Exam 3: Membrane and Action Potentials

Passive (leak) channels are walways open and permeability changes with conditions

Active Channels (Gated) open and close in response to stimuli
- most are closed at resting potential
- 3 states
- closed but capable of opening, open(activated), closed not capable of opening (inactivated)

Chemically gated - open in presense of specific chemicals (ACh) at a binding site - found on neuron cell body and dendrites
Voltage Gated: Respond to changes in membrane potential
- have activation gates (open) and inactivation gates (close)
- charactaristic of excitable membrane
- found in neural axons, skel muscle, sarcolemma, cardiac muscle
Mechanically Gated: Respond to membrane distortion
- found in sensory receptros for touch pressure vibration

Cytosol and ECF have different chemical/ionic balance
The plasma membrane is slectively premeable

Membrane potential changes with plasma membrane permeability in response to chemical or physical stimuli

Changes in membrane (resting) potential that cannot spread far from site of stimulation
- Any stimulus that opens a gated channel produces a graded potential
- The membrane potential is most changed at site of stimulation and effect decreases with distance
- the effect spreads passively due to local currents
- The graded change in membrane potential may involve hyper/depolarization based on charges that leave of enter the cell
- The stronger the stimulus, the greater the change in membrane potentiala dn the larger the area affected

A shift in membrane potential toward 0 mV and produces a local current
- a local current depolarizes adjacent portions of plasma membrane
- moevment of Na through channel - causes graded potential
- change in potential (increase) is proportional to stimulus
- membrane potential returns to normal when the stimulus is removed (repolarization)

Increasing the negativity of the resting membrane potential
- result of opening of K channel
- Poisitve ions move out of the cell

At cell dendrites or cell bodies: specific cell functions are triggered (exocytosislandular secretions)
At motor end plate: release ACh into synaptic cleft

Propagated changes in membrane potential that affect an entire excitable membrane
- link graded potentials at cell body with motor end plate actions

The initial stimulus is a graded depolarization of axon hillock large enough (10-15mV) to change resting potential (-70mV) to threshold level of voltage gates sodium channels (-60 to -55 mV)
- The all of none principle states that no matter how large the stimulus, if the stimulus exceeds threshold, the action potential is the same

Axolemma

1: Depolarizatino of thershold
2: Activiation of NA channels - NA ions rush into cytoplasm (inner membrane goes from neagitve to positive)
3: Inactivation of Na channels and activation of K channels
- at +30mV incativation gates close (Na channels) and K channels open, repolariazation begins
4: Return to neormal permeability
- k channels close, membrane reaches normal resting potential
- k channels finish closing and membrane is hyperpolarized to -90mV
- then returns to resting level, action potential is over

Time periodfrom beginning of action potential to the return to resting state
- during this time membrane will not respond normally to additional stimuli
Ex?
Absolute refractory period: Na channels open or inactivated - no ction potentials possible
Relative Refractory Period: membrnae potential almost normal - very large stimulus can initiate action potential

Maintains concentrations gradietns fo NA and K over time
- 1 ATP for each 2K in and 3Na out
- without ATP neurons stop functioning

Propagation moves action potentials generated in axon hillock along the entire length of the axon

Continuous propagation (unmyelinated axons): affects one segment of axon at a time
- in segment 1, depolarizes membrane to +30
- Depolarizes second segment to threshold from local current and develops action potential
- First segment enters refractory period
- local current depolarizes next segment
- cycle repeats - action potential travels in 1 direction (1m/sec)

Saltatory Propagation (myelinated axon): faster and uses less energy
- myelin insulates axons which prevents continous propagation
- local current jumps from node to node
- depolarization occurs only at nodes

Ion movement is related to cytoplsm concentration
Axon diameter affects action potential speed
Larger diameter - lower resistance

Type A: myelinated, large diameter, high speed (140m/sec)
- carry rapid info to/from CNS for position, balance, touch, motor impulses
Type B: myelinated, medium diameter, medium speed (18m/sec)
- carry intermediate signals like sensory info and peripheral effectors
Type C: Unmyelinated, small diameter, slow speed (m/sec)
- carry slower inof like involuntary muscle and gland controls

Information travels as propageted action potentials