The involuntary response of body to a particular type of stimulus without any conscious effort called a reflex action. The neural circuit for the reflex action is called a ‘reflex arc’.
The reflex pathway comprises of receptor organ, an afferent neuron (receptor) interneuron in the brain or spinal cord, an efferent (effector or excitor) neuron and effector organ (skeletal muscle, smooth muscle or glands) appropriately arranged in a series.
The afferent or sensory neuron receives signal from a sensory organ and transmits the impulse via a dorsal nerve root of spinal cord or sensory nerve to the spinal cord or brain (CNS). The efferent neuron then carries signals from ventral horn of spinal cord through efferent or motor nerve to the effector organ (Fig 43a). The reflexes may be monosynaptic reflex – directly synapses between afferent and efferent neurons, e.g. stretch reflex, or polysynaptic reflex – involving more than one interneurons between afferent and efferent neurons e.g. cough reflex.
Sensory receptors Nerve fibers carrying information to the CNS are called afferent or sensory fibers. The sensory receptors are distributed throughout the body and it can be of three types according to the responds to stimuli. They are;
Exteroceptors – respond to the stimuli outside the body. Ex. Light, sound, skin sensation, etc
Interoceptors – respond to the stimuli originating from internal body conditions. Ex. Blood pressure, glucose level in blood, etc
Propriorecetors – respond to the stimuli originating in the muscles and tendons.
The sensory receptors are broadly classified based on type of stimulus they receive as -
a) Mechanoreceptorsb) Thermoreceptorsc) Nociceptorsd) Photoreceptorse) Chemoreceptors
Mechanoreceptors consist of different types scattered in different parts of the body. Free nerve endings are distributed all over the skin that detects sensation of pain and some detect warmth, cold and touch. Merkel’s disc, tactile hairs, Pacinian corpuscles, Meissner’s corpuscles, Krause’s corpuscles and Ruffini’s end organ receptors are responsible to detect and respond to tissue deformation, touch and very low frequency tissue vibration, movement of any object on the surface of the skin. These receptors are present in finger tips, lips, skin and deep layers of the skin. Mechanoreceptor in muscle spindle is golgi tendon apparatus stimulated by tension produced by the muscle fibers and reflexly inhibit the tension. Besides these receptors, cochlear sound receptor, vestibular receptor for maintaining equilibrium, baroreceptors in the carotid sinus and aortic arch for sensing the blood pressure are some of the mechanoreceptors in the body (Fig 43b).
The temperature changes in the body stimulated the thermoreceptors. Cold receptor responds to decrease in body temperature (12 – 37°C), whereas warm receptor responds to increase in body temperature (25 – 45°C). The pathway that conveys the thermoreceptor sensation to brain is lateral spinothalamic tract. Skin or tissue damages stimulated the pain receptors or Nociceptors that are present as free nerve ending on the skin and other tissues. The pain in human consists of three types as pricking pain, burning pain and aching pain. Bradykinin or similar other polypeptide are responsible for stimulating nerve endings of pain sensation. The lateral spinothalamic tract conveys information about pain temperature through medial spinal cord to thalamus.
The photoreceptors detect the light and become hyperpolarized when struck by light. For example rods and cone cells of the retina of eye.
Chemoreceptors are widely distributed in the body. The important chemoreceptors are gustatory chemoreceptor for detection of taste, olfactory epithelium chemoreceptors for detection of smell and peripheral chemoreceptors (carotid body and aortic arch receptors) for sensing changes in the hydrogen ion level, carbon dioxide and oxygen tension in the blood and convey the information to the brain.