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Phys 1412 > Topic 3: Nervous System I > Flashcards

Topic 3: Nervous System I Flashcards

1
Q

Neurons

A

-Neurons are excitable (responsive to stimuli)

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2
Q

nerve impulse

A

when a neuron is stimulated (usually on cell body or dendrites) an electrical impulse may be generated and propagated along the axon

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3
Q

Electrical Properties of Cells due to:

A
  • ionic concentration differences across membrane (gradients)
  • permeability of cell membrane to ions
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4
Q

Important ions

A
  • K+, Na+, Cl-, Ca++

- large negatively charged organic ions (org-) – are non-diffusable proteins

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5
Q

Na+/K+- ATPase (pump) ions concentration

A
  • [Na+] + [K+] due to and maintained by activity of pump in cell membrane
  • [K+] is higher inside cell
  • [Na+] is lower inside cell
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6
Q

[Ca2+] low inside the cell

A

due to various transporters in cell and endoplasmic reticulum membranes

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7
Q

Cl-

A

repelled by org- (large, negatively charged organic ions) so is higher outside of the cell than inside

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8
Q

org-

A
  • large, negatively charged organic ions

- stay inside the cell

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9
Q

Permeability of cell membrane to ions

A

-determined by ion channels - ions diffuse through them down conc. gradients

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10
Q

Ion channel types

A
  • non-gated

- gated

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11
Q

Non-gated

A
  • always open
  • more K+ than Na+ ∴ cell membrane more permeable to K+ at rest (no stimulus)
  • these channels (especially K+ - more numerous) are important in establishing the resting membrane potential (RMP)
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12
Q

Gated

A
  • not involved at rest
  • open in response to stimuli:
  • membrane voltage changes = voltage gates
  • chemicals e.g. binding of hormone or neurotransmitter (nt) = chemical gates
  • temperature = thermal gates
  • mechanical deformation = mechanical gates
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13
Q

Resting Membrane Potential (RMP

A
  • At rest (not stimulated), a charge difference (potential difference) exists just across the cell membrane = membrane potential
  • ≈ -70 mV (inside of cell is more -ve)
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14
Q

Factors establishing RMP

A
  • Na+/K+-ATPase (Na+/K+ pump)
  • org- inside cell – can’t cross membrane
  • More non-gated K+ channels than non-gated Na+ channels (membrane more permeable to K+ than Na+ at rest ∴ K+ is the major determinant of RMP)
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15
Q

Na+/K+-ATPase (Na+/K+ pump)

A
  • breaks down 1 ATP and uses energy to pump 3 Na+ out and 2 K+ in ⇒ both ions are pumped against their concentration gradients ∴ active transport
  • effects:
  • -maintains concentration gradients of Na+ and K+
  • -contributes a little (a few mV) to RMP (pumping more +ve ions out than in)
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16
Q

More non-gated K+ channels than non-gated Na+ channels (membrane more permeable to K+ than Na+ at rest ∴ K+ is the major determinant of RMP)

A
  • K+ diffuses out of cell down concentration gradient ∴ cell loses +ve charge (inside becomes more –ve)
  • unlike charges attract and K+ diffusion slows as inside becomes increasingly -ve
  • Na+ diffusion into cell increases due to increasing attraction to –ve cell interior
  • until -70 mV reached, +ve out (K+) is greater than +ve (Na+) in – greater K+ permeability
  • once at -70mV, the amount of +ve (K+) moving out equals the amount of +ve (Na+) moving in – force on Na+ much higher than on K+
  • ∴ The net movement of charge (ions) is 0 (equal in both directions): RMP of -70 mV
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17
Q

Electrically Excitable Cells

A
  • ONLY muscle and nerve cells

- capable of producing departures from RMP in response to stimuli (= changes in the external or internal environment)

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18
Q

When a neuron is stimulated

A
  • gated ion channels open
  • MP changes = producing a graded potential. If the threshold potential is reached…
  • triggers an action potential
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19
Q

Graded Potentials (GPs)

A

stimulus causes a small change in RMP, usually on dendrite or cell body (no longer at rest!) by opening gated channels (changes membrane permeability)

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20
Q

GPs possible results:

A
  • more +ve than RMP = depolarization
    e. g. -70 mV to -65 mV (closer to zero)
  • more –ve than RMP = hyperpolarization
    e. g. -70 mV to -75 mV
21
Q

GPs characteristics

A
  • ions move passively (unlike charges attract (+,-)) = current flow, causing depol. or hyperpol. on adjacent membrane
  • GPs are short distance signals - die away quickly (short lived)
  • magnitude and distance travelled by potential varies directly with the strength of the stimulus
    i. e. larger stimulus ⇒ larger graded potential that travels further
  • GPs can summate - 1st GP present when 2nd stim occurs ⇒ these add (sum) together to create the resulting GP
22
Q

After a GP

A

repolarization = return to RMP after depolarization or hyperpolarization

23
Q

GPs to Action Potential (AP)

A
  • GPs are essential in initiating a nerve impulse (AP)
  • if the GP causes depol. and if it is large enough i.e. caused by a critical stimulus (or multiple GPs sum to be large enough) ⇒ leads to an AP
24
Q

GPs to Action Potential (AP) steps:

A
  • critical stimulus (or summating stimuli)
  • GP reaching threshold
  • Action Potential
25
Q

Action Potential (AP)

A
  • a nerve impulse (signal)
  • large change in MP that propagates along an axon with no change in intensity
  • initiates at trigger zone
    e. g. axon hillock of multipolar and bipolar neurons; just past dendrites of unipolar neurons
  • once K+ channels close ⇒ MP returns to RMP (e)
  • *NOTE: Na+/K+-ATPase always working to maintain gradients
  • takes 10,000s of APs to cause a measurable change in [ion] in the cell
26
Q

AP phases

A
  • depolarization phase
  • repolarization phase
  • After-hyperpolarization phase (below the RMP)
27
Q

Depolarization phase

A
  • voltage-gated Na+ channels respond to MP change (ie GP) and open – greatly increases Na+ permeability
  • as gates open more Na+ diffuses in (further changing MP) ⇒ causes even more Na+ gates to open (a +ve feedback mechanism)
  • Na+ diffuses in causing depolarization to +30 mV (inside membrane becomes +ve)
28
Q

Repolarization phase

A
  • Na+ channels close, become inactivated (decreased Na+ permeability) ⇒ Na+ movement returns to resting levels
  • voltage-gated K+ channels open (increased permeability) ∴ K+ diffuses out (+ve charges (K+) move out – decreases MP)
29
Q

After-hyperpolarization phase (below the RMP)

A

i. K+ channels are slow to close and remain open longer than necessary
ii. Na+ channels are reactivated – can respond to stimuli at this point

30
Q

Refractory Periods of an AP

A
  • Absolute Refractory Period (prevents AP summation)

- Relative Refractory Period (region d)

31
Q

Absolute Refractory Period (prevents AP summation)

A
  • NO AP can be generated, regardless of stimulus size
  • results from either:
  • -all Na+ channels being open (region b) or
  • -all Na+ channels being inactivated (can’t open until MP reaches RMP, region c)
32
Q

Relative Refractory Period (region d)

A
  • period when an AP can be generated but only by a greater than normal stimulus
  • Na+ channels are reactivated when MP passes RMP ∴ they are closed but can be reopened if threshold reached
  • K+ channels are open & membrane hyperpolarized
  • further to go to get to threshold ∴ need larger stimulus
33
Q

All-or-none Principle of APs

A
  • ALL: if threshold reached AP is produced - same every time (same max depol. etc)
  • NONE: below threshold ⇒ no AP
34
Q

Action Potential Propagation

A
  • to act as a communication device an AP must be propagated along the axon’s entire length
  • depolarization during AP (Na+ in) ⇒ +ve ions move toward more –ve on adjacent membrane ⇒ adjacent membrane depolarizes to reach threshold (voltage-gated Na+ open) ⇒ get AP on adjacent membrane
  • movement of charge occurs in both directions but APs move in 1 direction because preceding membrane is in the absolute refractory period
  • ∴ get sequence of APs along membrane, each one the same
  • AP propagates along its entire length to the axon terminal ends
35
Q

Rate of propagation depends on

A
  • fiber (axon) diameter

- myelination

36
Q

Fiber (axon) diameter

A

-larger diam. = faster propagation b/c less resistance to ion flow (= current)

37
Q

Myelination

A
  • unmyelinated fibers - APs all along the fiber (Na+ channels are adjacent to each other) = continuous conduction = slower
  • myelinated fibers - AP occurs at nodes of Ranvier (ion channels only present here) = saltatory (leaping) conduction ⇒ fast
38
Q

Fiber Types

A
  • Type A

- Type C

39
Q

Type A

A
  • large diameter
  • myelinated
  • propagate APs @ ~130 m/sec
  • most sensory neurons & motor neurons to skeletal muscles
40
Q

Type C

A
  • small diameter
  • unmyelinated
  • propagate APs @ ~0.5 m/sec
  • found in autonomic NS (ANS) and some pain fibers
41
Q

Synaptic Transmission (ST) at Neuronal Junction

A
  • NS depends on chains of neurons connected by junctions called synapses
  • presynaptic neuron to postsynaptic neuron transmission
42
Q

Synaptic Transmission (ST) at Neuronal Junction Steps

A
  • AP arrives at axon terminal (synaptic end bulb)
  • Ca++ voltage gates open (due to AP) and Ca++ enters (higher [Ca++] outside!)
  • rise in Ca++ triggers exocytosis of neurotransmitter (nt) containing vesicles
  • nt crosses synaptic cleft, binds to specific receptors on postsynaptic membrane
  • -receptors = chemically-gated ion channels ⇒ open in response to nt
  • gated ion channels open – allowing movement of ions into (or out of) postsynaptic membrane
  • -creates a graded potential (GP) called a postsynaptic potential (PSP).
43
Q

Postsynaptic Potentials (PSPs)

A
  • PSPs may be:
  • -Excitatory PSPs (EPSPs) = GP ⇒ depolarization.
  • -Inhibitory PSPs (IPSPs) = GP ⇒ hyperpolarization
  • PSPs occur on cell body or dendrites
44
Q

Excitatory PSPs (EPSPs)

A
  • due to opening of Na+ (or Ca++) channels, or closing of K+ channels
  • nt is often acetylcholine (ACh) or glutamate
45
Q

Inhibitory PSPs (IPSPs)

A
  • due to opening of K+ or Cl- channels
  • -inhibits neuron from reaching an AP
  • nt is often glycine or GABA
46
Q

PSPs occur on cell body or dendrites

A
  • many neurons can synapse onto one ⇒ if many EPSPs ⇒ summation ⇒ large area of membrane depolarized ⇒ spreads to axon hillock ⇒ if (sum of) EPSPs reaches threshold, get AP
  • However, some may be IPSPs ⇒ the sum of all EPSPs & IPSPs determines if AP will occur at the axon hillock
47
Q

Synaptic Transmission (ST) at Neuromuscular Junction

A

-junction between axon terminal of neuron & an individual muscle fibre

48
Q

Synaptic Transmission (ST) at Neuromuscular Junction Steps

A
  • neurotransmitter (nt) released = always ACh
  • Na+ chemical gates on muscle motor end plate (=sarcolemma of muscle fiber) open
  • -causes GP (= end plate potential (EPP)) on sarcolemma
  • EPP triggers AP on sarcolemma
  • -lots of ACh released in (a) ∴ always get an AP from an EPP

Phys 1412 (12 decks)

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