The control and coordination in human beings take place through nervous system and hormonal system which is called endocrine system.
The five sense organs in our body, eyes, ears, nose, tongue and skin are called receptors. This is because they receive information from the environment around us. Therefore, a receptor is a group of cells in sense organ which is sensitive to a particular type of stimulus like light, sound, smell, taste, heat, etc.
All the receptors send message in the form of electrical impulses to spinal cord and brain through sensory nerves. Another type of nerves called motor nerves then transmits response from brain and spinal cord to effectors. An effector is a part of the body which responds to a stimulus according to the instructions sent from the nervous system. The muscles and glands are effectors of the body.
Human Nervous System
Nervous system coordinates the activities of our body. It controls all our behaviour, thinking and actions. It is through nervous system only that all other systems of our body work. It passes information from one internal system to another. For example, when we put food in mouth, it causes release of saliva from the salivary glands.
The Unit of Nervous System: Neurons
A neuron (or nerve cell)
The cells that make up the nervous system are called neurons. Neuron is the largest cell in the body. The structure of neuron is such that it can carry messages in the body quickly. These messages are in the form of electrical impulses or nerve impulses. There are three components of neurons. They are:
i) Cell body
The cell body of a neuron contains cytoplasm and a nucleus. There are many long and thin fibres coming out of the cell body of a neuron. The short fibres are called dendrites and the long fibre is called axon. The axon is covered with an insulating and protective sheath called myelin. It is made of fat and proteins.
The messages transmit through nervous system is in the form of electrical impulses called nerve impulses. The dendrites pick nerve impulses or messages from receptors and send them to cell body and then to axon. The axon sends these impulses to another neuron through a junction called synapse. There are three types of neurons, sensory neurons, motor neurons and relay neurons.
i) Sensory neurons transmit messages from receptors towards the central nervous system that is the spinal cord and brain.
ii) The function of motor neurons is to transmit messages from central nervous system towards the muscle cells or effectors.
iii) Relay neurons serve as link between other neurons. They are present in central nervous system.
The small gap between the two neurons is called synapse. The nerve impulses are carried over this small gap through a chemical substance called neurotransmitter.
The sensory cells or receptors are in contact with dendrite of sense organs. When there is a stimulus which acts on the receptor a chemical reaction occurs which produces an electrical impulse in it. This impulse travels from dendrite of sensory neuron to its cell body and then along the axon. At the end of axon electrical impulse release tiny amount of chemical substance in synapse and similar electrical impulse is started in the dendrite of next neuron. In this way the electrical impulse is carried in neurons till it reaches the relay neurons in spinal cord and brain. The relay neurons and motor neurons are connected in the similar way and helps bring electrical impulses from brain and spinal cord to the effectors like muscles and glands. Synapse ensures that electrical impulse travel in one direction only. This is because the chemical substance is present on one side of the gap only.
Organs of Human Nervous System
The main organs of human nervous system are:
• Spinal cord
The brain is located inside the skull of the head. The spinal cord is a thick nerve which is located in the cavity of backbone. The upper end of spinal cord is attached to the brain. The nerves are distributed in all parts of the body. They are like wires.
The brain and spinal cord are connected to all the sense organs and other parts of the body through nerves.
The three types of nerves in our body are:
• Cranial nerves
• Spinal nerves
• Visceral nerves
The cranial nerves connect all parts of the head to brain, spinal nerves connect all the remaining parts of the body to spinal cord and visceral nerves connect internal organs of the body to spinal cord and some to brain.
The cranial nerves, spinal nerves and visceral nerves are of two types, sensory nerves and motor nerves. Sensory nerves are those which carry message from body to brain and spinal cord and motor nerves are those which carry message from brain and spinal cord to the body parts for action.
Working of Nervous System
When any sense organ of our body is affected it sends message to the brain in the form of electrical impulses. This message is send through sensory neurons. The brain analyses the message and decides the action to be taken. The brain then sends instructions for that body part through motor nerves.
Both brain and spinal cord are involved in complicated responses which require thinking and in simple responses which don’t require thinking only spinal cord is involved.
Parts of the Nervous System
The two main parts of nervous system are:
1) Central nervous system (consisting of brain and spinal cord)
2) Peripheral nervous system (consisting of all nerves of the body)
Peripheral nerves can further be divided into two parts.
i) Voluntary nervous system
ii) Autonomic nervous system
The Peripheral Nervous System
The peripheral nerves system is made of all the nerves of the body. There are three types of nerves which make the peripheral nervous system, cranial nerves, spinal nerves and visceral nerves. All these nerves enter or leave the central nervous system.
Reflex Action and Reflex Arcs
This diagram shows the reflex action and its path (which is called reflex arc)
Reflex action is the one which we perform automatically and is not under the voluntary control of the brain. Under reflex action same stimulus produces same kind of response every time. Knee jerk, movement of diaphragm, coughing, sneezing, yawning, blinking of eyes, immediately moving hand away on unknowingly touching hot plate, etc., are examples of reflex action.
These are the actions which we do without thinking to protect ourselves. Reflex action is an automatic response to a stimulus.
The pathway taken by nerve impulses in a reflex action is called reflex arc. The reflexes which involve only spinal cord are called spinal reflexes. The reflexes which involve brain are called cerebral reflexes.
Cerebral reflexes occur in the organs present in the head. These organs are directly connected to the brain. For example, when we are in dim light the pupil of our eye is big so that more light can enter into our eyes and when we are in bright light then the pupil of our eye automatically gets small. This automatic expansion and contraction of pupil is an example of cerebral reflexes.
How Effectors Cause Action or Movement
The effected muscles cause action because motor nerve impulses sent by the spinal cord or brain reach the effector organ.
A reflex Arc (This is actually a spinal reflex arc)
The muscles are able to move or act in response to electrical nerve impulse because muscles are made of muscle cells. These cells contain special protein which can change their shape and contract. So when the muscle cells contract, muscles also contract. And when the muscles contract they pull on the bones of that body part and thus, make it move.
A reflex arc showing cerebral reflex action
Autonomic Nervous System
Autonomic nervous system is a part of peripheral nervous system. It controls the activities of the organs inside our body automatically. This specific network of nerves controls the processes like breathing, heartbeat, digestion, sweating, etc. The nerves of autonomic nervous system are attaches with the smooth muscles of various internal organs of a body like head, heart, blood vessels, alimentary canal, lungs, kidneys, glands, skin, etc.
Voluntary Nervous System
Those actions which are performed by humans knowingly and need thinking are called voluntary actions. For example, writing, dancing, cycling, etc. Therefore, voluntary nervous system helps us take voluntary actions which are under the conscious control of brain.
Central Nervous System
Central nervous system consists of brain and spinal cord. It is responsible for the control and coordination of the activities of nervous system. The function of a central nervous system is to respond to the messages it receives. It directs the motor neurons that are connected to the part of the body which will respond to the stimulus.
The central nervous system collects information from all the receptors in our body.
The highest coordinating centre in our body is brain which is located inside the skull. Brain is protected by a bony structure in the skull called cranium. The three membranes called meninges surround the brain and protect it. The space between meninges is filled with a cerebro spinal fluid which helps the brain from mechanical shocks. Cranial nerves arise from the brain.
Brain is divided into three parts, forebrain, midbrain and hindbrain. The forebrain consists mainly of cerebrum, midbrain doesn’t have any further division and hindbrain consists of pons, cerebellum and medulla.
The main thinking part of the brain is cerebrum. It is a place for faculties such as learning, reasoning, intelligence, personality, etc. All our thoughts and actions are controlled by the cerebrum. The association area of cerebrum controls thinking and memory and stores information and experiences. The sensory area of cerebrum receives information from sense organs and gives sensation. The motor area of the cerebrum sends instructions to various muscles to perform different function. All the voluntary actions are coordinated by the cerebrum.
The function of hindbrain is to control reflex movements of head, neck and trunk in response to visual and auditory stimuli. It also controls the reflex movements of the eye muscles.
Pons helps in regulating respiration, cerebellum helps in maintaining posture and balance of the body and medulla controls various involuntary actions such as heart beat, breathing, blood pressure, coughing, sneezing, secretion of saliva, etc.
Spinal cord begins from medulla and extends downwards. Spinal cord is enclosed in bony structure called vertebral column. There are 31 pairs of nerves that arise from spinal cord. It is also surrounded by membranes called meninges. Spinal cord is concerned with the spinal reflexes and conducts nerve impulses to and from the brain.
Functions of Brain
• Brain receives nerve impulses which carry information from the sense organs of the body.
• Brain responds to these impulses by sending instructions to muscles and glands which function accordingly.
• Brain correlates stimuli from different sense organs and produce appropriate and intelligent response.
• It coordinates the activities of the body.
• It stores information.
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