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.
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