Membrane and Action Potentials Flashcards
Nervous system cells:
2 cells types:
-Neuroglia: small cells that surround and wrap (glue) delicate neurons, outnumber neurons 50-1
-Neurons (nerve cells): excitable cells that conduct electrical signals
Neuron special characteristics
-extreme longevity (last a persons lifetime)
-no mitosis
-high metabolic rate
–requires continuous supply of oxygen and glucose
-all have similar structure
Neuron
-cell body:
aka soma
rough ER and ribosomes (Nissl bodies) -> synthesizes proteins
nucleus with nucleolus
-processes
dendrites:
-receptive (input) region or neuron
-convey incoming messages toward cell body
axons:
-nerve fiber of variable length
-conduct action potentials
membrane potentials
-the voltage across the plasma membrane
-voltage (aka electrical potential) is the difference in electrical charge between two points
-this membrane potential exists in nearly all cells
Generating membrane potential:
-the concentration of ions is different inside and outside of the cell
–inside of the cell has more K+ than outside
–outside of the cell has more Na+ than inside
-differences in concentration are maintained by the Na+-K+ pump
–3 Na+ out of cell, 2 K+ in per ATP
-leakage channels allow the flow Na+ and K+ through the membrane (down concentration gradient)
-membrane is 100x more permeable to K+ then Na+
-more K+ is flowing out of cell than Na+ is flowing in
–causes the inside of the cell to be more negative relative to the outside
(biggest contribution to the membrane potential)
-Na+-K+ pumps transport one more + ion out than in
-together this creates a membrane potential
Neurons membrane potenital
-in a resting neuron, membrane potential ~-70–90 mV
-negative sign means that inside of the cell is negatively charge
-membrane is said to be polarized
Ways to change membrane potentials:
- anything that alters ion concentrations on both sides of the membrane
- anything that changes membrane permeability to any ion
deopolarization: potential difference becomes smaller
hyperpolarization: potential difference becomes greater
Graded potential
-short-lived, localized changes in membrane potential (depolarizations or hyper polarization)
-triggered by a change that opens gated ion channels
–chemical signals binding to receptors
–changes in charge across membrane
-spread as opposite charges attract each other
-magnitude of graded potentials declines with distance, current is lost due to leakage channels
what does “graded” potential mean?
-graded means that magnitude varies with stimulus strength or frequency
–stronger stimulus opens more voltage gated Na+ channels, current travels further
-graded potentials can summate or add onto each other
How are action potentials triggered?
-graded potentials are generated in the dendrites and cell body of a neuron
-they can trigger action potentials
Action potential:
-rapid changes in membrane potential
-responsible for transmission of nerve signals
-only occur in neurons and muscle cells
-unlike graded potentials, they do not decay over distance
-during an action potential, the membrane potential briefly reverses before returning to normal
4 Stages of an action potential:
- Resting: all gated Na+ and K+ channels are closed
2.Depolarization: Na+ channels open, allowing Na+ entry.
3.Repolarization: Na+ channels are inactivating. K+ channels open, allowing K+ to exit - Hyperpolarization: Some K+ channels remain open, and Na+ channels reset
what happens after an action potential?
-resting membrane potential has been restored
-resting ionic conditions are NOT restored
-Na+-K+ pump works to redistribute ions back to resting conditions
Propagating action potentials:
Propagation- spreading
-AP is transmitted from origin down entire axon length
-Occurs in one direction
-Na+ influx through voltage gates in one membrane area cause opening of Na+ voltage gates in adjacent membrane areas
–leads to depolarization of that area, which in turn causes depolarization in next area
-since Na+ channels closer to AP origin are still inactivated, no new AP is generated here. AP only occurs in forward direction
Refractory period + two types
time in which a neuron cannot trigger another AP
Two types:
1. absolute refractory period
2.relative refractory period
Absolute refractory period:
-time from opening Na+ channels until resting of the channels
-purpose:
-ensure that AP is an all-or-none event
-enforces one-way transition of nerve impulses
Relative refractory period:
-most Na+ channels in resting state, some K+ channels are still open
-repolarization occurring
-threshold for AP generation is elevated
–only exceptionally strong stimulus could stimulate AP
What 2 factors does speed of AP conduction depend on?
- Axon diameter:
larger-diameter fibers= less resistance t current flow
faster impulse conduction - degree of myelination:
two types of conduction depending on presence or absence of myelin
-1. continuous conduction (slower)
-2. salutary conduction (faster)
Myelin
-schwann cells wrap around axon in paper towel roll fashion
-the tight coil of wrapped plasma membrane is the myelin sheath
-nodes of ranvier: areas where schwann cells do not touch
Continuous conduction:
NONMYELINATED AXONS
-slow b/c
-one locale stimulates adjacent part of membrane to produce AP
-AP regenerated at each gate along axon
-takes time for ions and gates to move
Saltatory conduction:
MYELINATED AXONS
-30x faster than cantinas conduction
-myelin sheaths insulate and prevent leakage of charge
-voltage gated Na+ channels are located at nodes of ranvier
-APs generate only at gaps
Electrical signal appears to jump rapidly from gap to gap
Threshold:
-graded potentials depolarize membrane
-if depolarization is enough to open the gated sodium channels, an action potential will occur
-this voltage needed to create an action potential is called threshold
All-or-none principle: once a stimulus reaches the threshold, action potential will occur
Coding for intensity:
-all action potentials are alike in magnitude, regardless of stimulus intensity (unlike graded potentials.)
-CNS tells difference between a weak stimulus and a strong stimulus by frequency of impulses
-frequency is number of APs received per second
-higher frequency= stronger stimulus
Reflexes
-quick, involuntary, stereotyped reactions of glands and muscles to stimulation
-occur the same way every time
-involve reflex arc
5 components of a Reflex arc:
- arrival of stimulus and activation of receptor
- activation of sensory neuron
- integration-connection between neurons
- activation of motor neuron
- response by effector
reflex arc- receptors
-sensory receptor detects a stimulus
ex: thermoreceptors detect heat, when heat is detected, ion channels open up (creates graded potential -> action potential)
reflex arc- sensory neuron
-transmits afferent action potential to the CNS
reflex arc-integration center
-point of synaptic contact between neurons
-may include one more more interneurons
-synapse-> graded potential
reflex arc-synapses
-connections between neurons
-incoming AP triggers release of neurotransmitter molecules that travel across synaptic cleft
-ion channels open in response to binding neurotransmitter
reflex arc- motor neuron
-conducts efferent AP from the integration center to an effector organ
reflex arc- effector
muscle or gland responds to efferent AP
