Part 2 Portrait_anatomy and Physiology of the Nervous System

8/14/2019 Part 2 Portrait_anatomy and Physiology of the Nervous System

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The blood-brain barrier is thus a key element inthe normal functioning of the brain, and isolates it

from disturbances in the composition of the fluidsin the rest of the body.Malcolm Segal

1. Meninges

The meninges (singular meninx) is the system ofmembranes which envelops the central nervoussystem. The meninges consist of three layers: thedura mater, the arachnoid mater, and the piamater. The primary function of the meninges and of

the cerebrospinal fluid is to protect the centralnervous system.

Anatomy

Pia mater

The pia or pia mater is a very delicate membrane. Itis the meningeal envelope which firmly adheres tothe surface of the brain and spinal cord. As such it

follows all the minor contours of the brain (gyri andsulci). It is a very thin membrane composed offibrous tissue covered on its outer surface by asheet of flat cells thought to be impermeable tofluid. The pia mater is pierced by blood vesselswhich travel to the brain and spinal cord, and itscapillaries are responsible for nourishing the brain.

Arachnoid membrane

The middle element of the meninges is thearachnoid membrane, so named because of its

spider web-like appearance. It provides acushioning effect for the central nervous system.The arachnoid mater exists as a thin, transparentmembrane. It is composed of fibrous tissue and, likethe pia mater, is covered by flat cells also thoughtto be impermeable to fluid. The arachnoid does notfollow the convolutions of the surface of the brain


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and so looks like a loosely fitting sac. In the regionof the brain, particularly, a large number of finefilaments called arachnoid trabeculae pass from thearachnoid through the subarachnoid space to blendwith the tissue of the pia mater.

The arachnoid and pia mater are sometimestogether called the leptomeninges.

Dura mater

The dura mater (also rarely called meninx fibrosa,or pachymeninx) is a thick, durable membrane,closest to the skull. It consists of two layers, theperiosteal layer, closest to the calvaria and theinner meningeal layer. It contains larger bloodvessels which split into the capilliaries in the pia

mater. It is composed of dense fibrous tissue, andits inner surface is covered by flattened cells likethose present on the surfaces of the pia mater andarachnoid. The dura mater is a sac which envelopsthe arachnoid and has been modified to serveseveral functions. The dura mater surrounds andsupports the large venous channels (dural sinuses)carrying blood from the brain toward the heart.

Spaces

The subarachnoid space is the space whichnormally exists between the arachnoid and the piamater, which is filled with cerebrospinal fluid.

Normally, the dura mater is attached to the skull, orto the bones of the vertebral canal in the spinalcord. The arachnoid is attached to the dura mater,and the pia mater is attached to the centralnervous system tissue. When the dura mater andthe arachnoid separate through injury or illness, thespace between them is the subdural space.

2. Cerebrospinal Fluid (CSF)

The brain floats on a liquid cushion of Liquorcerebrospinalis, cerebrospinal fluid (CSF), within therigid bony skull. The CSF is contained between thearachnoid mater (the middle layer of the braincover, meninges) and the pia mater (the layer ofthe meninges closest to the brain). It constitutes

the content of all intra-cerebral (inside the brain,cerebrum) ventricles, cisterns and sulci (singularsulcus), as well as the cental canal of the spinalcord.


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Since the brain floats in CSF, the fluid acts in effectto reduce the weight of the brain from some 1000 gto about 50 g, and also protects the brain fromknocks on the head. However, since the brain canmove within the CS, it can be damaged on the

opposite side by a sudden deceleration such as in acar accident (contra coup injury).

The subarachnoid space on the outside of the brainis in continuity with a similar space around thespinal cord and also with the series ofinterconnected cerebral ventricles within the brain.Each of the paired lateral ventricles, in the cerebralhemispheres, contains a leaf-like, highly vascularchoroid plexus. It is from these structures, by the

modified ependymal cells in the choroid plexus thatthe bulk of the CSF is secreted and the remainder isformed around blood vessels and along ventricularwalls. From the lateral ventricles CSF drains into thecentral third ventricle, through the Foramen ofMonro, and thence through the cerebral aqueduct(Aqueduct of Sylvius) in the midbrain into the fourthventricle. Both the third and fourth ventriclescontribute to the flow from their own choroidaltissue. From the fourth ventricle, the CSF exits intothe subarachnoid space through several openings,and fills the basal cisterns beneath the brain.Thence the flow of CSF is mainly up and over thewhole brain surface, while some flows down aroundthe spinal cord. Completing the circuit back to thebloodstream, the fluid drains via the valve-likearachnoid granulations into the sagittal sinus, thelarge venous channel lying centrally on the top ofthe brain; some is also taken up into veins aroundspinal nerve roots and into the lymphatics of thenose.

The secretion of CSF is an active transport processthat moves fluid and solutes from the blood plasmainto the ventricles, the choroid plexuses being aspecialized part of the BBB. CSF secretion involvesthe pumping of ions and specialized ion channels,with the energy coming from glucose and oxygen inthe blood. In the adult human CSF is formed at arate of about 0.5 ml/min; the total volume is about200 ml, of which 30 ml is in the ventricles and the

remainder in the subarachnoid space. Thecirculation of CSF leads to the fluid beingcompletely replaced about every 4 hours.

CSF is a weak salt solution with similar inorganic ionconcentrations to plasma, but with small and


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significant differences, whereas the protein contentis about 100 times less than that of plasma.Abnormalities of the CSF can be important indiagnosis of some medical conditions; the fluid canbe sampled by lumbar puncture from the extension

of the subarachnoid space (the lumbar sac) belowthe lower end of the spinal cord. CSF is normally aclear, amazingly bright fluid, and if it is cloudy orcontains a raised level of protein or traces of bloodthis is usually an indication of brain infection, sometypes of brain or spinal cord tumor, or trauma.

The pressure within the brain, the intracranialpressure (ICP), is transmitted in the CSF around thespinal cord and down into the lumbar sac. With the

body horizontal, it is normally low; it is markedlyaffected by posture, and raised by straining orcoughing.

b. Spinal Cord

The spinal cord is the main pathway for informationconnecting the brain and peripheral nervous system(PNS).The spinal cord and medulla form a continuous structureextending from the cerebral hemispheres. Approximately45 cm (18 in) long and about the thickness of a finger, itextends from the foramen magnum at the base of the skullto the lower border of the first lumbar vertebra, where ittapers to a fibrous band called the conus medullaris.Continuing below the second lumbar space are the nerveroots that extend beyond the conus, which are called thecauda equina because they resemble a horses tail. Similarto the brain, the spinal cord consists of gray and whitematter. The butterfly-shaped, unmyelinated gray matter inthe brain is external and myelinated white matter is

internal; in the spinal cord, gray matter is in the center andis surrounded on all sides by white matter.


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The spinal cord issurrounded by themeninges, dura, arachnoid,and pia layers. Between thedura mater and the

vertebral canal is theepidural space. The spinalcord is an H-shapedstructure with nerve cellbodies (gray matter)surrounded by ascendingand descending tracts(white matter). The lowerportion of the H is broaderthan the upper portion andcorresponds to the anterior

horns. The anterior hornscontain cells with fibers thatform the anterior (motor)root end and are essentialfor the voluntary and reflexactivity of the muscles theyinnervate. The thinnerposterior (upper horns)portion contains cells withfibers that enter over theposterior (sensory) root end

and thus serve as a relaystation in the sensory/reflexpathway. The thoracic region of the spinal cord has aprojection from each side at the crossbar of the H of graymatter called the lateral horn. It contains the cells that giverise to the autonomic fibers of the sympathetic division.The fibers leave the spinal cord through the anterior rootsin the thoracic and upper lumbar segments.

Ascending and Descending Pathways: The Spinal

TractsThe white matter of the cord is composed of myelinatedand unmyelinated nerve fibers. The fast-conductingmyelinated fibers form bundles that also contain glial cells.Fiber bundles with a common function are called tracts. There are six ascending (Sensory) tracts. Two conductsensation, principally the perception of touch, pressure,vibration, position, and passive motion from the same sideof the body. Before reaching the cerebral cortex, thesefibers cross to the opposite side in the medulla. The twospinocerebellar tracts conduct sensory impulses frommuscle spindles, providing necessary input for coordinatedmuscle contraction. They ascend essentially uncrossed andterminate in the cerebellum. The last two spinothalamictracts are responsible for conduction of pain, temperature,proprioception, fine touch, and vibratory sense from theupper body to the brain. They ascend, cross to the opposite


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side of the brain, and terminate in the thalamus (Hickey,2003).

There are eight descending (Motor) tracts, seven of whichare engaged in motor function. The two corticospinal tractsconduct motor impulses to the anterior horn cells from theopposite side of the brain and control voluntary muscleactivity. The three vestibulospinal tracts descend uncrossedand are involved in some autonomic functions (sweating,pupil dilation, and circulation) and involuntary musclecontrol. The corticobulbar tract conducts impulsesresponsible for voluntary head and facial musclemovement and crosses at the level of the brain stem. Therubrospinal and reticulospinal tracts conduct impulsesinvolved with involuntary muscle movement.

Vertebral Column

The vertebral column or the spine is the central support ofthe body. The bones of the vertebral column surround andprotect the spinal cord and normally consist of 7 cervical,12 thoracic, and 5 lumbar vertebrae, as well as the sacrum(a fused mass of five vertebrae), and terminate in thecoccyx. Nerve roots exit from the vertebral column throughthe intervertebral foramina (openings). The vertebrae areseparated by disks, except for the first and second cervical,the sacral, and the coccygeal vertebrae. Each vertebra has

a ventral solid body and a dorsal segment or arch, which isposterior to the body. The arch is composed of two pediclesand two laminae supporting seven processes. Thevertebral body, arch, pedicles, and laminae all encase thevertebral canal.


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The Peripheral NervousSystem

The spinal cord is considerably shorter than the spinalcanal itself. The nerve roots angle downward increasinglyfrom top to bottom from where they exit to the spinal cordto where they exit from the canal. The spinal canal belowthe level of L1 contains no spinal cord, only nerve roots.

Outside the spinal canal, the nerve roots form intoperipheral nerves (part of the peripheral nervous system,or PNS). These nerves carry motor commands from thebrain and spinal cord to the individual muscles in the body.Sensory information is carried back along these nerves tothe central nervous system (CNS). A great deal of otherinformation controlling body function, but of which we areunaware (it is done automatically), is carried by the spinalcord and the attached network of the Autonomic Nervous

System.

The spinal cord regulates some important unconscious (orautomatic) body functions, including bowel and bladdercontrol, and normal sexual organ function. These controlsare governed by the conus medullaris, and are transmittedby the sacral nerve roots.

Most other reflexes in the spinal cord are far more complexin that they involve the participation of many more thantwo nerve cells. Many functions are served by reflexresponse, especially for the unconscious control of bodyfunction, such as blood pressure and body temperature.The information processing that takes place in the spinalcord and which occurs without conscious awareness iscalled a reflex response. While an able-bodied individualcan prevent his bladder or bowel from emptying when hedoes not wish it to, the actual emptying process in bothcases is largely by reflex and is automatic. The brain often

participates in reflex activities of the spinal cord. If an able-bodied person touches a hot stove, the pain causes themuscles to jerk the hand away. This action is caused by aspinal cord reflex. Another branch of the spinal nerve fibergoes up the spinal cord within the spinothalamic tract andcarries the pain sensation to the conscious awareness ofthe brain. In controlling blood pressure, reflexes occurwithin the spinal cord. Messages are also sent to the brain,although they are not consciously sensed.

II. Peripheral Nervous System

The peripheral nervous system (PNS) resides or extendsoutside the central nervous system (CNS), which consists ofthe brain and spinal cord. The main function of the PNS is to


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connect the CNS to the limbs and organs. Unlike the centralnervous system, the PNS is not protected by bone or by theblood-brain barrier, leaving it exposed to toxins andmechanical injuries. The peripheral nervous system is dividedinto the somatic nervous system and the autonomic nervous

system.

1. Somatic Nervous System

The somatic nervous system is responsible for coordinatingthe body movements, and also for receiving externalstimuli. It is the system that regulates activities that areunder conscious control. The somatic nervous systemconsists of 12 pairs of cranial nerves and 31 pairs of spinalnerves.

All of the spinal nerves are "mixed"; that is, they containboth sensory and motor neurons. All our consciousawareness of the external environment and all our motoractivity to cope with it operate through the sensory-somatic division of the PNS.


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The Cranial Nerves

Nerves Type FunctionI

Olfactorysensor

yolfaction (smell)

IIOptic

sensory

vision(Contain 38% of all the axons

connecting to the brain.)III

Oculomotormotor

*eyelid and eyeball muscles

IVTrochlear

motor*

eyeball muscles

V

Trigeminal

mixed Sensory: facial and mouth

sensationMotor: chewing

VIAbducens

motor*

eyeball movement

VIIFacial

mixed Sensory: tasteMotor: facial muscles and

salivary glands
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Taste.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Taste.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html


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VIIIAuditory

sensory

hearing and balance

IXGlossopharyn

geal

mixed Sensory: tasteMotor: swallowing

XVagus mixed main nerve of theparasympathetic nervous system(PNS)

XIAccessory

motor swallowing; moving head andshoulder

XIIHypoglossal

motor*

tongue muscles

*Note: These do contain a few sensory neurons that bring back signalsfrom the muscle spindles in the muscles they control.

2. Autonomic Nervous System

The autonomic nervous system consists of sensory neuronsand motor neurons that run between the central nervoussystem (especially the hypothalamus and medullaoblongata) and various internal organs such as the: heart,lungs, viscera and glands (both exocrine and endocrine).

The actions of the autonomic nervous system are largelyinvoluntary (in contrast to those of the sensory-somaticsystem). It also differs from the sensory-somatic system isusing two groups of motor neurons to stimulate theeffectors instead of one. The first, the preganglionicneurons, arise in the CNS and run to a ganglion in the body.Here they synapse with postganglionic neurons, which runto the effector organ (cardiac muscle, smooth muscle, or agland).

It is responsible for monitoring conditions in the internalenvironment and bringing about appropriate changes inthem. The contraction of both smooth muscle and cardiacmuscle is controlled by motor neurons of the autonomicsystem. The autonomic nervous system is then split intothe sympathetic division, parasympathetic division, andenteric division.

2.1. Sympathetic Nervous System

The sympathetic nervous system responds toimpending danger or stress, and is responsible forthe increase of one's heartbeat and blood pressure,among other physiological changes, along with thesense of excitement one feels due to the increase ofadrenaline in the system.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hearing.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#autonomichttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#parasympathetichttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/Hearing.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#autonomichttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#parasympathetichttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html#parasympathetic


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The neurotransmitter of the preganglionicsympathetic neurons is acetylcholine (ACh). Itstimulates action potentials in the postganglionicneurons. The neurotransmitter released by thepostganglionic neurons is noradrenaline (also callednorepinephrine). The action of noradrenaline on aparticular gland or muscle is excitatory is somecases, inhibitory in others. (At excitatory terminals,ATP may be released along with noradrenaline.)

The release of noradrenaline:

stimulates heartbeat

raises blood pressure

dilates the pupils

dilates the trachea and bronchi

stimulates the conversion of liver glycogen intoglucose

shunts blood away from the skin and viscera to

the skeletal muscles, brain, and heart

inhibits peristalsis in the gastrointestinal (GI)

tract

inhibits contraction of the bladder and rectum


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In short, stimulation of the sympathetic branch of theautonomic nervous system prepares the body foremergencies: for "fight or flight" (and, perhaps,enhances the memory of the event that triggered theresponse).

Activation of the sympathetic system is quite generalbecause:

A single preganglionic neuron usually synapses

with many postganglionic neurons;

the release of adrenaline from the adrenalmedulla into the blood ensures that all the cellsof the body will be exposed to sympathetic

stimulation even if no postganglionic neuronsreach them directly.

2.2. Parasympathetic Nervous System

The parasympathetic nervous system, on the otherhand, is evident when a person is resting and feelsrelaxed, and is responsible for such things as theconstriction of the pupil, the slowing of the heart, thedilation of the blood vessels, and the stimulation of

the digestive and genitourinary systems.

The main nerves of the parasympathetic system arethe tenth cranial nerves, the vagus nerves. Theyoriginate in the medulla oblongata. Otherpreganglionic parasympathetic neurons also extendfrom the brain as well as from the lower tip of thespinal cord.

Each preganglionic parasympathetic neuronsynapses with just a few postganglionic neurons,which are located near - or in - the effector organ, amuscle or gland. Acetylcholine (ACh) is theneurotransmitter at all the pre- and many of thepostganglionic neurons of the parasympatheticsystem. However, some of the postganglionicneurons release nitric oxide (NO) as theirneurotransmitter.

Parasympathetic stimulation causes:

slowing down of the heartbeat

lowering of blood pressure


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constriction of the pupils

increased blood flow to the skin and viscera

peristalsis of the GI tract

In short, the parasympathetic system returns thebody functions to normal after they have beenaltered by sympathetic stimulation. In times ofdanger, the sympathetic system prepares the bodyfor violent activity. The parasympathetic systemreverses these changes when the danger is over.

The vagus nerves also help keep inflammation undercontrol. Inflammation stimulates nearby sensory

neurons of the vagus. When these nerve impulsesreach the medulla oblongata, they are relayed backalong motor fibers to the inflamed area. Theacetylcholine from the motor neurons suppresses therelease of inflammatory cytokines, e.g., tumornecrosis factor (TNF), from macrophages in theinflamed tissue.

III. Autonomic Nervous System

The autonomic nervous system (ANS) is that part of theperipheral nervous system that largely acts independent ofconscious control or involuntary and consists of nerves incardiac muscle, smooth muscle, and exocrine and endocrineglands. It is responsible for maintenance functions such asmetabolism, cardiovascular activity, temperature regulationand digestion that have a reputation for being outside ofconscious control. The ANS affects heart rate, digestion,respiration rate, salivation, perspiration, diameter of thepupils, micturition (urination), and sexual arousal. Whereasmost of its actions are involuntary, some, such as breathing,work in tandem with the conscious mind.

The other main subdivision of the peripheral nervous system,the somatic nervous system, consists of cranial and spinalnerves that innervate skeletal muscle tissue and are moreunder voluntary control.

The autonomic nervous system is typically divided into twomain subsystems, the sympathetic nervous system and theparasympathetic nervous system.These tend to balance eachother, offering opposite and yet complementary effectsreflective of the philosophy of Yin and Yang. The efferent ANS


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fibers travel within some cranial and spinal nerves. These twosystems are highly integrated and interact with each other tomaintain a stable environment.

Just as Yin and Yang are opposing, yet complementary andinterdependent forces, the sympathetic and parasympatheticsystems complement each other and are both necessary tocreate overall harmony and balance in the living organism.

Unlike the somatic nervous system, which usually are singleneurons lining the CNS to a muscle or gland, the ANS has atwo-neuron chain prior to the effector organ. The terminal ofthe first neuron is located in the CNS and synapses with nerve

fibers whose cell bodies are within an autonomic ganglion.The axon of the second neuron carries impulses to the targetviscera. An exception is the adrenal medulla, which isinnervated directly by preganglionic fibers. The medulla isactually composed of postganglionic neurons that secreteadrenaline into the bloodstream during adrenalne rush.

The first division of the autonomic nervous system, called thesympathetic nervous system, dominates in times of stress. Itcontrols the "fight or flight" reaction, increasing blood

pressure, heart rate, breathing rate, and blood flow to themuscles. This coordiantes actions used to handle strain and isgeared for action as a whole for short periods of time. Thepreganglionic neurons of the sympathetic nervous systememerge from the spinal cord via the motor orventral roots ofthe thoracic and upper two lumbar spinal nerves.

Another division, called the parasympathetic nervous system,has the opposite effect. It is associated with conservation and

restoration of energy stores and is geared to act locally andand descretely for a longer duration of time. Thepreganglionic fibers emerge from the brain stem via thecranial nerves and from the spinal cord via the sacral spinalnerves. These preganglionic fibers have long axons thatsynapse with the postganglionic neurons in ganglia close to orlocated with organs to be innervated. Each post ganglionicneuron has a ralatively short axon. Most organ systems, butnot all have both sympathetic and parasympatheticinnervation.

The responsed of the different organs arerelated to the type ofneurotransmitter released. The preganglionic fibers aresympathetic and parasympathetic nerves and theprostaglandins fibers of the sympathetic nerves releaseacetylcholine. The prostaganglionic fibers of the sympatheticnerves release norepinephrine. Fibers secreting acetylcholine


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are called cholinergic fibers; fibers secreting norepinephrineare calledd adrenergic fibers.

The complexity of the sympathetic and parasympathetic

response also depends on the type of receptor that combineswith the neurotransmitter. The sympathetic nervous systemhas four types of receptors, alpaha1, alpah2, beta1 and beta2while parasympathetic nervous system has muscarinic andnicotinic receptors.

IV. Neurotransmitter

Each neuron can secrete several different transmittersubstances at its synapse. These neurons communicateinformation with one another by sending electrochemicalmessages from neuron to neuron, a process calledneurotransmission. These electrochemical messages passfrom the dendrites (projections from the cell body), throughthe soma or cell body, down the axon (long, extendedstructures), and across the synapses (gaps between cells) tothe dendrites of the next neuron. In the nervous system, theelectrochemical messages cross the synapses between theneural cells by way of special chemical messengers calledneurotransmitters.

Neurotransmitters are the chemical substances that transferan impulse from one neuron to another. They aid in thetransmission of information in the body. They either excite orstimulate an action in the cells or inhibit or stop an action. They fit into specific receptor cells embedded in themembrane of the dendrite. After neurotransmitters arereleased into the synapse and relay the message to thereceptor cells, they are either transported back from thesynapse to the axon to be stored for later use or metabolized

and inactivated by enzymes, primarily monoamine oxidase.The synaptic vesicles that store the neurotransmitters migrateto the presynaptic membrane of the transmitting neuron,where they release the neurotransmitter into the synapticspace. The neurotransmitter diffuses across the synapticspace to the postsynaptic membrane of the target neuron.Receptor sites on the postsynaptic membrane take up theneurotransmitters, and in this way the impulse is transferredto the next neuron.

Neurotransmitters:

1. Dopamine

Dopamine (DA) is primarily responsible for fine motormovement, sensory integration, and emotional behavior. Itis located primarily in the brain stem. It is generally


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excitatory and is synthesized from tyrosine, a dietaryamino acid. It is metabolized by monoamine oxidase(MAO). A plethora of dopamine receptors exist, including atleast five with pharmacologic significance including D1. D2.D3, D4, D5. Each D receptor carries out different degrees

of stimulation or inhibition of the postsynaptic response.D4 receptors have greater affinity for atypical antipsychoticagents. Hyperactivity of the dopaminergic system isimplicated in schizophrenia. Likewise, dopamine plays amajor role in addiction because drugs, such as cocaine,opiates and alcohol increase the amount of dopamine toact on D2 receptors and stimulate the reward system in thebrain. (Varcarolis, 2006)

2. Serotonin

Serotonin, a neurotransmitter found only in the brain, isderived from tryptophan, a dietary amino acid. Thefunction of serotonin is mostly inhibitory, and it is involvedin the control of food intake, sleep and wakefulness,temperature regulation, pain control, sexual behavior andregulations of emotions. It plays an important role inanxiety and mood disorders and schizophrenia. It has beenfound to contribute to the delusions, hallucinations, andwithdrawn behaviors in schizophrenia. (Videbeck, 2006)

3. Norepinephrine

Norepinephrine (NE) is also known as noradrenaline. It isthe most prevalent neurotransmitter in the nervoussystem, is located primarily in the brain stem and plays arole in changes in attention, learning, and memory, sleepand wakefulness, and mood regulation. It is closely relatedto its precursor, dopamine, and is secreted primarily bynoradrenergic neurons in the locus ceruleus in the pons,but also in scattered neuron bundles in the cerebral cortex,

limbic system and amygdala, thalamus and hypothalamus.NE is a precursor to adrenaline, the main ingredient in theflight or fight response to a real or perceived threat.

4. Acetylcholine

Found in the brain, spinal cord, and peripheral nervoussystem particularly at the neuromuscular junction of

skeletal muscle. It can be excitatory and inhibitory. It issynthesize from the dietary choline found in red meat andvegetables and has been found to affect the sleep/wakecycle and to signal muscles to become active. Studies haveshown that people with Alzheimers disease havedecreased acetycholine-secreting neurons, and people withmyasthenia gravis (a muscular disorder in which impulses


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fail to pass the myoneural junction, which causes muscleweakness) have reduced acetycholine receptors.

5. Gamma-Aminobutyric Acid (GABA)

GABA, an amino acid, is the major inhibitoryneurotransmitter in the brain and has been found tomodulate other neurotransmitter systems rather than toprovide a direct stimulus. Drugs that increase GABAfunction, such as benzodiazepines, are used to treatanxiety and induce sleep.


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CLASSIFICATION OF NEUROTRANMITTERS AND PATHWAYS

NEUROTRANSMITTER

CHEMICALTRANSMITT

ER

LOCATIONFOUND

MAJORPATHWAYS

Cholinergicsystems

Acetylcholine Myoneuraljunctions,

postganglionicneurons,

autonomicganglia,

parasympathetic

postganglionicneurons

Basal nucleus ofMeynert to

cerebral cortex,septal area(rostral to

hypothalamus)

to hippocampus)

Monoaminesystems

Catecholamines

Dopamine

Norepinephrine

Epinephrine

Indolamine

Serotonin

Locusceruleus

Raphei nuclei

Nigrostriatal(substantia nigra

to putamen)Mesolimbic

Mesocortical

Tuberoinfundibular

Locus ceruleus(in midbrain) to

Thalamus,cerebral cortex,

Cerebellum,and spinal cord

Lateralmidbrain to

Hypothalamus and basal

Forebrain.

Centraltegmental tract

Central

brainstem nucleiup to forebrainand down tospinal cord.

Neuropeptides Enkephalins

Endorphins

Substance P

Somastostatin,

neurotensin,angiotensin

II, and others

Spinal cord,hypothalamus

, midbrain


, midbrain


, and manyother places

Amino acids GABA Manyneurons,indicating


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Part 2 Portrait_anatomy and Physiology of the Nervous System

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