Efferent division of the nervous system

Somatic efferent nervous system

The somatic division of the efferent nervous system controls voluntary movement like grasping an object, standing up, and sticking out our tongue. Action potentials generated in the motor cortex travel through the brainstem to activate peripheral motor neurons located either in the cranial nerve nuclei or in the spinal cord. These second-order motor neurons reach the target skeletal muscle, synapsing directly with muscle fibers through neuromuscular junctions. Every action potential generated in the lower motor neuron results in an action potential in the muscle fibers it innervates, causing muscle fiber contraction. Therefore, somatic efferent pathways are exclusively excitatory. The main neurotransmitter is acetylcholine.

Autonomic nervous system

The autonomic division of the efferent nervous system regulates visceral functions like digestion and urination. These neural pathways start from centers within the brainstem and generally include two synapses: one in the spinal cord and another in autonomic ganglia of the peripheral nervous system. The postganglionic neuron then innervates the target organ such as glandular tissue, cardiac muscle or smooth muscle (like that found in blood vessels and hollow organs). Notable exceptions include cranial efferents and efferents to the suprarenal medulla that lack synapses in autonomic ganglia.

The functions of the autonomic nervous system are usually involuntary, with minimal involvement of the cerebral cortex and some functions are partially regulated through spinal autonomic reflexes (for example: continence).

Functionally, the autonomic efferent nervous system can be divided into sympathetic and parasympathetic nervous systems:

• The sympathetic nervous system, also called thoracolumbar division because of the location of its main centers in the spinal cord, is associated with ‘fight or flight’ responses that require heightened awareness and reactiveness. The main neurotransmitters involved are acetylcholine and norepinephrine.
• The parasympathetic nervous system, or craniosacral division, is instead associated with activities like ‘rest and digest’ and the main neurotransmitter is acetylcholine. The peripheral ganglia of the parasympathetic nervous system are often located within the target organ.

When neither response is particularly needed, the sympathetic and parasympathetic nervous systems are in a state of balance. This is possible because the two divisions of the autonomic nervous system can have excitatory or inhibitory effects and often have opposite effects on an organ. For instance, activation of the sympathetic nervous system reduces insulin secretion, whereas the activation of the parasympathetic nervous system facilitates it. Therefore, our nervous system can regulate how much insulin secretion should be prioritized by changing the relative activation of the sympathetic and parasympathetic systems.

Enteric nervous system

Neurons located in the digestive tract constitute the enteric nervous system, which controls smooth muscles and glandular tissues involved in digestion. In contrast to the somatic, sympathetic and parasympathetic nervous systems, the enteric nervous system is entirely located in the peripheral nervous system. It regulates digestion, and some of its functions do not require input from the central nervous system.