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Grade 8
Aug 20, 2022

Key Concepts

  • Brain
  • Nerves
  • CNS, PNS
  • Reflex action and reflex arc


Brain is the most complex and largest organ present in the human body. It is made up of many nerves which communicate with other connections called synapses. The brain is one of the parts of the central nervous system

central nervous system

All parts of the brain play a very important role in processing information. Nervous system and its parts such as CNS, PNS and neuron helps in the processing of information. 

Functions of the Nervous System 

  1. It gathers information from inside and outside the body. This is a sensory function. 
  1. It transmits information to the various areas of the brain and spine. 
  1. It processes the information in the brain and spine. This is an integration function. 
  1. It sends information to the various muscles, glands, and organs so that they can respond appropriately.  
  1. Mainly, it controls and coordinates all essential functions of the body, including all other body systems allowing the body to maintain homeostasis. 


The function of the brain is to receive information from the body and send out the required instructions to particular organs. It is the main control unit of the nervous system. The brain is enclosed in the brain box (Cranium) and is protected by a cerebrospinal fluid that acts as a shock absorber. It has several layers called meninges

Parts of brain: 

Brain is enclosed within the skull, which provides protection from all sides such as frontal, lateral and back. The brain consists of four major parts: 

  1. The brain stem 
  1. The cerebrum 
  1. The cerebellum 
  1. The diencephalon 
  1. The brain stem: It is also called the medulla oblongata. It connects the diencephalon to the spinal cord. Its size is only about one inch long. 
  1. The cerebrum:  The largest part of the brain is the cerebrum. It is the main part of memory and intelligence. Cerebrum is divided into two hemispheres, namely left and right hemispheres. The two hemispheres are separated by a groove called the interhemispheric fissure or longitudinal fissure.  

Function of hemisphere: Left hemisphere controls the activities of right side of the body and vice versa. The hemispheres are further divided into four lobes. Each lobe is associated with a different function.  


Lobes of the Brain and their function: 

  • Frontal lobe: It is responsible for problem solving, judgement and motor skills. 
  • Temporal lobe: It is involved in memory processes such as visual memory – facial recognition, verbal memory- understanding language, interpreting emotions and hearing. It is located on either side of the head. 
  • Parietal lobe: It helps to manage body position, handwriting, and sensation. 
  • Occipital lobe: It contains a visual processing system. It is located at the back of the brain. 
  1. The cerebellum: It coordinates muscular activities and maintains balance and posture.  This is located below and behind the cerebrum. 

The medulla controls involuntary activities like blood pressure and beating of the heart.  

  1. The diencephalon: It includes the thalamus and hypothalamus. The thalamus is where sensory and other impulses go and combine. Diencephalon helps in regulating consciousness, alertness, and sleep. It also helps in relaying sensory and motor signals to the cerebral cortex. 
Human Brain

The role of nervous system in responding to stimuli: 

 Existence of an organism depends on its ability to respond to changes in the environment. The nervous system allows an organism to act fast, but the response given by organisms are short lived.  

The control systems that allow organisms to respond to changes are very important. All control systems include: 

Receptor cells: These are a group of cells that find out changes in the environment called stimuli.  In the nervous system, an electrical impulse being made in response to the stimulus. Group of receptors are present in sense organs that respond to specific stimuli. 

Sense organ Stimulus receptor 
Skin Pain, temperature, touch 
Tongue Taste of food items 
Nose Chemicals present in air, dust 
Eye Light 
Ear Sound, position of head 

Coordinator center: Such as brain, spinal cord or pancreas, which receive information from receptors around the body, processes the information received and sends the response. 


Effectors: Bring about responses and restore optimum levels. Effectors include muscles and glands, and responses can include muscle contractions or hormone release. For example: a gland releasing hormone in the blood, contraction of muscle to move an arm. 

Receptors are connected to effectors as follows: 

Sensory receptors Detect the changes in the environment (stimuli). 
Sensory neurons Nerve cells send a signal from the sensory receptors to the CNS. 
Central nervous system It contains brain and spinal cord. This coordinates and sends signals to motor neurons. 
Motor neuron Nerve cells receive signals from CNS and transfer them to effector. 
Effector The muscle or glands respond to stimulus. 

The nervous system helps an organism respond quickly to changes in the internal or external environment. The responses to the stimuli do not last long.  


  • It is the basic functional cell or unit of nervous system. They are adapted to carry electrical impulses from one place to another. A bundle of neurons is called a nerve. 
  • It transmits impulses up to 250 mph. 

Parts of a Neuron 

  • Axon is a long fibre that carries messages up and down the body over long distances.  
  • An electrical nerve impulse passes along the axon when a neuron is stimulated.  
  • A fatty (myelin) sheath is a covering that protects the axon and increases the speed of the nerve impulses along the neuron. 
  • At each end of the neuron, tiny branches called dendrons are present, which branch even further into dendrites. The dendrites receive incoming nerve impulses from other neurons. 
  • Cell Body – It consists of nucleus and most of the cytoplasm. 
  • Schwann Cells- These are the cells that produce myelin (which is a fat layer) in the peripheral nervous system. 
  • Myelin sheath – It is a dense lipid layer that insulates the axon and makes the axon look grey. 
  • Node of Ranvier – These are gaps or nodes in the myelin sheath. 
  • Path of impulse — Impulses travel from dendrite to cell body to axon. 

Three types of Neurons 

  1. Sensory neurons – The function of sensory neurons is to send messages from the sense organs to the brain or spinal cord (CNS). 
  1. Motor neurons – The function of motor neurons is to carry messages back from the brain or spinal cord (CNS) to all the muscles and glands in the body. 
  1. Interneurons – Its function is to carry both sensory and motor nerves. 

Peripheral Nervous System 

The Peripheral nervous system is divided into two parts:  

i. Somatic nervous system   ii. Autonomic nervous system 

The Somatic Nervous System: 

The somatic nervous system is responsible for transmitting sensory information as well as for voluntary movement. This system contains two major types of neurons: 

Motor neurons (efferent neurons, Latin name “efferre” meaning “to bring away from”): The function of these neurons is to transmit the signals originating in the central nervous system to the various organs and muscles and put them into action as per the orders received from the brain. Motor neuron allows us to take physical action in response to the stimuli in the environment. 

Sensory neurons (afferent neurons, Latin name “afferre” meaning “to bring towards”): These neurons carry information from the nerves to the central nervous system. It allows us to take in sensory information and send it to the brain and spinal cord. 

Reflex action and Reflex arc 

Reflex actions are involuntary and very immediate movements in response to stimuli. For example: when a person accidentally touches a hot plate, he automatically jerks his hand away from the plate without thinking. A reflex does not require any thought input. Here the heat stimulates temperature and danger receptors located in the skin by triggering a sensory impulse that travels to the central nervous system. Then sensory neurons synapse with interneurons which connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal. 

Some natural pathways called reflex arcs act in an instinct before the impulse reaches the brain, and this makes a reflex or a reflex action. 

Reflex arcs are formed in the spinal cord itself, although the information input goes on to reach the brain. 

Effectors: Effectors can be muscles and glands. Their job is to produce a specific response to a detected stimulus. 

The flow of signal in a reflex arc is as follows: 

 reflex arc

Majority of the reflex arcs involve only three neurons: relay neuron, motor neuron and sensory neuron. 

The stimulus, such as a needle stick, stimulates the pain receptors of the skin, which initiate an impulse in a sensory neuron.  This impulse travels to the spinal cord and passes, by means of a synapse, to a connecting neuron called the relay neuron that is situated in the spinal cord. 

The relay neuron makes a synapse with motor neurons that transmit the impulse to the muscles of the limb and makes them contract and pull away from the sharp object. Reflex actions do not require any involvement of the brain, although in some cases, the brain can avoid reflex action. 

Reflex Action

In the above diagram: 

  • Receptor in the skin detects a stimulus (the change in temperature). 
  • Sensory neuron sends electrical impulses to a relay neuron, which is located in the spinal cord of the CNS. Relay neurons connect sensory neurons to motor neurons. 
  • Motor neuron sends electrical impulses to an effector. 
  • Effector produces a response (which is shown as muscle contracts to move the hand away).  

Organisms are able to adjust a reflex action and overcome it. For example, keeping grip of a hot object requires a nerve impulse to be sent to the motor neuron of the reflex arc to involve with the normal reflex action to drop the item. 

Synapses: It is a small gap where two neurons meet. 

Types of Reflex Arcs 

The reflex arcs are of two types.  

  1. Autonomic reflex arc, which affects inner organs. 
  1. Somatic reflex arc, which affects muscles.  

Stimulus and response related to various organs: 

Pupillary light reflex: In this type of reflex, if a light is flashed near one eye, we know that the pupils of both eyes contract. In this case, light is the stimulus, and the impulses reach the brain through the optic nerve. Then the response is taken to the pupillary musculature by autonomic nerves that connect the eye.   

Lacrimal reflex: When foreign particles irritate the conjunctiva or cornea of the eye, the lacrimal reflex causes nerve impulses to reach the midbrain. These nerve fibres stimulate the lacrimal glands of the orbit, causing the outpouring of tears.  

Cough and sneeze reflexes: These are the reflexes of the midbrain and medulla oblongata. The cough reflex is caused by a foreign particle in the trachea, and the reflex is given by a sneeze in the nose. In this case, the reflex response involves many muscles in order to remove the foreign particle. 


  • The nervous system is a complex, sophisticated system that regulates and coordinates body activities.
  • It is made up of two major divisions: Central nervous system (CNS) and peripheral nervous system (PNS)
  • CNS consists of the brain and spinal cord.
  • Reflex actions are involuntary and very immediate movements in response to stimuli.
  • Reflex arc is a special type of neural circuit that begins with a sensory neuron at a receptor and ends with a motor neuron at an effector.
  • Receptor cells are a group of cells that finds out changes in the environment called stimuli.
  • Effectors produce a specific response to a detected stimulus.
  • The reflex arcs are of two types: Autonomic reflex arc and somatic reflex arc.



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