Central Nervous System – The Brain and Spinal Cord

The study of marine Biology Topics reveals the incredible diversity and complexity of life in the oceans.

Explain Structure and Function of the Central Nervous System and What are Exocrine and Endocrine Glands

The central nervous system (CNS) consists of the brain and the spinal cord. Like a telephone exchange with ingoing and outgoing wires, it is responsible for the coordination and control of the activity of the nervous system. The work of the central nervous system is to direct incoming messages to the motor neurons that are connected to the part of the body which will respond to a stimulus. In complicated responses, the brain and spinal cord are both involved. That is, in complicated responses, the central nervous system is involved. The central nervous system enables a person to give a more appropriate and more intelligent response to various situations. By using the central nervous system, a person can vary his behaviour according to the changing situations. This point will become clear from the following example.

The central nervous system consists of the brain and the spinal cord.

If we pick up a very hot plate in the kitchen (without knowing that it is very hot), then our reflex action produced by the spinal cord alone says that we should pull away our hand (so that our hand is saved from burns). But if we pull away our hand, then the plate would drop and break into pieces (and our mother will definitely scold us for breaking the plate!). Now, it is here that the central nervous system involving the brain steps in.

When the message from our fingers saying that the ‘plate is too hot’ arrives at our central nervous system, there is already another message saying ‘but don’t drop it’ (This is due to the intelligence of the brain). The central nervous system will consider the two messages together. It may then decide to send a message to our muscles to tell them to put down the plate gently and not drop it. This intelligent response has been made possible only due to the central nervous system.

The job of the central nervous system is to collect all the information from all the receptors in our body. This information is added together before messages are sent out to the effectors. In this way, the best action can be taken in a particular set of circumstances. We will now describe the two organs of the central nervous system, the brain and spinal cord in detail.

Human Brain Anatomy and How the Brain Works

The brain is the highest coordinating centre in the body. The brain is located inside the skull of our body (at the top of the spinal cord). It is protected by a bony box in the skull called the cranium (see Figure). The brain is surrounded by three membranes called meninges, which help to protect it. The space between the membranes (or meninges) is filled with cerebrospinal fluid that protects the brain from mechanical shocks. Pairs of cranial nerves arise from the brain.

The brain is broadly divided into three regions: forebrain, midbrain, and hindbrain (see Figure). The forebrain consists mainly of the cerebrum. The midbrain does not have any further divisions. The hindbrain consists of three centres called pons, cerebellum, and medulla (see Figure). We will now discuss the functions of the forebrain, midbrain, and hindbrain. Let us start with the cerebrum which is in the forebrain.

The cerebrum (or forebrain) is the main thinking part of the brain. It is the site of our faculties such as learning, reasoning, intelligence, personality, and memory. All our thoughts, sensations, actions, and movements are controlled by the cerebrum. The cerebrum has different areas for performing different functions. ‘There are association areas in the cerebrum that control thinking and memory. These association areas also store information and experiences. There are sensory areas where information is received from the sense organs like eyes, ears, nose, tongue, and skin, and give us the ‘sensation’ or ‘feeling’. Similarly, the cerebrum has motor areas from which instructions are sent to muscles to do various types of jobs.

All the voluntary actions of the body are coordinated by the cerebrum. This happens as follows: The cerebrum receives sensory information through the receptors of sense organs. The cerebrum interprets this information in the light of previous experiences and takes a decision that it thinks is right. It then sends out instructions to the motor area (which controls the movement of voluntary muscles) so as to make voluntary muscles move to bring about the appropriate responses. We will now describe the functions of the midbrain. The midbrain controls reflex movements of the head, neck, and trunk in response to visual and auditory stimuli. It also controls the reflex movements of the eye muscles, changes in pupil size, and the shape of the eye lens.

We will now describe the functions of the parts of the hindbrain which are the pons, cerebellum, and medulla. The pons takes part in regulating respiration. The cerebellum helps in maintaining posture and balance of the body. It also enables us to make precise and accurate movements. The cerebellum coordinates smooth body movements such as walking, dancing, riding a bicycle and picking up a pencil, etc. Medulla controls various involuntary actions such as heartbeat (blood circulation), breathing, blood pressure, and peristaltic movements of the alimentary canal. Medulla is also the controlling centre for reflexes such as swallowing, coughing, sneezing, secretion of saliva, and vomiting.

The brain consists of millions of nerve cells like these, carrying millions of messages to and from the brain. These nerve cells are shown magnified many many times.

Our brain looks like this from above. The cerebrum is divided into two halves called cerebral hemispheres (which are separated by a deep groove).

Computer Tomography Scan (CT scan) of a healthy brain, showing both halves of the brain enclosed within the cranium. CT scan is used to detect abnormalities (like injury, etc.) in the human brain.

Anatomy of the Spinal Cord and How it Works

The spinal cord is a cylindrical structure. The spinal cord begins in continuation with the medulla and extends downwards. It is enclosed in a bony cage called a vertebral column (see Figure). The spinal cord is also surrounded by membranes called meninges. As many as 31 pairs of nerves arise from the spinal cord (see Figure). The spinal cord is concerned with spinal reflex actions and the conduction of nerve impulses to and from the brain.

This sketch shows the spinal cord enclosed in a bony cage called a vertebral column. The spinal cord is connected to the brain at the upper end.

This is a highly enlarged view of the spinal cord in the body of a person. Pairs of spinal nerves can be seen extending from the spinal cord.

The thin rope-like object held in the hands in this picture is a real human spinal cord.

Before we end this discussion, we would like to give the various functions of the brain. The various functions of the brain are as follows:

• The brain receives information-carrying nerve impulses from all the sensory organs of the body.
• The brain responds to the impulses brought in by sensory organs by sending its own instructions (through motor nerves) to the muscles and glands causing them to function accordingly.
• The brain correlates the various stimuli from different sense organs and produces the most appropriate and intelligent response.
• The brain coordinates the body’s activities so that the mechanisms and chemical reactions of the body work together efficiently.
• The brain stores ‘information’ so that behaviour can be modified according to the past experience. This function makes the brain the organ of thought and intelligence.

Before we describe the hormonal system or endocrine system for the coordination in human beings, we should know the meanings of two terms hormones and endocrine glands. So, let us first discuss hormones and endocrine glands.

Characteristics of Hormones

Hormones are chemical substances secreted in very small amounts by specialized tissues in the body called endocrine glands. These hormones coordinate the activities of living organisms and also their growth. So, we can now say that Hormones are the chemical substances that coordinate the activities of living organisms and also their growth. Hormones are made inside the body of an organism in very small amounts. The various characteristics of hormones are given below:

• The hormones are secreted in small amounts by the endocrine glands.
• The hormones are poured directly into the blood and carried throughout the body by the blood circulatory system.
• The hormones have their effect at the sites different from the sites where they are made. So, they are also called chemical messengers.
• The hormones act on specific tissues or organs (called target organs).
• The hormones coordinate the activities of the body and also its growth.

Endocrine Glands & Their Hormones

A gland is a structure that secretes a specific substance (or substances) in the body. A gland is made up of a group of cells or tissue. There are two types of glands in the body exocrine glands, and endocrine glands.

A gland that secretes its product into a duct (or tube) is called an exocrine gland. For example, the salivary gland secretes the saliva into a duct called the salivary duct, therefore, the salivary gland is an exocrine gland. Thus, exocrine glands are the glands having ducts (see Figure). A gland that does not have a duct and secretes its product directly into the bloodstream is called an endocrine gland. Thus, endocrine glands are ductless glands. An endocrine gland secretes a chemical substance called hormone. We can now say that A structure (group of cells or tissue) that makes hormones in the body is called an endocrine gland.

The various endocrine glands present in the human body are shown in Figure. The endocrine glands do not have ducts to secrete their hormones, so they are also called ductless glands. The endocrine glands release hormones directly into the blood of a person. These hormones reach the concerned body part through the blood and act on it. Hormones are a kind of chemical messengers. A hormone is produced in one part of the body but it acts on some other part of the body. The hormones are of different types and perform different functions.

Salivary glands are exocrine glands (having ducts). This picture shows the location of salivary glands in the human body.

Some of the glands in our body have both exocrine and endocrine functions. The pancreas, testes, and ovary are such glands. For example, the pancreas acts as an endocrine gland and secretes the hormone insulin. It also acts as an exocrine gland and secretes pancreatic juice containing digestive enzymes into the pancreatic duct that leads to the alimentary canal. The testes are glands that act as endocrine glands and secrete the hormone called testosterone. They act as exocrine glands and release sperm (male sex cells) into the duct. Similarly, ovaries are glands that act as endocrine glands and secrete the hormones ostrogen (read as ‘estrogen’) and progesterone. They act as exocrine glands and release ova or eggs (female sex cells) into the duct.