Afferent and Efferent Impulses
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Neural impulses which travel from sensory organs/receptors to the central nervous system (CNS) are known as afferent impulses, whereas those which travel from the CNS to the organs/glands are known as the efferent impulses.
Afferent: neural impulse carries signals from sensory receptors or organs to the brain or spinal cord (CNS) for their further processing/analysis and
Efferent: neural impulse carries signals from brain or spinal cord (CNS) to the organs (like limbs, muscle, glands, etc.) for displaying proper reaction.
Both afferent and efferent terms have been derived from French. Afferent from ad ferens (Latin ad literally means to and verb ferre means bring) = bring towards, and efferent from ex ferens (Latin ex means from and verb ferre means carry) = carrying away.
The PNS gathers information from the environment and directs it towards the CNS; the CNS processes this information and directs it back to the PNS for suitable reaction. For example, when you accidentally touch a hot iron-press, the heat sensation is transmitted by the peripheral nerve to the spinal cord (afferent impulse), upon which spinal cord sends back signals through the nerves to the limb to initiate some motor action like to withdraw your limb or to push away the iron-press (efferent impulse). (Ganong, 2005) In former case, sensory neurons are involved; whereas in latter case, motor neurons execute the desired actions. The sensory neurons receive a wide variety of stimuli such as taste, smell, light, pain, etc. through different senses and sends these signals upwards through the nerves to reach in the CNS. Hence, these are also known as afferent or ascending pathways. The motor neurons form the efferent or descending pathways as they pass the signals along the nerves to the effector organ, which are primarily the muscles and glands (Fig. 1).
As evident from fig. 1, there is structural difference between afferent and efferent neurons. Afferent impulses are transmitted by pseudo-unipolar nerve cell, that is, it contains a soma with long axon that splits into two branches; one branch runs to the periphery and the other toward the spinal cord. Therefore, no dendrites are present in this case. The soma of efferent neuron is satellite shaped and consists of several shorter dendrites projecting out of it along with a long axon. This axon generally forms a neuromuscular junction with the effectors. (Dharani, 2015) The motor neuron is present in the grey matter of the spinal cord and medulla oblongata and forms an electrochemical pathway to the effector organ or muscle.