Reflexes: principles and properties

The body responds to changing circumstances and environmental threats both consciously and subconsciously. The cognitive response to a physical threat normally involves movement mediated by skeletal muscle. There are a number of control mechanisms 'hardwired' into the nervous system that enable muscle systems to respond in an integrated fashion without involving a conscious decision, although the subject is usually conscious of what has happened. These include the stretch reflex, the withdrawal reflex and the crossed extensor reflexes. Muscle spindles, Golgi tendon organs and cutaneous nociceptors provide the sensory input to these reflexes, and muscle spindles also play a role in the control of voluntary movement. The autonomic nervous system controls the internal environment in response to environmental change. It consists of the parasympathetic division, which controls basal and vegetative mechanisms, and the sympathetic nervous system, which controls visceral adaptive responses to any sort of environmental change or threat.