Anatomy and Physiology Chapter 12 - Intro to Nervous Tissue Part 2

leak (passive) channels

channel randomly opens and closes

chemically (ligand) gated channels

chemical stimulus opens the channel

mechanically-gated channels

mechanical stimulus opens the channel

voltage-gated channels

change in membrane potential opens the channel

stimulation

a stimulus opens a gated channel

graded potential

localized changes in membrane potential (membrane channel most changed at site of stimulation, effect spreads passively due to local currents, either involves depolarization or hyperpolarization at site only, and the stronger the stimulus then greater the membrane potential)

depolarization

membrane potential becomes more positive

repolarization

returning back to resting potential (-70mV)

hyperpolarization

membrane potential becomes more negative

nerve impulse

once started affect entire membrane

all-or-none principle

properties of an action potential independent of relative strength of stimulus, just have to reach threshold

6 steps of the all-or-none principle

1. resting potential = at - 70mV and all gates are closed

2. graded potential = opens some Na+ channels; membrane threshold of -60mV reached

3. depolarization = at threshold, many Na+ channels open, Na+ floods into cell and potential rises to +30mV

4. repolarization = at +30mV Na+ channels close and K+ channels open; potential starts to decrease

5. hyperpolarization = near -70mV K+ channels start to close, but a lag allows this to occur briefly

6. resting potential = all voltage channels closed at rest, back at -70mV

4 things regarding action potentials propagation occuring in sections

1. as action potential develops in the initial segment, the transmembrane potential depolarizes to +30mV

2. a local current depolarizes the adjacent portion of the membrane to threshold

3. an action potential develops at this location, and the initial segment enters the refractory period

4. a local current depolarizes the adjacent portion of the membrane to threshold and the cycle is repeated

absolute refractory

period where new action potential can’t be generated; no stimulus can initiate another AP, meaning Na+ channels are open, then inactivated

relative refractory

period where a great stimulus is needed to generate an action potential; just after absolute-hyperpolarized then another AP possible (Na+ channels have reset), but minimum stimulus strength is now greater

continuous propagation

occurs in unmyelinated axons; slow, step-by-step movement of axon potential

saltatory propagation

occurs in myelinated axons; signal “leaps” from one node to the next and only ions are moved, causing it to be faster and less energetically costly

4 steps of graded potentials occuring at the synapse

1. Action Potential Arrives: AP depolarizes the axon terminal of a presynaptic neuron

2. Voltage-Gated Calcium Channels Open: Ca2+ ions enter the cytosol of the axon terminal; results in ACh release from teh synaptic vesicles by exocytosis

3. Neurotransmitters Released: ACh diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane; Na+ channels open, producing a graded depolarization

4. Signals Stop: depolarization ends as the ACh is broken down into acetate and choline by AChE; axon terminal reabsorbs choline from synaptic cleft and uses it to resynthesize ACh (synaptic delay = 0.2 to 0.5 sec but fewer synapses, faster response; synaptic fatigue = synapse response weakened until neurotransmitters replenished)

*other neurotransmitters can be a part of this process, but ACh used as an example