Nervous System C 45

8/6/2019 Nervous System C 45

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INTRODUCTIONTO

Nervous SystemBy

Prof. Dr. Abdul MajidMBBS, M.Phil, FCPS


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Composition1. Neurons or nerve cells which can

generate and propagate nerve signals>100 billion.

2. Neuroglial cells which can not generateand propagate nerve impulses 10-50times as many as neurons.


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Structure of a Large Neuron inthe Brain


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Functional Types of Neurons

1. Sensory or afferent neurons.2. Motor or efferent neurons.3. Inter neurons or connecting neurons .


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Neuroglial Cells in the Brain


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Astrocytes Astrocytes are found throughout the brain& are of two types.

1) Fibrous Astrocytes: The contain many intermediate filaments,

& are found primarily in the white matter.

2) Protoplasmic Astrocytes: The have granular cytoplasm & are found

in gray matter


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Functions of Astrocytes: Both types of astrocytes send process to

the blood vessels, where they inducecapillaries to form tight junctions thatform the blood brain barrier.

They also send process that envelop thesynapses and surface of nerve cells.


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They produce substances that are trophicto neurons. They help maintain the appropriate

concentration of ions andneurotransmitters by taking up K + ions andneurotransmitter glutamate & (GABA).


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PNS

Sensory (Afferent) Motor (Efferent )

SomaticTo skeletal muscles

for voluntaryControl

of movements

AutonomicTo viscera or

glands

SomaticAutonomic

Splanchanic or

visceral


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ANS

SympatheticThoraco lumbar outflow

(T1- L2 or L 3)Prepares the body to deal with

immediate tThreats to internal environment of

the bodyIt provides flight & fight responses

ParasympatheticCranio sacral outflow

(3 rd , 7 th , 9 th , 10 th ) cranialnerves,

Sacral nerves include S 2, S 3Coordinates body's normal

resting activities, henceCalled rest & repair division


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Cranial Nerves

1. Olfactory2. Optic3. Occulomotor 4. Trochlear 5. Trigeminal6. Abducent

7. Facial8. Vestibulo-cochlear 9. Glossopharyngeal10.Vagus11.Accessory12.hypoglossal


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Functionally 1, 2, 8 arepurely sensory nerves 3,4, 6, 11, 12 are purelymotor nerves 5, 7, 9, 10

are mixed nerves.


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Physiological Division of NS(General design or organization of NS)There are three divisions:

1. Sensory input division which includessensory receptors organs and or sensorynerves.

2. Central nervous system or integrative

division.3. Motor output division which includes

motor nerve supply to effectors.


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The Sensory Input Division

The sensory receptors detect state of thebody or state of the surroundings. For instance, receptors in the skin detectsensations of touch, pressure, pain &temperature. Eyes detect light while earsdetect sound. Sensory receptors are also

called transducers because they convertdifferent forms of energy (stimuli) intoaction potentials.


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Importance of sensory division: Most activities of the NS are initiated by

sensory experiences detected by sensoryreceptors or organs.

The sensory experience can either cause animmediate reaction of the body or Memory of sensory experiences can be storedin the brain for minutes, hours, weeks or yearsto determine bodily reaction at some futuredate.


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Sensory sensations may be:1. Somatic coming from entire body surface and

from some deep tissues.2. Autonomic or visceral sensation coming from

internal organs of the body or glands.


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Sensory areas of CNS

When somatic sensations travel throughperipheral nerves to the CNS they areconducted to multiple sensory areas.

1. All levels of spinal cord.2. Reticular substance of brain stem.

3. Cerebellum.4. Thalamus.5. Cerebral cortex.


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CNS or Integrative PortionIt is composed of brain and spinal cord .Brain can:1. Store information.

2. Generate thoughts.3. Create ambitions.4. Determine reaction of the body in response to

sensory sensation.

Spinal cord can:1. Conduct impulses to & from the brain.2. Generate reflex actions.


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CONT:

One of the most important functionsof the CNS is to process incoming

information in such a way thatappropriate mental & motor responses occur.

More than 99% of all sensoryinformation is discarded by the brainas irrelevant & unimportant.


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When important sensory informationexcites the mind it is immediatelychanneled into proper integrative andmotor regions of the brain to causedesired response, this channeling and

processing of information is calledintegrative function of NS. Thus, if a person places a hand on a hot

stove, the desired instantaneous responseis to lift the hand, associated responsesare moving of the entire body away fromthe stove and even perhaps shouting withpain.


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3. Facilitatory and inhibitory signals from

other areas in the NS can controlsynaptic transmission, sometimesopening the synapses for transmission

and at other times closing them.4. Some post synaptic neurons respondwith large numbers of output impulses &other respond with only a few.

5. Thus the synapses perform a selectiveaction, often blocking weak signals whileallowing strong signals to pass.


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6. At other times selecting & amplifying

certain weak signals.7. Often channeling these signals in many

directions rather than in one direction.

8. They store information called memoryfunction.


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Storage of memory:1.Cerebral cortex.2.In basal regions of brain.3.Spinal cord.

Mechanism: Each time certain type of informationpasses through sequences of synapses,these synapses become capable of transmitting the same type of signal thenext time, a process called facilitation.


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After the sensory signals havepassed a large number of times,the synapses become sofacilitated that signals generatedwith in the brain may be

perceived as if original sensationis there.


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Summary: After processing sensory information it ischanneled to:

1. Certain storage areas for future response& also for thinking process.

2. Certain integration areas.3. Certain motor areas for immediate

response.


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Motor Division of Nervous System

It controls:

contraction of appropriate skeletal muscles. Contraction of smooth muscles throughout the

body.

Secretion of chemicals from both exocrine &endocrine glands. These activities arecollectively called motor functions of NS, &muscles & glands are called effectors .


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Skeletal Motor Nerve Axis of NS


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Autonomic motor axis: Parallel to somatic motor axis is the

autonomic motor axis which controlsactivities of smooth muscles & glands.Skeletal muscles can be controlledfrom many levels of CNS.

1. Spinal cord.2. Reticular substance of brain stem.3. Basal ganglia.4. Cerebellum.5. Motor cortex.


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Major Levels of CNS Functions

1. Spinal cord level.2. Lower brain or subcortical level.

3. Higher brain level or cortical level.


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Cont:Spinal cord level:

Conducts sensory and motor impulses toand from brain.

In a spinal animal neuronal circuits withinspinal cord can produce. Walking movements.

Reflexes that withdraw portions of thebody from painful objects.

Reflexes that stiffen the legs to supportthe body against gravity.


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Lower Brain or Subcortical Level

Many of the subconscious activities of thebody are controlled by areas of the lower brainlike medulla, pons, mesencephlon, thalamus,hypothalamus, cerebellum and basal ganglia.

These activities includes:1. Subconscious control of arterial blood pressure

and respiration in the medulla and pons.

2. Control of equilibrium is the combined functionof older portions of cerebellum & reticular formation of brain stem.


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Cont:3. Feeding reflexes like salivation in

response to taste of food and the lickingof the lips are controlled by areas in themedulla, pons, mesencephlon, amygdala

and hypothalamus.4. Many emotional patterns such as anger,excitement, sexual response, reaction topain and reaction to pleasure can occur in animals after destruction of cerebralcortex.

5. Concerned with wakefulness of cerebralcortex.


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H igher Brain or Cortical Level

1. Large store house of memory.2. It nerve functions alone but always in

association with lower brain centersalthough without cerebral cortex thefunctions of lower brain centers are oftenimprecise.

3. It is essential for most of our thoughtprocesses.


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Physiological Classification of Synapses

Chemical Synapses:1. Transmission occurs by

releasingneurotransmitter.

2. Unidirectionaltransmission.

3. Gap junctions absent.

4. Width of synaptic cleft200-300 A.5. In human CNS 99%.

Electrical Synapses1. Transmission occurs by

direct electrical spread.

2. Bidirectionaltransmission.3. Gap junctions present.4. Width of synaptic cleft

20-30 A.5. In human CNS 01%.


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Mechanism of Action of aNeurotransmitter

Action of a neurotransmitter releaseddepends upon the receptor protein(excitatory or inhibitory) present on thepostsynaptic membrane.

1. A binding component that protrudesoutwards from the membrane intosynaptic cleft-here it binds theneurotransmitter coming from thepresynaptic terminal.


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Cont:2. An ionophore component that passes all the

way through the postsynaptic membrane to theinterior of the postsynaptic neuron. Theionophore in turn is one of the two types:An ion channel that allows passage of specific types of ions through the membrane or A second messenger activator that is not anion channel but instead a molecule that

protrudes into the cell cytoplasm & activatesone or more substances inside thepostsynaptic neurons.


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Cont: These substance serve as second

messengers to increase or decreasespecific cellular functions.

The ion channels in the postsynapticmembrane are of two types:1.1. Cation channelsCation channels that most often allowsodium ions to pass when opened, butsome times allow potassium and / or calcium ions as well and

2. Anion channels that allow mainlychloride ions to pass but also minutequantities of other anions.


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Cont: When cation channels open and allow

positively charged sodium ions to enter this will excite the neuron and theneurotransmitter which causes suchchange is called excitatoryneurotransmitter . Conversely when aneurotransmitter causes opening of anionchannels, the negatively charged ions

enter the cell and cause its inhibition. Sucha neurotransmitter is called inhibitoryneurotransmitter.


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Types of Neurotransmitter 1. Small molecule, rapidly acting

transmitter.Class - I Acetylcholine

Class - II The Amines(Norepinephrine, epinephrine,dopamine, serotonin, histamine)

Class - III Amino Acids (GABA, glycine,glutamate, aspartate)

Class - IV Nitric oxide (NO)


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Cont:2. Neuropeptide, slowly acting transmitters or growth

factors.I. H ypothalamic releasing hormones:Thyrotropin releasing hormone Luteinizing hormonereleasing hormone Somatostain (growth hormoneinhibitory factor)

II. Pituitary peptides: ACTHBeta endorphin

Alpha melanocyte stimulating hormoneProlactin

LHTSHGHVasopressinOxytocin


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Cont:III. Peptides that act on gut & brain:

Leucine enkephalinMethionine enkephalinSubstance P

GastrinCholecystokininVIPNerve growth factor

NeurotensinInsulinGlucagonBrain derived neurotropic factor


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Cont:

IV. From other tissues: Angiotensin IIBradykinin

CarnosineSleep peptidesCalcitonin


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Distribution of 03 Ions Across Neuronal SomalMembrane


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Electrical Events During NeuralExcitation

Resting membrane potential of theneural soma is about 65mv.This is some what less negative than the 90mv found in large peripheral nervefibers and in skeletal muscle fibers: thelower voltage is important because itallows both positive and negative controlof the degree of excitability of the neuron.


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Cont:Concentration differences of ions acrossthe neuronal somal membrane.

Sodium ions - Sodium pumpPotassium ions - potassium pumpChloride ions - weak chloride pump and

negative potential insidethe soma. (-65mv)

A potential that exactly opposesmovement of an ion is called Nernstpotential for that ion:


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Cont:Effect of synaptic excitation onpostsynaptic membrane excitatorypostsynaptic potential (EPSP).Figure A shows resting membranepotential inside the soma. (-65mV)Figure B shows hypo polarization due toinflux of sodium ions and rise in resting

membrane potential from -65 to -45mV.Figure C shows hyperpolarization due toinflux of chloride ions and or out flux of

potassium ions.


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Cont:The threshold level for excitation is -45mV. Only few presynaptic nerveterminals will not be able to excite thepostsynaptic neurons, for this at least 40-

80 presynaptic nerve terminals arerequired. When these many presynapticterminals send impulses to postsynapticmembrane at the same time or in rapidsuccession then summation occurs andaction potential is generated in the initialsegment which then travels along the

axon.


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Electrical Events DuringNeuronal Inhibition

As shown in figure C when inhibitoryneurotransmitter is released it causesinflux of chloride ions and or out flux of potassium ions from soma. This causesresting membrane potential to becomemore negative that is from 65 to 70mV

this is called hyperpolarization or inhibitorypostsynaptic potential. (IPSP)


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Cont:Presynaptic inhibition: inhibition of presynaptic neuron can also be achievedby another way that is by inhibitingpresynaptic nerve fiber or its terminal

before it releases its excitatoryneurotransmitter. This is calledpresynaptic inhibition. GABA is involved in

this type of inhibition which causesopening of anion channels. Presynapticinhibition occurs in many of the sensorypathways in CNS.


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Properties of Chemical Synapses

1. Dales Law single neurotransmitter.2. One way transmission presynaptic to

postsynaptic.

3. Minimum synaptic delay 0.5m/sec.Causes:

Discharge of neurotransmitter. Diffusion. Binding with receptor proteins. Increaser in membrane permeability. Inward diffusion of sodium ions to produce

action potential.


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Cont:Importance: By measuring delay time one

can estimate the number of neuronsinvolved in a circuit or whether the reflexaction is monosynaptic, bisynaptic or

polysynaptic.4. Convergence / divergence.5. Spatial & temporal summation.


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7 Occlusion


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7. Occlusion.


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8 . Fatigue of synaptic transmission.

When excitatory synapses are repeatedlystimulated at a rapid rate, the number of discharges by postsynaptic neuron is atfirst very great, but then it graduallydecreases or many case. This is calledfatigue or synaptic transmission.Mechanism of fatigue: It occurs due topartial exhaustion of stores of

neurotransmitter in presynaptic terminals.Importance: By this property the excessexcitability of brain during an epileptic fit isfinally subsided so that the seizure

ceases.


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Cont:9. Effect of alkalosis or acidosis on synaptic

transmission Alkalosis - neuronal excitability e.g epileptic

fits develop when pH of the ECF is from 7.4to 7.8 or 8.0

Acidosis - neuronal activity e.g during diabeticor uremic acidosis when pH from 7.4 to 7.0

10. Effect of hypoxia on synaptic transmission Adequate oxygen supply is required for normalexcitability. When there is hypoxia thisdecreases neuronal excitability. For examplelack of arterial supply to brain for 3 7seconds results in unconsciousness.


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Cont:11. Effect of drugs on synaptic transmission

CAFFEINE CoffeeTHEOPHYLLINE - TeaTHEOBROMINE Cocoa all increase

neuronal excitability by reducing the thresholdfor excitation of neurons.STRYCHNINE - neuronal excitability byinhibiting action of some of inhibitory

neurotransmitters like glycine in the spinal cord sever tonic muscle spasm. Lipid solubleanesthetics inhibit neuronal activity byincreasing threshold of excitation.

Nervous System C 45

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