Lecture 11: Action Potential Generation. Flashcards
Understanding Electrical Conductivity in the Heart:
- Within the atria and ventricles MYOCARDIAL CELLS areCONNECTED BY GAP JUNCTIONS .
- Gap junctions allow the CARDIAC ACTION POTENTIAL to PROPAGATE from cell to cell through a LOW RESISTANCE PATHWAY..
What are GAP JUNCTIONS?
ROLE?
FOUND?
FORMED FROM? = 5
1 ➢ Gap junctions are PORES between cells.
2 ➢ Allow small polar molecules to diffuse directly between cells.
3 ➢ Common in Heart, gastric smooth muscle.
4 ➢ Formed from transmembrane protein CONNEXON
5 ➢ Conenexon binds connexon of next cell forming tube.
Gap Junctions – pacemaker smooth muscle cell process
- Sponatenous action potential induced by pacemaker potential
- Action potential spread to nonpacemaker cell
- gap junction
- Non pacemaker smooth muscle cell
Initiation of Action Potentials = 5
- Action Potentials can start at pacemaker cells
- e.g. in the heart or intestine
- Potential can spread from cell to cell by GAP JUNCTIONS
- e.g. cardiac and smooth muscle cells - Electrical synapses of nerves are very rare
- Action potential in most tissues started by RECEPTOR POTENTIAL OR NEUROTRANSMITTERS.
INITIATION OF ACTION POTENTIALS IN …. Receptor potential specialised afferent ending
STIMULUS
- Sensory receptor (modified ending of afferent neuron)
2.Stimulus sensitive nonspecific cation channel
- Na+ enters through VOLTAGE gated Na+ channel
- Action Potential
- SUMMARY = DEPOLARISE afferent nerve ending – local current flow from receptor potential triggers action potential by opening voltage gated Na+ channels.
INITIATION OF ACTION POTENTIALS IN …. Receptor potential in SEPARATE receptor cell.
SUMMARY : Neurotransmitter binds to ligand-gated ion channels causing depolarization which opens voltage gated Na+ channels triggering an action potential.
- STIMULUS (Na+ entry depolarises)
- Separate receptor cell
- Stimulus sensitive nonspecific cation channel.
- Ca+2 enters through voltage gated Ca+2 channel
- Neurotransmitter via vesicle to afferent neuron via passing through cleft palate.
- Chemically gated receptor-channel
- Na+ goes inside the post synaptic neuron
- Action potential
- Action potentials: 3
carry neural signals along the axon.
- All the same size and actively regenerated.
Receptor potentials: 3
- generated in sensory cells in response to a stimulus.
- Stretch gated ion channels in touch receptors.
Graded potentials
synaptic potentials: 3
- are produced by neurotransmission.
- Binding of a neurotransmitter open ligand gated ion channels
- Graded potentials
Stimuli vs Permeability vs Potential
Stimuli open non-selective cation channels Receptor potential is graded
INCREASE stimulus - INCREASE permeability - INCREASE potential
Receptor Potential, How is is graded?
graded:
- in amplitude
- and duration,
- proportional to the stimulus.
what is integrative action?
transforms receptor potential to action potential.
—- Coded into frequency of AP firing.
Action potential is a …..
all- or-none response.
Understanding Output signal:
transmitter release from synaptic terminal.
Amount released is determined by the AP’s frequency.
Slide 10: Look at the various ways they are shown/ graphed.
- Receptor potential
- integrative action
- action potential
- Output signal
Diverse Neural Behaviour: Action potential,
Adaptation vs Rhythmic bursts
SLIDE 11
- Action potentials firing at steady frequency throughout the stimulus
- Adaptation: rapid firing frequency that slows over time
- Rhythmic Bursts:
rapid burst of firing followed by brief pause - Response of sensory receptor to sustained stimulation
Action Potential Initiation - sensory nerve ending
= 3
- Axon hillock or sensory nerve ending
- Spike Generating Zone or Trigger Zone
- High density of voltage gated Na+ and K+ channels
eg. Pyramidal cell
- spike-initiation zone: axon hillock
Sensory neuron
- initiation zone: sensory nerve ending
MEMBRANE WITH HIGHEST DENSITY OF VOLTAGE-GATED SODIUM CHANNELS.
Understanding Passive Potential decay….2
- Receptor & synaptic potentials decays away from source (length constant)
- Some neurons have voltage gated channels in dendrites to “boost” travelling synaptic potential
Understanding Synaptic Potentials …3
- High density voltage gated Na+ and K+ channels
- Lower threshold for action potential generation
- An action potential is generated where the amplitude of the synaptic potential crosses the threshold
understanding Excitatory Post-Synaptic Potentials
= 3
- Excitatory post-synaptic potentials – EPSP
- e.g. ACh-gated N-receptor or glutamate-gated ion channels
- Increase Na+ permeability
UNDERSTANDING Skeletal Muscle Nicotinic Receptors = 4
- In SKELETAL muscle ACh stimulates N-cholinergic receptors.
- Nicotinic receptors are LIGAND GATED cation channels.
- Produces a motor endplate potential (like an EPSP).
- DEPOLARISATION open Voltage gated sodium channels
UNDERSTANDING Inhibitory Post-Synaptic Potentials = 2
- Inhibitory post-synaptic potentials – IPSP
- e.g. Glycine-gated or GABA-gated ion channels Increase Cl- permeability
Synaptic Potentials: define
Synaptic potentials occur when a neuron signals another neuron
Post-synaptic potentials: 3
are the change in voltage produced in the post synaptic cell.
- EPSP are excitatory post synaptic potentials.
- IPSP are inhibitory post synaptic potentials
Junction potentials: 5
- are just the same as synaptic potentials but at a junction.
- Neuron to neuron connections are SYNAPSES
- Neuron to muscle or gland connections are called JUNCTIONS
- Neuron to skeletal muscle is called a MOTOR END PLATE
- END PLATE POTENTIALS in skeletal muscle
Summation of Synaptic Potentials:
SPATIAL SUMMATION VS TEMPORAL SUMMATION.
- Presynaptic action potential triggers a small EPSP in the postsynaptic neuron
- Spatial summation:
— 2 or more presynaptic inputs active at same time, individual EPSP’s add together - Temporal summation:
—Same presynaptic fiber fires AP’s in quick succession, individual EPSP’s add together
UNDERSTANDING Summation of Synaptic Potentials
- Summation of synaptic potentials can drive membrane potential to threshold (action potential)
- Synaptic potentials have long time course and summate readily
- Action potentials cannot summate
Summation of Synaptic Potentials:
Integration of EPSPs and IPSPs = 2
- EXCITATORY INPUT causes an inward postsynaptic current that spreads to the soma and recorded as an EPSP
- INHIBITORY SYNAPSE allows depolarizing current to leak out before it reaches the soma
Features of Cell Potentials:
Receptor Potential
size: SMALL (0.1 -10mV)
duration: Moderate (5-100ms)
Variability: Graded
Direction: Hyper- or depolarising
Propagation: PASSIVE
Typical channel: Stretch gated or specialised sensor
Origin: Sensory cell response to stimulation.
Features of CELL POTENTIALS:
SYNAPTIC POTENTIAL
size: SMALL (0.1 -10mV)
duration: WIDE RANGE (5ms - 20min)
Variability: Graded
Direction: Hyper- or depolarising
Propagation: PASSIVE
Typical channel: LIGAND GATED
Origin: Post synaptic cell response to neurotransmission
size: Large (70-110mV)
duration: Brief (1-10ms)
Variability: ALONE OR NONE
Direction: depolarising
Propagation: ACTIVE
Typical channel: VOLTAGE GATED
Origin: Axon hillock or spike generating zone, response to depolarization
Functional Circuit Example
Stretch Reflex pathway
- A receptor – muscle spindle
- An afferent fibre–muscle spindle afferent
- An integration centre – lamina IX of spinal cord
- An efferent fibre – α-motoneurones
- An effector – muscle
Summary: 5
- PACEMAKER cells, in gut or heart, can cause depolarisation and action potentials.
- Action potentials can spread from cell to cell through GAP JUNCTIONS, e.g. In the heart
- Activation of a receptor produces RECEPTOR POTENTIALS, neurotransmitters produce POSTSYNAPTIC (or post junctional) potentials
- SUMMATION of postsynaptic potentials at the SPIKE GENERATING or TRIGGER ZONE causes action potentials to form.
- Inhibitory transmitters produce INHIBITORY POST SYNAPTIC POTENTIALS potentials that hyperpolarize the neuron suppressing AP formation.
