WBCS Preliminary (Biology): Nervous System

The nervous system perceives the changes around us through our senses. It controls and coordinates all the activities of the muscles in response to the changes outside. It also conducts messages between different parts of the body.

• The units of nervous system are specialised cells called the neurons. A neuron consists of dendrites, a cell body and an axon. Around the cell body are short sensory projections called the dendrons. The fine branches of dendrons are called the dendrites. These short fibres receive messages and pass them on to the cell body or the cyton. The axon carries an impulse transmitted to it by the cell body to another neuron or to an effector muscle or gland. There are three types of neuron sensory, motor and association. A synapse is the junction between two neuron. A bunch of neurons together make up a nerve.
• In mammals these neuron comprise 2 types of nervous system i.e. Central nervous system and peripheral nervous system. The brain and spinal cord make up the central nervous system (CNS). The Central Nervous System (CNS) is composed of the brain and spinal cord.
• Nerves are of three types based on the types of neurons they carry. They are:
  1. Sensory Nerves or the Receptor Nerves: They are made up of only sensory neurons. For example, the cranial nerves that conduct impulses from the organs to the central nervous system.
  2. Motor Nerves or the Effector Nerves: They are made up of only motor neurons. For example, the cranial nerves that conduct impulses from the central nervous system to the muscles and glands (effector).
  3. Mixed Nerves : The nerves that are made up of both sensory and motor neurons. For example, all spinal nerves.
• The human nervous system can be divided into three main parts:
  1. Central nervous system: It is made up of the brain and the spinal cord which is the continuation of the brain. Brain and spinal cord are surrounded by membranes called the meninges.
     • The average adult human brain weighs 1.3 to 1.4 kg (approximately 3 pounds). The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The brain consists of gray matter (40%) and white matter (60%) contained within the skull. The brain has three main parts: the cerebrum, the cerebellum, and the brain stem (medulla). Cerebrospinal fluid (CSF) surrounds the brain. Although the brain is only 2% of the bodys weight, it uses 20% of the oxygen supply and gets 20% of the blood flow. Brain contained in the skull called cranium. The brain appears as three distinct but connected parts: the cerebrum, cerebellum and the brain stem a central core that gradually becomes the spinal chord.
     • Cerebrum: The cerebrum is the largest part of brain and makes up 85% of the brains weight. This is the thinking part of the brain.The cerebrum is made up of two halves, with one on either side of the head. The right half of the cerebrum controls the left side of your body, and the left half controls the right side. These two hemisphere is jointed by a nerve fibres known as corpus callosum. The surface of each cerebral hemisphere shows many convolutions called Gyri separated by sulci(depressions).
     • Thalamus: It is an area which coordinates the sensory impulses from the various sense organs - eyes, ears and skin and then relays it to the cerebrum.
     • Hypothalamus: The hypothalamus is composed of scattered masses of grey matter in white matter at the base of the brain. Although it is the size of only a pea (about 1/300 of the total brain weight), the hypothalamus is responsible for some very important functions. It is responsible for regulation of temperature, hunger, thirst and emotional reactions. The hypothalamus also controls the pituitary.
     • Mid Brain: It is a small portion of the brain that serves as a relay centre for sensory information from the ears to the cerebrum. It also controls the reflex movements of the head, neck and eye muscles. It provides a passage for the different neurons going in and coming out of the cerebrum.
     • Hind Brain : It consists of cerebellum, pons and medulla oblongata:
       • Cerebellum: The cerebellum is at the back of the brain, below the cerebrum. It is smaller than the cerebrum at only 1/8 of its size. It controls our balance, movement, and coordination (how your muscles work together). Because of our cerebellum, we can stand upright, keep your balance, and move around.
       • Pons: Pons literally means bridge. It serves as a relay station between the lower cerebellum and spinal cord and higher parts of the brain like the cerebrum and mid brain.
       • Medulla Oblongata :It is a small region of the brain. It is hidden as it is well protected because of its importance. It has the cardiovascular centre and the breathing centre. It also controls activites such as sneezing, coughing, swallowing, salivation and vomiting.
     • Spinal Cord: spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams. The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. Therefore, the spinal cord is much shorter than the vertebral column. It is covered by the same meninges as the brain and its main function is conduct impulses to and from the brain and acts as a reflex centre.
  2. Peripheral Nervous System: The peripheral nervous system is made up of nerves that connect the different parts of the body (peripheral tissues) to the central nervous system.
• Reflex Actions: When the body instantly responds to an external stimulus, it is called reflex action. Reflex may be natural or conditioned. Natural reflex does not involve the brain whereas conditioned reflex involves the brain. The path along which the impulses travel in a reflex are called the reflex arc.
• Animals have a nervous system for controlling and coordinating the activities of the body. But plants have neither a nervous system nor muscles.