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

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

Types of neuroglial cells :

1) CNS (Central Nervous System)

2) PNS (Peripheral Nervous System)

Properties of nerve fibres :

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

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 :

• 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 :

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.

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 :

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.

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.

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.

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 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.

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.

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.

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

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 :

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

(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 :

Types of exteroceptors and interoceptors, their locations and functions :

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 :

• 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 :

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 :

• 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 :

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.