Low Molecular Weight Neurotransmitters

Abstract

Our lives are critically dependent on our ability to sense the environment and world around us and to use this sensory information to guide our actions or physical movements. Our sensory system is designed to detect physical phenomena such as light, sound, heat or pressure and chemical stimuli by means of taste or smell. The devices employed to detect such stimuli are called primary sensory receptors. These sensory receptors convert the environmental stimulus into an electrical signal or nerve impulse which travels along a sensory neuron to a primary processing center where information from the particular form of sensing is initially evaluated. The frequency of the impulses transmitted along a particular sensory neuron reflects the intensity of the stimulation. For example, if a sensory receptor in the skin of the hand senses a very hot surface a burst of high frequency electrical impulses pass along the sensory nerve fiber, to which the receptor is attached, to the spinal cord which rapidly then initiates a burst of nerve impulses along appropriate motoneurons to muscle systems that act to remove the hand from the heat source. This is a very rapid reflex action designed to protect the hand from damage. However, the brain and spinal cord, i.e., the central nervous system, continuously receives sensory information which is ultimately processed in cortical regions of the brain. A combination of voluntary decisions based upon past experience, which is stored in the central nervous system, and information provided from all sensory inputs is employed to decide upon an appropriate motor action. This information is transferred to the peripheral musculature via motoneurons which often project from the spinal cord and, to a somewhat lesser extent, directly from encephalic structures in the brain-system. The motoneurons carry nerve impulses to the muscles and cause the muscle to contract. Thus, neurons carry sensory information to the central nervous system. After appropriate evaluation and processing, neurons then carry instructions from the central nervous system to muscles to effect appropriate movements. Clearly, therefore, neurons or nerve cells are the biological entities responsible for the transfer and processing of all sensory and motor information.