Neuronal Communication and Electroencephalography (EEG), Lecture notes of Neuroscience

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Applied

Neuroscience

Computational

Models of

Sleep

Fall 2017

Sleep

Objective: Computational Models of Sleep

Agenda:

1. Neurobiology of Sleep

• Action Potentials and Conduction
• Origin of Extracellular Currents

2. Computational Models

• Sleep Time-Frequency Spectra
• Seizure Prediction in Epilepsy

The Neuron

Signal Transmission and Interpretation

• Direction of signal transmission: neurons
  • transmit signals only in one direction (from dendrites to axon terminals), but
  • receive signals from different sources
    • Earlier or ‘lower’ processing stages (‘bottom-up’ or ‘feed-forward’)
    • Neighboring neurons in the same area (‘lateral’)
    • Subsequent or ‘higher’ processing areas (‘top-down’ or ‘feedback’)
• Combination of feed-forward and feed-back signal loops :
• Information is not just passively ‘forwarded’,
• But modified by everything else going on in the brain!

Ion Intracellular Extracellular Normal Plasma Value K

150 5 3.5-5. Na

12 140 135 - 145 Cl

• 10 105 100 - 108 Organic Anions

• Difference in ion concentration between compartments gives

rise to the resting membrane potential (RMP). Membrane

permeability to these ions also influences the RMP.

• Transient changes from the RMP produce electrical signals

which transmit information in nerve cells.

Changes in the Membrane Potential Produce Electric Signals in Nerve Cells

In the nervous system, different channel types are responsible for transmitting electrical signals over long and short distances: A. Graded potentials travel over short distances and are activated by the opening of mechanically or chemically gated channels. B. Action potentials travel over long distances and they are generated by the opening of voltage-gated channels. Gated Channels Are Involved in Neuronal Signaling Gated ion channels in the membrane open to a variety of stimuli:

• Mechanical force, eg. sensory neurons.
• Chemical ligands, eg. neurotransmitters.
• Voltage, eg. changes in the resting membrane potential.

Graded Potentials F8-

• Graded potentials are depolarizations or hyperpolarizations whose strength is proportional to the strength of the triggering event.
• Graded potentials lose their strength as they move through the cell due to the leakage of charge across the membrane (eg. leaky water hose).

§ A graded potential depolarization is called excitatory

postsynaptic potential (EPSP). A graded potential

hyperpolarization is called an inhibitory postsynaptic

potentials (IPSP).

§ They occur in the cell body and dendrites of the neuron.

§ The wave of depolarization or hyperpolarization which

moves through the cell with a graded potential is known as

local current flow.

Graded Potentials

Graded potentials travel through the neuron until they reach the trigger zone. If they depolarize the membrane above threshold voltage (about - mV in mammals), an action potential is triggered and it travels down the axon. Graded Potentials Above Threshold Voltage Trigger Action Potentials

Neural Transmission

• Neurons receive input from other neurons, especially through their dendrites
• Neurons send output in the form of an action potential (AP) along their axons
• When an AP arrives at the synaptic end bulb of a pre-synaptic neuron, neurotransmitter (NT) is released - NTs bind to receptors on the post-synaptic neuron, which often opens ion channels - The flow of ions causes an electrical current in the membrane - These graded potentials can be either - Positive/Depolarization/ Excitatory - Negative/ Hyperpolarization/ Inhibitory - The graded potentials are summed in the axon hillock - If the sum exceeds threshold, then the post-synaptic neuron will fire an action potential - When the action potential reaches the synaptic end bulb, NTs are released, and the cycle begins again

EPSPs and Action Potentials Source: Hausser et al, Science Vol. 291. 138- Neurons encode information and communicate via action potentials, which are generated by the summation of synaptic events. It was previously thought that APs reset the membrane potential completely. However, the strength of this reset is variable. EPSPs shunt, or diminish, the AP response in pyramidal neurons.

EPSPs and Action Potentials Source: Hausser et al, Science Vol. 291. 138- EPSP shunting depends on synaptic input kinetics. The rise and decay times differ between “fast” and “slow” EPSPs.

Electroenchaplography (EEG)

Current in the EEG measuring circuit depends on the nature and

location of the current sources, on the electrical properties of the

brain, skull and scalp and on location of both electrodes. Source:

Nunez et al (1891)

Test Your Understanding:

EEG activity is thought to arise from which of the following?

A. Cortical layers I and VI

B. Axonal action potentials

C. Horizontal dipoles

D. Excitatory and inhibitory post-synaptic potentials

Test Your Understanding:

EEG activity is thought to arise from which of the following?

A. Cortical layers I and VI

B. Axonal action potentials

C. Horizontal dipoles

D. Excitatory and inhibitory post-synaptic potentials

Explanation: EEG activity arises from the outermost cortex

layer I and does not directly capture axonal action

potentials. EEG is most sensitive to post-synaptic

potentials generated in the superficial layers of the cortex.