The Nervous System *dr. majid's edited notes (4 everyone's ease)

8/3/2019 The Nervous System *dr. majid's edited notes (4 everyone's ease)

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NERVOUS COMMUNICATION

The nervous system - centre for body

control and communication network

functions of the nervous system:

-Detect any changes (stimuli) that occur

inside and outside the body

-Define the changes

-Respond to the defined changes

Terms and Definition

Stimulus – any change in the externalor internal environment which

provokes a response

Receptor – specialized cells that

detect a stimulus

Neuron – cells which transmit nerve

impulses

Effector – organ that respond to the

stimuli and bring about a response

The organization of the nervous system

consists of 2 types of cells

1. Neuron

- basic functional unit of nervous system

- able to generate and transmit nerve

impulses

2. Neuroglia

-supporting cells

# Neuron (Nerve cell)

Divided into 3 parts:

i.Cell body

ii.Dendrites

iii.Axons

i.. Cell body @ sentron @ soma

~ Carries out maintenance activity, i.e.,

synthesizes materials required by neurons

~ Possesses organelles such as nucleus,

mitochondria, ribosomes, golgi apparatus,

endoplasmic reticulum, etc.

~ Cytoplasm contains Nissl’s granules rich in

RNA (for protein synthesis)

Various shapes, e.g., sphere or pyramid

ii. Dendrites

~ Short extensions from the cell body

~ Carry impulse towards the cell body

Nervous system + endocrine system + enzyme system

Maintain a stable internal environment in human



iii. Axons

~ Long extensions which carry impulses

away from the cell body~ Terminal end – branches with swollen

endings known as the synaptic knob

~Possess cytoplasm – axoplasm surrounded

by axomembrane

# Neuroglia Cell

-Provides structural support andmetabolism for neuron

E.g., Schwann cells form myelin

sheath surrounding the axon

(Myelin sheath)

- between 2 nodes of Ranvier

- increase the speed of impulse

transmission

(Nodes of Ranvier)

– small uncovered parts of

myelinated axon between the myelin

sheaths

3 Types of neurons according to function:

1. Sensory neuron (afferent neuron)

Long dendrites and short axons

Carries impulses from receptor to

CNS

2. Interneuron (in CNS)

Connects the sensory neuron to the

motor neuron

3. Motor neuron (efferent neuron)

Short dendrites and long axons

Receive nerve impulses from

interneuron and transmit to effector,

e.g., muscles and glands



3 Types of Neuron According to Structure

Depends on the number of extensions

leaving the cell body

1. Unipolar

Possesses a single extension from the

cell body

Characteristic of invertebrate

nervous systems

and sensory neurons

2. Bipolar

Possesses 2 extensions: dendrites

and axons,e.g., neuron in the retina

3. Multi-polar

Possesses a few extensions from the

cell body, generally in mammalian

nervous systems, e.g., pyramid cells,

Purkinje cells and motor neuron

Impulse Transmission

1.Along the axon

- as an

electrical signal

2.Across the synapse

– as a chemical signal



Impulse transmission along the axon

1. Resting Potential

2. Action Potential (depolarization and

repolarization)

# Resting Potential

The potential difference which exists

across the axon membrane when the

neuron is not conducting an impulse

or is at rest

It is caused by the unequal

distribution of charged ions inside

and outside the neuron membrane(inside more negatively charged

relative to the outside) – axon is

polarized

No stimulation – axon at rest – axon is

polarized

Axon is polarized when it is in resting

potential

Inner membrane –vely charged

[Na+] low, [K+] high

Presence of anion: Cl-

Negatively charged protein and

organic phosphate

Outer membrane +vely charged

[Na+] high, [K

+] low,

Cl-also present

These differences will cause electrical

potential difference across membrane -

resting potential ( – 70mV

How The Resting Potential Is

Maintained

3 types of ions play significant roles to

determine the resting potential

Sodium (Na+)

Potassium (K+)

Large negatively-charged organic

molecules (amino acids and proteins)

Involve 2 mechanisms:

K+/Na

+pump

Non-voltage gated K+/Na

+channel



The different concentrations of these types of

ions are maintained by an interplay of several

factors:

1. Diffusion

2. Electrical attractions and repulsions3. Active transport across the cell

membrane

4. Selective permeability of the axon

membrane to these three ions.

During the resting potential, Na+/K

+

pump actively transports Na+

and K+

across the membrane against their

concentration gradients

The presence of more non-voltage

gated K+ channels compared to those

for Na+

more K+

diffuse out than

Na+

diffuse in. Always some Na+

leaking in and this is reduced by the

Na+/K

+pump

3 Na+

are transported to the outside

membrane for every 2 K+

brought

into the cytoplasm of the axon.

These processes always more K+

inside so the resting potential is

maintained almost entirely by this K+ difference.

Presence of anions, e.g., proteins in

the cell which are too large to diffuse

out

The Na+/K

+voltage gated channels

are both closed

The net result – outer membrane is

+ve compared to inner membrane

Resting potential is established

# Action Potential

An action potential is the change in

the potential difference across an

axon membrane which occurs during

the passage of a nerve impulse

Nerve impulse

- an information that passes along the

axon, changes the potential difference

across the membrane and generate an

action potential

- only can be transmitted as a series of

electrical signals when the stimuli >

threshold intensity (> - 50 mV).

The action potential has 3 phases (2 - 3

msec)

1. Depolarization

2. Repolarization

3. Hyperpolarization



1. Depolarization (1 msec)

Stimulus reaches a resting neuron,

some voltage-gated Na channels

open

Na+

diffuse into the axon

The inside of the neuron becomes

more positive relative to the outside

The axon membrane is depolarized

More gates open more Na+

diffuse into the axon further

depolarization

When the membrane potential

difference reaches a threshold value,

many more gates open rapid

diffusion of Na+

sudden increasein the membrane potential

difference (+35 mV)

The action potential stimulates other

Na channels down the axon to open,

thus causing the impulse to travel

down the axon

2. Repolarization

Reversal in polarity to +35 mV

voltage-gated Na channels close

Voltage-gated K channels open

K+

diffuse out of the axon

The outside of the neuron becomes

more positive relative to the inside

The axon membrane is repolarized

Action potential alters from +35 to -

70 mV

3. Hyperpolarization

Voltage-gated K channels are slow to

close excess K+

leave the axon

Inner membrane becomes more –ve

the voltage falls slightly below -70mV hyperpolarization

Within a few msec, voltage-gated K

channels close

Resting potential (-70 mV) is re-

established

Factors Affecting Impulse Transmission

1. Diameter of the axon

- the larger the axon diameter the faster

the speed of impulse transmission- the smaller the diameter, the greater

the resistance created by the axoplasm

lower the speed of impulse

transmission

2. Myelinated neurone

- an action potential can only be generated

at nodes of Ranvier because Na+

and K+

are

able to move across the membrane

Hence, action potential jumps from 1 node

of Ranvier to another along the axon

increases the speed of impulse transmission



Information is transmitted along a

neuron as a nerve impulse which

consists of a series of actionpotentials

When a neuron is stimulated, Na+

flow into the neuron

depolarization of the inner

membrane action potential is

generated

This part of the membrane is more

positive relative to the adjacent part

(still at resting potential)

The difference in potential between

the active and resting membrane

parts creates a localized electric

current (LEC)

LEC stimulates the adjacent part (2nd

part) of the membrane

Na+

flow in, depolarize and generate

a second action potential

After the action potential, the first

part of the membrane is repolarizing

as K+

flow out

This process is repeated

Impulse is propagated as a series of

repolarization and depolarization

along the axon

Refractory Period

Period after an action potential has passed,

i.e., period when axon is not able to transmit

a new impulse (5 – 10 msec)

1. Absolute refractory period

the axon membrane is unable to

respond to another stimulus

action potential is not generated

1 msec

2. Relative refractory period

Resting potential is gradually

restored by the Na+/K

+pump

5 msec

All or Nothing Law

All action potentials are of the same

amplitude, i.e., after threshold is reached,

the size of the action potential producedremains constant and is independent of the

intensity of the stimulus



Synapse

Connection site between

1. neuron-neuron

2. neuron-muscle

Synaptic knob (at the end of axons) –

contain mitochondria and synaptic

vesicles

Synaptic vesicles contain

neurotransmitter

- important in impulse transmission

Neurotransmitter

- small chemicals found in the

synaptic vesicle

- helps to transmit an impulse across

the synapse

Neuron that carries impulse to

synapse – presynaptic neuron –

covered by presynaptic membrane.

Neuron that carries impulse away from

synapse - postsynaptic neuron –

covered by postsynaptic membrane



-Impulse that reach synaptic knob stimulates

opening of Ca channels

-Ca2+

(in the interstitial fluid) enter the knob

-Stimulate binding of vesicles and

presynaptic membrane

- Vesicles release neurotransmitter into

synaptic cleft (each synaptic vesicle contains

10 thousand molecules of neurotransmitter

-Neurotransmitter binds with receptor on

postsynaptic membrane

-Change configuration of protein on

postsynaptic membrane

-Na channels open

-Na+

enter and depolarize postsynaptic

membrane excitatory postsynaptic

potential (EPSP)-If EPSP reaches the threshold level, action

potential is generated and transmitted to

the 2nd neuron/muscle

*If acetycholine stays in the receptor sites,

Na channels remain open - continually

producing action potentials

-To prevent continuous production of action

potential – remove the neurotransmitter (nt)

i. Direct uptake of nt

e.g., noradrenaline is transported back into the

synaptic knob and inactivated by the enzyme

monoamine oxidase

ii. Enzymes are released to degrade nt

e.g., enzyme acetylcholinesterase splits

acetylcholine into acetyl coenzyme A and

choline taken up by the presynaptic

neurone combined to reform acetylcholine

Two main neurotransmitters used in the

vertebrate nervous system are

1. AcetylcholineNeurons releasing acetylcholine are called

cholinergic neurons. Found in most synapses

2. Nonadrenalin (norepinephrine)

Neurone releasing nonadrenalin are called

adrenergic neurons. Found specifically

in the synapses of the sympathetic nervous

system

3. Both nt can be inhibitory or excitatory,

depending on the type of receptor

4.Other neurotransmitters:Dopamine, serotonin (brain), glutamate, etc.

Functions of Synapse

1. Transmits information between neurons

2. Transmits nerve impulses in one direction

because nt are only released by the

presynaptic neuron

3. Filters out low-level stimuli of limited

importance4. Protects the effectors from damage by

overstimulation, i.e., by action potentials

continually being generated



Drugs

Chemical substances that cause

changes in the natural chemical

environment and functioning of the

body

Can be ingested, injected, inhaled or

put into the body in some other ways

Used in medicine to help prevent,

diagnose and treat diseasse or injuries

Psychoactive drugs (PAD) interfere

with the nervous system and cause

changes in the mental state and

behaviour

Overdose of PAD over

dependence (addiction) of the drug

Affect the nervous system by alteringthe mechanism of synaptic

transmission

i. Excitatory psychoactive drugs work in

various ways:

(a) Mimic a natural neurotransmitter,

fitting into the same receptors e.g.

nicotine mimics acetylcholine.

(b) Interfere with the normal enzyme

breakdown of a neurotransmitter. The drug (a)/neurotransmitter (b)

stays in the receptors & continues to

stimulate the postsynaptic membrane

- causes continuous stimulation &

contraction of muscles

E.g. Organophosphate insecticides.

ii. Inhibitory psychoactive drugs work in

various ways:

(a) They prevent the release of aneurotransmitter

E.g. Botulinum is a poisonous toxin

produced by the bacterium

Clostridium.

It will stop respiration muscles contraction &

resulted in impossible breathing

(b) They block the action of a

neurotransmitter at the receptors on the

postsynaptic membrane.

E.g. Curare is a natural poison.

It blocks the action of acetylcholine at

neuromuscular junctions

stop muscle contraction.



Skeletal Muscle Structure

Skeletal muscle is made up of

hundreds of muscle fibres.

Each muscle fibre

- surrounded by connective tissue

endomysium.

- long, cylindrical in shape & arranged

parallel to each other

- consists hundreds of myofibrils

- cytoplasm – sarcoplasm

- contain many mitochondria

Myofibrils

- thin threads that arranged parallel to

one another.

- made up of alternating light & dark

bands due to overlapping strands of contractile protein (myosin & actin).

- each contractile unit – sarcomere

Sarcomere

(myofibril basic unit)

i. e. region between one Z line

&another Z lineMyofibrils consist of:

Thick filament are composed of

protein - myosin

- long tail

- globular head – site for ATPase

enzyme.

Thin filament are composed of protein -

actin

- helical backbone consist of 2 strand.

- contain 2 other proteins

(tropomyosin & troponin).

Sarcoplasm of muscle fibre consists of

i. longitudinal interconnected tubules

between the myofibrils - sarcoplasmicreticulum.

ii. Transverse tubules which are

invaginations of sarcolemma membrane – T

tubules.

Ends of sarcoplasmic reticulum form

vesicles – terminal cisternae - involved

in the intake & release of Ca2+.



The neuromuscular junction (NMJ)

– a synapse between a motor

neurone & skeletal muscle fibres

Each muscle fibre has a region – motor end

plate where the axon of the motor neurone

divides & forms fine branches ending in

synaptic knobs. The NMJ includes the motor end plate & the

synaptic knob.

-On stimulation, the synaptic knob release

Ach which binds to the receptors on the

sarcolemma.

-This increases the permeability of the

sarcolemma to Na+.

-This depolarises the postsynaptic muscle

fibre & triggers an AP.-The AP passes along the sarcolemma

through the T tubules system, deep down

into the miofibril & results in muscle

contraction.

The Sliding Filament Theory

suggested by Huxley & Hanson

1. Muscle at rest

Outside of muscle membrane +ve charge.

Inside of muscle membrane -ve charge.

2. Muscle stimulation

Nerve impulse (action potential) travelsalong a motor neurone & reaches the

neuromuscular junction.

Acetylcholine (Ach) is released into the

synaptic cleft, diffuses to the

sarcolemma & bind with

receptor on the sarcolemma.

When action potential (AP) reaches it

threshold value, an AP is created in the

muscle fibre.

Ach in the cleft is then hydrolyzed & theproducts are reabsorbed into the motor

neurone.

AP travels along the sarcolemma, spreads

into the T tubules & stimulates the

release of Ca2+ from the cisternae

terminal at sarcoplasmic reticulum.

Ca2+ diffuse out to the sarcoplasm.



3. Actomyosin-Cross bridges formation

Ca2+ bind to troponin & alter its shape.

Tropomyosin strand moved to the sides &

exposed the binding sites.

A molecule of ATP binds to myosin head.

ATPase is activated.

ATP ADP + Pi + ENERGY

The energy is transferred to myosin head &

changes the myosin from low energy

configuration high energy

configuration.

Myosin heads attach to the actin binding

sites - actomyosin-cross bridges.

4. Slides

ADP & Pi are released.

Myosin head returns to it low-energy

configuration.

It bends & propels the actin towards the

centre of sarcomere.

Actin & myosin filaments slides between

each other.

5. Breakdown of the Actomyosin-Cross bridges A new ATP molecule binds to each myosin

head.

Each myosin head detaches from the actin &

returns to it low energy configuration.

Troponin reverts to its original shape &

tropomyosin block the binding site on the

actin filaments.

Myosin heads are ready to bind to the next

binding site on the actin filaments.

6. Repolarization

After contraction, Ca2+ is actively absorbed

back into the terminal cysterna.

Muscle relaxed.

The Nervous System *dr. majid's edited notes (4 everyone's ease)

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