Lecture 11: Action Potential Generation.

Question 1

Understanding Electrical Conductivity in the Heart:

Answer 1

• 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..

Question 1

What are GAP JUNCTIONS?
ROLE?
FOUND?
FORMED FROM? = 5

Answer 1

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.

Question 2

Gap Junctions – pacemaker smooth muscle cell process

Answer 2

1. Sponatenous action potential induced by pacemaker potential
2. Action potential spread to nonpacemaker cell
3. gap junction
4. Non pacemaker smooth muscle cell

Question 3

Initiation of Action Potentials = 5

Answer 3

1. Action Potentials can start at pacemaker cells

• e.g. in the heart or intestine

2. Potential can spread from cell to cell by GAP JUNCTIONS
   - e.g. cardiac and smooth muscle cells
3. Electrical synapses of nerves are very rare
4. Action potential in most tissues started by RECEPTOR POTENTIAL OR NEUROTRANSMITTERS.

Question 4

INITIATION OF ACTION POTENTIALS IN …. Receptor potential specialised afferent ending

Answer 4

STIMULUS

1. Sensory receptor (modified ending of afferent neuron)

2.Stimulus sensitive nonspecific cation channel

3. Na+ enters through VOLTAGE gated Na+ channel
4. Action Potential
5. SUMMARY = DEPOLARISE afferent nerve ending – local current flow from receptor potential triggers action potential by opening voltage gated Na+ channels.

Question 5

INITIATION OF ACTION POTENTIALS IN …. Receptor potential in SEPARATE receptor cell.

Answer 5

SUMMARY : Neurotransmitter binds to ligand-gated ion channels causing depolarization which opens voltage gated Na+ channels triggering an action potential.

1. STIMULUS (Na+ entry depolarises)
2. Separate receptor cell
3. Stimulus sensitive nonspecific cation channel.
4. Ca+2 enters through voltage gated Ca+2 channel
5. Neurotransmitter via vesicle to afferent neuron via passing through cleft palate.
6. Chemically gated receptor-channel
7. Na+ goes inside the post synaptic neuron
8. Action potential

Question 6

• Action potentials: 3

Answer 6

carry neural signals along the axon.

• All the same size and actively regenerated.

Question 7

Receptor potentials: 3

Answer 7

• generated in sensory cells in response to a stimulus.
• Stretch gated ion channels in touch receptors.

Graded potentials

Question 8

synaptic potentials: 3

Answer 8

• are produced by neurotransmission.
• Binding of a neurotransmitter open ligand gated ion channels
• Graded potentials

Question 9

Stimuli vs Permeability vs Potential

Answer 9

Stimuli open non-selective cation channels Receptor potential is graded

INCREASE stimulus - INCREASE permeability - INCREASE potential

Question 10

Receptor Potential, How is is graded?

Answer 10

graded:
- in amplitude
- and duration,
- proportional to the stimulus.

Question 11

what is integrative action?

Answer 11

transforms receptor potential to action potential.

—- Coded into frequency of AP firing.

Question 12

Action potential is a …..

Answer 12

all- or-none response.

Question 13

Understanding Output signal:

Answer 13

transmitter release from synaptic terminal.

Amount released is determined by the AP’s frequency.

Question 14

Slide 10: Look at the various ways they are shown/ graphed.

Answer 14

• Receptor potential
• integrative action
• action potential
• Output signal

Question 15

Diverse Neural Behaviour: Action potential,

Adaptation vs Rhythmic bursts

SLIDE 11

Answer 15

1. Action potentials firing at steady frequency throughout the stimulus
2. Adaptation: rapid firing frequency that slows over time
3. Rhythmic Bursts:
   rapid burst of firing followed by brief pause
4. Response of sensory receptor to sustained stimulation

Question 16

Action Potential Initiation - sensory nerve ending

= 3

Answer 16

1. Axon hillock or sensory nerve ending
2. Spike Generating Zone or Trigger Zone
3. 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.

Question 17

Understanding Passive Potential decay….2

Answer 17

1. Receptor & synaptic potentials decays away from source (length constant)
2. Some neurons have voltage gated channels in dendrites to “boost” travelling synaptic potential

Question 18

Understanding Synaptic Potentials …3

Answer 18

1. High density voltage gated Na+ and K+ channels
2. Lower threshold for action potential generation
3. An action potential is generated where the amplitude of the synaptic potential crosses the threshold

Question 19

understanding Excitatory Post-Synaptic Potentials
= 3

Answer 19

1. Excitatory post-synaptic potentials – EPSP
2. e.g. ACh-gated N-receptor or glutamate-gated ion channels
3. Increase Na+ permeability

Question 20

UNDERSTANDING Skeletal Muscle Nicotinic Receptors = 4

Answer 20

1. In SKELETAL muscle ACh stimulates N-cholinergic receptors.
2. Nicotinic receptors are LIGAND GATED cation channels.
3. Produces a motor endplate potential (like an EPSP).
4. DEPOLARISATION open Voltage gated sodium channels

Question 21

UNDERSTANDING Inhibitory Post-Synaptic Potentials = 2

Answer 21

1. Inhibitory post-synaptic potentials – IPSP
2. e.g. Glycine-gated or GABA-gated ion channels Increase Cl- permeability

Question 22

Synaptic Potentials: define

Answer 22

Synaptic potentials occur when a neuron signals another neuron

Question 23

Post-synaptic potentials: 3

Answer 23

are the change in voltage produced in the post synaptic cell.

• EPSP are excitatory post synaptic potentials.
• IPSP are inhibitory post synaptic potentials

Question 24

Junction potentials: 5

Answer 24

- are just the same as synaptic potentials but at a junction. 1. Neuron to neuron connections are SYNAPSES 2. Neuron to muscle or gland connections are called JUNCTIONS 3. Neuron to skeletal muscle is called a MOTOR END PLATE 4. END PLATE POTENTIALS in skeletal muscle

Question 25

Summation of Synaptic Potentials: SPATIAL SUMMATION VS TEMPORAL SUMMATION.

Answer 25

1. Presynaptic action potential triggers a small EPSP in the postsynaptic neuron 2. Spatial summation: --- 2 or more presynaptic inputs active at same time, individual EPSP’s add together 3. Temporal summation: ---Same presynaptic fiber fires AP’s in quick succession, individual EPSP’s add together

Question 26

UNDERSTANDING Summation of Synaptic Potentials

Answer 26

1. Summation of synaptic potentials can drive membrane potential to threshold (action potential) 2. Synaptic potentials have long time course and summate readily 3. Action potentials cannot summate

Question 27

Summation of Synaptic Potentials: Integration of EPSPs and IPSPs = 2

Answer 27

1. EXCITATORY INPUT causes an inward postsynaptic current that spreads to the soma and recorded as an EPSP 2. INHIBITORY SYNAPSE allows depolarizing current to leak out before it reaches the soma

Question 28

Features of Cell Potentials: Receptor Potential

Answer 28

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.

Question 29

Features of CELL POTENTIALS: SYNAPTIC POTENTIAL

Answer 29

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

Answer 30

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

Question 31

Functional Circuit Example

Answer 31

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

Question 32

Summary: 5

Answer 32

1. PACEMAKER cells, in gut or heart, can cause depolarisation and action potentials. 2. Action potentials can spread from cell to cell through GAP JUNCTIONS, e.g. In the heart 3. Activation of a receptor produces RECEPTOR POTENTIALS, neurotransmitters produce POSTSYNAPTIC (or post junctional) potentials 4. SUMMATION of postsynaptic potentials at the SPIKE GENERATING or TRIGGER ZONE causes action potentials to form. 5. Inhibitory transmitters produce INHIBITORY POST SYNAPTIC POTENTIALS potentials that hyperpolarize the neuron suppressing AP formation.