Nerves and action potentials Flashcards
Neuron Structure
- Dendrite - toward soma
– Input - Soma - cell body
– Integration - Axon - away from soma
– Conduction - Presynaptic Terminals
– Transmission - Synapse
– Site of communication
Supporting Cells
- 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
Transmission Between
Neurons
- Neuronal Circuit
– Signal (light, touch)
– Sensory neurons- Afferent fibers
– Interneurons - Link in CNS
– Motor neurons - Efferent fibers
– Effector
- Afferent fibers
Excitable Cells
- 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
Graded Potential
- 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
Action Potential
- All-or-None response
- Can be regeneratively
propagated - Can’t be summed
- Threshold
- Refractory Period
- Duration constant
- Depolarizing only
- Initiated by membrane
depolarization only
Modulating RP
- Generation of APs on Electrical Properties
of Nerves - Passive Electrical
– Resistance
– Capacitance - Active Electrical
– Voltage-gated
channels
The resting membrane potential in different cell
types are approximately
– Skeletal muscle cells: −95mV
– Smooth muscle cells: –60mV
– Astroglia: –80 to –90mV
– Neurons: –60 to –70mV
– Erythrocytes: –9mV
Action Potential Phases
- 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
Note
Study permeability changes during AP
page 3 slide 4
Voltage Gated Channels
- 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!
How is the AP propagated?
- 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
Conduction Velocity
- 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
- Larger diameter ® longer depolarization time
Invertebrates
- 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)
Need for Gated Channels
- Signal decays linearly
- Voltage gated channels spread along axon “gunpowder”
Schwann Cell - Oligodendrocytes
- Glial cell wrapped around neuron to form a
myelin sheath
Function of Myelin
- 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.
Myelin increases conduction velocity (a lot)
- 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
Conduction Velocity
- Conduction Velocity of Myelinated Fiber can
be up 120 m/sec OR 432 kmh (268 mph)
Nerve Trunk & Order of Firing
- 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.
Synapse
- 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.
Fast Chemical Synapses- Ionotropic
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
Slow Chemical Synapses – metabotropic
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
Possible Effects of NT
- 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.
EPSP/IPSP vs AP
GP can be summed, AP can’t be summed
Neuronal Integration
- 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
Sensory Perception
- 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
- External Environment
Types of Sensory Receptors
- Mechano- deformation
- Thermo- temperature
- Nociceptors- tissue damage/pain
- Electromagnetic- light
- Chemo- chemical concentration
Sensory Perception Process
- 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
Note study Tansduction and Action potential
Nerves page 7 slide 3
Encoding Information
- 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
Sensory Adaptation
- 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
Increase frequency of APs
– Increases with strength of stimulus
– Limited by size of graded potential sensor
can generate
– Limited by refractory period of neuron
Increase number of fibers with APs
– Threshold of Detection – 50% of time
– Range Fractionation - sensitivities of different
neurons vary increasing range of detection
Photoreceptors
Transduce energy
in photons to AP to be integrated in
CNS
Photoreception
- Fast Transmission
- Detailed information
– Color
– Pattern
– Movement
– Duty Cycle
– Plane of Polarization
Vision
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.
Basic Eye Design
- 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
difference between simple and compound eye
simple eye has 1 hole with multiple lenses, while compound eye has many different eye holes that are super-position eye light.
Parts of eye
Photoreceptors, Cornea, lens
Ommatidium
- Corneal lens
- Crytalline lens
- Rhabdome
– Rhabdomere – captureslight
* Densely packed microvilli
* Rhodopsin molecules
Note
study vertebrate eye
Focusing The Eye
- 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
Ciliary Muscle
- If relaxed = lens flat tension; suspensory
fibers - If contract = lens
Aquatic Organisms
- 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
Physiology of Detection
- Ciliary Structures
- Rod – visual pigment in membrane disk; isolated
– Vision in dim light - Cone – visual pigment in membrane continuous with plasma
membrane
– Color vision
