Nervous System: Control and Coordination

The nervous system and the endocrine system ensure that the body works in a controlled and coordinated manner. The nervous system includes the brain, spinal cord, sense organs and nerves while the endocrine system operates through certain chemicals called hormones which are produced by specialized glands and are secreted directly into the blood. The nervous system works with the endocrine system to communicate, integrate and coordinate the functions of various organs and systems in our body.

Nervous System

The functioning of the nervous system depends on detecting a stimulus in the internal or external environment and responding to it. A stimulus is an agent or an environmental change which can initiate a response in the body. The stimuli can be of several types. It could be physical (touch, prick, pressure), auditory, chemical, radiant (light), heat or cold, or electrical.

The nervous system has two main divisions: Central Nervous System (CNS) that includes brain and spinal cord and Peripheral Nervous System (PNS) which includes the nerves arising from the brain and spinal cord.

Neuron (Nerve Cell)

Neurons (individual cells of the nervous system) communicate with one another and other tissues to receive and transmit information throughout the body.  It consists of three parts - Dendrites, Cell body and Axon.

Dendrites

Dendrites are branched cytoplasmic projections from the cell body. The dendritic tip of the nerve cells receive impulses and sets off a chemical reaction that creates an electrical impulse which is further transmitted to the cell body.

Cell body

The cell body contains a well defined nucleus, surrounded by cytoplasm. It has cell organelles like any other cells. The cell body further transmits the impulse to the axon.

Axon

One branch arising out of the cell body is very long in comparison to others. This branch is called axon or nerve fibre. In most neurons, it is covered by an insulating fatty sheath called neurilemma.

Types of Neurons

There are three types of neurons:

  1. Sensory neurons convey the impulse from receptor (sense organ) to the main nervous system (brain or spinal cord).
  2. Motor neurons carry the impulse from the main nervous system to an effector (muscle or gland).
  3. Association (Connecting) neurons are located in the brain and spinal cord and interconnect the sensory and motor neurons.

Nerves

Nerves are thread-like structures which emerge from the brain and spinal cord and branch out to almost all parts of the body. A nerve is formed of a bundle of nerve fibres (axons) enclosed in a tubular sheath. There are three kinds of nerves:

Sensory nerves

Sensory nerves that contain sensory fibres. These nerves bring impulse from the receptors (sense organs) to the brain or spinal cord. Example: Optic nerve arising from the eye and ending in the brain.

Motor nerves

Motor nerves which contain motor fibres. These nerves carry impulse from the brain or spinal cord to the effector organ like muscles or glands. Example: a nerve arising from the brain and carrying impulse to the muscles of the eye.

Mixed nerves

Mixed nerves are those that contain both sensory and motor fibres and perform a mixed function. Example: a spinal nerve.

Central Nervous System

Central Nervous System (CNS) is regarded as the "information processor" in the body. It consists of the brain lying under the skull, and the spinal cord contained within the vertebral column.

Brain

The brain is a very delicate organ. It is well protected within the bony cranium (brain box). It is further protected by three meninges (membranous coverings) which continue backward over the spinal cord. These meninges are:

  1. Dura mater, the outermost tough fibrous membrane
  2. Arachnoid, the thin delicate middle layer giving a web-like cushion
  3. Pia mater, the innermost highly vascular membrane, richly supplied with blood

The space between the covering membranes is filled with a watery fluid known as cerebrospinal fluid which acts like a cushion to protect the brain from shocks.

The brain consists of three important parts - Cerebrum, Cerebellum and Medulla.

Cerebrum

It is the largest portion of the brain, vertically divided into two halves: right and left cerebral hemispheres. The outer portion or the cortex of the cerebrum contains cell bodies of the neurons which is the basic unit of nervous tissue. Being grayish in colour, it is called the gray matter. The inner portion of the cerebrum consists of white matter which mainly contains the axons or nerve fibres of the neurons.

The cerebrum is the seat of intelligence, consciousness and will-power. It controls all voluntary actions.

Cerebellum

The cerebellum is a much smaller area of the brain located below the cerebrum. It has no convolutions, but has numerous furrows. This also has an outer cortex made-up of gray matter and an inner white matter. The main function of the cerebellum is to maintain the balance of the body and coordinate muscular activity. The cerebrum and cerebellum work in close coordination.

Medulla

The medulla oblongata is the lowest portion of the brain located at the base of the skull. It is roughly triangular and is continued behind as the spinal cord. Its function is to control the activities of our visceral organs like the alimentary canal, movement, breathing, beating of heart and many other involuntary actions. Injury to the medulla generally results in death as the involuntary and vital functions like breathing and heart beat may be stopped.

Spinal Cord

The spinal cord is an integral part of the central nervous system. It extends from the medulla oblongata and continues downward almost throughout the length of the backbone, and lies within the neural canal of the vertebral column or the backbone.

In the spinal cord, the arrangement of the gray and white matter is reversed from that in the brain. The gray matter containing the cell bodies of motor neurons lie on the inner side, while the white matter on the outer side. The white matter contains axons running longitudinally to and from the brain and even crossing from one side to the other. There is a small central canal in the centre which runs through the entire length and continues with the cavities of the brain. It is filled with cerebrospinal fluid which acts as a shock proof cushion and forms a medium for the exchange of food materials, waste products, and respiratory gases with neurons.

Externally, the spinal cord is covered by the same three membranes - dura mater, arachnoid and pia mater in continuation with those of the brain.

Functions of Spinal Cord

The spinal cord is concerned with the following three functions:

  1. It controls the reflexes below the neck.
  2. It conducts sensory impulses from the skin and muscles to the brain
  3. It conducts motor responses from the brain to muscles of the trunk and limbs

Reflex Action

The word Reflex is used to convey sudden and immediate action in response to something. There are certain actions in the body that are spontaneous and do not need any processing by the brain. Such actions or responses are called reflex actions. Reflex actions are involuntary actions that occur without conscious thought processes. For example:

  1. When some particles fall into your eye, there is immediate flushing of tears to wash them out (glandular secretion)
  2. When your hand accidentally touches a hot pan, you withdraw it instantaneously (muscular movement)
  3. You shiver when it is very cold (muscular contractions) or sweat when it is too hot (glandular secretion)

Reflex Arc

Stimulus → receptor in the sense organ → afferent (sensory) nerve fibre → CNS → efferent (motor) nerve fibre → muscle (to contract) or gland (to secrete)

Types of Reflexes

Reflexes are of two types:

  1. Natural (inborn) reflexes
  2. Conditioned (acquired) reflexes

Natural (inborn) reflex

Activities in which no previous experience or learning is required are termed as natural reflexes. These reflexes are inborn, i.e. inherited at the time of birth. Examples are: swallowing, coughing and blinking of eyelids.

Conditioned (acquired) reflex

Actions which develop during the lifetime due to experience or learning are termed as conditioned reflex. For example, salivation occurs at the smell of the food as the brain is able to remember the taste of the food and works in an unconscious way. Such reflexes are not inborn and hence conditioned reflexes are acquired.

Peripheral Nervous System

It connects the central nervous system with the sense organs, muscles and glands of the body and, includes the sensory and motor nerves. The peripheral nervous system consists of two sub divisions:

  1. Somatic nervous system that conveys information from brain and spinal cord to skeletal muscles and regulates voluntary action.
  2. Autonomic nervous system which control the involuntary action of many internal organs, smooth muscles, heart muscles and glands.

Somatic Nervous System

This consists of two sets of nerves - the cranial nerves and the spinal nerves.

  • 12 pairs of cranial nerves emerge from the brain. For example, optic nerve (for eyes) and auditory nerve (for ears).
  • 31 pairs of spinal nerves emerge from the spinal cord.

Autonomic Nervous System

The autonomic nervous system (ANS) consists of a pair of chain of nerves and ganglia on either side of the backbone. This system controls the involuntary actions of the internal organs. There are two parts of the autonomic nervous system- Sympathetic and Parasympathetic.

The Sympathetic Nervous System (SNS) becomes more active during times of stress. It prepares the body for action. Its action during the stress response comprise the "fight-or-flight response" that is manifested largely under the influence of the hormone, adrenaline.

The Parasympathetic Nervous System executes actions that do not require immediate response, for example producing of saliva and tears, digestion, etc.

Endocrine System

Our body undergoes changes as long as we live. These changes are regulated by special glands in our body known as the endocrine glands. The main function of these glands is to produce chemical secretions called hormones.

Hormones play an important role in control, coordination and regulation of the functioning of tissues, organs and systems in the body. Well harmonized mechanisms regulate the release of very precise quantities of hormones to achieve optimal functioning of the human body.

The endocrine system is responsible for the chemical coordination in our body.

Pituitary Gland

This is a small gland located at the base of the brain. This gland plays an important role in the growth of a child from puberty to the full reproductive maturity. The pituitary gland secretes Gonad Stimulating Hormone, which regulates the activity of gonads (ovary in females and testis in the males).

There is an increase in the activity of this gland at the time of puberty which stimulates the ovary and testes to produce the sex hormones progesterone and oestrogen in females, and testosterone in males. These hormones initiate the development of secondary sexual characters.

The disorders caused by the increased or decreased activity of the pituitary gland include:

Cushing’s Disease: It is caused by the hyperactivity of pituitary gland. In the males, this disease may lead to excessive growth of hair. In some cases, it may even cause atrophy of testes leading to impotency. In the females, this disease causes sterility and masculinization, for example, growth of beard and moustaches.

Deficiency (hypoactivity) of growth hormone (GH) or Somatotropic Hormone (STH) secreted by pituitary gland causes dwarfism (retarded growth of the long bones) which adversely affects the height of a person. On the other hand, its excessive secretion or hyperactivity causes gigantism (excessive growth of long bones) making a person very tall.

Thyroid Gland

It is responsible for the speed of metabolism in our body. The thyroid gland is essential for life, growth and development. When the thyroid gland becomes overactive and produces more thyroid hormone than is necessary for optimal functioning, the condition is called Hyperthyroidism. When the thyroid gland becomes underactive and produces less thyroid hormone than is necessary, the condition is called Hypothyroidism.

Cretinism is a condition of severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormone (hypothyroidism) or from prolonged nutritional deficiency of iodine.

Goitre is a disease of the thyroid gland characterized by an enlargement of the gland, visible externally as a swelling on the front of the neck. Simple goitre is caused by a deficiency of iodine in the diet.

Pancreas

This gland secretes two hormones insulin and glucagon which help in the metabolism of glucose in our body. Hyposecretion of insulin causes diabetes mellitus in which glucose is present in excess in the blood.