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The Nervous System

A complete understanding of the human nervous system remain s a challenge. Several billion cells remain associated with this system. The varying functions of these cells and the nervous system are responsible for human behaviour and activities. Hence, scientists from different fields collectively are interested in understanding the functioning of this system. Studies on brain and other related structures began several years ago. Even to-day thousands of scientists are involved in researches for knowing the structure and functioning of the brain. For a thorough knowledge of this system, further works in anatomy, physiology, molecular,biology, psychology, medicine and other related fields are needed. Basically the nervous system is formed of nerve cells or neurons. Neurons are responsible for transmission of impulses. They also help in realising, analysing and storing messages. They can stimulate muscles to work. The network of interconnected neurons in the nerves, brain and spinal cord have highly complicated ethods of functioning. A neuron has a basic cell structure called the cyton. The projections of the cyton are the dendrites and the dendrons. The inter communicating long projection is the axon. There are variations in the shape of the cyton, number of dendrons and nature of axon. A neuron is interconnected with the dendrite of the neighbouring neuron through the endplate of the axon. Such specialized connections are called as synapses. In the terminal regions of the effector nerves the axon of the nerve cells are in contact with the muscle tissue. These joints are named as neuro - muscular junctions.

The structure of a peripheral nerve :
A nerve is made up of several nerve fibres. A nerve fibre contains axons with their coverings called schwann cells. The fibres are grouped into fasciculi. The number and pattern of fasciculi vary in different nerves. Thus a nerve trunk possesses many such fasciculi. Such a trunk is surrounded by an epineuruium. The individual fasciculi are enclosed by a multilayered perineurium. The perineurium surrounds the endoneurium or intra fascicular connective tissue. In a peripheral nerve the epineurium constitutes 30 -70 % of the total cross sectional area of the nerve bundle. The thickness is more when there are more fasciculi. A layer of fat in the epineurium provides a ‘cushion’ effect to the nerve. The perineurium contains alternating layers of flattened polygonal cells. The endoneurium remains condensed around axons. The components of the endoneurium remain bathed in endoneurial fluid. The fasciculi of the nerve are supplied blood by vasa nervosum. These minute blood vessels radiate upto the endoneurium.

Nervous System

The organs of the nervous system are continuous in nature. However,for study purposes it can be divided into systems and organs.

A. Central Nervous System (C N S)

This system includes the brain and the spinal cord or medulla spinalis. They are protected by surrounding bones. While the brain is located within the cranium, the spinal cord is placed within the vertebral canal of the vertebrae. Through an opening called foramen magnum, the spinal cord descends down from the brain.

B. Peripheral Nervous System.

It consists of nerves and ganglia. The nerves that are formed from the brain are called the cranial nerves. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves.

C. Autonomous Nervous System.

The nerves in this system transmit impulses from the C N S to smooth muscles, cardiac muscles and glands. It is also called the involuntary nervous system. It is subdivided into sympathetic and parasympathetic divisions.

Tonsils

These are the largest lymph nodules. They provide protection against bacteria and other harmful materials. In adults the tonsils decrease in size and may disappear. There are 3 groups of tonsils in the pharyngeal walls. Of the three, the palatine tonsils are usually refered to as “the tonsils”. These are larger lymphoid masses on each side of the junction between the oral cavity and the pharynx. The pharyngeal tonsil or adenoid are found near the junction between the nasal cavity and the pharynx. The lingual tonsil is a loosely associated collection of lymph nodules on the posterior surface of the tongue.

Spleen

The lymph enters the lymph nodes through afferent lymphatic vessels and exits through efferent vessels. The nodes contain open spaces called sinuses. The sinuses are lined with phagocytic cells. Spleen It is roughly the size of a clenched fist. It is located on the left side of the abdominal cavity. It has a fibrous capsule. The spleen contains two types of lymphatic tissues, namely the red pulp and the white pulp.

Lymph nodes

These are small round structures. Their size ranges from 1-25 mm. They are distributed throughout the course of the lymphatic vessels. These nodes are found all over the body. However they are concentrated as aggregations in 3 regions of the body. These are the inguinal nodes in the groin, the axillary nodes in the axillary region and the cervical nodes of the neck.

Thymus Gland

It is a roughly triangular, bilobed gland. It is located in the mediastinum (ie., between the lungs). It lies between the sternum and the pericardium. Its size varies with age. It is largest in the early part of life (upto 15 years). At birth it weighs 10 - 15 g. After puberty it greatly decreases in size. Each thymus lobe is surrounded by a thin capsule made of the connective tissue. It has 2 layers. The inner layer is the medulla, the outer layer is cortex. The lymphocytes are found only in cortex layer.

Lymphatic System

Lymphatic circulation along with blood circulation plays a key role in maintaining the fluidity in all regions of the body. It helps to maintain fluid balance in tissues and it absorbs fat from the digestive tract. It also functions as body’s defence system against micro organisms and other harmful substances. This system includes lymph, lymphocytes, lymphatic vessels, lymph nodules, lymph nodes, tonsils, the spleen and the thymus gland.

Lymphoid Cells And Tissues :

Lymphatic organs contain lymphatic tissues.These tissues primarily consist of lymphocytes. They also contain macrophages, dendritic cells and reticular cells. Lymphocytes are a type of white blood cells. They originate from red bone marrow and are carried by blood to lymphatic organs and other tissues. There are several classes of lymphocytes. The B-lymphocytes or B cells synthesize antibodies for recognizing and neutralising alien macromolecules. T- lymphocytes can recognize and selectively kill cells infected with viruses. B and T lymphocytes are produced from stem cells present in the bone marrow. The T lymphocytes get matured only after entering into Thymus, a lymphoid organ through circulation. Maturation and differentiation of B cells will occur in the bone marrow itself. Thus the thymus and bone marrow are described as central or primary lymphoid organs.

The Lymphatic Circulation :
The lymph fluid from the tissues is drained by lymphatic capillaries. These capillaries though present in many tissues are absent in epidermis, hairs, nails, cornea, cartilages, CNS and bone marrow. The lymphatic capillaries join into larger vessels. The larger vessels pass to local or remote lymph nodes. These vessels and associated lymph nodes are arranged in regional groups. Each group has its region of drainage. Nodes within a group are interconnected. Such regional groups with nodes and vessels are organised in (1) Head and neck (2) Upper limbs (3) Lower limbs (4) Abdomen and pelvis (5) thorax. The regional vessels return to the venous blood circulation via the right and left lympho venous portals. Nearly eight lymphatic trunks converge at the site of the vertebral column and open into the venous portal nearer to the neck.

Blood Vessels

Structure of blood vessels

Blood Vessels

The blood vessels show a vast range of structural modifications. However a few basic patterns can be studied. A blood vessel consists of a wall and a lumen or cavity. The wall of the blood vessels is made up of 3 distinct layers or tunica. They are the tunica intima, tunica media and tunica externa or tunica adventitia. The tunica intima is formed of an endothelium, a delicate connective tissue and elastic fibres. The tunica media contains smooth muscle cells. It causes vasoconstriction and vasodilation. The tunica externa is composed of connective tissue. The composition and thickness of layers varies with the diameter of the blood vessels and the type.

Types of blood vessels :

1. Large elastic arteries :
The walls of these arteries contain elastic fibres. The smooth wall measures about 1micron in thickness. It gets stretchedunder the effect of pulse and recoils elastically

2. Muscular arteries :
There are larger and smaller muscular arteries. The larger muscular arteries are inelastic and they have thick walls. The wall has 30-40microns in diameter in the layers of smooth muscles. Since they regulate blood supply, they are called distributing arteries. The small muscular arteries are capable of vasodilation and vasoconstriction.

3. Arterioles :
They conduct blood from the arteries to the capillary bed. These are small vessels capable of vasodilation and vasoconstriction.

4. Capillaries :
These are fine vessels found between arterioles and venules. They measure 5-8micron in diameter.

5. Venules :
These are tubes of flat, oval or polygonal endothelial cells. Each venule is formed by the convergence of two or more capillaries. Its diameter ranges upto 30micron.

6. Veins :
Veins seen in anatomy are medium veins. They run in between venules and large veins. Large veins transport blood to the heart. Veins with diameter above 2 mm have valves. They are of semilunar type. They allow movement of blood towards the heart. There are several valves in the medium veins.

Branching of blood vessels :
When an artery divides into two equal branches, the original artery ceases to exist. Hence the branches are called terminal branches. The smaller branching vessels formed on the sides are called the collateral branches. When arteries are joined to each other it is named as anastomosis.

Blood supply to blood vessels :
As any other region, the cells and tissue on the wall of the blood vessel require nourishment. Some amount can diffuse from blood in the lumen. For vessels having diameter greater than 1 mm, diffusion of nutrients may not be possible. Such vessels have very minute vessels called vasa vasorum spread over them. They penetrate into the wall of the blood vessels.

Innervation of blood vessels :
The walls of the blood vessels are innervated by sympathetic nerve fibres. They regulate the contraction of the musculature. They effect vasoconstriction.

Portal Circulation

In the systemic circulation the venous blood passing through spleen, pancreas, stomach and interstine is not carried back directly to the heart. It passes through the hepatic portal vein to the liver. This vein begins as capilaries from the visceral organs and ends in the liver again as capillaries. These capillaries converge to form the hepatic vein which joins the inferior vena cava, conveying blood to right atrium. This route is the portal circulation. Components of Circulatory system Blood vessels The blood vessels carrying blood away from the heart are the arteries.
The Veins carry blood towards the heart. The arteries and veins are named and classified according to their anatomical position. They can also be classified according to their size and wall structure. Functionally, arteries are subdivided into conducting, distributing and resistance vessels.

1. Conducting vessels :
These are large arteries from the heart and their main branches. the walls of these vessels are elastic in nature.

2. Distributing vessels :
These are smaller arteries reaching individual organs. They branch into the organs. They have muscular walls.

3. Resistance vessels :
These are mostly arterioles. While these vessels are smaller, their walls are highly muscular. Hence these vessels can reduce pressure of blood due to peripheral resistance.

4. Exchange vessels :
These are the capillaries. The walls of these vessels allow exchanges between blood and the tissue fluid surrounding the cells. The substances commonly exchanged are oxygen, carbon-di-oxide, nutrients, water, inorganic ions, vitamins, hormones, metabolic products and antibodies.

5. Capacitance or reservoir vessels :
These are the larger vessels and veins. These are of varying sizes. They collect and convey blood back to the heart. The higher capacitance of these vessels is due to their distensibility. Hence their blood content is more, even at low pressure. The number of such veins is also enormous.. Thus the veins are called as the “blood reservoirs”