lecture cns 1

8/12/2019 Lecture Cns 1

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Overview of

Nervous system Functions

Dr Che Badariah Ab Aziz

[email protected]


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Contents

Central nervous system

Peripheral nervous system

Somatic

Autonomic

Conduction of action potentials


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Organization of the Nervous

System

Sensory Division(afferent)

Peripheral Nervous SystemCranial Nerves

Spinal Nerves

Somatic Nervous System

(voluntary)

Autonomic Nervous System

(involuntary)

Motor Division(efferent)

Central Nervous System

Brain

Spinal Cord


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Design

Receives million of information from

different sensory organs

Information is processed

Necessary actions are taken


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Brain

Cerebrum

Cerebellum

Diencephalon

Brain stem


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The Brain

Cerebrum: site of mind and intellect,motor control, sensory input and

interpretation. Frontal Lobe: general intellect and motor control

Temporal Lobe: auditory input and its interpretation

Parietal Lobe: general sensory input and its

interpretation Occipital Lobe: visual input and its interpretation


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BRAIN


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The Diencephalon

Thalamus: interprets sensory input and relays

it to the appropriate area of the brain.

Hypothalamus: maintains homeostasis.

Thalamus

Hypothalamus


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The Brain and Spinal

Cord Cerebellum: movement control.

Brain Stem: relays information betweenthe brain and the spinal cord.

Spinal Cord: tracts of nerve fibers thatallow two-way conduction of nerve impulses.

afferent -vs- efferent


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Brain stem

The brain stemis the lower part of the brain, adjoining andstructurally continuous with the spinal cord.

Midbrain is involved in functions such as vision, hearing, eyemovement, and body movement.

Pons contains nerve fibres that connect the two halves of thecerebellum.

Vital in coordinating movements involving right and left sides of thebody.

Medulla oblongata transmits ascending and descending nerve fibersbetween the spinal cord and the brain.

The medulla also directly controls many involuntary muscular and

glandular activities, including breathing, heart contraction, arterydilation, salivation, vomiting.

The nuclei of some of the nerves that originate in the brain are alsolocated in the brain stem. (Medulla- IX, X, XI and XII; Pons- V, VI,VII and VIII; midbrain III and IV)

Nerve fibers in the brain stem do not readily regenerate, hence injury

may result in permanent loss of function.


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The Peripheral Nervous

System

The PNS contains 12 pairs of cranialnerves and 31 pairs of spinal nerves.

Sensory neuronsenter the spinal cord

through the dorsal root. mechanoreceptors (touch)

thermoreceptors (temperature)

nociceptors (pain)

chemoreceptors (oxygen,

glucose, electrolytes, etc.)

kinesthetic receptors (movement in joints, balance,etc.) ie. golgi tendon organs


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NOCICEPTORS

NEURAL PATHWAY

(A)

(B)


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Sensory division

Activities are initiated from sensory

receptors

The receptors transform the energy intoaction potentials (electrical energy)

Information is carried to various regions

on the CNS to be processed(interpreted)


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Motor division

To control various activities of the body

Contraction of appropriate mucles

(skeletal or smooth) Secretion of exocrine and endocrine

glands

Muscles and glandseffectors They perform functions dictated by

nerve signals


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Motor division

Skeletal muscles can be controlled by

various structures inc.

Spinal cord

Brain stem

Basal ganglia

Cerebellum

Motor cortex


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Role of the nervous system

To monitor the internal and external environment of

the body

To process this information

To direct behaviour and body processes


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Autonomic nervous system

Control smooth muscles, glands and

other internal system e.g. heart rate,

motility of the gastrointestinal tract Involuntary


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Sympathetic nervous

system

-fight and flight reaction

Parasympathetic nervous

system

-non-emergency


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Neurons The functional and structural unit ofthe nervous system

Specialized to conduct information from one part of the body to

another

There are many, many different types of neurons but most have

certain structural and functional characteristics in common:

Cell body

Dendrites

Axon

Nerve cell terminal


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Components Functions

Cell body Synthesis proteinsCoordinates cellular

activities

Dendrites Receive information fromthe surrounding

environment

AxonConduct nerve

impulses

Terminals Transmit informationto neighbouring

neurons via synapses


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Information

Information is carried in a form of actionpotentials

Nerve cells have plasma membrane that arecapable of producing action potentials

Their ability to generate action potentials isknown as excitability


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Resting membrane potential

-70mV (when they are not transmittingsignals)


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Resting membrane potential

(RMP)Factors that contribute to RMP

Large protein molecules (-ve charged) cannot

cross the membrane K+can cross the membrane easily (the

membrane is more permeable to K+) leak

channels.

The membrane is less permeable to Na+

The contribution of Na+-K+pump


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Na+-K+pump

Continuous pumping of 3 Na+ions to

the outside for each 2 K+ions pumped

to the inside Loss of positives charges from the cell

More negative in the inside


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Nerve action potentials

Rapid changes in the membrane

potential that spread rapidly along the

nerve fiber membrane


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Changes in the

voltages at different

parts of the axon and at

different time.

*AP is not weaken as it

travel


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Action Potentials


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Depolarization

Membrane becomes more permeable to Na+ions (Opening of voltage gated Na+channels)

More Na+enter the cells

Membrane potential is now more positive

Depolarization must exceed threshold value (1530mV more positive -65 mV) before it can

trigger an action potential More voltage gated Na+channels open

At the peak- voltage gated Na+channels close

- voltage gated K+channels open


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Repolarization

Voltage gated Na+channels close

Simultaneous increase in K+permeability

Afterhyperpolarization (membrane potentialbecomes more negative than RMP)

Closing of voltage gated K+channels-RMPwill be achieved

** Cl-permeability does not change during theaction potential


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Voltage gated Na+channels

Activation gate and inactivation gate

Fast opening

Fast closing

Inactivate spontaneously

The inactivation gate will not reopen

until the membrane potential returns to

or near RMP level


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Roles of other ions

Ca2+bind with the sodium channel proteinmolecule

The positive charges alter the electrical stateof the channel protein (alter voltage level foropening sodium channels)

Hypocalcaemia (low Ca 2+in blood) -thenerve becomes more excitable

The threshold to elicit an AP is reduced

Ca Ca


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Characteristics of AP

Propagation of action potential

The direction is away from the region

that has recently been active (refractoryperiod)

The velocity depends on the fiberdiameter and whether it is myelinated

Larger fiber and myelinated fibers-propagation of AP is faster


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In a myelinated fiber, ionic conduction only occurs at thenodes of Ranvier

Jumping of the AP- saltatory conduction is faster


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Follows All or None Law

Once membrane is depolarized to the

threshold, amplitude is independent ofthe initiating event

Cannot be summed


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Refractory period

Absoluterefractory period- a second stimulus

no matter how strong will not produce an AP Voltage gated Na+channels are opened and

inactivation gate blocks the channels

Relativerefractory period- the stimulus strength

should be stronger than the first one to producean AP

Some of the Na+channels have returned to itsresting stage

K+ channels are opened


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Relative Refractory Period

Could an AP be generated during the undershoot?

Yes! But it would take an initial stimulus that is much, much

stronger than usual.

WHY?

This situation is known as the relative refractory period.

Imagine, if you will, a toilet.

When you pull the handle,water floods the bowl. This event takes acouple of seconds and you cannot stop it in the middle. Once the bowl

empties, the flush is complete. Now the upper tank is empty. If you trypulling the handle at this point, nothing happens (absolute refractory).Wait for the upper tank to begin refilling. You can now flush again, but theintensity of the flushes increases as the upper tank refills (relativerefractory)


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Factors influencing impulse

conduction Size of the neuron

Myelinated/unmyelinated

Ions

1. Hypocalcaemia (low Ca2+)-excitable (tetany)2. Hyperkalaemia (high K+)-RMP less negativemore

excitable, easier to reach the threshold to generatethe AP

3. Hypokalaemia (low K+) -RMP more negative-

paralysis Drugs-anaesthetics e.g. procaine reduces Na+

permeability and prevents impulse transmission

K+ ECF


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Factors influencing impulse

conduction Temperature- If increase, rate of

conduction increases

Hypoxia (reduce in oxygen in blood)reduces excitability of neurons

Ouabain, dinitrofenol, cyanide inhibit Na+-K+pump

Toxin e.g. tetrodotoxin blocks Na +ionchannel

lecture cns 1

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