lecture 7/8- neurophysiology I and II Flashcards
nervous system divisions (general)
- CNS
- PNS
CNS divisions (2)
brain and spinal cord
PNS divisions
- afferent (sensory)
- efferent (autonomic and somatic)
efferent divisions
- autonomic –> parasympathetic and sympathetic
- somatic
cells of the nervous system
- neurons
(excitable)
-glial cells (not excitable)
4 types glial cells found in the CNS
-ependymal cells
-astrocytes
-microglia
-oligodenrocytes
2 types of glial cells found in PNS and functions
schwann cells= form myelin sheaths, secrete neurotropic factor
satellite cells= support cell bodies
neuron parts
dendrites= receive signals
cell body (soma)= integrates information
axons= carry info to axon terminal
signal is passed on to next neuron or target cell
what is a nerve?
neuron= 1 cell
nerve= bundle of axons from multiple neurons
what are two diseases caused by demyelination?
MS: multiple sclerosis, degeneration of CNS myelin, progressive
Guillain-Barre syndrome: autoimmune degeneration of PNS myelin, sudden onset, temporary
why does demyelination result in impaired functions?
demyelination causes impaired conduction of electrical signals alonf the axon
loss of function depends on the neurons/nerves affected
what is the resting membrane potential of a neuron
-70mV
what type of channels control the permeability of the neuronal membrane?
gated channels!
-mechanically gated, chemically gated, voltage gates
2 types of electrical signals in neurons
graded potentials
- depolarization or hyperpolarization
(EPSP or IPSP)
electrical signals travel as + or - charges
-lose strength as they travel
action potentials
- all or none
- only depolarization
- amplitude is stable (strength is conserved)
is depolarization excitatory or inhibitory?
excitatory!
Give an overview of neuronal electrical signalling
- signal input in dendrites
- change in ion permeability
- generates graded potential - graded potential spreads through soma
- reaches axon hillock (high density Nav channels)
-if threshold is reaches, AP initiates - AP conducted along axon
- At axon terminal, AP triggers neurotransmitter release
- NT binds to receptors on postsynaptic dendrites/membrane
graded potentials: what happens
if there is more stimulus?
of channel density increases?
more stimulus= more activation of channels
increased channel density= increased sensitivity to stimuli
Ohms law
V= IR
or
I= V/R
current depends on electrochemical gradient of ion and resistance that opposes the flow
two sources of resistance to current flow in cells?
- Rm, membrane resistance
- membrane is normally a good insulator (high Rm)
- open channels allow ion flow (decreases Rm) - Ri, internal resistance of cytoplasm
-inversely related to cell diameter
- larger diameter= lower Ri
do subthreshold graded potential trigger an AP?
NO
do suprathreshold graded potentials trigger an AP?
YES
spatial summation
currents from almost simultaneous graded potentials combine
temporal summation
currents from two graded potentials from one postsynaptic neuron occur close together in time
how does ion permeability change during an AP?
- Na channels open and close faster than K+ channels
-rise in permeability of Na (Pna) drives rising phase of an AP - rise in Pk drives falling phase
explain AP in terms of mV
at resting (-70mV), activation gate closes the channel
depolarizing stimulus arrives at channel, activation gates open (-55mV)
When activation gate opens, Na+ enters the cell (0mV)
Inactivation gate closes and Na+ entry stops (+30mV, hyperpolarization)
K+ leaves the cell, the gates reset to original position (-70mV, hyperpolarization)
what limits how soon after an AP another can be triggered?
refractory period
(absolute and relative)
absolute refractory period
most Nav channels are inactivated
No stimulus (regardless of strength) can trigger another AP
excitability=0
relative refractory period
some Nav channels recovered from inactivation
but Kv channels still open (slow to close)
stronger stimulus needed to account for fewer available Nav channels and hyperpolarizing K+ efflux
excitability still recovering
biological current convention
biological current is defined as the movement of positive charges
Na+ entry= inward current
K+ exit= outward current
AP is regenerated as the current moves
along axon and activates Nav channels
current can move backwards but AP…
does not due to refractory period
myelinated axons and saltatory conduction
high density of Na+ channels at the nodes of ranvier
3 ways that myelin is an insulator and enhances AP propagation
- it increases membrane resistance (Rm) and enhances current flow along the axon
- action potential are only generated at Nodes of Ranvier
- it decreases membrane capacitance
Membrane capacitance
a measure of how much charge needs to be separated across the membrane to produce a given voltage
