UPSC Biology The Nervous System / तंत्रिका तंत्र The Nervous System

The Nervous System

Divisions of the Nervous System

• The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops.

• The Central Nervous System (CNS) includes the brain and spinal cord.

• The Peripheral Nervous System (PNS) connects the CNS to other parts of the body, and is composed of nerves (bundles of neurons).

• Not all animals have highly specialized nervous systems.

• Those with simple systems tend to be either small and very mobile or large and immobile.

• Large, mobile animals have highly developed nervous systems: the evolution of nervous systems must have been an important adaptation in the evolution of body size and mobility.

Nervous System in Various Organisms

Coelenterates, cnidarians, and echinoderms have their neurons organized into a nerve net. These creatures have radial symmetry and lack a head. Although lacking a brain or either nervous system (CNS or PNS) nerve nets are capable of some complex behavior. Bilaterally symmetrical animals have a body plant that includes a defined head and a tail region. Development of bilateral symmetry is associated with cephalization of sensory organs at the front end of the organism. Flatworms have neurons associated into clusters known as ganglia, which in turn form a small brain. Vertebrates have a spinal cord in addition to a more developed brain. Chordates have a dorsal rather than ventral nervous system. Several evolutionary trends occur in chordates: spinal cord, continuation of cephalization in the form of larger and more complex brains, and development of a more elaborate nervous system.

The Neuron

Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.

The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape,

a. Parts of Neuron:

All neurons have three parts.

Dendrites receive information from another cell and transmit the message to the cell body.

The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells.

The axon conducts messages away from the cell body.

b. Types of Neuron:

Three types of neutrons occur, Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system.

Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands).

Interneurons are found only in the central nervous system where they connect neuron to neuron. Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann cells serve as supportive, nutritive, and service facilities for neurons. The gap between Schwann cells in known as the node of Ranvier, and serves as points along the neuron for generating a signal. Signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon. This allows our brain to communicate with our toes in a few thousandths of a second.

 The Nerve Message

The plasma membrane of neurons, like all other cells, has an unequal distribution of ions and electrical charges between the two sides of the membrane. The outside of the membrane has a positive charge, inside has a negative charge.

Resting potential results from differences between sodium and potassium positively charged ions and negatively charged ions in the cytoplasm.

Sodium ions are more concentrated outside the membrane, while potassium ions are more concentrated inside the membrane. This imbalance is maintained by the active transport of ions to reset the membrane known as the sodium potassium pump.

The sodium-potassium pump maintains this unequal concentration by actively transporting ions against their concentration gradients. The action potential begins at one spot on the membrane, but spreads to adjacent areas of the membrane, propagating the message along the length of the cell membrane. After passage of the action potential, there is a brief period, the refractory period, during which the membrane cannot be stimulated. This prevents the message from being transmitted backward along the membrane.

STEPS IN AN ACTION POTENTIAL

1. At rest the outside of the membrane is more positive than the inside.
2. Sodium moves inside the cell causing an action potential, the influx of positive sodium ions makes the inside of the membrane more positive than the outside.
3. Potassium ions flow out of the cell, restoring the resting potential net charges.
4. Sodium ions are pumped out of the cell and potassium ions are pumped into the cell, restoring the original distribution of ions.

Synapses

The junction between a nerve cell and another cell is called a synapse. Messages travel within the neuron as an electrical action potential. The space between two cells is known as the synaptic cleft. To cross the synaptic cleft requires the actions of neurotransmitters. Neurotransmitters are stored in small synaptic vesicles clustered at the tip of the axon. Neurotransmitters tend to be small molecules, some are even hormones. The neurotransmitters cross the cleft, binding to receptor molecules on the next cell, prompting transmission of the message along that cell’s membrane. Diseases that effect the function of signal transmission can have serious consequences. Parkinson’s disease has a deficiency of the neurotransmitter dopamine. Progressive death of brain cells increases this deficit, causing tremors, rigidity and unstable posture.

Endocrine System

You will know it in detail Chapter five.

Nervous System: The Telegraphic System of Communication and Co-Ordination

• The structural of the nervous system is a neuron. It has a nucleus and cytoplasm, forming the cell body. The cell body has elongated extension of cytoplasm. Those extension which are specialized for transmitting messages to two neuron are called ‘dendrites’ and the extension that transmits message from the neuron is called an ‘axon’. A reflex action, or simply a reflex is a fundamental function of the nervous system.

• The chain of neurons that participants in a reflex action is called a ‘reflex arc’. It consist of at least two neuron — (1) Sensory or afferent neuron which conveys the information about the stimulus to the central nervous system. (2) Motor or afferent neuron – which conveys the message from the central neuron system to the organ (effector organ) which has to respond to the stimulus.

• The function between two neurons is called a ‘ganglion’.