REFLEX ARC FOR SNEEZING, NERVOUS SYSTEM OF MAN

 

Q:    Trace by means of diagrams and description the possible reflex arc:

a) sneezing

b) watering of the mouth

Ans:-  REFLEX ARC FOR SNEEZING

                   Sneezing is caused due to some irritation mainly in the nasal cavity. This irritation may be because of entrance of some unwanted things in the nasal cavity. The receptors of the nasal cavity are stimulated and nerve impulses are carried by the sensory neurons to the spinal cord. From spinal cord impulses are carried via motor neurons to the diaphragm, intercostal muscles and ribs which cause rapid and sudden expiration with a jerk.

Watering of Mouth

                  The smell and sight of food stimulate the receptors present in the nasal cavity and eyes respectively. From these receptors the nerve impulses are carried by the sensory neurons to the spinal cord which sends message to the salivary glands via motor neurons. Thus salivary glands are stimulated and causes watering of mouth.

 

Q:   Describe Nervous System of man.

Ans:  NERVOUS SYSTEM OF MAN

                  The nervous system of man consists of two parts.

l- Central Nervous System

2- Peripheral Nervous system

CENTRAL NERVOUS SYSTEM (CNS)

                   The central nervous system has two parts; :

                     i)    Brain                  ii)    Spinal cord.

 

                  It can be compared to a switchboard of the telephone exchange for controlling, directing and integrating all messages but the CNS works at a much higher level than a switchboard. It can ma e judgements, inhibit, delay or make necessary adjustments, according to situations.

                 Human CNS is made up of nearly ten to twelve billion neurons with their cell bodies lying within the brain and spinal cord while the long fibers of most of them coming out to form nerves.

1) BRAIN

                 The brain can be divided into three main parts: (Fig., 5.10)

a) Forebrain             b) , Midbrain             c) Hindbrain

 

a) Forebrain

          The forebrain consists of:

 

i) cerebrum           ii) thalamus         iii) hypothalamus

 

Cerebrum:-   The cerebrum is the larges. part of the brain. Its surface is called cortex which covers and overlaps mos, of the other brain areas. It is composed of gray matter of some two billion cell bodies forming a folded mass which increases its surface area. The cerebrum is divided by a deep groove into two halves or hemispheres. The underlying area of cerebrum is formed of white matter having nerve tracts which connect with the cortex as well as with the spinal cord.

 

A section through the human brain.

                         Cerebrum controls many of the sensory areas such as sight, hearing and smell and motor areas such as voluntary muscles. Cerebrum is also concerned with many advanced functions such as memory, intelligence, reasoning, judgement etc.

Thalamus:-    The thalamus and hypothalamus form the posterior portion of the forebrain that lies above the midbrain. The thalamus serves as a relay station. It receives nearly all the impulses arriving from different sensory areas of the body before passing them on the cerebrum.

Hypothalamus.    The hypothalamus is a very important area possessing visceral reflex centers for controlling and integrating general metabolism and regulating body temperature, water balance, appetite, sleep etc.

 

b) Midbrain

         The midbrain is made up mostly of white matter surrounding central cavity. It lies just below the lower part of the cerebrum, midway between the two hemispheres. The dorsal part of the midbrain has a prominent mass of gray matter which along with cerebellum controls muscular activities. The ventral part of the midbrain has two rope-like masses of white matter that constitute the main connection between the hindbrain and the forebrain.

 

c) Hindbrain

       The hindbrain consists of three parts.

i) Pons    ii) cerebellum    iii) medulla oblongata.

Pons:-  It is a small lobe-like structure lying just above the medulla and is composed of largely white matter. It conducts nerve impulses between the cerebellum, medulla and cerebrum.

Cerebellum:- It is the second largest part of the brain but forms only about 10% of the weight of the brain. It has two lateral hemispheres and a central narrow lobe called vermis. Cerebellum is folded. It controls and coordinates the balancing organs and the muscles so movement becomes precise and accurate. If cerebellum is removed the movements of animals become jerky, shaky and poorly regulated.

Medulla oblongata:- It forms floor of the hindbrain and narrows down into the spinal cord. It consists of ascending and descending tracts of white matter with some gray matter in the interior. It controls many involuntary activities such as heart beat , breathing, constriction and dilation of blood vessels.

2) SPINAL CORD

           It is an oval-shaped hollow cylinder running through the vertebral column. It is made up of a very large number of neurons. The cross section of spinal cord shows on inner butterfly-shaped gray matter having a central canal, and an portion of white matter. The  gray matter consists of neuron cell bodies and non-myelinated parts of the fibers. The white matter is made up of bundles of myelinated nerve fibers or tracts. The spinal cord is the center for great many reflexes. It serves as a pathway for the conduction of impulses to and from different parts of the body and brain.

PERIPHERAL NERVOUS SYSTEM

              The peripheral nervous system consists of the two types of nerves:

i)                      Cerebral or cranial nerves that arise from brain.

ii)                     Spinal nerves that arise from spinal cord.

 

                      A nerve is a collection of nerve fibers dendrites or axons bound together with connective tissue. Nerves may be sensory, motor or mixed depending or their function. Mixed nerves have the fibers of both sensory and Motor neurons. There. are twelve pairs of cerebral nerves some of which are sensory, some motor and a few of them mixed. There are 31 pairs of spinal nerves all of which are mixed. The central nervous system depends on the nerves of the peripheral nervous system to receive and convey the nerve impulses.