1.0     Introduction

Phytochemicals are naturally occurring bioactive substances. They are found in different parts of plants in different concentrations. They are deemed valuable for their various functions (Onoja 2009) amongst which are: They play important protective and disease preventive role in plants and other organisms including humans, some of them attract pollinating agents, whereas others prevent predators. Due to the therapeutic properties of phytochemicals, most are now employed for culinary uses (Okwu, 2004). Recently, phytochemicals have attracted much interest from researchers in various fields because of their various applications (Onoja 2009). They are valuable biosource of drugs used frequently in traditional systems of medicine, modern medicine, pharmaceutical intermediates and as leads in drug synthesis (Ncube et al., 2008). They have been reported to be responsible for the antimicrobial, antibiotic, anticancer, antihelminthic and antisickling properties of many products used in medical practice (Preshant et al., 2008). The important bioactive compounds found in plants include: alkaloids, flavonoids, tannins, phenolic compounds etc [Preshant et al 2008]. The extraction of these phytochemicals and their application has been the focus of advances in science of late (Preshant et al., 2008]. The knowledge derived from their application is being harnessed daily in health care provision.

In pain, there is an uncomfortable sensitization due to actual or potential injury to tissues. There is usually an emotional experience which can be explained in terms of the damage done to tissues (Hill, 1952). Depending on the duration, pain may be acute or chronic. Chronic pain always has a long lasting sensation and may be of no identifiable cause. In acute pain, the duration of the pain may be limited and it is usually attributed to an identifiable cause. Nociceptive pain is evoked by specialized afferent fibers in the absence of sensitization. They are otherwise called physiological pain. Damage to the nerves of the nervous system results to neuropathic pain (Devor et al., 1999), when damage are done to tissues, stimuli are generated which are picked up and sent as signals via nociceptors to the dorsal aspect of the spinal cord. Inhibitory mechanisms to reduce the severity of perceived pain to tolerable levels is mediated via second order neurons of the spinothalamic tracts to higher brain centres. (Newman et al., 2000). Pain can be mediated via central or peripheral mechanisms. In central sensitization, there is a change in the functional state of neurons and nociceptive pathways throughout the neuraxis, caused by the increased membrane excitability and synaptic efficiency or by the decreased inhibition on the spinothalamic system. The series of processes that follow central sensitization include activation of wide range neurons, progressive increase of response provoked by a standard series of repeated stimuli, expansion of stimulus spatial extension and the triggering of changes lasting longer than the initial stimulus. Nociceptors play a vital role in peripheral sensitization. Action potentials are generated in the sensory endings of the healthy sensory nerve fibers once excited. Ectopicdischarges occur when nerve fibers are impaired. Nociceptors utilize inflammatory mediators which are short-term evoked. Once nociceptive nerve fibers are sensitized, second messenger systems are activated which influence ion channels.

1.1     Background of the Study

Natural products are generally either of pre-biotic origin or originate from microbes, plants or animal sources (Nakanishi, 1999). As chemicals, natural products include such classes of compounds as terpenoids, polyketides, amino acids, peptides, proteins, carbohydrates, lipids, nucleic acids-bases, ribonucleic acids, deoxyribonucleic acids etc. Natural products are not just accidents or products of convenience of nature; they are a natural expression of the increase in complexity of organisms (Jarvis, 2000). Increase in natural sources to provide treatments for pain, palliatives, or curatives for a variety of maladies or recreational use reaches back to the earliest point of history.

Nature has provided many things for humankind over the years, including the tools for the first attempt at therapeutic intervention (Nakanishi, 1999). Neanderthal remains have been found to contain the remnants of medicinal herbs (Holt and Chandra, 2002). The Nei Ching is one of the earliest health science anthologist ever produced and dates back to the thirtieth century BC (Nakanishi, 1999). Some of the first records on the use of natural product in medicine were written in cuneiform in Mesopotamia on clay tablets and date to approximately 2600 BC (Cragg and Newman, 2001a). Indeed many of these agents continue to exist in one form or another to this day as treatment for inflammation, influenza, cough and parasitic infestation. Chinese herbs guide document the use of herbaceous plants as far back in time as 2000 BC (Holt and Chandra, 2002). For a variety of reasons, the interest in natural products continues to this very day. The first commercial pure natural product introduced for therapeutic use is generally considered to be the narcotic morphine, marketed by Merck in 1826 (Newman et al., 2000). In