Medical Physics : Electrical Signaling

Question 1

What is an ECG?

Answer 1

Electrocardiography - voltages due to the heart measured at the body surface.

Question 2

What is an EEG?

Answer 2

Electroencephalography = voltages due to brain activity measured at the scalp

Question 3

What is an EMG?

Answer 3

An Electromyography = voltages due to muscle activity

Question 4

What is an MEG?

Answer 4

Magnetoencephalography = magnetic fields due to brain activity measured outside the head.

Question 5

What separates the inside of a nerve cell from the extra-cellular medium?

Answer 5

An electrically insulating, semi-permeable membrane.

Question 6

What do Ion-pumps do?

Answer 6

Ion-pumps sited in the cell membrane maintain a difference in the concentration of ions (particularly K+ and Na+) between the inside and the outside of the cell.

Question 7

What do ‘ion-channels’ do?

Answer 7

Ion channels allow small cations to diffuse through the membrane, but generally larger anions cannot pass through the membrane.

Question 8

What does the diffusion of ions cause?

Answer 8

The diffusion of ions causes a charge build up leading to a potential difference across the membrane, which also causes a flux of ions across the membrane.

Question 9

What equation describes the flux of ions across a membrane?

Answer 9

Question 10

What causes a potential difference across a membrane?

Answer 10

Since K+ ions carry positive charge, the diffusive flux out of the cell produces an excess negative charge insude the cell.

This charge seperation produces a potential difference across the capacitor.

Question 11

What happens at equilibrium at a cell membrane regarding the movement of ions?

Answer 11

The flow of charge due to diffusion balances the flow due to the electric fields.

Question 12

What formula describes the membrane voltage as a function of the concentration of ions inside and outside the cell.

Answer 12

Question 13

What causes the membrane voltage to change?

Answer 13

• Neurotransmitters are released in synapses and cause membrane permeability changes.
• If the potential rises above a critical voltage fast Na+ ion channels in the membrane open and Na+ ions rapidly diffuse into the cell producing a positive voltage
• When the voltage becomes positive the Na+ channels close and K+ channels open, allowing K+ ions to diffuse rapidly out of the cell restoring the negative voltage.

Question 14

What is the current flow due to depolarisation along a nerve?

Answer 14

Question 15

What is the voltage at a distance r from the nerve?

Answer 15

Question 16

What is the Biot-Savart Law that dictates the magnetic field at a distance r from a flow of current.

Answer 16

Question 17

How does an electrocardiogram work?

Answer 17

Three electrodes are placed on the body. These effectlvely measure the voltages at the vertices of a triangle in the chest. The magnitudes of the measured voltages at a particular times depend on the projection of the cardiac dipole vector on each lead.