Give structure of spinal cord, peripheral nerves, difference between spinal and cranial nerves


Structure of spinal cord:

Spinal cord is the part of central nervous system that extends from the brain to near of into the tail. A cross section shows a neural canal that contains cerebro spinal fluid. The gray matter consists of cell bodies and dendrites and is concerned with reflex connections at various levels of spinal cord. Extending from spinal cord are ventral and dorsal roots of spinal nerves. These roots contain main motor and sensory fibres (axon or dendrites) that contribute to the major spinal nerves. The white matter of spinal cord gets its name from the whitish myelin that covers the atoms.

Function:

It is the connecting link between brain and most of the body. It is involved in spinal reflex actions. A reflex is a predictable involuntary response to a stimulus. Thus both voluntary and involuntary limb movements as well as certain organ functions depend on this link.

Three layers of protective membrane called meanings surround the spinal cord. They are continuous with similar layers that cover the brain. The outer layer the dura mater is tough, fibrous membrane. Middle layer the arachnoid is delicate and connects to the innermost layer, the pia matter. The pia mater contains small blood vessels that nourish the spinal cord.

Peripheral nervous system:

It is composed of all nerves of the body outside the brain and spinal cord. These nerves are:

(1) Sensory (afferent) nerves which transmit information to central nervous system and motor (efferent) nerves, which carry commands away from central nervous system.

(2) Motor nerves divide into voluntary (somatic) nervous system, which relay commands to skeletal muscles and involuntary (visceral or automatic) nervous system which stimulates other muscles (smooth and cardiac) and glands of the body. The nerves of autonomic nervous system divide into sympathetic and parasympathetic system.

Nervous system pathways are composed of individual neuronal axons bundled together. In the central nervous system these bundles of nerve fibres are called tracts. In peripheral nervous system they are called nerves.

The cell bodies from which axons extend often cluster into groups. These groups are called nuclei if they are in central nervous system and ganlia if they are part of peripheral nervous system.

Difference between Spinal and Cranial nerves:

Spinal nerves: Extending from spinal cord are paired spinal nerves containing both motor and sensory fibres. Generally the number of spinal nerves is directly related to the number of segments in the trunk and tail of a vertebrate. For example a frog has evolved strong hind legs for swimming or jumping, a reduced trunk and no tail in the adult. It has only ten pairs of spinal nerves. By contrast a snake, which moves by lateral undulations of its long trunk and tail has several hundred pairs of spinal nerves.

Cranial nerves: In addition to paired spinal nerves peripheral nervous system of vertebrates includes paired cranial nerves. Reptiles birds and Mammals have 12 pairs of cranial nerves. Fishes and amphibians have only 10 pairs. Some of the nerves e.g. optic nerve contains only sensory axons which carry signals to the brain. Others contain sensory and motor axons and are termed mixed nerves. For example the vagus nerve has sensory axons leading to the brain as well as motor axons leading to the heart and smooth muscles of visceral organs in the thorax and abdoment.


Comments

Post a Comment

Popular posts from this blog

What is the Dynamic Theory of Profit

Image
Clark's Dynamic Theory of Profit And Its Criticism The Dynamic Theory of Profit is associated with the name of an American Economist, J. B. Clark. In the world of reality, according to J. B. Clark, profit arises only in a dynamic economy. An economy is said to be dynamic when there is a change in population growth or change in the method of production or a change in the consumer’s wants, etc. A society which is without these changes is called a static society. In a static society, only monopoly profits continue to exist. All other economic profits are gradually eliminated by competition. In a dynamic society, an entrepreneur is always confronted with continuous unpredictable changes in demand for his product. The variation in demand may take place due to changes in fashions, tastes, the standard of living, distribution of income, population, new inventions, international repercussions, technological advances, etc. A prudent entrepreneur will always keep an eye on future demand
Read more

Compare the anatomy of Bifacial and Isobilateral leaves

BIFACIAL LEAF: It is also called dorsiventral leaf. This leaf is more strongly illuminated on upper surface than on lower surface. This unequal illumination induces a difference in the internal structure between upper and lower sides. Its internal structure shows: (1)        Upper Epidermis This is single layer of cells with thick cuticle which checks excessive evaporation of water from the surface. It does not contains chloroplasts, stomata are also absent. (2)        Lower Epidermis This is also single layer but with a thin cuticle. It bears many stomata, two guard cells of which contain some chloroplast, none are present in the epidermal cells. Internal to each stomata is large cavity called respiratory cavity. Lower epidermis of leaf is meant for exchange of gases (oxygen and carbon dioxide) between the atmosphere and the plant body. Excess water also evaporates from lower epidermis. (3)        Mesophyll The ground tissue lying between upper epidermis and th
Read more

osmoregulation in terrestrial and aquatic animals

Mechanism of osmoregulation is very important for all groups of animals whether inhibting land or water. Unlike plant cells, animal cells when placed in hypotonic solution burst due to the continuous absorption of water. On the contrary they would shrink and die if constantly placed in hypertonic solution. Normally uptake and loss of water are in balance for proper survival of cell. (1)        Osmoregulation in terrestrial animals: Terrestrial animals are more likely to loose water by evaporation through their permeable surfaces exposed to amsophere. Among various animal groups only Arthropods and vertebrates became the most successful land dwellers. They have developed number of strategies to maintain osmoregulation of their body fluid. (a) Water proof external coverings: To prevent water loss through external surfaces, vertebrates like reptiles, birds and mammals have water proof keratinized epidermis. Similarly the insects have developed external water proof layer called
Read more