Section 4: Nerves

Nerves

Module 4

Excitable Cells - Can use resting potential to generate electrochemical impulse (action potential)

--> Ex. nerve & muscle cells

Action Potential--> rapid reversal of resting membrane

  1. Strong depolarization at axon hillock triggers of Na+
  2. Na+ rushes in
  3. Membrane depolarizes to +35mV
  4. Na+ channels inactivate, K+ open
  5. K+ rushes out
  6. Membrane re-polarizes +35mV --> 7-0mV
  7. K+ rushes out hyper polar -90mV
  8. K+ closes channel, slowly back to 7-0mV

Voltage Gated Na+ Channel V--> depolarization of membrane --> Na+ flow in cell ---> Na+ closes & returns to normal configuration ---> Channel won't open during absolute refractory period -

Voltage Gated K+ Channel

--> open when Na+ close

--> K+ flow out of cell

--> gate closes & returns to resting configuration

--> don't have an inactivation periodR

efractory Periods

Absolute --> Na+ channels can't be activated

Relative --> 1membrane yper polarized

2--> aused by K+ that slow to close

--> i is possible to fire stimulus but would need stronger stimulus


Threshold Staring Action Potential

--> initial depolarization must be strong enough to open almost all of the Na+ voltage gated channels

--> occurs when membrane potential depolarizes to -55mV -

-> once this reached will always have action potential


Action Potential Propagation

--> movement of action potential down axon

--> direction current flow (+) --> (-) (opposite attract)


Un myelinated Nerve

  1. inside membrane (+) (35mv) cause Na+ enter cell
  2. (+) charge attracted to (-) of near by resting membrane
  3. Nearby cell depolarizes, due to build up of (+) charge --> causes Na+ channels to open
  4. Na+ depolarizes, creates new action potential
  5. Repetition of this causes propagation along membrane

Ex. think of human wave at baseball game


Myelinated Nerve (Saltatory Conduction)

*Saltare - To jump*

--> much faster than Uu yelinated due to jumping

--> insulates axon to prevent ions leaking

--> voltage gated channels only at gaps (Nodes Ranvier)

  1. (+) charge from action potential attracted to adjacent node (-)
  2. Node depolarizes
  3. Triggers voltage-gated Na+ to open
  4. Na+ rushes in, depolarizes, starts new action potential
  5. Repeat & propagated

--> due to absolute refractory period, propagation can't & won't go backwards --> always forward

Multiple Sclerosis

--> myelinated sheaths under attack

--> body's natural immune system attacks & damages myelin

--> can stop transmission of action potentials

--> if nerve damaged, connected to muscle, muscle won't contract --> causes paralysis


Synaptic Transmission

--> connection of nerve cell to other nerve, muscle or organ called chemical synapse



euromuscular Junction (NMJ)

  1. synapse between neuron & muscle cell
  2. leads to contraction of muscle cell
  3. membrane presynaptic axon terminal contains Ca2+ channel

--> open when cell membrane depolarizes

  1. axon terminal contains vesicles with acetylcholine (ACh)
  2. basement membrane of axon terminal contains enzyme acetylcholinesterase (AChE)

muscle cell membrane (sarcolemma) under axon terminal --> called end plate (has receptors for acetylcholine

) --> associated with ligand-gated ion channels


Events at NMJ

  1. action potential triggers Ca2+ voltage gated channels open Ca2+ flows into cell
  2. Ca2+ triggers fusing synaptic vesicles to membrane & release of ACh into synaptic cleft via exocytosis
  3. ACh diffuses across synapse to receptors on muscle cell
  4. ligand-gated channels open, Na+ flows in, few K+ leave --> triggers local depolarization called end plate potential (EPP)
  5. depolarization EPP spreads to adjacent cells, channels open, large number of Na+ flows into muscle cell and triggers action potential
  6. ACh broken to acetic acid and choline by AChE --> choline back to axon terminal to recycle



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