Integrated Guidance Programme of General Studies for IAS
(Pre) - 2013
Subject - General Science
Chapter : The Nervous System
Multicellular animals must monitor and maintain a constant internal environment
as well as monitor and respond to an external environment. In many animals,
these two functions are coordinated by two integrated and coordinated organ
systems: the nervous system and the endocrine system. Three basic functions
performed by nervous systems are
- Receive sensory input from internal and external environments
- Integrate the input
- Respond to stimuli
Divisions of the Nervous System
The nervous system monitors and controls almost every
organ system through a series of positive and negative feedback loops.
The Central Nervous System (CNS) includes the brain and
The Peripheral Nervous System (PNS) connects the CNS to
other parts of the body, and is composed of nerves (bundles of neurons).
Not all animals have highly specialized nervous systems.
Those with simple systems tend to be either small and
very mobile or large and immobile.
Large, mobile animals have highly developed nervous
systems: the evolution of nervous systems must have been an important
adaptation in the evolution of body size and mobility.
Vertebrate Nervous System
The vertebrate nervous system is divided into a number of
parts. The central nervous system includes the brain and spinal cord. The
peripheral nervous system consists of all body nerves. Motor neuron pathways are
of two types: somatic (skeletal) and autonomic (smooth muscle, cardiac muscle,
and glands).The autonomic system is subdivided into the sympathetic and
Peripheral Nervous System
The Peripheral Nervous System (PNS) contains only nerves and
connects the brain and spinal cord (CNS) to the rest of the body. The axons and
dendrites are surrounded by a white myelin sheath. Cell bodies are in the
central nervous system (CNS) or ganglia. Ganglia are collections of nerve cell.
bodies. Cranial nerves in the PNS take impulses to and from the brain (CNS).
Spinal nerves take impulses to and away from the spinal cord. There are two
major subdivisions of the PNS motor pathways: the somatic and the autonomic.
Somatic Nervous System
The Somatic Nervous System (SNS) includes all nerves
controlling the muscular system and external sensory receptors. External sense
organs (including skin) are receptors. Muscle fibers and gland cells are
Autonomic Nervous System
The Autonomic Nervous System is that part of PNS
consisting of motor neurons that control internal organs. It has two
subsystems. The autonomic system controls muscles in the heart, the smooth
muscle in internal organs such as the intestine, bladder, and uterus. The
Sympathetic Nervous System is involved in the fight or flight response. The
Parasympathetic Nervous System is involved in relaxation. Each of these
subsystems operates in the reverse of the other (antagonism).
Both systems innervate the same organs and act in
opposition to maintain homeostasis. For example: when you are scared the
sympathetic system causes your heart to beat faster; the parasympathetic
system reverses this effect.
Central Nervous System
The Central Nervous System (CNS) is composed of the brain
and spinal cord. The CNS is surrounded by bone-skull and vertebrae. Fluid
and tissue also insulate the brain and spinal cord.. The brain is composed
of three parts: the cerebrum (seat of consciousness), the cerebellum, and
the medulla oblongata (these latter two are “part of the unconscious brain.
The medulla oblongata is closest to the spinal cord, and
is involved with the regulation of heartbeat, breathing, vasoconstriction
(blood pressure), and reflex centers for vomiting, coughing, sneezing,
swallowing, and hiccupping. The hypothalamus regulates homeostasis. It has
regulatory areas for thirst, hunger, body temperature, water balance, and
blood pressure, and links the Nervous System to the Endocrine System.
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During embryonic development, the brain first forms as a
tube, the anterior end of which enlarges into three hollow swellings that form
the brain, and the posterior of which develops into the spinal cord. Some parts
of the brain have changed little during vertebrate evolutionary history
The Brain Stem and Midbrain
The brain stem is the smallest and from an evolutionary
viewpoint, the oldest and most primitive part of the brain. The brain stem
is continuous with the spinal cord, and is composed of the parts of the
hindbrain and midbrain. The medulla oblongata and pons control heart rate,
constriction of blood vessels, digestion and respiration.
The midbrain consists of connections between the
hindbrain and forebrain. Mammals use this part of the brain only for eye
The cerebellum is the third part of the hindbrain, but it is
not considered part of the brain stem. Functions of the cerebellum include fine
motor coordination and body movement, posture, and balance. This region of the
brain is enlarged in birds and controls muscle action needed for flight.
The forebrain consists of the diencephalon and cerebrum.
The thalamus and hypothalamus are the parts of the diencephalon. The
thalamus acts as a switching center for nerve messages. The hypothalamus is
a major homeostatic center having both nervous and endocrine functions.
The cerebrum, the largest part of the human brain, is
divided into left and right hemispheres connected to each other by the
corpus callosum. The hemispheres are covered by a thin layer of gray matter
known as the cerebral cortex, the most recently evolved region of the
The Spinal Cord
The spinal cord runs along the dorsal side of the body and
links the brain to the rest of the body. Vertebrates have their spinal cords
encased in a series of (usually) bony vertebrae that comprise the vertebral
column. The gray matter of the spinal cord consists mostly of cell bodies and
dendrites. The surrounding white matter is made up of bundles of interneuronal
axons (tracts). Some tracts are ascending (carrying messages to the brain),
others are descending (carrying messages from the brain). The spinal cord is
also involved in reflexes that do not immediately involve the brain.
Nervous tissue is composed of two main cell types: neurons
and glial cells. Neurons transmit nerve messages. Glial cells are in direct
contact with neurons and often surround them. The neuron is the functional unit
of the nervous system. Humans have about 100 billion neurons in their brain
alone! While variable in size and shape.
The junction between a nerve cell and another cell is called
a synapse. Messages travel within the neuron as an electrical action potential.
The space between two cells is known as the synaptic cleft. To cross the
synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are
stored in small synaptic vessicles clustered at the tip of the axon.
Neurotransmitters tend to be small molecules, some are even hormones. The
neurotransmitters cross the cleft, binding to receptor molecules on the next
cell, prompting transmission of the message along that cell’s membrane.
Facts from NCERT
Eyes of a crab are quite small but they enable the crab
to look all around.
Butterflies have a large eye that seems to be made up of
thousands of little eyes. They can see not only in the front and the sides
but the back as well.
Owl can see very well in the night but not during the day
Kite, eagle can see well during the day but not in the
The owl has large cornea and a large pupil to allow more
light in its eye. Also it has on its retina a large number of rods and only
a few cones.
The day birds on the other hand have more cones and fewer
Louse Braille himself a visually challenged person
developed a system for visually challenged persons and published it in 1821.
Braille system has 63 dot pattern or characters.
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