What is stimuli?
The senses that evoke responses in organisms are called
stimuli.
Name some stimuli formed inside the body?
¢ Hunger
¢ Thirst
etc.
What are receptors?
There are specialized cells in the sense organs and other
parts of the body to receive stimuli. These cells are known as receptors. They
receive stimuli and generate suitable impulses.
How do responses occur?
Responses occur when the impulses generated by the receptors
on receiving the stimuli reach the brain. The nervous system controls and
co-ordinates these actions.
Neuron
What is the basic structural unit of the nervous system?
Neuron or nerve cell is the basic structural unit of the
nervous system.
Functions of the parts of neuron
Dendron
• Short filament from the cell body.
• Carries impulses from dendrites to the cell body.
Dendrite
• Branches of Dendron.
• Part that receives impulses from adjacent neuron.
Axon
• Longest filament from the cell body.
• Carries impulses from the cell body to outside.
Axonite
• Branches of axon.
• Carries impulses to the synaptic knob.
Synaptic knob
• Tip of axonite.
• Secretes neurotransmitter.
Schwann cell
• Encircles the axon.
Myelin sheath
Axons of most of the neurons are repeatedly encircled by myelin, a membrane containing lipid. This is called myelin sheath.
Formation of Myelin sheath
Myelin sheath in the nerves is formed of Schwann cells.
Myelin sheath in the brain and the spinal cord is formed of specialized cells
called oligodendrocytes.
Grey matter and White matter
The myelin sheath has a shiny white colour. The part of the brain and the spinal cord where myelinated nerve cells are present in abundance is called white matter and the part where non-myelinated nerves cells are present is called grey matter.
Functions of myelin sheath
To
provide nutrients and oxygen to axon
To
accelerate impulses
To
act as an electric insulator
To
protect the axon from external shocks
Generation and Transmission of Impulses
The nervous system manages control and coordination through
impulses. The outer surface of the plasma membrane of the neuron is positively
charged and the inner surface is negatively charged. This is due to the
difference in the distribution of certain ions. When stimulated, the
distribution of ions in that particular part changes and hence the inner
surface becomes positively charged and the outer surface becomes negatively
charged. This momentary charge difference stimulates its adjacent parts and
similar changes occur there too. As this process proceeds, impulses get
transmitted as electric charges. Nerve impulses are messages transmitted
through the neurons.
Synapse is the junction between two neurons or a neuron and
a muscle cell or a neuron and a glandular cell. When electric impulses from the
axon reach the synaptic knob, certain chemical substances are secreted from
there to the synaptic cleft. These chemical substances are called
neurotransmitters. They stimulate the adjacent dendrite or cell and new
electric impulses are generated. Acetylcholine and dopamine are examples of
neurotransmitters. Synapse helps to regulate the speed and direction of
impulses.
Different types of neurons
How are neurons classified? Differentiate them.
Ans:
On the basis of the direction of impulse, neurons can be
classified into sensory neurons and motor neurons. Sensory
neurons carry impulses to the brain and spinal cord. Motor neurons carry
impulses from the brain and spinal cord to various parts of the body.
Nerve
What are nerves? How are they classified? Write their
functions also?
Nerves are group of axons or nerve fibres covered by
connective tissue.
Nerves are classified into three based on their functions.
Nervous system
The nervous system consists of two parts namely the central
nervous system and the peripheral nervous system.
Brain
Which is the central part of nervous system?
Ans: Brain
How is brain protected?
The skull, three layered meninges and cerebrospinal fluid
filled within the inner membranes of meninges and the ventricles of the brain
protects the brain from external injuries.
What are the functions of cerebrospinal fluid?
The functions of the cerebrospinal fluid are to provide nutrients and oxygen to the tissues of the brain, regulate the pressure inside the brain and to protect the brain from injuries.
Characteristics and functions
Cerebrum
Cerebrum
is the largest part of the brain.
It
has numerous fissures and folds.
The
grey coloured outer part of cerebrum is called
Cortex and the white coloured inner part is called Medulla.
It
is the centre of thought, intelligence, memory and imagination.
It
evokes sensations.
It
controls voluntary movements.
Cerebellum
Cerebellum
is the second largest part of the brain.
It
is seen behind the cerebrum as two flaps.
It
has fissures and grooves.
It
coordinates muscular activities and maintains equilibrium of the body.
Medulla oblongata
The
rod shaped medulla oblongata is seen below the cerebrum, located near the
cerebellum.
Controls
involuntary actions like heart beat, breathing etc.
Thalamus
Thalamus
is situated below the cerebrum.
It
acts as relay station of impulses to and from the cerebrum.
Analyses
impulses from various parts of the body and sends the important ones to the
cerebrum.
Hypothalamus
Hypothalamus
is situated just below the thalamus.
It
plays a major role in the maintenance of homeostasis.
Spinal Cord
Structure of spinal cord
The spinal cord is the continuation of the medulla oblongata
is protected inside the vertebral column and also covered by meninges. The
central canal seen in the centre of the spinal cord is also filled with
cerebrospinal fluid. In the spinal cord, white matter is seen outside and grey
matter is seen inside. There are 31 pairs of spinal nerves arising from the
spinal cord. A dorsal root and a ventral root join to form a spinal nerve.
How is the dorsal root differ from ventral root?
Sensory impulses reach the spinal cord through the dorsal
root. Motor impulses go out of the spinal cord through the ventral root.
Impulses from different parts of the body are transmitted to and from the brain
through the spinal cord. It also coordinates the repeated movements during
walking, running etc.
Functions of spinal cord
Impulses
from different parts of the body are transmitted to and from the brain through
the spinal cord.
It
also coordinates the repeated movements during walking, running etc.
Effects
certain reflex actions.
Reflex Actions
The accidental and involuntary responses towards stimuli are
called reflex actions. They do not happen consciously. There are
cerebral and spinal reflexes.
What are the parts that involve in a reflex arc?
A reflex arc involves:
Stimulus
receiving receptor
Sensory
neuron
Inter
neuron
Motor
neuron
Effecting
muscles
Complete the pathway of
impulse during a reflex action when one touches a flame or a hot body accidently.
What do you mean by
spinal reflex? Give example.
Reflex actions in which
spinal cord acts as the centre are called spinal reflexes.
Eg: the reflex action
when one touches a flame or a hot body accidently.
What are called
cerebral reflexes? Give example.
The reflexes under the
control of cerebrum are called cerebral reflexes.
Eg: we blink our eyes
when light suddenly falls on our eyes
Autonomous Nervous
System
Autonomous nervous system
is a part of the peripheral nervous system which controls the activities that take place beyond the
conscious level. The sympathetic system and the parasympathetic
system together form the autonomous nervous system.
Certain diseases
affecting the nervous system
Let's assess questions and answers
3) Analyse the following situations and answer the questions.
• a thorn pierces the foot.
a)Write the stimuli and
responses.
b) Was the leg withdrawn
after sensing the pain? Identify the reflex action. Prepare an illustration
showing the parts through which the impulses were transmitted.
Ans:
a)Stimuli - A thorn
pierces the foot
Response – the leg is withdrawn
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