for class XII

Nervous system of man


As complexity in animals increases, the coordination of different parts of body among themselves and with the external environment becomes necessary. Human body is complex and highly developed. In human beings and other higher animals, the nervous system and endocrine system not only control and coordinate various cellular activities but also help animal to respond to stimulus. The nervous system is comparatively faster and localized in action.

Functions
• It receives changes in external environment by receptors and after interpreting and analyzing, sends the appropriate massages to effectors. The sensory organ like tongue, nose eye ears skin helps to receive stimulus from environment.
• It conducts information and transmits massages to various parts of body.
• It stimulates and inhibits the activities of various muscle and glands according to the kind of information received.
• It receives and coordinates the activities of various visceral organs in the body.
• By coordinating different activities it helps to maintain steady state of body.

Types
a. Central nervous system CNS which includes brain and spinal cord
b. Peripheral nervous system PNS which includes cranial nervous and spinal nerves.
c. Autonomic nervous system which includes nerve cells, ganglia etc

The central nervous system
The brain and spinal cord both are supported and protected by skeleton. The brain is protected by cranium and spinal cord is protected by vertebral column. Brain and spinal cord are covered by three membranes together known as meninges.
1. outer membrane duramater
2. middle membrane arachnoid
3. inner membrane piamater

Duramater is tough and made up of fibrous tissue. It is attached to skull. The middle membrane is soft and thin. Piamater is delicate and vascularised.
Subarachnoid space – it is space between arachnoid and piamater. An extracellular fluid called cerebrospinal fluid CSF is present in subarachnoid space, ventricles of brain and central canal of spinal cord. The CSF is slightly alkaline fluid. It is kept in circulation by beating of ciliated cells(ependymal) lining ventricles and central canal. It is secreted by anterior and posterior choroid plexus.


Functions of CSF
• It allows exchange of respiratory gases, nutrients and wastes between it and nervous tissue. It is both nutritive and excretory in function.
• It protects the CNS against mechanical shock and injury. It acts as cushion.
• It maintains the constant pressure inside cranium in spite of fluctuations in volume and pressure of blood.

Both brain and spinal cord show two distinct regions called Grey matter and White matter.

Differences between gray and white matter

Gray matter White matter
Gray in color White in color due to presence of fatty myelin sheath around nerve fiber.
Consist of cell bodies, dendrite and synapses of neuron. Consists of nerve fibers axon arising from or to nerve cell present in gray matter.
Contains numerous intermediate neurons Mainly consists of axons of neurons connecting various body of brain and links brain to spinal cord.
In brain, gray matter is found outside and white matter inside. The spinal cord, white matter forms outer layer and gray matter forms central core.


Human Brain
It is highly specialized delicate organ of the human body. It is present in a bony case called cranium. Cranium protects brain form external injuries. The brain of adult weighs about 1350 grams. It is mainly composed of soft nervous tissue. The cranial capacity of human is about 1450 cc.

The human brain can be divided into 3 parts
Fore brain (Prosencephalon)
Mid brain (Mesencepahlon)
Hind brain(Rhombencephalon)















Fore brain has 3 parts – Cerebrum, Thalamus and Hypothalamus

Cerebrum - it is largest part and divisible into right and left Cerebral Hemisphere (CH). Two CH are joined by broad curved and thick band of nerve fiber called Corpus Callosum. Each hemisphere has frontal lobe, parietal lobe and temporal lobe and occipital lobe. Outer layer is called cerebral cortex. It is made up of gray matter and has numerous fold like convolutions. The ridges of convolutions are known as gyri and depressions in between are known as sulci. These greatly increase surface area.

The cerebral cortex has different areas like motor area (movement), sensory area (heat and cold pain touch light pressure) auditory area (hearing), visual area (seeing), olfactory area (taste and smell), speech area etc.


Functions
1. Main center that governs all mental activities like intelligence, memory, reason, will, feelings, emotions etc.
2. Seat of consciousness, interpreter of sensations, origin of voluntary acts.
3. Also acts as control on many reflex acts that originates involuntary like weeping, laughing etc.

Thalamus
It lies between Cerebrum and mid brain. It consists of 2 rounded masses of gray matter.
It is found at the center of cerebrum.
Functions
1. Serves as relay center for sensory and motor impulses from spinal cord and brain stem to various parts of Cerebrum.
2. Regulates emotions and perceptions of heat cold pain etc.


Hypothalamus
It is present beneath Thalamus. It consists of gray matter scattered in white matter.
Functions
1. Controls internal mechanism or autonomic nervous system.
2. associated with temp regulation, water balance, hunger, blood pressure etc
3. In association with pituitary gland, secretes neurohormones.

Mid brain
It connects fore brain with hind brain. It possesses 2 pairs of round elevations called Corpora quadrigemina. The gray matter is found to be scattered in white matter. It is covered by CH. It controls the eye movements and auditory response. The floor of mid brain has thick crura cerebri. It contains bundles of fibers coming from hind brain and spinal cord passing forward.



Hind brain
It consists of Cerebellum on dorsal side and brain stem (pons and medulla) on ventral side.

Cerebellum - it is found at the back of head. It consists of 2 cerebellar hemispheres just like CH. It gives appearance of two halves of a large walnut. It is large reflex center for coordination of muscular body movements and maintained of posture or equilibrium. Cerebellar hemispheres are not convoluted but traversed by furrows. The central part is known as vermis.

Functions
• Coordinate muscular body movement, equilibrium and control posture.
• Control reflex action of skeletal muscle activities.

Pons varolii- it lies above medulla. It acts as bridge to connect two halves of cerebellum. It coordinates muscle movements on two sides.

Medulla oblongata – it is posterior most part of brain. It continues behind into spinal cord. Various ascending and descending tracts cross over from right to left and left to right. Any damage to one of its side causes paralyses on the opposite side of body.

Functions
• It carries nerve tracts connecting spinal cord to brain. All communications between brain and spinal cord pass through it.
• Center for vital activities
• Contains cardiac, respiratory and vasomotor center that control complex activities like heart action, respiration, coughing and sneezing etc.

Ventricles of brain.

These are cavities within the bran. They are 4 in number.
Right and left lateral ventricles lie within the CH. These communicate with the third ventricle (ventricle of Thalamus) by foramen of Monro. The third ventricle communicates with the fourth ventricle (ventricle of Medulla) by cerebral aqueduct or iter. The fourth ventricle is lozenge shaped and continues behind as central canal of spinal cord.

Spinal cord
It is tubular or cylindrical structure extending from Medulla Oblongata. It is situated in the neural canal of vertebral column. It is covered by all three membranes. The cerebrospinal fluid is present in the central canal and in between membranes. The white matter is found outside and Grey matter is found inside. The grey matter is in the the H shaped of butterfly shaped form. The dorsal and ventral horns come out from the Grey matter. In the middle, there is central canal. It is filled with cerebrospinal fluid and continuous with the ventricles of brain.


Peripheral nervous system
It consists of two types of nerves. Cranial nerves are arising from brain and spinal nerves arising from spinal cords.
A nerve is composed of many nerve fibers enclosed in a connective tissue sheath. Nerve fiber is long axon of a neuron . it could be myelinated or non myelinated.
Types of nerve fiber
Depending upon the direction of nerve impulse carried by nerve fiber, they are classified into
• Afferent nerve fiber which conduct impulses from peripheral tissue or organ (receptors) to CNS. It is also called sensory neuron
• Efferent nerve fiber which conduct nerve impulse from CNS to peripheral tissue or organs or effectors. It is also called motor neurons

Cranial nerves
There are 12 pairs of cranial nerves. They arise from different parts of Brain. These nerves innervate different tissue of organ to carry out different functions. They are sensory or motor or mixed in function.

Name of nerve nature Tissue innervated function
1. olfactory sensory Olfactory mucosa in nose smell
2. optic sensory Retina vision
3. oculomotor motor Eye muscle Eye movement
4. trochlear motor ,, ,, ,, ,,
5. trigeminal mixed Skin, teeth, mucosal membrane of mouth Sensation of head and face
6. abducens motor Eye muscle Eye movement
7. facial mixed Taste buds, salivary gland, muscle of face and neck Facial expression, saliva secretion
8. auditory sensory Internal ear Equilibrium, hearing
9. glossopharyngeal mixed Pharynx tongue Taste, sensation, swallowing
10. vagus mixed Pharynx, oesophagus, trachea, viscera Visceral reflex
11. spinal accessory motor Thoracic and abdominal viscera Visceral reflex, shoulder movement
12. hypoglossal motor Muscle of tongue movement


Sensory -- 1st , 2nd , 8th
Motor -- 3rd , 4th , 6th , 11th , 12th
Mixed -- 5th , 7th , 9th , 10th


Spinal nerves

There are 31 pairs of spinal nerves in man. They arise from the spinal cord. They innervate different parts of body. They are all mixed in nature.

Cervical nerves = 8 pairs
Thoracic nerves = 12 pairs
Lumbar nerves = 5 pairs
Sacral nerves = 5 pairs
Coccygeal nerves = 1 pair

C 8 Th 12 L 5 S 5 Co 1


Autonomic nervous system

It consists of nerves that control and coordinate activities of visceral organ like heart, lung, kidney, arteries, vein etc. It controls the functioning of visceral organ so, it is known as visceral nervous system.
Properties
• Basically motor system – they carry impulses from CNS to effectors
• Involuntary in action, controls activities like heart beat, peristaltic movement
• Consists of preganglionic fiber, autonomic ganglion and post ganglionic fiber etc
• Preganglionic nerve fiber is made up of motor neuron emerge from CNS and enter into autonomic ganglion
• Autonomic ganglion is swollen bulbous structure in which preganglionic fiber terminate and synapse with cell bodies of post ganglionic fiber
• Axon from cell bodies emerge out and pass to organ concerned

Sympathetic nervous system
They emerge from thoraco lumbar region. At the end of post ganglionic fiber, nor adrenaline is secrected. So, they are called adrenergic in function.
• Can stimulate many organ
• Prepare body for emergency
• Increase heart beat
• Dilate pupil
• Rise arterial BP

Parasympathetic nervous system
They emerge from cranio sacral out flow. At the end of post ganglionic fiber, acetyl choline is produced. So they are called cholinergic in nature.
Properties
• Associated with daily normal activities
• Stimulation of gastric, pancreatic secretion
• Gastro intestinal movement
• It has inhibitory effect on organ
• Brings about relaxation, comfort
• Prevent over working of organ
• Restore normalcy of organ


Transmission of nerve impulse

Prosser defined the nerve impulse as the sum of mechanical, chemical and electrical disturbances created by a stimuli in a neuron. The most accepted mechanism f nerve impulse conduction is Ionic theory prepared by Hodgekin and Huxley. This theory states that nerve impulse is an electrochemical event governed by differential permeability of neurilemma to Na + and K+ which in turn is regulated by the electrical field. The transmission of nerve impulse takes place always in one direction. It is very rapid process. About 1000 impulses can be carried in one second time along the same nerve fiber. It consists of following steps.
• Polarisation (resting potential) -- the extracellular fluid has high concentration of Na+ and low concentration of K+ . Out side the membrane, there are 10 times more Na+ and inside there are 25 times more K+ . The neurilemma is less permeable to Na+ and more permeable to K+ . the neurilemma shows – 70 to – 90 milli volts (mV). Out side the membrane there are + charges and inside there are – charges.
• Depolarization (action potential)-- when it receives stimuli, the permeability changes. It becomes more permeable to Na+ and less permeable to K+ . Now Na+ ions rush inside and K+ ions out side. Due to rapid inflow of Na+ the potential increases to 0 and then to +45 to +50 mV. Inside the membrane the + charges are developed and out side, - charges are developed. The newly developed potential difference is called action potential.
• Repolarisation -- after peak of action potential called spike potential, permeability to Na+ decreases and permeability to K+ increases. Na + ions are pumped out and K+ ions are taken in. This is called Sodium Potassium pump or simply Sodium pump. Sodium pump is a process of expelling out Sodium ions and drawing in Potassium ions against the concentration and electrochemical gradient. Out side the membrane there develop the + charges and inside, there develop the – charges. It is called repolarisation. The axon is made ready to receive another impulse.

Transmission of nerve impulse along the medullated nerve fiber

The medullary sheath is not permeable to both Na + and K+ ions. So, the ionic exchange occur only at the nodes of Ranvier. The action potential is conducted from node to node in a jumping manner. Nerve impulse is conducted faster ( about 20 times) in myelinated nerve fiber. It is called salutatory conduction.


Conduction of nerve impulse through synapse
In a synaptic cleft, the impulse is always carried from axon to dendrite. It is assisted by neurotransmitter like acetyl choline. It is purely a chemical event. Calcium ions help in this process.