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Nerves and action potentials Flashcards

1
Q

Neuron Structure

A
  • Dendrite - toward soma
    – Input
  • Soma - cell body
    – Integration
  • Axon - away from soma
    – Conduction
  • Presynaptic Terminals
    – Transmission
  • Synapse
    – Site of communication
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2
Q

Supporting Cells

A
  • Glial cells support neurons
    physically and metabolically
  • Examples in vertebrates
    – Schwann Cells -
  • Ensheathing cells
  • Peripheral NS
    – Oligodendricites
  • Ensheathing cells
  • Central NS
    – Astrocytes
  • Line capillaries
  • Metabolic intermediaries
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3
Q

Transmission Between
Neurons

A
  • Neuronal Circuit
    – Signal (light, touch)
    – Sensory neurons
    • Afferent fibers
      – Interneurons
    • Link in CNS
      – Motor neurons
    • Efferent fibers
      – Effector
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4
Q

Excitable Cells

A
  • Most cells maintain constant resting potential
  • Nerves modulate RP for cell-cell
    communication
  • Polarized - electrical potential
    – Depolarized-towards positive
    – Repolarized-towards negative, stops at neutral
    – Hyperpolarizedtowards negative past neutral
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5
Q

Graded Potential

A
  • Graded Response
  • Initial disturbance dies
    with distance or time
  • Can be summed
  • No threshold
  • No refractory Period
  • Duration varies
  • De or hyperpolarizing
  • Initiated by stimulus, NT,
    or spontaneously
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6
Q

Action Potential

A
  • All-or-None response
  • Can be regeneratively
    propagated
  • Can’t be summed
  • Threshold
  • Refractory Period
  • Duration constant
  • Depolarizing only
  • Initiated by membrane
    depolarization only
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7
Q

Modulating RP

A
  • Generation of APs on Electrical Properties
    of Nerves
  • Passive Electrical
    – Resistance
    – Capacitance
  • Active Electrical
    – Voltage-gated
    channels
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8
Q

The resting membrane potential in different cell
types are approximately

A

– Skeletal muscle cells: −95mV
– Smooth muscle cells: –60mV
– Astroglia: –80 to –90mV
– Neurons: –60 to –70mV
– Erythrocytes: –9mV

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9
Q

Action Potential Phases

A
  • Stimulus Phase
  • Rising Phase
    – Na+ Voltage-Gated Channel opens
    – Influx of Na+
    – Depolarize Cell
    – Overshoot
  • Termination Phase
    – Na+ Voltage Gated
    Channel closes
  • Repolarization
    Phase
    – K+ channels open
    – Reestablish E K+
  • Hyperpolarization
    – Refractory Period
  • Resting Phase
    – Na + K + Pump
    – No direct role in AP
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10
Q

Note

A

Study permeability changes during AP
page 3 slide 4

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11
Q

Voltage Gated Channels

A
  • Protein conduits
  • Specificity (only open to one type of ion)
  • Conformation of protein changes with
    membrane voltage
  • Conductance also changes
  • NOTE: Loss of ions during AP is miniscule
    because of short duration of AP
    – Membrane potential changes but not [ ] of
    ions in solution – i.e. ionic composition
    – Enough ions to support several million APs!
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12
Q

How is the AP propagated?

A
  • APs are the electrical “signal” used in cell-cell
    communication.
  • APs passively moved along the axon – self
    regenerating process.
  • Depend on 2 membrane (Cable) properties.
    – Capacitance
    – Resistance
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13
Q

Conduction Velocity

A
  • What determines the speed of the AP?
    – Capacitance – reduced by need to depolarize
    each section
    • Larger diameter ® longer depolarization time
      BUT
      – Resistance – larger diameter wires have lower
      resistance
      – RESULT: Larger nerve fiber ® faster AP
      • Velocity proportional to sq. root axon diameter
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14
Q

Invertebrates

A
  • Giant squid axons: 1 mm; 20 m/sec
  • Crab axons: 30 μm; 5 m/sec
  • Larger is faster
  • Costs
    – Larger proportion of body
    devoted to neurons
  • Velocity 4x® diameter 16x
    – Limits # of axons
    – All-or none (squid cannot move slowly)
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15
Q

Need for Gated Channels

A
  • Signal decays linearly
  • Voltage gated channels spread along axon “gunpowder”
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16
Q

Schwann Cell - Oligodendrocytes

A
  • Glial cell wrapped around neuron to form a
    myelin sheath
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17
Q

Function of Myelin

A
  • Insulates -
    – Electron signal (graded
    potential) – very fast,
    doesn’t depolarize every
    section of membrane
  • Nodes of Ranvier
    – Saltatory Conductance
    – APs only at Nodes of
    Ranvier
    – Regenerative depolarizations
    Myelinated has higher conduction which is faster.
    Unmyelinated has slower conduction which is slower.
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18
Q

Myelin increases conduction velocity (a lot)

A
  • Increases λ – Length
    – Decreases membrane leakage ( R m ,1000-10,000
    fold) and increases speed of local depolarization.
  • Maintains or Decreases τ - Speed
    – Thick sheaths ̄ C by 1000 fold
  • Nodes of Ranvier
    – Saltatory Conductance
    – APs only at Nodes of Ranvier
    – Regenerative depolarizations
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19
Q

Conduction Velocity

A
  • Conduction Velocity of Myelinated Fiber can
    be up 120 m/sec OR 432 kmh (268 mph)
20
Q

Nerve Trunk & Order of Firing

A
  • Largest fibers have large cell bodies & higher
    threshold than small cell bodies
  • It takes more current to depolarize large cells:
    thus small nerves fire first.
21
Q

Synapse

A
  • Coupling between 2 neurons or between a neuron
    and its effector.
  • Two basic types of synapses
    – Electrical - (less common) nerve cells connected
    by Gap
    Junctions. – allow ions (Ca++) to pass between
    nerves.
    – Chemical – (most common) nerves communicate
    via a transmitter substance.
22
Q

Fast Chemical Synapses- Ionotropic

A

1) Presynaptic AP → Depolarizes membrane
2) Activates Voltage Gated Channels
3) Ca2+ diffuses into cell; signals NT filled vesicles
to migrate to membrane.
4) Exocytosis of NT into
synaptic cleft
5) Transmitter binds directly
to receptor to activate
a response

23
Q

Slow Chemical Synapses – metabotropic

A

1) Presynaptic AP →
Depolarizes membrane
2) Activates Voltage Gated
Channels
3) Ca 2+ diffuses into cell; signals
NT filled vesicles to migrate
to membrane.
4) Exocytosis of NT into the
synaptic cleft
5) Binds to receptor that
initiates a second
messenger

24
Q

Possible Effects of NT

A
  • PSP = Post Synaptic Potential
  • PSP are graded
  • EPSP (excitatory postsynaptic potential) –
    induces changes in V m to increase the
    probability of initiating an AP.
  • IPSP (inhibitory PSP) - induces changes in
    V m to decrease the probability of
    initiating an AP.
25
Q

EPSP/IPSP vs AP

A

GP can be summed, AP can’t be summed

26
Q

Neuronal Integration

A
  • Events (Both IPSP
    and EPSP) are
    integrated at the
    axon hillock (where
    soma/axon join)
  • If combined
    stimulus reaches
    threshold!
    postsynaptic AP
  • Spatial Summation
  • Temporal
    Summation
27
Q

Sensory Perception

A
  • Gather, Process and Respond to Information
    – Events and conditions in the
    • External Environment
      – Exteroceptors – receives information from
      outside the body (i.e. 5 special senses) and
      communicates to CNS
    • Internal Environment
      – Interoceptors – receives information from
      inside the body and communicates to CNS
      “Nothing is in the mind that does not pass
      through the senses” Aristotle
28
Q

Types of Sensory Receptors

A
  • Mechano- deformation
  • Thermo- temperature
  • Nociceptors- tissue damage/pain
  • Electromagnetic- light
  • Chemo- chemical concentration
29
Q

Sensory Perception Process

A
  • Process involves:
    Detection → Amplification →
    Transduction → Transmission
  • Selectivity – respond only to specific stimulus
  • Sensitivity – amplify specific stimulus modality
  • Sensory Transduction – process by which stimulus
    energy is changed into the energy of a nerve impulse.
    – Results in changed ionic conductance; graded potential
30
Q

Note study Tansduction and Action potential

A

Nerves page 7 slide 3

31
Q

Encoding Information

A
  • Given than an AP is an “all-or-none”
    phenomenon, how do nerves encode
    information about the stimulus?
  • Sensory adaptation
    – Changes in perceived intensity of
    sensation even when the physical
    intensity of stimulation has not changed
32
Q

Sensory Adaptation

A
  • Changes in perceived intensity of sensation even when the physical intensity of stimulation has not changed.
    – Tonic Receptor – APs slow with continuous stimulation
    – Phasic Receptor – APs stop with continuous stimulation
  • Where can adaptation occur
    – Peripheral Filtration
    – Transduction
    – APs
33
Q

Increase frequency of APs

A

– Increases with strength of stimulus
– Limited by size of graded potential sensor
can generate
– Limited by refractory period of neuron

34
Q

Increase number of fibers with APs

A

– Threshold of Detection – 50% of time
– Range Fractionation - sensitivities of different
neurons vary increasing range of detection

35
Q

Photoreceptors

A

Transduce energy
in photons to AP to be integrated in
CNS

36
Q

Photoreception

A
  • Fast Transmission
  • Detailed information
    – Color
    – Pattern
    – Movement
    – Duty Cycle
    – Plane of Polarization
37
Q

Vision

A

Eyes have evolved many times,
with 2 distinct stages.
– Acquisition of photodetection
* Eye spots have evolved
35-60 times
– Image forming optic system
* 6 of 33 metazoan phyla
(Cnidaria, Mollusca,
Annelida, Onychophora,
Arthropoda, Chordata)
* These 6 phyla comprise
96% of all species on
earth, though not all
species in each phyla
have image forming eyes.

38
Q

Basic Eye Design

A
  • Physiology of photoreception highly conserved
  • Many different types of image forming eyes
    – Simple
    – Compound
  • Analogy – many different types of cameras but
    only one type of film
39
Q

difference between simple and compound eye

A

simple eye has 1 hole with multiple lenses, while compound eye has many different eye holes that are super-position eye light.

40
Q

Parts of eye

A

Photoreceptors, Cornea, lens

41
Q

Ommatidium

A
  • Corneal lens
  • Crytalline lens
  • Rhabdome
    – Rhabdomere – captureslight
    * Densely packed microvilli
    * Rhodopsin molecules
42
Q

Note

A

study vertebrate eye

43
Q

Focusing The Eye

A
  • At rest the eye focuses on INFINITY. To look at
    near by objects the eye needs to:
    – Convergence – both eyes converge on visual axis
    to they are both looking at same object
    – Accommodation – The object must be brought
    into focus on the retina
    * Some animals move lens or retina (more like
    camera)
    * Most vertebrates change the shape of the lens
44
Q

Ciliary Muscle

A
  • If relaxed = lens flat tension; suspensory
    fibers
  • If contract = lens
45
Q

Aquatic Organisms

A
  • Light detection get more difficult with depth
    – Light intensity decreases with depth
    – Light differentially absorbed
    • Uv, red, orange, yellow, green, blue (last to go)
  • Refractive index of water similar to cornea
    – How does cornea bend light to focus?
    • Thick spherical lens or multiple lenses
    • Such lenses have evolved at least 8 time in
      aquatic organisms, including mammals that have
      returned to water.
    • Accommodation by moving lens
46
Q

Physiology of Detection

A
  • Ciliary Structures
  • Rod – visual pigment in membrane disk; isolated
    – Vision in dim light
  • Cone – visual pigment in membrane continuous with plasma
    membrane
    – Color vision

animal phys (10 decks)

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