Chapter 4

Nervous System

The nervous system is the part of an animal's body that coordinates the voluntary and involuntaryactions of the animal and transmits signals between different parts of its body. Nervous tissue firstarose in wormlike organisms about 550 to 600 million years ago. In most types of animals it consists oftwo main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNScontains the brain and spinal cord. The PNS consists mainly of nerves, which are long fibers thatconnect the CNS to every other part of the body. The PNS includes motor neurons, mediating voluntarymovement, the autonomic nervous system, comprising the sympathetic nervous system andthe parasympathetic nervous system and regulating involuntary functions, and the enteric nervoussystem, a semi-independent part of the nervous system whose function is to control the gastrointestinalsystem.

At the cellular level, the nervous system is defined by the presence of a special type of cell, calledthe neuron, also known as a "nerve cell". Neurons have special structures that allow them to sendsignals rapidly and precisely to other cells. They send these signals in the form of electrochemical wavestraveling along thin fibers called axons, which cause chemicals called neurotransmitters to be released atjunctions called synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited,or otherwise modulated. The connections between neurons form neural circuits that generate anorganism's perception of the world and determine its behavior. Along with neurons, the nervous systemcontains other specialized cells called glial cells (or simply glia), which provide structural and metabolicsupport.

Nervous systems are found in most multicellular animals, but vary greatly in complexity.[1] The onlymulticellular animals that have no nervous system at all are sponges, placozoans and mesozoans, whichhave very simple body plans. The nervous systems of ctenophores (comb jellies) and cnidarians (e.g.,anemones, hydras, corals and jellyfishes) consist of a diffuse nerve net. All other types of animals, withthe exception of a few types of worms, have a nervous system containing a brain, a central cord (ortwo cords running in parallel), and nerves radiating from the brain and central cord. The size of thenervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billioncells in humans.

Functions of the nervous system:Sensory input: The sensory input Monitors changes/events occurring in and outside the body. Suchchanges are known as stimuli and the cells that monitor them are receptors.

Integration: Integration is responsible for the parallel processing and interpretation of sensoryinformation to determine the appropriate response

Motor output: The motor output is responsible for the activation of muscles or glands. This is typicallyachieved via the release of neurotransmitters.

Organization of the nervous system:

The central nervous system:

The central nervous system (CNS) is the part of the nervous system consisting of the brain and spinalcord. It is opposed to the peripheral nervous system (or PNS), which is composed of nerves leading toand from the CNS, often through junctions known as ganglia.

The central nervous system is so named because it integrates information it receives from, andcoordinates and influences the activity of, all parts of the bodies of bilaterally symmetric animals—thatis, all multicellular animals except sponges and radially symmetric animals such as jellyfish, and itcontains the majority of the nervous system. Arguably many consider the retina and the opticnerve (2nd cranial nerve) as well as the olfactory nerves(3rd) and olfactory epithelium as parts of theCNS, synapsing directly on brain tissue without intermediate ganglia. Following this classificationthe olfactory epithelium is the only central nervous tissue in direct contact with the environment, whichopens up for therapeutic treatments. The CNS is contained within the dorsal cavity, with the brain inthe cranial cavity and the spinal cord in the spinal cavity. In vertebrates, the brain is protected by theskull, while the spinal cord is protected by the vertebrae, both enclosed in the meninges.

Neuron:

A neuron is a nerve cell that is the basic building block of the nervous system. Neurons are similar toother cells in the human body in a number of ways, but there is one key difference between neuronsand other cells. Neurons are specialized to transmit information throughout the body.

These highly specialized nerve cells are responsible for communicating information in both chemical andelectrical forms. There are also several different types of neurons responsible for different tasks in thehuman body.

Sensory neurons carry information from the sensory receptor cells throughout the body to the brain.Motor neurons transmit information from the brain to the muscles of the body. Interneurons areresponsible for communicating information between different neurons in the body.

Neurons vs. Other Cells

Similarities with other cells:

Neurons and other body cells both contain a nucleus that holds genetic information.Neurons and other body cells are surrounded by a membrane that protects the cell.The cell bodies of both cell types contain organelles that support the life of the cell, includingmitochondria, Golgi bodies, and cytoplasm.

Differences that make neurons unique:

Unlike other body cells, neurons stop reproducing shortly after birth. Because of this, some partsof the brain have more neurons at birth than later in life because neurons die but are notreplaced. While neurons do not reproduce, research has shown that new connections betweenneurons form throughout life.

Neurons have a membrane that is designed to sends information to other cells. The axonand dendrites are specialized structures designed to transmit and receive information. Theconnections between cells are known as synapses. Neurons release chemicals known asneurotransmitters into these synapses to communicate with other neurons.

The Structure of a Neuron

There are three basic parts of a neuron: the dendrites, the cell body and the axon. However, all neuronsvary somewhat in size, shape, and characteristics depending on the function and role of the neuron.Some neurons have few dendritic branches, while others are highly branched in order to receive a greatdeal of information. Some neurons have short axons, while others can be quite long. The longest axonin the human body extends from the bottom of the spine to the big toe and averages a length ofapproximately three feet!

Structure of neuron

Dendrites are treelike extensions at the beginning of a neuron that help increase the surface areaof the cell body. These tiny protrusions receive information from other neurons and transmitelectrical stimulation to the soma. Dendrites are also covered with synapses.

Dendrite Characteristics - Most neurons have many dendrites- However, some neurons may have only one dendrite- Short and highly branched- Transmits information to the cell body

The soma is where the signals from the dendrites are joined and passed on. The soma and thenucleus do not play an active role in the transmission of the neural signal. Instead, these twostructures serve to maintain the cell and keep the neuron functional. The support structures of the cell include mitochondria, which provide energy for the cell, and theGolgi apparatus, which packages products created by the cell and secretes them outside the cellwall

The axon hillock is located at the end of the soma and controls the firing of the neuron. If thetotal strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire asignal known as an action potential down the axon.

The axon is the elongated fiber that extends from the cell body to the terminal endings andtransmits the neural signal. The larger the axon, the faster it transmits information. Some axonsare covered with a fatty substance called myelin that acts as an insulator. These myelinatedaxons transmit information much faster than other neurons.

Axon Characteristics- Most neurons have only one axon- Transmit information away from the cell body- May or may not have a myelin covering

The terminal buttons are located at the end of the neuron and are responsible for sending thesignal on to other neurons. At the end of the terminal button is a gap known asasynapse. Neurotransmitters are used to carry the signal across the synapse to other neurons.

Action Potentials

In order for neurons to communicate, they need to transmit information both within the neuron andfrom one neuron to the next. This process utilizes both electrical signals as well as chemicalmessengers.

The dendrites of neurons receive information from sensory receptors or other neurons. This informationis then passed down to the cell body and on to the axon. Once the information as arrived at the axon, ittravels down the length of the axon in the form of an electrical signal known as an action potential. Anaction potential is part of the process that occurs during the firing of a neuron. During the actionpotential, part of the neural membrane opens to allow positively charged ions inside the cell andnegatively charged ions out. This process causes a rapid increase in the positive charge of the nervefiber. When the charge reaches +40 mv, the impulse is propagated down the nerve fiber. This electricalimpulse is carried down the nerve through a series of action potentials.

Prior to the Action Potential

When a neuron is not sending signals, the inside of the neuron has a negative charge relative to thepositive charge outside the cell. Electrically charged chemicals known as ions maintain the balance ofpositive and negative charges. Calcium contains two positive charges, sodium and potassium containone positive charge and chloride contains a negative charge.

When at rest, the cell membrane of the neuron allows certain ions to pass through while preventing orrestricting the movement of other ions. In this state, sodium and potassium ions cannot easily passthrough the membrane. Potassium ions, however, are able to freely cross the membrane. Thenegatively ions inside of the cell are unable to cross the barrier. The cell must activity transport ions inorder to maintain its polarized state. This mechanism is known as the sodium-ion pump. For every twopotassium ions that pass through the membrane, three sodium ions are pumped out.

The resting potential of the neuron refers to the difference between the voltage inside and outside theneuron. The resting potential of the average neuron is around -70 millivolts, indicating that the inside ofthe cell is 70 millivolts less than the outside of the cell.

During the Action Potential

When an impulse is sent out from a cell body, the sodium channels open and the positive sodium cellssurge into the cell. Once the cell reaches a certain threshold, an action potential will fire, sending theelectrical signal down the axon. Action potentials either happen or they don't; there is no such thing asa "partial" firing of a neuron. This principle is known as the all-or-none law.

After the Action Potential

After the neuron has fired, there is a refractory period in which another action potential is not possible.During this time, the potassium channels reopen and the sodium channels close, gradually returning theneuron to its resting potential.

Communication Between Synapses

Once an electrical impulse has reached the end of an axon, the information must be transmitted acrossthe synaptic gap to the dendrites of the adjoining neuron. In some cases, the electrical signal canalmost instantaneously bridge the gap between the neurons and continue along its path.

In other cases, neurotransmitters are needed to send the information from one neuron to the next.Neurotransmitters are chemical messengers that are released from the axon terminals to cross thesynaptic gap and reach the receptor sites of other neurons. In a process known as reuptake, theseneurotransmitters attach to the receptor site and are reabsorbed by the neuron to be reused.

Neurotransmitters

Neurotransmitters are an essential part of our everyday functioning. While it is not known exactly howmany neurotransmitters exist, scientists have identified more than 100 of these chemical messengers.

What effects do each of these neurotransmitters have on the body ? What happens when disease ordrugs interfere with these chemical messengers? The following are just a few of the majorneurotransmitters, their known effects, and disorders they are associated with.

Acetylcholine: Associated with memory, muscle contractions, and learning. A lack of acetylcholine inthe brain is associated with Alzheimer’s disease.

Endorphins: Associated with emotions and pain perception. The body releases endorphins in responseto fear or trauma. These chemical messengers are similar to opiate drugs such as morphine, but aresignificantly stronger.

Dopamine: Associated with thought and pleasurable feelings. Parkinson’s disease is one illnessassociated with deficits in dopamine, while schizophrenia is strongly linked to excessive amounts of thischemical messenger.

The Spinal Cord

The spinal cord is a cylindrical shaped bundle of nerve fibers that is connected to the brain at the brainstem. The spinal cord runs down the center of the protective spinal column extending from the neck tothe lower back. The brain and spinal cord are the major components of the central nervoussystem (CNS). The CNS is the processing center for the nervous system, receiving information from andsending information to the peripheral nervous system. Peripheral nervous system cells connect variousorgans and structures of the body to the CNS through the cranial nerves and spinal nerves. Spinal cordnerves transmit information from body organs and external stimuli to the brain and send informationfrom the brain to other areas of the body.

The spinal cord is composed of nervous tissue. The interior of the spinal cord consists ofneurons,nervous system support cells called glia, and blood vessels. Neurons are the basic unit of nervoustissue. They are composed of a cell body and projections that extend from the cell body that are able toconduct and transmit nerve signals. These projections are axons (carry signals away from the cell body)and dendrites (carry signals toward the cell body). The neurons and their dendrites are contained withinan H-shaped region of the spinal cord called "grey matter." Surrounding the grey matter area is aregion called "white matter." The white matter section of the spinal cord contains axons that arecovered with an insulating substance called myelin. Myelin is whitish in appearance and allows electricalsignals to flow freely and quickly. Axons carry signals along descending and ascending tracts away fromand toward the brain.

Neurons are classified as either motor, sensory, or interneurons. Motor neurons carry information fromthe central nervous system to organs, glands, and muscles. Sensory neurons send information to thecentral nervous system from internal organs or from external stimuli. Interneurons relay signalsbetween motor and sensory neurons. The descending tracts of the spinal cord consist of motor nervesthat send signals from the brain to control voluntary and involuntary muscles. They also help tomaintain homeostasis by assisting in the regulation of autonomic functions such as heart

rate, blood pressure, and internal temperature. The ascending tracts of the spinal cord consist ofsensory nerves that send signals from internal organs and external signals from the skin and extremitiesto the brain. Reflexes and repetitive movements are controlled by spinal cord neuronal circuits that arestimulated by sensory information without input from the brain.

The axons that link the spinal cord to the muscles and the rest of the body are bundled into 31 pairs ofspinal nerves, each pair with a sensory root and a motor root that make connections within the greymatter. These nerves must pass between the protective barriers of the spinal column to connect thespinal cord to the rest of the body. The location of the nerves in the spinal cord determines theirfunction.

The spongy spinal cord is protected by the irregular shaped bones of the spinal column called vertebrae.Spinal vertebrae are components of the axial skeleton and each contain an opening that serves as achannel for the spinal cord to pass through. Between the stacked vertebrae are discs of semi-rigidcartilage, and in the narrow spaces between them are passages through which the spinal nerves exit tothe rest of the body. These are places where the spinal cord is vulnerable to direct injury. The vertebraecan be organized into sections, and are named and numbered from top to bottom according to theirlocation along the backbone:

Cervical vertebrae (1-7) located in the neckThoracic vertebrae (1-12) in the upper back (attached to the ribcage)Lumbar vertebrae (1-5) in the lower backSacral vertebrae (1-5) in the hip areaCoccygeal vertebrae (1-4 fused) in the tail-bone

The spinal cord is also organized into segments and named and numbered from top to bottom. Eachsegment marks where spinal nerves emerge from the cord to connect to specific regions of the body.Locations of spinal cord segments do not correspond exactly to vertebral locations, but they are roughlyequivalent.

Cervical spinal nerves (C1 to C8) control signals to the back of the head, the neck andshoulders, the arms and hands, and the diaphragm.Thoracic spinal nerves (T1 to T12) control signals to the chest muscles, some muscles of theback, and parts of the abdomen.Lumbar spinal nerves (L1 to L5) control signals to the lower parts of the abdomen and theback, the buttocks, some parts of the external genital organs, and parts of the leg.Sacral spinal nerves (S1 to S5) control signals to the thighs and lower parts of the legs, the feet,most of the external genital organs, and the area around the anus.

The single coccygeal nerve carries sensory information from the skin of the lower back.

The brain:

The anatomy of the brain is complex due its intricate structure and function. This amazing organ acts asa control center by receiving, interpreting, and directing sensory information throughout the body. Thebrain and spinal cord are the two main structures of thecentral nervous system. There are three majordivisions of the brain. They are the forebrain, the midbrain, and the hindbrain.

Anatomy of the Brain:

Brain Divisions

The forebrain is responsible for a variety of functions including receiving and processing sensoryinformation, thinking, perceiving, producing and understanding language, and controlling motorfunction. There are two major divisions of forebrain: the diencephalon and the telencephalon. Thediencephalon contains structures such as the thalamus and hypothalamus which are responsible for suchfunctions as motor control, relaying sensory information, and controlling autonomic functions. Thetelencephalon contains the largest part of the brain, the cerebrum. Most of the actual informationprocessing in the brain takes place in the cerebral cortex.

The midbrain and the hindbrain together make up the brainstem. The midbrain is the portion of thebrainstem that connects the hindbrain and the forebrain. This region of the brain is involved in auditoryand visual responses as well as motor function.

The hindbrain extends from the spinal cord and is composed ofthe metencephalon and myelencephalon. The metencephalon contains structures such asthe pons and cerebellum. These regions assists in maintaining balance and equilibrium, movementcoordination, and the conduction of sensory information. The myelencephalon is composed ofthe medulla oblongata which is responsible for controlling such autonomic functions as breathing, heartrate, and digestion.

Brain Lobes:

The frontal lobe is located at the front of the brain and is associated with reasoning, motorskills, higher level cognition, and expressive language. At the back of the frontal lobe, near thecentral sulcus, lies the motor cortex. This area of the brain receives information from variouslobes of the brain and utilizes this information to carry out body movements. Damage to thefrontal lobe can lead to changes in sexual habits, socialization, and attention as well as increasedrisk-taking.

The parietal lobe is located in the middle section of the brain and is associated with processingtactile sensory information such as pressure, touch, and pain. A portion of the brain known as thesomatosensory cortex is located in this lobe and is essential to the processing of the body'ssenses. Damage to the parietal lobe can result in problems with verbal memory, an impairedability to control eye gaze and problems with language.

The temporal lobe is located on the bottom section of the brain. This lobe is also the location ofthe primary auditory cortex, which is important for interpreting sounds and the language we hear.The hippocampus is also located in the temporal lobe, which is why this portion of the brain isalso heavily associated with the formation of memories. Damage to the temporal lobe can lead toproblems with memory, speech perception, and language skills.

The occipital lobe is located at the back portion of the brain and is associated with interpretingvisual stimuli and information. The primary visual cortex, which receives and interpretsinformation from the retinas of the eyes, is located in the occipital lobe. Damage to this lobe cancause visual problems such as difficulty recognizing objects, an inability to identify colors, andtrouble recognizing words.

Brain areas:

CerebrumCerebellumLimbic SystemBrain Stem

The Cerebrum: The cerebrum or cortex is the largest part of the human brain, associated with higherbrain function such as thought and action. The cerebral cortex is divided into four sections, called"lobes": the frontal lobe, parietal lobe, occipital lobe, and temporal lobe.

The cerebral cortex is highly wrinkled. Essentially this makes the brain more efficient, because it canincrease the surface area of the brain and the amount of neurons within it. We will discuss therelevance of the degree of cortical folding.

A deep furrow divides the cerebrum into two halves, known as the left and right hemispheres. The twohemispheres look mostly symmetrical yet it has been shown that each side functions slightly differentthan the other. Sometimes the right hemisphere is associated with creativity and the left hemisphere isassociated with logic abilities. The corpus callosum is a bundle of axons which connects these twohemispheres.

Nerve cells make up the gray surface of the cerebrum which is a little thicker than your thumb. Whitenerve fibers underneath carry signals between the nerve cells and other parts of the brain and body.

The neocortex occupies the bulk of the cerebrum. This is a six-layered structure of the cerebral cortexwhich is only found in mammals. It is thought that the neocortex is a recently evolved structure, and isassociated with "higher" information processing by more fully evolved animals.

The Cerebellum: The cerebellum, or "little brain", is similar to the cerebrum in that it has twohemispheres and has a highly folded surface or cortex. This structure is associated with regulation andcoordination of movement, posture, and balance.

In other words, animals which scientists assume to have evolved prior to humans, for example reptiles,do have developed cerebellums. However, reptiles do not have neocortex.

Limbic System: The limbic system, often referred to as the "emotional brain", is found buried withinthe cerebrum. Like the cerebellum, evolutionarily the structure is rather old.

This system contains the thalamus, hypothalamus, amygdala, and hippocampus. Here is a visualrepresentation of this system, from a midsagittal view of the human brain:

ThalamusHypothalamusAmygdalaHippocampus

Brain Stem: Underneath the limbic system is the brain stem. This structure is responsible for basic vitallife functions such as breathing, heartbeat, and blood pressure. Scientists say that this is the "simplest"part of human brains because animals' entire brains, such as reptiles (who appear early on theevolutionary scale) resemble our brain stem.

The brain stem is made of the midbrain, pons, and medulla.

MidbrainPonsMedulla

Lateralization of Brain

The human brain is separated into two distinct cerebral hemispheres along the longitudinal fissure, andthese hemispheres are connected by the Corpus Callosum. The two cerebral hemispheres exhibit strong,but not complete, bilateral symmetry in both structure and function.

The cerebral hemisphere to the left is known as the left hemisphere, while the hemisphere to the rightis known as the right hemisphere. It is believed that even though both the hemispheres contribute in allthe processes, there is lateralization of functions to a certain extent. This brain lateralization is evidentfrom the phenomenon of right- or left-handedness, and of right or left ear preference. Contralateralorganization is a principle issue in the study of the two hemispheres, and this purpose of contralateralityhas also contributed to the view that the two hemispheres carry out distinctly different functions. Thesensory data crosses over in the pathways leading towards the cortex. For example there is a visualcrossover; and the left visual field corresponds to the right hemisphere, while the right visual fieldcorresponds to the left hemisphere. Similar case exists with the other senses too. Furthermore,movements are controlled by the motor cortex, and the motor control is also contralateral in nature.The left side of the body is controlled by the right hemisphere, while the right side of the body iscontrolled by the left hemisphere.

Moreover, Split brain studies have also shown that there is a functional asymmetry in the twohemispheres to a certain extent. Split-brain is a term that is used to describe the result when theCorpus Callosum connecting the two hemispheres of the brain is severed to some degree.

Split brain research has been conducted among animals with the main focus on determining the differentfunctions associated with each hemisphere. In a particular study , is was seen that the cats that hadundergone such procedure behaved as if they had two brains, each of which was capable of attendingto, learning and remembering information independent of the other. Another such study was conductedon human patients with corpus Callosum damage. In this study, it was observed that when a patientwas given a common object such as a coin or a comb in their right hand, they could identify it verballysince the information from the right side crosses over to the left hemisphere where language processingis centralized. When the common object was given in the left hand of the subjects, it was observed thatthe patients could not describe it verbally; they could point it out but only with the left hand. Thesestudies and many more have indicated that the two hemispheres are lateralized in their functions. Theleft hemisphere is associated with functions such as language, conceptualization, analysis, andclassification; while the right hemisphere is associated with the integration of information, spatialprocessing, face recognition, etc. Although a number of such studies have indicated that the twohemispheres differ in their functions to a certain extent, the brain seems to operate as a holistic organ.

Peripheral Nervous system

The peripheral nervous system (PNS) is the division of the nervous system containing all the nervesthat lie outside of the central nervous system (CNS). The primary role of the PNS is to connect the CNSto the organs, limbs and skin.

These nerves extend from the central nervous system to the outermost areas of the body. The nervesthat make up the peripheral nervous system are actually the axons or bundles of axons from neuroncells. In some cases, these nerves are very small but some nerve bundles are so large that they can beeasily seen by the human eye. The peripheral nervous system is divided into two parts: the somaticnervous system and the autonomic nervous system

The Somatic Nervous System

The somatic system is the part of the peripheral nervous system responsible for carrying sensory andmotor information to and from the central nervous system. The somatic nervous system derives itsname from the Greek word soma, which means "body." The somatic system is responsible fortransmitting sensory information as well as for voluntary movement. This system contains two majortypes of neurons: sensory neurons (or afferent neurons) that carry information from the nerves to thecentral nervous system, and motor neurons (or efferent neurons) that carry information from the brainand spinal cord to muscle fibers throughout the body.

The Autonomic Nervous System

The autonomic system is the part of the peripheral nervous system responsible for regulatinginvoluntary body functions, such as blood flow, heartbeat, digestion and breathing. This system isfurther divided into two branches: the sympathetic system regulates the flight-or-fight responses, whilethe parasympathetic system helps maintain normal body functions and conserves physical resources.

The sympathetic nervous system responds to impending danger, and is responsible for theincrease of one's heartbeat and blood pressure, among other physiological changes, along withthe sense of excitement one feels due to the increase of adrenaline in the system. ("Fight orflight" responses).

The parasympathetic nervous system, on the other hand, is evident when a person is resting andfeels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of theheart, the dilation of the blood vessels, and the stimulation of the digestive and genitourinarysystems. ("Rest and digest" responses).

Neuro-imaging techniques

Neuro-imaging includes the use of various techniques to either directly or indirectly image the structure,function/pharmacology of the brain. Neuro-imaging falls into two broad categories:

Structural imaging, which deals with the structure of the brain and the diagnosis of gross (largescale) intracranial disease (such as tumor), and injury.Functional imaging, which is used to diagnose metabolic diseases and lesions on a finer scale(such as Alzheimer's disease) and also for neurological and cognitive psychology research andbuilding brain-computer interfaces. Functional imaging enables, for example, the processing ofinformation by centers in the brain to be visualized directly. Such processing causes the involvedarea of the brain to increase metabolism and "light up" on the scan.

Hemodynamic techniques are the techniques that measure the blood flow, blood oxygenation in thebrain, using the methods of optical imaging. The techniques include PET (Positron EmissionTomography), fMRI (Functional Magnetic Resonance Imaging)

Electrical/Magnetic Techniques are the techniques that measure the electrical activity, or themagnetic fields that are produced by this electrical activity in the brain. These measures includeEEG/ERP (Electroencephalography/Event-Related Potential, MEG (Magnetoencephalography).

Blood Flow and Brain Activity are correlated, and an increase in cognitive activity is correlated withincreased brain blood flow. The spatial resolution is the localization capability of the method, rangingfrom individual synapses to the brain as a whole. And the temporal resolution: is the time scale overwhich the method can take measurements, ranging from milliseconds to life-times. All these neuro-imaging methods and techniques make use of some degree of invasiveness, which is the extent towhich foreign substances are introduced to the body.; and are more or less, really expensive

Single Cell Recording

Single cell recording is an invasive method of measuring the electro-physiological responses of a singleneuron using a microelectrode system. This method measures the number of action potentials persecond.

When a neuron generates an action potential, the signal propagates down the neuron as a currentwhich flows in and out of the cell through excitable membrane regions in the soma and axon. Amicroelectrode is inserted into the brain, where it can record the rate of change in voltage with respectto time. These microelectrodes must be fine-tipped, high-impedance conductors; they are primarilyglass micro-pipettes or metal microelectrodes made of platinum or tungsten. Microelectrodes can becarefully placed within (or close to) the cell membrane, allowing the ability to record intracellularly orextracellularly. This method enables the researchers to understand how individual neurons codeinformation.

Positron Emission Tomography (PET)

Positron emission tomography or PET scan is a technique that relies on injection of a radioactive isotopeto measure cerebral blood flow. The activity levels are determined as (very mild) radioactivity levels aremeasured by subtracting activity levels at rest from activity levels during a particular task. Thistechnique maps wide range of cognitive activities including complex tasks, and can give a reasonablelocation of active areas (3-4 millimeters).

This technique measures emissions from radioactively labeled metabolically active chemicals that havebeen injected into the bloodstream. The emission data are computer-processed to produce 2- or 3-dimensional images of the distribution of the chemicals throughout the brain. The positron emittingradioisotopes used are produced by a cyclotron, and chemicals are labeled with these radioactive atoms.The labeled compound, called a radiotracer, is injected into the bloodstream and eventually makes itsway to the brain. Sensors in the PET scanner detect the radioactivity as the compound accumulates invarious regions of the brain. A computer uses the data gathered by the sensors to create multicolored2- or 3-dimensional images that show where the compound acts in the brain.

The greatest benefit of PET scanning is that different compounds can show blood flow and oxygen andglucose metabolism in the tissues of the working brain. These measurements reflect the amount of brainactivity in the various regions of the brain and allow learning more about how the brain works. PETscans were superior to all other metabolic imaging methods in terms of resolution and speed ofcompletion (as little as 30 seconds), when they first became available. The improved resolutionpermitted better study to be made as to the area of the brain activated by a particular task. The biggestdrawback of PET scanning is that because the radioactivity decays rapidly, it is limited to monitoringshort tasks.

Functional Magnetic Resonance Imaging (fMRI)

Neural activity consumes oxygen as well as generates electrical signals. In order to compensate forincreased oxygen consumption, more blood is pumped into the active region. This is called the BOLDresponse (Blood Oxygen Level Dependent contrast). The change in BOLD response over time is calledthe haemodynamic response function and it has a number of distinct phases. The HaemodynamicResponse Function peaks in 6–8 seconds and so this is the temporal resolution of fMRI. Thus fMRImeasures the concentration of deoxyhaemoglobin in the blood.

This technique relies on the paramagnetic properties of oxygenated and deoxygenated hemoglobin tosee images of changing blood flow in the brain associated with neural activity. This allows images to begenerated that reflect which brain structures are activated (and how) during performance of different

tasks or at resting state.

Most fMRI scanners allow subjects to be presented with different visual images, sounds and touchstimuli, and to make different actions such as pressing a button or moving a joystick. Consequently,fMRI can be used to reveal brain structures and processes associated with perception, thought andaction. The resolution of fMRI is about 2-3 millimeters at present, limited by the spatial spread of thehemodynamic response to neural activity.

Transcranial magnetic stimulation (TMS)

Transcranial magnetic stimulation (TMS) is a noninvasive method to cause depolarization orhyperpolarization in the neurons of the brain. TMS uses electromagnetic induction to induce weakelectric currents using a rapidly changing magnetic field; this can cause activity in specific or generalparts of the brain with little discomfort, allowing for study of the brain's functioning andinterconnections. An electromagnetic coil is held against the forehead and short electromagnetic pulsesare administered through the coil. The magnetic pulse easily passes through the skull, and causes smallelectrical currents that stimulate nerve cells in the targeted brain region. And because this type of pulsegenerally does not reach further than two inches into the brain, scientists can select which parts of thebrain will be affected and which will not be.

Single Photon Emission Computed Tomography (SPECT)

Single-photon emission computed tomography (SPECT) is similar to PET and uses gamma ray-emittingradioisotopes and a gamma camera to record data that a computer uses to construct two- or three-dimensional images of active brain regions. SPECT relies on an injection of radioactive tracer, or "SPECTagent," which is rapidly taken up by the brain but does not redistribute. Uptake of SPECT agent is nearly100% complete within 30 to 60 seconds, reflecting cerebral blood flow (CBF) at the time of injection.This technique has less spatial resolution than PET, but far less expensive. Often, it is used for earlydetection of dementias, which has been evidenced by hypoperfusion in a given area.

Electro-encephalogram (EEG)

Electroencephalography (EEG) is the recording of electrical activity along the scalp. EEG measuresvoltage fluctuations resulting from ionic current flows within the neurons of the brain. In clinicalcontexts, 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. The electricpotential generated by an individual neuron is far too small to be picked up by EEG or MEG. EEG activitytherefore always reflects the summation of the synchronous activity of thousands or millions of neuronsthat have similar spatial orientation.

In conventional scalp EEG, the recording is obtained by placing electrodes on the scalp with a conductivegel or paste, usually after preparing the scalp area by light abrasion to reduce impedance due to deadskin cells. Many systems typically use electrodes, each of which is attached to an individual wire. Somesystems use caps or nets into which electrodes are embedded; this is particularly common when high-density arrays of electrodes are needed.

Electrode locations and names are specified by the International 10–20 system for most clinical andresearch applications (except when high-density arrays are used). This system ensures that the namingof electrodes is consistent across laboratories.

The wave patterns that can be observed in an EEG can be described as follows –

Alpha Waves – These waves have the characteristic frequency of 8-13 Hz and amplitude of 20-60 µV. These waves are easily produced when quietly sitting in relaxed position with eyes closed(few people have trouble producing alpha waves). While alpha blockade occurs with mentalactivity.Beta waves – These waves have the characteristic frequency of 14-30 Hz and amplitude of 2-20µV. These are the most common form of brain waves, and are present during mental thought andactivity.Theta waves – These waves have the characteristic frequency of 4-7Hz and amplitude: 20-100µV. These waves are believed to be more common in children than adults.Delta Waves – These waves are characteristics of frequency of 0.5-3.5 Hz and amplitude of 20-200µV. These waves are found during periods of deep sleep in most people. They arecharacterized by very irregular and slow wave patterns. Also, these waves are useful in detectingtumors and abnormal brain behaviors.Gamma Waves – These waves have a characteristic frequency of 36-44Hz and amplitude of 3-5µV. Theses waves occur with sudden sensory stimuli.

EEG/ ERP equipment

Electrodes – Electrodes are placed on scalp, and conduct electrical current. These aresystematically placed, according to the 10-20 system, and generally consist of 128 channelsystem.Montage: The representation of the EEG channels is referred to as a montage. It is the particulararrangement of electrodesComputers: STIM and SCANI/O ports: These send trigger from STIM to SCAN, allows SCAN to know timing of events.

Raw EEG is the composite of all neural events happening in the brain at any time. However, it is nearlyimpossible to pick out responses to one event from the others in the raw EEG due to low signal to noise

ratio (SNR). The amplitude of spontaneous EEG (noise) is higher than the individual event of interest(signal). The ERP component is 10 μv at scalp, while the spontaneous EEG is 50-80 μv at scalp. In theprocess of signal averaging, multiple samples of EEG epochs (e.g. 1 sec long) are averaged together.The signal of interest is boosted and the (essentially random) noise is averaged out.

Since ERP always occurs in response to stimuli at certain time, the averaging is time locked to thestimulus or the response. An event-related potential (ERP) is the measured brain response that is thedirect result of a specific sensory, cognitive, or motor event.[1] More formally, it is any stereotypedelectrophysiological response to a stimulus. The study of the brain in this way provides a noninvasivemeans of evaluating brain functioning in patients with cognitive diseases. ERPs are measured withelectroencephalography (EEG). ERP measures peaks; Amplitude is measured in terms of voltage, andLatency is measured from the time of the stimulus to the time of the peak. Peaks at a particular latencyand time are called components. Early (before 100 ms) includes sensory reaction to physicalcharacteristics, and late (after 100 ms) involves interpretation of stimulus.

Sensory ERP components – These components are related to the presentation of the stimulus.Some such components include –

N1 (N100): Selective attentionP2 (P200): Early processing of stimulusN2 (N200): Mismatch negativity – oddball paradigm (mismatch based on pitch, intensity,duration)P3: Deviant items & memory updating: largest component, most research (in yourreading)N400: semantic evaluation

Response related components – These components are related to the subject’s responses. Theseinclude -

Lateralized readiness potential (LRP): It is a negative potential and starts 800 ms prior tovoluntary movement.Error-related negativity (ERN): It is a negative component occurs with incorrect response.

Magnetoencephalography (MEG)

Magnetoencephalography (MEG) is an imaging technique used to measure the magnetic fields producedby electrical activity in the brain via extremely sensitive devices. It is the measurement of the magneticfields naturally present outside the head due to electrical activity in the brain. Sensors make no contactwith scalp. This technique has better spatial resolution than EEG/ERP However, this is an expensivetechnique and requires very low noise environment (e.g. no lorries driving by).

MEG offers a very direct measurement of neural electrical activity (compared to fMRI for example) withvery high temporal resolution but relatively low spatial resolution. The advantage of measuring themagnetic fields produced by neural activity is that they are likely to be less distorted by surroundingtissue (particularly the skull and scalp) compared to the electric fields measured byelectroencephalography (EEG).

Polygraph

A polygraph (popularly referred to as a lie detector) measures and records several physiologicalindices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked andanswers a series of questions. The belief underpinning the use of the polygraph is that deceptiveanswers will produce physiological responses that can be differentiated from those associated with non-deceptive answers.

Traditional polygraph measures the bodily response (e.g. sweating, heart rate). The anterior cingulatecortex becomes active when asked to generate false answers to questions relative to truthful ones (e.g.“where was your last vacation?”). This region is believed to be involved in monitoring conflicts betweenresponses; but it is not necessarily active if lie is memorized in advance.