5.2 Nervous tissue: Action potentials Flashcards
Describe action potential in neurons
Principal way neurons send signals
* Means of long-distance neural communication
- Occur primarily in muscle cells and axons of neurons
- Brief reversal of membrane potential with a change in voltage of ~100 mV
*Action potentials (APs) do not decay over distance
In neurons, also referred to as a nerve impulse
Involves opening of specific voltage-gated channels
What channels regulate action potential
- Only leakage channels for Na+ and K+ are open at rest
- Each voltage-gated K+ channel has one voltage sensitive gate
- Closed at rest
- Opens slowly with depolarization
- Each voltage-gated Na+ channel has two voltage sensitive gates
- Activation gates: closed at rest; open with depolarization, allowing Na+ to enter cell
- Inactivation gates: open at rest; block channel once it is open to prevent more Na+ from entering cell
- *two gates 3 states: closed, open and inactivated
Stages of action potential
describe the absolute refractory period
*size of AP is not important, no addition action portnetial can be generated during the absoulte refractory period
- Time from opening of voltage-gated Na+ channels until resetting of channels (involves opening the inactivation gate)
- Ensures that each AP is an all-or-none event
- Enforces one-way transmission of nerve impulses
relative refractory period
Most voltage-gated Na+ channels have returned to resting state
- Some voltage-gated K+ channels are still open
- Threshold required for initiating an AP generation is elevated
what is the role of the Na+/K+ pump in the action potential
- Repolarization restores resting electrical conditions of neuron
- Only a small amount of Na+ and K+ cross the membrane during an AP (e.g. 0.012% percent of the Na+ outside the cell enters)
- Ionic redistribution is restored by the Na+/K+ pumps
describe Threshold and the All-or-None Phenomenon
Not all depolarization events produce APs
- For an axon to “fire,” depolarization must reach threshold voltage to trigger AP
- Threshold:
- Membrane is depolarized by 15 to 20 mV
- Na+ permeability increases
- Na+ influx exceeds K+ efflux
- The positive feedback cycle begins
• All-or-None: An AP either happens completely, or does not happen at all
Describe propogation of action potential
- propogation allows AP to be transmitted from origin down entire axon length twds terminal
- Na+ influx through voltage gates in one membrane area -> local currents that cause opening of Na+ voltage gates in adjacent membrane areas
- Once initiated, an AP is self-propagating
* In nonmyelinated axons, each successive segment of membrane depolarizes, then repolarizes
*Propagation in myelinated axons differs
• Since Na+ channels closer to the AP origin are still inactivated, no new AP is generated there
– AP occurs only in a forward direction
How does strength of stimulus intensity relate to action potential
*APs are alike & are independent of stimulus intensity
-Strong stimuli can generate APs more often than weaker stimuli
• CNS determines stimulus intensity by frequency of impulses
Conduction velocity
- vary widely • Effect of axon diameter
– Larger diameter fibers: less resistance to local current flow and faster impulse conduction
• Effect of myelination – Myelin sheaths insulate & prevent leakage of charge –
Saltatory conduction: Voltage-gated Na+ channels located at nodes of Ranvier • APs jump rapidly from node to node (30X faster than non-myelinated)
How does voltage decay in bare plasma membrnae, unmyelinated axn and myelinated axon?
Bare PM: doesnt have voltage gated channels like dendrites, voltage decays bc current leaks across
Unmyelinated: voltage gated Na and K channels regenerate ap at point along axon, votlage does not decaay
*Conduction is slow because movements of ions & of the gates of channel proteins take time & must occur before voltage regeneration occurs.
Myelinated axon: keeps current in axons (voltage doesn’t decay much). APs are generated only in nodes of Ranvier & appear to jump rapidly from node to node.
What are the classifications of nerve fibers
Group A:
- 150 m/s (~300mph)
- large diameter, myelinated
- Ex: somatic sensory motor fibers
Group B:
- 15 m/s (30 mph)
- intermediate diamter, lightly myelinated
- ANS motor fibers serving isceral organs and smaller somatic sensory fibers from skin (pain and touch)
Group C
- 1 m/s (2 mph)
- Smallest diameter, unmyelinated
- include some ANS motor fibres serving visceral organs and smaller somatic sensory fibers from skin (pain &
how does multiple sclerosis related to AP
Multiple sclerosis (MS) = autoimmune disease -> affects primarily young adults
• Myelin sheaths in CNS are destroyed when immune system attacks myelin
– Turns myelin into hardened lesions called scleroses -> Impulse conduction slows and eventually ceases -> Demyelinated axons increase Na+ channels, causing cycles of relapse and remission
- Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, incontinence
- Treatment: drugs that modify immune system activity
describe the synpase
- junction that mediates information transfer from one neuron to another or an effector cell
- Presynaptic neuron: conducts impulses toward synapse
- Postsynaptic neuron: transmits impulses away from synapse
*in PNS: postsynaptic cell may be a neuron, muscle cell, or gland cell