Prof. J.P.N. Mishra
The central nervous system and the peripheral nervous system are subdivisions of the nervous system, instead of separate organ systems as their names suggest. The central nervous system (CNS) consists of the brain and the spinal cord. The brain is located within the skull, and the spinal cord is located within the vertebral canal, formed by the vertebrae. The brain and spinal cord are continuous with each other at the foramen magnum.
The peripheral nervous system (PNS) consists of sensory receptors, nervous ganglia, and plexuses. Sensory receptors are the endings of nerve cells or separate, specialized cells that detect temperature, pain, tough, pressure; light, sound, odors, and other stimuli. Sensory receptors are located in the skin, muscles, joints, internal organs, and specialized sensory organs such as the eyes and ears. A nerve is a sum of axons and their sheaths that connects the CNS to sensory receptors, muscles, and glands. Twelve pairs of cranial nerves originate from the brain, and 31 paird of spinal nerves originate from the spinal cord. A ganglion is a collection of neuron cell bodies located outside the CNS. A plexus is an extensive network of axons and, in some cases, also neuron cell bodies, located outside the CNS.
The PNS is further divided into two groups. The first group is sensory (afferent), division which transmits electric signals, called action potentials, from the sensory receptors to the CNS. The cell bodies of sensory neurons are located in ganglia near the spinal cord or near the origin of certain cranial nerves. The second group is motor (efferent), division which transmits action potentials from the CNS to effector organs, such as muscles and glands.
The motor division of PNS is may be further divided into the somatic and autonomic nervous system (ANS). The somatic nervous system transmits action potentials from the CNS to skeletal muscles. Skeletal muscles are voluntarily controlled through the somatic nervous system. The cell bodies of somatic motor neurons are located within the CNS, and their axons extend through nerves to form synapses with skeletal muscle cells. A synapse is the junction of a nerve cell with another cell. The neuromuscular junction, the synapse between a neuron and skeletal muscle cell. Nerve cells can also form synapses with other nerve cells, smooth muscle cells, cardiac muscle cells, and gland cells.
The ANS transmits action potentials from the CNS to smooth muscle, cardiac muscle, and certain glands. Subconscious, or involuntary, control of smooth muscle, cardiac muscle, and glands occurs through the ANS. The ANS has two sets of neurons that exist in a series between the CNS and the effector organs. Cell bodies of the first neurons are within the CNS and send their axons to autonomic ganglia, where neuron cell bodies of the second neurons are located. Synapses exist between the first and second neurons within the autonomic ganglia, and the axons of the second neurons extend from the autonomic ganglia to the effector organs.
The ANS is divided into two subgroups the sympathetic and the parasympathetic divisions The sympathetic division prepares the body for physical activity, whereas the parasympathetic division regulates resting or vegetative functions, such as digesting food or emptying the urinary bladder. Another specialized enteric nervous system consists of plexuses within the wall of the digestive tract. Although the enteric nervous system is capable of controlling the digestive tract independently of the CNS, it's considered part of the ANS because of the parasympathetic and sympathetic neurons that contribute to the plexuses.
The sensory part of the PNS works to detect stimuli and transmit information in the form of action potentials to the CNS. The CNS is the major site for processing information, initiating responses, and integrating mental process. It's much like a highly sophisticated computer with the ability to received input, process and store information, and generates responses. The motor division of the PNS conducts action potentials from the CNS muscles and glands (Seeley at. el., 2003).
