N. System & Tissue Ch. 11, Pt. Flashcards
Neuron Function
They are highly irritable & respond to adequate stimulus by generating an action potential
Role of Membrane Ion Channels
- Leakage (nongated) channels: always open
2. Gated channels [3 types]
3 types of gated channels
- Chemically-ligand-gated channels
- Voltage gated channels
- Mechanically gated channels
When gated channels are open, ions:
diffuse quickly across membrane along electrochemical gradients & ion flow creates an electrical current & voltage changes across the membrane
Resting Membrane Potential
Potential difference cross membrane of resting cell
RMP is generated by two things:
Difference in ion concentration in ICF & ECF
Differential permeability of the plasma membrane
Difference in ion concentration:
- Lower concentration of Na+ & Cl- in ICF than in ECF
- Higher concentration of K+ & A- than in ECF
Differential permeability of membrane
Impermeable to A-
Slightly permeable Na+
More permeable to K+
The sodium potassium pump
stabilizes RMP by maintaining concentration gradients for Na+ & K+
There is a negative interior of a cell at RMP due to
greater diffusion of K+ out of the cell than Na+ into the cell
Changes in membrane potential include:
Depolarization & hyperpolarization
Depolarization is when
the cell becomes more +
A reduction in membrane potential; toward zero
Increase chances of producing a nerve impulse
Hyperpolarization is when
the cell becomes more - than at rest
An increase in membrane potential; away from zero
Reduces chance of producing a nerve impulse
Membrane potentials that act as signals… two types
Graded & Action potential
Graded Potential signals are
Incoming short-distance
Action Potential signals are
Long distance of axons
Graded Potentials
Short- lived local changes in membrane potential.
Depolarizations or hyperpolarizations
Spread locally; adjacent regions; wash out/die out
Graded Potentials happen when
a stimulus causes gated ion channels to open.
The magnitude in graded potentials
vary directly with the stimulus
Decrease in magnitude w/ distance as ions flow/diffuse through leak channels
Action Potential is a
- Brief reversal of membrane potential with an amp of ~100mV [large disturbance]
- Occurs in muscle cells & axons of neurons
- Principal means of long distance neural communication
Generation of AP includes
Resting state
Depolarization
Repolarization
Hyperpolarization
Resting state
Only leakage channels are open.
All gated channels are closed.
Depolarization
Depolarization local current open VG Na+ channels
Na+ influx
Threshold -55 to -50 mV positive feedback leads to opening of all Na+ channels & reversal of membrane polarity to +30 mV
Repolarization
Na+ channel gates close
Membrane permeability to Na+ declines to resting levels.
Voltage-sensitive K+ gates open.
K+ exits cell & internal negativity is restored.
Na+/K+ pump
Hyperpolarization
Some K+ channels remain open, excessive K+ efflux.
Slow closing gate.
At threshold..
mebrane potential is depolarized by 15-20 mV
Potassium permeability increases
More Na in than K out
Positive feedback cycle begins
Subthreshold stimulus
weak loceal depolarization that doesn’t reach threshold
Threshold stimulus
Strong enough to push the membrane potential toward/beyond threshold.
Stimulus intensity
Difference between weak/strong stimulus? Frequency of AP
Conduction velocity
vary widely on effect of axon diameter
(the larger the faster)
& on degree of myelination
Myelination effects
- Myelin sheaths insulate & prevent leakage of charge
- Saltatory conduction is 30x faster
- VG-Na channels are located at nodes and APs jump from node to node.
Multiple sclerosis
Autoimmune disease that affects mainly young adults
Short circuiting of nerve impulses; slows & eventually stops
Symptoms of MS
weakness
loss of muscular control
speech disturbances
Regeneration of Peripheral Nerves
Cell body & neurilemmal tube is intact
Stranded end of axon and myelin sheath degenerate
Axon stump puts out several sprouts
Regeneration tube guides lucky sprout back to its original destination; schwann cells produce nerve growth factors
The synapse is a..
junction that transfers information from a neuron to another or to an effector cell.
presynaptic neuron
conducts impulses towards the synapse
Delivers a message [NT]
postsynaptic neuron
transmits impulses away from the synapse
Neuron responds to the stimulus & becomes more - or +
Postsynaptic Potentials are
- Graded potentials
* Strength determined by amount of neurotransmitter release, time neurotransmitter is in the area
Types of postsynaptic potentials
1.EPSP—excitatory postsynaptic potentials
Any NT that causes depolarization
2.IPSP—inhibitory postsynaptic potentials
Hypolarization prevents muscle contraction
EPSP
NT binds to and opens chemically gated channels that allow simultaneous flow of Na+and K+in opposite directions
•Na+influx is greater that K+efflux, causing a net depolarization
•Helps trigger AP at axon hillock if it has threshold strength and opens the voltage-gated channels
IPSP
- NT binds to and opens channels for K+or Cl–
- Causes hyperpolarization
- Reduces postsynaptic neuron’s ability to produce an action potential
Integration: Summation; two types
temporal & spatial
Temporal summation
1+ presynaptic neurons transmits impulses in rapid fire order
Spatial summation
Postsynaptic neuron is stimulated by a large number of terminals at the same time
Neurotransmitters
Most neurons make 2+ neurotransmitters, which are released at different stimulation frequencies
•50+ NT have been identified
