The Nervous System

Our nervous system consists of the brain, spinal cord, sense receptors and nerves.

Functions of the Nervous System

  • Keeps us informed about the outside world through sensory organs.
  • Controls and harmonises all voluntary muscular activities, e.g. running and writing.
  • Enables us to remember, think and reason.

Neuron: The Unit of the Nervous System

Structure of the Neuron:

  • Cell Body: It has a well-defined nucleus and granular cytoplasm.
  • Dendrites: They are the branched cytoplasmic projections of the cell body.
  • Axon:
    • It is a long process of the cell body.
    • The axon is covered by a myelin sheath.
    • The myelin sheath shows gaps throughout its length known as Nodes of Ranvier.


  • A synapse is the point of contact between the terminal branches of the axon of a neuron and the dendrites of another neuron.
  • As the nerve impulse reaches the axon terminal of one neuron, the neurotransmitter acetylcholine is released by the bulbs present in the axon.
  • Acetylcholine is then broken down by an enzyme to ensure that the synapse is ready for them transmission of the next nerve impulse.
  • Also Read Photosynthesis

Transmission of Nerve Impulse

In the resting condition, the outer side of the nerve fibre carries a positive charge, i.e.
more Na+ ions outside the axon membrane. This is called the polarised state or polarisation of the nerve fibre.
On stimulation, the axon membrane at the site of stimulation becomes more permeable to Na+ ions. Thus, Na+ ions move inwards and results in loss of polarisation which is known as depolarised state or depolarisation of the nerve fibre. Such a region of the nerve fibre is known as the excited region.
The point of depolarisation becomes the stimulus for the next region of the axon
membrane which in turn becomes depolarised.
The previous region on the membrane becomes repolarised due to the active
transport of Na+ ions to the outside of the membrane.

Types of Neurons

  • Sensory Neurons: Convey the impulse from the receptors (sense organs) to the main nervous system (the brain or spinal cord).
  • Motor Neurons: Carry impulse from the main nervous system to an effector, i.e. muscle or gland.
  • Associated Neurons: They interconnect sensory and motor neurons.
  • Also Read The Circulatory System

Types of Nerves

  • A nerve is a bundle of nerve fibres (axons) of separate neurons enclosed in a tubular sheath.
  • Ganglia are an aggregation of the nerve cells (cell bodies) from which the nerve fibres may arise or enter.
  • Types of Nerves
    1. Sensory Nerves (contain sensory fibres)
    2. Motor Nerves (contain motor fibres)
    3. Mixed Nerves (carry sensory as well as motor fibres)

Division of the Nervous System

  1. Central Nervous System (CNS)
  2. Peripheral Nervous System (PNS)

The Central Nervous System

The central nervous system includes the brain and the spinal cord.

The Brain:

  • The human brain is well protected inside the cranium or the skull.
  • In adults, it weighs about 1.35 kg
  • It is protected by three meninges—dura mater, arachnoid and pia mater.
  • The space between the covering membranes, central spaces of the brain and the central canal of the spinal cord consists of cerebrospinal fluid which protects the brain from shocks.

Three Primary Regions of the Brain:

  1. Forebrain
    • The cerebrum is the centre of intelligence, memory, consciousness, will power and voluntary actions.
    • The thalamus relays pain and pressure impulses to the cerebrum.
    • The hypothalamus controls the body temperature and the activity of the pituitary gland.
  2. Midbrain
    • This small tube-like part is responsible for reflexes involving the eyes and ears.
  3. Hind Brain
    • The cerebellum coordinates muscular activity and balance of the body.
    • The pons carries impulses from one hemisphere to the other hemisphere and coordinates muscular movements on both sides of the body.
    • The medulla oblongata controls the activities of internal organs, heartbeat, breathing etc.

Parts of the Brain:

  • Cerebrum
    • The largest portion of the brain.
    • It is divided into two cerebral hemispheres connected to each other by the corpus callosum.
    • The cortex contains cell bodies of the neuron and is greyish in colour; hence, it is called grey matter.
    • The grey matter has many folds (i.e. gyri) and grooves (i.e. sulci).
    • The medulla consists of the axons of the nerve fibres and is called white matter.
  • Cerebellum
    • Located at the base of the cerebrum.
    • In a median section, its white matter appears like a branching tree.
  • Medulla Oblongata
    • Located at the base of the skull.
    • It is roughly triangular.
    • It continues behind the brain as the spinal cord.
    • Injury to the medulla oblongata results in death.

The Spinal Cord:

  • Lies within the neural canal of the vertebrae.
  • The grey matter is on the inner side and the white matter is on the outer side of the spinal cord.
  • Similar to the brain, it is covered with three meninges—dura mater, arachnoid and pia mater.
Functions of the Spinal Cord:
  • Responsible for reflexes below the neck.
  • Conducts sensory impulses from the skin and muscles to the brain.
  • Conducts motor responses from the brain to muscles of the trunk and limbs.

Peripheral Nervous System

The peripheral nervous system consists of nerves which carry impulses to and from the central nervous system.

  • Peripheral Nervous System
    • Somatic Nervous System (Cranial and Spinal Nerves)
    • Autonomic Nervous System
      • Sympathetic System
      • Parasympathetic Nervous System

Somatic Nervous System:

  • Cranial Nerves: 12 pairs emerge from the brain.
  • Spinal Nerves: 31 pairs: 8 pairs in the neck region, 12 pairs in the thorax, 5 pairs in the lumbar region, 5 pairs in the sacral region and 1 pair in the coccygeal region.

Autonomic Nervous System:

  • Sympathetic Nervous System
    • Nerves arise from the spinal cord between the neck and waist regions.
  • Parasympathetic Nervous System
    • Located anteriorly in the head and neck while posteriorly in the sacral region.


The reflex action is an automatic, quick and involuntary action in the body brought about by a stimulus.

Difference between Reflexes/Involuntary Actions and Voluntary Actions

Reflexes (Involuntary Actions)Voluntary Actions
Initiated by some stimulus such as touch, pain, pressure, heat, light etc.Initiated by a willing thought.
Commands originate in the spinal cord,
autonomic nervous system and a few in
the brain as well.
Commands originate in the brain.

Some examples of reflexes:

  • Shivering when it is too cold or sweating when it is too hot.
  • Non-stop beating of the heart.
  • Instant withdrawal of the hand when it accidently touches a hot iron.
  • Dilation of the pupil in eyes when looking in the dark.

The Sense Organs

  • The sense organs enable us to be aware of the condition of the environment.
  • A receptor is any specialised tissue or cell sensitive to a specific stimulus.
MechanoreceptorsReceptors of touch, i.e. pressure on the skin due to mechanical change.
ChemoreceptorsReceptors of taste of the tongue and smell of the nose due to chemical influence.
PhotoreceptorsReceptors of light present in rods and cones of the retina of eyes.
ThermoreceptorsHeat and cold receptors in the skin due to change in temperature.

The Eyes

  • The two eyes are located in deep sockets called orbits.
  • The upper and lower moveable eyelids protect the front surface of the eyes.
  • There are 6–12 tear glands.
  • Functions of the tear glands are
    • Lubricate the surface of the eye
    • Wash away the dust particles
  • A thin membrane which covers the entire front part of the eyes is called conjunctiva.
  • Due to viral infection of the conjunctiva, we suffer from eye disease called conjunctivitis.

Comparison between Rods and Cones:

More in number.Less in number.
Located at the periphery of the retina.Located in the centre of the retina.
Rapid generation of light-sensitive
pigment rhodopsin
Slow generation of light-sensitive
pigment iodopsin.


  • It is transparent, biconvex and crystalline.
  • It is held by a suspensory ligament which attaches the lens to the ciliary body.

Light and Dark Adaptation

Dark Adaptation
When we pass from a brightly lit area to a dark area, we experience difficulty in seeing the objects for a short while. This is called dark adaptation.

Light Adaptation
When we pass from a dark area to a brightly lit area, we experience a dazzling effect for a short period. This is called light adaptation.

Common Defects of the Eyes

Myopia (Short-sightedness)1. Near objects are seen clearly, but distant objects appear blurred.
2. The lens is too curved.
3. Myopia is corrected by suitable concave lens
Hyperopia (Hypermetropia/ long-sightedness)1. Difficulty in seeing nearer objects.
2. The lens is too flat.
AstigmatismSome parts of the object are seen in focus, while others appear blurred.
PresbyopiaObserved in older people. Near objects cannot be
seen clearly.
CataractThe lens turns opaque and the vision is reduced.
Colour blindnessColour blind people cannot distinguish between certain colours such as red and green.
Night blindness1. Difficulty in seeing in dim light.
2. Due to non-formation of rhodopsin in rod cells.
SquintThe eyes converge leading to cross eyes.

Stereoscopic Vision

Humans, monkeys and apes can perceive depth or the relative distance of objects. This is due to simultaneous focusing of an object in both eyes. The images of both eyes are overlapping and give a 3-dimensional effect.


When one looks at a brightly coloured object and then looks at a dark surface, an image of the object in the same colour will persist. This is known as persistence image or after-image.

The Ear

The human ear has the three following main divisions- Outer Ear, Middle Ear, Inner Ear.

Functions of the Ear:

  1. Hearing
    The pinna collects sound waves and conducts them through the external auditory canal. They finally strike
    on the ear drum and the vibration is set.
  2. Body Balance
    • The sensory cells in the semicircular canals are concerned with dynamic equilibrium, i.e. when the body is in motion.
    • The sensory cells in utriculus and sacculus are concerned with static equilibrium, i.e. when the body is stationary.

The Sense of Taste

  • The sense of taste is located in the taste buds of the tongue.
  • A taste bud is an ovoid group of sensory cells.
  • Substances enter the pore and stimulate the sensory hair of the sensory cells.

The Sense of Smell

  • The sense of smell is located in the epithelial layer of the nasal chamber.
  • The sense cells for smell have hair-like projections.
  • These hair-like projections respond to particles dissolved in the mucous secretion of the nose.
  • The impulse from these cells is then transmitted to the brain via the olfactory nerve.

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