The spinal cord and brain are the central part of the nervous system, and they work receiving and interpreting the signals that the peripheral system sends to them, via the nervous cell called neuron. The following are some of the attributes of each of them:
The spinal cord is a thin tubular collection of nerves protected by the vertebral column, and it runs as an extension of the CNS from the medulla oblongata in the brain to the fibrous extension known as the filum terminale, prolonged downward from the apex of the conus medullaris (Fillum terminale, online web 2006). It consists of thirty-one pair of nerves covered by three connective tissues called the meninges. It carries information via electric impulses from the arms, legs and the rest of the body, as well as from the brain to the body (Definition of spinal cord, online web; Spinal cord, from wikipedia).
It contains one hundred billion nerve cells, it is considered the most complex organ of the body and it is the centerpiece of the CNS. It is organized in three interconnected layers: the central core, limbic system and the cerebral cortex. Basic life processes such as breathing, pulse, arousal, movement, balance, sleep, and the early stage of processing sensory information are fine-tune through the central core. The limbic system regulates body temperature, blood pressure and blood sugar level, as well as motivated behaviors, emotional states (as anger) and memory processes related with emotional feelings. The cerebral cortex determines intelligence, personality, touch sensation, assists in motor function, initiation of voluntary movement, auditory and visual information, comprehension of spoken language, and cognitive activities, as well as emotional function (Cardoso, 1997; Phillips, 2006; The human brain, online web 2001).
According to their functions, neurons can be classified in three groups: sensory neuron, motor neuron and interneuron. Sensory neurons are the ones that receive information from the outside (e.g. Light), motor neurons pass on messages to the muscles, and the interneuron which receives and sends the messages from and to other neurons (Palmer, 2003).
The parts of a neuron are the soma, dendrites and axons. The soma contains mitochondria and all the necessary elements for the cell to survive. The dendrites receive the information from other neurons or stimulus as heat, and the axons are the ones that send these signals away from the soma. The axons can be very long, and they are insulated with a membrane of myelin sheath along them. This semi-permeable membrane selectively limits the passage of charged particles, and when the cell is excited an explosive way of depolarizing current moves along the axon and out into the pre-synaptic terminals. If myelin sheath is stripped off, a process of demyelization occurs and multiple sclerosis can be present (Axon, online web 2001; Palmer, 2003).
Neurons send messages electrochemically, and if the stimulus is strong enough and the myelin sheath is active along the axon, a rapid and quick change in electrical activity passes along to other neurons, muscles or other body organs, creating a nerve impulse (see illustration 1) (Nerve impulse, 2007). However, many