Topic 4 - Recording and Stimulating Neural Activity

Recording Neural Activity

Axons produce action potentials and terminal buttons elicit postsynaptic potentials in the membrane of the cells with which they form synapses. These electrical events can be recorded e.g. during decision-making or motor activities. Recordings can be made chronically or acutely.

FMRI

Best functional brain imaging method in terms of spatial and temporal resolution is the ____ scan (mod of MRI that permits measurement of regional metabolism in the brain - this is done by detecting levels of oxygen in brain's blood levels). Functional MRI scans have a higher resolution than PET scans and can be acquired faster - more detailed info about activity of particular brain regions.

Stimulating Neural Activity

Don't always only just want to measure the activity but sometimes we want to artificially change the activity of specific regions to see the effects on animal's behaviour e.g. perhaps with the VMH thing we may make up for the lack of female sex hormones if we activate the VMH and the rats will copulate again.

Acutely

For a short period during which the animal is kept anaesthetised. Acute recording limited to studies of sensory pathways and seldom involve behavioural observations since the behavioural capacity of anaesthetised animal is very limited.

Metabolic Rate

If neural activity of a brain region increases so does the ___________ _____ largely as a result of increased operation of transporters in the cell membranes - increased metabolic rate can be measured.

fMRI

Images produced by MEG are cruder than ____ but can be acquired more rapidly and consequently reveal fast-moving events.

Advantage of Chemical Stimulation

It activates cell bodies but not axons because only cells bodies (and their dendrites) contain glutamate receptors (principal excitatory NT in the brain) and can be sure that the injection of glutamate into a particular regions is exciting the cells there and not the axons of other neurons passing through the region (effects of chemical more localised than effects of electrical). Back to VMH - VMH stimulation does sub for female sex hormones; perhaps then the female sex hormones exert their effects in this nucleus.

Disadvantage of Chemical Stimulation

It is more complicated than electrical: chemical requires cannulas, tubes, special pumps or syringes and sterile solutions of excitatory amino acids to inject the chemical.

Recording Neural Activity with Magnetoencephalography

MEG is performed with neuromagnetometers, devices that contain superconducting detectors (SQUIDs), orientated so that a computer can examine their output and calculate the source of particular signals in the brain. Can be used clinically e.g. find sources of seizures so they can be removed surgically and used in experiments to measure regional brain activity that accompanies the perception of various stimuli or the performance of various behaviours or cognitive tasks.

Functional Imaging

Metabolic rate of specific brain regions can be measured in human brains too by means of ___________ ________ (computerised method of detecting metabolic/chemical changes in the brain) - first FI was the PET scan (reveals localisation of a radioactive tracer in the living brain) but this very expensive as need an atomic particular accelerator (cyclotron) on top of the cost of the actual PET scanner.

Chemical Stimulation

Method of stimulating neural activity. Can activate neurons by injecting small amount of excitatory amino acid e.g. kainic or glutamic into the brain.

Electrical Stimulation

Method of stimulating neural activity. Can activate neurons by passing electrical current through a wire inserted into the brain.

Transcranial Magnetic Stimulation

Method of stimulating neural activity. TMS. Neural activity induces magnetic fields that can be detected by means of magnetoencephalography, similarly magnetic fields can be used to stimulate neurons by inducing electrical currents in brain tissue. TMS uses a coil of wires to stimulate neurons in the human cerebral cortex - effects of TMS similar to those of direct stimulation of the exposed brain e.g. stimulation of a particular region of the visual association cortex will disrupt a person's ability to detect movements in visual stimuli. TMS also used to treat symptoms of mental disorders like depression. Depending on the strength and pattern of the stimulation, TMS can either excite the region of the brain over which the coil is positioned or interfere with its functions.

Optogenetic Methods

Method of stimulating neural activity. They are the use of a genetically modified virus to insert light-sensitive ion channels into the membrane of particular neurons in the brain; can depolarise or hyperpolarise the neurons when light of the appropriate wavelength is applied.

Recording Neural Activity with Microelectrodes (2)

Need durable electrodes when recording in unaesthetised animals. Electric signals detected by microelectrodes are small so need to be amplified so that the signals can be displayed on an oscilloscope. The results of the neurons are that firing rates fall almost to 0 during REM suggesting these neurons have an inhibitory effect on REM i.e. REM can't occur when these neurons firing.

Chronically

Over extended period of time after the animal recovers from surgery.

Recording Neural Activity with Microelectrodes

Serotonergic and noradrenergic neurons affect REM sleep - to find out whether the activity of these neurons change we record them with microelectrodes, can even record individual neurons (process called single-unit recording).

Measuring Metabolic Rate

This is done by injecting radioactive 2-DG into animal's bloodstream, as it resembles glucose (principal food for the brain) it is taken into cells 0 so most active cells for glucose rate take highest concentration of 2-DG nut unlike glucose it can't be metabolised so stays in cell. Experimenter then kills animal and performs autoradiography where sections of the brain are mounted on microscope slides and taken to a darkroom for several weeks can then see that the most active regions of the brain contain the most radioactivity (shown in form of dark spots on developed emulsion).

Recording and Stimulating Neural Activity

This looks at studying the brain by recording or stimulating the activity of particular regions not by looking at effects of damage.

Stereotaxic Surgery

To record awake and freely moving animals we attach electrodes to miniature electrical sockets and bond the sockets to the animal's skull, this is done by means of ________________ _________.

Magnetic Field

When electrical current flows through a conductor it induces a ________ _______ - as action potentials pass down axons or as postsynaptic potentials pass down dendrites or sweep across the somatic membrane of a neuron, magnetic fields are also produced.

Recording Neural Activity with Macroelectrodes

When recording the activity of a region of the brain as a whole, not the activity of individual neurons, we use macroelectrodes. These don't detect the activity of individual neurons but record the postsynaptic potentials of many 1000s or millions of cells in the area of the electrode. The electrical activity of a human brain is displayed on a polygraph, these recordings are called EEGs - can be used to diagnose epilepsy or study stages of sleep. EEG can also monitor the condition of the brain during surgery.