CHAPTER 12 NERVOUS SYSTEM

12.1 The nervous system12.2 The transmission of impulse12.3 The synapse12.4 Neuromuscular junction 12.4.1 Structure of the neuromuscular junction 12.4.2 Structure of the skeletal muscle 12.4.3 The sliding-filament theory 12.4.4 Mechanism of the muscle contraction 12.4.5 The autonomic nervous system12.5 Drug abuse

OBJECTIVES

At the end of the lesson, students should be able to :

1) Build the organization chart of the human nervous system

2) Explain the rising of a resting potential in neurons

3) List down the characteristics of impulse4) Explain the propagation of impulse along the

axon

NERVOUS SYSTEM

• To synchronize the activities of the inner body parts (by cooperating with the endocrine / enzymatic systems) towards general balance.

• Performs the three overlapping functions of sensory input, integration & motor output.

• Impulse is transmitted from one receptor to an effector specifically.

Sensory input

Brain & spinal cord

Motor input

Sensory receptor

EffectorPeripheral nervous system (PNS)

Central nervous system (CNS)

THE RELATIONSHIP BETWEEN THE SENSORY INPUT, INTEGRATION & MOTOR OUTPUT

ORGANIZATION

NERVOUS SYSTEM

CENTRAL NERVOUS SYSTEM

PERIPHERAL

NERVOUS SYSTEM

BRAIN SPINAL CORD SENSORYMOTOR

(EFFERENT DIV)

SOMATIC

AUTONOMIC

PARASYMPATHETIC

SYMPATHETIC

CENTRAL NERVOUS SYSTEM

Comprises of :

1) the brain

2) the spinal cord

PERIPHERAL NERVOUS SYSTEM

consists of neurons that interconnect the brain to all parts of the body ;

a) the body muscle

b) the sensory organs

c) the organ built systems

the neurons are :

a) motor neurons – somatic & autonomic

b) sensory neurons

MOTOR NEURONSSOMATIC

Controls the voluntarily responses which involves the skeletal muscles

AUTONOMIC

Regulates the internal environment by controlling smooth & cardiac muscles

Controls the involuntarily responses of all the internal organs & glands

Actions are controlled in the medulla & hypothalamus

Consists of sympathetic & parasympathetic division.

Both act on the same target but very often antagonistic in the effect they bring.

Neuron

basic unit of nervous system

NeuronFunctions :

a) receive information from the inner/ outer environment ; and/or from other neurons. b) integrates information received & produces the appropriate output signals. c) guiding the signals until it reaches the far end/terminal of a neuron. d) sending signals to other nerve cells, glands or muscles.

Comprises of plasma membrane

The selective permeability of the membrane ensures the information from the environment reaches the desired target

RESTING POTENTIAL & THE GENERATION OF THE ACTION POTENTIAL

Unstimulated neuron maintains a different charge condition across the membranes.

The different charges develops a localized electrical gradient, which is called the resting potential.

The resting potential develops when the charge is more negative within the cell than from the outside (which is more positive).

The voltage measured across the plasma membrane (the membrane potential) is about -70 mV.

RESTING POTENTIAL

The intracellular & extracellular fluid of a neuron contains all kinds of solutes; including ions (cations &anions)

The fluid within the neurons contains mostly potassium ions (K+) & a lower concentration of sodium ions (Na+)

In contrast, the extracellular fluid contains a higher concentration of sodium ions.

In it’s unstimulated stage, the membrane of the neuron is highly permeable to K+ ions which passively diffuse across the membrane according to the concentration gradient (from the inside, out of the membrane)

A slow diffusion of Na+ ions occur across the membrane because the permeability to these ions is lower than to the K+ ions.

These diffusions do not achieve an equilibrium since the sodium-potassium pump transports these ions against their concentration gradient.

This results in the resting potential condition or the polarization stage.

THE ACHIEVEMENT OF RESTING (POLARIZED) STAGE

THE RISING OF ACTION POTENTIAL

The neuron is stimulated by the change in the environment (inner / outer).

The electrical potential across the membrane will change form it’s resting stage; the charge within the cell becomes more positive because :

the sodium-potassium pumps stop functioning

Na+ ions rush into the cell, changing the membrane potential to a more positive state

The change in the electrical potential is called depolarization.

If graded potentials sum to – 55mv, a threshold potential is achieved. This triggers an action potential (impulse).

At the peak of the action potential, the sodium-potassium pumps continue functioning; the conductivity of the Na+ ions decreases while the conductivity of the K+ ions increases again

The K+ ions diffuse out passively from the cell; resulting a more negative state within the cell.

The neuron is said to undergo repolarization which ultimately reaches the resting stage.

CHANGES IN THE MEMBRANE POTENTIAL : DEPOLARIZATION, ACTION POTENTIAL, REPOLARIZATION

• In the resting state, both the sodium channel and potassium channel are closed, and the membrane’s resting potential is maintained.

• During the depolarizing phase, the action potential is generated as activation gates of the sodium channels open, and the potassium channel remains closed.

THE TRANSMISSION OF IMPULSE

THE TRANSMISSION OF IMPULSEIs an electrical phenomena that occurs through the dendrite, dendron & the axon.

Involves 2 important phases :

1) the resting stage 2) the action potential

The features of action potentials / impulse are :

1) stimulation 2) all-or-nothing event 3) refractory period 4) speed of conduction

STIMULATION

There are 2 kinds of stimulation that affect the nerves:

1) common stimulation

- involves the stimulation of the receptor organs

- e.g light, sound, taste, smell

2) situational stimulation

- all the stimulation that are capable of depolarizing

the axons.

- e.g mechanical, chemical, heat, pressure, electrical

stimulations.

ALL-OR-NOTHING EVENTThe size of a nerve impulse is not determined by the size of the stimulation received.

The action potential is triggered only if the depolarization of the membrane is above the threshold level.

Below the threshold level, the stimulation is not sufficient to depolarize the membrane & thus triggering the action potential.

If an action potential is achieved, a stronger intensity of a stimulus won’t increase the size of it.

THE ALL-OR-NOTHING EVENT

REFRACTORY PERIOD

Impulse ‘travels’ one-way along the axon from the excitable region to the resting region next to it.

The previously active region undergoes a recovery phase which is known as the refractory period.

Two phases are involved in this very short period of about 5-10ms :

1) absolute refractory period

2) relative refractory period

Absolute refractory period

• the previously active region undergoes a recovery phase during which the axon cannot respond to a depolarization even if the stimulus intensity is increased.

• during this period the axon membrane goes through hiperpolarization; the membrane’s permeability to K+ ions increases dramatically.

• these ions diffuse out very highly making the charge within the neuron becomes too negative.

Relative refractory perioda phase following the absolute refractory period where a high-intensity stimulus may produce a depolarization.

the axon membrane reaching its normal permeability state, allowing the Na+ ions into the cell; the charge within the cell slowly becomes less negative; nearing its resting state.

SPEED OF CONDUCTIONDepends on : a) the presence of myelin

sheath

• act as an electrical insulator

• depolarization only occurs at the nodes of Ranvier where no myelin sheath is present.

• local circuits are set up at these points & current flows across the axon membrane generating the next action potential.

• in effect, the action potential ‘jumps’ from node to node & passes along the myelinated axon faster.

• this type of conduction is called saltatory.

• the conduction velocities is increased up to 50x as compared to in the unmyelinated axon.

b) The axon diameter

the bigger the diameter, the higher the velocity of the propagated action potential.

the resistance is reduced when the diameter of the axon is big

THE SALTATORY CONDUCTION ALONG THE MYELINATED AXON

GENERATION & PROPAGATION OF IMPULSE

The action potential is produced locally in axon; the membrane is depolarized at a specific area in the axon.

The action potential ‘flows’ along the axon because it is self-propagated.

An action potential achieved at one region of the membrane is sufficient to depolarized a neighboring region above threshold because the depolarized area has a different charge from the inactive area next to it; thus a local circuit is produced.

The current flow from one activated region to the inactivated area enables depolarization to occur, thus produces the action potential.

The continuous occurrence of depolarization from one area to the one next to it, ensures the transmission of impulse even in a great distance.

THE PROPAGATION OF IMPULSE ALONG THE AXON

SYNAPSE

Objectives

At the end of the lesson, students should be able to:

Draw out and label a picture of a synapse

Explain the mechanism of the impulse transmission across the synapse

Compare the transmission of impulse across the synapse with the transmission along the axon

Synapse

• An area of functional between neurons for transferring information.

• Found between fine terminal branches (axon), dendrites or cell body.

• 2 types of synapses

a) electrical

b) chemical

Structure of the chemical synapse

Commonly found in vertebrates

Consist a bulbous expansion of a nerve terminal called synaptic knob

The membrane of the synaptic knob is thickened and form the presynaptic membrane

The thickened membrane of the dendrite is termed the postsynaptic membrane

The two membranes are separated by a gap called synaptic cleft (20nm)

Structure of the chemical synapse

The cytoplasm of the synaptic knot contains mitochondria, smooth endoplasmic reticulum, microfilaments,and numerous synaptic vesicles

Each vesicle contains a chemical neurotransmitter substance

Two main neurotransmitter substances area) acetylcholine - secreted by parasympatheticb) noradrenaline - secreted by sympathetic nerves

• Protein channels are found on the postsynaptic membrane and they have receptors for the neurotransmitter substance

• These channels allow the movement of ions into the postsynaptic neurons

Mechanism of synaptic transmission

Mechanism of synaptic transmission

The arrival of nerve impulses at the synaptic knob depolarizes the presynaptic membrane.

The permeability of the membrane to Ca2+ ions is increased, and they easily enter the knob.

The entrance of those ions causes the synaptic vesicles to fuse with the presynaptic membrane and rupture; discharging their contents into the synaptic cleft.

The vesicles then return to the cytoplasm where they are refilled with neurotransmitter substance.

The neurotransmitter substance diffuses across the synaptic cleft and attaches to a specific receptor site on the postsynaptic membrane; causing the opening of the protein channels.

Na+ ions enter the postsynaptic neurons, followed by the leaving of K+ ions down their respective concentration gradient.

This leads to a depolarization of the postsynaptic membrane.

The depolarization response is known as an excitatory postsynaptic potential (EPSP)

Having produced a change in the permeability of the postsynaptic membrane, the neurotransmitter substance is then hydrolyzed by a specific enzyme

acetylcholine is hydrolyzed by acetylcholinesterase

noradrenalin is hydrolyzed by monoaminoxidase

The depolarizing effect of the EPSP is additive, a phenomenon called summation:

two or more EPSP arising simultaneously at different regions on the same neuron may produce collectively sufficient depolarization (spatial summation).

rapid release of transmitter substance from several synaptic vesicles by the same synaptic knob produces individual EPSP close together, they summate and give rise to action potential in postsynaptic neuron (temporal summation).

Fig 12.11 : The relationship between EPSP with the achievement of action potential

The action potential produced is then transmitted along the postsynaptic axon by the flow of an electric current.

The summation effect of EPSP delays the transmission of impulse in neurons; but it ensures the flow of impulse is unidirectional.

Because: the synaptic vesicles only exist in the presynaptic membrane side.

Comparison of impulse transmission : across the synapse and along the axon

Synapse Axon

1. Impulse is chemically transmitted. 1. Impulse is electrically transmitted.

2. Involves the neurotransmitter substances. 2. No neurotransmitter substances are involved.

3. Impulse transmission is slower because :

-the neurotransmitter need to diffuse across the synaptic cleft

-the summation of EPSP is needed to reach the threshold level

3. Impulse transmission is very fast.

4. Involves the diffusion of Ca+ ions into the synaptic knob to activate the vesicles.

4. Ca+ ions are not involved.

5. The diffusion of Na across the membrane is needed.

5. The diffusion of Na across the membrane is needed.

NEUROMUSCULAR JUNCTION

OBJECTIVES

At the end of the lesson, students should be able to:

• draw out a label picture of a neuromuscular junction completely

• explain the rising of end-plate potential

• explain the fine structure of the skeletal muscle which consists of myofibril, actin, myosin and sarcomere.

STRUCTURE OF THE NEUROMUSCULAR JUNCTION

1. SPECIALIZED FORM OF SINAPSE FOUND BETWEEN NERVE TERMINALS & MUSCLE FIBRES

2. THE CYTOPLASM OF THE MOTOR NEURON TERMINAL RELEASES ACETYLCHOLINE ON STIMULATION

3. THE REGION WHERE THE THE AXON OF THE MOTOR NEURON DIVIDES & FORM NON-MYELINATED BRANCHES ON THE MEMBRANE CELL SURFACE IS CALLED THE THE MOTOR END- PLATE

4. POSTSYNAPTIC MEMBRANE (MUSCLE MEMBRANE ) CALLED SARCOLEMMA HAS MANY FOLDS CALLED JUNCTIONAL FOLDS

5. A LOCAL DEPOLARIZATION AT EACH MOTOR END PLATE PRODUCES END-PLATE POTENTIAL (EPP)

6. EPP IS SUFFICIENT TO LEAD TO A PROPAGATED ACTION POTENTIAL ALONG SARCOLEMMA DOWN INTO THE MUSCLE FIBRE VIA TRANSVERSE TUBULE SYSTEM (T-SYSTEM)

STRUCTURE OF THE SKELETAL MUSCLE

• The major components are the cylindrical muscle fibres; which measure about 10 – 100µm in diameter and 1 – 40 mm in length

• Within the muscle fibres are numerous thin myofibrils

• Each myofibril is composed of two types of proteinaceous myofilaments, actin (thin filaments) and myosin (thick filaments)

• The cytoplasm of the myofibril is called sarcoplasm and contains a network of internal membranes termed the sarcoplasmic reticulum

• The skeletal muscle is observed to be striated; a regular alternation of light and dark bands

• The light and the dark bands are called the I and A bands respectively

• The bands are due to the regular arrangement of actin and myosin

• Traversing the middle of each I band is a dark line called the Z line

• The section of a myofibril between two Z line is called a sarcomere; the functional unit of muscle contraction

• In certain regions of the sarcomere, actin and myosin filaments overlap

• Myosin and actin filaments constitute the A band, whilst actin filament alone constitute the I band

• The centre of the A band is lighter than its other regions because it constitutes only the filament myosin, and is called the H band

• The H band is bisected by a dark line, the M line; which joins adjacent myosin filaments together at a point halfway along their length

• Running transversely across the fibre and between fibrils is a system of tubules known as the T system

• The T system is in contact with the surface of the sarcolemma

• At certain points the T tubules pass between the pairs of vesicles which are components of the sarcoplasmic reticulum

• The vesicles are involved in the uptake and release of Ca2+ ions which controls the contractile behaviour of the muscle fibre

MECHANISM OF MUSCLE CONTRACTION

• AT REST, TROPOMYOSIN BLOCKS THE MYOSIN ATTACHMENT TO ACTIN

• UPON STIMULATION BY NERVE IMPULSE, Ca2+ IONS ARE RELEASED INTO THE SARCOPLASM

• THE IONS BIND TO THE TROPONIN COMPLEX

• TROPONIN COMPLEX CHANGES ITS CONFORMATION

• NOW THE MYOSIN BINDING SITES ARE EXPOSED

• MYOSIN HEADS ATTACHED TO THE ACTIN FILAMENT

• CROSS BRIDGES ARE FORMED

• MUSCLE CONTRACTS

• IN SOME MUSCLES Ca2+ ALSO STIMULATES THE MYOSIN ATPASE ACTIVITY.

• THE ATPASE HYDROLIZES THE ATP & CHANGE THE MYOSIN HEAD TO A HIGH ENERGY CONFIGURATION.

• • THIS ALLOWS THE FORMATION

OF CROSS BRIDGES.

• WHEN THE EXCITATION OF THE SARCOMERES CEASES, Ca2+ ARE PUMPED BACK INTO THE VESICLES.

• • SARCOMERES RELAXES

THE MECHANISM OF MUSCLE CONTRACTION

The Sliding Filament Theory

The Sliding – Filament TheoryProposed by Huxley and Hanson in 1954.

They suggested that the muscle contracts when the thin filament (actin) and the thick filament (myosin) slide past each other.

During contraction the actin filament move inwards towards the centre of the sarcomere, making it (the sarcomere)looks shorter without changing the length of the A band.

The myosin head is the centre of bioenergetic reactions that power muscle contraction.

The high energy configuration heads of the myosin filament operate like a hook attaching to the specific sites on actin in a particular way to form cross bridges.

The high energy configuration state is achieved when the ATP molecules bound to the heads been hydrolyzed into ADP and inorganic phosphate, and releasing high energy used to change the configuration.

• The myosin heads then change their relative configuration such that the actin molecules are pulled further into the A band.

• After the process is completed, the myosin heads, bound to another ATP molecules, detach from the actin.

• They then split the new ATP molecules to revert to the high energy configuration and attach to another sites further along the actin filament.

• The cross bridge attachment/detachment cycles could be repeated many times depending on the speed of shortening.

• The pulling of the actin filament repeatedly towards the centre is unidirectional in a mechanism called the ratchet mechanism.

AUTONOMIC NERVOUS SYSTEM

OBJECTIVES

• At the end of the lesson, students should be able to :

Explain the structure and the functions of the sympathetic and parasympathetic systems

Compare both the sympathetic and the parasympathetic system

MAMALIAN AUTONOMIC NERVOUS SYSTEM (ANS)

A part of peripheral nervous system controlling the involuntary activities of internal environment ;eg: heart rate, peristalsis, sweating

Consist of motor neuron passing to the smooth muscles of internal organ and cardiac muscle

Most activities of ANS is integrated locally within the spinal cord or brain by visceral reflexes

ANS composed 2 types of neurons :Preganglionic neuron (mylienated) – emerges from CNSPostganglionic neuron (unmylienated) – leading the effectors

• 2 division of ANS :

Symphatetic nervous system

Parasymphatetic nervous system

The two systems differ primarily in the structural organization of their neurons

SYMPATHETIC NERVOUS SYSTEM (SNS)

Neurons are originated from spinal cord( the thoracic + lumbar region)

Synapses + cell bodies of postganglionic neurons in the trunk region are situated in ganglia close to spinal cord

Adjacent segmental symphatetic ganglia on each side of spinal cord are linked together by the sympathetic nerve tract

They form chain of symphatetic ganglia running alongside the spinal cord

Its preganglionic neurons are shorter than postganglionic neurons

The chemical transmitter substance released at postganglionic effector synapses in noradrenalin (Ad)

The effect s spread to all part of the body and takes time to decease

The symphatetic nervous system is especially dominant under stress or at time danger

Eg.of its effects :Dilates pupil

Increase amplitude and rate of heart beat

Increase ventilation rate

Constricts arterioles to gut and smooth muscle

PARASYMPHATETIC NERVOUS SYSTEM (PNS)Neurons originated from the cranial and the sacral region of the CNS

The ganglia of PNS situated close to or within the effector organ

Its preganglionic neurons longer than its postganglionic neurons

Chemical transmitter substance secreted at the postganglionic synapses is AceKoa

Its effect – locally and short

PNS controls the routine activities of the body at rest ; a compensation for the symphatetic effect

Eg :decrease the amplitude + rate of heart beat,ventilation rate Maintains steady muscle tone in atrioles to gut,smooth muscle,brain and skeletal muscle

Differences between symphatetic & parasymphatetic nervous system

Feature Symphatetic Parasymphatetic

Origin of neuron Emerges from thoracic and lumbar regions of CNS

Emerges from cranial + sacral regions of CNS

Position of ganglion Close to spinal cord Close to effector

Length of fibres Short preganglionic , long postganglionic fibers

Long preganglionic fibers,short postganlionic fibers

Numbers of fibers Numerous postganglionic fibres Few ganglionic fibers

Distribution of fibers Preganglionic fibers innervate a wide area

Preganglionic fibers innervate a restricted region

Area of influence Effect difuse Effect localized

Feature Symphatetic Parasymphatetic

Transmitter substance Noradrenalin released at effector

AceKoa released at effector

General effects •Increase metabolite

•Lowers sensory threshold

•Restores sensory threshold to normal level

•Decrease metabolite level

Overall effect Excitatory homeostatic effects

Inhibitory homeostatic effect

Condition when active Dominant during danger,stress + activity

•Dominant during rest

•Controls routine body activities

DRUG ABUSE

OBJECTIVES

At the end of the lesson the students should be able to :

• Give the definition of drugs

• List and give explanations of the 5 types of drugs

• Explain the effect of cocaine on the synapses and neuromuscular junction

DRUGS……………Definition

Any chemical substance that alters the physiological state of a living organisms

Also known as psychoactive substance which could lead to addiction if abused

giving harmful effect to mental & physical activities

AddictionChemically dependent on drugs resulting from the body tolerance more dosage is needed to get the same effect

Individual is said to be addicted when the drugs has taken over the important role in his biochemical reaction

Most drugs interfere with the impulse transmission by :– Changing– Hindering synthesis the neurotransmitter substance– Releasing and absorbing

TYPES OF DRUGS

DEPRESSANT

STIMULANT

HALLUCINOGEN

ANTI-DEPRESSANT

INHALANT

STIMULANT• Small dosage - activities of CNS (feeling more energetic)

• High dosage/prolong consumption – lead to depression

• Eg :1. Caffeine – prevents the hydrolyses of neurotransmitter substances –

continuous depolarization occurs at the postsynapse membrane

• Small dosage – stimulates the cerebrum cortex-increase alertness

• High dosage – influence medulla oblongata – interfering motor and intellectual coordination

2. Nicotine – mimics the effect of AceKoa on receptor and stimulates the sensory receptors

- short term use – change in heart beat rate and blood pressure

3. Amphetamines and cocaine• Block the reabsorption of neurotransmitters from the postsynapses

membrane – continuous depolarization of the postsynapse membranes in long period

DEPRESSANT

Lowering activities in CNS – lowering body activities as a whole

Eg : Barbiturates

Gives different based on dosage consumed– Low dosage

• stimulate synaptic activities, so persons would in state of euphoria/excited

– High dosage• synaptic action is hindered ( a feeling of depression is experienced by the

individual)

HALLUCINOGEN

Change the perception of the senses especially sight and hearing

Some acts by imitating/inhibiting the action of neurotransmitter substance

Most involve in causing disorientation and hallucinating – not fatal

Eg: – LSD (Lysergic acid diethylamide)– Marijuana

ANTI-DEPRESSANT(TRANQUILIZERS)

Use as pain killer & to lessen anxiety

Mimic the action of endorphin and enkephalins which are neuromodulators that assist the action of the neurotransmitter substances Endorphin + enkephalins inhibit the transmission of pain signals to brain

Eg : Narcotics ( heroin, morphine)

Prolong consumption of narcoticsincrease the receptors for enkephalins – bind to the receptor used

by enkephalins –therefore, pain signals are prevented from reaching brain

Prolong consumption of narcoticsincrease the receptors for enkephalins – bind to the

receptor used by enkephalins –therefore, pain signals are prevented from reaching brain

High dosage – needed when body becomes tolerant to the drug

When drug withdrawn from body – pain pathway neurons become extremely sensitive

Because number of enkephalins receptor has been increased, more receptors are left unbound by enkephalins and pain impulses are not blocked

The withdrawing addict experiences greater pain than normal until the number of receptors reaches its preaddiction level

INHALANT

Drugs that taken by means of inhalingEg:

Organics solventEtherChloroformOrganic based adhesive substance

Causes :HallucinationHigher heart beat rateAnaesthetize conditionA near fainted feelings

COCAINE : THE MECHANISM OF ACTION

As stimulant, cocaine effects the brain’s limbic system (the body’s “pleasure centre”) by imitating a neuromodulator that blocks the reabsorption of dopamine (neurotransmitter substance) back into the presynapse membranes

Result of blockagedopamine stays in synaptic clefts and continually binds to the receptors in postsynaptic membraneDepolarization occurs repeatedly which result in continuous impulse transmission – causes????????

Causes :intense pleasure, increase energy and feeling of power

Neuron respond to continual stimulation by reducing the number of dopamine receptor in postsynaptic membranes

Thus, more and more drug is needed for the addict to experience the pleasurable effects that the dopamine binding elicits

Addiction build, cocaine addicts find that their pleasure centres can’t function at all without the stimulation of drugs

The drug’s effect wear off and the addicts begins to suffer deep depression

When drug again introduced into the body, the mood of depression swings to euphoria (intense feeling of happiness and pleasant excitement)

That’s all for this topic