Notes-Part-1-Class-12-Biology-Chapter-9-Control and Coordination-Maharashtra Board

Control and Coordination

Maharashtra Board-Class-12th-Biology-Chapter-9


Topics to be Learn : Part-1

  • Introduction
  • Nervous Coordination
  • Nervous System in Hydra
  • Nervous System in Planaria (flatworm)
  • Neural tissue
  • Synapse
  • Transmission of nerve impulse along the axon
  • Human Nervous System
  • Sensory Receptors
  • Disorders of nervous system

Topics to be Learn : Part-2

  • Chemical Coordination
  • Endocrine system
  • Major endocrine glands

Introduction : Plants and animals both show a control and coordination mechanism.

  • In plants this done by sending chemical signals and bringing about various types of movements.
  • Animals, show a gradual increase in the complexity of their control and coordination by giving both electrical and chemical singles.
  • The nervous system and endocrine control system are two coordinating systems in them.

In this chapter you will study about development of nervous system in different animal groups and details of the system in humans.

Nervous system in different animal groups :

Nervous System in Hydra : Phylum : Cnidaria

  • Hydra, a cnidarian shows the diffused nervous system in the form of nerve net.
  • It is the most primitive nervous system.
  • There are two nerve nets in the mesoglea one connected towards the epidermis and second towards the gastro-dermis.
  • Hydra lacks sensory organs, but the .sensory cells scattered in the body wall.
  • The nerve impulse shows no polarity or direction. As all neurons are interconnected the response is seen throughout the body

Nervous System in Planaria (flatworm) : Phylum :Platyhelminthes

  • Planaria is a flatworm and belongs to the phylum Platyhelminthes.
  • It is the most primitive animal with a Central Nervous System (CNS) located on the ventral side of body.

  • Nervous system consists of a mass of cerebral or cephalic ganglion appearing like an inverted U-shaped brain.
  • Ventrally from below the ganglia arise a pair of Ventral Nerve Cords (VNC) or long nerve cords. These are interconnected to each other by transfer nerve or commissure in a ladder like manner.
  • The PNS include sensory cells arranged in lateral cords in the body.

Sponges : Phylum : Porifera

  • Nervous System : Lack nervous system

Earthworm : Phylum : Annelida

  • Nervous System :Ganglionated
  • Position :Ventral, solid
  • Composition: CNS, PNS, Sympathetic NS

Cockroach : Phylum : Arthropoda

  • Nervous System : Ganglionated
  • Position :Ventral, solid
  • Composition : CNS, PNS, and ANS

Human : Phylum : Chordata

  • Nervous System : Non-ganglionated
  • Position : Dorsal, hollow
  • Composition : CNS, PNS, and ANS


Neural tissue:

  • Two types of cells in neural tissue — the neurons and the neuroglia or glial cells.
  • Nerve is bundle of axons. Outside the CNS, it is called nerve while inside it is called tract.

Types of nerves : Sensory (with sensory fibres), motor (with motor fibres) and mixed type (with both sensory and motor fibres).

Neurons/Nerve cells :

  • Neuron is structural and functional unit of the nervous system.
  • Each multipolar neuron has three parts — cyton or cell body, dendron and axon.

Grey matter and white matter :

  • Grey matter is darker part of CNS due to presence of cytons.
  • White matter is lighter part of CNS. This is due to presence of myelin sheath axons.

In PNS however, the accumulation of cyton causes a swelling on the nerve. Such a swelling is called ganglion.  [cytons within CNS form nuclei while those in PNS form ganglia]

Connective tissue layers in a nerve are :

  • Endoneurium : covers each nerve fibre
  • Perineurium : covers each nerve bundle having a number of neurons
  • Epineurium : covers many nerve bundles to form a peripheral nerve

Neuroglial cells :

  • More in number than the neurons.
  • They are supporting cells of the Central Nervous System (CNS) and Peripheral Nervous System (PNS).
  • Neurilemma is the plasma membrane of Schwann cell.

Types of neuroglial cells :

Types of neuroglial cells :

1) CNS (Central Nervous System)

CNS –glial cells Functions
Oligodendrocytes These cells have few branches and mainly form the myelin sheath around the central axons, which form the white matter of CNS.

Myelin an isulating sheath is made up of protein and fatty substances. It allows quick transmission of electrical impulses.

Microglia or brain macrophages Small sized cells with few branches. These are derived from monocytes and act as macrophages.

They go to the site of injury, dead neurons and cell debris in the CNS. They mediate immune response in the CNS.

Astrocytes Star shaped cells and the most abundant glial cells of CNS.

They have varied roles in the brain, secretion and absorption of neural transmitter and maintenance of blood-brain barrier BBB.

Regulate the transmission of electrical impulses with the brain.

Ependymal cells Form single layer of squamous or columnar, often ciliated epithelial cells lining the ventricles or brain cavities and central canal of spinal cord. Mainly responsible for production and probably also for circulation of CSF in brain ventricles and central canal.

2) PNS (Peripheral Nervous System)

PNS –glial cells Functions
Schwann Cells These are the most abundant glial cells of PNS. They produce myelin sheath around medullated nerves of PNS.
Satellite cells They support the functions of neurons.


Synapse :

Synapse is a microscopic functional gap between two successive neurons.

  • In this telodendrites of pre-synaptic neuron are in close proximity with dendrites of post-synaptic neuron.
  • This gap is also called synaptic cleft.
  • During transmission of nerve impulse, the synapse get filled with neurotransmitters like acetyl choline.
  • When the telodendria are connected to muscle fibre, it is called motor end plate or neuro-muscular junction.

Properties of nerve fibres :

Properties of nerve fibres :

  • Excitability/ Irritability
  • Conductivity
  • Stimulus
  • Summation effect
  • All or none law
  • Refractory period
  • Synaptic delay
  • Synaptic fatigue
  • Velocity


Types of synapses :Electrical synapse and Chemical synapse.

  • Electrical synapses are found in those places of the body requiring fastest response as in the defence reflexes.
  • A chemical synapse between a motor neuron and a muscle cell is called a neuromuscular junction or motor end plate.

There are three components of a typical chemical synapse.

  • The pre-synaptic terminal
  • The synaptic cleft
  • The post-synaptic neuron

Transmission of nerve impulse across a synapse :

Transmission of nerve impulse across a synapse :

  • This transmission takes place with the help of neurotransmitters.
  • Once the neurotransmitters bind to the receptors of the post-synaptic cell, the action is either excitatory or inhibitory depending on the type of neurotransmitter.
  • The enzyme like acetyl cholinesterase destroys the neurotransmitter after the transmission and the synapse is ready to receive a new impulse.


Transmission of nerve impulse along the axon :

Transmission of nerve impulse along the axon :

  • The excitable neurons transmit the impulse through changes in electrical charges across the neuronal membrane.
  • The external tissue fluid has both Na+ and K+ ions.
  • This process is called sodium pump or Na-K exchange pump.
  • Generation of nerve impulse : Occurs through depolarization.
  • Saltatory conduction takes place in medullated nerve fibres.


Steps in generation and conduction of nerve impulse :

Steps in generation and conduction of nerve impulse :


Human Nervous System :

Central nervous system (CNS) :

Brain is enclosed within the brain box/cranium of the skull, whereas the spinal cord lies in the vertebral canal of the vertebral column.

Inner to these bony structures, there are 3 protective membranes called meninges.

Meninges of CNS :

Meninges of CNS :

Meninges are the connective tissue membranes that cover the brain and the spinal cord. There are three meninges, viz. dura mater, arachnoid mater and pia mater that cover the Central nervous system.

  • The outermost tough, thick and fibrous meninx is dura mater. It is protective in function as it is attached to the inner side of the cranium.
  • The middle, thin and vascular membrane formed of reticular connective tissue is called arachnoid mater. It carries out nutritive function and also gives protection to the brain.
  • The innermost highly vascular and thin membrane is pia mater. It lies in contact with CNS. It is nutritive in function.
  • There is subdural space between the dura mater and arachnoid mater. It is filled with serous fluid.
  • Besides, there is a sub-arachnoidal space lying between arachnoid mater and pia mater. It is filled with cerebrospinal fluid.


CSF (cerebrospinal fluid) :

  • About 100—120 cc lymph like extra cellular fluid with specific gravity of 1.005, present in and around the CNS.
  • It is secreted by the pia mater, the choroid plexuses and the ependymal cells lining the ventricles of the brain and central canal of spinal cord.

Functions of meninges and CSF :

  • Shock absorber, protection, prevention of desiccation.
  • Maintaining constant pressure inside as well as outside the CNS.
  • Exchange of nutrients and wastes between blood and brain tissue.
  • Supply of oxygen to the brain.

The Human brain :

  • The study of all aspects of the brain is called encephalology.
  • The brain can be divided into three main parts - forebrain, midbrain and hindbrain.
  • About 1300-1400 g in weight and 1300-1500 cc in volume

Parts of the brain :

Parts of the brain :

There are three divisions of the brain, viz. forebrain (prosencephalon), midbrain mesencephalon) and hindbrain (rhombencephalon).

  • Forebrain is divided into cerebrum (telencephalon) and diencephalon (thalamencephalon). Underdeveloped olfactory lobes (rhinencephalon) can also be seen in the anterior region.
  • Midbrain consists of corpora quadrigemina and crura cerebri.
  • Hindbrain has cerebellum (metencephalon) and brain stem. It is divided into pons varolii and medulla oblongata (myelencephalon).


Functional areas of cerebrum :

Functional areas of cerebrum :

There are three functional areas in cerebrum viz., sensory, association and motor area.

  • In sensory area, sensory receptors bring the sensory inputs. These inputs are analysed in sensory area.
  • The sensory speech area is located in parietal lobe. It is called Wernicke’s area.
  • Association area forms the major portion of the cerebrum. It processes, analyses and stores the information given by the inputs.
  • Power of reasoning, will, understanding, memory, etc. are the faculties present in the cerebral cortex.
  • Motor area is present in the frontal lobe lying anterior to the premotor area. In the lower part of the motor area just above the lateral sulcus lies the Broca’s area or motor speech area. The Broca’s area controls the movements necessary for speech.


Forebrain : Forebrain consists of olfactory lobes, cerebrum and diencephalon.

Parts of the orebrain (Prosencephalon) & Functions :

Parts of the Forebrain (Prosencephalon) & functions.

(i) Olfactory lobes (Rhinencephalon)

  • Paired, small bodies, lying ventrally in the forebrain
  • Two parts : Olfactory bulbs and olfactory tracts
  • Functions : Sense of smell

(ii) Cerebrum (Telencephalon) 85% of brain

Structure associated :

  • Corpus callosum : band connecting two hemispheres.
  • Pallium : Roof of brain.
  • Corpora striata : Ventro lateral walls.

Peculiarities :

  • Cerebral fissure divides it into two cerebral hemispheres.
  • Outer cortex, inner medulla.
  • Gyri-ridges and sulci-depressions
  • Three deep sulci : Central, lateral and parieto-occipital.
  • Four lobes of cerebrum : Anterior frontal, middle parietal, posterior occipital and lateral temporal

Functions of cerebrum :

  • The cerebrum controls the voluntary activities.
  • The cerebrum perceives various sensory stimuli received through vision, taste, smell, sound, touch, speech, etc.
  • It is the centre of memory, will-power, intelligence, reasoning and learning.
  • The cerebrum is the centre for emotions, thoughts and feelings, pain, pleasure, fear, fatigue, pressure, temperature, etc.
  • It is also the centre for micturition, defecation, weeping, laughing, etc.

(iii) Diencephalon (Thalamencephalon) :

Structure associated :

  • Epithalamus : Roof has anterior choroid plexus.
  • Thalami, Reticular activating system (RAS) is in thalami.
  • Hypothalarnus Floor of diencephalon

Peculiarities :

  • Present below corpus callosum and above midbrain.
  • Epithalamus shows pineal body attached to pineal stalk.
  • Habencular commissure connects two thalami.
  • Pituitary gland is attached by infundibular stalk.

Functions of diencephalon :

  • Diencephalon acts as a relay centre for motor and sensory impulses between spinal cord, brainstem and various areas of cerebral cortex.
  • Diencephalon consists of epithalamus, thalami and hypothalamus. Therefore it acts as a centre for homeostasis and higher centre of autonomous nervous system.
  • Hypothalamic nuclei secrete neurohormones which influence the pituitary gland.
  • Diencephalon regulates heartbeats, blood pressure and water balance.
  • Anterior choroid plexus which is located in the diencephalon secretes cerebrospinal fluid.
  • Hypothalmic regions control many involuntary functions such as hunger, thirst, thermo-regulation, fear, anger, sleep, sexual desire, etc.


Mid brain : It is located between diencephalon and the pons varolli. It contains the cerebral aqueduct or iter that connects the third and fourth ventricles.

Parts of the Midbrain (Mesencephalon) & Functions :

Parts of the Midbrain (Mesencephalon) & Functions :

 (i) Corpora quadrigemina :

Structure associated :

  • Connection of cerebrum to cerebellum

Peculiarities :

  • Two pairs of lobes.
  • 1st pair superior colliculi : Receives optic nerves.
  • 2nd pair inferior colliculi : Receives auditory nerves.

Functions of Corpora quadrigemina :

  • Control and coordination of eye movement and head movement.
  • Control and coordination of auditory reflexes.


Hind brain : The posterior region of the brain is called hind brain.

Parts of the Hindbrain (Rhombencephalon) & Functions :

Parts of the Hindbrain (Rhombencephalon) & Functions :

(i) Cerebellum (Metencephalon) : 11% of the brain second largest region.

Structure associated :

  • Arbor vitae : Tree like processes of inner white matter extending into the outer grey matter.

Peculiarities :

  • Posterior most part.
  • Three lobes : Median vermis, two lateral cerebellar hemispheres.

Functions of Cerebellum :

  • Cerebellum is a primary centre for the control of equilibrium, posture, balancing and orientation.
  • Neuromuscular activities are regulated by the cerebellum.
  • Coordination of walking, running, speaking, etc. is under the control of hindbrain.

(ii) Pons Varolii :

  • Nerve fibres that form bridges between cerebrum and medulla oblongata.
  • Outer white and inner grey matter.
  • Nerve fibres cross over here.

Functions of Pons :

  • Activities of two cerebellar hemispheres are coordinated by pons.
  • Nerve fibres cross over in this area and thus the right side of the brain controls the left part of the body and vice versa.
  • Pons controls the consciousness of the brain.
  • Breathing centre is located in pons along with medulla.

(iii) Medulla oblongata (Myelencephalon) :

  • Posteriormost part that continues as a spinal cord.
  • Outer white and inner grey matter.
  • It has a posterior choroid plexus.
  • Cranial nerves arise from medulla.

Functions of Medulla oblongata :

  • Medulla oblongata controls all the involuntary activities such as heartbeats, respiration, vasomotor activities.
  • Peristalsis and reflex actions such as coughing, sneezing, swallowing, etc. are also under the control of medulla oblongata.
  • Medulla oblongata is essential for all the vital functions of the body.


Ventricles of brain :

Ventricles of brain :

Ventricles are the cavities present in different parts of the brain.

  • There are four ventricles in the human brain. All the ventricles are connected with each other.
  • They are filled with cerebrospinal fluid.
  • Paracoel or lateral ventricles-I and-II are present inside the cerebral hemispheres.
  • The diencephalon has ventricle-III.
  • Ventricle-III is in connection with lateral ventricles by foramen of Monro.


Important terms associated with brain :

Important terms associated with brain.

Corpus callosum : Transverse band of nerve fibres which connects right and left cerebral hemisphere. It is the largest commissure of the brain.

Cerebral cortex : The outer surface of cerebrum. composed of grey matter.

Cerebral medulla : Inner part composed of white matter.

Gyri (elevations) and Sulci (depressions) : convolutions and grooves on the surface of cerebrum.

Central sulcus : Between frontal lobe and the parietal lobes.

Parieto-occipital sulcus : Between parietal and occipital lobes.

Lateral or Sylvian sulcus : Between temporal lobe and frontal and parietal lobes.

Insula or insular cortex : Fifth lobe which is folded deep within the lateral sulcus.

Foramen of Monroe : Narrow opening through which two lateral ventricles communicate with diocoel (third ventricle).

Pineal gland : Vestigial 3rd eye and an important endocrine gland, producing hormones melatonin and serotonin.

Habenular commissure : Connects two thalami.

RAS (Reticular Activating System) : Relay centre as it transmits all sensory impulses except those of olfactory to the cerebrum.Situated in thalami.

Aqueduct of Sylvius or iter : Connection between third and fourth ventricle through hypothalamus and midbrain.

Limbic system : A complex neuronal circuit formed by the hypothalamus, amygdala, parts of epithalamus and thalamus, hippocampus and other areas.

Optic chiasma : Crossing of the two optic nerves.

Corpora quadrigemina : Four rounded elevations on the dorsal surface of the midbrain. The two superior colliculi are involved in visual reflexes and the two inferior colliculi are for auditory reflexes.

Crura cerebri : Two thick fibrous tracks, also called cerebral peduncles, situated in the floor midbrain.

Red nucleus : Grey matter near the centre of the midbrain, controlling posture and muscle tone, modifying some motor activities and motor coordination.

Pons varolii : Rounded bulge on the underside of the brain stem.

Brain stem : Consist of midbrain, pons and medulla.

Arbor vitae : The mixing of white matter with the grey matter showing‘ a branched tree-like pattern.

Cerebellar peduncles : Three pairs of myelinated nerve bundles connecting cerebellum to the other parts of CNS.

A pair of lateral—foramina of Luschka and a median - foramen of Magendie: apertures on the posterior choroid plexus.


Spinal Cord : Spinal cord is the part of central nervous system and forms the lower extension of the medulla oblongata of the brain.

  • It lies within the neural canal of the vertebral column and is surrounded by three meninges.
  • Externally, the spinal cord appears as long cylindrical rod.
  • It is 42 to 45 cm long and 2.0 to 2.5 cm broad.
  • Conus medullaris : Terminal nervous part of the spinal cord.
  • Filum terminale : Thread like non-nervous extension.
  • 31 pairs of spinal nerves arise from lateral sides of the spinal cord.
  • Cauda equina — Filum terminale with some spinal nerves running parallel to it. (appearing like a horse-tail)

T.S. of spinal cord & Function :

T.S. of spinal cord :

  • The spinal cord has a deep, narrow dorsal fissure and a broad ventral fissure.
  • The inner grey matter is H-shaped and the outer white matter surrounds it.
  • Grey matter is divisible into six horns, namely dorsal, lateral and ventral horns.
  • The white matter is divisible into 6 columns or funiculi, namely dorsal, lateral and ventral funiculi.
  • The dorsal and ventral horns extend out of the spinal cord as dorsal root and ventral root.
  • The dorsal root has dorsal root ganglion which is a collection of unipolar sensory neurons. No such ganglia on ventral root.
  • The adjustor/association or inter-neurons lie inside the grey matter.
  • The white matter consists mainly of bundles of myelinated nerve fibre called ascending and descending tracts.

Functions :

  • The spinal cord is the main centre for the most reflex actions.
  • It provides pathway for conduction of sensory and motor impulses.


Peripheral Nervous System (PNS) ; The peripheral nervous system connects the central nervous system to the different parts of the body having receptors and effectors.

Two types of peripheral nerves :

  • Cranial nerves : arise from the brain.
  • Spinal nerves : arise from the spinal cord.

Cranial nerves - nature and functions :

Cranial nerves - nature and functions :

  • These nerves develop from the brain, in all amniotes (reptiles, birds and mammals).
  • There are 12 pairs of cranial nerves.
  • These nerves originate from or terminate into the brain.
  • According to their function, these are classified as sensory, motor  and mixed nerves.
  • The details of cranial nerves present in the human body is presented in below Table
Name Type Origin Organs Innervated Functions
1-Olfactory Sensory Olfactory bulb Epithelium of Nose Smell
2-Optic Sensory Side of


Retina of Eye Vision
3-Occulomotor Motor Floor of mid brain Eye muscles (4 of 6 eye muscles) Movement of eye ball
4-Pathetic Motor Floor of mid brain Eye muscles (1 of 6 eye muscles,  forehead scalp) Rotation and movement of eye ball

(Dentist’s nerve)


b. Maxillary

c. Mandibular





c. Mixed

Ventral side of pons



a. Nasal cavity, Upper eyelids, forehead, scalp, conjunctiva,

lacrimal gland, scalp

b. Mucosa of nose,

palate, upper teeth,

upper lip, lower eye lid parts of pharynx

c. Lower teeth, skin

over mandible cheek,

side of head in front

ear, muscles of


Sensation of skin touch, taste, jaw movement.
6-Abducens Motor Pons Muscles of eye ball, lateral rectus muscle Movement of eye
7-Facial (bearing



Mixed Pons facial, scalp and

neck muscles,

lacrimal, sublingual,

submandibula, nasal

and palatine glands

Facial expression,

movement of

neck, secretion of tears, taste,

salivary secretion.

8-Auditory (vestibulo-


i. Vestibular

ii. Cochlear

Sensory Pons




Internal Ear Hearing and equilibrium
9-Glossopharyngeal Mixed Side of medulla oblongata Pharynx, tongue, salivary glands Taste,        salivation and swallowing
10-Vagus (Pneumogastric) Mixed Side of medulla oblongata Larynx, trachea, pharynx, alimentary canal, heart, lungs, pancreas, blood vessels, Visceral sensations and visceral

movements like breathing cardiac,  slowing, gastric and pancreatic secretion, gastrointestinal


11-Spinal accessory Motor Side of medulla oblongata Neck and shoulder muscles, reflexes of thoracic and abdominal vicera larynx, pharynx Movements of larynx, pharynx, neck and shoulder
12-Hypoglossal Motor Side of medulla oblongata Tongue muscles Movement of tongue


Spinal Nerves :

  • Thirty-one pairs of spinal nerves originate from the spinal cord.
  • Spinal Nerves : All spinal nerves are mixed nerves.

Formation of a typical spinal nerve :

  • Each spinal nerve is formed inside the neural canal of vertebral column.
  • The dorsal sensory and the ventral motor nerves together form the mixed spinal nerve.

As soon as it emerges out of vertebral column, it shows three branches, viz.

  • Ramus dorsalis : from skin and to muscles of dorsal side
  • Ramus ventralis : the largest of the three supplies the organs and muscles on lateral and anterior side
  • Ramus communicans : the smallest of the three and given out from 1st thoracic upto 3rd lumbar (L3) spinal nerve. It joins the sympathetic ganglia.

Reflex Action :

Reflex action is defined as a quick, automatic involuntary and often unconscious action brought about when the receptors are stimulated by external or internal stimuli.

The path along which the action is carried out is called reflex arc.

Reflex arc :

Reflex arc : Reflex actions are controlled by CNS. Reflex arc is the structural or functional unit of reflex action. Simple reflex arc is formed of the following five components.

  • Receptor organ : The sensory part that receives the stimulus is called receptor organ. It can be any sense organ that receives the stimulus and converts it into the impulse. e.g. skin, eye, ear, tongue, nasal epithelium, etc.
  • Sensory neuron or afferent neuron : Sensory part carrying impulse from receptor organ to CNS is called sensory neuron. Its cyton is located in dorsal root ganglion. Its dendron is long and connected to receptor while the axon enters in the grey matter of spinal cord to form a synapse.
  • Association, adjustor or intermediate neuron : It is present in the grey matter of spinal cord. Receiving impulse from sensory neuron, interpreting it and generating motor impulse are done by association neuron.
  • Motor neuron (effector) : The cyton of motor neuron is present in the ventral horn of grey matter and axon travels through ventral root. It conducts motor impulse from spinal cord to effector organ.
  • Effector organ : Effector organ is specialized part of the body which is excited by receiving the motor impulse. It gives proper response to the stimulus, e.g. muscles or glands. The path of reflex action is followed by the unidirectional impulse. It originates in the receptor organ and ends in effector organ through CNS.


Types of reflexes :

Types of reflexes :

Based on the location of their action : The reflexes are divided into somatic reflexes and visceral reflexes.

  • Somatic reflex : When effector is located in body structures such as skeletal muscles, it is called a somatic reflex.
  • Visceral reflex : When the effector is located in the visceral organs such as glands or smooth muscles then it is called a visceral reflex.

Based on the basis of number of neurons : Reflexes are of two types, viz. monosynaptic reflexes and polysynaptic reflexes.

  • Simple or monosynaptic reflexes are those in which one sensory and one motor neuron are involved in the reflex action.
  • Polysynaptic or complex reflexes are those when more than two neurons are involved in the reflex action.

Based on inheritance and experience of learning: The reflexes are subdivided into unconditional or inborn and conditional or acquired.

  • Unconditional or inborn reflexes are inborn or hereditary. They are permanent, never disappear and need no previous experience, e.g. blinking of eyes, suckling, swallowing, knee jerk, sneezing, coughing, etc.
  • Conditional or acquired reflexes are acquired during life by experience or learning. They are based on individual learning or experience. These are not heritable, temporary and may disappear or reappear, e.g. driving, cycling, etc.

On the basis of control over the actions :

  • Cranial reflexes : carried out by brain , slow action response • eg. watering of mouth on sight or smell of good food
  • Spinal reflexes :  Carried out through spinal cord.  Urgency for response is required so these are quick acting. E.g. withdrawal of leg while stepping on something hot or pointed


According to recent studies, the ANS is under the control of CNS and nerves arising from it (PNS).

According to this view, the PNS is divided into

  • Somatic nervous system
  • Autonomic nervous system

The somatic nervous system relays impulses from CNS to the skeletal or voluntary muscles of the body.

Autonomic Nervous System (ANS) : Autonomic nervous system transmits impulses from CNS to the involuntary organs and smooth muscles of the body.

It includes — autonomic ganglia, preganglionic fibres and postganglionic fibres.

Autonomic ganglia include

  • Sympathetic ganglia-present near CNS in the form of sympathetic cord.
  • Parasympathetic ganglia - present near or on the effector organs.

Preganglionic fibres arise from grey matter of CNS and end at autonomic ganglia.

Postganglionic fibres arise from autonomic ganglia to the effector organs.

Autonomic nervous system consists of sympathetic and parasympathetic nervous system.

Sympathetic and Parasympathetic Nervous System :

(i) Sympathetic Nervous System [SNS) :

  • Also called thoraco-lumbar outflow.
  • Consists of 22 pairs of sympathetic ganglia which lie near vertebral column.
  • Post ganglion is neuron which produce adrenaline. Hence they are called adrenergic fibres.
  • It works in emergencies. It is also called 3 Fs system [fright, fight and flight]. It has excitatory and stimulating effect on most organs of the body.

(ii) Parasympathetic Nervous System :

  • It is also called cranio-sacral outflow.
  • It consists of ganglia which are very close or within the wall of the effector organs.
  • Acetylcholine is produced at the terminal end of postganglionic nerve at the effector organ. Hence these are also called cholinergic fibres.
  • All activities which are stimulated by the sympathetic system are brought back normal by this system. Hence it is also called housekeeping system.

Comparison between Sympathetic and Parasympathetic Nervous System :

Organ/Region Sympathetic effect Parasympathetic effect
Heart beat Increases Decreases
Blood vessels Constricts Dilates
Arterial B.P. Increases Decreases
Pupil of Eye Dilates Constricts
Gastrointestinal movements

(stomach and intestine)

Retards peristalsis Accelerates peristalsis
Urinary bladder Relaxes the bladder Contracts the bladder


Sensory Receptors

Specialised structures in the body modified to receive the various stimuli from the external or internal environment.

Classification of receptors : Receptors are classified on the basis of their location, function and their sensitivity to specific stimuli. Their classification is given in the following chart.

Types of exteroceptors and interoceptors, their locations and functions :

Types of exteroceptors and interoceptors, their locations and functions :

No. Name/Type of receptor Location Function
I. Exteroceptors : Receive external stimuli
a. Phonoreceptors Internal Ear - organ of corti Sound reception
b. Statoreceptors Internal Ear- semicircular canals Receptors for maintaining balance and equilibrium
c. Photoreceptors Retina of Eye Receives sensory stimuli for vision
d. Thermoreceptors Skin Receives sensory stimuli for heat (caloriceptors) and cold (trigidocetptors)
e. Mechanoreceptors Skin Sensitive to mechanical stimuli like touch, pain, pressure, deep pressure, etc.
f. Chemoreceptors

•         Gustatoreceptors

•         Olfactory receptors


Taste buds of tongue Olfactory Epithelium of Nose

Sensitive to taste of sweet, salt, sour, bitter and umami. Sensitive to about 10,000 different smells
II. Interoceptors : Receive stimuli coming from within the body
a. Enteroceptors from internal body organs Sensitive to stimuli coming from internal organs like hunger, thirst, pain, osmotic change
b. Proprioceptors Joints, muscles and tendons Detect changes in the movements of joints, tendons and muscles; pain, tension and sensitive to vibrations
c. Baroreceptors

(*These are also considered as mechanoreceptors, receiving signals from internal organ)

Present in walls of atria, venae cavae, aortic arch, carotid sinus Sense changes in B.P. so as to restore homeostasis through vasodilation or vasoconstriction


Eye :

  • The eyes are a pair of sensory organs of vision located in the orbit of skull
  • Each eye is spherical/rounded and called eyeball.

Structure of eye :

Wall of the eyeball is made up of 3 layers : (1) sclera, (2) choroid (3) retina.

Sclera is the outer layer of dense connective tissue with anterior transparent cornea.

Choroid is the middle layer. It is bluish in colour containing many blood vessels. The anterior region is thick and forms the ciliary body. Posterior 2/3rd region is thinner.

Iris is the forward segment of the ciliary body which is pigmented and opaque. This part is the visible coloured portion of the eye.

Lens is present anteriorly inside the iris and is held in position by the ligaments of ciliary body.

The aperture surrounded by the iris in front of the lens is known as pupil. The movement of the pupil is regulated by the muscle fibres of iris.

Retina : The innermost layer of the eye is the retina having three sub-layers formed by ganglion cells, bipolar cells and photoreceptor cells, which are sensitive to light.

Photoreceptor cells : There are two types of photoreceptor cells, viz. rods and cones containing light sensitive proteins. They are termed as photo pigments, rhodopsin which is a derivative of vitamin A (in rods) and iodopsin (in cones).

  • The cones are responsible for daylight or photopic vision and colour vision.
  • The rods function in dim light giving scotopic vision.
  • The cones are of three types, each containing its own characteristic photopigments that respond to red, green and blue lights.
  • The optic nerve leaves the eye at a point slightly away from the median posterior pole of the eyeball. In this region, the rods and cones are absent therefore this region is known as blind spot. Macula lutea, a yellowish pigmented spot is present lateral to the blind spot.

Fovea is a central pit present beside it. Fovea is a thinned out portion of the retina where only the cones are densely packed and therefore have greatest visual acuity (resolution).

A space between the cornea and the lens is called aqueous chamber. It contains a thin watery fluid known as aqueous humor.


Generation of image :

Generation of image :

  • The light rays from the object pass through the conjunctiva, cornea through the pupil upon the lens and is focused on the retina to form an image.
  • In the visual area of cerebrum, the nerve impulses are analysed and the image formed is recognized.

Mechanism of vision :

1-Nerve impulse is transmitted by optic nerve to brain ↓
2-Nerve impulse in the axons of ganglion cells converge and leave via the optic nerve ↓
3-Nerve impulse transmitted to ganglion cells ↓
4-Nerve impulse transmitted to bipolar nerve cells ↓
5-Stimulation of rod and cone cells and generation of nerve impulse ↓
6-Breaking up of light sensitive pigments by specific wavelength of light ↓
7-Light falls on rod and cone cells in Retina ↓
8-Changes in retina when light rays fall on it ↓
9-Perception of image by the brain cells (in the visual area of cerebrum) ↓


Ear :

The human ear is called statoacoustic organ as it has two functions - hearing and body equilibrium

Anatomically the ear is made up of three divisions : the external ear, middle ear and inner ear,

Structure of ear :

External ear : It consists of ear pinna, auditory canal and tympanic membrane.

  • The ear pinna is an immovable part, supported by elastic cartilage structure. It leads into an auditory canal.
  • The pinna collects and sends the sound waves into the auditory canal.
  • The auditory canal ends at the ear drum. It transfers the sound waves to the ear drum.

Middle ear : It consists of chain of three ossicles called malleus, incus and stapes.

  • The malleus is attached to the tympanic membrane and the stapes is connected to the oval window of the internal ear. They help in the transmission of sound waves from external auditory canal to internal ear.
  • Connecting middle ear with the pharynx is eustachian tube which helps in equalizing the air pressure on either side of the tympanic membrane.

Internal ear : It is fluid filled structure called labyrinth. It has two parts, bony and the membranous labyrinth.

  • The outer bony labyrinth is formed by the series of channels in which the membranous labyrinth containing endolymph fluid is present.
  • The membranes consist of coiled cochlea, the reissner’s membrane and basilar membranes. These membranes divide the surrounding perilymph filled bony labyrinth into an upper scala vestibule and a lower scala tympani.
  • The space within cochlea which is known as scala media is filled with endolymph. The scala vestibule ends at the oval window at the base of cochlea.
  • The scala tympani terminates at the round window which opens to the middle ear. The organ of corti is located on the basilar membrane. It contains the hair cells which act as auditory receptors.
  • The hair cells are columnar cells present in rows. The basal ends of the hair cells are in close contact with the afferent nerve fibres while their apical end contains numerous cilia. A thin elastic membrane projects above the rows of the hair cells called tectorial membrane.
  • Above the cochlea, the internal ear also contains vestibular apparatus. it consists of three semicircular canals and the otolith organ formed of the sacculus and utriculus. The semicircular canals lie in different plane at right angles to each other and are suspended in the perilymph.
  • The bases of canals are swollen and are called ampullae, which contain a projecting ridge known as crista ampullaris which contain hair cells.
  • The sacculus and utriculus also have projecting ridge called macula. The crista and macula are the specific receptors of vestibular apparatus. They are responsible for maintenance of body posture and the balance.


Mechanism of Hearing :

Mechanism of Hearing :

  • Pinna of the ear receives the sound waves and directs them to eardrum.
  • Eardrum vibrates and these vibrations are amplified and transmitted through the ear ossicles to the endolymph inside cochlea.
  • This generates, wave in the endolymph.
  • These waves induce ripples in the basilar membrane.
  • These movements in the basilar membrane cause the hair cells to press against tectorial membrane.
  • This generates nerve impulse in the afferent neurons. Impulse is sent to the brain via the auditory nerve.
  • Auditory cortex of the brain decodes the sound.


Disorders of nervous system :

Disorders of nervous system :

Psychological disorders :

Commonly called mental disorders. There is a wide range of conditions that affect the mood, thinking or behaviour.

Some of the major categories of psychological disorders are :

  • Intellectual disability (earlier known as mental retardation),
  • Autism spectrum disorder
  • Bipolar disorder
  • Depression
  • Anxiety disorder
  • ADHD (Attention Deficit Hyperactivity Disorder)
  • Stress related disorders.

Parkinson’s disease :

  • Degeneration of dopamine-producing neurons in the CNS causes Parkinson’s disease.
  • Symptoms develop gradually over the years.
  • Symptoms are tremors, stiffness, difficulty in walking, balance and coordination.

Alzheimer’s disease :

  • It is the most common form of dementia.
  • Its incidence increases with the age.
  • Symptoms include the loss of cognitive functioning—thinking, remembering, reasoning and behavioural abilities. It interferes with the person's daily life and activities.

The tympanic membrane (ear drum) is a delicate, membranous structure which transmits the sound waves to the middle ear.


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