32 terms

Nerve Conduction

Characteristics of a neuron
-ability to generate and conduct an impulse
-limited ablility to regenerate
resting membrane potential
-the electrical difference between the inside of the neuron and the interstitial fluid when nerve is at rest
-neg. 70 mV for most cells neg. 40mV for cardiac cells
resting membrane potential chemical distribution
-the membrane has a 50-100 times greater permiability to K+ than Na+, so the K+ keeps leaking out to leave a negative atmosphere in the cell.
-in the cell; protein sacks that are stuck (to big to get out) and K+ that are getting pumped in.
-outside the cell; Na+ and Cl-
sodium potassium pump
-needs ATP to run
-located in the plasma membrane
-transports Na+ out of cell and K+ into cell
-maintains a relatively constant Na+ concentration
2 types of non-gated channels
-potassium channels:more numerous, allows K+ to exit the cell
-sodium channels: sodium has more difficulty entering
4 types of gated channels
-chemical gated ion channels
-voltage gated or voltage regulated ion channels
-mechanically gated ion channels
-light gated channels
chemical gated channels
-sensitive to chemicals like neurotransmitters, hormones, and foods
-found in cell bodies, dendrites, receptors for taste and smell
voltage gated channels
-activated by changes in the membrane potential charge (-55mV is the magic number)
-located in the axon trigger zone and the nodes of Ranvier
mechanically gated channels
-activated by mechanical vibration and pressure
-located in the dendrite receptors like touch and pressure receptors in skin
light gated channels
-activated by light
-located in the rods and cones of the retina
the nerve ability to respond to a stimulus and convert it to an impulse
-the negative atmosphere of -70mV makes the cell capable of excitability
-40% of all ATP goes to Na+ pumps in order to keep the resting potential state!
-any condition capable of altering the membrane potential
-can be negative or positive
negative stimulus
shifts the membrane potential away from the threshold for action by inviting K+ to leave the nerve
positive stimulus
shifts membrane potential toward the threshold for action potential
threshold stimulus
strong enough to initate an action potential (magic number!)
sub-threshold stimulus
too weak of an intensity to generate an action potential, so some positivity is reached, but not enough
can be combined with other stimulus to achieve action potential
action potential
-wave of negativity that dominoes along the outer membrane of a neuron (inside has depolarized)
-a rapid change in membrane potential that involves a move from - to + (depolarization) followed by a repolarization (returns to resting state of -70mV)
absolute refractory period
-depolarization to +30 mV
-coincides with the action potential and lasts for a msec.
-cell membrane not responsive to other stimulus at this point
relative refractory period
-repolarization that can dip to -90mV because the K+ pumps are slower to close
-lasts for 10-15 msec
-cell membrane will only respond to stimulus that takes it above threshold
depolarization sequence
1. chemical gates open allowing Na+ into the cell
2.a positive shift in electrical diff. moves towards threshold (-55mV)
3. ACTION POTENTIAL! with result as the voltage gates open at the axon trigger zone
4. voltage gate channels are stimulated to let even more Na+ in to the cell, similar to a positive feedback system
5. voltage shifts from -70mV to +30mv = NERVE IMPULSE!
repolarization sequence
1. during the 2nd half of the AP plasma membrane becomes highly permeable to K+ and relatively impermeable to Na+ (also Na+ close)
2. as K+ diffuses out, more leaves than Na+ came in because the K+ gates are slow to close. this causes hyperpolarization (-90mV)
3.sodium-potassium pumps restore the membrane to resting potential
all or none principle
-action potential occurs maximally or not at all
-duration and magnitude of AP is the same under normal conditions
-if a stimulus causes AP the impulse is transmitted along the whole neuron at a constant strength
2 types of nerve conduction
saltatory and continuous
saltatory conduction
-can be up to 300mph
-along myleninated nerve fibers
-AP propagates at the exposed segments (nodes of Ranvier)
-conduction is fast because no ions can escape through the myelin so the depolarization can only happen at the exposed neurofibrils
continuous conduction
-more like 2mph so for less "emergency" responses
-along unmyelinated fibers
-AP occurs in one segment of the membrane and then triggers the next segment and so on . . .
2 factors that determine speed of impulse transmission
fiber diameter and degree of myelination
fiber diameter
larger = faster propagation of impulse
larger= less resistance to intracellular current flow (big hose theory)
degree of myelination
more myelination= faster impulse because the current can't leak out of the axon's plasma membrane wherever the myelin is
3 types of nerve fibers
A fibers, B fibers, C fibers
A fibers
-largest in diameter with short absolute refractory period (faster recovery for next stimulus)
-myelinated and can conduct up to 268mph
-motor neurons and sensory neurons relating to touch, pressure, joint position and balance
B fibers
-medium size
-myelinated and conduct impulses at 40mph
-motor neurons bring commands to the autonomic ganglia, and snesory information from the internal organs as well as pain and temperature information (?)
C fibers
-smallest in diameter and have the longest refractory period
-unmyelinated conduct impulses at 2mph
-motor neurons transmitting information from the autonomic ganglia to smooth muscle, cardiac muscle and gland and sensory fibers bringing info from the internal organs, along with pain and temp. info