SAMACHEER CLASS 10 UNIT 15 NOTES

UNIT 15

  samacheer SCIENCE
CLASS 10

BIOLOGY

NERVOUS SYSTEM

 

INTRODUCTION

·     Responding to stimuli – characteristic feature of all living organisms

·     Stimulus – Changes in the environmental condition – detected by receptors in our body

·     Relevant changes in the activities of an organism – to a particular stimuli – called reactions / responses

 

RESPONDING TO STIMULI

·     Living organisms – show response to different stimuli like – light, heat, cold, sound, smell, taste, touch, pressure, pain or force of gravity, etc

·     Example:

·     Withdrawal of hand when we touch hot object – response to stimulus heat

·     Closing the eyes when flashed with bright light – response to stimulus light

·     Our body responds to stimulus – in most appropriate manner – for its survival & functioning

 

CO-ORDINATION

·     To provide correct response – all organs should work together – in a conditioned manner

·     Working together of various organs – systematic, controlled & efficient way – to produce response to stimuli – called Co-ordination

·     In animals including humans – co-ordination between cells & organs – essential to maintain  - physiological balance - Homeostasis

 

NERVOUS SYSTEM

·     Nervous system – made of nervous tissues

·     3 distinct components

·     (i) Neurons     - (ii) Neuroglia         - (iii) Nerve fibres

 

(I) NEURON / NERVE CELL

·     Neuron – structural & functional unit of Nervous system

·     Longest cell of human body – length over 100µm

·     Specialised cells – detect, receive & transmit – different stimuli

·     Information – conducted through neurons – from one part of the body to another – as electrical impulses

 

(II) NEUROGLIA

·     Also called glial cells

·     Non-exciting, supporting cells – of nervous system

·     Do not initiate or conduct nerve impulses

 

(III) NERVE FIBRES

·     Nerve fibres – long, slender processes of neurons

·     Nerve fibres – bundle up together – form nerves

 

STRUCTURE OF NEURON

·     Neuron – 3 basic parts

·     (i) Cyton          - (ii) Dendrites &     - (iii) Axon

 

(I) CYTON

·     Also called cell body or perikaryon

·     Central nucleus with abundant cytoplasm – called Neuroplasm

·     Cytoplasm – has large granular body – Nissl’s granules

·     Other cell organelles – Mitochondria, ribosomes, lysosomes & ER – present

·     Neurons – do not divide

·     Several neurofibrils – in cytoplasm – transmits nerve impulse to & from cell body

 

(II) DENDRITES

·     Numerous branched cytoplasmic processes – project from – surface of cell body – Dendrites

·     Conduct nerve impulses – towards cyton

·     Branched projections – increase surface area – for receiving signals – from other nerve cells

 

(III) AXON

·     Single, elongated, slender projection

·     End of axon – fine branches – terminate in knob like swellings – Synaptic Knob

·     Plasma membrane of axon – Axolemma; Cytoplasm – Axoplasm

·     Axon – carries impulses – away from cyton

·     Axon – covered by – protective sheath – Myelin sheath – further covered by – Schwann cells – Called Neurilemma

·     Depression in Myelin sheath – nodes of Ranvier

·     Region between nodes – Internode

·     Myelin Sheath – acts as insulator – ensures rapid transmission of nerve impulse

 

SYNAPSE

·     Junction between synaptic knob of axon of one neuron – & dendron of next neuron – Synaptic junction

·     Information from one neuron – pass to another neuron – through these junctions – by releasing chemicals – Neurotransmitters (from synaptic knob)

 

TYPES OF NEURONS

·     Different types of Neurons – based on structure & function

·     Structurally – neurons are of 3 types

o  Unipolar Neurons

o  Bipolar Neurons &

o  Multipolar Neurons

 

UNIPOLAR NEURONS

·     Only one nerve process arise from cyton – acts as both axon & dendron

 

BIPOLAR NEURONS

·     Cyton – gives rise to 2 processes

·     One – Axon; Another - Dendron

 

MULTIPOLAR NEURONS

·     Cyton – gives rise to – many dendrons & 1 axon

 

WHERE ARE THEY FOUND?

 

TYPES OF NEURONS – BASED ON FUNCTION

·     Based on function – 3 types of Neurons

o  Sensory or Afferent neurons

o  Motor or Efferent neurons

o  Association neurons

(I) SENSORY/AFFERENT NEURONS

·     Carry impulse from sense organs – to CNS

 

(II) MOTOR/EFFERENT NEURONS

·     Carry impulse from CNS – to effector organ (muscle / gland)

 

(III) ASSOCIATION NEURONS

·     Conduct impulse – between sensory & motor neurons

 

TYPES OF NERVE FIBRES

·     2 types – based on presence or absence of Myelin Sheath

 

(I) MYELINATED NERVE FIBRE

·     Axon – covered with Myelin sheath

 

(II) NON-MYELINATED NERVE FIBRE

·     Axon – not covered by Myelin Sheath

·     Myelinated & Non-myelinated fibres – form white matter & grey matter of the brain

 

TRANSMISSION OF NERVE IMPULSE

·     All information – from the environment – detected by – receptors in sense organs – Eyes, nose, skin, etc.,

·     Information – from receptors – transmitted as electrical impulse – received by dendrites of neuron

·     Impulse travels – from dendrites – to cell body – then to axon – to its terminal end

·     At the axon end – causes nerve endings to release – a chemical (Neurotransmitter)

·     Neurotransmitter – diffuses across the gap (synapse) – reach next neuron – starts similar electrical impulse

·     Like wise – electrical signal – reaches brain / spinal cord

·     Similarly response from brain / spinal cord – passed on to effector organs (muscle / gland cell) – undergoes the desired response

SYNAPTIC TRANSMISSION

·     Flow of nerve impulse – from axon end of one neuron – to dendrite of another neuron – through synapse – called Synaptic transmission

 

DO YOU KNOW?

·     Each neuron – transmit 1000 nerve impulses per second

·     Each neuron – can make 10,000 synaptic contacts – with other neuron

 

NEUROTRANSMITTERS

·     Neurotransmitters – chemicals – allow transmission of nerve impulse – from axon end of one neuron – to dendron of another neuron / to effector organ

·     Neurotransmitter – that is released by neurons – Acetylcholine

 

HUMAN NERVOUS SYSTEM

·     During evolution – Nervous system – become complex

·     Human beings – different from animals – humans have the ability to think & take actions – due to well developed Nervous system

·     Human Nervous system – differentiated into

o  Central Nervous System (CNS)

o  Peripheral Nervous System (PNS)

o  Autonomic Nervous System (ANS)

 

CNS

·     Centre for information, processing & Control

·     Consists of brain & spinal cord

 

PNS

·     Made of nerves – connects brain & spinal cord – with all other parts

 

ANS

·     Formed of – Sympathetic & Parasympathetic nerves

 

CENTRAL NERVOUS SYSTEM (CNS)

·     Brain & spinal cord – delicate organs

·     Protected by bony structures

·     Brain – Skull

·     Spinal Cord – Vertebral Column

·     CNS – formed of – 2 types of matter

·     White matter – Myelin sheath present

·     Grey matter – Myelin sheath absent

·     Brain – controls all body activities

·     Brain – covered by 3 connective tissue membrane / meninges

o  Duramater

o  Arachnoid Membrane

o  Piamater

 

DURAMATER

·     Dura – tough; mater – membrane

·     Outermost, thick, fibrous membrane

 

ARACHNOID MEMBRANE

·     Arachnoid – Spider

·     Middle, thin, vascular membrane

·     Provide web like cushion

 

PIAMATER

·     Pia – soft or tender

·     Innermost, thin, delicate membrane

·     Richly supplied with blood

 

DO YOU KNOW?

·     Meningites – Inflammation of meninges

·     Occurs when fluid surrounding the meninges – infected

·     Causes of Meningites – Viral / Bacterial infection

 

HUMAN BRAIN

·     Human brain – 3 main parts

·     (a) Forebrain     - (b) Midbrain      - (c) Hindbrain

 

(A)   FOREBRAIN

·     Formed of Cerebrum & Diencephalon

·     Diencephalon – has dorsal thalamus & ventral hypothalamus

 

CEREBRUM

·     Largest portion – 2/3^rd of the brain

·     Longitudinally divided by deep median cleft – divides the cerebrum into – right & left Cerebral Hemispheres

·     2 Cerebral hemispheres – interconnected by – thick band of nerve fibres – Corpus Callosum

·     Outer portion – formed by grey matter – Called Cerebral Cortex

·     Inner portion – formed of white matter – called Cerebral Medulla

·     Cortex – extremely folded elevations – Gyri; depressions – Sulci (increase surface area)

·     Each cerebral hemisphere – divisible into – frontal lobe, parietal lobe, temporal lobe & occipital lobe – also called Cerebral lobes

·     Cerebral lobes – have specific functions

·     Damage in any lobe – affects its functions

·     Cerebrum – responsible for – thinking, intelligence, consciousness, memory, imagination, reasoning & will power

 

THALAMUS

·     Present in Cerebral Medulla

·     Major conducting centre – for sensory & motor signalling

·     Acts as relay centre

 

HYPOTHALAMUS

·     Lies at the base of thalamus

·     Controls involuntary functions – hunger, thirst, sleep, sweating, sexual desire, anger, fear, water balance, blood pressure, etc.,

·     Acts as thermoregulatory centre (controls body temperature)

·     Controls hormone secretion – from anterior pituitary gland

·     Important link – between Nervous system & endocrine system

 

(B)   MIDBRAIN

·     Located between thalamus & hind brain

·     Dorsal portion – consists of – 4 rounded bodies – Corpora quadrigemina – controls visual & auditory reflexes

DO YOU KNOW?

·     Human Brain – constitutes 60% of fat

·     Brain’s integrity & ability – determined by – Essential Fatty Acids (EFAs)

·     EFAs – cannot be synthesized – obtained from food

·     Fish, green leafy vegetables, almond, walnut – rich in EFAs

 

(C)    HIND BRAIN

·     3 parts

o  Cerebellum 

CEREBELLUM

o  Pons &

o  Medulla Oblongata

 

CEREBELLUM

·     Second largest part of Brain

·     Formed of – 2 large hemispheres & middle Vermis

·     Co-ordinates  voluntary movements & maintains body balance

 

PONS

·     ‘Pons’ – Latin word – means ‘Bridge’

·     Bridge of nerve fibres – connects lobes of Cerebellum

·     Relay signals between – Cerebellum, Spinal cord, Mid brain & Cerebrum

·     Controls respiration & sleep cycle

 

MEDULLA OBLONGATA

·     Posterior most part of brain

·     Connects spinal cord & various parts of brain

·     It has

·     Cardiac centres – controls heart beat

·     Respiratory centres – controls respiration

·     Vasomotor centres – controls contractions of blood vessels

·     Also regulates vomiting & salivation

 

HUMAN BRAIN – FUNCTIONS

 

MORE TO KNOW

·     Electroencephalogram (EEG) – instrument – records electrical impulses of Brain

·     EEG – detects abnormalities in brain waves

·     Helps in diagnosis of – Seizures, epilepsy, brain tumors, head injuries, etc.,

 

SPINAL CORD

·     Cylindrical structure – lies in neural canal of vertebral column

·     Also covered by meninges

·     Extends from – lower end of medulla oblongata – to first lumbar vertebrae

·     Posterior region – tapers into thin fibrous thread like structure – Filum terminale

·     Internally – contains a cavity – called Central Canal – filled with Cerebrospinal Fluid

·     Grey matter – ‘H’ shaped

·     Upper end of ‘H’ forms posterior horns

·     Lower end – anterior horns

·     Bundle of fibres – pass into posterior horn – forming dorsal or afferent root

·     Fibres pass outward – from anterior horn – forming ventral or efferent root

·     2 roots – joins to form – spinal nerves

·     White matter – external – have bundle of nerve tracts

·     Spinal cord – conducts sensory & motor impulses – to & from brain

·     Controls – reflex actions of body

 

CEREBROSPINAL FLUID

·     Brain – suspended in special fluid environment – Cerebrospinal fluid (CSF)

·     Lymph like, watery fluid – surrounds & protects brain – within the skull

·     Also fills – Central canal of spinal cord

 

FUNCTIONS OF CSF

·     Acts as shock absorbant – protects brain from damage – during sudden jerk

·     Supplies nutrients to brain

·     Collects & removes waste – from brain

·     Responsible for maintaining – constant pressure inside Cranium

 

REFLEX ACTION

·     Reflex – any response that happens automatically – without consciousness

·     2 types of reflexes

o  Simple / Basic reflexes

o  Acquired / Conditioned reflexes

 

SIMPLE / BASIC REFLEXES

·     Inbuilt & unlearned responses

·     Many actions – performed in day to day life – are simple reflexes

·     Example: Winking of eyes – when dust enters, sneezing, coughing, yawning, etc.,

·     These actions performed – without thinking

ACQUIRED / CONDITIONED RESPONSES

·     Reflexes – result of practice & learning

·     Playing harmonium – by striking particular key – seeing music notes – example of conditioned reflexes – requires constant effort

·     Most reflex actions – monitored & controlled by – Spinal Cord – ‘Spinal reflexes’

·     Pathway taken by nerve impulse – to accomplish reflex action – Reflex arc

 

HOW BODY EXECUTES REFLEX ACTION?

·     When we touch a hot pan – stimulus heat – sensed by receptor – ‘heat receptors’ / ‘Thermo receptors’ (in hand)

·     Sensory neuron – transmits the message – to spinal cord

·     Spinal cord – interprets the stimulus – impulse passed to relay neuron – impulse transmitted to motor neuron

·     Motor neuron – carry command – from spinal cord to arm

·     Arm muscles – contracts – withdraw the hand immediately from hot pan

·     Muscle – Effector organ – responded to heat

PERIPHERAL NERVOUS SYSTEM (PNS)

·     PNS – Formed by – nerves from brain & spinal cord

·     Nerves arising from brain – Cranial nerves

·     Nerves arising from spinal cord – spinal nerves

 

CRANIAL NERVES

·     12 pairs of Cranial nerves – in human

·     Some cranial nerves – sensory – Example: Optic nerve – innervates the eye

·     Some cranial nerves – motor nerves – helps in rotation of eye ball

·     Also innervates – eye muscles, muscles of Iris & tear gland

 

SPINAL NERVES

·     31 pairs of spinal nerves

SPINAL NERVES

·     Each spinal nerve – has a dorsal sensory root & ventral motor root

·     Dorsal spinal root – direction of impulse – towards spinal cord

·     Ventral spinal root – away from spinal cord

AUTONOMIC NERVOUS SYSTEM (ANS)

·     ANS – also called visceral nervous system

·     Regulates the function of visceral organs (internal delicate organs)

·     Visceral organs – regulated through – 2 antagonistic (opposite) components – Sympathetic & Parasympathetic systems

·     Enable the body – perform rapid & specific visceral activities – maintains a steady state

·     Controls involuntary functions of visceral organs