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Reflex Arc


The reflex arc is a neural circuit that provides a framework for a reflex action. The reflex arc is the simplest nervous pathway. The reflex arc is made up of three neurones, namely: the sensory or afferent neurone, the relay or intermediate neurone and the motor or efferent neurone. A receptor e.g. sensory nerve endings, (often within a sense organ), receives a stimulus, while an effector e.g. a muscle or a gland, carries out a response. The sensory neurone transmits impulses from a receptor. it enters the spinal cord through the dorsal root of a nerve. The cell bodies of sensory neurones are concentrated in an area of the dorsal root called the dorsal root ganglion. impulses, therefore, travel from the receptor to the spinal cord or brain. The intermediate or relay neurone lies in the spinal cord or brain. it receives impulses from the Sensory neurone and passes them to the motor neurone, which then sends the impulses to an effector through the ventral root so that a very quick response is produced. A reflex arc that passes through the brain, it is termed a crani reflex arc and the action involved is a crani reflex action. Spinal reflex arcs, however, pass through the Spinal cord, producing spinal reflex actions. Some neurones run longitudinally through the white matter of the spinal cord to the brain. During a spinal reflex action, impulses are sent to the brain through these neurones. The brain then, takes note of reflex action, records it and the individual then, becomes aware of what has happened.

If the hand, for instance, incidentally touches a hot sharp object, e.g. a pin, receptor cells in the skin sensitive to pain are stimulated. These stimulated receptor cells generate impulses that move along the sensory neurone through the dorsal root of the spinal nerve.The impulses then pass through the dorsal root ganglion to the end of the sensory neurone. The impulses travel across a synapse and along the relay neurone, and then across another synapse to the motor neurone. This leads to a muscle (effector) in the arm, which contracts, and jerks the hand away. The above describes the pathway of a typical reflex action.

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