I. Central Nervous System (CNS) (Brain Spinal Cord)
1. Brain:
           In man, brain is lodged and protected in a bony box in the skull called cranium. Brain is covered by three membranes called meninges, namely, outer duramater (double layered), middle arachnoid layer and inner piamater (thin and attached to outer surface of brain). Dura matter and arachnoid layer are separated by sub dural space and arachnoid layer and pia mater are separated by sub arachnoid space. Sub dural space is filled with cerebro spinal fluid (alkaline and colour less).
It acts as a shock absorbing medium.   
           Brain is divided into three parts, namely, fore brain (Prosencephalon), mid brain (Mesencephalon) and hind brain (Rhombencephalon).

A) Prosencephalon: It is the anterior and the largest part of the brain and is formed by olfactory bulbs, cerebrum and diencephalon.
a. Olfactory bulbs: In the anterior part of brain is a pair of olfactory bulbs. They receive sensory impulses of smell from olfactory epithelium.
b. Cerebrum: It is the largest part of the brain and is divided into two cerebral hemispheres (left and right) by a deep longitudinal fissure. Beneath the cortex, left and right cerebral hemispheres are connected by a transeversly arranged bundle of myelenated nerve fibres called corpus callosum/ colossal commissure. It co - ordinates the left and right cerebral hemispheres. The outer part of cerebrum is formed by grey matter called cortex and inner part by white matter called medulla. Cortex contains nerve cell bodies medulla contain myelenated axons.
            Cerebral cortex contains many folds (gyri) among which fissures (deep grooves) and sulci (shallow grooves) are present. Cortex has three functional areas.
I. Sensory areas - Receive and interpret sensory impulses.
II. Motor areas - Control voluntary muscular movements.
III. Association areas - Deal with integrative functions (such as memory and com munications)

        Each cerebral hemisphere is divided into four lobes- frontal, parietal, temporal and occipital lobes.
c. Diencephalon/ Thalamencephalon: It has an epithalamus, thalamus and a hypothalamus
i. Epithalamus is non nervous. It is fused with pia mater and form the anterior choroid plexus. Behind the anterior choroid plexus, is a pineal stalk, which bears a pineal body at its tip.
ii. Thalamus acts as a co - ordinating centre for sensory and motor signaling.
iii. Hypothalamus ventrally has a hallow out growth called infundibulum. It bears the pituitary gland.
Infundibulum also contains several neurosecretory cells. Hypothalamus acts as an osmoregulatory, thermoregulatory, hunger, thirst and satiety centre.
 The amygdala, hippocampus (deep structures of cerebral hemispheres) and inner parts of cerebral hemispheres form the limbic system. Along with hypothalamus, the limbic system is involved in sexual behaviour and emotions.

B) Mesencephalon: It lies between the thalamus and pons varoli. Ventrally, it contains a pair of longitudinal bands of nervous tissue called crura cerebri. They connect the fore brain and hind brain. Dorsally, the mid brain has two pairs of spherical lobes called corpora quadrigemina (four optic lobes). The anterior pair is called superior colliculi (concerned with vision) and the posterior pair is known as inferior colliculi (concerned with hearing).
C) Rhombencephalon: It is formed by cerebellum, pons and medulla oblongata.
a. Cerebellum:  It is the second largest part of the brain and is composed of a median vermis and two lateral cerebellar lobes. Each cerebellar lobe consists of an anterior, a posterior and a floscular lobe. The inner white matter of cerebellum spreads like a branching tree. It is called arbor vitae. Cerebellum (the little brain/ gyroscope of the body) control and co - ordinate locomotor movements and maintains equilibrium (Damage to cerebellum results in ataxia).
b. Pons varoli: Beneath the mid brain, just in front of cerebellum and above the medulla oblongata, pons varoli is present. It is in the form of transervese bridge of nerve fibres connecting the two cerebellar lobes. It acts as a relay centre between cerebellum, spinal cord and the remaining part of brain. It contains a pneumotoxic centre (controls respiratory muscles).

c. Medulla oblongata: It is the posterior part of the brain and is continued as spinal cord. Its roof is thin and vascular and is called posterior choroid plexus. Medulla oblongata consists of cardio vascular and respiratory centres and the centres of swallowing, cough, vomiting, sneezing, hiccuping.
Mid brain, pons varoli and medulla oblongata together form the brain stem.

Cavities of brain: Cavities of brain are known as ventricles. The cavities of right and left ventricles are known as 1st and 2nd ventricles respectively. They are also called paracoels or lateral ventricles. Diencephalon contains the third ventricle or diacoel and medulla oblongata contains the fourth ventricle or myelocoel. The two lateral ventricles open into the diacoel through separate openings called foramina of Manro. Diacoel and myelocoel are connected by a narrow tube called iter or ductus sylvius. 4th ventricle is continuous with central canal of spinal cord. All the ventricles are filled with cerebrospinal fluid. It was flushed four times in a day to remove metabolities and toxins.

2. Spinal Cord: Spinal cord is lodged and protected in the neural canal of vertebral column. In neural canal, spinal cord is surrounded by three meninges (like brain), namely, outer duramater, middle arachnoid mater and inner pia mater. Spinal cord extends between medulla oblongata and upper boarder of 2nd lumbar vertebra. Spinal cord has two median grooves, namely, anterior or ventral fissure and posterior or dorsal fissure. Spinal cord also has two enlargements viz., cervical and lumbar enlargements. Below the lumbar enlargement, spinal cord tapers to a conical portion known as conus medullaris. Extension of conus medullaris into the coccyx as a non nervous fibre is called filum terminale.
        Spinal cord contains a central canal which is lined by ependymal epithelium. It is surrounded by butterfly wings like or H - shaped grey matter (contains cell bodies of neurons, neuroglia, dendrites and non myelenated parts of axons), which is surrounded by white matter (contains myelenated axons). White matter is organised into a dorsal funiculus, lateral funiculus and ventral funicular on each side. Spinal cord is a middle man between receptors and effectors. It also acts as coordinating centre for simple reflexes.

II. Peripheral Nervous System (PNS) (Cranial nerves and spinal nerves)
1. Cranial nerves: Nerves that are associated to brain are known as cranial nerves. There are 12 pairs of cranial nerves in man. Among these 12 pairs.
         I, II and VIII cranial nerves are sensory in nature.
         III, IV, VI, XI and XII cranial nerves are motor in nature.
         V, VII, IX and X cranial nerves are mixed in nature.

2. Spinal nerves: Nerves associated with spinal cord are known as spinal nerves.
       There are 31 pairs of spinal nerves in man. They are classified into 5 types

 Each spinal nerve has two roots viz., a dorsal root (sensory) and a ventral root (motor) in the spinal cord.  They are united in the neural canal of vertebral column and form the spinal nerve. Spinal  nerve come out from vertebral column through inter vertebral foramina.
  Before emerging out, the lumbar, sacral and caudal nerves extend posteriorly along with filum terminale as a bunch called cauda equina.
  Branches of first four cervical nerves on each side form cervical plexus.

 Branches of 5th to 8th cervical nerves and 1st thoracic nerve on each side form brachial plexus.
 Branches of 1st lumbar and fourth lumbar on each side form the lumbar plexus.
 Branches of first three lumbar, a branch of 4th lumbar, a branch of 5th lumbar nerves an each side form sacral plexus.
 4th and 5th sacral nerves and coccygeal nerves form the coccygeal plexus.
 All spinal nerves are mixed nerves.
 

III. Somatic Neural System
1. It includes both sensory and motor neurons.
2. Sensory neurons transmit impulses from sensory neurons to central nervous system. These impulses are processed in CNS.
3. Somatic motor neurons innervate the skeletal muscles and produce voluntary movements. The effect of somatic motor neuron is always excitation.

IV. Autonomic Nervous System

        Neurons of autonomic nervous system are associated with the receptors located in the walls of visceral organs. These receptors are known as interoceptors (e.g.: chemoreceptors). They sense the internal stimuli.
 

1. Sympathetic division:
i. As the preganglionic neurons arise from the thoracic and lumbar regions of spinal cord, sympathetic division is called thoraco lumbar division. It is said to exhibit thoraco lumbar out flow.
a. There is a pair of sympathetic chains that extend from the base of the skull to the pelvis, one on each side of the dorsal aorta.
b. These chains have a series of sympathetic ganglia.
c. The preganglionic nerve fibres may be connected to post ganglionic neurons or directly to the collateral ganglia.
d. There are three collateral ganglia, namely, coeliac, superior musenteric and inferior mesenteric ganglia.

e. Post ganglionic fibres from sympathetic trunk mostly innervate the organs anterior to diaphragm. Whereas the post ganglionic axons from collateral ganglia innervate organs posterior to diaphragm.
 

2. Parasympathetic division:
a. As the cell bodies of preganglionic neurons of parasympathetic division are located in brain and scaral region of spinal cord, it is called cranio sacral division. It is said to be cranio sacral outflow.
b. The cranial out flow includes the ciliary, pterygo palatine, submandibular and otic ganglia. They receive preganglionic fibres from III, VII and IX cranial nerves and send post ganglionic fibres to eye ball, nasal mucosa, palatine, pharynx, lacrimal glands and salivary glands.
c. The sacral outflow includes the pelvic plexus that receives preganglionic fibres from 2 - 4 sacral spinal nerves and supply nerves urino genital system.

Differences between Sympathetic and Parasympathetic divisions:

Effects of sympathetic and parasympathetic Divisions (Comparision):

Generation And Conduction of Nerve Impulse

          The signal that travels along a nerve fibre and hence in the release of neurotransmittors is called a nerve impulse. Neurons conduct nerve impulses (action potentionals) because membrane potential is established across the neuronal membrane. Ions move in and out through axolemma of a neuron through channels - such as:
i. Leakage Channels - (Na+, K+): K+ leakage channels are more than Na+ leakage channels.
ii. Ligand - Gagted Channels - Located in post synaptic membrane and open or close in response to chemical stimuli.
iii. Voltage Gated Channels - Open in response to a change in membrane potential. They are Na+ and K+ voltage gated channels. Na+ channels are of 2 types (Activation channel, Inactivation channel).
Resting Membrane Potential
  The membrane potential of a neuron that is not transmitting signals is called the resting membrane potential. It depends on the ionic gradients that exit across the axolemma (membrane) and the differential permeability of axolemma.

    At the resting potential voltage gated channels of Na+ or in resting stage. Voltage gated channels of K+ are closed. As the axolemma has more K+ leakage channels than Na+, the cell is roughly hundred times more permeable to K+ than Na+. Consequently the resting membrane potential is about - 70 mV on the inner side of axolemma. At this phase, the axolemma (membrane) is said to be polarised. If the inner side becomes less negative, it is said to be depolarised. If the inner side becomes more negative, it is said to be hyperpolaraised.
The membrane potential can be measured by a voltmeter or oscilloscope.

Action potential
       The momentary change (about 1 - 2 milli seconds) in electrical potential on the membrane (axolemma) of a neuron that occurs when it is stimulated, resulting in the transmission of an electrical impulse, is termed as action potential.

         A stimulus depolarises the axolemma. When it is depolarised, the voltage gated channels of K+ open more slowly than those of Na+. As a result, actions potential begins to increase in membrane permeability to Na+, which depolarises the membrane, followed by an increased permeability to K+, which repolarises the membrane.
i. Depolarising (resting) phase: When a nerve fibre is stimulated when the depolarization reaches the threshold level (-55 mV), voltage gated channels of Na+ (both activation and inactivation channels) and voltage gated channels of K+ close. Due to rapid influx of Na+, the membrane potential shoots rapidly upto + 45 mV (spike potential).
         If the level of depolarization is less than the threshold potential, the membrane potential will normally drop back to resting levels without further consequences. When depolarization is just equal to or above threshold potential, the action potential of equal amplitude is initiated. It means the nerve impulse is either conducted totally or not conducted at all. This is called all or none principle.

ii. Repolarising (fallling) phase:
     Once the membrane potential has risen to peak, the voltage gated channels of Na+ are inactivated and voltage gated channels of K+ open. Efflux of K+ repolarises the membrane (axolemma).
iii. Hyperpolarization or undershoot:
         During this phase, membrane potential briefly becomes even more negative (-90 mV) that it normally is at rest. This is called hyperpolarization. This occurs because of increased permeability of K+ that exit while voltage gated K+ channels remain open. Na+ channels come to resting state. When the membrane potential returns to normal ( -70 mV), the K+ channels will close.
Refractory periods
        Few milli seconds after initiation of an action potential, the neuron can not generate another action potential to a normal threshold stimulus. This brief period is called refractory period.
i. Absolute refractory period - During this period, even a very strong stimulus can not initiate a second action potential. This period is equal to periods of depolarization and repolarization.

ii. Relative refractory period - The time during which a second action potential can be initiated by a larger than normal stimulus. It is equal to the period of hyper polarization.
Speed of conduction
    The speed of conduction is directly propotional to the diameter of the axon. In myelenated neurons, the voltage gated channels of Na+ and K+ are concentrate at nodes of Ranvier. As a result the impulse jumps from one node to another. Hence it is called jumping or saltatory transmission. In non myelenated nerve fibres the conduction is continuous.

Sodium - Potassium pump:

       The Na+ and K+ ions diffuse in and out across the axolemma (membrane). If this diffusion is unchecked, the resting membrane potential may be disturbed. These flows of ions are offect by sodium - potassium pumps located in the walls of axons. These pumps expel three Na+ ions for every two K+ ions imported.

Synaptic Transmission

    The place at which nerve impulse is relayed from the axon terminal of a neuron to the dendrite of adjacent neuron is known as synapse. It is formed by pre synapticmembrane (of axon terminal) and post synaptic membrane (of dendrite). Pre and post synaptic membranes are separated by a narrow gap known as synaptic cleft synapses are of two types, viz., electrical synapses and chemical synapses.
A. In electrical synapse, the pre and post synaptic membranes are in close proximity. They have electrically conductive links called gap junctions. Impulse transmission across the electrical synapse is much faster when compared to that of chemical synapse.
B. In chemical synapse, the pre and post synaptic membranes are separated by a synaptic cleft. In these synapses, neurotransmitters (chemicals) are involved in impulse transmission. The axon terminals contain synaptic vesicles (filled with neuro transmitters) and mitochondria besides some other cell organells.

i. When the action potential reaches the axon terminal, it depolarises the presynaptic membrane. Thus voltage gated channels of calcium open. Ca++ ions stimulate the synaptic vesicles towards the presynaptic membrane where they fuse with it. Thus the neurotransmitter is released into the synaptic cleft. These neuro transmitter bind to the specific receptors of post synaptic membrane.
ii. The most common neurotransmitter is the acetyl choline. Epinephrine, norepinephrine, serotonin, dopamine etc., are either excitatory or inhibitory neurotransmitters. Glycine, Gamma Amino Butyric Acid (GABA) are inhibitory neurotransmitters.

iii. The post synaptic membrane has ligand gated channel (They are ion channels and respond to chemical signals - ligand). The entry of ions can generate a new action potential in the post synaptic membrane - which may be excitatory or inhibitory.
iv.  Excitatory post synaptic potentials (EPSP) cause depolarisation and inhibitory post synaptic potentials (IPSP) cause hyperpolarisation in post synaptic membrane.
v.  Then the neurotransmitter (e.g.: acetyl choline) is degraded by an enzyme ( e.g.: acetyl choline esterase).
vi.  The post synaptic potentials are graded potentials and summation of these potentials occur at the axon hillocks.
 Summation of inputs from many pre synaptic membranes is called spatial summation.

Summation of successive inputs from a single presynaptic membrane is called temporal summation.

REFLEX ACTION AND REFLEX ARC

1. An automatic, involuntary, instantaneous and uncoscious action brought about by the involvement of central nervous system is known as reflex action.
e.g.: Salivation of mouth at the thought of good food, pulling of hand away on touching a hot object.
2. The neural pathway that controls a reflex action is known as reflex arc.
3. Reflex arc consists of afferent neuron (sensory), internuncial neuron (in CNS) and efferent neuron (motor).
4. Afferent neuron receives signals from a sense organ and transmits the impulse though dorsal root of a spinal nerve. The efferent neuron carries signals to effector organ through the ventral root of spinal nerve. The afferent and efferent neurons are linked in CNS by the internuncial neuron.

SENSORY RECEPTION AND PROCESSING

EYE

        Eyes are the photoreceptors and are located in the orbits of the skull. Each eye has two components, viz., accessory structures and the eye ball.
1. Accessory structures:
a. Each eye is provided with a pair of movable eye lids (upper and lower) with eye lashes. They are useful for protection of eye.
b. Associated with the eye is a lacrimal apparatus (lacrimal glands/tear glands and their ducts) that produce and drain the tears (lacrimal fluid). Tears contains salts, mucus and an enzyme lysozyme (bactericidal in nature).
c. Associated with the eye ball there are six muscles, viz., superior, inferior, lateral, medial rectus muscles and superior and inferior oblique muscles. They are responsible for movements of eye ball. These muscles are innervated by III, IV and V cranial nerves.
 

2. Eye ball
a. Eye ball is spherical and its wall consists of three layers, viz., outer fibrous tunic, middle vascular tunic and inner nervous tunic.

b. Fibrous tunic: It is the outer wall of the eye ball. In front part, it is transparent (cornea) and the remaining part is known as sclera. Cornea is non vascular and covers the coloured iris. Cornea is covered by a thing transparent conjunctiva. Sclera provides support and shape to the eye.
 At the function of sclera and cornea is a channel called canal of Schlemm or scleral venous sinus.

c. Vascular tunic: It is the middle layer of eye ball and is also known as uvea. It has three portions
i. Choroid - Vascular and bluish.
ii. Ciliary body - In front, choroid forms the ciliary body. The muscles of it hold and alters the shape of lens.
iii. Iris - It is suspended between the cornea and the lens and is attached to outer margin of ciliary body. Iris is the coloured portion of eye ball. The aperture in the centre of iris is called pupil. Iris regulates the amount of light entering the vitreous chamber.

d. Nervous tunic: It is also known as retina. It is the inner layer of eye ball. Its non visual part is formed by pigmented epithelium and visual part is formed by neurons. These neurons are arranged in three layers, viz., photoreceptor layer (contains rods and cones), bipolar cell layer, ganglion cell layer.
  The process by which the eye changs optical power to focus on an object as its distance varies is known as accommadation.

 Rods contain rhodopsin or visual purple, a derivative of vitamin A. It is important for vision of eye under low light (Scotopic vision).

 Cones contain iodapsin (made of a protein - Photopsin) and is necessary for day light vision and colour vision. Cones are of three types, and they are sensitive to red, green and blue light respectively.
 Light passes through ganglion layer, bipolar cells and then reaches the photoreceptor layer.

 Ganglion cells only are capable of sending action potentials to brain.

e. The centre of the posterior part of retina is called yellow spot or macula lutea and the depression present in the centre of yellow spot is known as fovea centralis. (it contains cones only). It is responsible for sharp and central vision (useful in walking, reading, driving etc.)
f. All the axons of ganlion cells exit the eye ball as an optic nerve and it ends in visual cortex in the occipital lobe of brain.
g. The point of retina where the optic nerve exits the eye ball is known as blind spot (optic disc) (No photoreceptor cells and no vision at blind spot)
h. Just behind the pupil and iris, is a non vascular transparent lens. It was held in position by suspensory ligaments.
i. Due to iris and lens, the cavity of eye ball is divided into two chambers. The anterior chambers is known as aqueous chamber (filled with aqueous humor secreted by ciliary process and nourishes lens and cornea) and the posterior chamber is known as vitreous chamber (filled with vitreous humor - it contributes intra occular pressure and maintains shape of eye ball).
j. When the light rays focussed on retina (through cornea and lens). They generate impulses in rods and cones light induces the dissociation of photopigments. As a result, action potentials develop in ganglion cells. They are transmitted to the brain through optic nerve where the images are processed and the image is formed on retina.

EAR (Stato Acoustc Receptor)

        Ear is the Phonoreceptor (hearing) and also helps in maintenance of equlibrium. Ear consists of three parts, viz, external ear, middle ear and internal ear.
1. External ear: It consists of pinna, external auditory meatus and tympanum.
a. Pinna is a flap of elastic cartilage covered by skin. It collects the vibration in air that produce sound.
b. External auditory meatus is a curved tube that leads inwards and extend upto tympanum.
c. Tympanum or ear drum is a thin, semi transparent diaphragm located between the external auditory meatus and middle ear. It has connective tissue on the outer surface and mucus membrane on the inner surface.
 External ear contains fine hair and ceruminous glands. Hair and cerumen (ear wax) help in preventing dust particles in the ear.

2. Middle ear:
a. It is in the form of an air filled cavity in the temporal bone. This cavity is called tympanic cavity. It has ear drum on the outer side and is separated from internal ear on the inner side by a thin bony partition, having two membrane covered openings, viz fenestra ovalis and fenestra rotunda.
b. Middle ear contanis a chain of three small bones, viz, malleus, incus and stapes (these are called ear ossicles)
c. Connecting cavity of middle ear and pharynx, there is an eustachian tube. As a result, equal pressure is maintained on either side of the tympanum.
 

3. Internal ear (labyrinth):
a. Internal ear consists of two parts, namely, bony labyrinth and membranous labyrinth.
b. The bony labyrinth is a series of channels that can be divided into three areas, namely, cochlea, vestibule and semi circular canals.
i. Cochlea is a coiled portion like a watch spring. It contains three tubes, namely, scala vestibule, scala media and scala tympani/ cochlea duct.

ii. Scala vestibuli and scala media are seperated by Reissner's membrane and scala media and scala tympani are seperated by basilar membrane.

iii. Scala vestibuli and scala tympani are filled with perilymph and scala media is filled with endolymph.
iv. At the base of cochlea, the scala vestibuli ends at fenestra ovalis and scala tympani terminates at fenestra rotunda.
v. The cochlear epithelium from as a thickening called organ corti. It contains hair cells. They acts as auditory receptors from these cells cochlear branch of VIII cranial nerve arises and ends in the junction of pons and medulla.
vi. There are three semi circular canals, each one is at right angles to the other. The base of each canal is swollen (ampulla) and contains a ridge (crista - contain hair cells).
vii. Semi circular canals provide angular acceleration
C. The membranous labyrinth has two sacs, known as saccule and utricle. Saccule and utricle together constitute vestibular organ.
i. Saccule and utricle contain a projecting ridge, macula. It contains receptors of gravity.
ii. Saccule and utricle provide a sense of linear acceleration.
iii. Saccule percieves vertical movement and utricle provides horizontal movement.

iv. The three semi circular canals and the otolith organ together constitute the vestibular apparatus.
v. From the vestibular apparatus vestibular branch of VIII cranial nurve arises and ends in the floor of fourth ventricle.
   In response to the sound waves, the tympanum vibrates. These vibrations reach the fluid of cochlea through the ear ossicles through fenestra ovalis. Thus waves are generated in perilymph and endolymph, that generate ripple in the basillar membrane. These movements bend the hair cells pressing them against tectorial membrane. Hence, nerve impulse is generated, reach the brain through VIII cranial nerve, where the impulses are processed and sound is recognised.
 

Disorders of Nervous System

1. Meningitis:
 Inflammation of meninges of brain and spinal cord.
 May be caused by virus and rarely by certain drugs.
2. Parkinson's disease:
 Progressive disorder of CNS.

 Affects the movement, producing motor symptoms, which include autonomic disfunction, neuropsychiatric problems and sleep disturbances.
3. Stroke or cerebro vascular accident:
 Rapid decrease of brain functioning due to disturbance in blood supply to brain (may be due to ischemia or haemorrhage in brain)
 As a result, the affected area can not function, which leads to disability in the organs under the control of that area.
4. Alzeimeir disease:
 Common in people over 65 years of age.
 It is a progressive disease of brain due to loss of neurons, leading to loss of intellectual abilities, including memory.