The Human Nervous System | Functions of neurons | Cell body of a neuro | Functions of the central nervous system | Function of ciliary muscles | Blind spot of the eye | What is blind spot | Brain structure and function | Functions of spinal cord | What is an axon | Iris of the eye | Parasympathetic and sympathetic nervous system

Brain structure and function

Brain structure and function

Nervous system is fully restricted to animals and not found in plants. It is mainly ectodermal in origin and functions in an integrated manner for control and coordination of various body functions (responding, understanding and memorising) through neurons and hence also called as neural system.

The neural system and endocrine system work together to control and coordinate various body parts to maintain physiology. The neural system is basically a control system which involves highly specialised cells, called neurons.

Neurons (Basic unit of nervous system)

It functions by detecting and receiving information from different sense organs, i.e., receptors in the form of stimuli and transmit the stimuli to the central nervous system. Neuron or nerve cell is a structural and functional unit of neural tissue. They are known to be the longest cells present in the human body. Human neural system has about 100 billion neurons and they are mostly present in brain.


Structurally, neuron has following two main parts

Cell Body

It is also known as cyton or soma that vary in size and form. It may be irregular, spherical, oval, round, star-shaped or pyramidal. They have neurofibrils and Nissl’s granules that play a characteristic role in the body. Neurofibrils play a significant role in the impulse transmission while Nissl’s granules or Nissl’s bodies are like rough endoplasmic reticulum with numerous attached free ribosomes and polysomes, i.e., synthesise proteins for the cell.

Some neuro-tubules are also present that help in maintaining the shape of the neuron. Function Thess are mainly associated with metabolic maintenance and growth.


The processes associated with neurons are called neurities. These are mainly of two types

1.Dendrites  ::- 

These are usually shorter, tapering and much branched structures, which contains neurofibriles, neurotubules and Nissl’s granules. They conduct nerve impulse towards the cell body and this process is called afferent process (i.e., the receiving process).

 2. Axon

It is a very long structure of uniform thickness. The part of cyton from where the axon arises is known as axon hillock. It is mainly dependent on cell body for the supply of protein.

Each branch of axon fibre terminates as a bulb like structure called synaptic knob that possess some chemicals called neurotransmitters. Axons mainly conducts nerve impulse away from the cell body and therefore the process is called as efferent process.

Axons are mainly of two types

Myelinated   Axons are nerve fibres enveloped with Schwann cells that form a myelin sheath around the axon. It mainly found in spinal and cranial nerves.

Unmyelinated    Axons are nerve fibre enveloped by a Schwann cell that does not form a myelin sheath around the axon and is commonly found in autonomous and motor neurons somatic neural system.

Types of Neurous

There are mainly three types of neurons
  1. Sensory Neurons   (Afterent neurons) They conduct impulses from receptors to central nervous system. The terminal ends of dendrites become modified to form receptor and acts as sensory.
  2. Inter Neurons   They act as connectors and functions in connecting the sensory and motor neuron. It mainly carries stimuli in brain and spinal cord.
  3. Motor Neurons    They conduct impulses from central nervous system to the effectors i.e.. they mainly transmit impulse from brain and spinal cord to the muscle or gland, which will respond to the stimulus.
On the basis of number of dendrites and axons neurons can be of other three types
  1. Multipolar (with one axon and two or more dendrites)
  2. Bipolar i.e., (with one axon and one or more dendrites)
  3. Unipolar i.e., with one axon only.

Parts of Human Nervous System

 Human nervous system mainly constitutes three different parts

  1.  Central Nervous System (CNS) 
  2. Peripheral Nervous System (PNS) 
  3. Autonomic Nervous System (ANS)

Central Nervous System (CNS)

 It is mainly comprised of brain and spinal cord.

 1. Brain 

It is the anterior portion of the CNS which is lodged in the cranial cavity, i.e., cranium of the skull. It weighs from 1220 to 1400 grams.


Stratenlly it consists of three membranes (meninges)

  1. Piamater membrane     linnamast thin very delicate, vascular and inverts the brain closely)
  2. Arachnoid membrane   (outer to piameter, thin, spider webby structure) 
  3. Durameter membrane    (outermost, tough fibrous membrane, adhering closely to the inside of the skull).

Human brain structurally consists of three main parts

  1. Fore brain (Prosencephalon)
  2. Mid brain (Mesencephalon) 
  3. Hind brain (Rhombencephalon)

Forebrain It includes olfactory lobes, cerebrum and diencephalon.

  1. Olfactory Lobes    are the anterior part of the brain formed by a pair of short club shaped structures. They are fully covered by the cerebral hemisphere . Function of these olfactory Lobes are concerned with the sense of smell.
  2.  Cerebrum   is the largest and most complex part of the brain. It mainly have two hemispheres, (i.e., left and right) connected by a large bundle of myelinated fibres, the corpus callosum and small bundle of fibres.


 The outer portion of cerebrum is called the cerebral cortex. The surface of the cortex is greatly folded. The upward folds are known as gyri. There is a cavity present inside the cerebrum whose outer part is known as grey matter and the inner part (i.e., beneath the grey matter) is the white matter.

Functions of Cerebrum   It has sensory areas that receives impulse from receptor and motor area that transmits impulse to the effectors.


The main parts involved in diencephalon are epithalaums, thalamus and hypothalamus.

  • Thalamus   represents the lateral walls of diencephalon and mainly formed of grey matter. It is the relay station of sensory inputs and does not produce hormones.
  • Epithalamus   thin membrane tissue that acts as a posterior segment of the diencephelon
  • Hypothalamus    contains a number of centres that control urge for eating and drinking. It also called as thermostat of the body, because it acts as thermoregulatory centre. It also integrates and controls the visceral activities.

Functions of Diencephalon  It control and coordinate various functions like body temperature, feeling of satisfaction, appetite, thirst etc.

Mid Brain

It is mainly the portion located in between the thalamus/hypothalamus of the forebrain and pons of the hind brain. There is a canal known as cerebral aqueduct that passes through the mid brain.

Mid brain and hind brain together forms the brain stem.

Functions of Midbrain
  • It relays impulse back and forth between cerebrum, cerebellum, pons and medulla .
  •  It is also concerned with sense of sight.

Hind Brain

 It involves some parts like cerebellum and pons, meduila .


It is the second largest part of the human brain. It has convoluted surface in order to provide more space for many more neurons.

Functions of Cerebellum 

It controls rapid muscular activities such as running typing and even talking. Nearly all activities are involuntary in nature but com involve learning in early stages. 


 It is located below mid brain and above the medulla oblongata.

Functions of Pons  ::

 It relays impulses between medulla to regulate breathing.

Medulla (Oblongata)

It extends from the pons varolii in above direction and is continuous with the spinal cord in below direction.

 Functions of Medulla

It functions as receiver, integrates signals from spiral cord and sends resulting signal to the cerebellum and thalamus. It mainly have centres that regulate heart rate, blood pressure, breathing. swallowing salivation, sneezing, coughing, vomiting and other involuntary movements etc.

Spinal Cord (Myelon)

 It extends from the base of the brain and is continuous to second lumbar vertebra In adult, the spinal cord ranges from 42 to 45 cm in length. It mainly lies in the neural canal of the vertebral column. It is basically the posterior part of CNS which runs mid-dorsally within the vertebral column. The three meninges, i.e., duramater arachnoid and piameter which covers the brain, also continue over the spinal cord.

The two indentations, i.e., posterior median sulcus and the anterior median tissues separates the spinal cord into left and right halves. The inner area is the grey matter while outside to it are white columns called the white matter. 

Functions of Spinal Cord

  • The stimuli passes from and to the brain through the spinal cord.
  •  It also act as the centre of spinal reflex action.

Peripheral Neural System (PNS)

The nerves that originate from central nervous system connect either with receptor or effector organs from peripheral neural system and those nerves which arises from brain are called cranial nerves while the nerves originating from the spinal cord are termed as spinal nerves. It relays impulse from the CNS to skeletal muscles. In human body there are 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Autonomic Neural System (ANS)

It transmit impulse from the CNS to the involuntary organs and smooth muscles of the body.This system was dicovered by Langly  1921 . It is further divided into two types 

Sympathetic Nervous System

Accelerales heart beat, enlarge pupils, supply blood to muscles, contract nerves of urinary bladder, lowers the intestinal digestion activities, helps in blood clotting increased secretion of sweat glands, make breathing easier and promote liver to release sugar and decrease bile production are some activities controlled by this nervous system. 

Parasympathetic Nervous System

Works just analogus to the sympathetic nervous system, i.e., slows down heart beat. dilates arteries and lower blood pressure, speeds up peristalsis, stimulate salivary gland secretion contracts gall bladder, promotes pancreas for secretion, decreases the secrection of sweat glands etc.

 Sensory Reception and Processing

 Sensory organ includes the organs that respond to the stimuli and send signal to the CNS where all the inputs are processed and analaysed, signals are then sent to different parts or centres of the brain. e.g., for vision-eye, for hearing-ear, for smell-nose, for taste -tongue and for touch-skin.


Humans have a pair of eyes situated in the sockets of the skull called orbits. The eye ball is mainly spherical in structure.

A human eye has the following main parts .

Cornea  ::- 

It is the transparent spherical membrane covering the front of the eye. Light enters the eye through this membrane  Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea.

 Lens ::- 

 It is the central part of the eye that facilitates the image formation.

Iris ::- 

It is dark muscular diaphragm between the cornea and the lens. It controls the size of the pupil. It is the colour of the iris that we call colour of the eye.

 Pupil ::- 

It is a small hole between the iris through which light enters the eye. In  dim light, it opens up completely due to contraction of iris muscles, but in bright light it becomes very small due to relaxation of iris muscles.

Ciliary  Muscles ::- 

They hold the lens in position and help in modifying the curvature of the lens.

Retina ::- 

It is the light-sensitive surface of eye on which the image is formed it hind light-sensitive cells rods and cones. Rod cells respond to the intensity of light and cones respond to the illumination. There number is around 125 million. Number of rod cells is greater than number of cone cells. These cells generate signals which are transmitted to the brain through optical nerves.

Optic Nerves ::- 

It transmits visual information from the retina to the brain.

 Sclera ::- 

 It is an opaque, fibrous, protective, outer layer of an eye containing collagen and elastic fibre. It is also known as white of the eye.

Blind Spot ::- 

It is the point at which the optic nerves leave the eye. It contains no rods  and cones, so an image formed at this point is not sent to the brain.

Aqueous Humour

Behind the cornea, we have a space filled with a transparent liquid called the aqueous humour and behind this there is a crystalline lens, which helps to refract light to be focused on retina and maintains intraocular pressure.

Vitreous Humour 

The space between eye lens and retina is filled with another liquid called – vitreous humour.

Mechanism of Vision

The process of vision take place in the following steps

  • The light rays in visible wavelength focuss on retina through cornea and lens generate impulse in rods and cones. 
  • Human eyes are composed of opsin (protein) and retinal (aldehyde of vitamin-A). Light induces dissociation of retinal from opsin resulting changes in the structure of opsin. This cause change in membrane permeability.
  • Consequently, the potential difference generated in the photoreceptor cells. 
  • This produces a signal that generates an action potential in ganglion cells through bipolar cells.
  • The action potential or impulses are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analysed.
  • The image formed on the retina is recognised based on earlier memory and experience.


The human ear participate in the following two functions, i.e., hearing and maintenance of balance of the body. Structurally, an ear is divided into three major sections, i.e., Outer ear, middle ear and the inner ear.


Outer ear further consists of two main parts

  • Pinna  ::–  It collects vibrations in the air that produce sound.
  • External Auditory Meatus  ::- It leads inwards and extends up to the tympanic  ear drum). Middle ear contains three ear ossicles, i.e., malleus, incus and stapes, which are attached to one another in a chain like manner. The ear ossicles mainly increase the efficiency of transmission of sound waves to the inner ear. There is an eustachian tube that connects the middle ear cavity with the pharynx and equalise the pressure on either sides of the ear drum. The fluid-filled in the inner part of the ear is called labyrinth which consist of two parts, i.e., bony labyrinth and membranous labyrinth. The inner portion of ear also contains a complex system called vestibular apparatus, which is mainly responsible for the maintenanance of balance of the body and posture .

Mechanism of Hearing

(i) The external ear receives sound waves and direct them to  ear drum.

(ii) It vibrates in response to the sound waves and these vibrations are transmitted through ear ossicles to oral window (attached to stapes).

(iii) The vibrations are passed through the oval window on the fluid of the cochlea, where waves generate.

(iv) Nerve impulses are generated in associated afferent neurons.

(v) These impulses are transmitted by the afferent fibres via auditory nerves to the auditory lartex of the brain, where the impulses are analysed and the sound is recognised.

Other Sense Organs

Nose (Smell)

The nose is the organ responsible for the sense of smell. The cavity of the nose is lined with mucous membranes that have smell receptors connected to the olfactory nerve. The smell themselves consist of vapours of various substances. The smell receptors interact with the molecules of these vapours and transmit the sensations to the brain. The nose also has a structure called the vomeronasal organ whose function has not been determined, but which is suspected of being sensitive to pheromones that influence the reproductive cycle. The sense of smell is sometimes temporarily lost when a person has a cold. Dogs have a sense of smell that is many times more sensitive than that of a man.

Skin (Touch)

The sense of touch is distributed throughout the body. Nerve endings in the skin and other parts of the body transmit sensations to the brain. Some parts of the body have a larger number of nerve endings and therefore, are more sensitive. Four kinds of touch sensations can be identified cold, heat, contact, and pain. Hairs on the skin magnify the sensitivity and act as an early warning system for the body. The fingertips and the sexual organs have the greatest concentration of nerve endings.

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