C2.2.8 - C2.2.16 : HL (Neural Signalling)

Depolarization

Voltage-gated sodium channels open, and the influx makes membrane potential more positive

• The area causes next area to depolarize → propagation of action potential through axon

• “self propagation”: impulse in one dendrite → action potential through axon → synaptic terminal

Threshold potential

Minimum change and membrane polarity for an action potential to occur

Usually starts when first receptor neuron (converts physical stimulus into first action potential; e.g: Retina = minimum light) begins chain of events

All-or-nothing action of depolarization

Refers to principle that if minimum threshold potential is reached (-55mV), then full action potential reached; if not, nothing happens

Repolarization

voltage-gated K⁺ channels open and K⁺ diffuse out of axon (creates negative value)

Sodium-potassium pump actively transports Na⁺ and K⁺ across membrane

Refractory period

Period of recovery after depolarization during which neuron is unable to respond to additional stimulation

Oscilloscope

An electronic test instrument to measure membrane potential across neural membrane

Data is displayed as a graph with time (in milliseconds) on x-axis and membrane potential (millivolts, mV) on y-axis

Main oscilloscope stages

1. Resting state: voltage-gated Na+ and K+ channels are closed; resting potential is maintained by ungated channels

2. Depolarization: a stimulus opens some Na+ channels; if threshold reached (-55mV), an action potentially triggered

3. More Na+ channels open; K+ channels remain closed; interior of cell becomes more positive; membrane polarity becomes the reverse of resting state

4. Repolarization: Na+ channels close and inactive; K+ channels open, and K+ rushes out; interior of cell becomes more negative than outside

5. K+ channels close relatively slowly, causing brief under shoot

1. Return to resting state

Salatory conduction

Name given to phenomenon where an action potential jumps from one node of Ranvier to the next as an impulse progresses along a myelinated axon

Allows much faster, transmission, velocity because ion movements happen only at the notes → depolarization, and repolarization only happen at nodes

Exogenous chemicals

Chemicals produced outside the body

Endogenous chemicals

Chemicals produced inside the body

2 Examples of exogenous chemicals

• Neonicotinoid insecticides

• Cocaine

Neonicotinoid insecticides (as an example of exogenous chemicals)

Class of insecticides chemically similar to nicotine and structurally similar to acetylcholine

• Binds to the postsynaptic receptor of acetylcholine irreversibly and cannot be decomposed by acetylcholinesterase

• Ion channels stay open, causing over stimulation of neurons → continue depolarization, leads to convulsions and death

• Insects have higher proportion of acetylcholine receptors than mammals → effective pesticide

Cocaine

Drug that prevents the removal of docking from synapse and stimulates dopamine-releasing neurons to release more dopamine

• Usually dopamine is removed by dopamine transporter (a protein)

• Cocaine binds to dopamine, transporter, and blocks removal → dopamine floods brain

• Repeated use can cause reward pathways to alter, reinforcing addiction

Excitatory neurotransmitters

Generate action potential by increasing membrane, permeability of postsynaptic neuron to positive ions (e.g: acetylcholine)

• Increase permeability to Na+ causes more Na+ to diffuse in postsynaptic neuron

  Neuron depolarizes due to positive charge inside axon → impulse carried forward

Inhibitory neurotransmitters

Inhibit action potential by hyperpolarization

• Makes the inside of neuron more negative because Cl- moving in or K+ moving out

• Results in neuron, being hyperpolarized, allowing inhibition of impulse

→ Binds to a specific receptor

(e.g: GABA)

Summation of inhibitor

If the sum of signals is inhibitory, then impulse is not carried forward

If some of signals is excitatory, the signal is carried forward

Class of sensory receptors

Nociceptors

Sensory receptors of pain

• Have chemical chemicals for positive ions that open in response to stimuli like temperature, chemicals…

• If threshold potential is reached → action potential generated and conducted to CNS for interpretation

Hot chili have Capsaicin → combined to nociceptors opening Ca2+ channels, causing impulse

Reductionism

Approach that reduces the complex phenomenon of organisms to the interactions of their parts → some of parts makes organism

Emergence

Believes that the hole is greater than the sum of parts