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Subjects /Biology / The Nervous System

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INTRODUCTION
08 May 2021

  • All animals respond according to the environment surrounding them. For example, if it is cold outside, the body itself starts generating more heat so as to maintain the body temperature.
  • Or, when we touch a hot cup of tea, our mind simulates to remove our hand from the surface.
  • In above examples, we saw the control of our body and the coordination among different body parts.
  • All multicellular organisms have a specialised system of organs that maintain the control and coordination in such organisms.
  • In humans or animals, such control and coordination is provided by the neurons and muscular tissues.
  • In nervous system, there is a wide network of nerves that are spread throughout the body. These nerves carry the messages of environmental changes, if any, from the sensitive organs to the brain that help in coordination among different organs.

Nervous system of human is divided into three parts:

  • Central Nervous System
  • Peripheral Nervous System
  • Automonic Nervous System

Function of Receptors

  • All the information from our environment is detected by our sense organs i.e. inner ear, nose, tongue or skin.
  • These sense organs has specialised receptors that detect any change in our environment.
  • These receptors are located at the end of some nerve cells present in the sense organs.
  • For example, gustatory receptors detect taste while olfactory receptors detect smell and so on.
  • Olfactory receptors help us to detect the smell of an incense stick (agarbatti) by sending the signals to the brain. Then based on the information stored in our brain, we identify the smell of the incense stick.

The function of the receptors include:

  • Acquiring information through sensory organs from the outer environment.
  • The information so acquired at the end of the dendritic tip of a nerve cell sets off a chemical reaction that creates an electrical impulse.
  • They convert the energy from the external and internal environment into electrical impulse.
  • Receptors are connected to the central nervous system through the nerve fibres.

The electric impulse travel in the body through the following process:

  • The electric impulse travels from the dendrite to the cell body, and then along the axon to its end.
  • At the end of the axon, the electrical impulse sets off the release of some chemicals.
  • These chemicals cross the gap or synapse and start a similar electrical impulse in a dendrite of the next neuron.
  • A similar synapse finally allows delivery of such impulses from neurons to other cells, such as muscles cell or gland.
  • Nervous tissue is made up of an organised network of nerve cells or neurons, and is specialised for conducting information via electrical impulses from one part of the body to another.

Reflex Actions

  • The word ‘reflex’ means some sudden action in response to something in the environment.
  • For example, when you put your hand on something very hot, you immediately pull back your hand. This pulling back of hand is your reflex action.
  • Thus, there are some actions that we do without thinking or without feeling in control of our reactions.
  • The process of control and coordination of such actions involves a complicated interaction of many nerve impulses from many neurons.
  • For this purpose, the body consists of dense networks of intricately arranged neurons. It sits in the forward end of the skull, and receives signals from all over the body which it thinks about before responding to them.
  • In order to receive these signals, this thinking part of the brain in the skull must be connected to nerves coming from various parts of the body.
  • Similarly, if this part of the brain is to instruct muscles to move, nerves must carry this signal back to different parts of the body.

Reflex Arc

  • The nerves that detect heat are connected to the nerves that move muscles in a simpler way, the process of detecting the signal or the input and responding to it by an output action might be completed quickly. It is faster than sending the impulses to the brain and getting back the reaction.
  • Such a connection is called a reflex arc.
  • The reflex arc connections are made between the input nerve and the output nerve, where they first meet each other.
  • Nerves from all over the body meet in a bundle in the spinal cord on their way to the brain.
  • Reflex arcs are formed in this spinal cord itself, although the information input also goes on to reach the brain.
  • The control and coordination of reflex actions is conducted by the spinal cord.
  • Reflex arcs have evolved in animals because the thinking process of the brain is not fast enough. In fact, many animals have very little or none of the complex neuron network needed for thinking.
  • So, it is quite likely that reflex arcs have evolved as efficient ways of functioning in the absence of true thought processes. However, even after complex neuron networks have come to existence, reflex arcs continue to be more efficient for quick responses.

What is the role of brain in reflex actions?

  • The brain does not have any significant role in reflex actions.
  • The purpose of the reflex actions is to provide quick and involuntary response to any sudden change in the environment.
  • Reflex arcs are formed in the spinal cord and are found in the muscular tissue that contain the necessary information to react as per the changing circumstances.
  • However, the brain also stores such information and remember it for future use.
  • For example, a 4-year-old kid when touches the hot iron for first time, there is no such reflex arc formed. He burns his hand and now that event is stored in his brain for future. Also, to respond to such an event reflex arc connection is also formed. Next time, when he will see a hot iron, he would remember what would happen next.
  • Also, mid-brain controls the simple reflex actions like increasing the size of pupil to absorb more light in case of darkness.

Central Nervous System

  • It is the part of the nervous system which keeps control on the body as well as the nervous system. It receives information from all parts of the body and integrate it.
  • Central nervous system consists of:
    • Brain
    • Spinal Cord
  • Brain is located in a bony box called craninum which protects it from external injuries.
  • Spinal cord is made up of nerves which supply information to brain and other body parts.

Brain


  • Brain is the main coordinating centre of the body.
  • The brain and spinal cord constitute the central nervous system.
  • The brain allows us to think and take actions based on our surrounding.
  • Different parts of the brain are responsible for integrating different inputs and outputs.
  • Brain sits inside a bony box containing fluid-filled balloon which provides shock absorption in case of injury.

The brain has three such major parts or regions, namely:

  • Fore-brain
  • Mid-brain
  • Hind-brain

Fore-brain

  • It is the main thinking part of the brain.
  • It has regions which receive sensory impulses from various receptors.
  • Separate areas of the fore-brain are specialised for hearing, smell, sight and other things.

How decision is made in fore-brain?

  • There are separate areas of association where this sensory information is interpreted by putting it together with information from other receptors as well as with information that is already stored in the brain.
  • Based on all this, a decision is made about how to respond and the information is passed on to the motor areas which control the movement of voluntary muscles, for example, our leg muscles.
  • However, certain sensations are distinct from seeing or hearing for example, sensation of feeling full, feeling associated with hunger, which is in a separate part of the fore-brain.

Fore-brain consist of:

  • Cerebrum:
    • It is the most developed part of the brain.
    • It is the centre of wisdom memory, movements knowledge and thinking.
    • The analysis and coordination of muscular movement received from sense organs.
  • Thalamus
    • It is the centre of cold, pain and heat.
  • Hypothalamus
    • It controls the hormonal secretion of endocrine glands. Blood pressure is also controlled by it.
    • It is the centre of hunger, thirst, temperature control, love, hate and other feelings.

Mid-brain

  • It controls the involuntary actions like heart-beat, breathing, digestion.
  • It also controls simple reflex actions like change in the size of the pupil.

Corpora quadrigemina

  • It is a part of mid-brain.
  • It is the centre of control over vision and hearing power.

Hind-brain

  • Involuntary actions like blood pressure, salivation and vomiting are controlled by the hind-brain.

The parts of the hind-brain are:

  • Cerebellum
    • It is located at the back of the head.
    • It is responsible for precision of voluntary actions and maintaining the posture and balance of the body. (Guess what! When someone consumes alcohol, his cerebellum part is affected. That is why he is not able to walk properly).
    • It is large reflex centre for coordination of muscular body movements.
  • Pons
    • It functions as a bridge between the brain and the spinal cord.
  • Medulla
    • It connects and communicate the brain with spinal cord.
    • It controls all the involuntary actions like blood pressure, salivation and vomiting.

Movement

  • Nervous Tissue collects information, sends it around the body, processes information, makes decision based on information, and conveys decisions to muscles for action.
  • In short, nerves send the impulses and the muscle tissues perform the last action or movement.
  • When the nerve impulse reaches the musclem the muscle fibre move by changing the shape of its cells. Muscle cells have special proteins that change both their shape and their arrangement in the cell in response to nervous electrical impulses.
  • When this happens, new arrangements of these proteins give the muscle cells a shorter form and help in the movement of the muscles.

Peripheral Nervous System

  • It is made up of the nerves arising from brain and spinal cord.
  • Cranial & Spinal nerves constitute the peripheral nervous system.
  • The primary role of the peripheral nervous system is to connect the central nervous system to the organs, limbs and skin.
  • It allows the brain and spinal cord to receive and send information to other areas of the body, which allows us to react to stimuli in our environment.

Autonomic Nervous System

  • It is the part of the peripheral nervous system that is responsible for regulating involuntary body functions, such as blood flow, heartbeat, digestion, and breathing.

It is further divided into two branches:

  • Parasympathetic system
    • It helps in maintaining normal body functions and conserve physical resources.
    • It constricts the pupils.
    • It creates contraction in the muscles of the urinary bladder.
    • It increases the secretion of saliva and other digestive juices.
  • Sympathetic system
    • By regulating the flight or fight response, the sympathetic system prepares the body to expend energy to respond to environmental threats.
    • When action is needed, the sympathetic system triggers a response by accelerating heart rate, increasing breathing rate, boosting blood flow to muscles, activating sweat secretion and dilating the pupils.
    • It helps in clotting of blood.

Did You Know

  • The unit of nervous system is Neuron.
  • The membrane covering the brain and spinal cord is called Meninges.
  • The nerve that transmits impulses from the eye to brain is optic nerve.