Vertebrate Nervous System

1.0The Vertebrate Nervous System consists of two parts

(1) Central Nervous System  

(2) Peripheral Nervous System

Central Nervous System

The central nervous system consists of two parts (i) upper large brain situated in the head region and (ii) the lower long and narrow spinal cord situated in the neck and trunk region is in continuation with brain downwards.

(i) Brain: It is the part of the central nervous system that is present in the head region. The bony box that houses the brain within the skull is called the cranium. It has three main regions namely the fore brain, the mid brain and the hind brain. The three regions have different parts that have specific functions.

Fore Brain: It is made up of cerebrum, hypothalamus and many other parts. 

(a) Cerebrum: It is the largest and main thinking part of the brain and is made up of two hemispheres called the cerebral hemispheres. The cerebrum has sensory areas, association areas and motor areas. Sensory areas receive the messages. There are different areas for hearing, smell, sight and so on in cerebrum. There are separate areas of association where this sensory information is interpreted by putting it together with information from the other receptors as well as with information that is already stored in the brain. 

The motor areas are responsible of the action of the voluntary muscles. The Cerebrum is also responsible for the intelligence, memory, consciousness and will power.

(b) Hypothalamus: Hypothalamus is an important region of the brain. It receives the taste and smell impulses, coordinates message from the autonomous nervous system, controls the heart rate, blood pressure, body temperature. It also forms an axis with the pituitary which is the main link between the nervous and the endocrine systems. It also has centres that control emotions, hunger, thirst, fatigue, sleep, body temperature and sweating. It secretes neurohormones which regulate the secretion of anterior lobe of pituitary. 

Note: Olfactory lobes are a part of the fore brain. They are related with smell sensation.

Mid Brain: It is a small portion of the brain that serves as a relay centre for sensory information from the eyes and ears to the cerebrum. It also controls the reflex movements of the ears and eyes muscles. It provides a passage for the different neurons going in and coming out of the cerebrum.

Hind Brain: It consists of cerebellum, pons varolii and medulla oblongata.

(a) Cerebellum: Cerebellum is the second largest part of the brain. It is responsible for maintaining balance while walking, swimming, riding, etc. It is also responsible for precision and the fine control of the voluntary movements. For example, we can do actions like eating while talking or listening. The action of eating, while talking is done automatically. This is controlled by the cerebellum. Alcohol adversely affects the cerebellum.

(b) Pons: Pons literally means bridge. It is hidden as it is well protected because of its importance. It has a breathing centre. 

(c) Medulla oblongata: Medulla oblongata is the posterior most part of the brain which lie below the cerebellum. It controls activities such as sneezing, coughing, swallowing, salivation and vomiting. It contains centre which control respiration and cardio-vascular reflexes and gastric secretions. It also controls the rate of heartbeat and expansion and contraction of blood vessels to regulate the blood pressure.

(ii) Spinal cord: It is a collection of nervous tissue running along the backbone. It is protected by the vertebral column. The spinal cord begins in continuation with medulla oblongata and extends downwards.

The functions of the spinal cord are –

1. Coordinating spinal reflexes. 

2. It conducts sensory and motor impulse to and from the brain via sensory and motor nerve fibers respectively.

Peripheral Nervous System

All the nerves arising from the brain and spinal cord are included in the peripheral nervous system. It is divided into a somatic neural system and autonomic neural system. 

Somatic neural system consists of two sets of nerves:

(i) Cranial nerves: The nerves arising from the brain are called cranial nerves. The nerves can be sensory, motor or mixed. 12 pairs of cranial nerves are found in humans.

(ii) Spinal Nerves: The nerves arising from spinal cord are called spinal nerves. Each spinal nerve is of mixed type. In humans, 31 pairs of spinal nerves are found.

Autonomous Nervous System: Sympathetic and parasympathetic nervous system are involved in it.

Endocrine System: A group of endocrine glands which produce various hormones form the endocrine system. In addition to the nervous system, the endocrine system also helps in coordinating the activities of our body.

Endocrine glands - The ductless glands which pour their secretions directly into the blood are called endocrine glands.

Hormones - They are secretions of the endocrine glands and one of the important substances that control the body chemistry. They are also known as “Chemical messengers”.

• The term hormone was introduced by Bayliss and Starling.

• Hormones have their effect at sites different from the sites where they are made. They act on specific areas called target organs.

Physical and chemical properties of hormones

(i) These are secreted by the endocrine glands.

(ii) Hormones are secreted only when required. 

(iii) Their secretion is regulated by feedback mechanisms.

(iv) These are generally released in the blood stream.

(v) The molecules of most of the hormones are small. 

(vi) The secretion of hormones is always in very small quantity.

(vii) Hormones get destroyed after use i.e. hormones cannot be stored in the body. Thyroxine is an exception.

2.0Transfer of information in multicellular organisms

Electrical impulses are an excellent means for the transfer of information rapidly. But there are some limitations to the use of electrical impulses. They will reach only those cells that are connected by nervous tissue. Once an electrical impulse is generated in a cell and transmitted, the cell will take some time to reset its mechanisms before it can generate and transmit a new impulse. So, multicellular organisms use another means of communication between cells namely, chemical communication. This will be slower and it can be done steadily and persistently because it can potentially reach all cells of the body.

In chemical communication, stimulated cells release a chemical compound (hormone) which diffuse all around the original cells. If other cells around have the means to detect this compound using special molecules on their surfaces, then they would be able to recognize information and even transmit it.