 4 of non-pacemaker cells

resting membrane potential is -90 mV
K+ channels are open, and K+ ions
leave the cell

what is phase 0 of non pacemaker cells

depolarization

Membrane potential of -70 mV
triggers voltage-gated Na
channels to open → Na enters
the cell → membrane potential goes
to around +30 mV

phase 1 non pacemaker cells

initial repolarization

At +30 mV, the voltage-gated
K channels open →
K leaves the cell →
membrane potential drops slightly

phase 2 non pacemaker

plataeu

Ca influx (slow L type) balances K+ efflux —>prolonged depolarization

phase 3 non pacemaker

repolarization

voltage-gated Ca channels close
The voltage-gated K channels remain open
Repolarization to -90 mV

why is plateau important?

lengthens refractory period

prevents tetany

ensures heart chambers relax and refill between beats

pacemaker cells dont have 2 things that non pacemaker cells do

a plateau phase and a stable resting potential

pacemaker cells phase 4

pacemaker potential

Na channels open slowly (Hcn) let Na in, reduced K efflux —→ slowly depolarizes cell

phase 0 pacemaker cells

depolarization

threshold (-40 Mv) → Ca influx via L type calcium channels

pacemaker cells phase 3

K efflux brings cell back to -60 mv.

repolarization

what is an L type channel

long lasting voltage gated Ca channels

what makes phase 4 of pacemaker cells different than non pacemaker cells?

they dont sit at -90mV like contractile cells they sit at -60mV and slowly depolarize

the end result is membrane potential slowly rises from -60mV to threshold until its ready to fire an action potential

this matters because this phase makes the heart autorhythmic

what makes the cell positive/negative

K efflux/influx

when it makes the cell negative, it activates Na channels at -90mV and leads to depolarization.

SA nodes

• sinoatria

(right atrium)

• sets heartrate

• fastest

• fires first

AV nodes

atrioventricular nodes

lower right atrium

delays conduction

atria finish contracting before ventricles start before ventricles start

lub

closing of the atrioventricular valves during ventricular contraction.

dub

the closing of the semilunar valves. 

SA node function

heart's natural pacemaker, which generates an electrical impulse that spreads through the atria, causing them to contract first. The signal then travels to the atrioventricular (AV) node, which sends it to the ventricles, causing them to contract and pump blood out of the heart

SA electrical conduction

initiates heartbeat, natural pacemaker.

atrial muscle cells - electrical conduction of heart

Spread depolarization via gap junctions → atria contract.

AV Node (atrioventricular node, interatrial septum)

Slows conduction (~0.1 sec delay). Ensures atria finish contracting before ventricles begin.

Bundle of His

carries impulse into interventricular septum.

conducts into interventricular septum

carry signal to apex.

Purkinje Fibers

spread impulse rapidly through ventricles for coordinated contraction

why sa node starts heartbeat

Spontaneously depolarizes fastest (~100 bpm intrinsic). Starts each heartbeat.

why does the sa node pace the heart

• Intrinsic rate higher than AV node or Purkinje.

• Fastest pacemaker overrides slower ones (overdrive suppression).

• Parasympathetic input slows resting HR to ~70 bpm .