ELECTROPHYSIOLOGY Single Neuron Recording Patch Clamp Recording ECG

ELECTROPHYSIOLOGY Single Neuron Recording Patch Clamp Recording ECG EEG- Brain activity Recording PET, MRI, f. MRI , CAT

Single-unit recording • It is the use of an electrode to record the electrophysiological activity (action potentials) from a single neuron.

• An electrode introduced into the brain of a living animal will detect electrical activity that is generated by the neurons adjacent to the electrode tip. If the electrode is a microelectrode, with a tip size of 3 to 10 micrometers, the electrode will often isolate the activity of a single neuron.

• The activity consists of the voltages generated in the extra cellular matrix by the current fields outside the cell when it generates an action potential. Recording in this way is generally called "single -unit" recording.

• The recorded action potentials look very much like the action potentials that are recorded intracellularly, but the signals are very much smaller (typically about 0. 1 m. V).

• Recordings of single neurons in living animals have provided important insights into how the brain processes information, following the hypothesis put forth by Edgar Adrian that unitary action potential events are the fundamental means of communication in the brain.

• Microelectrodes used for extra cellular single-unit recordings are usually very fine wires made from tungsten or platinumiridium alloys that are insulated except at their extreme tip and are less often glass micropipettes filled with a weak electrolyte solution similar in composition to extra cellular fluid.

• Hubel and Wiesel were awarded the Nobel Prize in Physiology or Medicine in 1981.

The patch clamp technique • It is a laboratory technique in electrophysiology that allows the study of single or multiple ion channels in cells.

• The technique can be applied to a wide variety of cells, but is especially useful in the study of excitable cells such as neurons, cardiomyocytes, muscle fibers and pancreatic beta cells. It can also be applied to the study of bacterial ion channels in specially prepared giant spheroplasts.

• It is a laboratory technique in electrophysiology that allows the study of single or multiple ion channels in cells

• Erwin Neher and Bert Sakmann developed the patch clamp in the late 1970 s and early 1980 s. • This discovery made it possible to record the currents of single ion channels for the first time, proving their involvement in fundamental cell processes such as action potential conduction.

• Neher and Sakmann received the Nobel Prize in Physiology or Medicine in 1991 for this work.

• Patch clamp recording uses, as an electrode, a glass micropipette that has an open tip diameter of about one micrometer, a size enclosing a membrane surface area or "patch" that often contains just one or a few ion channel molecules

• In some experiments, the micropipette tip is heated in a microforge to produce a smooth surface that assists in forming a high resistance seal with the cell membrane. • The interior of the pipette is filled with a solution matching the ionic composition of the bath solution, as in the case of cellattached recording, or the cytoplasm for whole-cell recording

• Unlike traditional two-electrode voltage clamp recordings, patch clamp recording uses a single electrode to record currents.

• ELECTROCARDIOGRAPH

• As cardiac impulses pass through the heart, electrical currents spread into the tissues surrounding the heart, and a small portion of these currents spread throughout the surface of the body.

• If electrodes are placed on the skin on opposite sides of the heart, electrical potential generated by these currents can be recorded.

• Electrocardiograph is an instrument which records the electrical activity of the heart during a cardiac cycle.

• A record of the minute electrical pulses generated by the heart used to determine the condition of the patient’s heart is the Electro. Cardio. Gram.

• Electrodes are placed on the chest and limbs, and the impulses which they detect are amplified by the electrograph to which the electrodes are connected.

• The ECG was developed by William Einthoven of Leiden University, England between 1903 and 1910

• Electrical impulses in the heart originate in the sinoatrial node and travel through the heart muscle where they impart electrical initiation of systole or contraction of the heart. • The electrical waves can be measured at selectively placed electrodes (electrical contacts) on the skin.

• Electrodes on different sides of the heart measure the activity of different parts of the heart muscle. • An ECG displays the voltage between pairs of these electrodes, and the muscle activity that they measure, from different directions, also understood as vectors.

• This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle. • It is the best way to measure and diagnose abnormal rhythms of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by levels of dissolved salts (electrolytes), such as potassium, that are too high or low. In myocardial infarction (MI), the ECG can identify damaged heart muscle

• The ECG cannot reliably measure the pumping ability of the heart; for which ultrasound-based (echocardiography) or nuclear medicine tests are used.

• A typical electrocardiograph runs at a paper speed of 25 mm/s, although faster paper speeds are occasionally used. Each small block of ECG paper is 1 mm². • At a paper speed of 25 mm/s, one small block of ECG paper translates into 0. 04 s (or 40 ms). • Five small blocks make up 1 large block, which translates into 0. 20 s (or 200 ms). • Hence, there are 5 large blocks per second.

• A standard signal of 1 m. V must move the stylus vertically 1 cm, that is two large squares on ECG paper.

Leads used in ECG • Limb Leads • Leads I, II and III are the so-called limb leads • Lead I is a dipole with the negative (white) electrode on the right arm and the positive (black) electrode on the left arm. • Lead II is a dipole with the negative (white) electrode on the right arm and the positive (red) electrode on the left leg. • Lead III is a dipole with the negative (black) electrode on the left arm and the positive (red) electrode on the left leg.

Augmented limb • Leads a. VR, a. VL, and a. VF are 'augmented limb leads'. They are derived from the same three electrodes as leads I, II, and III. However, they view the heart from different angles • Lead a. VR or "augmented vector right" has the positive electrode (white) on the right arm. The negative electrode is a combination of the left arm (black) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the right arm.

• Lead a. VL or "augmented vector left" has the positive (black) electrode on the left arm. The negative electrode is a combination of the right arm (white) electrode and the left leg (red) electrode, which "augments" the signal strength of the positive electrode on the left arm. • Lead a. VF or "augmented vector foot" has the positive (red) electrode on the left leg. The negative electrode is a combination of the right arm (white) electrode and the left arm (black) electrode, which "augments" the signal of the positive electrode on the left leg

Precordial • The precordial leads V 1, V 2, V 3, V 4, V 5, and V 6 are placed directly on the chest. Because of their close proximity to the heart, they do not require augmentation

Waves and intervals • A typical ECG tracing of a normal heartbeat (or cardiac cycle) consists of a P wave, a QRS complex and a T wave. • A small U wave is normally visible in 50 to 75% of ECGs. The baseline voltage of the electrocardiogram is known as the isoelectric line. • Typically the isoelectric line is measured as the portion of the tracing following the T wave and preceding the next P wave.

Schematic representation of normal ECG

P wave • During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. • This turns into the P wave on the ECG, which is upright in II, III, and a. VF and inverted in a. VR. A P wave must be upright in leads II and a. VF and inverted in lead a. VR to designate a cardiac rhythm as Sinus Rhythm.

• The relationship between P waves and QRS complexes helps distinguish various cardiac arrhythmias. • The shape and duration of the P waves may indicate atrial enlargement

QRS complex • The QRS complex is a structure on the ECG that corresponds to the depolarization of the ventricles. • Because the ventricles contain more muscle mass than the atria, the QRS complex is larger than the P wave.

• In addition, because the His/Purkinje system coordinates the depolarization of the ventricles, the QRS complex tends to look "spiked" rather than rounded due to the increase in conduction velocity. • A normal QRS complex is 0. 08 to 0. 12 sec (80 to 120 ms) in duration represented by three small squares or less, but any abnormality of conduction takes longer, and causes widened QRS complexes

PR/PQ interval • The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. • It is usually 120 to 200 ms long. On an ECG tracing, this corresponds to 3 to 5 small boxes. In case a Q wave was measured with a ECG the PR interval is also commonly named PQ interval instead. • A PR interval of over 200 ms may indicate a first degree heart block.

• A short PR interval may indicate a preexcitation syndrome via an accessory pathway that leads to early activation of the ventricles, such as seen in Wolff. Parkinson-White syndrome. • A variable PR interval may indicate other types of heart block.

• The duration, amplitude, and morphology of the QRS complex is useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.

• "Buried" inside the QRS wave is the atrial repolarization wave, which resembles an inverse P wave. • It is far smaller in magnitude than the QRS and is therefore obscured by it.

ST segment • The ST segment connects the QRS complex and the T wave and has a duration of 0. 08 to 0. 12 sec (80 to 120 ms). • It starts at the J point (junction between the QRS complex and ST segment) and ends at the beginning of the T wave.

• The typical ST segment duration is usually around 0. 08 sec (80 ms). • The normal ST segment has a slight upward concavity.

T wave • The T wave represents the repolarization (or recovery) of the ventricles. • The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period.

• The last half of the T wave is referred to as the relative refractory period (or vulnerable period). Tall or "tented" symmetrical T waves may indicate hyperkalemia. • Flat T waves may indicate coronary ischemia or hypokalemia.

QT interval • The QT interval is measured from the beginning of the QRS complex to the end of the T wave. • Normal values for the QT interval are between 0. 30 and 0. 44 seconds.

• The QT interval as well as the corrected QT interval are important in the diagnosis of long QT syndrome and short QT syndrome. • The QT interval varies based on the heart rate, and various correction factors have been developed to correct the QT interval for the heart rate.

• The QT interval represents on an ECG the total time needed for the ventricles to depolarize and repolarize

U wave • The U wave is not always seen. It is typically small, and, by definition, follows the T wave. U waves are thought to represent repolarization of the papillary muscles or Purkinje fibers. Prominent • An inverted U wave may represent myocardial ischemia or left ventricular volume overload

Computed Axial Tomography (CAT) • Computed tomography (CT) is a medical imaging method employing tomography. • Digital geometry processing is used to generate a three-dimensional image of the inside of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.

• CT scan was invented by Sir Godfrey Hounsfield • He got Nobel prize in 1979 for the discovery

CT Scanner

CT Image of Brain

Brain Images

• The word "tomography" is derived from the Greek tomos (slice) and graphein (to write). • Computed tomography was originally known as the "EMI scan" as it was developed at a research branch of EMI, a company best known today for its music and recording business. • It was later known as computed axial tomography (CAT or CT scan) and body section röntgenography.

Positron emission tomography (PET) • Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a threedimensional image or picture of functional processes in the body

• The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule.

• Images of tracer concentration in 3 dimensional space within the body are then reconstructed by computer analysis. • In modern scanners, this reconstruction is often accomplished with the aid of a CT Xray scan performed on the patient during the same session, in the same machine.

• If the biologically active molecule chosen for PET is FDG, an analogue of glucose, the concentrations of tracer imaged then give tissue metabolic activity, in terms of regional glucose uptake.

• To conduct the scan, a short-lived radioactive tracer isotope, is injected into the living subject (usually into blood circulation). • The tracer is chemically incorporated into a biologically active molecule • The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.

• As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, a particle with the opposite charge of an electron. • After traveling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions.

• These are detected when they reach a scintillator in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes

PET

PET IMAGE

PET Image of Brain

PET Acquisition Process

Radioisotopes • Radionuclides used in PET scanning are typically isotopes with short half lives such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min), and fluorine-18 (~110 min).

• These radionuclides are incorporated either into compounds normally used by the body such as glucose (or glucose analogues), water or ammonia, or into molecules that bind to receptors or other sites of drug action. Such labelled compounds are known as radiotracers.

Applications • PET is both a medical and research tool • It is used heavily in clinical oncology (medical imaging of tumors and the search for metastases), and for clinical diagnosis of certain diffuse brain diseases such as those causing various types of dementias • PET is also an important research tool to map normal human brain and heart function.

Magnetic resonance imaging (MRI) • Magnetic resonance imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a medical imaging technique most commonly used in radiology to visualize the structure and function of the body. It provides detailed images of the body in any plane.

MRI

• NMR was discovered by Bloch and Purcell in 1952

MRI Image of Brain

• MRI provides much greater contrast between the different soft tissues of the body than computed tomography (CT) does, making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging

• Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogen atoms in water in the body.

• Radiofrequency fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. • This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.

• The body is mainly composed of water molecules which each contain two hydrogen nuclei or protons. • When a person goes inside the powerful magnetic field of the scanner these protons align with the direction of the field.

• A second radiofrequency electromagnetic field is then briefly turned on causing the protons to absorb some of its energy. • When this field is turned off the protons release this energy at a radiofrequency which can be detected by the scanner.

• The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.

• Diseased tissue, such as tumors, can be detected because the protons in different tissues return to their equilibrium state at different rates.

Applications • In clinical practice, MRI is used to distinguish pathologic tissue (such as a brain tumor) from normal tissue. • One advantage of an MRI scan is that it is harmless to the patient. It uses strong magnetic fields and non-ionizing radiation in the radio frequency range • Compare this to CT scans and traditional X-rays which involve doses of ionizing radiation and may increase the risk of malignancy, especially in a fetus.

MRI Signs Safe Conditional Unsafe

Functional MRI (f. MRI) • It is a type of specialized MRI scan. It measures the haemodynamic response related to neural activity in the brain or spinal cord of humans or other animals

• It is one of the most recently developed forms of neuroimaging. • Since the early 1990 s, f. MRI has come to dominate the brain mapping field due to its low invasiveness, lack of radiation exposure, and relatively wide availability.

f MRI Image

Electroencephalography (EEG) • EEG is the recording of electrical activity along the scalp produced by the firing of neurons within the brain.

• In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a short period of time, usually 20 -40 minutes, as recorded from multiple electrodes placed on the scalp

• EEG used to be a first-line method for the diagnosis of tumors, stroke and other focal brain disorders, but this use has decreased with the advent of anatomical imaging techniques such as MRI and CT.

• EEG was discovered in 1929 by a German psychiatrist Hans Berger • In 1932 by a British electrophysiologist, Edgar Adrian won Nobel prize for the demonstration of electrical impulses from brain

EEG

1 Second EEG

• It is generally accepted that the activity measured by EEG is electrical potentials created by the post-synaptic currents, rather than by action potentials. • More specifically, the scalp electrical potentials that produce EEG are due to the extracellular ionic currents caused by dendritic electrical activity (whereas the fields producing magnetoencephalographic signals are associated with intracellular ionic currents).

Wave patterns • delta waves. • Delta is the frequency range up to 3 Hz. It tends to be the highest in amplitude and the slowest waves. It is seen normally in adults in slow wave sleep. It is also seen normally in babies

Delta Wave

• Theta is the frequency range from 4 Hz to 7 Hz. Theta is seen normally in young children. It may be seen in drowsiness or arousal in older children and adults; it can also be seen in meditation

Theta Wave

• Alpha is the frequency range from 8 Hz to 12 Hz. Hans Berger named the first rhythmic EEG activity he saw, the "alpha wave. " This is activity in the 8 -12 Hz range seen in the posterior regions of the head on both sides, being higher in amplitude on the dominant side. It is brought out by closing the eyes and by relaxation.

Alpha Wave

• Beta is the frequency range from 12 Hz to about 30 Hz. • It is seen usually on both sides in symmetrical distribution and is most evident frontally. • Low amplitude beta with multiple and varying frequencies is often associated with active, busy or anxious thinking and active concentration. Rhythmic beta with a dominant set of frequencies is associated with various pathologies and drug effects, especially benzodiazepines.

Beta Wave

• Gamma is the frequency range approximately 26– 100 Hz. • Gamma rhythms are thought to represent binding of different populations of neurons together into a network for the purpose of carrying out a certain cognitive or motor function.

Gamma Wave

Medical Imaging Instruments • • Barium X ray CT Scan Bronchography Endoscopy PET MET Echocardiograph X ray Optical Nuclear Ultrasound

• ECG • Electromyograph EMG • EEG Bioelectricity