Grade 8 Science Assessment 1 – Unit 2 Nervous System

 Grade 8 Science Assessment 1 – Unit 2   Nervous System 

 

NERVOUS SYSTEM

Whenever a person gets injury on his foot while walking, he feels pain and his hand immediately reaches the injured site. Who asked the hand to reach the site? In fact, there is an organ system in our body which carries messages from one part of the body to another and coordinates body functions. This system is called nervous system. Human nervous system consists of central nervous system (CNS) and peripheral nervous system (PNS) (Figure 2.1). The central nervous system is composed of brain and spinal cord. Peripheral nervous system consists of a network of nerves which connect the central nervous system to all parts of the body.

Neuron or Nerve Cell

Neuron or nerve cell is the basic structural and functional unit of the nervous system. All parts of the nervous system, i.e., brain, spinal cord and nerves are made up of neurons. Neurons transmit messages in the form of electrochemical waves called nerve impulses.

The part of a neuron which contains nucleus and most of the cytoplasm is called cell body. The fine projections of the cell body which receive messages are called dendrites. A long projection of the cell body which conducts messages away from the cell body is called axon. Terminal ends of the axon transmit the messages to the next cells.

Nerve

A nerve is cable-like bundle of axons enclosed in a common sheath. Nerve transmits messages from one part of body to another.

Types of Neurons

On the basis of their functions, neurons are of three types, i.e. sensory neurons, motor neurons and interneurons.

• Sensory neurons carry nerve impulses from sense organs (ears, eyes, skin, tongue, nose etc.) to the central nervous system.
• Motor neurons carry nerve impulses from central nervous system to effectors (muscles and glands), i.e., the parts which respond.
• Inter-neurons are present in central nervous system (brain and spinal cord). They form a link between sensory and motor neurons (Figure 2.3).

Interesting Information:

Impulses may travel as fast as 150 meters per second or as slow as 0.2 meters per second.

Central Nervous System (CNS)

Central nervous system acts as a control center of the whole nervous system. It comprises brain and spinal cord.

Brain

Human brain (Figure 2.4) is enclosed in a bony skull called cranium, and consists of billions of inter-neurons. It is divided into the following parts:

1.     Forebrain:

Forebrain is the largest part of the brain. It consists of three main parts, i.e., cerebrum, thalamus and hypothalamus.

                     I.            Cerebrum is the topmost and the largest part of the brain. It is divided into right and left cerebral hemispheres. Each hemisphere controls the opposite side of the body. In general, the left hemisphere controls speech, comprehension, arithmetic, and writing. The right hemisphere controls creativity, spatial ability, artistic, and musical skills. Inside cerebrum there is a small structure called

                   II.            Thalamus. It controls many sensory functions.

                  III.            Hypothalamus lies at the base of thalamus. It controls body temperature, hunger and thirst.

2.     Midbrain:
Midbrain is a small part of the brain which is present below the cerebrum. It receives information from sense organs which is then passed to the appropriate part of the forebrain.

3.     Hindbrain:
Hindbrain consists of three parts, i.e., cerebellum, pons and medulla oblongata.

                                I.            Cerebellum:

Lies under the back part of the cerebrum. It acts as a controller for maintaining the body balance and making precise and accurate movements.

                             II.            Pons:

Is an oval structure present beneath midbrain. It controls many functions like sleep, swallowing, equilibrium and taste, etc.

                           III.            Medulla oblongata:

Forms the posterior part of the brain where it is connected with the spinal cord. Medulla oblongata controls heartbeat, breathing and digestion, etc. Medulla oblongata keeps on working when rest of the brain goes to sleep. 

Spinal Cord: 

Spinal cord is an extension of medulla oblongata (Figure 2.5). It runs backwards inside the backbone up to its lower end. It is also made up of interneurons.

Spinal cord creates a link between brain and different body parts. It also controls some reflex actions (immediate and involuntary actions) and some other involuntary actions. 

Peripheral Nervous System:

Peripheral nervous system (PMS) consists of a network of nerves which spread in the body to connect all the body parts to the central nervous system (brain and spinal cord) (Figure 2.1). The nerves which arise from brain are called *cranial nerves. The nerves which arise from spinal cord are called **spinal nerves*. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves in human body. 

Working Model of the Nervous System

Nervous system coordinates all body functions. It also detects the changes in environment and produces response to the changes. Figure 2.6 shows the working of the nervous system as described below: 

 Any change in the environment (external or internal) that can be detected by a receptor to initiate a nerve impulse is called *stimulus* (Plural: stimuli). Heat, cold, pressure, sound waves, etc. are the examples of stimuli. The special organs, tissues or 

Sensory and Motor Neurons:

·        The sensory neurons carry the messages regarding stimuli in the form of nerve impulses from receptors to central nervous system. 

·        The central nervous system processes the messages and transmits the nerve impulses to motor neurons. 

·        The motor neurons carry the nerve impulses to the parts of the body which produce responses. Such parts are called effectors. Muscles and glands in the body act as effectors. 

Sensory neuron → Processing (interneuron) → Motor neuron → Effectors 

Actions Controlled by the Nervous System

Voluntary actions:

The body actions which are performed under conscious control, i.e., which are done after thinking over them are called voluntary actions. For example; speaking, eating, reading, walking, running, clapping, etc., are voluntary actions. 

Involuntary actions

The body actions which are performed without involvement of thinking process are called involuntary actions. Involuntary actions are not performed under conscious control. 

Examples of Involuntary Actions 

Heartbeat, breathing, blinking of eyes, movement of small intestine, etc., are the examples of involuntary actions. 

REFLEX ACTION 

An immediate and involuntary response to a stimulus is called reflex action. Quick pulling of hand just after touching the hot object is a common example of reflex action. 

In this example of reflex action shown in temperature of hot object is a stimulus which is received by the cells (thermoreceptors) of the skin. A nerve impulse is created in the sensory neuron present in skin. The nerve impulse is carried by the sensory neuron to the spinal cord. The inter-neuron of the spinal cord transmits the impulse to the motor neuron. The motor neuron carries the impulse to the arm muscles (effectors). The arm muscles contract and the hand is pulled back. The pathway of nerve impulses which complete a reflex action is called reflex arc. It consists of receptor-a sensory neuron, an inter-neuron, a motor neuron and effector. 

Reflex Arc Pathway: 

Thermoreceptor in skin → Sensory neuron → Spinal cord (inter-neuron) → Motor neuron → Effectors (Muscles of arm) 

Reflex arc representing reflex action. Touching a hot object (flame in this example) results in immediate withdrawal of hand through contraction of muscles of arm.