Neuronal Cell Excitability Flashcards
Two major components of nervous system:
- CNS: brain and spinal cord
- PNS: nerves that enter and leave from CNS
How many pairs of nerves are in the PNS?
- 12 pairs cranial
- 31 pairs spinal
Sensory nerves in PNS:
- afferent
- from skin and skeletal muscle
Motor nerves in PNS:
- efferent
- somatic: to skin and skeletal muscle
- autonomic: to endocrine glands and visceral organs
Autonomic motor nerves have two parts:
- parasympathetic (PNS): lower heart rate and contraction
- sympathetic (SNS): higher heart rate and contraction
Enteric nervous system:
- gastrointestinal
- has local sensory and motor of its own
Neuron:
- nerve cell
- functional unit of NS
- excitable
- cell membrane: plasmalemma/neurilemma
4 basic components of neuron:
- soma / cell body (1)
- dendrites (many)
- axon: has a hillock and terminal (many and many)
Function of soma/cell body:
directs synthesis of neurotransmitter
T/F: Dendrites have areas w/ receptors for neurotransmitters
T
Function of axon:
carries action potential (AP) to other nerve cells / effectors
T/F: axon hillock is where axon enters the cell body
F, it’s where axon leaves the cell body
Axon hillock:
- site where AP are generated
- has increased [ ] of VG Na+ channels
T/F: axon terminal contains neurotransmitter vesicles that transmit into other nerve cells at synapses
T
Glial cells:
- nonexcitable
- myelin forming glial cells: increase rate of signal movement
- Schwann cells in PNS
- oligodendrocytes in CNS
Types of glial cells:
- satellite cells
- astrocytes
- microglia
- ependymal
Satellite cells:
- nonmyelin-forming
- supportive
Astrocytes:
- at synapses in blood brain barrier
- secretes ions/chem
- helps w/ ECF homeostasis by maintaining chem environment
Microglia:
- helps w/ immune system
- may contribute to neurodegenerative diseases
- scavengers in CNS
Ependymal:
- in epithelial
- stem cells
- contribute to CSF: part of ECF
Graded potential (GP):
- involves gates ion channels
- amp is directly proportionate to strength of stimulus
- decreases in strength as it spreads from origin b/c of current leak / cytoplasmic resistance
Hyperpolarization:
- caused by efflux of K+
- makes cell more neg/polar than RMP
- makes it less likely to generate AP than at RMP
Depolarization:
- caused by influx of Na+
- reverses RMP from negative to positive
- causes local reduction in membrane potential b/c of movement of positive ions into cell
- enough reduction = AP
Action potential (AP):
- rapid and large change in membrane potential followed by return to RMP
- occurs in excitable cells
- VGC in plasma membrane responsible for AP
T/F: AP don’t have an all or nothing response
F, threshold has to be reached to send signal
RMP can be altered by ________ resulting in AP
stimulus
Stimulus examples of how RMP can be altered:
- electrical (lightning/electrocution)
- chemical (neurotransmitters/hormones)
- mechanical (pressure/stretch)
What happens if there’s not enough reduction in AP?
- subthreshold/local response occurs: membrane becomes depolarized over small distance (nonpropagated potential)
Nonpropagated potential:
- aka synaptic/generator/electrogenic potentials
- size of potential change decreases exponentially w/ distance from initiation site
- potential will die out and not conducted
Threshold point:
- minimal amount of depolarization needed to convert MP into AP
- positive feedback occurs
- needs 15-30 mV depolarization of RMP to initiate AP
- causes rapid increase in rate of depolarization
- spike potential: decreases potential from 0 mV and then to 30 mV
T/F: positive mV causes change in membrane conductance of ions and continues depolarization
F, it would stop depolarization
Steps for AP at membrane:
- threshold point
- membrane then repolarizes
- AP depolarizes adjacent membrane
How does membrane repolarize?
- membrane returns to RMP
- neurons go through hyperpolarization
T/F: AP is self propagating through electrical impulse.
T
Suprathreshold stimulus:
- bigger than one needed for threshold
- doesn’t increase size of AP
VG Na+ channel:
- “fast” acting
- rapid activation time
- has double gating system
VG K+ channel:
- slower activation time
- has single gate that can be opened / closed
Step one of mechanism w/ Na+ and K+ (at rest):
- VG Na+ and K+ are closed
- Na+ not moving
- K+ leak channels open: K+ moves
Step two of mechanism w/ Na+ and K+ (depolarization stimulus applied):
- many VG Na+ open: Na+ moves into cell w/ gradient
- mem slowly depolarizes to threshold point
Step three of mechanism w/ Na+ and K+ (threshold point):
- AP initiated, so no going back
- positive feedback mechanism: Na+ entry causes more Na+ to open until all are open
Step four of mechanism w/ Na+ and K+ (depolarization phase):
- Na+ entry moves membrane potential towards Na+ equilibrium potential
- membrane rapidly depolarizes and overshoots to isoelectric point
- inside becomes positive
Step five of mechanism w/ Na+ and K+ (VG K+ channels open):
- K+ moves out of cell w/ electrochemical gradient
- slows depolarization, which counteracts impact of Na+ entry
Step six of mechanism w/ Na+ and K+ (repolarization phase):
- begins when membrane potential reaches 30 to 50 mV
- Na+ conductance decreases: less Na+ into cell
- K+ conductance increases: more K+ leaves cell
Step seven of mechanism w/ Na+ and K+ (hyperpolarization phase):
- membrane potential moves toward K+ equilibrium potential
- more negative than resting level
- VG K+ channels are slow to close, so K+ still open while Na+ closed
- K+ will eventually close but K+ has leak channels still open
- Na+/K+ ATPase pump restores ion balance and RMP
Refractory period:
- minimum time required after AP is generated before membrane can respond to another stimulus
- short refractory period = can conduct impulses more frequently
Absolute refractory period:
- most Na+ gates haven’t reset yet, so can’t reopen
- Na+ conductance is too slow to generate enough flow to trigger AP
- no stimulus here can initiate another AP
- occurs about 1 ms after AP initiated (from beginning of AP to 2/3 of repolarization)
Relative refractory peroid:
- after absolute refractory period
- lasts until RMP is established
- needs stronger stimulus than normal to reach threshold b/c has to overcome hyperpolarization effect of VG K+ channel
- amplitude of second AP is lower than normal
T/F: AP is a widespread response that occurs at multiple areas on membrane
F, it’s a local response that occurs at one specific area on membrane
Propagation:
- AP initiated at one part of membrane triggers APs in neighboring areas
Self propagation:
- domino effect
- AP can’t travel back to previous activated areas because of refractory period
Nerve impulse:
- wave of depolarization followed by wave of repolarization along axon
- basically wave of AP
T/F: increase in diameter of axons leads to decrease in velocity of AP
F, should lead to increase in velocity b/c of lower resistance to conduction
Insulation around core conductors prevents…
- loss of current to surrounding media
- plasma membrane: acts as insulator
- cytoplasm: core conductor
Myelin:
- type of insulator
- is a lipid (poor conductor of ion current)
- covers plasmalemma of axon in most neurons
Myelin prevents the loss of…
current and flow of ion between ECF and ICF
Myelin sheath:
- produced by myelin producing cells around axon
- covers axon
- Schwann cells: covers nerves in PNS
- oligodendrocytes: covers nerves in CNS
Nodes of Ranvier:
- gaps that are between myelin producing cells
- place where ion exchange can occur
- has saltatory conduction: AP “jumps” from node to node
- increases velocity of AP by reducing the resistance to conduction
- 50x faster than unmyelinated neurons
Nodes of Ranvier conserves E because…
- uses less ATP
- Na+/K+ ATPase needed to return to RMP b/c ions only cross membrane at nodes
T/F: Axon fiber type affects speed transmission
T
Order of largest to smallest axon types:
- larger axon = faster transmission
- alpha
- beta
- gamma
- delta
A alpha neuron for sensory / afferent is…
- myelinated
- largest axon
- fastest conduction velocity out of the other types
A alpha fiber for sensory / afferent carries info from…
- proprioceptors
A alpha fiber for sensory / afferent subtypes:
- indicate area of origin
- type 1a: from muscle spindle
- type 1b: from Golgi tendon organ
A beta fiber for sensory / afferent:
- type 2
- from muscle spindles, rapid touch, pressure receptors
A gamma fiber for sensory / afferent:
- type 3
- relays info about fast pain, cold temp, non-specific touch
C fibers for sensory / afferent:
- unmyelinated
- type 4
- relays info about slow pain and warm temp
A alpha fiber for motor / efferent:
- somatic motor neurons
- myelinated
- for skeletal muscle
A beta fiber motor / efferent:
- preganglionic fibers
- for autonomic motor output
C fiber motor / efferent:
- postganglionic fibers
- for autonomic motor output
VGC in plasma membrane is responsible for…
AP
Resting state of VG Na+ channel:
gates closed
- cell not permeable to Na+
Activation state of VG Na+ channel:
gates opened
- cell permeable to Na+
Inactivation state of VG Na+ channel:
one closed and the other opened
- cell not permeable to Na+
