Ch. 12 - Nervous Tissue II Flashcards
What are leak channels?
- randomly open/close; allow ions to move down conc gradient
- exist in dendrites, axons, cell bodies
What are ligand-gated channels?
- require binding of specific chemical (e.g. NT, hormone) to open/close
- exist in dendrites
What are mechanically gated ion channels?
- open/close in response to mechanical stimuli (e.g. vibration, pressure, stretching)
- in sensory neurons
What are voltage-gated channels?
- open/close in response to changes in membrane potential
- in axons (b/c this is where APs are propagated!)
What are graded potentials?
- occur in response to a stimulus
- local deviations form resting potential of -70mV
- if sufficient to reach threshold, will trigger an AP
What are action potentials?
- occurs when local membrane potential becomes more positive than -55mV
Where do graded potentials usually occur? What type of ion channels do they involve?
dendrites and cell body of a neuron; mechanically gated + ligand-gated
C/C hyperpolarization and depolarization
H - membrane becomes more negative than -70mV (resting potential); further away from threshold –> less likely to trigger AP
D - membrane becomes less negative than -70mV; closer to threshold –> more likely to trigger AP
Why does a stronger stimulus cause a larger graded potential than a weaker stimulus?
because more ion channels will open up
What kind of ion channels do APs involve?
voltage-gated Na+ and K+ channels
Where is an AP initiated? What does it result with?
axon hillock and travels down length of axon; NT release at next synapse
What are the phases of an AP?
depolarizing and repolarizing phases
What occurs during the depolarizing phase of an AP?
- voltage-gated Na+ channels open
2. Na+ enters cell and changes MP to +30mV
What occurs during the repolarizing phase of an AP?
- Na+ channels inactivate (inflow stops)
- K+ channels open to let K+ out of cell
- MP returns to -70mV (repolarization)
*MP can hyperpolarize (reach -90mV)
What is the difference between absolute refractory and relative refractory period?
absolute - inactivated Na+ channels must return to resting state before they can be reopened; strong stimulus will not trigger another AP
relative - Na+ channels have returned to resting state (closed but can be opened again); suprathreshold stimulus can trigger an AP
What must happen to ‘reset’ the Na+ and K+ gradient so the neuron can fire another AP?
Na+/K+ pump
What can increase the intensity of an AP?
higher frequency of APs + number of sensory neurons activated
How do local anesthetics manipulate APs?
- blocks voltage-gated Na+ channels of neurons from opening
- prevents AP generation so pain sensations don’t get transmitted to brain
What is the difference between an electrical synapse and a chemical synapse?
E - ionic current spreads via gap junctions
C - transfer from presynaptic to postsynaptic cells; involve NT + receptors
What are the 2 effects of a NT?
excitatory and inhibitory
How does an excitatory effect affect ion channels?
NT causes opening of ligand-gated Na+ channels; triggers depolarization
How does an inhibitory effect affect ion channels?
NT causes opening of ligand-gated K+ or Cl- channels; triggers hyperpolarization
What is the difference between spatial summation and temporal summation?
S - NTs released from several end bulbs onto one neuron
T - NTs released from multiple firings of same end bulb
What are the two classes of NT?
- small-molecule NTs
2. neuropeptides
What are some examples of small-molecule NTs?
- Ach
- Glutamate (excitatory)
- GABA + glycine (inhibitory)
- norepinephrine, dopamine, serotonin
- NO
- adenosine
What are some examples of neuropeptides?
- endorphins & other opioid peptides
- substance P
How do opioid peptides relate to substance P?
substance P enhances perception of pain; opioid peptides are natural painkillers by blocking release of substance P
What are 3 mechanisms that prevent over-stimulation of postsynaptic cell? (terminate signal)
- diffusion
- enzymatic degradation
- uptake
What are the 3 steps involved in the repair process of PNS neurons?
- chromatolysis
- Wallerian degeneration
- formation of a regeneration tube
