Part 3 Flashcards
What are the steps of an action potential? (Just name each)
a) Resting state
b) Depolarization
c) Repolarization
d) Hyperpolarization
When is the absolute refractory period?
During depolarization/repolarization; Na+/K+ channels are either opened or inactivated so no new action potential can be generated
What is a relative refractory period?
The time after an action potential during which a neuron can fire again, but only in response to a stronger-than-normal stimulus because the membrane potential is hyperpolarized and some sodium channels are still recovering.
What are characteristics of the action potential?
a) all or nothing - degree of depolarization does not vary with the strength of stimulus
b) once started, passes all the way down the axon without fading
Explain the steps of the action potential: at resting membrane potential
Resting membrane potential:
- intracellular environment has a high [K+] while the extracellular environment has a higher [Na+]
- neuron is negatively charged relative to the outside; -70mV
- at rest 3 Na+ goes out, 2 K+ goes in, and 1 ATP is used by the sodium potassium pump as it goes against the concentration gradient to maintain the resting potential by ensuring there is more [Na+] outside the cell and [K+] inside
- neuronal membrane is semi-permeable, allowing some ions to pass through
- K+ leaky channels allow K+ to slowly leak out of the cell. The membrane in the neuron is more permeable to K+ while not for Na+, so very few Na+ will leak into the neuron
- gated Na+/K+ channels are closed
Requires a stimulus to start depolarization
Explain the steps of the action potential: depolarization
Depolarization:
- stimulus triggers depolarization, making the membrane potential less negative
- once depolarization reaches threshold of -55mV, Na+ voltage gated channels open
- Na+ ions rush into the neuron due to the concentration and electrical gradient
- Makes inside of the cell less negative, moving towards +30mV
- at the peak of +30mV, Na+ channels start to close and K+ channels open
Explain the steps of the action potential: repolarization
Repolarization:
- Na+ channels are inactivated and K+ channels open
- K+ ions flow out of the cell due to electrochemical gradient
- membrane potential moves back towards resting potential
Explain the steps of the action potential: hyperpolarization
Hyperpolarization:
- K+ voltage gated channels take longer to close, causing excess outflow of K+ ions
- this causes membrane potential to become more negative than at resting membrane potential = -80/90mV
- relative refractory period: neuron could potentially fire another action potential but would require a stronger than usual stimulus because the membrane potential is further from threshold
- following hyperpolarization, K+ channels eventually close and Na+/K+ pump restores ion gradient, returning the resting membrane potential to -70mV
Where does the action potential start in a neuron?
Initial segment; and travels down the axon
Where does the graded potential start?
Axon hillock
How is the action potential conducted without fading out like a graded potential?
It is an all or nothing event - that is mediated by voltage gated channels and Na+/K+ pumps that open as soon as they are up to threshold
Why does the action potential normally only move in one direction?
The axon hillock does not have any voltage gated Na+ channels so it cannot respond with an action potential, so to begin with, the action potential propagates in one direction only
- it can never reverse as it always travels down the axon as the previous segment is always still in the absolute refractory period
- all voltage gated sodium channels are either open or deactivated
What are the steps to conduction of an action potential along the membrane?
- As an action potential develops in the initial segment, the transmembrane potential depolarized to +30mV
- A local current depolarizes the adjacent portion of the membrane to threshold
- An action potential develops at this location, and the initial segment enters the refractory period
- A local current depolarizes the adjacent portion of the membrane to threshold, and the cycle is repeated
Why are myelinated cells faster in impulse transmission?
The spaces between the sheath are formed by individuals cells called nodes of ranvier
in cells that are myelinated, the action potential skips the internode and so travels rapidly from node to node
Saltatory propagation, conduction in a myelinated neuron:
- No voltage sensitive Na+ channels under the myelin
- No leakage of ions across the membrane
What affects conduction velocity?
- presence of myelin sheath:
- saltatory conduction = very rapid - axon diameter
- large diameter axons conduct faster than smaller ones
What are the 3 types of nerve fibers in terms of conduction velocity?
Type A : the largest axons, 4-20um diameter, fastest AP speed of 120m/s or 268mph
function: conduct sensory info from skin, joints, skeletal muscle into CNS, and conduct instructions to skeletal muscle
Type B: small, myelinated axons, 2-4um diameter, AP speed of 18m/s or 40mph
Type C: small, unmyelinated axons, 2um diameter, AP speed of 1m/s or 2mph
functions of B and C: transmit sensory information from sensory visceroreceptors (heart, lungs) and from pain, temperature, and general touch and pressure receptors. Carries instructions to smooth and cardiac muscle, glands
What is the function of Type A nerve fibers?
Type A : the largest axons, 4-20um diameter, fastest AP speed of 120m/s or 268mph
function: conduct sensory info from skin, joints, skeletal muscle into CNS, and conduct instructions to skeletal muscle
What is the function of Type B and C fibers?
Type B: small, myelinated axons, 2-4um diameter, AP speed of 18m/s or 40mph
Type C: small, unmyelinated axons, 2um diameter, AP speed of 1m/s or 2mph
functions of B and C: transmit sensory information from sensory visceroreceptors (heart, lungs) and from pain, temperature, and general touch and pressure receptors. Carries instructions to smooth and cardiac muscle, glands
What is multiple sclerosis?
Involves immune system attacking myelin
symptoms:
1. muscle weakness of spasms
2. coordination problems
3. visual problems
4. fatigue, chronic pain
5. bladder, bowel problems
6. cognitive impairment
What are the (2) types of synapses?
- electrical: less common depolarization spread from cell to cell through direct ion flow through channels - very fast! no synaptic cleft, no vesicles.
- fasted type of synapse
- chemical: two parts
a) axon terminal = excitatory/inhibitory neurotransmitter
b) receptor region on postsynaptic membrane
Process of information transfer at chemical synapses:
ex. a chonlinergic synapse (ACh)
- an action potential arrives and depolarizes the synaptic knob
- extracellular Ca++ enters the synaptic cleft, triggering the exocytosis of ACh, released in synaptic vesicles
- ACh binds to sodium channel receptors on the postsynaptic membrane, producing a graded depolarization
- depolarization ends as ACh is broken down into acetate and choline by AChE
- the synaptic knob reabsorbs choline from the synaptic cleft and uses it to synthesize new molecules of ACh
What is synaptic delay?
0.2-0.5msec
= time between the arrival of the action potential at the synaptic knob and the effect on the postsynaptic membrane
- includes: neurotransmitter release and Ca+ influx
What is synaptic fatigue?
under intense stimulation, resynthesis and transport may be unable to keep pace with the demand for neurotransmitter
How do neurotransmitters and neuromodulators work?
Are excitatory or inhibitory and work by:
a) opening/closing chemically gated ion channels (ionotropic)
b) indirectly activating G protein coupled receptors (GPCR)
c) indirectly through activating/inhibiting intracellular enzymes/channels
What are neuromodulators?
Chemicals released by neuron axon terminals that alter:
1) rate of neurotransmitter release
2) cells response to neurotransmitters
adjusts the sensitivity of a neuron to specific neurotransmitters
- effects are long term and slow to appear
- released alone or with a neurotransmitter
a) neuropeptides
b) opiods - inhibit release of neurotransmitter substance P (which relays pain sensations)
What are postsynaptic potentials?
Graded potentials generated on postsynaptic cell at the synapse
excitatory postsynaptic potentials (EPSP)
pg. 75
What are IPSPs?
Inhibitory post synaptic potentials
pg.76
Where must action potentials be started?
At the initial segment
Where do postsynaptic potentials spread?
Towards the initial segment, fading with distance
a single epsp is unlikely to cause an action potential because it is not enough to reach threshold
What determines whether the postsynaptic cell fires?
Whether local currents depolarize that region by at least 10mV
- from -70mV to -60mV
What is temporal summation?
Frequent EPSP (multiple) along 1 axon
if a second depolarization occurs before recovery from the first, more ACh will be released, increasing local depolarization
multiple EPSPs occurring in quick succession can have a cumulative effect and can generate an action potential in the postsynaptic cell
What is spatial summation?
If two postsynaptic potentials (2 axons) occur at the same time in different places, the effect is increased
multiple stimuli occurring in different places on the cell at the same time may increase to generate an action potential
What are axosomatic synapses?
Connection between axon and cell body
What are axodendritic synapses?
Connection between axon and dendrites
What are axoaxonic synapses?
Connection between axon and axon
A postsynaptic cell may receive input at hundred of synapses. The pattern of action potentials generated depends on whether the membrane potential reaches treshold
PG 82
True or false; neuromodulators and certain hormones can change the postsynaptic membrane’s sensitivity to excitatory/inhibitory neurotransmitters?
TRUE
What is presynaptic inhibition?
Inactivation of Ca++ channels of presynaptic cell make it less likely that neurotransmitter release will occur
A given cell has a resting membrane potential of -70mV, and a threshold potential of -60mV
when neuron a fires it depolarizes D by 5 mV
when neuron b fires it depolarizes D by 5 mV
when neuron c fires it hyperpolarized D by 5 mV
A. if a fires will d generate an action potential?
B. if a and b fire will d generate an action potential?
C. if a, b and c fire will d generate an action potential?
A. no
B. yes
C. no
